Process-induced Residual Stresses Research Articles

Finite Element Simulation and Experimental Validation of Residual Stresses in High Speed Dry Milling of Biodegradable Mg-Ca Alloys

Magnesium-Calcium (Mg-Ca) alloys have become attractive biodegradable orthopedic biomaterials recently. Residual stresses are proven to be very influential on the degradation rate of an Mg-Ca implant in the human body. Due to the time and cost reasons, development of finite element models to predict residual stress profiles in cutting of Mg-Ca implants is highly desirable. In this study, a finite element simulation model of orthogonal cutting without explicit chip formation has been developed by using the plowing depth approach in order to predict process-induced residual stresses in high speed dry cutting of Mg-Ca0.8 (wt %) alloy using diamond tools. Mechanical properties of Mg-Ca0.8 biomaterial at high strain rates and large strains were determined using the split-Hopkinson pressure bar test. The internal state variable (ISV) plasticity model has been implemented to model the dynamic material behavior under cutting regimes. The residual stress evolution and effects of plowing speed and plowing depth on residual stress profiles are studied. Residual stress measurements were also performed utilizing the X- ray diffraction technique for validation purposes.

Finite element analysis of the microdroplet test method using cohesive zone model of the fiber/matrix interface

This paper presents a finite element (FE) modeling methodology to simulate and assess the importance of various factors affecting the failure mechanisms in the microdroplet test used to measure the interface shear strength (IFSS) of S-glass fiber epoxy matrix. These factors include the effect of processing-induced residual thermal stresses, progressive debonding with interfacial friction, unstable crack propagation and frictional fiber sliding on mixed-mode loading at the interface. Cohesive zone modeling approach is employed to simulate the crack propagation and interfacial failure. Mode II dominated traction separation laws are determined through numerical simulations of microdroplet test results. The importance of including the effect of residual stresses and interfacial friction in determining the cohesive traction–separation laws is illustrated. The sensitivity of the results for mode I traction–separation parameters is studied. The developed holistic modeling methodology allows for accurate determination of traction–separation behavior of the interface. This modeling effort also attempts to identify improvements to the microdroplet test method which is widely used to assess the degree of fiber/matrix adhesion.

Evaluation of Stress Intensity Factor of 316 SS LN Circular Grid Plate Hardfaced with Colmonoy 5 for Various Loads

In sodium cooled fast breeder reactors, the high operating temperature necessitates hardfacing of grid plate to avoid galling of the base material. Nickel based cobalt-free alloy called Colmonoy-5 has been chosen as the hard facing material for the Prototype Fast Breeder Reactor (PFBR) grid plate. The grid plate (GP) which is a critical component made of SS316 that holds the core subassemblies are hard faced, to prevent galling caused by subassembly insertion/removal and erosion due to high sodium velocity at 670 K. Colmonoy 5 was hardfaced on circular grid plate along the annular groove by Plasma Transferred Arc Welding (PTAW). The process-induced residual stress generated due to difference in coefficients of thermal expansion between the deposit and substrate material was removed by annealing. In this paper, in order to evaluate Stress Intensity Factor (SIF) near the crack tip of the crack caused by the remote load or residual stresses generated due to thermal cycling owing to on/off of nuclear reactor, a grid plate model hard-faced with colmonoy 5 was prepared and then annealed to remove process-induced residual stress. Considering the plane strain condition to the same geometry of the model, a 3mm crack was considered at the vertical interface between the steel and the colmonoy. By supporting the specimen on the left and right corners of the upper side, load was applied exactly on the opposite side of crack in the positive direction, simulating mode I loading. After loading the model and getting the solution, KIwas obtained at the crack tip. This step was repeated for various loads to obtain the relationship between SIF and load for fracture characteristics.

