The Essential Guide to Composites Testing | R-TECH Materials

The Essential Guide to Composites Testing

Tips And Pitfalls From Our Testing Team

With the rapid growth of the composites industry globally expected to reach over $100 billion by 2021, the demand for high-quality composites testing and analysis has never been greater. The advantages of these materials across a whole range of engineering applications has led to the rapid adoption of composites by industries across the world.

Composites are complex materials with unique mechanical properties, requiring skill and expertise to produce accurate and reliable test results.

At R-Tech Materials, we have been developing our laboratories for testing composites for a number of years and have gained a wealth of knowledge and experience in the finer details of composites testing. As experts in the field, we offer here our essential guide to composites testing. Adapted from a range of articles written by our Composites Manager, Dr. Geraint Havard, this E-book addresses some of the common pit falls, frequently asked questions and a few dark arts for effective composites testing. We trust that it will be helpful! Should you require any further support, or have any questions, please contact us on +44 (0) 1656 748 000 or email us info@r-techmaterials.com.

Contents

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Best Practice Machining of Composites Materials

A Practical Guide to Compression Testing of Composites

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Why Aren’t Composite Tensile Specimens Dog Bones

Strain Gauging of Composites Dark Art of Simple Science?

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When Does a Composites Flexure Test Become an Interlaminar Shear Test?

Straightening Out 5 Common Areas of Testing Misalignment to Deliver Good Quality Test Results

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End tabbing of composite test specimens – Theory

End tabbing of composite test specimens – Practical Tips

Best Practice Machining of Composite Materials We are often asked about the best practice for machining of composite test specimens . Machining handbooks generally do not cover composite materials, so it’s not possible to look up the correct machining tools or feed rates. This lack of information can lead to the machining of composites being a bit of a dark art. With this in mind, what effect does poor machining have on test results?

And, what pitfalls should you watch out for when machining composites?

Heat dissipation issues Typically, when machining metals, the waste material chips away, but with composites there isn’t so much of a chipping effect, as a shattering process. Rather than shearing the material away, the impact of the cutting tool edge fractures the carbon fibres. Because the material doesn’t chip away and due to the low thermal conductivity of composite materials, there are significant difficulties in dissipating heat when machining, leading to high local temperatures. This can lead to local damage and distortion of the composite as the matrix melts. With this in mind, it is highly recommended that machining operations are water cooled. Alongside the heat dissipation benefits, the water prevents the fine carbon fibre dust becoming airborne and the associated health effects this would bring. [3]

Figure 1: Effect of surface finish on tensile strength of carbon fibre composite

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When wet cutting is used however, steps must be taken to ensure that specimens are adequately dried and that no water damage is introduced. Moisture can damage composites by reducing the strength of the interface between the fibres and the matrix, increasing the residual stresses within the matrix and in the case of glass fibres, leaching salts out from the fibres. [5]

What effect does surface finish have on the measured test results?

While there can be a small advantage in having a rough surface finish for composites to prevent extensometer slips, consideration must be given to the notch sensitivity of composite materials. Therefore, in general, the rougher the surface finish, the lower the measured result, in terms of strength or ductility.

In “Mechanical Testing of Advanced Fibre Composites�, J Hodgkinson investigated the effect of surface finish on tensile strength of carbon fibre using three different surface finishes. Samples were cut using a diamond blade with a 600 grit finish. Samples were then abraded with 80 grit aluminium oxide paper until all visible cutting marks were removed.

It can be seen in Figure 1, that by performing this abrasion, the tensile strength increased by 20%. Any edge polishing must be carried out carefully in order to avoid radiusing the edge of the specimen, or introducing localised width variations. Unfortunately, details of the grinding operation are not given by the author, but it can be seen that ground samples exhibit a greater tensile strength, however they also show a greater degree of standard deviation. [1]

Figure 2: Waterjet cutting With this in mind it is generally accepted that machining with a diamond blade is optimal.

Rough machining could be carried out by band saw, however this can damage fibres and gives a rough finish. Early versions of ASTM D3039 [2] used to recommend cutting test pieces 3mm wider than required, before grinding them back to the required finished width, however the latest version of the standard requires that the final dimensions are reached using water-lubricated sawing, milling or grinding. Waterjet cutting is now a popular technique, and it allows for the machining of more complex shapes. Samples can be machined out of a laminate in such a way that they are easily traceable to their location within the original laminate. If the laminate has been analysed by ultrasonic c-scan prior to machining, it is then possible to determine whether a weaker specimen within a test batch is due to a defect within the laminate in that location. Great care must be taken with waterjet cutting to tailor the method to the requirements of composite materials. A poor waterjet cutting method can lead to fibres being knocked out of the laminate, resulting in a poor surface finish, even delamination, giving significantly lower results. A further drawback of waterjet cutting, is that it leaves a tab of material attached to the sample,

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Best Practice Machining of Composite Materials which will need some post-processing to remove. An example of a poorly cut waterjet specimen can be seen below. Such a sample should be discarded due to the presence of obvious notches and machining damage, which would give unrepresentative results.

Waterjet cutting can’t always deliver the same level of precision as general machining tools, which can be critical with some composite specimens. [3]

Figure 3: An example of a notched specimen prepared by a poor waterjet cutting process Accuracy of machining – compression specimens The accuracy of cutting of test samples is obviously a key requirement. This is especially true for compression test specimens, where the relevant standards demand a +/- 0.02mm tolerance to the specified specimen dimensions. With ASTM standards there are also strict requirements on the parallelism and perpendicularity of test specimens. This means that machining must be highly accurate. The easiest way to achieve high accuracy is by cutting the samples to near net shape and then using a surface grinder to achieve the final specimen geometry.

In recent years there has been a significant amount of development in the field of CNC plate saws, by manufacturers such as Extec and Sharp & Tappin. These saws allow for geometries to be programmed into the machine which will then automatically machine samples to the correct geometry, claiming that no post processing is required. This facility would allow a lab to increase its workshop throughput, while also increasing operator safety. However these machines do have a sample size limitation, meaning an element of pre-processing would be required in order to ensure that the laminates for machining fit on the machine bed.

