Presence Of Residual Stresses Research Articles

Feasibility on accurate measurement of non-uniform strain field through contact methods

When straight material filaments do not remain straight after deformation, the strain varies spatially. These spatially varying strain fields arise because of (1) non-uniform distribution of boundary traction, (2) presence of residual stress in the reference configuration, (3) the body being inhomogeneous. The strain measured using devices such as extensometer and strain gauge assumes that the strain is uniform over the gauge length, irrespective of the strains actual variation. Here the accuracy of the strain estimated using contact strain measurement techniques is investigated by assuming that the uniaxial strain (not stress) varies periodically along the measurement axis. A tight bound for the error in the measured strain is obtained and verified through numerical simulation. This study’s significant finding is that for gauge length of strain measurement about two times the periodicity of the strain variation, the measured strain’s value would be repeatable but with a significant error. Thus, one has to be cautious about the magnitude of the estimated non-uniform strain.

FEM assessment of the local side compression technique efficiency as applicable for notched prismatic specimens

A crack front straightness is one of the test result validity criteria for fatigue precracked static fracture toughness specimens. Actually, the ideally straight crack front cannot be reached due to the presence of residual stress. This is particularly actual for specimens cut out of welded joints containing the residual welding stress (RWS). One of the techniques allowing to lower the RWS effect is a local side compression of specimens. Its efficiency has been proved in physical testing however no quantitative assessments are known in the literature. This work comprises FEM simulation of welding, sampling and side compression processes. The effect of local compression on base metal containing no residual stress is also investigated.It has been found that in the course of local side compression the initial residual stress field caused by welding and specimen making is replaced by another field showing stress gradients more favourable for getting the fatigue crack shape meeting the validity criteria of test results as per approved test methods. The calculation results show that the complete removal of residual stress as in base metal as in welded specimens is not feasible in the range of actual practicable degrees of compression.

Nanomechanical Response of Pulsed Tungsten Inert Gas Welded Titanium Alloy by Nanoindentation and Atomic Force Microscopy

Nanohardness and Effective Elastic Moduli were measured for pulsed-Gas Tungsten Arc Welded Ti-5Al-2.5Sn alloy using autogenous mode through nanoindentation and atomic force microscopy. Experiments were conducted using a Berkovich tip on nanoindentor with Berkovich tip and elliptical pile ups were measured using an Atomic Force Microscope. Nanohardness and effective elastic moduli were calculated in the base metal, heat affected zone and fusion zone of the weldments using different approaches namely Oliver–Pharr method, AFM analysis and work of indentation. A significant difference was observed in the nanomechanical response using these approaches which was attributed to the pile up morphology of the nano indents. The presence of residual stress in the weldments also significantly influenced the nanohardness profile across the weld joint. The present research suggested that the work of indentation is most suitable for assessment of nanomechanical properties of Ti-5Al-2.5Sn alloy weldments among the three techniques studied in this investigation.

Residual stress effects on fatigue crack propagation in Butt–Welded joints for 304 stainless steel sheets

Welded joints are sensitive to fatigue failure due to cyclic loading, as well as fatigue crack propagation influenced by the distribution of welding residual stress. In this study, the fatigue crack propagation rates in butt-welded joints for 304 stainless steel sheets were evaluated in the presence of welding residual stresses. The analysis consisted of two separate models: first, a 3D-finite element (FE) model was used to predict the residual stresses due to welding; second, a numerical study was undertaken to predict fatigue crack propagation in the presence and absence of residual stress using the extended finite element method (XFEM). The crack growth model (NASGRO) and available experimental data were applied to verify the simulation results. The XFEM without residual stress effects shows good agreement with the experimental data and the NASGRO model. However, in the presence of residual stress, the simulation results show less agreement with the NASGRO model. The level and the nature of residual stress have significant effects on crack growth. A faster crack propagation rate is recognized due to the effect of tensile residual stress at the crack tip, while a higher resistance to crack growth is developed due to a compressive residual stress field.

