Initial Residual Stress State Research Articles

Effect of DED process parameters on distortion and residual stress state of additively manufactured Ti-6Al-4V components during machining

One of the major challenges in high-deposition rate Directed Energy Deposition processes is the resultant residual stresses generated during material deposition, often leading to distortion and poor material characteristics. Important part families suitable for DED process in aerospace sector are thin-wall components, characterized by a large base surface area with rib-like strengthening structures. Here, the substrate plate can be designed to be a part of the final component. The integration of substrate plate into final component results in possible deformation due to residual stress release during machining. This paper therefore investigates the effect of various powder-based Laser Metal Deposition process parameters and strategies on the residual stress state of the additively manufactured Ti-6Al-4V components and the resulting stress release during machining process. The analysis has been carried out during the machining process by including in-process strain measurements of the substrate. The embraced layer removal method allows the determination of machining zone specific stress release mapping, based on an analytical and FEM-model. Hence, the initial residual stress state of the builds was calculated, which revealed that although the heat treatment resolved most of the residual stresses, also in heat treated parts residues were identified depending on the part clamping during the treatment. Furthermore, the study revealed that the significant residual stresses are present in the layers close to the substrate.

Application of a nonlinear kinematic-isotropic material model for the prediction of residual stress relaxation under a cyclic load

The effect of residual stress in the fatigue analysis of a formed products must be considered with great care. The relaxation of residual stress under a cyclic load is a transient phenomenon that is described in this paper using classic plasticity theory by employing a combined, non-linear, kinematic and isotropic hardening material model. Results were compared with experimental observations by means of contour method and digital-image-correlation method on specimens made of S235JR steel, with focus on near surface state. This research showed reasonable prediction of initial residual stress state and an over-prediction of strain ratcheting as cyclic load progresses.

The Influence of Residual Stresses Induced by Near-Net-Shape Blanking Processes on the Fatigue Behavior under Bending Loads

Gears manufactured by blanking can be found in many different products like hammer drills or automobiles. Here, only the functional surface, the clean shear, can be used to transmit torque. Therefore, parts in serial production demand a high amount of clean shear, so the required torque can be transmitted with a minimal thickness and part weight. To achieve this, these parts are usually manufactured by fineblanking or related Near-Net-Shape Blanking processes (NNSBPs). Furthermore, the gears are subjected to cyclic loading which can, especially in the highly stressed tooth root, lead to tooth breakage. The effect of different process variants and process parameters on the residual stresses and the fatigue behavior under a pulsating bending load has not been investigated yet. Due to the potential of endurance improvement of blanked gears, this topic is addressed in this paper. To accomplish this, C-shaped profiles are manufactured by five different Near-Net-Shape blanking processes. The investigated processes are fineblanking, precision blanking with and without blank holder, and blanking with a small die clearance with and without a v-ring. The sheet metal material, S355MC (material number 1.0976) with a thickness of 6 mm, is first subjected to a stress relief heat treatment to minimize residual stresses induced by the specimen preparation and to ensure a defined initial residual stress state. After blanking, the residual stresses of the parts are measured. Finally, fatigue strength tests are carried out under a pulsating bending load on the C-shaped profiles with shear-cut edges. The results show that the residual stress state, as well as the part’s fatigue behavior are strongly influenced by the chosen blanking process.

Influence of Initial Residual Stress on Growth and Pattern Creation for a Layered Aorta