Figure 4: Example of CNC composite plate saw

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Other machining considerations, notches and drilled holes Machining quality for composite test samples is not limited to simple straight edges; the quality of machining of notches for v-notched shear testing can have a significant effect on measured shear strength. Similarly the quality of drilled holes can also have a major effect on test results because delaminations can be introduced by drilling.

Figure 5: examples of poor and good quality drilled holes. Hole in the left shows signs of delamination. Centre hole shows splintering/fraying of carbon fibre. Hole on the right is an example of a good quality hole.

It is also difficult to prevent breakout where the drill bit enters and exits the laminate. It is worth noting that it is difficult to drill holes in unidirectional laminates without causing longitudinal splitting. A smaller drill point angle tends to produce better exit quality.

However, too small a point angle can give the tool poor edge strength. For CFRP, the optimal compromise seems to be a point angle of 90 degrees. By comparison, a more typical point angle for drilling metal is 135 degrees (meaning the composites drill is “pointier�). [4] Entry defects can often be reduced by increasing the feed rate—although this poses the danger of exacerbating exit defects. The optimum feed rate balances quality on both the entry and exit sides. A solution that we have developed here at R-TECH is to use a sacrificial piece of composite material both above and below the laminate which needs a hole drilling in it. This leads to entry and exit defects being contained within the sacrificial samples. As with machining, it is important that coolant is used during the drilling process in order to prevent undue damage to the composite samples.

At R-TECH we have a machine shop manned by highly experienced and skilled operators, who are able to carry out composite machining operations to the highest quality, giving you confidence in our test results. To find out more about our composites preparation capabilities, from laminate manufacture to sample polishing for microsection analysis visit, contact us today. on +44 (0) 1656 748 000 or email us info@r-techmaterials.com.

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A Practical Guide to Compression Testing of Composites Compression testing of composites can be a minefield. With countless different standards based on three different means of load introduction, many fixture variations and a seemingly endless multitude of standards. But which is the ultimate compression testing method and what pitfalls should you look out for? Despite decades of research and use, no single compression test method has become generally favoured by the composites community. How has this situation come to pass, when the options for tensile testing are so straightforward?

Figure 6: Clockwise from top left, IITRI (Wyoming Test Fixtures), Celanese, Mixed mode compression, modified D695 and NASA end block compression fixtures.

Metals exhibit the same properties in both tension and compression and in the early days of composite technology, the belief was held that composites should do the same. However, when experimental results didn’t reflect this assumption, rather than question the science, technologists concluded that there must be something wrong with the compression test method leading researchers to develop many compression test methods! Samples would be seen to buckle at what was believed to be premature failure and a whole host of anti-buckling solutions were developed in order to prevent Euler column buckling. Despite the development of these anti-buckling fixtures, the compression strengths remained significantly short of the tensile strengths. In fact the truth behind the lower compression strength is actually caused by a difference in failure modes. In tension, the failure is caused by an overload of the fibres, while in compression we see micro-buckling of the individual fibres.

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This micro buckling is due in part to the difference in thermal expansion properties of the fibre and matrix during curing. A typical thermal expansion of a matrix is 70µε/°C, while the fibre would typically be -5µε/°C. This difference in thermal expansion leads to the fibres becoming slightly wavy during curing. In tension, these fibres straighten up and are able to experience their full load capacity, but in compression this waviness encourages micro-buckling [6]. This micro-buckling is not to be confused with Euler Buckling, which is a macroscopic phenomenon.

Figure 7: Microbuckling failure observed during composites compression testing [7]

During the period of doubt and discussion around this issue, a number of methods gained seniority over others, but in the early days, these methods were not without their problems. To ensure consistency in load application and ease of use, the standard ASTM D695 fixture, which was used for plastics testing, was modified to include an L-shaped stand, attached via mechanical fasteners [8]. This became known as the Boeing modified ASTM D695 (despite not actually becoming recognised as an ASTM standard in itself ), which underlined the end loading method also used for EN 2850 and SACMA SRM 1.

One of the downsides of this method was that the consistency of the test results was influenced by how tightly the fasteners were torqued. An overly torqued fixture can lead to a significant friction effect through a redundant load path, giving excessively high strength results. Due to this, the Boeing procedure recommends that the fasteners should be torqued between 0.68 and 1.13Nm. Similarly the first standardised compression test method, the Celanese shear loaded method used in ASTM D3410 was also not without it’s faults. The method essentially used an inverted wedge grip system, that increased the clamping forces as the compressive force increased. As the load is introduced through side acting wedge grips, the compressive force is introduced onto the end tabs of the composite specimen through a shear action. The challenge with the Celanese design was that a lack of alignment rods meant it was easy for the wedges to become misaligned which introduced a bending stress on to the specimen, promoting a premature failure [9]. This issue was overcome through the presence of alignment rods in the IITRI fixture but eventually these misalignment issues saw the Celanese fixture removed from the ASTM D3410 standard in recent years.

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A Practical Guide to Compression Testing of Composites

Figure 8 – Summary of methods using either shear, end or combined loading methods and their load introduction technique

Figure 9: Schematic diagram of the IITRI shear loaded compression fixture used for ASTM D3410

During the 1990’s, in reaction to the perceived weaknesses of the end loading and shear loading methods, the combined loading test method was developed. The theory behind this method is that by introducing the load through both the end and through shear, there is a reduction in the stress concentration introduced, giving more consistent results and minimising the deficiencies of each loading method [10-11]. At around 5kg in weight, this also leads to an easier to handle and lighter weight fixture than the IITRI fixture, which weighs around 40kg. The level of shear introduced can be

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controlled by the level of torque applied to the clamping bolts, with the minimum amount of torque to prevent end crushing failures being recommended. Generally this torque level is specified as being between 2.5-3Nm, however this can be increased if excessive end failures occur. It is not recommended to tighten the bolts to an excessive torque, as there are no positive benefits and it only serves to increase the detrimental clamping forces, which increase the stress concentration in the gauge area and causes through thickness distortion of the test specimen. [8]

Figure 10: Effect of bolt torque on measured compressive strength using combined loading compression fixture [8] All of this history and theory is nice, but which method actually gives the best results? Well the answer in short is all of them! A number of studies have been conducted comparing the results obtained using the IITRI and the modified ASTM D695 method and have showed they largely deliver the same results when used correctly [12]. Likewise, a similar initial study comparing the combined loading compression method with the IITRI method on a small number of samples showed similar results [13]. This highlights the golden rule of compression testing of composites: It is not what method you choose, so much as ensuring it is used correctly that leads to accurate and reliable test results.

