Papers by Keyword: Crack Initiation

Abstract: Spur gears are the most common type of gears for industry, due to its simple structures and low costs of manufacture. Under the complex loading conditions, failures can easily occur in the form of de-bonding, pitting, spalling or crushing of coating structures. Failure may originate from initiation of cracks, and its growth and propagation, however, basic failure mechanism is still not clear. In order to investigate the failure mechanism of coating structure failure for the spur gears, this paper presents some understandings about the coating damage at the teeth flank of a spur gear, based on a novel Finite Element simulation-procedures. This modeling procedure was developed based on several modeling approaches including: parameterized FE modeling, Cohesive-Zone Model and sub-modeling technique. The numerical model of spur gear was based on 42CrMo4 steel with PVD coating deposited as TiN/CrN multilayer structures. It was found that greater load bearing capacity exist for spur gears with the coating of nitride states deposited on 42CrMo4 steel.

Abstract: Widespread Fatigue Damage (WFD) issue is a major threat to the structural integrity of the aging aircrafts, So WFD assessment is a key measurement and necessary task to ensure the continuing airworthiness. A methodology using probabilistic framework to assess WFD was developed. The major feature of this method is that a stochastic process which is introduced to crack initiation and crack growth as well as yield stress and fracture toughness. Stress Intensity Factor (SIF) was incrementally calculated using Finite Element Method (FEM). Numerical studies were performed for panels with five holes using Monte Carlo Simulation (MCS). The WFD Average Behavior, include inspection, monitoring and repair period, was presented, and the convergence of the MCS was also discussed. The effectiveness of this methodology was verified by comparing the experimental data. This approach simulated and evaluated the WFD behavior and got the relative statistic data, which provided a practical engineering method for WFD evaluation.

Abstract: Selective laser melting (SLM) is a relatively new additive manufacturing (AM) technology which uses laser energy for manufacturing in a layered pattern. The unique manufacturing process of SLM offers a competitive advantage in case of very complex and highly customized parts having quasi-static mechanical properties comparable to those of wrought materials. However, it is not currently being harnessed in dynamic applications due to the lack of reliable fatigue data. The manufacturing process shows competitive advantages particularly in the aerospace and medical industry in which Ti-6Al-4V is commonly used, especially for high performance and dynamic applications. Therefore, in this exploratory research, high cycle fatigue (HCF) tests were performed for as-built, polished and shot-peened samples to investigate the capability of SLM for these applications. As-built samples showed a drastic decrement of fatigue limit due to poor surface quality (R_a ≈ 13 µm) obtained from the SLM process. Polishing improved the fatigue limit to more than 500 MPa, the typical value for base material. The effect of shot-peening proved to be antithetical to the expected results. In this context, fractographic analysis showed that very small remnant porosity (less than 0.4%) played a critical role in fatigue performance.

Abstract: This paper based to study of the dynamic and fatigue behaviors with numerical and experimental parts of a composite material manufactured by stacking two layers of E-glass fiber in different angle orientations (0°/90°) immersed in polyester resin with total thickness 1.39mm. In Finite Element Analysis (FEA) using ABAQUS was used to evaluate the maximal (displacement, strain and stress) and the natural frequencies (mode shapes) of composite behavior under crack initiation condition. In the experimental part the laminate plate specimen with two layers is tested under one types of cyclic load in fully reversible tensile at (R=0), the fast fracture occur phenomenon and the fatigue life are presented , the fatigue testing exerted in INSTRON 8801 machine.

Abstract: In the range of linear elastic fracture mechanics, the critical loading assessment of structures made of two dissimilar materials is usually based on the assumptions of the prevailing normal mode of loading. However, in engineering practice there are cases of loading and failure close to the shear mode of loading. The aim of the work is to study the stress distribution in the vicinity of a bi-material notch subjected to a combination of normal and shear modes of loading. Then the stability criteria use knowledge of common fracture mechanics properties for normal I and shear II modes of loading. The assessment of crack initiation conditions is shown on a specimen with two different bi-material notches under loading of a varying direction.

Abstract: In order to investigate the effect of bio-diesel fuel (BDF) on fatigue strength of a squeeze cast Al alloy, AC4CH-T6, rotating bending fatigue tests were carried out using plain specimens in BDF made by rapeseed oil. The results were compared with those in turbine oil and in ambient air. Fatigue life was longer in turbine oil and shorter in BDF than that in ambient air. Fatigue life in each environment tested was mainly occupied by the growth life of a crack smaller than 1mm, meaning that the difference in fatigue strength affected by environment was mainly yielded in the growth process of a small crack. The crack growth rates were suppressed in turbine oil and accelerated in BDF in comparison with those in air. BDF contained much water dissolved from air in the early stage of fatigue process. The suppression of crack growth was caused by wedging effect of oil and the acceleration was caused by corrosion due to dissolved water in BDF, which was larger than the suppression due to the wedging effect.

Abstract: Landfills come with a cover barrier which includes a compacted clay liner essential to safety on site. Unfortunately differential settlement may cause stress in the clay layer leading to the development of cracks. This is why tensile stress damage and shearing are observed on the cap cover. Due to difficulties of strains determination in clayey soil during direct tensile tests a 3-dimensional digital image correlation is used. Two digital cameras are used at a constant distance from the sample tested. Pictures taken at preset intervals are post-processed to extract specimen strain and strain localization. The suitability of this method is discussed in regards of experimental results. This technique is particularly well adapted to the clayey soils when conventional measurements are impossible.

Abstract: A plain-strain finite element (FE) model of the rail is developed by ABAQUS. The 1070 rail steel is considered in the model by Jiang and Sehitoglu's cyclic plastic constitutive equation. The wheel-rail rolling contact is represented by a moving load applied on the contact surface. Based on the FE results the initiation of rolling contact fatigue (RCF) crack is further evaluated by Jiang's fatigue model. Effects of the rail material inner defect and the friction coefficient are also investigated. The results show that the stress-strain state of the rail surface material becomes stable after about thirty passages. The maximum residual stress and strain are located in the subsurface. The life of a defected rail can be as low as 1/27~1/17 of a qualified rail, and the most probable location for crack initiation is highly dependent on the inner defect. The fatigue life is found to considerably decrease with the friction coefficient, while the location of crack initiation and its direction of propagation are less affected.

Abstract: The purpose of this paper is to estimate the number of cycles for fatigue crack initiation in notched plate under constant amplitude loading through tensile residual stress field of Aluminum alloy plate. Residual stress field was generated by plastic deformation using finite element method. Simulation of fatigue behavior was made on AFGROW code. It was shown that the fatigue crack initiation and propagation were affected by level of residual stress filed. In this investigation, the presence of tensile residual stresses reduces considerably the total fatigue life. Loading parameter namely stress ratio was studied. The decreasing in this parameter reduces the fatigue crack growth rate (FCGRs).

Abstract: A newly developed single-ball RCF testing machine was used in order to investigate crack initiation direction within SUJ2. From empirical data, it was found that the distribution of crack initiation direction at N = 1.0x106 cycles is very close to that at N = 1.0x107. This means that the cracks that do not cause failure stop growing by 1.0x106 cycles. Some cracks however continue to grow towards the surface, at an angle of 135 degrees.