Papers by Keyword: Crack Initiation

Abstract: Energy efficiency is a key issue worldwide, and not confined solely to the realm of engineers. Past failures of mechanical power system components must be examined carefully in order to minimise future occurrences and increase energy efficiencies. Improved design procedures have been highly sought by engineers and researchers over the past few decades. The latest verified method with strong application potential within the power industry is that of the Theory of Critical Distances (TCD). TCD is not one method, but a group of methods that have a common feature; the use of a characteristic material length parameter, the critical distance L, for calculating the influence of notch-like stress raisers under static and fatigue loading. A case study from a hydro power plant turbine shaft was chosen to illustrate the development of this methodology. The paper illustrates the application of TCD to the fatigue life assessment of a turbine shaft with stress concentrations due to pitting corrosion.

Abstract: Notch effect on the fracture behavior of the Ni3Al foils has been investigated as a function of notch radius and depth. Tensile tests along the rolling direction showed that notch weakening occurred with introduce of notch. The effective stress concentration factor (ke) was much lower than the theoretical stress concentration factor (Kt), which was due to the local plastic deformation at the notch root. Cracks initiated along the shear band in the RD tension.

Abstract: Using the ultrasonic fatigue testing technique, specimens of Q345 bridge steel with two types of shape (round and plate) were performed in ambient air at room temperature with a stress ratio of R=-1. The very long life fatigue property of Q345 was studied and the initiation mechanisms of fatigue cracks were investigated and analyzed with SEM. The results show that the two types of specimen present different S-N curve characteristics in the region of 105-109 cycles. The S-N curve of round specimens shows continuously decreasing tendency, while the S-N curve of plate specimens has a steep decreasing step and an asymptotic horizontal one. The fatigue strength of round specimen is higher than that of plate specimen. Fatigue failure initiates from the surface before 107cycles with high stess level while at internal inclusion in the very long life fatigue regime.

Abstract: In this paper, the mechanical behaviour of extruded AZ31 magnesium alloys under multiaxial fatigue loading conditions is studied. The monotonic properties of the AZ31 magnesium alloy were determined by tests on the specimens which were machined from extruded rods. Then, the cyclic deformation under multiaxial loading conditions was simulated by ANSYS and a plasticity program with the Jiang & Sehitoglu plasticity model. The fatigue lives were estimated by the critical plane models coupled with Coffin-Manson rule, such as Findley, Fatemi-Socie, Brown-Miller, SWT and Liu models. Four loading paths were considered with different levels of non-proportionality, the results show significant loading path dependence.

Abstract: For modern high strength steels, instead of metal instability, ductile damage triggered by the formation of microvoids or microcracks resulting from the complex material microstructure, has become the key factor responsible for the final failure in the forming process of such steels. The target of this study is to describe the initiation and evolution of damage in a dual-phase (DP) steel (DP600). By applying a newly proposed approach that is able to indicate the onset of damage in an engineering sense and quantify the subsequent damage evolution, to predict the forming limits for DP600 are predicted by simulating Nakajima test. Accordingly, two forming limit curves (FLC) are numerically computed to characterize two moments: when damage becomes pronounced and when the final failure is triggered by the accumulation of damage. Comparing with the conventional experimentally calibrated FLC at necking, the limit at crack initiation predicted by modeling gives a lower but defect-free forming boundary. The forming limit at final fracture is well captured by allowing the subsequent damage evolution to a critical value.

Abstract: A ductile failure is characterized by pronounced plastic deformations which involve significant plastic strains. The modeling of this failure behavior requires a precise description of the material plasticity starting from the crack initiation, its propagation through the material to the final fracture. The classical theory of metal plasticity based on the von Mises or Tresca formulations assumes that the effect of hydrostatic pressure on the flow potential is insignificant. Furthermore, it postulates that the flow stress is independent of the third stress invariant of the deviatoric stress tensor. The scientific findings from last few years show, however, that these both quantities should be considered for the precise description of plasticity, especially, of the real materials [1-4].

Abstract: The present paper deals with experimental studies on the tension-tension fatigue properties of 316L stainless steel by using a servo-valve controlled electro-hydraulic testing machine at room temperature. The low cycle fatigue properties of 316L stainless steel were studied and the initiation mechanisms of fatigue cracks were investigated and analyzed with scanning electron microscopy (SEM). Preliminary results indicate that the S-N curve of 316L stainless steel descends linearly in the low cycle regime and fatigue failure initiates from inclusions/defects on the specimen surface.

Abstract: The failure of rock materials in compression test suffers three characteristic stages: crack initiation, crack damage threshold, and macro-failure. There are some methods which can be adopt to identity the characteristic strengths in uniaxil or triaxial compression such as crack strain identifying method, AE activity and longitudinal wave velocity monitoring methods. In this paper, an creative experiment of recording stress-strain data, AE spectrums, and velocity of longitudinal wave is proposed and put into practice. Test results show that the mean initiation strength and damage strength by ratio of UCS are between 0.38 to 0.52 and 0.82 to 0.86 respectively with AE and longitudinal wave velocity methods synthetically. The AE monitoring method and the lateral P-wave or the axial S-wave test is an effective indicative of identifying the damage threshold when subject an axial compression failure.

Abstract: In order to obtain experimental data to investigate the mechanism of crack initiation and propagation, an innovative rolling contact fatigue (RCF) machine was developed. Compared to the conventional thrust type RCF machine the new device enables more efficient RCF testing and observation of subsurface cracks. Experimental data and information on inclusions and micro-cracks were obtained through observation by a laser confocal microscope and comparison with stress analysis. The depth of detected crack initiation is strongly correlated with the stress distribution.

Abstract: It is impossible to keep pipelines free from defects in the manufacturing, installation and servicing processes. In this paper, pre-tension deformation of X60 pipeline steel was employed to experimentally simulate the influence of dents and the mechanism of fatigue crack initiation of X60 pipeline steel after per-tension deformation under cyclic loading were investigated. The results indicate that the mechanism of fatigue crack initiation is the typical cleavage fracture characteristics and the cracks mainly initiates from the non-metallic inclusions which was the local brittle fracture materials such as MnS inclusion. With the pre-tension deformation increase, the yield strength of the matrix was increased and the toughness decreased due to the work-hardening effect. With the effects of the non-metallic inclusions larger, the fatigue cracks initiated from the non-metallic inclusions easier.