In light of this, what parameters should we pay close attention to that can influence compression testing results? With end loading compression, the flatness and parallelism of the ends of the samples, along with that of the compression platens is of vital importance in order to ensure an even load transfer across the ends of the samples. This is why standards often state a requirement that the ends of the samples and the working area of

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A Practical Guide to Compression Testing of Composites your compression platens should be flat to within 0.02mm. If the samples aren’t parallel, you will end up with the load transfer being off axis in comparison to the fibre direction, which will promote bending and a premature buckling failure.

Likewise, with shear loaded compression samples, the flatness and parallelism of the tabbed surfaces on the side of the sample are of upmost importance. Again, this is in order to prevent excessive bending. It is highly recommended that back to back strain gauges are used on compression samples to ensure excessive bending does not occur. Unfortunately, strain gauges are the only reliable way to detect bending, as visual or microscopic examination is unable to give a reliable indicator of bending during testing. But how much bending is too much? Well, even though the ASTM standards acknowledges that bending strains of up to 40% may not significantly affect the measured compressive strength, a test is only considered valid if bending strains are less than 10% [14]. This is to ensure that your sample preparation and machine/fixture alignment is of sufficient quality to deliver consistent results.

Figure 11: Compressive strengths of various unidirection carbon/ epoxy composites as a function of slenderness ratio using different test types [12] Another consideration that is often overlooked is the hardness of the compression platens.

Insufficiently hardened platens will deform over time and lead to uneven load transfer at the ends of the samples. In practise this leads to end crushing rather than gauge length failures.

For this reason, it is recommended that the compression platens are regularly checked for any sign of excessive wear or damage. One thing that the wide range of compression test methods have in common is the use of a very short gauge lengths? Typical gauge lengths for compression samples are 10-13mm, although they can be as low as 5mm. In comparison, the typical gauge length of a tensile specimen is around 160mm. This is due to the requirement to minimise a premature buckling failure. But what is the ideal gauge length to deliver the highest measured compressive strength for your samples? In truth, the shorter the gauge length the better, as it minimises the possibility of buckling failures caused by poor specimen preparation. This is where the old SACMA SRM 1 & EN2850 standard come into their own. 9

Unfortunately, the SACMA standard has not been updated since 1994 and with SACMA having disbanded in 1999 [15] is unlikely to be updated in the future.

Figure 12: Suddenly diverging strain gauge readings indicating buckling failure [12]

There are, however, drawbacks to using short gauge lengths. Gauge lengths shorter than 10mm mean that a separate specimen needs to be used for strength and modulus determination, as it is incredibly difficult to bond a strain gauge accurately on this scale.

It is worth noting that, while tensile samples perform best with a tapered tab termination region, this is inadvisable for compression samples. While the benefits of a tapered tab in reducing the stress concentrations at the tab termination area are clear and well researched, the requirement to prevent micro buckling failures outside of the gauge length is of greater significance. For that reason, it is preferred that samples are not tapered in order to offer full structural support to the sample outside of the gauge area. Due to this, the majority of compressive failures occur at, or very near to the tab termination region. One way of reducing the tab termination stresses however is to use a thicker adhesive bondline, with adhesive layer thicknesses of 0.3-1.3mm recommended.

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One final consideration is the thickness of composite test specimens. As thin rectangular specimens are used for testing, these are typically prone to buckling failure. In order to overcome this, a minimum specimen thickness is required, which can be calculated from the equation given in ASTM D3410 and ASTM D6641M, based on the gauge length, the expected compressive strength, the compressive strength of the fibre, the fibre volume fraction, the expected flexural modulus and the expected through-thickness shear modulus (We told you it can be a minefield!). When buckling failure occurs, the only thing that has been determined is that the compressive strength of the material is higher than the stress at which buckling has occurred.

In summary, as long as your sample preparation and test set up is such that you can get consistently reliable test results from the fixture you are using, then the best compression test method is the one that works for you. At R-TECH Materials we are accredited by UKAS to perform compression testing to a multitude of standards and would be happy to help you with your compression testing enquiries.

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Why aren’t composite tensile specimens dog bones?

The tensile strength and modulus of materials are crucial characteristics that engineers use for component design. This information is determined by means of a simple tensile test. For most materials, a dog bone shape is used in order to promote a failure within the gauge length of the extensometer and prevent grip end failures, but not for composites. Why?

Figure 13: Dog ears shearing off a carbon fibre dog bone specimen, leading to failure in the grips

With a metal or a polymer, the material is isotropic, meaning that the material exhibits the same behaviour in all directions. Composites however are different. The strength of a composite material exists in the orientation of the fibres within. Because of these fibres, running the length of the sample, the removal of material within the gauge length to form the familiar dog bone specimen shape. This may at first seem of little relevance, however, the problem with this approach is that the shear stresses exerted during a tensile test are greater than the shear strength of the matrix. In practise, this results in the dog ears simply shearing from the specimen, leaving what is essentially a straight sided specimen in the grips.

As the applied through thickness force exerted within the grips (assuming wedge grips are used) is typically 20% higher than the applied force, this will result in grip breaks. This is undesirable as it gives an artificially low failure load. In order to overcome this issue in metals dog bone specimens are typically used. It has been calculated that in order to prevent the shear stresses overcoming the shear strength of the matrix, the radius of the dog ears would have to be one metre [1]. Obviously, this would make gripping of the specimen impractical.