Experimental Analysis of the Influence of Process Parameters on Residual Stress in AA2024-T3 Friction Stir Welds

Friction stir welding (FSW) is an innovative joining technique, in which a solid state weld is obtained by means of frictional heating and plastic deformation of the processing material. In recent years, an increasing application of the FSW process has been observed, however, for the effective implementation of the technique in safety-critical components and to predict the fatigue behavior of FSWed assemblies, an accurate knowledge of the process-induced residual stress is needed. In this paper results, provided by an experimental analysis on the influence of process parameters on the residual stress in 4mm AA2024-T3 butt joints are reported and discussed. The contour method has been applied to evaluate the residual stress. An asymmetric stress distribution has been found in all samples. A significant dependence of the tensile and compressive stress peaks on the feed rate has been found, while a non monotonic influence of the rotating speed has been evidenced.

On the development of microstructures and residual stresses during laser cladding and post-heat treatments

In this article, laser cladding process with a blown powder feeding was used to deposit nickel-based IN-625 superalloy, cobalt-based hardfacing Stellite 6 alloy and high-vanadium CPM 10V tool steel onto a similar or dissimilar base material, respectively, to investigate the development and controllability of process-induced residual stresses in the clad and to analyse their correlation with microstructural evolutions of the clad and heat-affected zone (HAZ) during cladding and post-heat treatments. The residual stresses were evaluated using the hole-drilling method as per ASTM E837-95, whereas the microstructures were studied using X-ray diffractometer, optical microscope and scanning electron microscope. A particular attention was paid to combined effect of both clad and HAZ on the build-up of residual stresses in the clad. It is expected that the experimental results will form a useful addition to the existing knowledge with respect to the topic and, more significantly, to promote confidence on industrial applications of laser-clad IN-625, Stellite 6 and CPM 10V materials.

Influence of mechanical surface treatments on the high cycle fatigue performance of TIMETAL 54M

TIMETAL 54M (in the following Ti-54M) is a newly developed (α+β) titanium alloy with nominal composition Ti–5Al–4V–0.6Mo–0.4Fe. The alloy can provide a cost benefit over Ti–6Al–4V due to improved machinability and formability. These attractive properties might be a driving force for replacing Ti–6Al–4V in many aircraft as well as biomedical applications. Since HCF performance is one of the most important requirements for these applications, it is essential to improve this property by microstructural optimization and by mechanical surface treatments such as shot peening or ball burnishing. The latter improvement is mainly the result of induced near-surface severe plastic deformation which results in work-hardening and the generation of compressive residual stresses that retard fatigue crack propagation. The main aim of the present study was to investigate the potential fatigue life improvements in Ti-54M due to shot peening and ball-burnishing. The process-induced residual stresses and stress–depth profiles were determined by energy-dispersive X-ray diffraction (ED) of synchrotron radiation with the beam energy of 10–80keV. Results on Ti-54M and Ti–6Al–4V will be compared and correlated with the mean stress and environmental sensitivities of the fatigue strengths in the microstructures.

Fatigue endurance of Ti-6Al-4V alloy with electro-eroded surface for improved bone in-growth

Ti-6Al-4V hour-glass shaped rotating beam specimens with duplex microstructure were processed by electric discharge machining (EDM). A comparatively high peak current of 29A was utilized in order to increase surface roughness for improved osteointegration. High cycle fatigue (HCF) tests were performed in rotating beam loading ( R = − 1 ) on these EDM specimens and results were compared with electrolytically polished specimens serving as reference. As expected, the HCF performance of EDM specimens was inferior to the electrolytically polished specimens. A detailed study of fatigue crack nucleation and microcrack growth was carried out on failed specimens by SEM. The poor HCF strength of EDM specimens is explained by early crack nucleation due to the high notch sensitivity of Ti-6Al-4V. In addition, process-induced residual tensile stresses and microstructural effects may also account for the drastic loss in HCF performance relative to the electropolished baseline.

