Full-Field Experimental Analysis of Ductile and Fatigue Fracture and the Accompanying Thermal Effects

This work presents the experimental testing of fatigue and ductile failure for the purposes of computational validation. A precipitation hardening martensitic stainless steel known as 15-5PH is utilized for the experiments in multiple heat treated conditions. Experimental investigations of ductile failure and the accompanying thermal effects are presented with an emphasis on computational model validation in the spirit of the first Sandia Fracture Challenge (SFC). An argument against using isothermal assumptions during model calibration is presented. To further this claim, a FLIR a6753sc infrared detector is utilized to provide full-field temperature measurements on the surface of deforming specimens. In addition, strain fields are measured using digital image correlation (DIC) techniques. Utilizing a modified compact tension (mc(t)) specimen, such as that studied in SFC, and a dogbone geometry for tensile testing, the failure mechanisms are studied. Interesting results show that strain-to-failure in uniaxial testing does not clearly indicate how a mc(t) specimen will fail. The failure strain in mc(t) specimens is correlated with the material hardening rates. Implications of isothermal assumptions become apparent when the hardening rates are compared between isothermal and finite conductivity responses. Fatigue failure is then studied in the same material for the purposes of validating ZFEM, an abaqus-implemented finite element routine studying fatigue fracture paths. Utilizing swiss cheese specimens, unique fracture paths are produced based on the starter notch location. After the nominal swiss cheese geometry is tested, the specimens are slightly modified then locally heated to study the effect of temperature gradients on the fracture paths.