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So as a composites test lab, how do we overcome this issue? The answer in short is end tabbing. End tabbing involves bonding glass fibre, sometimes carbon fibre, to the specimen using an adhesive that has a shear strength greater than the exerted shear stress. Glass fibre is preferable because it is soft enough to enable the grips to bite into the material surface and of course, is cheaper than a carbon fibre alternative. In addition, its low stiffness in comparison with carbon fibre allows for a better distribution of stresses.

By end tabbing, the thickness, rather than the width of the material in the grip regions is increased, resulting in a failure in the gauge length. An additional consideration that can be introduced is the tab termination angle. A tapered tab angle is recommended in order to minimise the stress concentration in the tab termination region and as such prevent failures in the tab termination region. Despite this, theoretically a perfect composite should fail at the tab termination angle as ultimately this will be the area of highest stress.

In practise however, specimens contain flaws within the gauge length and as such, this is where failure will occur. Whilst this can all seem a little bit theoretical, in practise, it makes a huge difference in achieving accurate, reliable and repeatable results.

Figure 14: Tabbed specimen being placed into machine ready for testing

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Strain gauging of composites - dark art or simple science?

The relationship between stress and strain is the most fundamental aspect of mechanical testing. One of the primary means of measuring the strain is through the use of bonded strain gauges. Strain gauging is an ability that every test engineer should have in their toolbox and in theory, it should be a relatively easy tool to use. Simply stick the gauge to the specimen, solder on some lead wires and away you go. In reality, however, life is never that easy!

Without knowing a few simple techniques, a world of heartache caused by detached gauges and broken terminals, stripped from the gauge by the lead wires, will quickly follow.

In order to successfully gauge a sample, a few key aspects need to be considered.

Choosing the correct size gauge to fit on the specimen will improve the measurement of the strain. Alignment of the gauge is also critical; ensuring alignment along the straining axis. Misalignment of 2° can lead to an error in measurement of 15%. Having a larger gauge makes it easier to ensure accurate alignment. Additionally, having a gauge length larger than the inhomogeneity of the material ensures that the measured strain is representative of the material specimen.

A lack of heat dissipation from the gauge into the composite can also cause issues, such as errors in strain measurement, so it is vital to ensure the right type of gauge is chosen. 350Ω gauges are generally considered the best choice for composites, as strain gauges with a higher resistance produce less heat for the produced voltage. Careful consideration must also be given to the choice of adhesive, and the preparation of the sample before applying adhesive will significantly reduce the chances of a failed test.

Figure 15: Strain gauge attached to carbon fibre test specimen

Much consideration must go into these factors in order to achieve optimum results, as without a properly adhered gauge, you will not get accurate strain readings. Without proper guidance, this can seem like a dark art, but proper knowledge and experience can quickly change your luck. As they say, “the more you practise, the luckier you get.”

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A final tip…take great care with the positioning of the lead wires. Ensure that there is a proper stress relief in the wire so that there are no problems caused by a sudden, even slight tug of the wires, which can rip off the terminals and leave you with a useless gauge.

There is no worse feeling than witnessing such a situation after spending a great deal of time and care ensuring that the gauges are properly bonded to the specimens!

Due to these challenges, strain gauging is generally too impractical and time consuming for routine testing, where modern extensometry is generally sufficiently accurate.

However, there are a number of standard tests that do require strain gauging; usually for modulus determination (see below). There will also always be a need for bespoke testing, where more automated methods would not be applicable. As such, it is always crucial to keep strain gauging in your tool box and to ensure that you can do it well. R-TECH Materials staff have been fully trained in practical strain gauging techniques by Vishay Micro Measurements Group, and can offer this service to our clients where specified.

Examples of standard tests requiring strain gauging: • • • • • • • •

BS EN ISO 14126 (Compression) ASTM D695 (Compression) prEN 2850 (Compression) SACMA SRM (Compression) ASTM D6641 (Compression) BS EN ISO 14129 (In plane shear) ASTM D3518 (In plain shear), ASTM D3039 (test machine alignment check)

Figure 16: Back to back strain gauged specimen in combined loading compression fixture

Contact Us To find out more about testing of composite materials at R-TECH Materials please visit www.r-techmaterials.com or email myself at g.havard@r-techmaterials.com

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When does a composites flexure test become an interlaminar shear test?

Three or four-point bend testing has long been a go-to method for the composites engineer to gather relevant design data for their material or component. Depending on the span used, and with some alterations to the diameter of the loading rollers, the same test set-up can be utilised to determine both the interlaminar shear and the flexural properties of a composite material.

Although interlaminar shear testing doesn’t provide a pure shear state (where there are no additional compressive or tensile forces), it remains very popular in the industry.

This popularity is due to the test providing useful information on the interlaminar shear behaviour of the laminate, while also being a cheap and easy test to undertake. However, because of this imperfect shear state, ASTM has removed the word shear from the title of ASTM D2344 [26], simply referring to it as short beam strength.

Whilst we all refer to the relevant standards to tell us what spans and radii to use for these tests, what actually determines whether the test will produce a delamination or a fibre breaking failure? Additionally, at what point will the span become so narrow that we observe an invalid failure due to crushing of the specimen under the rollers? By answering these questions and understanding the parameters that determine these transitions, we can react to the failure modes being exhibited during testing; ensuring that we adjust the test to get valid results.

Figure 17: Type of failures recorded by 3 point testing of a composite sample with different span to thickness ratios.

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Figure 18: Interlaminar shear testing

The answers to these questions are determined by the shear strength of the fibre matrix interface. The shear stresses in a beam during 3-point bending are the same regardless of specimen length. Therefore, if a flexural failure mode is desired, then an increase in the bending moment during testing will ensure that flexural failure is achieved prior to reaching the shear strength of the composite.

Experiments carried out on carbon fibre specimens with different matrices have identified that shear failure is most likely to occur is in the span-to-thickness range of 4:1 and 9:1.

