Accumulation Of Residual Stresses Research Articles

Effect of hygral swelling and shrinkage on mechanical durability of fuel cell membranes

Membrane mechanical durability is one of the primary factors limiting the lifetime of ionomer membranes in polymer electrolyte fuel cells. Due to the importance of membrane dimensional stability during relative humidity (RH) cycles on the magnitude of mechanical stress, the present work contributes to an in-depth understanding of the hygral swelling and shrinkage of state-of-the-art membranes and catalyst coated membranes (CCMs). Through conducting ex-situ hydration and dehydration tests, the swelling and shrinkage characteristics for each membrane were determined. An overall, progressive shrinkage in the planar dimension was observed in certain membranes suggesting the creation of residual tensile stress during in-situ operation, hence diminishing the overall membrane mechanical durability. The accumulation of residual stress during successive RH cycles was therefore measured explicitly using a custom-designed constrained shrinkage test and compared to the tensile properties of each individual membrane. Finally, the hygral swelling and shrinkage behavior of each membrane was compared to the corresponding CCM and the reinforcement offered by the catalyst layers was determined. The present findings indicate critical impact of the membrane hygral swelling and shrinkage on the in-situ mechanical stresses and the associated membrane mechanical durability.

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 utilizing topology optimization to design support structure to prevent residual stress induced build failure in laser powder bed metal additive manufacturing

Metal additive manufacturing (AM) as an emerging manufacturing technique has been gradually accepted to manufacture end-use components. However, one of the most critical issues preventing its broad applications is build failure resulting from residual stress accumulation in manufacturing process. The goal of this work is to investigate the feasibility of using topology optimization to design support structure to mitigate residual stress induced build failure. To make topology optimization computationally tractable, the inherent strain method is employed to perform fast prediction of residual stress in an AM build. Graded lattice structure optimization is utilized to design the support structure due to the open-celled and self-supporting nature of periodic lattice structure. The objective for the optimization is to minimize the mass of sacrificial support structure under stress constraint. By limiting the maximum stress under the yield strength, cracking resulting from residual stress can be prevented. To show the feasibility of the proposed method, the support structure of a double-cantilever beam and a hip implant is designed, respectively. The support structure after optimization can achieve a weight reduction of approximately 60%. The components with optimized support structures no longer suffer from stress-induced cracking after the designs are realized by AM, which proves the effectiveness of the proposed method.

Prediction Model for Weld Hydrogen Cracking in High Strength Steel Weld

Prediction model for weld hydrogen cracking (so called cold cracking) in high strength steel weld was developed by a coupled thermo-elastic-plastic and hydrogen diffusion analysis in the y-grooved weld joint. Critical conditions of cracking was given as the function of principal stress and accumulated hydrogen concentration in the root region where the cracking occurs. In order to clarify the critical conditions of cold cracking, y-grooved cold cracking tests were first conducted using the steel plate with tensile strength level of 780MPa. Plate thickness of the plates were 25 mm and 50 mm. Hydrogen concentration in the weld metal was changed by applying hydrogen into the shield gas in the gas-metal arc welding (GMAW), and different pre-heating conditions were applied to evaluate the critical pre-heating temperature to prevent cold cracking. It was found that the critical pre-heating temperature was increased with thicker plate thickness and higher hydrogen concentration in the weld metal. The cracks were initiated at the weld metal in the root region which exhibits significant tensile residual stress by the welding. Hydrogen accumulation into the root region may have enhanced cracking. Based on the above y-grooved weld cold cracking tests, a simulation model for estimating the critical pre-heating temperature was developed. The y-grooved weld joints with different plate thickness were modeled by the FEM with the actual weld configuration and material properties in the base metal, heat affected zone (HAZ) and weld metal. Hydrogen was put in the weld metal as an initial condition, then welding simulation was conducted by coupled thermo-elastic-plastic and hydrogen diffusion analysis. In the hydrogen diffusion analysis, the α-multiplication method, which stress gradient term in the diffusion law was multiplied, was applied to express realistic hydrogen diffusion enhanced by hydrostatic stress field. Hydrogen accumulation occurred in the root region where showed highest hydrostatic stress. The point where showed the hydrogen accumulation was well corresponded to the crack initiation site. It was found that cracking was enhanced with thicker plate and higher initial hydrogen concentration because of higher tensile residual stress and higher hydrogen accumulation in the root region, respectively. These models were implemented into the Materials Integration (MI) System to estimate the critical pre-heating temperature in the y-grooved weld tests with different welding conditions. Although, further validation and optimization are needed for the models, this hydrogen cracking prediction system has a lot of potential as a material development support tool for reducing experimentation and speeding up the development.

