Three-dimensional (3D) reconstruction and finite element method are combined to study the damage behavior of aluminum alloy resistance spot-welded joints. Fatigue damage of spot-welded joints under different cyclic loading stages was obtained by X-ray microcomputed tomography (X-ray micro CT). Then, avizo software was used to reconstruct the scanned data of joints with different damage degrees, and the distribution and variation of defects in the joints are obtained. On this basis, 3D finite element damage models were established. Finite element calculations were carried out to analyze the fatigue damage of spot-welded joints by adopting the effective elastic modulus as the damage parameter. The results show that the effective elastic modulus is consistent with the experimental results. The method of combining 3D reconstruction with the finite element method can be used to evaluate the internal damage of spot-welded joints and provide theoretical basis for the prediction of fatigue life.

Arising from long-term high temperature service, the microstructure of nickel-base (Ni-base) superalloy components undergoes thermally and deformation-induced aging characterized by isotropic coarsening and directional coarsening (rafting) of the γ ′ precipitates. The net result of the morphological evolutions of the γ ′ particles is a deviation of the mechanical behavior from that of the as-heat treated properties. To capture the influence of a rafted and isotropic aged microstructure states on the long-term constitutive behavior of a Ni-base superalloy undergoing thermomechanical fatigue (TMF), a temperature-dependent crystal viscoplasticity (CVP) constitutive model is extended to include the effects of aging. The influence of aging in the CVP framework is captured through the addition of internal state variables that measure the widening of the γ channels and in-turn update the material parameters of the CVP model. Through the coupling with analytical derived kinetic equations to the CVP model, the enhanced CVP model is shown to be in good agreement when compared to experimental behavior in describing the long-term aging effects on the cyclic response of a directionally solidified (DS) Ni-base superalloy used in hot section components of industrial gas turbines.

Next-generation, reusable hypersonic aircraft will be subjected to extreme environments that produce complex fatigue loads at high temperatures, reminiscent of the life-limiting thermal and mechanical loads present in large gas-powered land-based turbines. In both of these applications, there is a need for greater fidelity in the constitutive material models employed in finite element simulations, resulting in the transition to nonlinear formulations. One such formulation is the nonlinear kinematic hardening (NLKH) model, which is a plasticity model quickly gaining popularity in the industrial sector, and can be found in commercial finite element software. The drawback to using models like the NLKH model is that the parameterization can be difficult, and the numerical fitting techniques commonly used for such tasks may result in constants devoid of physical meaning. This study presents a simple method to derive these constants by extrapolation of a reduced-order model, where the cyclic Ramberg–Osgood (CRO) formulation is used to obtain the parameters of a three-part NLKH model. This fitting scheme is used with basic literature-based data to fully characterize a constitutive model for Inconel 617 at temperatures between 20 °C and 1000 °C. This model is validated for low-cycle fatigue (LCF), creep-fatigue (CF), thermomechanical fatigue (TMF), and combined thermomechanical-high-cycle fatigue (HCF) using a mix of literature data and original data produced at the Air Force Research Laboratory (AFRL).

Shot peening is well known as a surface deformation process which can induce compressive residual stresses into the subsurface of materials in order to improve the fatigue life. In this paper, the effects of the process conditions for both single and double shot peening on the fatigue life of AISI 4340 low alloy steel is investigated. The fatigue tests revealed that the shot peening process could significantly enhance the fatigue life of the treated components. However, a side effect of the process was an increase in surface roughness which was more prevalent under higher peening pressures and led to a reduction in the fatigue life. Therefore, to maximize the performance of the process, the peening parameters need to be carefully selected. Microstructure analysis of the shot peened parts indicated that the nucleation cracks or initiation cracks occurred in the subsurface at depths of 10–20 μ m in the case of as-received samples but moved up to the free surface for the shot peened parts.

