Residual Stresses In Order Research Articles

Mechanical Properties of Tensile Cracking in Indium Tin Oxide Films on Polycarbonate Substrates

The electro-mechanical behaviors of transparent conductive oxide film on polymer substrate are of great concern because they would greatly affect the stability and lifespan of the corresponding devices. In this paper, indium tin oxide (ITO) films with different thicknesses were deposited on a polycarbonate (PC) sheet; meanwhile, in situ electrical resistance, in situ scanning electron microscopy and profilometry were employed to record the electrical resistance, morphologies and residual stress in order to investigate the fracture behavior and electrical-mechanical properties of ITO films under uniaxial tension loading. The electrical resistance changes, crack initiation, crack propagation and crack density evolution of ITO films were systematically characterized by in situ tests. Three fracture stages of ITO films were summarized: Ⅰ crack initiation, Ⅱ crack propagation, Ⅲ crack saturation and delamination. The crack initiation and electrical failure in a thinner ITO film occurred at relatively higher applied tensile strain; namely, the ductility of the film decreased as the film thickness increased. Residual compressive stress was recorded in the ITO films deposited on PC at room temperature and increased as the film thickness increased. Intrinsic crack initiation strain (CIS*) showed an opposite thickness dependence to residual strain (εr); the increase in residual compressive strain was counteracted by the decrease of intrinsic cohesion, leading to an overall decrease in effective crack initiation strain (CIS) when the film thickness increased. In addition, integrated with a formulated mechanics model and the analysis of the three fracture stages under tension, the fracture toughness and interfacial shear strength were quantitatively determined. As the film thickness increased (in the range of 50~500 nm), the fracture toughness decreased and the films were more prone to crack, whereas the interfacial shear strength increased and the films were less likely to delaminate.

Eksperimentalno određivanje zaostalih napona u konstrukciji metodom bušenja rupe

A very important segment, which occurs as a result of welding, is the occurrence of residual stresses in welded structures. The effects of residual stresses can be beneficial or harmful to the structure, depending on their size, sign, and stress distribution in relation to the stresses caused by the external load itself. Therefore, the examination of residual stresses is very important because they significantly affect the formation and growth of cracks, the appearance of brittle fractures, material fatigue, etc. The paper presents the test procedure for measuring residual stresses by the method of "hole drilling" and the possibility of practical application of knowledge about residual stresses in order to improve the quality of machine parts and the structure itself.

Effects of shot peening with different coverage on surface integrity and fatigue crack growth properties of 7B50-T7751 aluminum alloy

Engineering design methods based on the assumption of crack growth and the application of fracture mechanics principles are commonly used in aerospace engineering for the design of aluminum structures. In aerospace manufacturing, surface shot peening is one of the most widely used techniques for improving the fatigue performance of structural components. This work reviews the effect of different shot peening coverage on surface integrity and fatigue crack growth (FCG) properties of 7B50-T7751 aluminum alloy. The shot peening was applied to middle-crack tension (MT) specimens with four kinds of peening coverage: 100%, 300%, 600% and 1000%, respectively. The results showed that shot peening treatment reduced specimen surface roughness, moreover, surface shot peening promoted a maximum microhardness increased by 28.1% and induced compressive residual stresses in order of −412 MPa compared to untreated specimens. FCG experiment results showed that the effect of shot peening coverage on da/dN-ΔK curves was quite limited at constant amplitude loading. With the increase of shot peening coverage from 100% to 1000%, the FCG rate decreases first and then increase. At 300% of surface shot peening coverage, the FCG rate is the lowest, which results in the longest fatigue crack growth life.

