The continual drive for improved safety and increased fuel efficiency within the automotive industry hasn't escaped anyone; the message is repeatedly broadcast via news outlets and advertised by auto manufactures themselves. The majority of new car models compete fiercely for the most impressive fuel efficiency (mpg or L/100km) figures in class. As auto manufactures strive for improved fuel economy, it is clear that material selection will continue to be an important factor in weight reduction.
One route that is being aggressively investigated by auto manufactures involves increasing the strength and formability of steel. Increasing the strength of steel allows the safety requirements to be met while reducing the mass (or gauge/thickness) of material used. In 2010, the mass of the typical lightweight vehicle was reported to be 1,752 kg (3,863 lbs.); of that, 58% was steel. Although the estimated mass savings from utilizing new designs of high-strength steel vary, a structural mass saving of 39% has been reported.
These new generations of Advanced High Strength Steels (AHSS) introduce additional challenges, such as gripping and strain measurement, when performing a tensile test. As material strength increases, typically hardness also increases, and this leads to gripping challenges around grip slippage and premature wear of jaw faces. In order to reduce the likelihood of slippage during a tensile test, high clamping forces can be utilized at the initial stages of the test. Using a hydraulic wedge grip design gives an ideal balance of adjustable initial gripping pressure to prevent premature failure and, if desired, fixed proportional gripping as the test load increases. With the increased strain seen in Twinning-Induced Plasticity (TWIP) steels and high tensile strengths seen in Martensitic (MS) steels, a highly accurate yet robust extensometer is required for the measurement of potentially relatively high strains (compared to other steels). This combination of hydraulic wedge grips and an automatic extensometer offer the ideal solutions for the latest generations of steels.
