13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- Table 1. Test matrix for calibrating failure parameters Test No. Loading speed (m/s) Strain rate (/s) Stress Triaxiality range Uniaxial tension 0.0002 0.01/s 1/3 0.2 10/s Simple shear 0.0001 0.01/s 0 0.1 10/s Notch tension 0.0001 0.01/s 1/3-2/3 0.1 10/s Punch 0.001 / 2/3 3.5 / 3. Material constitutive law A talc-filled and impact-modified polypropylene was chosen for the experimental and simulation study in this paper. All the test coupons in this paper were milled from an injecting molding plate with thickness of 3.4mm. The material model SAMP-1 [4] in LS-DYNA developed for polymers was utilized to construct material constitutive. A quadratic form in the stress tensor is used to describe the yield surface and SAMP-1 model is restricted to isotropic formulations. The expression for the yield surface is as follows: =σvm 2 −A0 −A1 −A2 2 ≤0 (6) Uniaxial tension, shear, compression and biaxial tension are four basic tests for construction of the yielding surface. Hence, all the above four tests at approximate strain rate of 0.01/s were carried out to obtain plastic true stress-strain curve as input. Fig. 2 shows equivalent stress-strain curves of four types of tests. Based on the four basic stress-strain curves, quadratic fitting formulation was selected to fit yielding surface, which is plotted in Fig. 3. Figure 2. Input curves of SAMP-1 model Figure 3. Yielding surface A proper visco-plastic consideration of the rate effects is included in SAMP-1 model. In order to 0.0 0.2 0.4 0.6 0.8 0 10 20 30 40 50 Equivalent stress (MPa) Equivalent strain Tension Shear stress Compression Biaxial tension -12-8 -4 0 4 8 121620 0 10 20 30 40 50 Biaxial Tension Shear Vonmises (MPa) Pressure (MPa) Test result Quadratic fitting Compression
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