3D laser shock peening as a way to improve geometrical accuracy in selective laser melting

One of the major drawbacks of selective laser melting (SLM) is the accumulation of tensile residual stresses (TRS) in the surface and subsurface zones of produced parts which can lead to cracking, delamination, geometrical distortions, and a decrease in fatigue life. 3D laser shock peening (3D LSP) is a novel hybrid method which introduces a repetitive LSP treatment during the manufacturing phase of the SLM process. In this paper, the ability of 3D LSP to convert TRS into beneficial compressive residual stresses and their subsequent effect on the geometrical accuracy of produced parts were investigated. Samples made of Ti6Al4V were manufactured with the 3D LSP process and treated with different processing parameters. Cuboidal samples were used for residual stress measurements, and the evolution of residual stresses was evaluated. Geometrical distortions were measured on bridge-like samples, and the influence on the final sample geometry was quantified. A significant improvement in geometrical accuracy resulting from reduced distortions was observed in all selected 3D LSP processing conditions.

Experimental and numerical investigations of the behaviour of footing on geosynthetic reinforced fill slope under cyclic loading

This paper investigates the cyclic loading responses of a strip footing supported by a geosynthetic reinforced fill embankment. A series of large-scale model footing tests were conducted first to investigate the accumulation of permanent footing displacement and residual vertical soil stress over large number of load cycles. The embankment fill was a heavily compacted silty sand and the reinforcement was a flexible geogrid, so that the model test configurations were representative of actual field conditions. Both permanent displacement and residual stress accumulated asymptotically with load cycles and majority of the build-up occurred over the first few hundred cycles. The potential effect of load interruptions was part of the study. Depending on how cyclic load interruption was implemented, it may or may not induce a trailing effect on subsequent cyclic loading responses. To have more in-depth understanding, these footing tests were also investigated numerically based on a soil model that can capture the unload-reload stress-strain loop over large number of load cycles. Reasonably good agreement between experimental observations and numerical predictions was also achieved.

Influence of coating thickness on residual stress and adhesion-strength of cold-sprayed Inconel 718 coatings

In the cold spray process, deposition of particles takes place through intensive plastic deformation upon impact in a solid state at the temperatures well below their melting point. The high particle impact velocity causes high local stresses which lead to deforming the particles and the substrate plastically in the proximity of the particle–substrate interface. As a result, high residual stresses are introduced in cold spray coatings due to the peening effect of the particles collisions with the substrate. In this study, a powder based on the chemical composition of IN 718 was cold-sprayed on IN 718 substrates by using nitrogen gas for an application as a repair tool for aero engine components. The magnitude of the residual stress and its distribution through the thickness were measured by using the hole-drilling and the bending methods. Residual stress was also estimated by using an approach based on the physical process parameters. Mainly compressive residual stresses were observed in cold-sprayed IN 718 coatings. Accumulation of residual stresses in the coatings is highly affected by peening during deposition and it decreases with increase in thickness. It has been observed that the adhesion-strengths of cold-sprayed IN 718 coatings are highly influenced by coating thickness and residual stress states of the coating/substrate system. In the presence of residual stresses in the coatings, adhesion-strength decreases with increasing coating thickness. The energy-release-rate criterion has been used to predict adhesion-strength with increasing coating thickness. Predicted bond-strength values are close to the measured adhesion-strength values and decrease with increase in coating thickness.

Modeling and Optimization of Layer-by-Layer Structures

In the present paper a differential-geometric approach is developed to modeling of residual stresses in layered (LbL) structures obtained as a result of successive curing of thin layers of material. The objects of modeling are the structures obtained by sequential adsorption of a large number of thin layers. During this assembling, the internal (residual) stresses appear in the multilayered structure while local deformations turn out to be incompatible. This leads to accumulation of residual stresses and distortion of the final shape of the LbL structure. To reduce these factors, geometric compensation is used, the calculation of which is extremely laborious with a large number of layers. Geometric methods allow us to implement fast algorithms for obtaining the compensation. They are based on the “smoothing” of the multilayer LbL structure and its representation by introducing a smooth body-manifold with non-Euclidean connection that characterizes the incompatibility of deformations. Connection is determined from the solution of the evolutionary problem, which formalizes the course of the technological process. The classical fields of the mechanics of a continuous medium are put in correspondence with their non-Euclidean counterparts. As an example, model problems for cylindrical LbL elastic structures are considered. A model problem of structural optimization is solved to determine the optimal strategy of the technological process.