Technological prerequisites for the energy efficiency increase of the engine cylinder bore processing

An example of the rationalization of the current technology for manufacturing a cylinder of a small-sized diesel engine at PA “TULAMASHZAVOD” from a cast iron is given. It is shown that lowpower obsolete equipment leads to an increase in the number of technological operations and equipment used. However, even if all the generally accepted technological recommendations are followed, only 80% of the parts meet the requirements for the required parameters of accuracy and quality of the cylinder bore surface. Studies have found that the main reasons for insufficient quality are the presence of residual stresses in the casting, which cannot be removed during artificial aging, as well as insufficient accuracy and rigidity of metal cutting equipment at turning and boring operations. The use of more powerful and accurate modern equipment is theoretically and experimentally justified, which, due to increased refinement, allows reducing the number of machining operations. Replacement of artificial thermal aging by natural, combined with the replacement of shaft furnaces for heat treatment of workpieces with chamber furnaces of lower power and higher capacity, reduce energy consumption for the manufacturing by almost 4 times.

Numerical Simulation Of Nanoindentation Process On Pre-stressed Ti6Al4V Alloy For Residual Stresses Evaluation

Finite Element analysis can be a valid alternative to expensive, time consuming and destructive experimental tests. However, the reliability of a numerical model needs to be properly verified by a robust calibration and validation method, able to demonstrate the effectiveness of the proposed technique. A topic of considerable scientific interest is the estimation of residual stresses in the engineering field, since the available experimental techniques present a limited range of applications, because of the complexity of measurement as well as problems associated with measurement accuracy and applicability to a broad range of materials. This paper presents the development of a numerical model able to calculate residual stresses due to the effects of machining, using the nanoindentation method. Numerical simulations of indentation tests on stressed and stress-free samples were carried out, calibrated and validated using experiments. Results concerning the presence of residual stresses are presented with a good agreement with the experimental evidences.

Kontrolisanje zaostalih napona - razvoj najnovijih inženjerskih metoda za merenje bez razaranja i povoljnu preraspodelu zaostalih napona

Welding is used widely in the automotive industry for joining a variety of structural components and parts. Of paramount importance in these structures are their engineering properties, such as fatigue life, distortions, dimensional stability and corrosion resistance that can be affected considerably by the presence of residual stresses (RS). The knowledge of RS and the ability to control their distribution in welded structures is critical when evaluating their fatigue life and preventing catastrophic failures. An engineering concept of residual stress management (RSM) has been developed that addresses all aspects of residual stresses in structural elements. RSM includes three major stages in stress management, i.e. RS determination, RS analysis and RS redistribution. Using this approach, stresses in structures and materials can be evaluated in each specific case either theoretically or experimentally and the performance and fatigue behavior of such structures optimized. This paper is built as an overview of the RSM concept and its application in the fields of non-destructive measurement of residual and applied stresses and in treatment of structures with residual stresses to achieve better performance and longer fatigue life. All three stages of the RSM concept will be discussed and for each of the stages practical engineering approach examples will be given. An example of a project in which the residual stress distribution in a filet welded joint was measured, analyzed and changed by post-weld treatment will be presented to demonstrate the effectiveness of the RSM approach. The advancements in the modern tools used in this project for non-destructive measurement of residual stresses using an ultrasonic computerized complex for residual stress measurement and the ultrasonic peening for redistribution of the residual stresses that allowed improving the quality of the welds and increasing their fatigue life will also be presented.

Numerical analysis of the effect of residual stress on transient creep in SENB specimen

Many manufacturing processes can induce residual stresses in produced components for high-temperature application. Residual stresses, combined with the primary mechanical stresses, play an important role in structural integrity assessments of high-temperature components. In this paper the elastic and elastic-plastic creep behaviour of mechanically loaded shallow cracked SENB specimen in the presence of residual stress has been analyzed. The transient crack tip parameter under creep conditions, C(t), has been numerically determined at different primary mechanical stress and residual stress levels. The analysis was performed for general material properties where different plastic and creep stress exponents were systematically varied. Based on the performed analyzes, the expressions for relaxation curves of C(t) for the considered elastic creep and elastic plastic creep cases are proposed. These expressions show good agreement with the results of the numerical analysis conducted for the arbitrarily selected input data, which gives confidence in their reliable application for estimating the C(t) values of shallow cracked SENB speciemens.