Cold expansion (CX) is a material processing technique that has been widely used in the aircraft industry to enhance fatigue life of structural components containing holes. CX introduces compressive hoop residual stresses that slow crack growth near the hole edge. The objective of this paper is to predict residual stresses arising from cold expansion using two different finite element (FE) approaches, and compare the results to measurement data obtained by the contour method. The paper considers single-hole, double-hole, and triple-hole configurations with three different edge margins. The first FE approach considers process modeling, and includes elastic–plastic behavior, while the second approach is based on the eigenstrain method, and includes only elastic behavior. The results obtained from the FE models are in good agreement with one another, and with measurement data, especially close to the holes, and with respect to the effect of edge margin on the residual stress distributions. The distribution of the residual stress and equivalent plastic strain around the holes is also explored, and the results are discussed in detail. The eigenstrain method was found to be very useful, providing generally accurate predictions of residual stress.

The aim of this work is to design a lightweight, creep-resistant blade for an axial single-stage micro-gas turbine. The selected process was casting of an intermetallic titanium/aluminum alloy. All the project phases are described, from the preliminary thermodynamic and geometric stage design, to its three-dimensional (3D) modeling and the subsequent finite element method–computational fluid dynamics (FEM-CFD) analysis, to the manufacturing process of the single rotor blade. The blade making (height 7 cm and chord 5 cm, approximately) consisted in a prototyping phase in which a fully 3D version was realized by means of fused deposition modeling and then in the actual production of a full-scale set of blades by investment casting in an induction furnace. The produced items showed acceptable characteristics in terms of shape and soundness. Metallographic investigations and preliminary mechanical tests were performed on the blade specimens. The geometry was then refined by a CFD study, and a slightly modified shape was obtained whose final testing under operative conditions is though left for a later study. This paper describes the spec-to-final product procedure and discusses some critical aspects of this manufacturing process, such as the considerable reactivity between the molten metal and the mold material, the resistance of the ceramic shell to the molten metal impact at high temperatures, and the maximal acceptable mold porosity for the specified surface finish. The CFD results that led to the modification of the original commercial shape are also discussed, and a preliminary performance assessment of the turbine stage is presented and discussed.

Hole cold expansion and bolt clamping force are usually applied to improve the fatigue performance of bolted joints. In order to investigate the effects of hole cold expansion and bolt clamping force and reveal the mechanism of these two factors on the fatigue damage of bolted joint, a continuum damage mechanics (CDM) based approach in conjunction with the finite element method is used. The damage-coupled Voyiadjis plasticity constitutive model is used to represent the material behavior, which is implemented by user material subroutine in abaqus . The elasticity and plasticity damage evolutions of the material are described by the stress-based and plastic-strain-based equations, respectively. The fatigue damage of joint is calculated using abaqus cycle by cycle. The fatigue lives of double-lap bolted joints with and without clamping force at different levels of hole cold expansion are all obtained. The characteristics of fatigue damage corresponding to the different conditions are presented to unfold the influencing mechanism of these two factors. The predicted fatigue lives and crack initiation locations are in good agreement with the experimental results available in the literature. The beneficial effects of hole cold expansion and bolt clamping force on the fatigue behavior of bolted joint are presented in this work.

Cobalt-based Tribaloy alloys are strengthened mainly by a hard, intermetallic Laves phase consisting of Co 3 Mo 2 Si or/and CoMoSi; therefore, silicon content plays a large role in the microstructure and performance of these materials. In this research, the microstructures of two cobalt-based Tribaloy alloys that are largely different in Si content are studied using scanning electron microscopy (SEM) with an EDAX energy dispersive X-ray (EDX) spectroscopy, and X-ray diffraction (XRD), fatigue strength under rotating-bending test, mechanical behavior under nanoindentation, and hardness at room and elevated temperatures using a microindentation tester. It is revealed that with higher silicon content (2.6 wt. %), T-400 has a hypereutectic microstructure with Laves phase as primary phase, whereas with lower silicon content (1.2 wt. %), T-401 has a hypoeutectic microstructure with solid solution as primary phase. T-400, containing lager volume fraction of Laves phase, exhibits better fatigue strength, in particular, at high stresses, while T-401, with less volume fraction of Laves phase, has improved ductility, exhibiting better resistance to fatigue at low stresses. The hardness of both alloys decreases with temperature, and T-401 shows higher reduction rate. T-400 is harder than T-401.