Residual stress is ubiquitous and indispensable in most biological and artificial materials, where it sustains and optimizes many biological and functional mechanisms. The theory of volume growth, starting from a stress-free initial state, is widely used to explain the creation and evolution of growth-induced residual stress and the resulting changes in shape, and to model how growing bio-tissues such as arteries and solid tumors develop a strategy of pattern creation according to geometrical and material parameters. This modelling provides promising avenues for designing and directing some appropriate morphology of a given tissue or organ and achieve some targeted biomedical function. In this paper, we rely on a modified, augmented theory to reveal how we can obtain growth-induced residual stress and pattern evolution of a layered artery by starting from an existing, non-zero initial residual stress state. We use experimentally determined residual stress distributions of aged bi-layered human aortas and quantify their influence by a magnitude factor. Our results show that initial residual stress has a more significant impact on residual stress accumulation and the subsequent evolution of patterns than geometry and material parameters. Additionally, we provide an essential explanation for growth-induced patterns driven by differential growth coupled to an initial residual stress. Finally, we show that initial residual stress is a readily available way to control growth-induced pattern creation for tissues and thus may provide a promising inspiration for biomedical engineering.

On the analysis of post weld heat treatment residual stress relaxation in Inconel alloy 740H by combining the principles of artificial intelligence with the eigenstrain theory

Post weld heat treatment (PWHT) process has an important role on fabrication of advanced ultra-supercritical power plant turbines. This process relieves the residual stresses formed as a result of welding by converting elastic strains into creep strains. In order to analyse the residual stress relief mechanism during the PWHT process, a novel simulation approach based on experimental data was developed for the analysis of residual stress states from complex manufacturing processes which are welding and heat treatment. This model uses permanent plastic strains (eigenstrains) formed as a result of welding process to set the initial mechanical state of the sample. The distribution of eigenstrains in the whole body was determined using displacement data obtained from contour measurements. The use of eigenstrains to set the initial residual stress state of the creep model reduced the number of uncertainties. This allowed the use of the principles of artificial intelligence for the development of a new fuzzy finite element model (fFEM) that determines the eigenstrain-creep model parameters through an evolution process. Subsequent to the determination of the model parameters, conditions of the PWHT process are investigated to analyse residual stress relaxation in Inconel Alloy 740H weldments.

Residual stresses in parts manufactured by near-net-shape-blanking

Near-net-shape-blanking processes enable to manufacture functional surfaces in a single manufacturing step, which offers great potential in the production of gears, for example. Among these processes are fineblanking, blanking with a small die clearance, blanking with a v-ring, precision blanking with and without a blank holder. All of these are able to produce almost identical cut-surfaces, this means a high proportion of clean-shear almost without a fracture zone, but plastic deformation in the shearing zone varies due to the different tool setups. Therefore, the residual stresses in the produced parts, which are highly relevant for the fatigue behavior of shear-cut gears, vary as well. These process-induced residual stresses are addressed in this paper. First, sheet metal strips made of S355MC (1.0976) were subjected to a stress relief heat treatment to get a defined and low initial residual stress state. Afterwards, a circular hole was cut in the strips by the five different blanking processes. The cut surfaces of the thus produced parts were measured as well as the hardness. The residual stresses in tangential and axial direction were measured for a depth of up to 0.34 mm by X-ray diffraction. Finally, a comparison between the different manufacturing techniques regarding the residual stresses is carried out.

Influence of pre-machining on the surface integrity after processing by mechanical surface treatment

The functional performance of workpieces is strongly influenced by the surface and subsurface properties which are affected and can be adjusted by manufacturing processes. The finishing process determines the final form and the surface roughness of a machined component. Nonetheless, the final surface integrity is a result of the total material modification along the whole process chain. Previous manufacturing processes affect the initial residual stress state and the work hardening in the subsurface layers. Furthermore, the resulting material modifications may vary depending on the initial or pre-machined material state. In this paper, the influence of pre-machining processes on the resulting surface integrity after mechanical finishing processes is analyzed. As finishing processes, deep rolling and so called “grind-strengthening” with varied process parameters are chosen. The initial material state is generated by heat treatment followed by pre-machining using three different processes: turning, grinding and electrochemical etching leading to strong variations in the surface-near properties. For the analysis of the influence of initial material state residual stresses as well as the shear stress were considered. Both processes, deep rolling and grind-strengthening, achieve comparable compressive stresses at the surface, whereby the residual stress states can deviate for constant parameters under variation of pre-machining. The residual stress depth profiles show a significant influence of the initial state for the grinding process but minor influence for deep rolling. The investigations contribute to a better understanding of mechanisms causing material modification during mechanical surface treatment and along the process chain.