Lifetime estimation of a BWR core shroud in terms of IGSCC

The continued operation of aging Boiling Water Reactors (BWRs) worldwide requires gradually increasing cost of inspection, maintenance, and repair. Intergranular Stress Corrosion Cracking (IGSCC) in sensitized austenitic stainless steel piping first became a major issue for BWRs in the 1980s, resulting in recognition of the susceptibility of reactor internals to IGSCC. Shroud cracking identified in 1993–1994 confirmed that IGSCC of internals is a significant issue for BWRs. IGSCC is a time-dependent, material degradation process, which is caused and accelerated by the presence of residual stresses, material sensitization, irradiation, cold work, elevated temperature, and corrosive environments. This paper emphasizes the importance of accounting for corrosive environments, or more exactly, electrochemical phenomena in modelling IGSCC and predicting the service life of BWR in-vessel components, and stresses the necessity of performing such modeling not just for a single state point under full power conditions, but for the whole operating history of the reactor, including startups and shutdowns. This paper demonstrates that ignoring electrochemical considerations may result in underestimating component lifetime and lead to unnecessary expenses for inspection and repair.

Sub-Structure and Residual Stress in Rotary Swaged Cu/Al Clad Composite Wires

This study investigated the prospective application of the advantageous intensive plastic deformation method of rotary swaging for production of Al-Cu composite wires. Such materials are perspective to be used within a wide range of commercial and industrial branches, from transportation to electrotechnics. Cu-Al laminated wires with two unique different stacking sequences were rotary swaged down to 5 mm diameter at room temperature to minimize the development of brittle intermetallics at the interfaces. The analyses primarily focused on the mutual comparison of both the stacking sequences (Al sheath reinforced with Cu wires vs. Al sheath and Al core reinforced with Cu inter-layer) from the viewpoints of mechanical properties, sub-structure development, and occurrence of residual stress. While the individual Cu wires exhibited bimodal structure and the presence of residual stress within the growing grains, the Cu inter-layer featured recrystallized grains and homogeneous stress distribution. The mechanical properties for both the composites were enhanced by the swaging technology; the composite reinforced with Cu wires exhibited slightly higher ultimate tensile strength than the one with Cu inter-layer (258 MPa vs. 276 MPa). However, the latter featured significantly higher plasticity.

Prediction of interfacial debonding between stiff spherical particles and a soft matrix with the coupled criterion

The tensile load at debonding onset between stiff particles and a soft matrix is predicted with the coupled criterion which combines a stress and an energy conditions. A finite element model of a representative volume element is implemented in order to consider the influence of the size of the particles, the particle volume fraction, mixed mode conditions at the interface and the presence of residual stresses. It is also shown how this approach can be used in order to analyze experimental data for the identification of the interfacial fracture parameters.

Failure Analysis of Cooling Tower Fan-Arm

This work presents failure investigation of cooling tower fan-arms commissioned in a chemical processing plant. The analysis aims at understanding the mechanism and root-cause of the failure. The investigation involves site visits, microstructural analysis, fracture surface analysis, hardness measurements, numerical stress analysis and experimental simulation. Work concludes that the fan-arms failed due to the lack of post-weld heat treatment, which caused localized stress-corrosion and pitting at critical locations that served as crack initiation sites. Fatigue loading condition and presence of residual stresses at the weld enabled easy propagation of cracks that led to recurring premature failure. Based on the root-cause and the mechanism identified in this analysis, proper pre-heating and post-weld heat treatment is recommended to relieve the residual stresses at the critical locations and thus to avoid/minimize such recurring failures in future.

Factors Influencing the Prediction of Pile Driveability Using CPT-Based Approaches

This paper investigates the applicability of Cone Penetration Test (CPT)-based axial capacity approaches, used for estimating pile static capacity, to the prediction of pile driveability. An investigation of the influence of various operational parameters in a driveability study is conducted. A variety of axial capacity approaches (IC-05, UWA-05 and Fugro-05) are assessed in unmodified and modified form to appraise their ability to be used in estimating the driveability of open-ended steel piles used to support, for example, offshore jackets or bridge piers. Modifications to the CPT-based design approaches include alterations to the proposed base resistance to account for the resistance mobilized under discrete hammer impacts and the presence of residual stresses, as well as accounting for the effects of static capacity increases over time, namely ageing. Furthermore, a study on the influence of various operational parameters within a wave equation solver is conducted to ascertain the relative impact of uncertain data in this respect. The purpose of the paper is not to suggest a new design procedure for estimating pile driveability, rather to investigate the influence of the various operating parameters in a driveability analysis and how they affect the magnitude of the resulting predictions. The study will be of interest to geotechnical design of piles using CPT data.