Below 4:1 there is a high likelihood of crushing, or combined failures. 10:1 or above and you would expect a flexural failure. [9] With this information in mind, it is possible, within an R&D environment, to tailor the span to thickness ratio to suit the desired failure mode. If crushing failures are occurring, it is recommended to increase the span to thickness ratio to promote a shear failure. Similarly, if a shear failure is observed when flexural failure is desired, a further increase in the spanto-thickness ratio will promote a flexural failure.

An interesting inconsistency in this discussion is the differing instructions given by the ISO and ASTM standards for Interlaminar Shear tests. ASTM has chosen a span-to-thickness ratio of 4:1 for short-beam testing, while the equivalent ISO standard states a 5:1 ratio should be used. Given the discussion above, there seems to be wisdom in ISO 14130 [27] choosing 5:1 as the span ratio. Not only does this span reduce the likelihood of achieving a crushing or mixed mode failure, but the specimens are longer, easier to work with and it becomes that little bit easier to determine the failure mode.

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Straightening Out 5 common areas of testing misalignment to deliver good quality test results In the world of fibre composite testing, alignment is king. Unlike metals, which are isotropic in nature, the properties of composites vary significantly according to the directionality of the specimen’s fibres. Furthermore, in compression testing, any sources of misalignment can induce buckling, producing invalid results regardless of the specimen appearing to fail by a valid failure mode.

All this means that ensuring and maintaining alignment throughout the whole test setup, is the only means of delivering good quality, repeatable results. Now you might be thinking, ‘well how hard can it be?’. We have all seen photos of drastically wonky samples being tested by inexperienced and naive technicians. But don’t be fooled, guaranteeing alignment takes a lot more than just a good chunk of common sense, and in some cases, satisfactory alignment can be impossible to achieve. Here are 4 areas to straighten out in your test method to guarantee reliable, high quality, consistent test results.

Fibre Alignment and Specimen Machining: Fibre misalignment is generally introduced due to a lack of care and detail during machining. Failing to ensure that samples are machined such that the fibres are oriented perfectly can lead to a reduction in measured values.

A 1° misalignment of the fibres leads to fibres terminating off the edge of your specimen prior to the end of the sample. These fibres that were intended for load bearing will therefore be unable to bear the loads required of them, ultimately reducing the load bearing capacity of the specimen and delivering reduced results. Alongside diminished reported strengths, misaligned machining also results in an uneven stress distribution amongst the remaining fibres, leading to premature failures in the high stress region, illustrated by figure 1. [1]

Figure 19: Stress distribution due to misaligned fibres

The effect of misalignment can be seen in a graph taken from the book Mechanical Testing of Advanced fibre composites in figure 2. Not only does misalignment deliver reduced results, but also a significant increase in the standard deviation of the results. Bizarrely the standard deviation caused by a misaligned specimen is reduced by using side acting hydraulic grips in comparison with wedge grips, however it is clear that it leads to a reduction in the measured tensile strength.

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So how do we guarantee that we machine our samples in such a way that we get well aligned fibre? Aligning unidirectional composites can be easily achieved by using what is called the ‘chocolate bar’ snap test. Using the edge of the bench, you can snap the bar along the length of the fibre, due to the unsupported matrix between the fibres being so brittle. With a non-unidirectional specimen, things aren’t quite as straight forward however. The only way to guarantee this alignment would be by visibly tracing the fibre direction and marking it with an indelible marker. This line can then be used as a reference for machining.

Figure 20 – The effect of misalignment on tensile strength of composites

Grip alignment You have now spent a great deal of care to make sure your samples are aligned, however this can all be undone by a poorly aligned machine. Misalignment of the load train or the specimen leads to non-uniform force application resulting in additional bending or twisting forces being imparted on the specimen and erroneously low breaking forces.

This is why many tensile standards, such as ASTM D3039 [24-25] and ISO 527, require you to prove the alignment of your machine during testing by using a strain gauged specimen.

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Straightening Out 5 common areas of testing misalignment to deliver good quality test results The methods outlined in both standards require that three strain gauges be placed around the midpoint of a specimen, two on the front face and one on the rear face to measure the bending stresses applied on your specimen during the modulus determination range. In order to produce a valid test, the applied bending in any direction must not exceed 5%.

Moving into the world of testing for aerospace applications and the requirements become more stringent again. Nadcap stipulate that the alignment of your testing machine meets the requirements of ASTM E1012. In this standard mis-alignment is defined as “the greatest difference in axial strain under load between the average strain (of 3 or 4 strain gauges in any circumferential set) and any individual gauge in that set, divided by the average strain, times 100�. Using a specimen with a minimum of 8 strain gauges, the recorded mis-alignment of all directions combined must not exceed 8% when testing composites.

A great deal of attention must go in to ensuring the grips are correctly set and if the load train changes for whatever reason, for example by changing the grips being used, the alignment must be verified again.

Figure 21: Zwick Roell grip alignment fixture

Traditionally small degrees of misalignment could be over come by the use of carefully placed metal shims, however this involves a great deal of time and effort. These days, some machine manufacturers have helped ease the pain by producing an alignment fixture. This fixture allows for fine adjustments of the alignment in each direction to give the highest possible degrees of alignment. An example of which can be seen in figure 3 above. Some manufacturers even provide a software package to accompany the fixture, which allows you to easily determine the precise amount of adjustment needed based on the results given by ASTM E1012. [27]

Figure 22: Example of a specimen misaligned within the grips of a tensile machine

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Specimen gripping So, you have made sure that your fibres and your grips are aligned, but that is only half the battle. If your specimen is misaligned within the grips, it can undo all the good work of ensuring your grips are aligned and some. One way to ensure the specimen is aligned within the grips is by eye, but this can be quite unreliable and time consuming. Another more consistent approach is to use physical back stops. You can set the back stops to the required distance and by placing the sample in up against the back stops, you can ensure that the specimen is perfectly aligned every time. An example of this can be seen in figure 5.