EVALUACIÓN MEDIANTE NANOINDENTACIÓN INSTRUMENTA DE LA ALTERACIÓN DEL MÓDULO DE YOUNG CAUSADA POR LA INDUCCIÓN DE CAMPOS DE TENSIONES RESIDUALES

This work evaluates the alteration of Young’s modulus caused by the induction of residual stress fields in a set of SAE 1045 Steel specimens, in which the measurements were carried with instrumented nanoindentation. First, the specimens were subjected to a stress relief to eliminate prior history. Previous residual stresses are accumulated with the ones generated during manufacturing processes, this accumulation of residual stresses may produce sudden failures. After the stress relief process, the specimens were subjected to the induction of residual stress by applying a controlled bending load and the magnitudes of the stresses were measured with the Crack Compliance Method (CCM). Finally, measurements of the Young modulus alterations were performed with instrumented nanoindentation by using a Berkovich nanoindenter and following the Oliver-Parr method. A differential segment was extracted from the centre of the beam which contains the same stress field distribution as it was also demonstrated with a numerical analysis carried in ANSYS®. A total of four cases were analysed considering: a) Non previous history, b) with the induction of residual stress, c) with the application of the Bauschinger effect and d) with the induction of residual stresses plus the application of the Bauschinger effect. Differences between Young´s modulus and surface hardness values were found for each case showing the effect of the induced residual stress in the specimens.

A quantitative investigation of distortion of polylactic acid/PLA) part in FDM from the point of interface residual stress

In order to reveal the distortion mechanism of PLA part, the bilinear elastic-softened cohesive model is adopted and the micromechanics is utilized for the first time to describe the interface state. The residual stress models of the elastic interface state, elastic-softened state, and the elastic-softened-de-bonding interface state are established quantitatively according to the cohesive zone model to investigate the relationship between building parameters and residual stress, respectively. An experimental is proposed to validate the theoretical residual models. The experimental results indicate that the layer thickness plays the dominant role in determining the distortion compared with deposition velocity. The distortion of PLA part is mainly caused by accumulation of residual stresses resulting from non-uniform temperature gradients in continuous heating and cooling cycles. The temperature of the filament with different building parameters in the fabrication process is collected to observe the relationship between part distortion and temperature and the temperature files show a good accordance with the distortion trend. The qualitative analyzed results of the residual stress models also coincide with the experimental results well, which indicates the effectiveness of the residual stress model and shows that it can express the distortion mechanism well.

Analytical and Numerical Temperature Prediction in Direct Metal Deposition of Ti6Al4V

Direct Metal Deposition (DMD) is an additive manufacturing (AM) process capable of producing large components using a layer by layer deposition of molten powder. DMD is increasingly investigated due to its higher deposition rate and the possibility to produce large structural components specifically for the aerospace industry. During fabrication, a complex thermal history is experienced in different regions of the workpiece, depending on the process parameters and part geometry. The thermal history induces residual stress accumulation in the buildup, which is the main cause of distortions. In order to control the process and enhance the product quality, the understanding of the workpiece temperature is substantial. In this study, two methods to predict temperature evolution during the DMD process are introduced based on analytical and finite element methods. The objective is to compare these methods to experimental results and to provide more insights about their capabilities to predict accurately the temperature gradient, the cooling rate, and the melt pool geometry. A comparison of the computational time is also provided. Based on the results of the investigation, It appears that the analytical method provides an effective and accurate method to understand the influence of the process on the workpiece temperature.

Analysis of the current state of additive welding technologies for manufacturing volume metallic products (review)

The current state of 3D technologies for manufacturing of volumetric metal products is analyzed. It is shown that the main advantages of additive welding technologies for obtaining three-dimensional metal structures of complex shape in comparison with SLM - technologies are: – increase of the process productivity by 1–2 orders with the same power consumption; 3–10 times reduction in the cost of equipment; – the possibility of increasing the overall dimensions of the created parts by a factor of 10 or more; – expansion of the range of used consumables (powders, wires, composite materials); – increase in the utilization factor of consumables by 20–50 %. The main drawbacks of additive welding technologies for the production of three-dimensional metal structures are quite large dimensions of the heat-affected zone and the build-up layer. This leads to the emergence of undesirable temperature gradients, the accumulation of residual stresses and, as a result, a decrease in performance. One of the methods for eliminating these drawbacks is to increase the thermal locality of the energy source. For example, the use of non-transferred arc plasma. The analysis of additive plasma-arc welding technologies and own research has shown that their advantages are: – high (5...50 kg/h and more) performance; – the possibility of obtaining sufficiently thin (1.5...5.0 mm) walls with a relatively small overheating; – about 5-fold saving of materials in combination with the increase in the quality (for example, strength and density) of the resulting metal parts, in comparison with the traditional methods of mechanical manufacture.