Experimental study of residual stresses in laser clad AISI P20 tool steel on pre-hardened wrought P20 substrate

Laser cladding is to deposit desired material onto the surface of a base material (or substrate) with a relatively low heat input to form a metallurgically sound and dense clad. This process has been successfully applied for repairing damaged high-value tooling to reduce their through-life cost. However, laser cladding, which needs to melt a small amount of a substrate along with cladding material, inevitably introduces residual stresses in both clad and substrate. The tensile residual stresses in the clad could adversely affect mechanical performance of the substrate being deposited. This paper presents an experimental study on process-induced residual stresses in laser clad AISI P20 tool steel onto pre-hardened wrought P20 base material and the correlation with microstructures using hole-drilling and neutron diffraction methods. Combined with X-ray diffraction and scanning electron microscopic analyses, the roles of solid-state phase transformations in the clad and heat-affected zone (HAZ) of the substrate during cladding and post-cladding heat treatments on the development and controllability of residual stresses in the P20 clad have been investigated, and the results could be beneficial to more effective repair of damaged plastic injection molds made by P20 tool steel.

Prediction of Process-induced Geometrical Deformations for Stiffened Thermosetting Composite Panels

Stiffened thermosetting composite panels are primary structure of aircraft and fabricated with co-curing processing. The process-induced residual stresses resulted in geometrical deformation when the stiffened composite panel was fully cured and removed from the tool. An investigation into the three-dimensional cure simulation of T-shape stiffened thermosetting composite panels was presented to predict the process-induced deformation during co-curing. And flexible tools and locating tools were considered in the cure simulation. Simulation examples were presented to demonstrate use of the present finite element procedure for predicting composite parts deformations. The T300/BMI structure was analyzed provide insight into the non-uniform curing process. The temperature was higher in the interface between web and flange during the entire cure cycle. And the distortions of skin and webs for T-shape stiffened thermosetting composite panel were remarkable.

Prediction of Process-Induced Residual Stress for Laminated Composites with Metallic Plate

The process-induced residual stress for laminated composite with metallic plate is investigated by numerical simulation during the curing process. A two-dimensional finite element model is used to compute process-induced residual stress of the laminated composite. Interface formation process between the fiber and the matrix in the longitudinal direction of composites is described by user-defined exponential function and considered in the numerical model. Numerical results show that the prediction is more accurate in the longitudinal direction of composites when the interface formation process between the fiber and the matrix is considered , which has been confirmed in the test.

Numerical Simulation of a Stretch Bending Process

There are several difficulties associated with the numerical simulation of the stretch bending process of extruded components; the main ones are non-linear material behavior, geometrical non-linearities, modeling of boundary conditions, contact between die and specimen, springback during the unloading phase. Another very complex aspect is the calibration of the numerical model, as rather few experimental results are generally available. This paper deals with a numerical simulation of a complete stretch bending process with the aim to select the most suitable numerical procedure oriented to the evaluation of the process-induced distortions and residual stresses; to validate the proposed procedure, numerical–experimental comparisons have been performed. The activities reported have been developed in collaboration with ALENIA Finmeccanica s.p.a. within the VI FP’s research project called COMPACT.

Investigation on the spring-in phenomenon of carbon nanofiber-glass fiber/polyester composites manufactured with vacuum assisted resin transfer molding

Abstract Process-induced residual stress arises in polymer composites as a result of mismatched resin contraction and fiber contraction during the cure stage. When a curved shell-like composite part is de-molded, the residual stress causes the spring-in phenomenon, in which the enclosed angle of the part becomes smaller than the angle of its mold. In this paper, a new approach is presented to control and reduce the spring-in angle by infusing a small amount of carbon nanofibers (CNFs) together with liquid resin into the glass fiber preform using vacuum assisted resin transfer molding (VARTM) process. The experimental results showed that the spring-in angles of the L-shaped composite specimens were effectively restrained by the CNFs. An analytical model and a 3-D FEA model were developed to predict the spring-in phenomenon and to understand the role of CNFs in reducing the spring-in angle. The models agreed with the experimental results reasonably well. Furthermore, the analytical model explains how the CNF-enhanced dimensional tolerance control is accomplished through the reductions in the matrix’s equivalent coefficient of thermal expansion and linear crosslinking shrinkage.