Increasing interest in using aluminum as the structural component of light-weight structures, mechanical devices, and ships necessitates further investigations on fatigue life of aluminum alloys. The investigation reported here focuses on characterizing the performance of cruciform-shaped weldments made of 5083 aluminum alloys in thickness of 9.53 mm (3/8 in.) under constant, random, and bilevel amplitude loadings. The results are presented as S/N curves that show cyclic stress amplitude versus the number of cycles to failure. Statistical procedures show good agreements between test results and predicted fatigue life of aluminum weldments. Moreover, the results are compared to the results obtained from previous experiments on aluminum specimens with thicknesses of 12.7 mm (1/2 in.) and 6.35 mm (1/4 in.).

Simulation plays a critical role in the development and evaluation of critical components that are regularly subjected to mechanical loads at elevated temperatures. The cost, applicability, and accuracy of either numerical or analytical simulations are largely dependent on the material model chosen for the application. A noninteraction (NI) model derived from individual elastic, plastic, and creep components is developed in this study. The candidate material under examination for this application is 2.25Cr–1Mo, a low-alloy ferritic steel commonly used in chemical processing, nuclear reactors, pressure vessels, and power generation. Data acquired from prior research over a range of temperatures up to 650 °C are used to calibrate the creep and plastic components described using constitutive models generally native to general-purpose fea . Traditional methods invoked to generate constitutive modeling coefficients employ numerical fittings of hysteresis data, which result in values that are neither repeatable nor display reasonable temperature dependence. By extrapolating simplifications commonly used for reduced-order model approximations, an extension utilizing only the cyclic Ramberg–Osgood (RO) coefficients has been developed. This method is used to identify the nonlinear kinematic hardening (NLKH) constants needed at each temperature. Single-element simulations are conducted to verify the accuracy of the approach. Results are compared with isothermal and nonisothermal literature data.

The fatigue properties of high-pressure die-casting (HPDC) magnesium (Mg) alloys AZ91 exhibit a high variability, due primarily to the porosity that is inherent in the injection process. In the 94% of the studied samples, the porosity in which crack nucleation originates is at the surface or adjacent to the surface. The threshold stress intensity factor amplitude and the limit of fatigue have been calculated following the classical models of parameterization of defects. A new set of samples were prepared by machining the surface slightly, in order to conserve the microstructure, and the fatigue behavior at low level of stress was improved. All the samples were produced in molds with the final shape by HPDC process, which allowed a realistic study of the surface effect and the influence of grain size variation from the edge to the center of the samples.

Hastelloy X (HX) and 304 stainless steel (304SS) are widely used in the pressure vessel and piping industries, specifically in nuclear and chemical reactors, pipe, and valve applications. Both alloys are favored for their resistance to extreme environments, although the materials exhibit a rate-dependent mechanical behavior. Numerous unified viscoplastic models proposed in literature claim to have the ability to describe the inelastic behavior of these alloys subjected to a variety of boundary conditions; however, typically limited experimental data are used to validate these claims. In this paper, two unified viscoplastic models (Miller and Walker) are experimentally validated for HX subjected to creep and 304SS subjected to strain-controlled low cycle fatigue (LCF). Both constitutive models are coded into ansys Mechanical as user-programmable features. Creep and fatigue behavior are simulated at a broad range of stress levels. The results are compared to an exhaustive database of experimental data to fully validate the capabilities and performance of these models. Material constants are calculated using the recently developed Material Constant Heuristic Optimizer ( macho ) software. This software uses the simulated annealing algorithm to determine the optimal material constants through the comparison of simulations to a database of experimental data. A qualitative and quantitative discussion is presented to determine the most suitable model to predict the behavior of HX and 304SS.