Cyber-physical approach toward semiautonomous postprocessing of additive manufactured parts and components

Additive manufacturing (AM), often referred to as 3D printing, is a generic term describing the layered build-up of material in near net shape frequently attributed with a freedom of design that cannot be achieved otherwise. AM focuses basically on the fabrication of parts for different fields in complex high-tech applications. Examples include components for jet engines, turbines blades, and implants in the medical sector. This is often justified with tool cost savings, shorter lead-time, and overcoming the “design for manufacture” paradigm. On the other hand, a machining allowance is frequently required to counteract the inherent surface roughness and the widespread challenge of part distortion due to residual stresses. At this point, geometrical complexity and small batch sizes transform into strong cost drivers compared to conventional subtractive processing. In fact, these parts are simply hard-to-clamp and hard-to-probe. Moreover, iterative processing is frequently required due to remaining residual stresses in order to reach the target geometry; even the part envelope changes unintentionally. The current paper explores the novel approach of semiautonomous postprocessing of AM parts and components based on flexible clamping, geometry acquisition in the as-clamped position using cooperating laser profile sensors, and an adaptive milling path planning strategy to counteract unforeseen change of the part envelope.

Influence of SLM-process parameters on the formation of the boundaries of parts of heat-resistant nickel alloy Inconel 718

We consider the improvement is considered of the modes of selective laser melting technology based on the design model to reduce the level of residual stresses and prevent deviations in the geometry of the part. Simulation results are presented on a universal voxel structure and a simplified object to predict metal behavior depending on the specific energy density in the region of the boundaries of a metal part made of Inconel 718. An experiment was carried out to study the influence of different strategies and process modes on the curvature of parts as a result of the effect of residual stresses in order to minimize them. Printing was carried out on a 3-D printer "Alfa-150" (LLC "ALT Ukraine") at constant power (P, W) and distance between tracks (d, mm) in each zone (up-skin, down-skin, in - skin) with a change in the speed (V, mm / s) of the laser beam movement, as well as a different pattern of sample growth by 3-D printing with 67 degrees rotation of each new layer relative to the previous one. To identify defects and deviations from the original model to the solid (sample), metallographic analysis was performed using optical microscopy (Carl Zeiss AXIOVERT 200M). It was found that the simulation of printing processes, performed on the Magics platform by breaking the model into a voxel structure, allows an analytical assessment of stresses and strains. Analysis of the appearance of the prototypes showed that the best down-skin indicators are formed at a power of 80 W and a specific energy density (40 ... 38 J / mm3). By using the 67 degrees staggered printing strategy at the optimum specific energy density, it is possible to minimize the residual internal stresses leading to distortion of the product. In the future, the results can be supplemented by studies of the effect of residual stresses of compressive forces when exposed to a laser beam at constant applied power. Using a computational model that allows calculating the residual stresses during the deposition of the next layer, depending on the speed of the laser, the power and the distance between the applied tracks, it is possible to obtain high-precision parts with specified properties. The adaptation of the model, which allows us to obtain a quantitative estimate of the residual thermal stresses depending on the speed of movement and the laser power for the Inconel 718 heat-resistant alloy, has been carried out. Optimal modes have been determined to minimize these stresses and reduce the curvature of the part.

Multi-Frequency Resonance Behaviour of a Si FractalNEMS Resonator.

Novel Si-based nanosize mechanical resonator has been top-down fabricated. The shape of the resonating body has been numerically derived and consists of seven star-polygons that form a fractal structure. The actual resonator is defined by focused ion-beam implantation on a SOI wafer where its 18 vertices are clamped to nanopillars. The structure is suspended over a 10 μm trench and has width of 12 μm. Its thickness of 0.040 μm is defined by the fabrication process and prescribes Young’s modulus of 76 GPa which is significantly lower than the value of the bulk material. The resonator is excited by the bottom Si-layer and the interferometric characterisation confirms broadband frequency response with quality factors of over 800 for several peaks between 2 MHz and 16 MHz. COMSOL FEM software has been used to vary material properties and residual stress in order to fit the eigenfrequencies of the model with the resonance peaks detected experimentally. Further use of the model shows how the symmetry of the device affects the frequency spectrum. Also, by using the FEM model, the possibility for an electrical read out of the device was tested. The experimental measurements and simulations proved that the device can resonate at many different excitation frequencies allowing multiple operational bands. The size, and the power needed for actuation are comparable with the ones of single beam resonator while the fractal structure allows much larger area for functionalisation.

