Papers by Keyword: Crack Closure

Abstract: Reliable damage tolerant design of airframe structures relies on the accurate life prediction of fatigue cracks propagating from a detectable size to the critical size, which is challenging due to the complex load sequence effect under spectrum loading. This paper aims at gaining a further understanding of the complex influence of the loading history on fatigue damage through a detailed numerical simulation of the near-tip crack behaviour using the crack closure concept. The spectrum loading is broken down into a number of simple yet representative loading scenarios with overload/underload superimposed onto the baseline constant amplitude fatigue loading. Detailed finite element (FE) simulation of the plasticity-induced crack closure (PICC) has been carried out to catch the transient behaviour of PICC and link it to the fatigue damage. The load interaction effect has been analysed with the aim to identify the possible dominant loading cycle which could simplify the fatigue life prediction process in the industry. It is concluded that more reliable damage tolerant design can be achieved if the load sequence effect on fatigue damage can be taken into account more accurately for a structure under spectrum loadings.

Abstract: The fatigue crack propagation behaviour of a new third generation Al-Cu-Li alloy type 2050-T84 developed for aeronautical applications is studied in comparison to a new generation Al-Cu-Mg alloy type 2022-T851. The alloy resistance against crack growth is shown to depend on alloy composition, aging condition and atmosphere environment. The crack path and the growth rate at moderate DK and in the near-threshold domain are discussed in terms of the slip morphology with respect to the microstructure. The different crack propagation regimes, as identified by mean of micro-fractographic observations and EBSD analysis are discussed on the basis of a modelling framework elaborated for conventional metallic alloys.

Abstract: Based on the theory of linear elastic fracture mechanics, formulae for the circumferential stress and the initial fracture point around flattening elliptical hole under compression were derived. When considering the expansion of closed crack under pressure, by introducing the effective friction coefficient f obtained the advantages derived crack angle and the friction coefficient of the crack surface had inversely proportional relationship. Use the software RFPA2D to simulate the rock samples with pre-fracture. The results show that the plane of weakness occurred fracture firstly that lead to the weakness plane through and then formed to be cracks. With the axial displacement exerting, the through crack occurred closure, the end of the rock fracture produced a curved path expansion at the same time. The final destruction face of the rock is approximately a 450 slip-line. It shows that the final failure mode of the rock is compression and shear failure. Different crack angles make the differences of initial cracking angle, but the crack propagation mode and its ultimate failure mode are similar, while have no affect on the carrying capacity.

Abstract: According to standard test method for fatigue crack growth rates of metallic materials, the crack growth rate of 30NCD16 at three stress ratio (R=0.1, 0.3 and 0.5) were measured. Based on linear elasticity fracture mechanics theory, the fatigue crack growth rate was studied through the nonlinear least squares fitting method. The Paris model parameters at steady growth region and near threshold growth region and NASGRO model parameters were obtained. The effective stress intensity factors versus curves at three stress ratios were determined by crack closure effect. The results show that the Paris equation can preferably describe relations at steady growth region. At this region the model parameter m lies 2.5-4. This result is consistent with the known statistical facts of most metallic materials. NASGRO equation can preferably describe relations from near threshold growth region to high values region. all the test data at three stress ratio was able to correlate and . Crack closure was the major factor in correlating stress ratio and crack growth rate, the degree of crack closure weaken with increasing stress ratio.

Abstract: In this paper, the repair capacity of SMA wire by using internal and external installation was studied. The SMA wires were embedded in epoxy resin and silicon cement mortar beams made of polymer clay, and then three-point bending test was conducted on the composite beam. The ability of beams which embedded SMA wires was also studied. On the other hand, the SMA wires were installed in the external of the epoxy polymer clay cement mortar, then three point bending test was also used and its self-repair ability was also studied. The results show that the crack can be repaired very well under these two methods. If there are cracks in existing buildings, we can use an external installation method to repair them. In order to ensure the new buildings crack free, both methods are feasible.

Abstract: Because of the high actuation stress and high recovery strain, NiTi shape-memory alloy (SMA) has been proposed for shape/position control and crack closure in structures for many years. In this paper, we demonstrate the feasibility to use NiTi SMA for not only crack closure, but also shape restoration in a silicon/nanoclay composite beam. Instead of embedding SMA into the beam, we use a piece of external SMA wire so that the expensive NiTi SMA can be reused. In addition, both shape restoration and crack closure can be achieved even when the beam is still in working condition, i.e., with external load applied.

Abstract: It has been recognized that the threshold stress intensity factor range Kth of a short crack is lower than that of a long crack. The short crack behavior in plain specimen has been studied by many researchers. However, the behavior of a short crack at the root of a long notch is not yet clear. The crack closure behavior is considered to be affected by the constraint at notch root and it is dependent on the length and the root radius of notch. In this study, fatigue tests on specimens with short pre-crack at long notch were done and the difference in crack closure behavior was studied. As a result, short crack effect appeared in any notch root radius. In a sharp notch, the crack opening point easily reached its stable condition after a small amount of crack extension. On the contrary in a dull notch, the opening point was lower than the stable condition and consequently short crack effect lasted in relatively wide range of crack extension. The small crack effect of notched specimen was discussed based on crack closure behavior.

Abstract: In this paper, fatigue crack growth rate (FCGR) analyses were conducted on compact specimens of an AISI 4340 alloy to study the behavior over a range in load ratios (0.1 ≤ R ≤ 0.95) and constant maximum stress intensity factor (Kmax) condition. Previous study had indicated that high R > 0.7 and constant Kmax test conditions near threshold conditions were suspected to be free of crack-closure and that any differences were caused by Kmax effects, from threshold to near fracture conditions. Cracks in high-cycle fatigue (HCF) components spend a large portion of their fatigue life near threshold conditions. In order to characterize the evolution of damage and crack propagation during these conditions, fatigue crack growth rate (FCGR) data at threshold and near-threshold conditions are essential in predicting service life and in determining the proper inspection intervals. Fatigue crack growth model, namely the Forman model were examined, this model implicit the effect of R ratio and ease of curve fitting to measured data. The Forman model may be suggested for use in critical applications in studying fatigue crack growth for different load ratios.

Abstract: There are situations where baseline fatigue crack propagation (FCP) data are needed but the available material for its evaluation is far from enough to fabricate full sized standard specimens. Miniature specimen testing techniques have been proposed but published data seems to be slightly but consistently slower under the same stress intensity than that from standard specimen results. In this work, an aluminum alloy and a steel, which have vastly different mechanical behaviors, are used to generate different stress states and premature crack closure to allow the validity of miniature specimen FCP data to be critically examined.

Abstract: The numerical simulation of crack closure is employed to assist on the prediction of crack growth rate. Under fatigue load, the stress-strain response of metals is altered due to cyclic loading. For this reason, the material properties characterization is of prime concern as an input parameter to obtain reliable results. From numerical simulations, it was observed that simple material models do not provide accurate data for long crack lengths. In this paper, the effect that different hardening models have on the opening response of a cracked component when it is subject to variable amplitude loading is analyzed. The interaction effects (crack arrest/acceleration) for long crack length simulation are specially highlighted. For this purpose, a 6082-T6 aluminium alloy was analyzed experimentally and numerically in order to measure crack closure, and then, those data were used to predict fatigue crack growth rate under different patterns of overload. The Paris equation and the Elber crack closure concept were employed. The results showed that small variations in the opening stresses obtained from different material models produce high overestimated simulations of crack growth rate. Also, it was proved that the crack closure mechanism is able to take into account interaction effects due to variable amplitude loading.