13th International Conference on Fracture June 16–21, 2013, Beijing, China -8- ∫ Δ = l B C p l n mean A l N 0 max , , ( ) 10 1 σ ε . (5) One key issue in the critical distance method is determining the critical distance, d. Until now there is no effective method except searching to determine the critical distance for elasto-plastic problem. The most popular method is to determine the distance by minimizing the deviations between prediction and experiments. For instance, one may use the least square method to minimize ||Np,d-Nf|| or ||Np,l-Nf|| and to find the most appropriate distance. It follows that the critical distance for the PM is 0.437mm and 0.7mm for the LM. Generally, one may assume that the distance should be independent of loading ratio and temperature. Table 3 lists the results for Cruse-Meyer PM and LM models. Results confirm significant improvement in predictions, in comparing with prediction results at root surface. The critical distance concept introduces a new variable into the model and makes it more flexible. However, applicability of it needs more detailed experimental and computational efforts. From the results we can observe LM predicted a little better than PM. Table 3. Notched fatigue life comparison results Notched Specimen No. Experimental Life (cycles) Predicted Life (cycles) Root Surface PM, d=0.437mm LM, l=0.7mm 1 16201 923 12351 12450 2 31248 2439 44232 38132 5. Conclusions It’s been shown that the strength of Inconel 718 EB weldment is lower than that of the base metal, the weldment material properties cannot be obtained from simple tension test. Nano-indentation tests are useful to investigate the mechanical properties of weld joint. By assuming a power law plastic model, the elastoplastic properties of weldment material were derived from inverse nano-indentation analysis with help of finite element simulation. To verify the results, a comparison of nominal stress-strain curve between calculated and test results was carried out. The accuracy of the results is satisfactory. To predict the fatigue life of weldment, the simple Cruse-Meyer model was extended to multiaxial fatigue directly and applied for this prediction. The predicted smooth fatigue lives were found to agree with the experimental data. Combining it with the critical distance concept provides a reasonable prediction for notched specimen fatigue. The basic assumption here is that the critical distance should be constant for the specific material and fatigue model which has to be verified further. Empirical determination of the critical distance needs to be refined. References [1] H. Schultz. Electron beam welding. Abington publishing, Cambridge, 1993. [2] A.C. Fischer-Cripps, Nanoindentation. Vol. 1. Springer, 2011. [3] W.C. Oliver, G.M. Pharr, An improved technique for determining hardness and elastic modulus using
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