A strategy to improve tool life by controlling cohesive failure in thick TiAlN coating during turning of CGI

Beside machining condition and parameters, coating properties is one of the main factors affecting tool performance and wear pattern. These properties include coating hardness and elastic modulus, adhesion to substrate, roughness, defects, and most importantly residual stress. In general, the properties of a physical vapor deposition (PVD) coating can be controlled by adjusting deposition parameters such as substrate bias voltage and nitrogen pressure. Therefore, a coating can be tailored to withstand a specific wear mode which is dominant in machining the workpiece material. In this work a strategy is suggested to decrease adhesion wear and formation of built-up edge during machining of compacted graphite iron (CGI). This strategy involves deposition of thick TiAlN coating with low residual stress in order to promote cohesive failure within the coating. For this purpose, monolayer Ti40Al60N PVD coatings with high thickness (5–10 μm) were deposited under various deposition parameters. A comprehensive coating characterization was done using scanning electron microscope (SEM), X-ray diffraction (XRD), nanoindentation, and scratch test. Tool life of selected coatings were then tested under dry finishing of compacted graphite iron, and wear pattern was investigated with optical and scanning electron microscope. The results showed that coating under low compressive residual stress is successful in reducing sticking and built-up edge formation. This phenomenon happens with gradual cohesive failure of the coating under adhesion wear, resulting in a higher tool life under adhesive wear.

The effect of laser stitch welding residual stress on the dynamic behaviour of thin steel structure

Laser stitch welding is one of the most reliable and efficient permanent metal joining processes in the automotive industry, particularly in the manufacturing of a car body-in-white (BIW). It is widely known that this welding process induces the generation of residual stresses that can influence the dynamic behaviours of welded structures. In order to accurately predict the dynamic behaviours of these welded structures, it is important to experimentally understand the influence of residual stress. Therefore, this study addresses the finite element modelling method of thin steel welded structures with and without the influences of residual stress in order to identify its effect towards dynamic behaviours. The finite element models of thin steel welded structures are developed by employing the area contact model (ACM2) format element connector. The accuracy of the finite element models is then compared in terms of natural frequencies and mode shapes with the experimental counterparts. The dynamic behaviours of the measured structure are obtained using an impact hammer with free-free boundary conditions. The results demonstrate the importance of considering the influence of laser stitch welding residual stress in predicting the dynamic behaviours of thin steel welded structure.
 

Influence of processing parameters on the fatigue life time of specimens made from quenched and tempered steel SAE 4140H

Abstract The demands placed on highly stressed components are constantly increasing. For this reason, components with a higher degree of efficiency are required for the competitiveness of metallic components. Therefore, a comprehensive understanding of the fatigue behavior of metallic materials and components as well as the development of new methods for the reliable calculation of service life are of increasing importance. The fatigue properties of components are multi-parameter sets and are strongly influenced by surface integrity, as surface and near-surface zones are the most stressed regions of a component. Surface integrity is influenced by each individual manufacturing process, its parameters, the process order and the process combination. Due to their high complexity, the influence-cause-effect relationships of process chains have not yet been sufficiently understood. In order to analyze the interactions between manufacturing process chains, their design, variations in material properties, in particular the microstructure and the resulting component properties, specimens were processed with specifically different machining parameters with and without polished surface. The specimens were characterized by different analyzing methods, such as X-ray for residual stress measurements and phase analysis as well as surface characterizations through confocal microscopy. The results from these analytics are used as an input for a semi-empirical model and finite element analysis providing effective stress amplitudes which include factors such as stress concentration and residual stress in order to predict their influence on fatigue life. This additional material information can be used in currently existing short-time procedures or for conventional determined SN curves to expand them to a multi-parameter dataset. The investigations presented in the scope of this paper were performed on specimens made from quenched and tempered steel SAE 4140H (42CrMoS4) machined with various processing parameters.