Figure 23: Example of back stops used for specimen alignment

Strain Gauging Finally, you have done everything you can to test your samples with perfect alignment, but the struggle doesn’t necessarily end there! Strain gauging can be another pitfall of misalignment. Errors of 15% can arise from a 2°degree misalignment of a strain gauge. [1] Great care must go into ensuring the correct alignment of a gauge on the specimen. This is where is becomes essential to have highly trained technical staff to carry out the strain gauging, or to outsource to a company that specialises in applying strain gauges, as strain gauging of composites can be difficult. This can be helped by the use of larger gauges however.

In summary, if you want to produced accurate and reliable results, you need to minimise all potential sources of misalignment. The combined errors introduced can have significant effects on the measured results.

To find out more about factors which can negatively effect your testing results, please check out my previous articles on why aren’t composite tensile samples dog bones, when does a composite flexure test become an interlaminar shear test and strain gauging of composites – dark art or simple science? Additionally, sign up to our popular training course on mechanical testing of composite materials.

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End tabbing of composite test specimens – Theory Previously, we have covered why we do not use dog bone shaped specimens when testing composites. In order to ensure we get gauge length breaks, as a test lab we instead turn to end tabbing.

There seems to be a sense of unnecessary mystery in the composites industry surrounding the end tabbing of composite test specimens. This view is not helped by the following statement in ASTM D3039-17 for tensile testing [28]: The reality however, is that there is something of an industry consensus on how to end tab specimens. I will go into further detail as to why I do not categorically state that there is a definitive way, as there are some small differences between laboratory methods. Despite this, best practice tabbing of composite materials can be found in DOT/FAA/AR-02/106 Tabbing Guide for Composite Test Specimens [29].

Why do we end tab?

Before we delve into the how, lets first explore why end tabbing of composites is necessary. Assuming that wedge grips are used, at a typical wedge angle of 10°, the through-thickness clamping force applied on an untabbed specimen will be 28% higher than the applied tensile force. This causes a significant stress concentration at the edge of the grips. To counter this and to promote a failure in the central gauge section of a specimen, tensile specimens are usually dog boned in shape. However, as covered in a previous article, this isn’t possible for composite materials, so an alternative way of alleviating the stress concentration is required. An additional consideration is the damage that the grip serrations cause to the surface of the composite. These are usually sufficiently aggressive that they can break through the resin-rich outer surface and damage the underlying load bearing fibres. With this in mind, it is clear that both the stress concentration needs to be limited and the surface protected.

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Figure 23: Zwick Roell grip alignment fixture

That is not to say that end tabbing is always necessary. As a rule of thumb, the more orthotropic a material is, and the higher the strength, the greater the need for tabbing becomes [30]. Additionally, the higher strength the material, the more impact end tab design has on the test results. It is perfectly reasonable not to end tab fibreglass constructed from chopped strand mat for example, as it’s relatively low strength and irregular fibre distribution, can promote gauge length failures without the need for any additional measures. Similarly, it may be possible to use a sheet of emery cloth as the cushion between the grips and the specimen for intermediate strength composite materials. However, with unidirectional composites, end tabbing becomes a necessity.

What material should we use?

Material choice for end tabs can influence both the measured properties and the ease of use through future operations. The chosen material should be as compliant as possible, without losing the strength necessary to transfer the load from the grips to the specimen’s gauge section. It must also be sufficiently tough to withstand the harsh tearing action of the grips.

Metallic end tabs have a long history of use with composite test specimens, with carbon steel or aluminium being the preferred metallic material. Both of these materials have issues though. The high toughness of steel, means that when machining specimens with pre bonded steel tabs, the tabs will pop off. This means that the only way to accurately tab specimens with steel is to tab specimens after machining. Tabbing in this way is incredibly time consuming and to guarantee the alignment a special fixture must be used.

While aluminium end tabs do not suffer with popping off in the same way, aluminium requires special surface treatment to prepare it for bonding. Due to the oxide film that forms on aluminium, a chemical etch is required to promote bonding. This involves the use of chromic acid or phosphoric acid to remove the oxide film prior to bonding. Extreme caution must be used when using chromic acid, as there are significant health concerns related to its use.

The most commonly used tabbing material is fibreglass printed circuit board material, ideally in a +/-45° orientation. The material is easily bonded to the composite panel and does not pop off during machining. The relatively low stiffness of the fibreglass in this orientation helps to minimise the tab termination stress concentrations and it

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End tabbing of composite test specimens – Theory is sufficiently soft to allow the grips to bite into the surface of the tab. Printed circuit board material is also relatively low cost and is readily available in various thicknesses.

An additional benefit of using fibreglass end-tabs is the material is translucent and any gaps in the glue line can easily be seen by visual examination. In concluding their review of tabbing materials, Adams and Adams concluded that “Glass fabric/epoxy should be considered as a tabbing material unless there is a clear reason why another material is preferred [29].”

The final material option for tabbing is carbon fibre. Carbon fibre is an expensive tabbing material and the additional stiffness of the carbon fibre means that it doesn’t have the same stress concentration reduction capabilities as fibreglass.

Figure 24: A selection of samples with fibreglass end tabs

Adhesive choice As the tensile load is introduced into the specimen via shear, it is important to choose an adhesive that has a suitably high shear strength. The old CRAG standard specified that the adhesive used should have a shear strength of more than 30MPa [31]. Beyond that, the choice of adhesive is driven by the user and the Tg of the material being tested.

Film adhesives are generally easier to use and give a controlled bond-line thickness, however they require an elevated temperature and pressure to cure. This is either achieved through a hot press, or by vacuum bagging within a curing oven. In some cases, the cure temperature of the adhesive is greater than the Tg of the test material, in which case, using a film adhesive can have a deleterious effect on the mechanical properties.

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Alternatively, a two part epoxy paste adhesive can be used. These do not require any additional equipment to use and can be cured at room temperature, negating any concerns over the Tg of the material, however steps need to be taken in order to control bond-line thickness.

End tab design

Figure 25: End tab design A final consideration when it comes to tabbing material is the design of the tab itself.

