Fatigue and Crack Growth under Constant- and Variable-Amplitude Loading in 9310 Steel Using “Rainflow-on-the-Fly” Methodology

Fatigue of materials, like alloys, is basically fatigue-crack growth in small cracks nucleating and growing from micro-structural features, such as inclusions and voids, or at micro-machining marks, and large cracks growing to failure. Thus, the traditional fatigue-crack nucleation stage (Ni) is basically the growth in microcracks (initial flaw sizes of 1 to 30 μm growing to about 250 μm) in metal alloys. Fatigue and crack-growth tests were conducted on a 9310 steel under laboratory air and room temperature conditions. Large-crack-growth-rate data were obtained from compact, C(T), specimens over a wide range in rates from threshold to fracture for load ratios (R) of 0.1 to 0.95. New test procedures based on compression pre-cracking were used in the near-threshold regime because the current ASTM test method (load shedding) has been shown to cause load-history effects with elevated thresholds and slower rates than steady-state behavior under constant-amplitude loading. High load-ratio (R) data were used to approximate small-crack-growth-rate behavior. A crack-closure model, FASTRAN, was used to develop the baseline crack-growth-rate curve. Fatigue tests were conducted on single-edge-notch-bend, SEN(B), specimens under both constant-amplitude and a Cold-Turbistan+ spectrum loading. Under spectrum loading, the model used a “Rainflow-on-the-Fly” subroutine to account for crack-growth damage. Test results were compared to fatigue-life calculations made under constant-amplitude loading to establish the initial microstructural flaw size and predictions made under spectrum loading from the FASTRAN code using the same micro-structural, semi-circular, surface-flaw size (6-μm). Thus, the model is a unified fatigue approach, from crack nucleation (small-crack growth) and large-crack growth to failure using fracture mechanics principles. The model was validated for both fatigue and crack-growth predictions. In general, predictions agreed well with the test data.

Interaction Analysis of Collinear Two Cracks with Unequal Length under Uniaxial Traction

The interaction among different sizes of cracks in materials is one of the key factors leading to the damage of brittle materials. Based on the Kachanov method, the expressions for the stress intensity factors of two collinear cracks with unequal length were obtained and the interaction effect was analyzed. A compression test of a cement mortar specimen containing two cracks and numerical analysis using RFPA2D were performed. The results indicate that the crack interaction can almost be neglected when the crack distance reaches the length of the large crack; the two respective collinear cracks in the specimen grow and do not affect each other when the crack distance reaches the large crack length. The results of compression test and numerical analysis are both in agreement with the theoretical result.

Effect of Crack Healing of SiC according to Times of SiO2 Colloid Coating

Crack healing behavior of SiC ceramics with large crack width has been studied as a function of coating and heat treatment. The SiO2 colloid coating was carried out on two types: hydrostatic pressure coating and roll coating. The crack healing was one hour at 1173 K in air. The crack part formed SiO2 oxides until the critical times by a hydrostatic pressure method. The crack does not anymore heal if it exceeds the critical times. The crack part and the base part have many O components and Si components regardless of the times of coating and heat treatment. The combined hydrostatic and rolling coating method did not have nearly an effect on crack-healing for large crack width over 1.4 μm. The study for more effective healing of a large crack width must be carried out in the future.

Analysis and Solutions of Deformation Behavior of Fault Zone Side Slope Hazard

The side hill cut of certain road cutting has a gradual slope and low slope height. But due to the location in fault zone, serious slump damage occurs during construction, leaving large crack. Aiming at the deformation damage of the side slope of the road cutting, this essay provides analysis of the deformation behavior and mechanism causing the slump through the geographic environment and monitoring data, and through summing up the experience in design and construction management, provides engineering analogy case and experience reference for similar side slope management.

Assessment of the Hodograph Cone Method (HCM) for the Evaluation of the COD of Off-Centered Crack in Pipes Under Bending for LBB Assessment

In this work the applicability and the accuracy of the hodograph cone method (HCM) for estimating the COD distribution of off-centered cracks in pipe under bending has been verified by means of extensive finite element analysis and experimental investigation. The need to develop appropriated numerical model and the guideline to follows in order to get accurate results is discussed. Experimental measures performed by means of direct imaging correlation confirmed both the accuracy of the FEM model adopted as well as the robustness of the HCM. A correction for further improvement of the HCM accuracy and extension to large crack length is also proposed.