Abstract: This paper proposes a new three-layer artificial neural network (ANN) to predict the fatigue crack length under constant amplitude mode I cyclic loading. It is shown that the proposed model predicts the crack length with an error of less than 0.05%, and more accurately than the current commonly-used models.
Abstract: Fracture studies of components involving mode III loading are rare. This paper concentrates on mixed-modes I, II and III fracture testing of brittle PMMA material and summarises an extensive experimental investigation. Karpour and Zarrabi have developed a special pair of grips for which they have received a patent. These grips allow the application of various mixtures of modes, including mode III, using a universal testing machine. To measure the three-dimensional crack-tip opening and sliding displacements, Karpour and Zarrabi have also developed a novel stereo machine vision system whose description is included in the paper. The measured critical crack-opening displacements presented in normalised format with potential of being used to assess the integrity of any component against elastic fracture and they compare well with limited pertinent data available from literature.
Abstract: To date the analysis of fatigue crack growth from an open hole subjected to contained cyclic plastic deformation is problematic due to considerable computational effort involved. This paper presents the integration of a new efficient method for calculating the elastic-plastic notch stress field under cyclic loading into a crack growth analysis tool. The anticipated crack path is defined along a line emanating from an open hole, and the line is discretized into a series of elements on which the elastic-plastic stress distribution is solved for the uncracked body. This solution is then used in conjunction with a weight function approach to determine the stress intensity factors used in the crack growth analysis. This technique has been incorporated into a Defence Science and Technology Organisation proprietary crack growth analysis tool based on plasticity-induced crack closure. Results are presented for cases involving service load spectra from F/A-18 and F-111 aircraft and are compared to those obtained from a commonly used crack growth code that does not account for the effect of contained notch plasticity. It is hoped that this generic approach will lead to improved predictions for crack growth in plasticity-affected zones.
Abstract: The near-threshold fatigue behavior of small, semi-elliptical surface cracks in a bearing steel was investigated under cyclic shear-mode loading in ambient air. Fully-reversed cyclic torsion was combined with a static axial compressive stress to obtain a stable shear-mode crack growth in the longitudinal direction of cylindrical specimens. Shear stress amplitude was gradually decreased with an increase in crack length and the crack finally became non-propagating. Abrasive wear on the crack faces was inferred by debris and also by changes in microstructure in the wake of crack tip. Further, it was found that these effects resulted in a significant decrease in the crack growth rate. In this study, we shed light on the important role of the crack size and crack face interference on the crack growth behavior.
Abstract: During the mid 1990s earthquakes in Northridge, California, and Kobe, Japan, illustrated a lack of understanding of the behaviour of structural steels exposed to seismic loads. Under this type of load regime, structural steel members are subjected to fully plastic load cycles and unexpectedly brittle failures resulted. This paper presents a simple, yet powerful, method for predicting the accumulation of damage in a steel element, based on its toughness. In addition the damage parameter chosen provides an accurate prediction of when failure of the element can be expected to occur. The damage accumulation model developed allows for the deconvolution of complex load histories, such as could be expected to occur during a seismic event, in a systematic, stepwise manner. This approach is ideally suited to automation and could readily be implemented into a finite element model.
Abstract: In order to study the effect of post-ECAP (equal channel angular pressing) mild annealing treatment on fatigue strength of ultrafine grained (UFG) copper, the stress-controlled fatigue tests of UFG copper with and without post-ECAP annealing treatment were performed for a wide range of fatigue life. The evolution of surface damage and growth behavior of surface cracks were monitored by plastic replication technique. The effect of post-ECAP annealing on the fatigue strength was discussed through the observation of changes in surface morphological features.
Abstract: Fatigue failure of metal components containing notches, cracks and other defects has been a very active research topic for well over seven decades because of its important practical and theoretical implications. Recently, Taylor and his colleagues have re-visited this topic and proposed the Theory of Critical Distance (TCD), which summarizes the early work by Neuber, Peterson and others in a unifying theory and predicts fatigue fracture with the use of a critical distance, L0. In this paper, an experimental and numerical study of the fatigue of notched and unnotched 6061 aluminium alloys is used to verify the TCD and some of the limitations of the TCD are discussed on this basis.
Abstract: The rotating components in the hot sections of land-based gas turbine are exposed to severe environment during several ten thousand hours at above 1100 oC operating temperature. The failure mechanism of the hot gas components would be accompanied by material degradation in the properties of high temperature and creep rupture strength. Many hot gas components in gas turbine are made of Ni-based superalloy because of their high temperature performance. In this work, we surveyed the time and temperature dependent degradation of Ni-based superalloy. We prepared the specimens from GTD111 that are exposed at 871 oC and 982 oC in 1,000 ~ 10,000 hours. We carried out the mechanical test and microstructural observation.
Abstract: Replication, or repeated tests at the same stress amplitude, is used to provide statistical confidence in life data during the development of S-N curves. This paper discusses the effects of replication on the measurement of S-N curves and presents an alternative to traditional replication methods for the determination of S-N curves, particularly for the development of preliminary S-N curves. Using specimens made out of the extruded bars of a magnesium alloy, it is demonstrated that the S-N curve estimated using the data from non-replication tests is almost same as that from replication tests. The advantage of using non-replication fatigue tests is that it uses fewer specimens, in this instance, only half of that required for 50% replication fatigue test, to achieve the same estimation as that of the replication fatigue tests. Another advantage of using non-replication fatigue tests is that it can detect the non-linearity using limited specimens.