Investigation of the residual stress in UO2-Mo composites via a neutron diffraction method

UO2-Mo composites with a core-shell structure have been considered candidates for the thermal conductivity (TC)-enhanced UO2 pellets and have demonstrated commercial potential for use in novel high-level safety reactors. Nevertheless, UO2-Mo composites tend to form micro-cracks that are caused by the presence of residual stress (RS) during manufacturing. In this work, neutron diffraction measurements were employed to analyse the RS in UO2-Mo core-shell structured composites fabricated by spark plasma sintering (SPS) for the first time. It was found that in the UO2-Mo composites, the RS state present in the UO2 matrix was tensile in nature. The RS in the UO2 matrix increased with increaseing Mo content. There was a maximum value of 148 ± 15 MPa in the UO2-10 vol% Mo composite. The micro-cracks produced in the high-Mo content composites were explained by the results of the neutron diffraction measurements. These results could provide significant guidance for the manufacturing and improvement of the operational performance of UO2-Mo composites as next-generation fuels.

Investigation on Pull-in Voltage, Frequency Tuning and Frequency Stability of MEMS Devices Incorporating Casimir Force with Correction for Finite Conductivity

This paper studies the influence of Casimir force on the static and dynamic characteristics of electrostatically actuated fixed–fixed microbeam. The theoretical formulation for the study is obtained using the Euler–Bernoulli beam theory. The formulation of governing equation includes the influence of mid-plane stretching of microbeam, fringing field and Casimir force incorporating correction for finite conductivity. The governing differential equation is solved using the Galerkin discretisation method and reduced order model technique. The results of the methodology implemented in current work are validated through comparison with reported numerical and experimental results. Subsequently, the effect of the Casimir force incorporating correction for the finite conductivity on the pull-in voltage and natural frequency is investigated in the presence of the residual stresses in a microbeam. The investigation shows that neglect of the Casimir force significantly overestimates the pull-in voltage; however, inclusion of corrections for the finite conductivity to the Casimir force indicates reduction in amount of overestimation in the pull-in voltage. The linear free vibration characteristics depict that microbeam with high mid-plane stretching parameter in the presence of the compressive residual stress offers tuning of natural frequency for more than two times. Further, the microbeam with high mid-plane stretching parameters provides resonant frequency stability characteristics at high voltage parameters though there is variation in the residual stress due to temperature variation during application. The present work also shows that these frequency tunability and stability characteristics are significantly influenced by the Casimir force. The results of the current work are expected to be valuable for the design of microbeam devices.

Effect of Crack Inclination and Residual Stress on the Stress Intensity Factor of Semi-elliptical Crack in a Pressure Vessel

Pressure vessels are widely used in products ranging from consumer artifacts to engineering components. Ensuring safety and reliability of pressure vessel is therefore imperative. In cases where conducting experiments is not viable, analytical or numerical methods are used. In this work, a horizontal pressure vessel is analyzed using finite element method for estimating the failure pressure and stress intensity factor in the presence of residual stress. A three-dimensional finite element model was created with a commercially available software, and a semi-elliptical crack was introduced in the pressure vessel on the outer and inner surfaces. In addition, the crack orientation was also changed, and the stress intensity factor was determined both analytically and numerically. The results showed a reasonably good agreement.

Interaction of Cyclic Loading (Low‐Cyclic Fatigue) with Stress Corrosion Cracking (SCC) Growth Rate

