Issue 33

J.M. Ayllon et alii, Frattura ed Integrità Strutturale, 33 (2015) 415-426; DOI: 10.3221/IGF-ESIS.33.46 423 Therefore, based on the SIF obtained at a certain crack length, a given increment of  D a in the crack size at the crack front is imposed. To determine the crack aspect ratio at this new length, and hence the SIF, the following relation is applied           n S S D D K a a K (2) With this value,  S a , the new crack length at the surface point is obtained and a new FE problem can be solved to obtain the SIF. This process is repeated until the final crack length is reached. Under the previously exposed propagation conditions, as can be seen in Fig. 10, the aspect ratio decreases strongly at the beginning of the crack growth. This shows that the almost circular crack grows faster at the surface than inside at the central point. This is due mainly to the effect of the stress concentration induced by the external thread of the implant body. The stress concentration always affects the crack at the surface, but its effect tends to disappear at the crack center as the crack grows. It can also be observed that the aspect ratio tends to an almost constant value as the crack length increases. Figure 10: Evolution of the crack aspect ratio when it grows. Figure 11: Range of the stress intensity factor vs. crack length in the test with F = 220N. Figure 12: Mode I, II and III stress intensity factors along the crack front. Fig. 11 shows the SIF vs. crack length at the deepest point of the crack. The high stress gradient close to the surface makes the SIF increase rapidly at the beginning and at a much lower pace afterwards. Therefore, more simulations had to be performed at small crack lengths to get a better estimation of the SIF evolution with the crack length.