Presence Of Residual Stresses Research Articles

Fatigue life estimation of the overloaded notched components

This paper presents an estimation of fatigue life for notched components in the presence of residual stress due to overloading. The proposed method is based on the stress distribution analysis around the notch root. Despite many prediction methods, which are emphasizing stress value to define the hot spot, this method is more concerned with mean stress distribution and local stress ratio as controlling parameters of the fatigue mechanism. Mean and amplitude of effective stress play a vital role in the proposed method. This feature helps to take into account the effect of high plastic strains that appeared around the notch root due to the stress raisers. Some experiments were carried out in which specimens with different notch geometries underwent a wide range of loading conditions. Notches were designed to cover an extended range of the stress concentration factors from 2.06 to 6.93. Eventually, the experimental and predicted fatigue lives were compared to evaluate the model reliability. A good agreement between the results was observed. The method was found to be appropriate and conservative in predicting the fatigue life of both U- and V-notched components with low- and high-stress concentration factors.

Effect of thermal residual stresses on ductile-to-brittle transition of a bi-material specimen by using the CAFE method

The effect of residual stress on fracture of materials or structures has been widely studied. However, its influence on ductile-to-brittle transition (DBT), a crucial phenomenon of structural materials, has rarely been investigated so far. In the present study, employing the eigenstrain method residual stresses are introduced into a bi-material specimen, where two configurations of crack and interface, e.g., one with interface perpendicular and one parallel to the crack extension, are designed to study the influence of residual stress. The DBT of the bi-material specimen in the presence of residual stresses is numerically studied by using the CAFE method where temperature dependent surface energy is implemented to calculate absorbed energy of Charpy impact testing specimen. It is found that residual stress generated in the two configurations affect the DBT in a similar manner. The DBT curves generally shift to higher temperature due to the decrease of absorbed energy with the increase of residual stress. It is found that the decrease of absorbed energy in both configurations is caused by the additional constraint on the notch root, which is induced by the residual stress and can facilitate the fracture.

Non-focusing dense plasma focus device based alternative synthesis technology for ZnO thin films

Dense plasma focus (DPF) device is conventionally operated in focus mode, to achieve pinch plasma with highest possible temperature and density to maximize the soft and hard x-rays and charged particles. In this paper, we report the first ever application of non-focus mode of DPF device, which is free of magneto-hydrodynamics (MHD) instabilities, for the deposition of zinc oxide thin films (ZnO TFs) on silicon substrates for various number (5, 10, 15 and 20) of non-focused deposition shots (NFDS). The X-ray diffraction (XRD) patterns of as-deposited ZnO TFs confirms the growth along (0 0 2) orientation only. The ZnO TFs are then annealed at 600 °C temperature for 2 h. The XRD patterns of annealed ZnO-TFs confirm the wurtzite phase of ZnO with (1 0 0), (0 0 2) and (1 0 0) planes with improved crystallinity. The up and down shifting of ZnO (0 0 2) diffraction plane indicates the presence of residual stresses which are reduced in annealed ZnO TF. The surface morphology, like shape, size and the distribution of rounded nano-particles, is strongly associated with increasing number of NFDS. Raman analysis shows the development of downshifted E2 (high) and upshifted A1 longitudinal optical (LO) modes centered at 430 cm−1 and 580 cm−1 compared to bulk ZnO (430 and 575 cm−1) indicating the presence of tensile residual stress due to mismatch of thermal expansion coefficient of ZnO TF and Si substrate and due to the presence of oxygen vacancies and Zn interstitials, respectively. The XPS analysis confirms the presence of Zn, Zn–O, C–O, and Zn–OH bonds. The energy band gap and refractive index of annealed ZnO-TF are found to be 3.30 eV and 1.88, respectively. A new method of high quality ZnO TFs synthesis using MHD instability free non-focusing mode of DPF device will open a new alternative synthesis technique.

