Starting from James Rice’s classical work on cyclic plastic stresses and deformations in the plastic zone of a Mode III loaded crack, it will be shown that the crack tip opening displacement of a Mode I crack in a work hardening material can be written in analytical form. This result was then used to formulate the blunting line for J-integral testing and to estimate the fatigue crack propagation rate of a number of materials. In a similar manner—based on the strain distribution within the plastic zone of a work hardening material—the initiation of crack extension under static loading was estimated. The stress distribution ahead of a crack and the Ritchie, Knott. and Rice model were applied to the ductile-to-brittle transition of ferritic steels as well as the transition temperature shift due to neutron irradiation. Inspired by Fong Shih’s contribution to the Electric Power Research Institute Handbook, a simple but straightforward method for expressing the δ5 crack opening displacement as a crack driving force for fully plastic conditions was developed, finally ending up in a comprehensive assessment method for cracked components. The application to mismatched welded joints was demonstrated to be possible if the yield load for mismatch is available; this latter task was performed using both slip line theory and finite element (FE) analyses. Application examples of these models will be shown, and it will be seen that estimates using these models are in reasonable agreement with experimental results and FE analyses. Several elements of these models have made their way to international codes and standards.
Fatigue-crack-growth rate tests were conducted on compact specimens made of a 7050-T7451 aluminum alloy to study the behavior over a range in load ratios (0.1≤R≤0.9) and constant Kmax test conditions. Previous research had suggested that differences in the threshold regime at high load ratios were attributed to Kmax effects. But recent measurements of crack-closure behavior under high R and constant Kmax test conditions near threshold conditions on a variety of materials have indicated that these tests may not be crack-closure free as suspected. Strain gages were placed near and ahead of the crack tip to measure crack-opening loads from local strain records. In addition, a back-face strain (BFS) gage was used to monitor crack sizes and to measure crack-opening loads from remote strain records during the same tests. The 7050 alloy produced very rough crack-surface profiles. For R=0.1, the BFS and local gages indicated very similar high crack-opening loads. For R≥0.7 and Kmax test results in the threshold regime, the BFS gages indicated lower crack-opening loads than the local gages. Based on local measurements, crack-closure-free fatigue-crack-growth data (ΔKeff against rate) were calculated. These results indicated that the ΔKeff against rate relation is nearly a unique function over a wide range of R values even in the threshold regime, if crack-opening loads were measured from local strain gages. At low R, all three major shielding mechanisms (plasticity, roughness, and fretting debris) are suspected to cause crack closure. But for high R and Kmax tests, roughness and fretting debris are suspected to cause crack closure above the minimum load. A strip-yield model was also used to correlate the data over a wide range in load ratios and rates, but required a very low constraint factor (α=1.3), due to the high crack-opening loads.
This paper presents the results of a long-term research program aimed at developing qualitative and quantitative design guidelines for the use of mechanical surface treatments designed to improve the fatigue life of structural components. High cycle fatigue tests were performed on planar four-point bending specimens derived from Ti-6Al-4V pancake forgings with a mill-annealed microstructure. The high cycle fatigue behavior of specimens with different surface conditions (as-forged and machined) in both an unpeened and a shot peened state was compared. In order to assess the fatigue failure mechanisms, detailed investigations of the surface layers were carried out. The as-forged surface state exhibits a stress distribution with significant compressive stresses near the surface, resulting in equilibrium tensile stresses in the depth. When the tensile stresses were exposed by machining a bordering surface, a distinct decrease in the fatigue strength was observed. In such cases, a shot peening treatment was shown to improve the fatigue strength. Square edges lead to a decrease in the fatigue strength, which could be aggravated by shot peening.
Fatigue crack propagation (FCP) rates in submerged arc welding (SAW) seam welds of 1524 steel (API 5L X42) pipe were measured by using an arc-shaped bend specimen with the same radius of curvature of the body of the pipe so the material did not have to be cold worked to get a flat shape nor extensively machined. The test direction was girth radial and the stress intensity factor (K) function was calibrated for this type of nonstandard specimen. The FCP tests were carried out in air at room temperature, testing the three zones: The base metal, the deposited metal, and the heat affected zone (HAZ). A fractographic analysis was done to analyze the role of the microstructure in the FCP in the three zones. It was found that the zone of greater resistance to FCP was the base metal, whereas the deposited metal showed the least resistance to crack propagation. FCP in the deposited metal and the HAZ behaved according to the Paris law, unlike the base metal, which showed a high data dispersion. The behavior in the base metal was attributed to the propagation of the crack in the transverse direction of the preferential alignment of the microstructure, while the deposited metal and the HAZ had a more homogeneous microstructure.
The so-called EURO data set is the largest set ever assembled, consisting of fracture toughness results obtained in the ductile-to-brittle transition region. It was the outcome of a large project, sponsored by the European Union, which involved ten European laboratories in the second half of the 1990s. Several post-project investigations have identified one of the blocks from which specimens were extracted (block SX9) as macroscopically inhomogeneous and significantly tougher than the remaining blocks. In this paper, the variability of block SX9 has been investigated using the conventional master curve (MC) methodology and some recent MC extensions, namely, the SINTAP (structural integrity assessment procedure) lower tail, the single point estimation, the bi-modal MC, and the multi-modal MC. The basic MC method is intended for macroscopically homogeneous ferritic steels only, and the alternative approaches have been developed for the investigation of inhomogeneous materials. Therefore, these methods can be used to study the behavior of block SX9 within the EURO data set. It has been found that the bi-modal and multi-modal MC approaches are quite effective in detecting the “anomaly” represented by block SX9 but only when analyses are performed on data sets of comparable size.
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