Selection of container materials for modern planar sodium sulfur (NaS) energy storage cells towards higher thermo-mechanical stability

Sodium sulfur (NaS) cell is recognized as a promising candidate for advanced grid-scale large energy storage systems (ESS). In this work, we study the impacts of planar NaS cell container materials on the accumulation of residual stresses in the cell joints and solid electrolyte during the cell assembly and operation processes. Concentration of such thermo-mechanical stress in these vulnerable areas in the modern NaS cells can lead to catastrophic cell failures, which can present a huge challenge for developing large planar NaS cells towards commercial deployment. Here, we employ the finite-element analysis (FEA) computational technique to quantitatively assess the thermo-mechanical stress accumulation using prototype planar NaS cells. Relevant experimental procedures with corresponding thermal cycling conditions for the cell assembly, operation, and maintenance processes are incorporated into the FEA model. The influences of Al alloy (Al3003), stainless steels (STS304 and STS340), and iron-nickel-cobalt superalloy (KOVAR) on the residual stress accumulation are tested and thoroughly discussed. The computation results show that high stress concentration can be developed in the cell joint area. Through the comprehensive computational analysis, it is suggested that applying smaller CTE (less than ∼12×10−6K−1) alloys is necessary to secure the thermo-mechanical stability of NaS cells that can be implemented in a large scale ESS.

Modeling of Temperature Field Evolution During Multilayered Direct Laser Metal Deposition

It is of great importance to thoroughly explore the evolving temperature fields of direct laser metal deposition (abbreviated as LMD) in vertical thin wall manufacturing. It is helpful to control the temperature gradient, and even to adjust to forming microstructures and accumulation of residual stress. In this paper, a comprehensive three-dimensional transient model is developed for evolving temperature fields. The manufactured material is DS superalloy Rene80. The laser-powder interaction during the powder flowing process is simulated first, and its possible effect on the temperature field of the melting pool is analyzed. Then a 3D numerical simulation for the evolving temperature field is carried out based on considering transport phenomena during LMD such as the change in phase, powder injection and liquid flow. The applied deposition parameters are derived from experimental investigation with optimized vertical wall manufacturing. The simulated results explain why a balance between heat input and dissipation could form inside the vertical thin wall. These reconstruct the instability at an early phase of the building process without any temperature control unit and exhibit the influence of parameters such as laser power, deposition velocity and laser beam deposition pattern. The simulation results of temperature evolution are consistent with experimental investigation.

Технология снятия остаточных напряжений при многоцикловой обкатке колец подшипников

The substantiation of the new technology of multi-cycle racer running-in is shown. A technological method for racer resi-dual stresses removal by means of multi-cycle centerless running-in is described. For the computation of running-in basic time which is necessary for the complete removal of residual stresses there is developed a simulator. A simulator takes into account geometrical and mechanical conditions of the contact of rollers and a racer, physical-mechanical and geometrical parameters of a racer. On the basis of the simulator obtained there is carried out an investigation of the dependence of residual stress upon the duration of racer multi-cycle running-in for various stressed states. It is established that for the efficient stress removal the running-in time must be optimum. If time is lesser than optimum, then there is no a complete removal of residual stresses.
 At processing time longer of optimum one after a complete removal of residual stresses the accumulation of new residual stresses takes place.

Residual stress measurement in Fused Deposition Modelling parts

Fused Deposition Modelling (FDM) has become one of the most employed technologies to build complex 3D prototypes directly from a computerized solid model. In this process, the model is built as a layer-by-layer deposition of a feedstock wire. One of the most important issues in the FDM process is the distortion of the part during the printing. This issue is due to the rapid heating and cooling cycles of the feedstock material that could produce accumulation of residual stress during part building up. The aim of this work is to measure the residual stress in FDM parts made of ABS employing the hole-drilling method. In order to avoid the local reinforcement of the strain gage, an optical technique, i.e. ESPI (electronic speckle pattern interferometry), is employed to measure the displacement of the surface due to the stress relaxation. Furthermore, the effect of the stacking sequence and the residual stress distribution on each side of the specimen have been investigated.

