13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- average was used to determine the crack length. We compare the model predicted crack growth with experimental measurements at the same applied load levels and find very good agreements between the two. For examples, C(T)-17 has crack extensions of 6.81 mm (measured from post fatigue marks) when the test was stopped. The finite element analyses predict the amount of crack extension of 7.11 mm at the same load level. Figure 5. Fracture surface and stress triaxiality distribution in the side-grooved C(T) specimen. 4.2. Fracture tests with tensile residual stress The C(T) specimens considered hereafter are all side-grooved. Two levels of compression forces, 182 kN and 220 kN, were used in the experiments to generate tensile residual stresses. The average total indentation depths (after the punches were removed) are 0.083 mm and 0.244 mm for 182 kN and 220 kN respectively. The finite element analyses results in 0.089 mm and 0.259 mm total indentation depths for these two cases. Since the LOPC method creates residual stress field by introducing plastic strain into structure, too much side compression may results in the crack extension. Several methods, such as dye injection and SEM observation of the fracture surface, are used to verify if this has happened. From result of the dye and also SEM observations, the tensile residual side compression does result in crack initiation and growth when 220 kN load was applied. SEM observations also showed clear damage, i.e., inclusions broken and pulled loose from the matrix. However, this was not observed for the 182 kN case. The simulation results for both cases confirm the experimental observations. Figure 6 shows the crack front region after side compression, suggesting crack does extend about 1.5 mm when 220 kN of compression force is employed, whereas crack extension does not happen when 182 kN compression force is applied. The contours of residual stresses normal to the crack plane for both cases are shown in Figure 6. The high positive residual stress is confined in a small region close to the crack tip, within about 1.3 mm from the crack tip, and the residual stress distribution is fairly uniform along the crack front except in the region close to the free surface. (a) (b)
RkJQdWJsaXNoZXIy MjM0NDE=