Inconel 718 Superalloy Controlled Surface Integrity for Fatigue Applications Produced by Precision Turning

An original approach is introduced to discriminate the effects of residual stress and surface roughness on turned Inconel 718 alloy parts. This could be used for assessing or studying the performance of these specimens during fatigue tests. This method consists of building surface integrity maps in terms of surface roughness parameters and residual stress in order to select specific turning conditions with adequate machining parameters. The surface integrity maps were generated by using response surface methodology based empirical models and precision turning tests on coupons. The proposed approach was therefore successfully applied to manufacture rotating bending fatigue specimens with the surface axial residual stress varying from − 239 to 446 MPa while keeping the arithmetic average roughness and the surface hoop residual stress constant. The effect of the surface axial residual stress on the fatigue life could thus be discriminated from the other surface integrity characteristics effects. The proposed methodology is independent of the manufacturing process and can be extended to other machining processes such as milling, grinding, and broaching.

Shot peening effects on residual stresses redistribution of offshore wind monopile multi-pass weldments

In some industrial applications, welding is the only alternative to join different parts. However, the major problem in welded structures is the tensile residual stresses that are inevitably produced during welding process. Surface tensile stresses can threaten the performance of the weldments as they act as an accelerant in fatigue crack initiation and failure. Shot peening is a well-known method which can enhance weldments' fatigue performance and longevity by inducing surface compressive residual stresses in order to eliminate or limit tensile residual stresses.In this study, the effect of shot peening on redistribution of multi-pass welding residual stresses was numerically and experimentally investigated on very large components typical of welded joints used in offshore wind turbine monopiles. In experimental part of the study, the residual stresses were measured using the Incremental Centre Hole Drilling (ICHD) method before and after shot peening. Finite element studies were carried out using 3D welding models and random shot peening analyses. Moreover, extensive finite element analyses were conducted to study the effect of model dimensions and the number of passes on prediction of welding residual stresses. Interesting set of results obtained from both the numerical studies and the ICHD measurements were in good agreements and showed that shot peening can be advantageous even for large components with multi-pass welded joints. Additionally, reducing the number of weld passes in finite element models could considerably lower the computational time without affecting the accuracy of results at surface regions of the models.

Prediction of residual stress in precision milling of AISI H13 steel

Surface integrity describes the attributes of a surface and it influences the functional performance of a work piece significantly. Residual stress is one of the major characterization parameters of surface integrity. Non-favorable residual stresses on a machined surface can reduce the fatigue life and performance of the machined part. It therefore requires a prediction model for residual stress in order to establish machining strategy to obtain favorable residual stress for prolonged fatigue life. Hardened tool steels have been widely used to make molds and dies by precision milling in aerospace and automotive industries. Knowledge of the relationship between residual stress on the machined molds and machining conditions is very important for process control. In this work, a prediction model for residual stress was developed by using a model-based approach on an Artificial Neural Network. This model is expected to predict the residual stress based on cutting parameters such as cutting speed, feed rate, depth of cut and tool lead angle. Several precision milling trials were carried out using a central composite design method. The networks have been trained and validated by experimental results. The performance of a feed forward neural network model with backpropagation was assessed and compared with a radial basis function network model by criterion of least mean squared error. Furthermore, the neural network prediction model was supported by the finite element simulation of the milling process to understand the formation mechanism of the residual stress in the machined surface. It was found, that the predicted values by the neural network model matched well with the experimental results. The radial basis function network showed better results than the feed forward network and was therefore chosen to take forward in the analysis. The feed rate was in this case the most influential factor, because it contributes significantly to heat and deformation on the work piece. The model could be used to optimize machining processes to obtain machining strategy for generating favorable residual stress and increasing fatigue life performance of the machined parts.