Physical Phenomena of Thermoplastic Articles in the Vicinity of Their Glass Transition Temperatures

It is well known that environmental conditions impact the performance of thermoplastic articles, therefore requiring selection of appropriate raw materials. Nevertheless, frequently due to business reasons, when selecting a material, the rigor of understanding the fundamental physical events responsible for the desired performance is shortchanged. In this study, we investigate the effects of cyclic heat history on the dimensional stability of articles made with commercial grade rigid thermoplastic resins, particularly, in the temperature range close to the material’s glass transition temperature, as more materials are used in hot & arid environments. The two physical events identified as most relevant to desired article performance are the relaxation of the processinduced residual stresses within the material and the dynamic phenomena brought about by the material’s coefficient of thermal expansion, CTE. We find that the periodic development of convexconcave surfaces on an article matches the periodic development of a non-uniform temperature gradient within the article. We demonstrate that this periodic heat exposure leads to an irreversible concave deformation of the article’s exposed surface. Moreover, articles exhibit this deformation phenomenon regardless of the level of the initial process-induced stresses. We offer explanations of these behaviors and discuss their practical implications.

Elastoplastic damage micromechanics for elliptical fiber composites with progressive partial fiber debonding and thermal residual stresses

Incorporating the interfacial damage and thermal residual stresses, an elasto-plastic damage formulation is proposed to predict the overall transverse mechanical behavior of continuous elliptical-fiber reinforced ductile matrix composites within the framework of micromechanics and homogenization. Based on the concept of equivalent inclusion and taking the progressive interfacial debonding angle into consideration, partially debonded fibers are replaced by equivalent orthotropic, perfectly bonded fibers. Three interfacial damage modes are considered. The Weibull's probabilistic function is adopted to describe the varying probability of progressive partial fiber debonding. The effective elastic moduli of four-phase composites, composed of a ductile matrix and randomly located yet unidirectionally aligned fibers are derived by a micromechanical formulation. Thermal residual stresses are taken into account through the concept of thermal eigenstrain to investigate the effects of the manufacturing processinduced residual stresses. Employing the micromechanical approximation, the overall stress-strain responses and the effective yield function are formulated with the thermal eigenstrain. When comparing with the available experimental data, significant effects of thermal residual stresses are discussed. Moreover, the effects of the interfacial strengths and the cross- sectional shapes of fibers on the mechanical behaviors of composites are systematically investigated.

Riveting Process Induced Residual Stresses Around Solid Rivets in Mechanical Joints

The interference fit provided by solid rivets introduces a residual stress field beneficial to the fatigue life of riveted joints. Evolution in riveting technology has led to force-controlled riveters which provide greater consistency over the rivet installation process and the resulting residual stress field. By reexamining the rivet installation process and its effects on the formation of residual stresses, the fatigue benefits of rivets could be further exploited. Using a 3-D finite element model, installation of universal and countersunk rivets in monolithic aluminum sheet has been studied. Aspects of accepted riveting practice, including the degree of rivet flushness and the rivet squeeze force were found to play significant roles in the formation of residual stresses. Residual stresses beneath the rivet head were also found to be influenced primarily by through-thickness compression of the joined sheets during riveting, challenging the traditional analogy of riveting to radial expansion processes.

Residual stresses in rolled and machined nickel alloy plates: Synchrotron X-ray diffraction measurement and three-dimensional eigenstrain analysis

Most engineering components made from wrought metallic alloys undergo complex sequences of manufacturing operations. These processing steps frequently include extrusion, forging, or rolling, followed by machining and heat treatment. Since such components will be subjected to service loading as part of engineering assemblies, their durability must be assessed using suitably reliable life prediction models. The present study is aimed at the investigation of a combination of experimental and modelling techniques that involves microstructural investigation, diffraction measurement of residual elastic strains, and finite element simulation of residual stress distributions. Eigenstrain-based modelling approach to the analysis of processing-induced residual stresses has been previously presented in the two-dimensional approximation, i.e. under the assumption that the equivalent permanent plastic strain field induced by processing is equibiaxial. Several different formulations were considered and compared, including plane stress, plane stress, and three-dimensional models. In the present study a further development of the eigenstrain-based analysis approach that incorporates the experimental data obtained from synchrotron X-ray diffraction measurements of residual elastic strains in two complementary cross-sections of a forged and machined nickel superalloy plate is reported. The microstructure was assessed using electron backscattered diffraction, and near-surface residual stresses evaluated using laboratory X-ray diffraction. It is found that the results of fully three-dimensional formulation differ from two-dimensional approximations particularly in the vicinity of machined surfaces, having potentially significant implications for durability assessment and fatigue life models.