Microstructural and mechanical characterization investigations on three variants of a through-hardened M50 bearing steel are presented to compare and contrast their performances under rolling contact fatigue (RCF) loading. Baseline (BL) variant of M50 steel bearing balls is subjected to: (i) a surface nitriding treatment and (ii) a surface mechanical processing treatment, to obtain distinct microstructures and mechanical properties. These balls are subjected to RCF loading for several hundred million cycles at two different test temperatures, and the subsequent changes in subsurface hardness and compressive stress–strain response are measured. It was found that the RCF-affected subsurface regions grow larger in size at higher temperature. Micro-indentation hardness measurements within the RCF-affected regions revealed an increase in hardness in all the three variants. The size of the RCF-affected region and intensity of hardening were the largest in the BL material and smallest in the mechanically processed (MP) material. Based on Goodman's diagram, it is shown that the compressive residual stress reduces the effective fully reversed alternating stress amplitude and thereby retards the initiation and evolution of subsurface plasticity within the material during RCF loading. It is quantitatively shown that high material hardness and compressive residual stress are greatly beneficial for enhancing the RCF life of bearings.

Fiber-reinforced polymer (FRP) composites used in the construction of composite-based civil and military marine crafts are often exposed to aggressive elements that include ultraviolet radiation, moisture, and cyclic loadings. With time, these elements can individually and more so cooperatively degrade the mechanical properties and structural integrity of FRP composites. To assist in increasing the long-term reliability of composite marine crafts, this work experimentally investigates the cooperative damaging effects of ultraviolet (UV), moisture, and cyclic loading on the structural integrity of carbon fiber reinforced vinyl-ester marine composite. Results demonstrate that UV and moisture can synergistically interact with fatigue damage mechanisms and accelerate fatigue damage accumulation. For the considered composite, damage and S–N curve models with minimal fitting constants are proposed. The new models are derived by adapting well-known cumulative fatigue damage models to account for the ability of UV and moisture to accelerate fatigue damaging effects.

Fatigue analysis of a simply supported composite plate with laminate configuration of [0n/90n]s under central patch impulse loading is presented using an analytical method. The method mainly consists of two steps, one, evaluation of vibration induced stresses for the given central patch impulse loading using modal analysis, and two, fatigue analysis using S–N curve approach, residual strength approach as well as failure function approach. The stress state in the plate was evaluated using viscous damping model as a function of time. The stress-time history was converted into block loading consisting of many sub-blocks. In the present study, the block loading consisted of four sub-blocks and a total of 175 numbers of cycles. The block loading was repeated after every 5 s. Next, fatigue analysis was carried out based on the block loading condition evaluated. Number of loading blocks for fatigue failure initiation and the location of failure were obtained. Studies were also carried out using two-dimensional (2D) finite element analysis (FEA). Number of loading blocks required to cause fatigue failure initiation under central patch impulse loading was found to be 3120 and 3170 using the analytical method and 2D FEA, respectively.

The fatigue properties of two variants of AISI 1018 steel samples were measured in a series of 33 experiments using new kinds of magnetic diagnostics. An MTS-810 servohydraulic test machine applied sinusoidal fully reversed (R = −1) loads under strain (Є) control in the range of 0.0008 ≤ (Є)≤ 0.0020. In 28 experiments, the number of cycles to fatigue failure Nf varied between 36,000 < Nf < 3,661,000. By contrast, in five runs extending over 107 cycles, the specimens showed no detectable signs of weakening or damage. The corresponding “S-N” or classical Wöhler plots indicated that the transitions from fatigue failure to nominally infinite life (i.e., the fatigue limit) occurred at strains of about Є = 0.0009 and Є = 0.0010, respectively, for the two types of steel. Every loading cycle of each test was instrumented to record continual values of stress and strain. Flux gate magnetometers measured the variations of the piezomagnetic fields near the specimens. A 1000-turn coil surrounding the test pieces detected the piezo-Barkhausen pulses generated by abrupt rearrangements of their internal ferromagnetic domain structures. Analyses of the magnetic data yielded four independent indices each of which located the fatigue limits in complete agreement with the values derived from the Wöhler curves.