Evaluation of Residual Stresses in Butt Welded Joint of Dissimilar Material by FEM

The study proposed within this paper deals with an application of finite element techniques to the thermo-structural analysis of a dissimilar butt-welded joint. Residual stresses induced by the fusion arc-welding of steel joints in power generation plants are a concern to the industry. Nowadays, the application of finite element method appears to be a very efficient method for the prediction and the investigation of the weld-induced residual stresses, nevertheless the detailed modelling of all phenomena involved in such process is still challenging. The structural integrity assessment of welded structures strongly requires a deep investigation of weld-induced residual stresses in order to be compliant with safety requirement of power plant. The longitudinal and transversal residual stresses in dissimilar material butt joints of 8 mm thick for V-groove shape were studied. The developed thermo-mechanical FE model as well as the simulation procedures are detailed and results are discussed. As a result of such work, it has been found out that residual stresses in the two dissimilar plates are characterized by very different magnitudes and distribution.

A modification to the cross slitting method for measuring two components of residual stresses

Residual stresses measurement techniques are constantly modified and progressed. There have been many residual stresses measurement methods developed and practiced by researchers. Slitting method is one of the destructive techniques that can measure residual stresses deep within the components. However, the conventional slitting method can only measure the component of residual stress normal to the cut plane. Furthermore, the cross slitting has been developed to measure two components of residual stresses simultaneously. In the current research, a modification to the cross slitting method is proposed to enhance the accuracy of the technique. The samples were prepared exactly similar to previous study and were quenched at different temperatures to create different levels of residual stresses in order to compare the outcomes. The proposed modification was examined both numerically and experimentally. The results confirmed the success of the modification in reducing the errors. Results from experiments and predictions agreed well.

Robust methodology to simulate real shot peening process using discrete-continuum coupling method

Shot peening is widely used in automotive and aeronautic industries to improve fatigue life of metallic components. Its beneficial effects are mainly due to the residual stress field caused by the plastic deformation of the near-surface region resulting from multiple shot impacts. It is therefore important to know the values of the induced residual stresses in order to predict the mechanical strength of the peened component, and to know how these stresses vary by changing the shot peening parameters. The problem is that experimental measurement of residual stress is costly and time-consuming, and generally involves semi-destructive techniques. These difficulties make assessment of compressive residual stresses in real (industrial) peened components very challenging. On the contrary, numerical simulation can provide an alternative way to deal with this task. Consequently, several shot peening models have been developed in the literature. Although these models were successfully applied to investigate important physical phenomena encountered in shot peening, their application to assess residual stresses resulting from a real shot peening test is still not within reach. Indeed, due to computation costs and the complexity of the process, they cannot be directly applied to simulate a complete shot peening experiment. Development of a robust methodology allowing these models to properly simulate such an experiment at minimal cost (i.e. using simplifying assumptions) is thus needed. The present paper aims to meet this need. First, a new discrete-continuum coupling model combining the strengths of the existing shot peening models was developed. To avoid expensive computation times, only major shot peening features are included in this model. Then, a comprehensive methodology explaining how this model can be applied to simulate a real shot peening experiment was proposed. To validate the developed model as well as the associated methodology, they were applied to simulate a real shot peening experiment from the literature. Relatively good results were obtained compared to experimental ones, with relatively little computation effort.

Fatigue Crack Propagation in Shot Peened al 7475-t7351 Alloy Specimens

The approach to engineering design based on the flaws propagation assumption applying the principles of fracture mechanics is commonly used in aluminum structures for aerospace engineering, in which surface shot peening is an attractive method of improving fatigue performance, because it promotes the retardation of the crack initiation and earlier crack growth. The main purpose of present work was to analyze the effect of the surface shot peening on the fatigue crack propagation of the 7475 aluminum alloy with a T7351 heat treatment.Two types of fatigue tests were performed: constant amplitude and variable amplitude loading in which periodic overload blocks of 300 cycles are applied with intervals of Nint cycles. Surface micro shot peened promoted an increasing in micro-hardness only in order or 6% and created negative surface residual stresses in order of -174MPa, which compare with the positive residual stresses of +291MPa on the machined specimens.For tests at constant amplitude loading the effect of surface peening on da/dN-ΔK curves is quite limited, particularly for R = 0.4. However, this beneficial effect increases significantly near the threshold. Repeated overload block reduces significantly the fatigue crack propagation rate, being this effect particularly dependent of the intervals between the blocks. The maximum reduction of crack propagation rate and retardation effects were obtained for Nint = 7500 cycles.