Influence of Residual Stresses Introduced by Manufacturing Processes on Bearing Endurance Time

By enhancing surface properties and by reducing weight and size of machine elements, future trends in resource efficiency for machines can be fulfilled. Rolling element bearings are widely used machine elements. By improving their boundary zone properties, there is a potential to extend maintenance-free operating periods, postpone replacements or increase the lifespan of the complete system. The aim is to set favorable boundary zone properties by enhancing the residual stress state within the manufacturing process. For this, the processes hard turning and deep rolling were investigated. The influence of the initial residual stress state on fatigue life was calculated using a model based on the approach of Ioannides and Harris. The resulting residual stresses after the manufacturing process and their changes during operation in a bearing test rig are discussed and compared to those of standard bearings. The tests prove that pre-induced residual stresses increase is able to significantly delay rolling contact fatigue-related bearing failures.

Experimental measurements of surface damage and residual stresses in micro-engineered plasma facing materials

The thermomechanical damage and residual stresses in plasma-facing materials operating at high heat flux are experimentally investigated. Materials with micro-surfaces are found to be more resilient, when exposed to cyclic high heat flux generated by an arc-jet plasma. An experimental facility, dedicated to High Energy Flux Testing (HEFTY), is developed for testing cyclic heat flux in excess of 10 MW/m2. We show that plastic deformation and subsequent fracture of the surface can be controlled by sample cooling. We demonstrate that W surfaces with micro-pillar type surface architecture have significantly reduced residual thermal stresses after plasma exposure, as compared to those with flat surfaces. X-ray diffraction (XRD) spectra of the W-(110) peak reveal that broadening of the Full Width at Half Maximum (FWHM) for micro-engineered samples is substantially smaller than corresponding flat surfaces. Spectral shifts of XRD signals indicate that residual stresses due to plasma exposure of micro-engineered surfaces build up in the first few cycles of exposure. Subsequent cyclic plasma heat loading is shown to anneal out most of the built-up residual stresses in micro-engineered surfaces. These findings are consistent with relaxation of residual thermal stresses in surfaces with micro-engineered features. The initial residual stress state of highly polished flat W samples is compressive (≈ -1.3 GPa). After exposure to 50 plasma cycles, the surface stress relaxes to −1.0 GPa. Micro-engineered samples exposed to the same thermal cycling show that the initial residual stress state is compressive at (- 250 MPa), and remains largely unchanged after plasma exposure.

Remaining stress-state and strain-energy in tempered glass fragments

When tempered glass breaks, it shatters into relatively small pieces depending on the residual stress state in the glass. This has been known for centuries and is currently used in standards for classifying whether a piece of glass is tempered or not. However, the process of fragmentation is complex and only a few, relatively simple, models have been suggested for predicting the fragment size. The full theoretical explanation is still to be found and this work aims at providing another brick to the puzzle. The strain-energy present in tempered glass is obviously contributing to the fragmentation process and some authors e.g. Barsom (J Am Ceram Soc 51(2):75, 1968), Gulati (Glass processing days, Tamglass Engineering Oy, Tampere, 1997), Warren (Fractography of glasses and ceramics IV, Alfred University, Alfred, 2001) and Tandon and Glass (Fracture mechanics of ceramics—active materials, nanoscale materials, composites, glass and fundamentals, Springer, Houston, 2005) have proposed models for the fragments size based on an energy approach. Often an estimate of the remaining strain energy in the fragment is used; which leaves the questions: (a) what parameters are important for the remaining strain energy? (b) what is the magnitude of the remaining strain energy? (c) is there a simple way to estimate the remaining strain energy? The present paper applies a quasi-static finite element model in order to answer these questions. In the present paper an example on the deformation and the stress redistribution in a fragment is given. Furthermore, a parametric investigation on the strain energy remaining in cylindrical- and prismatic fragments is given. It is shown, that there exists a simple relation between the thickness of the glass pane and the remaining strain energy in the fragment. A simple method for estimating the remaining strain energy in a fragment of a given shape and initial residual stress state is presented.