Research and analysis of the stressed-strained state of metal corrugated structures of railroad tracks

We conducted research into, and analysis of, bearing capacity of metal corrugated structures (MCS), which are in service in the railroads of Ukraine. To assess the stressed-strained state of MCS, a calculation was carried out of equivalent forces that arise from the rolling stock when there is an irregularity in a railroad track, formed in the course of using a pipe. We ran analysis of the MCS bearing capacity at boundary load from the rolling stock of railroads by the indicator of influence of the type of corrugation (corrugattion dimensions) and characteristics of soils on their stressed-strained state. A calculation of equivalent forces was conducted by the method of calculation of a railroad track by strength and stability. A mathematical algorithm was programmed by the Peterson method to calculate the stressed-strained state of MCS. An analysis of multi-choice calculations of the MCS strength, which is made from the corrugated structure Multiplate MR150 with thickness of corrugated sheet 6 mm and dimensions of the corrugation waves 150 × 50 mm and 380 × 140 mm, demonstrated that its bearing capacity is provided. The degrees of compaction of the soil backfill are from 0.9 to 1.0. A metal corrugated structure that is made from corrugated sheets of size 200 × 55 mm, at the degree of compaction of the soil backfill 0.9, there occur stresses that exceed the permissible. Further accumulation of residual stresses from the action of dynamic wheel load, taking into account time parameter, may affect the occurrence of fluidity of material of a metal pipe. This may lead to the formation of its plastic irreversible deformations. Results of the MCS bearing capacity that we received are necessary for the optimal design of MCS, to establish the causes of occurrence of defects, to make timely relevant engineering decisions to increase bearing capacity of MCS and substantiate reasonable use of funds for the construction or renovation of existing transport facilities using metal corrugated pipes. These studies may be used by engineers at the bridge-testing stations of Ukrainian Railroads and Ukravtodor and by design organisations engaged in the design of metal corrugated structures of large diameters.

A Study on Optimal Design of Process Parameters in Tube Drawing Process of Rectangular Parts by Combining Box–Behnken Design of Experiment, Response Surface Methodology and Artificial Bee Colony Algorithm

Tube drawing process is comprised of many factors that have great impact on the formed part quality and die life. In producing polygonal sections from round tube, due to accumulation of residual stress in corners, the final shape of the drawn part is to some extent different from what is expected. Therefore, selection of optimal combination of drawing parameters for enhancing the parts quality and die life is crucial. In the present paper, a complete multi criteria optimization procedure starting from numerical modeling, and leading to the search of robust optimal process parameters is proposed. A numerical model of tube drawing process of aluminum alloy for rectangular section is developed by means of ABAQUS and validated experimentally. Then a total set of 27 numerical tests are conducted based on four-factors and three levels Box–Behnken design to correlate empirical relationship between factors (i.e., die angle, bearing length, friction coefficient and drawing velocity) and responses (i.e., drawing force, linear and peripheral thickness distributions and dimensional error). Response surface model of each quality characteristic is obtained and analysis of variances is performed to check the adequacy of developed models. Finally the obtained RSM models are integrated with their weight factors and the objective function is formed. This function is then associated with artificial bee colony (ABC) algorithm to find factor combination regarding minimum drawing force and dimensional error, as well as maximum thickness distributions. Results indicate that the optimal result obtained through RSM–ABC is in agreement with those derived from simulations and confirmatory experiments.

Evolution of grains to relieve additional compressive stress developed in Al–Mg alloy films during thermal annealing

In this work, a possible mechanism for grain evolution in Al–Mg alloy films during thermal annealing is suggested on the basis of the phase transition and the related residual stress. Al–Mg alloy films with compositions of 14.0 and 18.0wt.% Mg content were deposited on cold-rolled steel substrates by the direct current co-sputtering method using Al and Mg targets. After the deposition, the samples were thermally annealed at 400°C for 10min. The featureless, dense cross-sectional microstructure of the as-deposited films turned into a grainy microstructure after the thermal annealing. According to the residual stress evaluated by using the X-ray diffraction (XRD)-sin2ψ technique and the phase analysis by XRD, it is likely that grains were created in order to relieve the additional accumulation of residual stress originating from the phase transition from face-centered cubic Al (α) to Al3Mg2 (β) and Mg (δ) phases, suggesting an interplay between the microstructure and residual stress.

Dynamic mechanical response of polyetheretherketone (PEEK) exposed to cyclic loads in the high stress tensile regime

Tensile fatigue tests of PEEK at high load levels were carried out and analyzed regarding the dynamic mechanic behavior of the material during the tests. The experiments were conducted in the high stress tensile regime of PEEK. After a short period of unsteady temperature increase distinct material changes were observed during the tests at medium and high load levels. The storage modulus increased continuously while the loss modulus, loss factor (tanδ) and the dissipation energy rate decreased continuously. It was concluded that the accumulation of residual stresses because of partially irreversible deformation causes this effect.