Fatigue Life Prediction for Specimens with an Open Hole with a Pre-Compressed Boundary Via the Nominal Stresses Under Regular Cyclic Loading

The method of predicting the fatigue life of structural elements containing open holes with a preliminary compressed material in the hole vicinity under cyclic loading is proposed. The method is based on the commonly accepted approach of the life prediction via nominal stresses. The presence of residual stresses caused by plastic deformation of the material in the hole zone is taken into account by adjusting the cyclic load ratio in calculations. The concept of “fictitious compression stresses” is introduced for this purpose; its value is determined by solving the contact problem in an elastoplastic formulation in a finite element package. A functional relation between the value of residual stresses and the fatigue life of specimens with pre-compressed hole surface is established. The calculation results via the proposed method are compared with fatigue test results for specimens with pre-compressed hole surface. Satisfactory correlation between calculated and experimental data is obtained. The advantage of the proposed method is the reduction of required input data to construction of the fatigue curve for standard specimens with a stress-free hole. Considering that for the same alloy with identical heat treatment such curves may differ for different material batches, testing of standard specimens under regular loading is mandatory. Despite a usual scatter in fatigue life experimental values, the proposed method allows one to obtain a reliable and conservative assessment of the effect of hole boundary zone pre-compression on the fatigue life at the design stage.

Numerical Investigation of the Effect of Rolling on the Localized Stress and Strain Induction for Wire\u2009+\u2009Arc Additive Manufactured Structures

Cold rolling can be used in-process or post-process to improve microstructure, mechanical properties and residual stress in directed-energy-deposition techniques, such as the high deposition rate wire + arc additive manufacturing (WAAM) process. Finite element simulations of the rolling process are employed to investigate the effect of rolling parameters, in particular rolling load and roller profile radius on the residual stress field as well as plastic strain distribution for the profiled roller. The results show the response to rolling of commonly used structural metals in WAAM, i.e., AA2319, S335JR steel and Ti-6Al-4V, taking into account the presence of residual stresses. The rolling load leads to changes in the location and the maximum value of the compressive residual stresses, as well as the depth of the compressive residual stresses. However, the roller profile radius only changes the maximum value of these compressive residual stresses. Changing the rolling load influences the equivalent plastic strain close to the top surface of the wall as well as in deeper areas, whereas the influence of the roller profile radius is negligible. The plastic strain distribution is virtually unaffected by the initial residual stresses prior to rolling. Finally, design curves were generated from the simulations for different materials, suggesting ideal rolling load and roller profile combinations for microstructural improvement requiring certain plastic strains at a specific depth of the additive structure.

Analysis of cracks in the welded zone of stainless steel pipe used in high-pressure decomposer equipment

ASTM A276 was the material used in a stainless steel pipe inside High-Pressure Decomposer (HPD) equipment that was being used in the fertilizer industry. The pipe had 3 mm of thickness and was operated at a pressure and temperature of approximately 1.7 MPa and 130°C respectively. Leaking was observed from a crack in a welded area of the Heat Affected Zone (HAZ) and caused the factory to shut down. From the study results, the stress intensity factor (KI) value at the crack tip was calculated to be approximately 1244 Mpa.m1/2, whereas the fracture toughness (KIC) of the pipe material value is 499.9 MPa.m1/2. It can be inferred that due to the KI value being higher than the KIC value, then crack propagation occurred in the welded zone until it penetrated through the pipe wall. The cracking was also influenced by the presence of residual stresses and corrosive fluid flowing in the pipe which produced Stress Corrosion Cracking (SCC). It was further found that the welding process used to join the pipe produced porosity defects, which in turn caused porosity coalescence under the pressure of the pipe in operation, allowing crack propagation to occur and penetration of the pipe wall.

Experimental and numerical investigation of residual stress effects on fatigue crack growth behaviour of S355 steel weldments

Fatigue crack growth tests have been conducted on S355 G10+M structural steel which is widely used in the fabrication of offshore structures. Fracture mechanics tests have been performed on compact tension specimens with the crack tip located in the heat affected zone. All tests were performed at room temperature in air and the obtained results are compared with the literature data available on a range of offshore structural steels and also the recommended BS7910 trends using the 2-stage law and simplified law. The specimen orientation, with respect to the location of the extraction within the welded plate, has also been examined and discussed in this work. Residual stress measurements have been performed prior to testing by using the neutron diffraction technique. Finally, a numerical model has been developed in order to calculate the effective stress intensity factor range in the presence of residual stresses. The results have shown that the residual stresses play a key role in the fatigue life of the welded structures, especially in the near threshold region.