Methodology for aluminium part machining quality improvement considering mechanical properties and process conditions

The manufacturing of structural aluminium alloy parts requires several steps of both forming processes and heat treatments. Before machining, which is usually the last step of the manufacturing, the workpiece has thus undergone multiple manufacturing steps involving unequal plastic deformations which are source of residual stresses. During machining, where up to 90% of the initial workpiece volume can be removed, the mechanical equilibrium of the part evolves constantly with the redistribution of the initial residual stresses. For thick, large and complex parts in highly alloyed aluminium, this redistribution of the residual stresses can leads to an unexpected behaviour of the workpiece and is the main reason for both workpiece deflections (during machining) and post-machining distortions (after unclamping). These two phenomena can lead to the nonconformity of the part with the geometrical and dimensional tolerance specifications and therefore to the rejection of the part or to additional conforming steps. As a consequence, the mechanical behaviour of the workpiece has to be considered during the definition of the machining process plan to improve the machining accuracy and the robustness of the process and thus to ensure the conformity of the machined part with the dimensional and geometrical specifications, i.e. to ensure the desired machining quality. In this paper, the numerical tool developed in [1] is used to conduct an analysis on the influence of the initial workpiece residual stress state, of the fixture layout as well as of the machining sequence on the machining quality. This analysis is performed on a part which has been specially designed and which can be considered as being representative of real aerospace parts. Several comparisons with experimental results are performed, one of them using digital image correlation (DIC) measurements. Results obtained show a good agreement, validating both the prediction of the behaviour of the workpiece during machining and the prediction of the machined part geometry. Based on the results of this analysis, a classification of the parameters has been performed depending on their influence on the machining quality. A first methodology allowing to define machining process plans adapted to the initial workpiece stress state has then been created based on the previous classification. This methodology is composed of a procedure and basic guidelines which are both presented in detail. An example of an application of this methodology is then introduced, demonstrating the benefits of the approach developed in this work.

A multi-scale self-consistent model describing the lattice deformation in austenitic stainless steels

Differently oriented grains within a polycrystalline material exhibit different micro-mechanical lattice responses for a given macroscopic stress due to the local elastic and plastic anisotropy. A physical understanding of the lattice deformation mechanism during plastic flow is still lacking. In this study, a three-dimensional multi-scale self-consistent model is developed to examine the micro-mechanical deformation behaviour of F.C.C. polycrystalline austenitic stainless steels under uniaxial tensile loading at ambient temperature. The model is formulated in a crystal based plasticity framework and takes into account the detailed kinematics of dislocation slip and its influence on the evolution of the dislocation distribution on different crystallographic planes of individual grains within a polycrystalline material. The effect of athermal solute strengthening is also incorporated. Predictions of the microscopic lattice response developed within individual grains are compared with various available experimental results obtained using neutron diffraction (ND). There are significant differences in the way in which the lattice strain evolves with macroscopic strain obtained by different groups. This is explained in terms of the variation in initial residual stress state in the samples tested by these different groups.

An Experimental Procedure to Determine the Interaction between Applied Loads and Residual Stresses

This paper presents a novel experiment to quantify both the initial residual stress state in a specimen and its redistribution due to plasticity induced by in-situ loading. The rate of relaxation of the residual stress with respect to permanent deformation is a measure of the elastic follow-up associated with the residual stress field. Residual stress measurements were made using high energy dispersive X-ray diffraction. Digital image correlation, verified by strain gauges, was used to measure full-field deformation on the specimen. The specimen was loaded and unloaded in-situ incrementally to promote plasticity, allowing the relaxation rate of the residual stress to be quantified. An elastic follow-up factor was calculated for the residual stress field, that indicated loading conditions of the residual stress field between fixed-displacement and fixed-load.

