|Ph.D Student||Wang Zonggang|
|Subject||Experimental Study of Adiabatic Shear Band Formation|
|Department||Department of Mechanical Engineering||Supervisor||Professor Daniel Rittel|
|Full Thesis text|
It’s found that the accepted concept of a critical strain for ASB formation does not accurately reflect the overall mechanical history of a material. The homogeneous specimen temperature (either adiabatic rise or from test conditions) prior to strain localization has a very minor influence on the subsequent ASB formation. The dynamic deformation energy has a constant value (toughness) at the onset of ASB formation, for a large range of quasi-static pre-strain levels and initial test temperatures. Consequently, the dynamic deformation energy of cold work is experimentally identified as the governing factor for ASB formation. Next, a systematic separation of the effect of the length and the radius of short notches shows that neither the failure strain nor the dynamic deformation energy does exhibit a clear correlation with the length of the imperfection. By contrast, the notch-root radius appears to be the governing parameter for the two materials. Numerical simulations show that the dynamic deformation energy tends to concentrate in the vicinity of the notch tip over part of the gauge length. This length of influence occupies 25% of the gauge length of the specimen for Ti6Al4V alloy, which has a small dynamic strain hardening capacity. By contrast, it reduces to about 3% for the high dynamic strain hardening AM50 alloy. In addition, three distinct steps of the ASB formation were identified, using thermal data. The stage at which an ASB is fully developed can be tentatively identified as the point at which the measured and the calculated temperature curves intersect and diverge. This is also the strain level at which the homogeneous strain assumptions are no longer valid. The temperature rise of the 2 investigated alloys was found to be relatively modest until the onset of stage 2. At the onset of stage 3, it remained small for the AM50 alloy and much more significant for the Ti6Al4V. For AM50, thermal softening is probably not the main factor leading to ASB formation. Other factors, such as dynamic recrystallization can potentially destabilize the deformation. On the other hand, for Ti6Al4V, thermal softening appears to be more influential.