Effects of Material Bending and Hardening on Dynamic Dent Resistance

In the first part of this paper, a previously published acceleration compensation methodology for dynamic dent testing [1] was successfully applied to calculate dent loads and applied energy in dynamic dent testing. This procedure was validated utilizing a hydraulic controlled dynamic dent tester on a number of low carbon and bake hardenable steels. In the second part of this study, the impact of strain rate on material bending and hardening in high-speed dynamic dent resistance testing was studied. Previous work [2] investigated these factors in static dent resistance. The procedure utilized in that research was further developed and adapted for high speed testing and used as a basis for a new, single loading incremental dynamic dent test. This new test was used to investigate the effects of material bending and hardening in high-speed dynamic dent resistance.

Testing incorporated laboratory produced stretch dome panels with 2% biaxial strains as test specimens. For the first part of the experiments, four different steel grades were used: DQSK, BH210, BH280 and DP600. For the second part of the study, the same steels were used except BH250 was used in place of BH280 for availability reasons. The results revealed a significantly higher applied energy for the single loading condition than for the incremental loading condition for both baked and unbaked materials of the same dent depth. Although the single loading dent test condition more realistically simulates the dent condition in real world situations, the incremental loading dent test condition provides a more conservative result. Due to higher impact energy in the dynamic testing, the 0.1 mm visible dent depth commonly used as static dent test criterion may need to be modified.