PSI - Issue 52

Ben M B Sargeant et al. / Procedia Structural Integrity 52 (2024) 472–479 Ben M B Sargeant , Catrin M Davies and Paul A Hooper / Structural Integrity Procedia 00 (2023) 000 – 000

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3.1.3. Comparison to Plane Strain and Plane Stress Conditions Experimental data was compared with historical data from the original 25 mm wide and 25 mm thick samples [5] (Fig. 5). Each data set was normalised for fair comparison. Bending stress and J-integral were used for normalised loading from the influence of sample size. Plastic load line displacement to width ratio (LLD p / W ) removed differences in machine compliance and enable a comparison of different thicknesses. Since the crack growth was not monitored on the 25 mm thick samples, J can only be measured at the maximum load achieved in the test, which is sometimes considered a conservative estimate of J IC [7]. Maximum loading in historical data (25 mm sample) was at the end of test but the trend would suggest proximity maximum load. The observed J IC , from maximum loading, were 0.378 MPa and 0.641 MPa, for the 10 mm and 25 mm samples respectively. The elastic component of these toughness’s were 3.1% and 10.2%, showing further indicating a dominance of plasticity and hence ductile fracture. This demonstrates a higher toughness for the larger sample and closer to plane strain (100% elastic) condition. Fig. 5 also shows the point indicated by critical voltage ( V 0 ), at bending stress of 316 MPa and J-integral of 0.403 MPa. This is 6.6% greater than the critical point suggested by maximum loading. The proximity of these points gives evidence to the accuracy of this result and agreement to the prior statement that J measured by maximum load is a conservative estimate. For this material and temperature, the stress-displacement response and fracture surface clearly indicate a high dependency on sample size making them invalid to treat results as intrinsic material properties or apply purely plane stress or plane strain assumption. The smaller sample is noticeably ductile, with shear sides, necking and tearing. Therefore, a detailed study to relate the results from small Charpy sized samples to larger thicknesses is required if the toughness of thicker structures is to be predicted from small samples.

Fig. 5 Comparison of experimental data, from 10 mm and 25 mm SEN(B) samples and identified critical points. a) Comparison made using normalised load measurement as bending stress and normalised displacement measurement (plastic load line displacement). b) Comparison made using J-integral and bending stress.