Effects of Material Bending and Hardening on Static Dent Resistance 2003-01-0605

A standardized quasi-static dent test has been used in evaluating the dent resistance of automotive body panels for many years. In order to predict the onset of a visible dent, cyclic loading with small load increments was adopted into standard automotive test procedures. Although researchers have investigated the effects of sheet thickness, yield strength, strain aging and prestrain on the static dent resistance of sheet steel in the past, material bending and hardening effects on static dent resistance were assumed to be inconsequential, and were therefore largely ignored.

In this study, the impacts of material bending and hardening on static dent resistance are investigated. A fixed load, single loading condition was carefully designed for different materials and incorporated into the quasi-static dent test. For comparison, the incremental quasi-static loading condition is also examined. Experiments are conducted on a hydraulic controlled dent tester incorporating a laboratory produced stretch dome panel with 2% biaxial strains as the test specimen. Three different steel grades are used: a mild steel, a bake-hardenable steel and an advanced high strength steel. The results showed a significantly higher denting load for the single loading condition than that of the incremental loading condition for unbaked materials of the same dent depth. The difference of the denting load is reduced for materials which are exposed to a paint baking cycle. However, for an advanced high strength steel, the difference in denting load is similar for both baked and unbaked conditions. Based on these experiments, the relationship between quasi-static dent test procedures and static denting phenomena appears to need further investigation.