Challenging design criteria for improved fatigue performance

Sheared or laser-cut, Amstrong® Ultra MCL series were proven to be a durable solution, surpassing the design curves proposed by the current standards.

Okan Yılmaz

There has been an increasing demand for ultra-high-strength steels, especially in applications where weight reduction, consequent fuel and energy savings, higher payloads, and easier road homologation are desired. ArcelorMittal’s Amstrong® Ultra MCL series – with a minimum yield strength ranging from 650 to 1100 MPa – is ideal for thinner, lighter, and smarter solutions. Target applications include (but are not limited to) yellow and green goods, trucks, tippers, cranes, and heavy mechanical equipment, where stringent fatigue and toughness requirements must be met during design.

Decreasing the thickness of current products for weight reduction results in increased stresses, which make the use of ultra-highstrength steels essential. These materials have higher yield and tensile strengths than conventional mild steels and, although not always proportional, they have higher fatigue limits resulting in better durability. To ensure safety under repeated loading, it must be known that fatigue failure typically originates from stress raisers. These stress raisers can be inherent in the microstructure (non-metallic inclusions), due to surface condition (poor surface quality, high roughness), or they result from joining and cutting procedures. Naturally, before the start of their service life, steel parts of designed components must be machined to their final shape, as they are supplied in standardised shapes and sizes. Different cutting strategies induce various physical phenomena, such as tensile residual stress pattern, micro-defects at surface, and heataffected zones when cut thermally within the cut material, which affect the behaviour under repeated dynamic loading. It is essential to correctly identify and link the effects of cutting methods to an important in-use property, fatigue performance. Available standards address the proper cutting selection to a limited extent. Overlooking the material strength and cutting effects results in conservative designs. OCAS’s recent experimental campaign, focusing on shearing and laser-cutting effects, demonstrated that, especially at high-cycle and infinitefatigue life regimes, Amstrong® Ultra MCL series were superior to the design curves proposed by the current standards, which demonstrates its potential for thickness and weight reduction in designs.