Generally speaking the tab should be the same width as the test specimen and for tensile testing it should be at least 40mm long and up to 90mm long. The length chosen in that range depends entirely on the length of the wedges in the grips of the machine you are using.

While ISO 527-4/5 categorically state that a 90° tab termination angle should be used [32], ASTM D3039 is more nuanced in its approach to this. This is due to research which has shown that using tabs having a tapered termination angle of between 10-15° can exhibit an increase in tensile strength of up to 18%. This is because tapering the tabs reduces the stress concentration at the tab termination region and thus, promotes failures within the gauge section. Shallower angles than 10° however are not recommended, as this leads to the tabs peeling off during testing [30].

Conversely, when compression testing of laminates, it is recommended not to taper the tab termination area. The reason for this, is that the potential for buckling has more of a bearing on results than the stress concentration caused by the tab termination region.

We therefore compromise on the design in compression, in order to minimise the effects of buckling on results. More information on this can be found in the previous article “A Practical Guide to Compression Testing of Composites.”

Finally, the thickness of both the adhesive and the tabbing material has an affect on test results. Both materials act as a cushioning layer, with the closest to the material, the adhesive having the greatest effect. As a rule of thumb, a tab thickness of 1.5-2mm thick and a bond line thickness of 0.5-1.3mm generally deliver optimum results, while avoiding the additional cost of using more material.

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End tabbing of composite test specimens – Practical tips Previously, we covered the importance of alignment when testing composite specimens. For that reason, it is vitally important that steps are taken to ensure correct alignment of the test coupons from the start of the tabbing process Alignment Aligning a unidirectional composite can be easily achieved by using the ‘chocolate bar’ snap test, where you can easily snap the laminate along the length of the fibre. With a non-unidirectional sample however, you must physically trace the fibre direction using an indelible marker prior to machining. Ensure that the laminate is machined with a square edge along the fibre direction, when machining down to the required tabbing geometry.

Pre-treatment In order to get a good quality bond between the end tab and the test specimen, there is a need to first ensure there is a suitable bonding surface. There are two ways to ensure this.

The most common way is to mechanically abrade the bonding surface of both the end tab material and the test material. The goal is to both remove any surface contamination, such as mould release agents, greases or oils and to key the surface, in order to aid mechanical bonding.

This can be done by either grit blasting, or by hand sanding. It is important that during this operation, only the outer resin layer of the laminate is removed, without causing any damage to the underlying fibres. Once abraded, make sure that the bonding surface is suitably cleaned and solvent dried before proceeding to applying the adhesive. A useful tip In order to check if the surface is suitably abraded is to carry out a water break test. This involves pouring water on the surface of the abraded material. If it is not suitably abraded, then the water will gather in a number of small puddles on the surface, whereas a well abraded sample will lead to the water gathering as a solid film on the surface of the end tab. An example of this can be seen below, where the water has been dosed with red food colouring in order to more clearly show the water agglomeration on the surface of the tab material.

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Figure 26: Water break test carried out on non-abraded sample on the top showing coloured water patches and an abraded sample below, showing a solid film of coloured water An alternative solution is to chemically etch to bonding surfaces. This comes with the benefit of reducing the amount of fibreglass dust that becomes airborne from the pre-treatment process. An example of such a chemical etch can be found from Oxford Advanced Surfaces, with their ONTO adhesion promotion range.

Bonding the end tab Adhesive selection is just as important as tab material selection. The adhesive must be able to transmit the applied load into the test specimen via shear and must withstand the compressive force applied by the grips. The ability to control bond line thickness is also desirable, as this can allow for the design of the tab region to be manipulated in such a way as to further reduce the stress concentrations.

Either a film or a paste adhesive can be used to bond the end tabs to the surface of the laminate. The choice of adhesive really depends on the application. For example, paste adhesives tend to have a smaller window of operating temperatures, however their application is simpler.

Film adhesives require a combination of temperature and pressure in order to cure, whereas paste adhesives will cure readily under ambient conditions. The benefits of film adhesives are the ease with which you can control the bond line thickness and the larger operating window of test temperatures. However, care must be taken when using a film adhesive, to ensure that the cure temperature of the adhesive does not exceed the post cure temperature of the test material or the end tab material.

As far as paste adhesives are concerned, commercially available two part epoxy resin systems have been found to be suitable for bonding end tabs. Whichever adhesive is used, it should be tough, with a greater strain to failure than the test material and have a shear strength in excess of 30MPa. It is important while using a paste adhesive, to ensure that the adhesive is applied to both the end tab and the composite panel during tabbing. Steps should also be made to control the bond line thickness. This can be done through the use of wire spacers, fishing wire, or using a layer of vacuum infusion distribution mesh.

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End tabbing of composite test specimens – Practical tips How thick should the bond line be? Well it depends on the nature of the end tab. For tapered end tabs, the bond line thickness has no significant effect on tab termination stress concentrations, however with un-tapered specimens, such as for compression testing, a bond line thickness of 1.3mm is recommended.

Tab alignment Alignment of the tab on to the material surface can have an impact on the alignment of the specimen within the grips. A well aligned tab can aid this, however, oversized or poorly aligned tabs can lead to the sample becoming misaligned within the grips of the machine. Furthermore, some compression standards specify the maximum misalignment between the tabs, from one side of the specimen to the other. Therefore, it is important to take steps to ensure good quality alignment during the bonding process. In order to achieve this alignment, it is recommended to use a fixture for the tabbing process. An example of which is commercially available from the Wyoming Test Fixtures website. The fixture is aluminium, which allows for suitable heat transfer through the fixture should an elevated temperature be required for the adhesive.

Figure 27: End tabbing fixture from Wyoming Test Fixtures

The fixture’s base plate is fitted with pins, against which the strips of tabbing material can be aligned, to ensure a consistent specimen gauge length. After applying the adhesive, place the test material on top of the tabbing material, between the alignment pins. Repeat the process for the other side and then finally, attach the top plate. After this point, carry out the cure schedule as required.