Effect of Initial Residual Stress on Stress Relaxation in Cold-Drawn Steel Rods

This work shows the effect of the initial residual stress state on the stress relaxation behavior of cold-drawn steel rods. The evolution of residual strains at several locations along the rod diameter was measured in-situ by neutron diffraction during a stress relaxation experiment. It was found that if residual stresses are significant, stress relaxation is not homogeneous in the cross-section of the rods. This also explains the higher stress losses found in the rods with high residual stresses.

Experimental and numerical rolling contact fatigue study on the 32CrMoV13 steel

ABSTRACTThe aim of this work is to study pure rolling contact fatigue in 32CrMoV13 quenching and tempering steel. The study involves both experimental and numerical work. The influence of the roughness and the residual stresses on the mechanisms and zones of cracking were studied. The results show a rapid reduction in roughness during the first minute of test but even so there will be specimen deterioration. The residual stress profile after rolling contact tests have high compression values in the surface and at a depth of approximately 240 μm, which is related with the Hertzian maximal shear stress. The numerical simulation of the Hertzian loading was used both to determine the elastic shakedown of the material and to apply a high‐cycle multiaxial fatigue criterion. The three‐dimensional finite element analysis used in the numerical calculation includes elastic‐linear kinematic hardening plastic material and allows the introduction of an initial residual stress state. Taking into account the elastoplastic load induced by the Hertz pressure, low‐cycle fatigue tests were used to characterize the mechanical properties of the material. In order to validate the numerical simulation, the results of the calculation after elastic shakedown were compared with the values measured by X‐ray diffraction after rolling contact tests. The results showed a reasonable agreement between calculated and measured stresses. The Dang Van high‐cycle multiaxial fatigue criterion showed a good relationship with the experimental findings.

Stress effects in sol-gel derived ferroelectric thin films

Residual stress development during processing of sol-gel derived ferroelectric thin films influences electromechanical properties and performance. The present work investigates the effects of stress on field-induced polarization switching in ferroelectric Pb(Zr0.52Ti0.48)O3 (PZT) (52/48) thin films. Film response is measured as a function of externally applied mechanical stress using a double-beam laser-Doppler heterodyne interferometer. This apparatus successfully eliminates any displacement contribution from flexural vibration of the substrate and enables measurement of the strain-electric field hysteresis loops as a function of applied stress. The field-induced strain in the PZT film increases with increasing compressive stress, while the opposite trend is observed for applied tensile stress. The dependence of electromechanical response on the external stress is attributed to the initial tensile residual stress state in the film. Tensile stress creates an in-plane clamping effect on the domains in the film, hindering polarization switching. The application of a compressive stress reduces tensile residual stress in the film and the constraint on the domains, leading to higher field-induced strains. Applied tensile stress amplifies the clamping effect, leading to lower field-induced strains.

03/02501 Long-term fatigue life loss of turbine blades owing to non-characteristic harmonic currents in asynchronous HVDC links: Tsai, J.-T. et al. Electric Power Systems Research, 2003, 65, (2), 135–147

Foreign-object damage associated with the ingestion of debris into aircraft turbine engines can lead to a marked degradation in the high-cycle fatigue (HCF) life of turbine components. This degradation is generally considered to be associated with the premature initiation of fatigue cracks at or near the damage sites; this is suspected to be due to, at least in part, the impact-induced residual stress state, which can be strongly tensile in these locations. However, recent experimental studies have shown the unexpected propensity for impact-induced fatigue crack formation at locations of compressive residual stress in the vicinity of the impact site. To address this issue, in situ and ex situ spatially-resolved X-ray diffraction and numerical modeling are utilized to show that the initial residual stress state can be strongly relaxed during the fatigue loading process. The magnitude and rate of relaxation is strongly dependent on the applied loads. For a Ti–6Al–4V turbine blade alloy, little relaxation was observed for an applied maximum stress of 325 MPa (0.35σy, where σy is the yield stress), and cracks tended to form in subsurface zones of tensile residual stress away from the damage sites. In contrast, at an applied maximum stress of 500 MPa (0.54σy), equal to the smooth-bar 107-cycle endurance strength, cracks tended to form at the damage sites in zones of high stress concentration that had initially been in strong compression, but had relaxed during the fatigue loading.

