Notch strain hardening

Clearly, the homogeneous deformation response shown in Fig. 12.17 also applies in inhomogeneous flow where notches and other local strain concentrations are present. There, through the double effect of enhancement of the local strain hardening and of the strain rate, global embrittlement will result. 

W. Guo, C. H. Wang, and L. R. F. Rose Elastoplastic analysis of notch-tip fields in strain hardening materials. Technical Report AR-010-065, Aeronautical and Maritime Research Laboratory, Melbourne, 1998. 

In the stress relaxation process, JNC assumes a strain hardening rule for the calculation of the creep strain rate.The loading conditions, the geometry of the plates and the dimensions of the initial notches are given in Table 1. The membrane stress Gm and the bending stress Gb can be calculated elastically. The material properties are given in Table 2. The cyclic stress-strain curve at 650°C is expressed as follows [5]. 

In order to start the multiscale modeling, internal state variables were adopted to reflect void/crack nucleation, void growth, and void coalescence from the casting microstructural features (porosity and particles) under different temperatures, strain rates, and deformation paths [115, 116, 221, 283]. Furthermore, internal state variables were used to reflect the dislocation density evolution that affects the work hardening rate and, thus, stress state under different temperatures and strain rates [25, 283-285]. In order to determine the pertinent effects of the microstructural features to be admitted into the internal state variable theory, several different length scale analyses were performed. Once the pertinent microstructural features were determined and included in the macroscale internal state variable model, notch tests [216, 286] and control arm tests were performed to validate the model s precision. After the validation process, optimization studies were performed to reduce the weight of the control arm [287-289]. 

The fibrils (as seen in Fig. 4) are the remains of blunting at the notch tip. The formation of fibrils is also an indication of work hardening due to orientation. Strain induced crystallization may even further enhance this work hardening. 

We start with the fully plastic, non-hardening plane-strain flow field of a deeply double-edge-notched plate of a rigid plastic material where the two symmetrically placed notches simulate two opposing sharp cracks shown in Fig. 12.12. The material is considered to have a tensile yield strength (To (= T) or a yield strength in shear of k = (To/ /3-... 

We found that the true axial stress-strain curves in the three studies showed similar trends when comparing notched specimens to smooth specimen controls (Figure 31.6). All of the notched specimens showed an elevation of the yield stress and a reduction of the ultimate stress. Also, the stress-strain curves of the notched specimens appear truncated when compared to the smooth specimens in that they lack the steep final portion of the curve that is typically associated with orientation hardening. In all of the studies and notched conditions tested, the crosslinked materials showed decreased ductility when compared to the conventional material. 

FIGURE 31.6 True stress-strain curve from study II showing the orientation hardening region in smooth specimens and showing the truncation of this region and the elevated yield stress in notched specimens. 

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