Computation of Residual Stress Distributions and Opening Angles of 3D Patient‐Specific Arterial Walls

AbstractA method to incorporate residual stresses in arteries, based on the assumption of smooth transmural stress measure distributions with slight slopes, is discussed. The artery is first loaded with the internal blood pressure without considering any residual stresses. With help of suitable stress invariants and volume averaged mean values on specific sectors domains, the stress gradient is iteratively decreased in radial direction. In order to assess the accuracy of the method, a three‐dimensional patient‐specific arterial geometry, suffering from atherosclerosis, is considered. These was reconstructed from ultrasound based medical imaging, see [2]. Moreover, the radially sliced arteries can be loaded exclusively with the calculated residual stresses in order to predict the opening angle. (© 2015 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Influence of residual stresses on the fatigue crack growth from surface anomalies in a nickel-based superalloy

The present study evaluates the influence of initial residual stresses surrounding scratch or dent type surface anomalies on the fatigue crack growth of Direct Aged (DA) Inconel 718 alloy. A heat treatment has been used to at least partly relax these residual stresses in order to evaluate their effect on the fatigue crack initiation and propagation starting from scratches and dents. In the presence of compressive residual stresses under the surface anomaly, the first stages of crack propagation are slowed down. The evolution of the crack front shape is also strongly influenced by the residual stress field and leads to the formation of an “ear-shaped” crack front for dents and initiation in the corners for scratches. The initiation fatigue life is short for both type of surface anomalies and a competition between the stress concentration and the initial microstructural state was noted for the control of this stage of the fatigue life. These results highlight the need to characterize the residual stress field resulting from the introduction of the surface anomaly in order to predict accurately the fatigue life of DA Inconel 718 alloy components containing scratches and dents. A preliminary finite element simulation is developed in this study to address this issue.

The Factors Impacting the Surface Quality of Machining and Modifying Measures

In this paper, the machining surface quality is analyzed. The factors impacting the surface quality of machining are pointed out such as wear resistance, fatigue strength, corrosion resistance, cooperate with quality etc. Measures to improving the machining surface quality are put forward including cutting tool, work piece material, cutting condition, processing method; reduce the machining deformation of surface strengthening, and residual stress in order to apply the rule to control the machining process, finally achieve the purpose of improving surface quality and product performance. The cold work hardening of the surface layer is analyzed. It will be certain guidance for engineering practice.

Intragranular Residual Stress Evaluation Using the Semi-Destructive FIB-DIC Ring-Core Drilling Method

Titanium aluminide (TiAl) is a lightweight intermetallic compound with a range of exceptional mid-to-high temperature mechanical properties. These characteristics have the potential to deliver significant weight savings in aero engine components. However, the relatively low ductility of TiAl requires improved understanding of the relationship between manufacturing processes and residual stresses in order to expand the use of such components in service. Previous studies have suggested that stress determination at high spatial resolution is necessary to achieve better insight. The present paper reports progress beyond the current state-of-the-art towards the identification of the near-surface intragranular residual stress state in cast and ground TiAl at a resolution better than 5μm. The semi-destructive ring-core drilling method using Focused Ion Beam (FIB) and Digital Image Correlation (DIC) was used for in-plane residual stress estimation in ten grains at the sample surface. The nature of the locally observed strain reliefs suggests that tensile residual stresses may have been induced in some grains by the unidirectional grinding process applied to the surface.