The structural integrity analysis of nuclear power plants (NPPs) is an essential procedure since the age of NPPs is increasing constantly while the number of new NPPs is still limited. Low‐cyclic fatigue (LCF) and stress corrosion cracking (SSC) are the two main causes of failure in light‐water reactors (LWRs). In the last few decades, many types of research studies have been conducted on these two phenomena separately, but the joint effect of these two mechanisms on the same crack has not been discussed yet though these two loads exist simultaneously in the LWRs. SCC is mainly a combination of the loading, the corrosive medium, and the susceptibility of materials while the LCF depends upon the elements such as compression, moisture, contact, and weld. As it is an attempt to combine SCC and LCF, this research focuses on the joint effect of SCC and LCF loading on crack propagation. The simulations are carried out using extended finite element method (XFEM) separately, for the SCC and LCF, on an identical crack. In the case of SCC, da/dt(mm/sec) is converted into da/dNScc (mm/cycle), and results are combined at the end. It has been observed that the separately calculated results for SCC (da/dNScc) and LCF (da/dNm) of crack growth rate are different from those of joint/overall effect, . By applying different SCC loads, the overall crack growth is measured as SCC load becomes the main cause of failure in LWRs in some cases particularly in the presence of residual stresses.

Investigation into residual stresses inherited during manufacturing of carbon-epoxy composites with delamination

Last decades, 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 damage of the product at the stage of manufacturing. The presence of residual stresses in the composite material affects the occurrence of such defects as delamination, warping, as well as the appearance of microcracks in the matrix of the material, which has a significant impact on the static and fatigue strength of the final product made of composite material.In this paper, we consider the change of the distribution of the residual stress field caused by manufacturing process in the specimen with initial delamination defect (crack) due to appearance of free edge cut. Different length of initial crack has been analyzed with well-known composite material AS4/8552-1.The curing model is numerically implemented using ABAQUS finite element modeling package with special user subroutines. The coupled thermal and stress problem under plane strain conditions is solved for the modelling of curing. The history of energy release rates under mode I, II (GI, GII) were analyzed over time during process of polymerization and free edge cut appearance using seam crack technique.A slight increase in the GI and GII values at the crack tip during polymerization and a multiple excess of these values after a mechanical cut is shown. On the contrary obtained by modelling levels of stress components are essential and cannot be ignored in consequent structural analysis. The results of the study can be applied for prediction of residual stresses in composite structure by means of simulation and further understanding the nature of fracture of composites.

Measurement and Prediction of Residual Stresses in Low Carbon Steel Pipes Welded Shielded Metal Arc Welding

Welding is carried out with a very complex thermal cycle, which results in irreversible elastic-plastic deformation, and residual stresses in and around fusion zone and heat-affected zone (HAZ). A residual stress due to welding arises from the differential heating of the pipes due to the weld heat source. However, due to the presence of residual stresses in and around the weld zone the strength and life of the component is also reduced. The objective of this work is to measure the welding residual stress in ASTM (A-106 Gr. b) steel pipes with 4" diameter and 6 mm thickness welded manually (SMAW) in three-pass butt joint where shielded metal arc welding process consists of heating, melting and solidification of parent metals and a filler material in localized fusion zone by a transient heat source to form a joint between the parent metals. The welding process will have accrued without preheating and heat treatment. This measuring of residual stress occurs by using hole-drilling strain gauge method according to (ASTM E-873) then the experimental results for residual stresses obtained from welded carbon steel pipes used to provide validation for finite element simulations. The welding process and welding residual stress distribution is calculated by Ansys Finite Element techniques. Theoretical considerations can be assessed by a mechanical model. The overall, there is good agreement between the predicated and measured distributions of residual stress, but the magnitude of predicted stress tend to greater in welding region.

Undercooling versus stress induced martensitic phase transformation: The case of MgO – partially stabilized zirconia

The martensitic tetragonal-to-monoclinic (t→m) phase transformation is the primary inelastic deformation mechanism in zirconia ceramics. In this study, a Ginzburg-Landau type phase-field model is utilized to simulate phase transformations in MgO-partially stabilized zirconia, clearly distinguishing between the effects of undercooling and stress. The simulation results reproduce characteristic lamellar microstructures observed in experiments. Our study shows distinctive differences in microstructure and evolution path between undercooling-induced and stress-induced t→m transformations. In the latter case, sequential growth of monoclinic lamellae is observed, whereas undercooling induced microstructure evolution is characterized by an almost homogeneous and simultaneous nucleation of the monoclinic phase. We show that the different behavior can be explained by the presence of an energy barrier in the Gibbs free enthalpy. It is found that the presence of residual stresses in the tetragonal phase provides such a barrier and contributes to the stability of the tetragonal phase at low temperatures.