An experimental/computational approach to identify moduli and residual stress in MEMS radio-frequency switches

In this paper, we identify the Young's modulus and residual stress state of a free-standing thin aluminum membrane, used in MEMS radio-frequency (rf) switches. We have developed a new methodology that combines a membrane deflection experiment (MDE) and three-dimensional numerical simulations. Wafer-level MDE tests were conducted with a commercially available nanoindenter. The accuracy and usefulness of the MDE is confirmed by the repeatability and uniformity of measured load-deflection curves on a number of switches with both wedge and Berkovich tips. It was found that the load-deflection behavior is a function of membrane elastic properties, initial residual stress state and corresponding membrane shape. Furthermore, it was assessed that initial membrane shape has a strong effect on load-deflection curves; hence, its accurate characterization is critical. Through an iterative process and comparison between MDE data and numerical simulations, the Young's modulus and residual stress state, consistent with measured membrane shape, were identified. One important finding from this investigation is that variations in membrane elastic properties and residual stress state affect the load-deflection curve in different regimes. Changes in residual stress state significantly affect the load-deflection slope at small values of deflection. By contrast, variations in Young's modulus result in changes in load-deflection slope at large deflections. These features are helpful to decouple both effects in the identification process.

Mechanical relaxation of localized residual stresses associated with foreign object damage

Foreign-object damage associated with the ingestion of debris into aircraft turbine engines can lead to a marked degradation in the high-cycle fatigue (HCF) life of turbine components. This degradation is generally considered to be associated with the premature initiation of fatigue cracks at or near the damage sites; this is suspected to be due to, at least in part, the impact-induced residual stress state, which can be strongly tensile in these locations. However, recent experimental studies have shown the unexpected propensity for impact-induced fatigue crack formation at locations of compressive residual stress in the vicinity of the impact site. To address this issue, in situ and ex situ spatially-resolved X-ray diffraction and numerical modeling are utilized to show that the initial residual stress state can be strongly relaxed during the fatigue loading process. The magnitude and rate of relaxation is strongly dependent on the applied loads. For a Ti–6Al–4V turbine blade alloy, little relaxation was observed for an applied maximum stress of 325 MPa (0.35σy, where σy is the yield stress), and cracks tended to form in subsurface zones of tensile residual stress away from the damage sites. In contrast, at an applied maximum stress of 500 MPa (0.54σy), equal to the smooth-bar 107-cycle endurance strength, cracks tended to form at the damage sites in zones of high stress concentration that had initially been in strong compression, but had relaxed during the fatigue loading.

Intergranular stresses in Zircaloy-2 with rod texture

The intergranular strains in thermally and mechanically anisotropic Zircaloy-2 with rod texture have been measured in situ under uniaxial tension along the rod axis by neutron diffraction. An in-plane biaxial stress state was developed in the grains as deformation proceeded. The tensors describing the stress state of the grains throughout the deformation process have been determined experimentally in the natural crystallographic axes system. After 5% deformation, the stress tensor components of the grains, σ 11 and σ 33—stresses acting along the 〈 a〉-axes and 〈 c〉-axes lying in the transverse plane—are 230±20 MPa and −241±21 MPa, respectively. The angular dependence of the initial thermal residual stress state and the final deformed state have also been obtained. The elasto-plastic self-consistent model has been used to simulate the experiments and exhibits agreement with experiment. It appears that the introduction of basal slip {0001}〈1 2 ̄ 10〉 considerably improves the agreement with experiment.