Transverse fatigue behaviour and residual stress analyses of double sided FSW aluminium alloy joints

AbstractFriction stir welding (FSW) since its invention has been attracting relevant interest for joining aluminium alloys. Due to the nature of this process, the materials can be joint without melting. Thanks to this peculiar characteristic, the issues associated with the cooling from liquid phase are avoided or considerably reduced, such as cracking, porosity, and defects.However, as well as other well‐established welding techniques, the FSW process gives rise to formation of residual stress in the welding region and surrounding volume: heat and thermo‐mechanical affected zones. Presence of residual stress in a mechanical component is well‐known to affect its performance, particularly regarding fatigue at high number of cycles. Another aspect that influences the fatigue life is the underlying microstructure.In this work, we firstly study the residual stress field and the underlying microstructural features arising in FSW butt joints and their effect on the fatigue performance of this type of weldments. The evaluation of residual stress field is carried out by means of modern experimental techniques. In the first instance, synchrotron X‐ray powder diffraction was employed for two‐dimensional full field maps of residual stress. Corroboration of these measurements was done by exploiting the capability of focused ion beam and digital image correlation (FIB‐DIC), which is able to deliver pointwise absolute measurement of residual stress. A set of FSW samples were then tested under uniaxial fatigue loading at several loading ranges, in the high cycle fatigue regime, in order to understand whether the severity of loads affects the crack path and life endurance. Fractographic and electron backscattered diffraction (EBSD) analysis then revealed crack nucleation site and propagation mechanisms with the respect of the underlying microstructure. Outcome of these experimental studies is then thoroughly discussed.

Principal Stress Ratio Effect at Residual Stress Determination Utilizing the Variation of Indentation Hardness

The determination of residual stresses is an important issue when it comes to material failure analysis. The variation of global indentation properties, due to the presence of residual stresses, can serve as a guideline for the size and direction of such stresses. One of these global indentation properties, the material hardness, is unfortunately invariant of residual stresses when metals and alloys are at issue. In this situation, one has to rely on the size of the indentation contact area for residual stress determination. For other materials such as ceramics and polymers, where elastic deformations are of greater importance at indentation, such invariance is no longer present. Here, this variation is investigated based on finite element simulations. The aim is then to determine how the indentation hardness is influenced by the principal residual stress ratio and also discuss if such an influence is sufficient in order to determine the size and direction of such stresses in an experimental situation. It should be emphasized that this work does not suggest a new approach to residual stress determination (by indentation testing) but investigates the applicability of previously derived methods to a situation where the surface stress field is not simplified as equi-biaxial or uniaxial. For simplicity, but not out of necessity, only cone indentation of elastic-perfectly plastic materials is considered.

A Thermoelastic Stress Analysis General Model: Study of the Influence of Biaxial Residual Stress on Aluminium and Titanium

All the studies on the thermoelastic behaviour of materials, including the revised higher order theory on the thermoelastic effect, are based on several assumptions that limit the application of such theory to the cases of isotropic materials in the presence of uniaxial residual stresses and undergoing uniaxial applied loads. These assumptions lead to some discrepancies in the description of the real thermoelastic behaviour of materials in the presence of residual stresses. In this work, by rewriting the thermoelastic equation in a different way, it was possible to study the behaviour of homogeneous and non-isotropic materials undergoing any loading conditions and residual stresses. Firstly, the error made by the calibration procedures of thermoelastic stress analysis (TSA) data in the presence of residual stresses has been investigated. Then, a statistical analysis was carried out to determine the minimum value of residual stress which would lead to significant and measurable variations in the thermoelastic signal. The simulations involved two non-ferrous metals: AA6082 and Ti6Al4V, which exhibit a specific thermoelastic behaviour.

Non-contact stress measurement in PET preforms

ABSTRACTResidual stresses in PET preforms is thought to be an important cause in the stress cracking of blown bottles but there is no data yet that can be linked to stress and cracking. An important step in this link is the measurement of stresses in the preform base and neck region of the preforms. This paper reports on the usage of photoelastic methods to measure residual stresses in preforms. It is found that the residual stresses vary from preform to preform and how cracks develop is due to the qualitative and quantitative presence of residual stresses and its orientation.

Influence of manufacturing process on the strength of composite materials

Residual stresses in laminated composites induced by non-uniform matrix solidification and shrinkage through manufacturing process may lead to the shape distortion and possible damage of the final product. In this work, the phase transition model for matrix material was numerically implemented into Abaqus software to simulate evolution of stiffness properties and development of residual stresses of composite laminate during manufacturing temperature cycle. This simulation shows the presence of significant residual stresses in composites even before mechanical processing of the material. As an example, the residual stresses were computed for the plate with cross-ply layup. As a last step two specimens with presence of residual stresses and without one were loaded up to failure using progressive damage model. Modelling results show essential influence of residual stresses on distribution and development of matrix cracks, failure modes and strength values.