In the present investigation, the applicability of a previously developed closed form energy based framework to predict low cycle fatigue (LCF) life of aluminum 6061-T6 was extended from room temperature to elevated temperature. The three different elevated temperatures considered in the present investigation were 75 °C, 100 °C, and 125 °C which were below the creep activation temperature for aluminum 6061-T6. Like the room temperature life assessment framework, the elevated temperature life assessment framework involved computation of the Ramberg–Osgood cyclic parameters from the average plastic strain range and the average plastic energy obtained from an axial isothermal-mechanical fatigue (IMF) test. The temperature dependent cyclic parameters were computed for 25 °C (room temperature), 75 °C, and 100 °C and then extrapolated to 125 °C utilizing functions describing the dependence of the cyclic parameters on temperature. For aluminum 6061-T6, the cyclic parameters were found to decrease with increase of temperature in a quadratic fashion. Furthermore, the present energy based axial IMF framework was found to be able to predict the LCF life of aluminum 6061-T6 at both room and elevated temperatures with excellent accuracy.

A previously developed energy based high cycle fatigue (HCF) life assessment framework is modified to predict the low cycle fatigue (LCF) life of aluminum 6061-T6. The fatigue life assessment model of this modified framework is formulated in a closed form expression by incorporating the Ramberg–Osgood constitutive relationship. The modified framework is composed of the following entities: (1) assessment of the average strain energy density and the average plastic strain range developed in aluminum 6061-T6 during a fatigue test conducting at the ideal frequency for optimum energy calculation, and (2) determination of the Ramberg–Osgood cyclic parameters for aluminum 6061-T6 from the average strain energy density and the average plastic strain range. By this framework, the applied stress range is related to the fatigue life by a power law whose parameters are functions of the fatigue toughness and the cyclic parameters. The predicted fatigue lives are found to be in a good agreement with the experimental data.

A microstructure-based fatigue model is employed to predict fatigue damage in 4140 steel. Fully reversed, strain control fatigue tests were conducted at various strain amplitudes and scanning electron microscopy was employed to establish structure-property relations between the microstructure and cyclic damage. Fatigue cracks were found to initiate from particles near the free surface of the specimens. In addition, fatigue striations were found to originate from these particles and grew radially outward. The fatigue model used in this study captured the microstructural effects and mechanics of nucleation and growth observed in this ferrous metal. Good correlation of the number of cycles to failure between the experimental results and the model were achieved. Based on analysis of the mechanical testing, fractography and modeling, the fatigue life of the 4140 steel is estimated to comprise mainly of small crack growth in the low cycle regime and crack incubation in the high cycle fatigue regime.

This study presents a multiscale computational model and its application to predict damage dependent mechanical behavior of bituminous mixtures subjected to cyclic loading. Two length scales (global and local) are two-way coupled in the model framework by linking a homogenized global scale to a heterogeneous local scale representative volume element. Based on the unique two-way coupled multiscaling and the use of the finite element technique incorporated with the material viscoelasticity and cohesive zone fracture, the model approach can successfully account for the effect of mixture heterogeneity, material viscoelasticity, and damage accumulation due to cracks in the small scale on the overall performance of larger scale mixtures or structures. This step requires only the properties of individual constituents. To demonstrate the model and its features, bending beam fatigue testing of a bituminous mixture, which is composed of elastic aggregates and viscoelastic bitumen, is simulated by altering the mixture's constituent properties. The model clearly presents progressive damage characteristics with repetitive loading cycles and the analysis clearly demonstrates the sensitivity of the approach to constituent material properties. The multiscale model presented herein is expected to drastically reduce time-consuming and expensive fatigue tests, which, when performed in the traditional manner, require many replicates and do not define the cause of microstructural fatigue, damage, and failure.