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It is vital to prevent brittle cracks in large structures. This is particularly important for a number of industry sectors including offshore wind, Oil & Gas, and shipbuilding where structural failure risks loss of human life and loss of expensive assets. Some modern steels exhibit high Charpy energy – i.e. high initiation fracture toughness, but poor resistance to crack propagation – i.e. low crack arrest toughness. The correlation between initiation and arrest toughness measured through small-scale testing is investigated in five different steels, which include S355 structural steel (with two different thicknesses), X65 pipeline steel, two high strength reactor pressure vessel steels and EH47 shipbuilding steel. Small scale mechanical tests were carried out to characterise the materials’ properties and were compared to the materials’ microstructures. A wide range of tests were carried out, including instrumented Charpy, drop weight Pellini, fracture toughness, tensile testing, and optical microscopy. Nil ductility transition temperature (NDTT) is used to characterise a material’s arrest properties. Initiation fracture toughness correlated with higher upper shelf Charpy energy and smaller average grain sizes, as expected, however none of these correlated well with the arrest toughness measured through NDTT. The NDTT correlated most strongly with the T27J temperature which indicates the start of lower shelf of the Charpy curve. This correlation held for all materials including those where the NDTT lies on the upper shelf of the Charpy curve. While initiation fracture toughness can be predicted through high Charpy toughness and operation temperatures on the upper shelf, crack arrest behaviour should be predicted from characteristics of the ductile to brittle transition temperature, for example by using the T4kN from instrumented Charpy tests or T27J.

Laser shock peening (LSP) is one of the life enhancement processes by inducing compressive residual stresses into the material. The effects of repeating LSP pattern on the residual stress is investigated numerically in this study. A 3-D finite element model was developed to obtain residual stress field in the material. FEM results have shown that increasing the number of peening impacts increase the magnitude and depth of compressive residual stress. Also the average surface residual stress in the impact region increased from −291 after 1 impact to −326, −343 and −356 MPa after 2, 3 and 4 impacts respectively. Moreover, the effects of repeating LSP pattern on fatigue crack growth properties were investigated experimentally by performing tests on C(T) specimens made of 316L stainless steel. The test results have shown that the fatigue crack growth life is very sensitive to the number of peening impacts and increased from 84,380 cycles in untreated sample to 104,130 cycles in sample with 1 time LSP and 170,500 cycles to sample with 2 times LSP. Also the fatigue crack growth rates decreased by repeating LSP patterns on the test specimens.

The reliability analysis of defective pipes, which operate under creep-fatigue loading conditions has been studied. An austenitic stainless steel pipe containing an axially external semi-elliptical surface crack, subjected to variable internal pressure at elevated temperature has been considered as the benchmark problem. Different parameters in material behavior, geometry parameters, and operational condition have been assumed as random variables in the probabilistic assessments. The reliability of the cracked pipes have been evaluated at various operation times, using different reliability methods. Sensitivity analyses have been conducted to investigate the importance measure of the random variables involved in the problem. Moreover, the effects of reducing the scattering of the random variables on the variations of the reliability index, β, has been examined.

The crack growth process in columnar nanocrystalline samples is simulated using the molecular dynamics method. The effects of grain size, grain boundary, crystallographic orientation and crack tip position on the crack growth behavior are investigated. Different sets of samples with mean grain sizes ranging from 4 nm to 14 nm are prepared. Samples with a similar number of grains and identical dimensions are considered for examining the impact of grain boundary and crystallographic orientation. To assess the effect of the grain boundary, no constraint is considered on the position and orientation of grains, while only the grain orientations are changed to examine the effect of crystallographic orientation. Also, samples with scaled dimensions are created to study the effect of grain size on the crack growth process. In such samples, the relative position of seeds and crystallographic orientation of grains remain constant. The results show that as the mean grain size is reduced, a higher level of stress is required for the initiation of crack growth. Moreover, the reduction of grain size from 14.24 nm to 11.65 nm increases the critical stress intensity factor. Further reduction of grain size decreases the critical stress intensity factor. For grain sizes smaller than 11.65 nm, the volume of stacking fault in the grains near the crack-tip is not significant. Grain size and grain boundary volume have a simultaneous impact on the initial slope which can be observed around the grain size of 11.65 nm. As a result, in nanocrystals with mean grain sizes of less than 11.65 nm, the crack grows in a brittle manner to reach the grain boundary, then progresses intergranularly with high speed. Consequently, grain boundaries and crystallographic orientations are not effective on the crack growth mechanism in nanocrystals with grain sizes less than 11.65 nm, whereas these factors can blunt or arrest the crack in nanocrystals with mean grain sizes of larger than 11.65 nm.