A thermo-viscoelastic analysis of process-induced residual stress in fibre-reinforced polymer–matrix composites

Process-induced residual stress in fibre-reinforced thermoset polymer–matrix composites was analysed using a thermo-viscoelastic micromechanical model and the finite element method. A three-dimensional unit cell with glass fibre and epoxy polymer–matrix, representing the periodic microstructure of unidirectional fibre-reinforced composites, was considered to compute cure residual stress of fibre composites induced by chemical shrinkage of the epoxy resin and thermal cooling contraction of the whole fibre and resin system. The constitutive behaviour of the epoxy matrix was described by a cure and temperature-dependent viscoelastic material model. Compared to an elasticity solution, a reduction in residual stress was predicted due to the stress-relaxation caused by the viscoelastic behaviour of the epoxy matrix. Calculated residual stress shows strong dependency on the fibre volume fraction and fibre packing. After the cure process is complete, residual stress tends to relax to a constant value. The effect of residual stress on damage and failure of the model was also studied using the maximum stress failure criterion combined with a post-failure stiffness reduction technique. Damage onset, in terms of the location and the load level, was shown to be clearly influenced by the residual stress for both normal and shear loading. Initial and final failure envelopes, predicted for biaxial normal (longitudinal and transverse) loading and combined shear (longitudinal) and normal (transverse) loading, were shown to be shifted and contracted by the inclusion of residual stress. For final failure, residual stress was seen to have little effect on the load levels for longitudinal failure but greatly affected the load levels for transverse and shear failure. Residual stress could be detrimental or beneficial depending on the state of existing residual stress and the loading conditions.

Residual-stress-induced subsurface crack nucleation in titanium alloys

Abstract This investigation has considered the influence of heat treatment, environment and residual stress level on the response of titanium alloys to mechanical surface treatment. It has been shown that the high cycle fatigue benefits to be gained by mechanical surface treatment will depend upon all of these variables. For example, shot peening of Ti–6Al–7Nb a/b solution treated, water quenched and aged condition results in a substantial increase in the high cycle fatigue performance in laboratory air. In contrast, shot peening of Ti–6Al–7Nb a/b solution treated, air cooled and aged condition has only a relatively small impact on the high cycle fatigue performance. This study also shows that high cycle fatigue crack nucleation of mechanically surface treated titanium alloys within the low stress, high cycle regime is associated with subsurface fatigue crack nucleation. Further, that this phenomenon may be related to the presence of a process-induced residual tensile stress necessarily present below the mechanically treated surface and required to balance the outer process-induced residual compressive stresses.

Numerical prediction of process-induced residual stresses in glass bulb panel

A numerical simulation model for predicting residual stresses which arise during the solidification process of pressed glass bulb panel was developed. The solidification of a molten layer of glass between cooled parallel plates was used to model the mechanics of the buildup of residual stresses in the forming process. A thermorheologically simple thermoviscoelastic model was assumed for the material. The finite element method employed was based on the theory of shells as an assembly of flat elements. This approach calculates residual stresses layer by layer like a truly three-dimensional calculation, which is well suited for thin pressed products of complex shape. An experimental comparison was employed to verify the proposed models and methods.

Influence of prestress on the velocities of plane waves propagating normally to the layers of nanocomposites

The paper is concerned with longitudinal and transverse waves propagating at a right angle to the layers of a nanocomposite material with initial (process-induced residual) stresses. The composite consists of alternating layers of two dissimilar materials. The materials are assumed nonlinearly elastic and described by the Murnaghan potential. For simulation of wave propagation, a problem is formulated within the framework of the three-dimensional linearized theory of elasticity for finite prestrains. It is established that the relative velocities of waves depend linearly on small prestresses. In some composite materials, however, the thicknesses of the layers may be in a ratio such that the wave velocities are independent of the prestress level