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References: [1] - Hodgkinson, J. (2000). Mechanical testing of advanced fibre composites. Cambridge: Woodhead Pub. [2] - ASTM D3039M-17 Standard Test Method for Tensile Properties of Polymer Matrix Composite Materials. (2017). ASTM International.

[3] - Zelinski, P. (2018). How To Machine Composites, Part 1-5 — Waterjet Cutting. [online] Mmsonline.com. Available at: https://www.mmsonline.com/articles/how-to-machine-composites-part-5—-waterjet-cutting-of-composites [Accessed 17 Aug. 2018].

[4] - Sandvik Coromant (2010). User’s Guide Machining carbon fibre materials [5] - Hull, D & Clyne, T. (1996). An Introduction to Composite Materials. 2nd Cambridge University Press [6] - IRT Saint Exupéry. (2018). Graphene in Composites, unexpected science from a pencil trace by Constantinos Soutis. [Online Video]. 6 September 2017. Available from: https:// www.youtube.com/watch?v=HDxv40K6Dk8. [Accessed: 4 April 2018] [7] - Lemanski SL, J Wang, Sutcliffe MPF, KD Potter, MR Wisnom. Modelling failure of composite specimens with defects under compression loading. Composites A (2013) [8] - Adams, D. (2007). Testing Tech: The Modified D 695 Compression Test Method.

Composites World. [online] Available at: https://www.compositesworld.com/articles/testingtech-the-modified-d-695-compression-test-method [Accessed 20 Aug. 2018].

[9] - Adams, D. (2017). Test Methods for Composite Materials.

[10] - ASTM D6641M-16 Standard Test Method for Compressive Properties of Polymer Matrix [11] - Carlsson, L. Adams, D. & Pipes, R. (2014). Experimental characterization of advanced composite materials. 4th Florida: CRC Press pp.131-145 [12] - Charterjee, S. Adams, D. & Aplinger, D. (1993). Test methods for composites a status report volume II. Compression test methods. FAA Technical Center [13] - DOT/FAA/AR-00/26 Verification of the combined load compression (CLC) test method [14] - Adams, D. (2014). Bending of composite material compression specimens. Composites World. [online] Available at: https://www.compositesworld.com/articles/bending-ofcomposite-material-compression-specimens [Accessed 20 Aug. 2018].

[15] - O’DAY, P. (2011). Time for a new “SACMA”. [online] Compositesworld.com. Available at: https://www.compositesworld.com/columns/time-for-a-new-sacma [Accessed 20 Aug. 2018].

[16] - Adams, D. and Adams, D. (2018). DOT/FAA/AR-02/106 Tabbing Guide for Composite Test Specimens. [online] Washington: U.S. Department of Transportation Federal Aviation Administration. Available at: http://www.tc.faa.gov/its/worldpac/techrpt/ar02-106.pdf

References: [Accessed 20 Aug. 2018].

[17] - Composite Materials Using a Combined Loading Compression (CLC) Test Fixture (2016).

ASTM International.

[18] - ASTM D695 -15 Standard Test Method for Compressive Properties of Rigid Plastics (2015). ASTM International.

[19] - ASTM D3410M-16 Standard Test Method for Compressive Properties of Polymer Matrix Composite Materials with Unsupported Gage Section by Shear Loading (2016). ASTM International.

[20] - BS EN ISO 14126:1999 Fibre reinforced plastic composites – Determination of compressive properties in the in-plane direction [21] - BS EN ISO 604:2003 Plastics – Determination of compressive properties [22] - prEN 2850-1997 Aerospace series carbon fibre thermosetting resin unidirectional laminates compression test parallel to fibre direction [23] - D O Adams & D F Adams – Tabbing Guide For Composite Test Specimens, US Department of Transportation Federal Aviation Division, report DOT/FAA/AR-02/106, 2002 [24] - ASTM D3039/D3039M-17 – “Standard test method for Tensile Properties of Polymer Matrix Composite Materials” [25] - BS EN ISO 527-4:1997 – “Plastics – Determination of tensile properties – Part 4: Test conditions for isotropic fibre-reinforced plastic composites” [26] - ASTM D2344-16 Standard Test Method for Short-Beam Strength of Polymer Matrix Composite Materials and Their Laminates 1 [27] - BS EN ISO 14130:1998 - “Fibre Reinforced Plastic Composites - Determination of apparent interlaminar shear strength by short-beam method [28] - ASTM D3039/D3039M-17 Standard Test Method for Tensile Properties of Polymer Matrix Composite Materials [29] - Adams D.O. & Adams, D.F. DOT/FAA/AR-02/106 Tabbing Guide for Composite Test Specimens, October 2002 [30] - Don Adams, Tabbing composite test specimens, when and why, Composites World, 2011 [31] - Hodgkinson, J.M. Mechanical testing of advanced fibre composites, CRC Press, 2000 [32] - BS EN ISO 527-5:2019 Plastics – Determination of Tensile Properties, Part 5: Test conditions for unidirectional fibre-reinforced plastic composites.

About the Authors

The R-TECH Materials composites laboratory is an award winning and leading laboratory for the testing of composite materials in the UK and Europe.

Accredited to ISO 17025 and the requirements of Nadcap, the lab provides one of the largest scopes of accredited testing in the UK and Europe with facilities for composite manufacture, machining, characterisation and analysis alongside static and dynamic mechanical testing.

Dr. Geraint Havard is the Composites Manager of R-TECH Materials and sits on the BSI and ASTM technical committees for Composites testing on behalf of Composites UK. Alongside his responsibilities for running the R-TECH testing laboratory, Geraint has become a recognised technical voice in the composites industry. Geraint delivers composite testing courses on behalf of Zwick Roell and has presented on behalf of R-TECH Materials in a number of other technical conferences. He has also been heavily involved in the research that led to R-TECH Materials gaining a JEC innovation award in 2017.

C O N TA C T U S 01656 748000 info@r-techmaterials.com www.r-techmaterials.com

Testing House, Kenfig Industrial Estate, Margam, Port Talbot, UK. SA13 2PE