Creep crack growth tests, conducted on contoured double cantilever beam (DCB) specimens are described for aluminium alloy RR58 and a chromium-molybdenum-vanadium steel. The results are analyzed in terms of ˙J, the rate equivalent of the J contour integral, which is a nonlinear fracture mechanics parameter. Direct proportionality is found between crack growth rate, ˙a and ˙J. The treatment is shown to reveal a unification of the linear elastic fracture mechanics and net section or reference stress descriptions of creep cracking.

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In this work the time dependent failure assessment diagram (TDFAD) approach is applied to the study of crack initiation in Type 316H stainless steel, a material commonly used in high temperature applications. A TDFAD has been constructed for the steel at a temperature of 550 o C, and was found to be relatively insensitive to time. The TDFAD procedure is then applied to predict initiation times, at increments of creep crack growth Δa = 0.2 mm and Δa = 0.5 mm, for tests on compact tension specimens and the results compared to experimentally determined values. It has been found that initiation time predictions are sensitive to the creep toughness values, and to the limit load (or reference stress) solution used. Conservative predictions of initiation times have been achieved through the use of the lower bound creep toughness values in conjunction with the plane strain limit load solution. The plane stress limit load solution has given conservative predictions for all bounds of creep toughness used.

Results obtained from a European collaborative project aimed at producing a draft code of practice for creep crack growth testing are presented. The tests were performed on an Austenitic Type 316H Stainless Steel at 550°C and a carbon manganese steel at 400°C. A range of specimen test geometries has been examined. These are the single edge notched specimen in tension and bending, the double edge notched specimen in tension, the middle cracked specimen in tension, and the C-shaped specimen in tension. The results obtained are compared to those obtained using the standard compact tension specimen following the ASTM Standard Test Method for Measurement of Creep Crack Growth Rates in Metals (E 1457). Creep crack creep growth rates and initiation times have been correlated with the fracture mechanics parameter C*. The applicability of the stress intensity factor K to describe the creep cracking behavior is also assessed. Recommendations are made to support the use of different geometries in creep crack growth and/or creep crack initiation testing.

Selective laser melting (SLM) is a promising additive manufacturing (AM) process for high-strength or high-manufacturing-cost metals such as Ti-6Al-4V widely applied in aeronautical industry components with high material waste or complex geometry. However, one of the main challenges of AM parts is the variability in fatigue properties. In this study, standard cyclic fatigue and monotonic tensile testing specimens were fabricated by SLM and subsequently heat treated using the standard heat treatment (HT) or hot isostatic pressing (HIP) methods. All the specimens were post-treated to relieve the residual stress and subsequently machined to the same surface finishing. These specimens were tested in the low-cycle fatigue (LCF) regime. The effects of post-process methods on the failure mechanisms were observed using scanning electron microscopy (SEM) and optical microscopy (OM) characterization methods. While the tensile test results showed that specimens with different post-process treatment methods have similar tensile strength, the LCF test revealed that no significant difference exists between HT and HIP specimens. Based on the results, critical factors influencing the LCF properties are discussed. Furthermore, a microstructure-based multistage fatigue model was employed to predict the LCF life. The results show good agreement with the experiment.