High strain rate response of automotive alloys (tensile + shearcharacterization):
1、DP980 (SMDI), DP600
2、Fully quenched Usibor? 1500-AS, Ductibor? 500-AS
Effect of strain rate on fracture?
1、Shear conditions (zero triaxiality)
2、Positive triaxiality conditions
Weld failure characterization – “spot weld groups”
? Achieves shear strains easily on the order of unity with relatively constant triaxiality of zero .
? Appropriate for high rate testing in Hopkinson bar or high speed hydraulics (1-1000/s).
Issue today lies in the majority of fracture characterization for crash CAE being doneat quasi-static rates…
Potential sources of rate effects:
? Elevated strain rates
? Temperature increase through adiabatic heating (~90% of plastic work convertedto heat) *
? Inertial effects
? Rate sensitivity is initially positive, but becomes negative atlarge strains
? Promotes earlier localization under shear loading
? Onset of shear cracking is determined as the point where thehardening rate is exhausted 1
? Repeatable and eliminates the need to detect fracture based on visible cracking 2
In tension, positive rate sensitivity promotes higher failure strains as strain rate increases.
In shear, thermal localizationreduces failure strain (as measuredusing DIC) with increasing strain rate.
? Given the coarse meshesnecessitated in vehicle CAE, failurestrains should be measured using alength scale on the order of theelement size
? DIC virtual strain gauge length: 0.3mm) versus 3-5 mm element size
? For these materials, shear strain tofailure input to FEA decreases withstrain rate!
? Less impact on positive triaxialityregime, however, confoundingresults exist in the literature
? Earlier localization,narrower thermal fieldfor the higher strengthconditions
? Thermal localizationenhanced for higherstrength alloys
? The strong workhardening of the DP600is clearly beneficial
? Interaction between nugget, HAZ and parent metal strength(single weld)
? Test method for weld group testing (Mode I Caiman)