On-machine Characterization of Bulk Residual Stresses on Machining Blanks

The manufacturing of ribbed aluminum structural parts presents several difficulties due to the appearance of geometrical distortions after machining. The presence of residual stresses on the aluminum blanks before machining is reported to be the main source of distortions for aluminum alloys. The variability of these stresses and the inability to measure them in every aluminum blank is the main limitation on the optimization of the machining processes of these parts. In this paper, a layer-removal based residual stress characterization method is proposed for its application in manufacturing environment. This method enables the identification of the bulk stresses on the machining blank and still allow the final machining of the component, making possible the definition of an optimized machining process that would yield a minimum component distortion.

Effects of residual stresses and localised strain-hardening on the fracture of ductile materials

The strain history of a ductile material can affect its apparent initiation fracture toughness as well as its resistance to subsequent crack growth. Using neutron diffraction or synchrotron X-ray diffraction in conjunction with digital image correlation, it is possible to map both the total strain around the tip of a propagating crack, and the elastic strain component. Separating the elastic and irreversible strain components allows us to experimentally investigate the effects that residual stresses and prior strain-hardening have on crack propagation. This article presents the results of two experiments using these techniques. In the first experiment, it was shown that residual stresses in a 7xxx-series aluminium alloy affect not only the initiation of fracture but also fracture propagation. During crack propagation, residual stress relaxation occurs due to extension of the crack and plastic deformation in the region ahead of the crack tip. This affects the material’s apparent crack growth resistance in a non-linear manner and hence changes its fracture stability. In a second experiment, it was shown that the plastic zone ahead of a crack in a ductile material (in this case a ductile ferritic steel) can be modified by localised strain-hardening prior to fracture. Localised strain-hardening using an indentation tool was observed to cause small increases in the specimens’ peak load capacity and its energy absorption prior to the onset of unstable tearing. Using synchrotron diffraction, it was shown that is because hardening a region ahead of the crack tip causes stress redistribution to this region and away from the crack tip itself during fracture loading. These findings can inform how ductile and semi-ductile fracture is treated in structural integrity assessment procedures, particularly in the presence of residual stress. They also suggest that new localised peening treatments to improve the fracture resistance of structures in certain areas may be possible.

АНАЛИЗ ОСТАТОЧНЫХ НАПРЯЖЕНИЙ В СЛОИСТЫХ КОМПОЗИТАХ НА ПРИМЕРЕ СИММЕТРИЧНОЙ СХЕМЫ АРМИРОВАНИЯ [0°/90°

Recently an increased attention has been paid to the deep understanding of the processes caused by residual stresses in laminated composites, specifically, shape distortion and possible product damages at the stage of manufacturing. The presence of residual stresses in a composite material affects the occurrence of such defects as delamination, warping, as well as the appearance of microcracks in a matrix of a material, which has a significant impact on the static and fatigue strength of the product made of composite materials. This paper considers a change in the distribution of the residual stress field in a specimen from the known material AS4/8552-1 with the layers [0°/90°]n in the curing process and the stresses on the free edge after the cut of the sample along the reinforcement direction. The curing model is numerically implemented using ABAQUS finite element modeling package with special user subroutines. The coupled thermal and strength problem under plane strain conditions is solved to model the curing. A significant difference in residual stresses at the free edge before and after the mechanical cutoff is shown. Computed residual stresses are further used to analyze the development of the composite fracture under uniaxial tension. As a result of modelling the loading of the composite up to failure with a consideration of the residual stresses, it is shown that tension leads to cracking of the matrix in layers where the orientation of the reinforcement coincides with the direction of the load application. At the same time, when ignoring residual stresses, the appearance of such defects is not observed. This research shows that residual stresses affect both the strength of the material and the mode of failure.

ЧИСЛЕННОЕ МОДЕЛИРОВАНИЕ НАПРЯЖЕННО-ДЕФОРМИРОВАННОГО СОСТОЯНИЯ УЧАСТКА ГАЗОПРОВОДА С ОТВОДАМИ ХОЛОДНОГО ГНУТЬЯ ПО ДАННЫМ ВНУТРИТРУБНОЙ ДИАГНОСТИКИ

Knowledge of the current stress-strain state of any section of the pipeline allows you to make informed decisions on its operation, maintenance and repair, as well as on the prediction of the technical condition. The task of determining the characteristics of the stress-strain state of a gas pipeline section that has cold bend offsets (CBO) according to in-line inspection (ILI) is considered. The bent part of CBO is characterized by the presence of residual stresses and deformations in the wall of the offset, which contribute to the overall level of the stress-strain state of the gas pipeline operating under external and internal loads. Using the results of in-line diagnostics, numerical modeling and a solution, the change in the values of longitudinal stresses, is determined and the need to take into account residual stresses in the zone of elastic-plastic deformations of cold bend offsets is shown.

