AbstractAbstractFinite element (FE) models have been developed to predict residual stress distributions resulting from the friction stir welding (FSW) process. Plates 250 mm × 50 mm × 5 mm in dimension of an Aluminium alloy (AA2024-T4) were butt-joined through FSW. The thermal profiles were monitored in-situ during the welding process using thermocouples. Sequentially-coupled thermo-mechanical simulations have been performed using an instantaneous relative linear velocity based heat source. Post weld residual stress measurements have been obtained using the neutron diffraction technique and were used to verify the finite element results. The thermal profiles measured during welding have been simulated in the FE model. Increasing the tool traversal speed is found to reduce the peak temperatures experienced during welding for a given tool rotational speed. The general trends and magnitudes of the residual stress distributions measured along the weld line have been predicted by the FE model. The residual stress distributions measured and predicted are relatively symmetric and uninfluenced by the range of tool traversal speeds for the cases considered. Further refinement of the material and process model may be required to improve the predictions accuracy. However, effective predictions have been obtained in the FE model presented by treating the material as a continuum without additional complexities.Keywords: AA2024 Al alloyresidual stressfriction stir welding
In service fatigue loading of composite components damage prediction needs to be addressed, using experimental and numerical methods. However the majority of fatigues loading experimental data for characterizing composite materials are taken from constant amplitude tests. This is mainly due to the expensive and time-consuming nature of variable amplitude experiments. The fatigue effects are often assessed by performing block loading experiments with various high-to-low and low-to-high sequences. The purpose of this study is to predict using probabilistic methods, the fatigue life of the coupon and eventually plot the relationship between the damage fractions accumulated during the first and the second fatigue test for different block loading sequences. A thermodynamic based cumulative damage theory is used to provide an effective and efficient methodology to accurately characterize the effects of load sequence, load level, and fatigue damage history on the multi-stage fatigue loading of composites. The probabilistic predictions are used to identify the influence of different input variables on the predictions. The results have been verified with the tensile fatigue experimental data measured for a 3D 3TEX woven Fiberglas reinforced laminate system by Delphi/ORNL. The probabilistic risk mitigation is performed to seek the range of fatigue life extension.
A recently developed equilibrium and equivalency (E2) mechanisms based curve fitting method is extended to surface fitting for linear function z = f (x, y) = a + bx + cy with two independent variables x and y. The concept of equilibrium of ‘force’ and ‘moment’ is adopted to derive surface fitting formulae, which are exactly the same as that obtained with traditional least squares (LS) method for the linear function. However, E2 method has obvious physical meaning and therefore is more intuitive in quickly and correctly identifying data pattern and subsequent data analysis. Furthermore, the formula based on perpendicular offsets method in terms of data variation along surface normal direction is derived and the results are compared with the traditional methods. Finally, the application of these methods to data of fatigue and creep lives is presented.
A new method is presented for the design of preform die shapes in a multistage forging process. The method incorporates a backward linear Lagrange interpolation scheme into the finite element method. In the backward deformation method, the final component shape is taken as the starting point and the die is moved in the reverse direction. The nodal coordinates in the backward direction are interpolated using the linear Lagrange interpolation method, which can specify the location of the nodes in each backward time increment. The method also uses the constant volume concept in conjunction with geometrical features, such as backward die/workpiece overlap attributes, to determine the shape of the perform die.
Abstract Understanding the buckling behavior of fiber-reinforced composites (FRCs) is critical for the design of composite structures. In this study, finite element (FE) models of FRC buckling behaviors were developed and validated. The validated FE models could accurately predict the numerical and experimental observations in the literature. The effect of the specimen geometric imperfections was included in the model to secure a realistic FE model; to this end, linear buckling analyses were employed before beginning the nonlinear buckling analyses. The FRCs’ mechanical properties and buckling behavior of FRCs can be temperature-dependent. Because the presence of a hole in the design of composite structures may be inevitable in a few applications, the temperature-dependent buckling responses of open-hole glass/epoxy, glass/polyester, carbon/epoxy, and carbon/polyester composites were compared with those of the plain specimens. The effects of the fiber and resin types, temperature, and the presence of holes on buckling behavior were investigated and discussed in detail. Five different temperatures, 25, 0, −50, −100, and −180 °C were considered. The cryogenic temperatures raised Young’s moduli and consequently raised the critical buckling loads. The validated models and results on the open-hole composites can be used as benchmarks in composite structure designs involving buckling behavior.
In this paper a finite element simulation of fine blanking process has been demonstrated. The commercial finite element code ABAQUS has been used to simulate the large deformation in punch/die interaction area while a user written visual FORTRAN program has been developed to calculate the variations of void volume fraction (VVF) and incorporate it into crack initiation by specifying the crack propagation time. To prevent element distortion at the crack tip, the finite element mesh is locally remeshed. After each remeshing, the program maps solutions from the previous deformed mesh to the new model. The value of VVF at each element is obtained from Gurson [1] and Tvergaard damage model. Since the height and location of V-ring indenter have a great influence on the fine blanking process conditions, in the present research work, comparisons are made for different V-ring locations and heights. Describing the variations of VVF would lead to estimate crack initiation time and consequently the quality of sheared surface.
Failure of pressure vessels and piping systems that operate at high temperatures can occur by net section rupture, creep crack growth or a combination of both processes. Several design and assessment procedures are available for dealing with this situation. These include the ASME Pressure Vessel and Piping, French RCC-MR (Appendix 16) and British R5 and BS7910 codes. Each of these procedures uses a combination of continuum mechanics and fracture mechanics concepts to make an assessment. Although the procedures adopt the same basic principles, often different formulae are employed to make an assessment. The main parameters that are used are reference stress, σref, stress intensity factor, K, and the creep fracture mechanics term C*. In this paper, an analysis is performed to estimate the sensitivity of the predictions of creep crack growth in a pressurised pipe to the choice of formulae used and materials properties employed. It is shown that most sensitivity is obtained to choice of expression employed for calculating σref and to whether batch specific or more generic materials properties data are selected.
Residual stress distributions in ferritic steel T-plate weldments have been obtained using the neutron diffraction method. It is shown that the transverse residual stress distribution for two plates of different yield strength are of similar shape and magnitude when normalized appropriately and peak stresses are on the order of the material yield strength. The resultant linear elastic stress intensity factors for these stress distributions have been obtained using the finite element method. It has been shown that the use of the recommended residual stress distributions in UK structural integrity procedures leads to a conservative assessment. The stress intensity factors for the welded T-plate have been shown to be very similar to those obtained using a smooth edge cracked plate subjected to the same local stress field.
Advanced steels are designed and produced to be used in engineering applications in which thermo-mechanical fatigue could be a main factor in causing failure in components operating at elevated temperatures. In this paper thermo-mechanical fatigue properties of these steels are studied under the influence of creep and fatigue damage evolution. Development of different models and simulation techniques are reviewed to predict material behaviour. Numerical simulations are carried out to predict experimental tests on parent material notched bar specimens. Numerical predictions are introduced in advance of experimental test to assess the experimental test procedure. This is usually done to enhance the experimental result integrity and expectations. A local ductile damage development methodology is employed using the kinematic hardening criterion and compared to previously used strain hardening material property. The modelling on notched bar geometries is extended to geometries with cracks in which a local damage criterion will be used to predict virtual crack extension in compact tension specimens.
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