COMPOSITES

Grips and Fixtures

Composite specimens are extremely susceptible to misalignment in the load string as well as improper gripping, both potentially causing a reduction in material properties. Considerable care should be taken in specimen preparation, and the load string should be optimized to produce repeatable results. Tensile tests require proper loadstring alignment to avoid bending, and 2716 028/-030 precision manual wedge grips or 2742/2743 Series hydraulic wedge grips are recommended as they are approved for meeting Nadcap standards. Composite materials often require special fixturing, and a full range of composite test fixtures are shown in Section 2.1, which includes devices for tension, compression, shear, flex, and peel according to all the major standards.

Alignment

Alignment is critical when testing composites. Instron® testing machines use precision guidance columns along with adjustable AlignPRO™ alignment fixtures to meet and maintain the highest levels of alignment demanded by the aerospace industry. The Instron alignment system improves the accuracy of test data by ensuring that the load string is correctly aligned under load and consists of the AlignPRO fixture, a strain gauged alignment specimen (fitted with 8 or 12 gauges), and dedicated AlignPRO software. The fixture allows the load cell and grip assembly to be aligned quickly and precisely while allowing both concentric and angular misalignment to be adjusted. The software monitors the output from the strain gauges mounted on either flat or round alignment cells and identifies which adjustments are required, displaying in real time the effect of the adjustments, enabling accurate machine setup in minutes.

Extensometry

Extensometry is used extensively for composites due to their need for precise strain measurement. Historically, composite specimens are tested using applied strain gauges, and strain gauge adapter boxes allow for the seamless strain measurement in Bluehill® Software from a standard gauge. 2620 and 2630 Series clip-on extensometers can be used for measuring modulus before failure. 2650 Series averaging extensometers are designed to minimize bending errors on thin laminates. The determination of Poisson’s Ratio requires the simultaneous monitoring of axial and transverse strain. The very small deformations require very high accuracy devices. 2650-500 Series bi-axial extensometers measure both axial and transverse directions on both sides of the specimen and automatically average each side to offset any bending errors. Non-contacting solutions such as the AVE are especially effective at measuring strain until failure on specimens that exhibit high energy breaks. The monitoring of multiple strain gauges on a specimen or structure can be achieved via special multi-channel data acquisition units, such as the 2210-920 expansion channel module for 5900 or 6800 Series systems.