Load distribution and damage evolution in bending and stretch dominated Ti-6Al-4V cellular structures processed by selective laser melting

Cellular structures produced by additive manufacturing techniques, such as selective laser melting, enable for realizing parts of high specific strength and stiffness. In dependence of the prevalent deformation mechanism, i.e. bending or stretch dominated behavior, differences in the mechanical performance are observed. However, predictability of the mechanical behavior by current analytical and numerical approaches is limited, as local phenomena, such as process related geometrical inaccuracies and/or the local microstructure, are not incorporated in respective models. Thus, little knowledge on the actual load distribution and the damage evolution within cellular structures is available, although a high dependency of the structural performance on these aspects can be expected.The current work aims at closing this gap. By means of digital image correlation profound insights into the local strain distribution and damage evolution on the level of individual struts were obtained and correlated to the phenomena observed on the macroscopic scale under quasi-static and cyclic loading. By contemplating as-built as well as heat treated Ti-6Al-4V structures revealing fundamentally different mechanical performance, the observations made are not only related to the geometrically induced deformation mechanism, but also to process induced material characteristics, such as pronounced brittleness and the presence of residual stresses in the as-built condition.

Synthesis and characterization of TiN / Ti / AISI 410 coatings

The synthesis and characterization of Ti and TiN thin films of different thicknesses was carried out on a martensitic stainless steel AISI 410 substrate used for tool manufacturing. The mechanical parameters between the interacting surfaces such as thickness, adhesion and hardness were measured. By means of the scanning electron microscope (SEM) the superficial morphology of the Ti/TiN interface was observed, finding that the growth was of columnar grains and by means of EDAX the existence of titanium was verified. Using X-ray diffraction (XRD) it was possible to observe the presence of residual stresses (~ -3.1 GPa) due to the different crystalline phases in the coating. Under X-ray photoemission spectroscopy (XPS) it was possible to observe the molecular chemical composition of the coating surface, being Ti-N, Ti-N-O and Ti-O the predominant ones.

Relation between residual stresses and microstructure evolution in Al–Si alloys based on different casting parameters

ABSTRACTMost designers take into consideration the stresses that act on a material but despite safety considerations, failure may occur due to other factors that were neglected in their design. These factors can be a pre-existing flaw, microstructure deficiency or the presence of residual stresses. Depending on the stress type, residual stresses combined with applied stresses can aid or hinder failures. Consequently, reducing the amount of residual stress can have a promising effect on the life of the component. Different casting parameters can change the microstructure and residual stresses of castings. In this research, the relation between residual stresses and microstructure evolution under the influence of different casting parameters was investigated, using both Al–Si–Mg (Al-356) and Al–Si–Mg–Cu (Al-319) alloys. Solidification rate, quenching rate, aging temperature and aging time were the main parameters considered for this study. The results indicate that the magnitude of the residual stresses increases with increasing solidification rate and quenching rate. Also, the residual stress relieving is proportional to the aging temperature

Incorporating grain-level residual stresses and validating a crystal plasticity model of a two-phase Ti-6Al-4 V alloy produced via additive manufacturing

Titanium alloys, produced via additive manufacturing techniques, offer tremendous benefits over conventional manufacturing processes. However, there is inherent uncertainty associated with their properties, often stemming from the variability in the manufacturing process itself along with the presence of residual stresses in the material, which prevents their use as critical components. This work investigates Ti-6Al-4 V produced via selective laser melting by carrying out crystal plasticity finite element (CPFE) simulations and high-resolution digital image correlation (HR-DIC) on samples subject to cyclic loading. This is preceded by detailed material characterization using electron backscatter diffraction, back-scattered electron imaging and transmission electron microscopy, whose results are utilized to inform the CPFE model. A method to incorporate the effect of grain-level residual stresses via geometrically necessary dislocations is developed and implemented within the CPFE framework. Using this approach, grain level information about residual stresses obtained spatially over the region of interest, directly from the experimental material characterization, is utilized as an input to the model. Simulation results match well with HR-DIC and indicate that prior β boundaries play an important role in strain localization. In addition, possible sites for damage nucleation are identified, which correspond to regions of high plastic strain accumulation.