Characterization Of Residual Stresses Research Articles

Characterization of residual stresses from cold expansion using spatial statistics

AbstractResidual stress fields from cold expansion have been widely used to extend the fatigue life of aircraft structures. However, the spatial statistical character of these residual stress fields has not been established and has not been incorporated in current analysis methods. The objective of this study was to establish a spatial statistical method to quantify the residual stress field around a cold expanded hole. A framework called the Spatial Analysis of Residual Stress (SpARS) was developed utilizing spatial correlation, response surface modelling techniques and statistical resampling methods to characterize the residual stress field. Our results showed that tolerance bounds on residual stress can be quantified using this method. We also demonstrated the SpARS method using recently published round robin case studies. The newly developed model will be useful for aircraft structural fatigue crack growth analyses to incorporate residual stress fields for extending inspection intervals for fatigue and fracture critical structures.

Neutron diffraction measurements of weld residual stresses in three-pass slot weld (Alloy 600/82) and assessment of the measurement uncertainty

This paper describes in detail two neutron diffraction residual stress measurements, performed on the ENGIN-X neutron scattering instrument at the ISIS facility in the UK and on the SALSA instrument at the Institut Laue-Langevin in Grenoble, France. The measurements were conducted as part of the NeT Task Group 6 (TG6) international measurement round robin on an Alloy 600/82 multi-pass weldment - a slot in an Alloy 600 plate filled with three Alloy 82 weld beads, simulating a repair weld. This alloy/weld combination is considered challenging to measure, due to the large grain size and texture in the weld, and large gradients in the stress-free lattice parameter between the parent and weld metal. The basic principles of the neutron diffraction technique are introduced and issues affecting the reliability of residual stress characterization are highlighted. Two different analysis strategies are used for estimation of residual stresses from the raw data. Chemical composition studies are used to measure the mixing of parent and weld metal and highlight the steep lattice parameter gradients that arise as a consequence. The inferred residual stresses are then compared with three sets of measurements performed on the same plate by other NeT partners on E3 at the HZB in Berlin, STRESS-SPEC at the FRM II in Munich and KOWARI in Sydney. A robust Bayesian estimation average is calculated from the combined five-instrument data set, allowing reliable best estimates of the residual stress distribution in the vicinity of the weldment. The systematic uncertainties associated with the residual stress measurements are determined separately in the weld and parent materials, and compared with those in the NeT TG4 benchmark. This is a three-pass slot-welded plate fabricated from American Iron and Steel Institute AISI 316L(N) austenitic stainless steel, and is normally considered less challenging to measure using diffraction techniques than all nickel welds. The uncertainties in the stress measurements by neutron diffraction for these two weldments seem to be comparable.

Thermal stress relaxation in shot peened and laser peened nickel-based superalloy

The effects of cold work produced by peening processes in residual stress relaxation under elevated temperatures were investigated. Two types of peening processes namely shot peening and laser peening were used to generate different amount of cold work and residual stresses in the study. Characterisation of residual stress induced by cold working were carried out using X-ray diffraction coupled with electrochemical polishing method to measure the variation of residual stress with depth. Residual stresses in shot peened samples were observed to relax more rapidly than that of laser peened ones when subjected to thermal exposure. The data collected from the experimental investigation was then used to build a correlation function to model the effects of cold work on residual stress relaxation. A parameter describing cold work’s contribution to stress relaxation is introduced into the well-known Zener-Wert-Avrami (ZWA) model. The model was validated against experimental measurements and showed good agreement in both shot peened and laser peened samples.

Structural and mechanism study on enhanced thermal stability of hydrogenated diamond-like carbon films doped with Si/O

a-C:H and a-C:H:Si:O films with two different Si/O co-doping contents had been deposited using a PECVD system by the mixture of C2H2 and HMDSO gas. The structure evolution of as-deposited and annealed films had been characterized by the Raman spectroscopy, XPS and FTIR. A progressive increase of sp2 carbon sites and a reduction of sp3 with the increase of the annealing temperature were expected. However, the Si/O co-doping was found to be able to reduce the graphitization degree of the annealed films. After annealing at 400 °C, the decrease rate of sp3 fraction of a-C:H film was 14.2%, while the a-C:H:Si:O (0.93 at.% Si) film was 8.16% and the a-C:H:Si:O (3.62 at.% Si) film was 6.8%. To understand the mechanism on the improved thermal stability by Si/O co-doping, the structure and residual stress of the a-C:H and a-C:H:Si:O films were analyzed. The results revealed that silicon atoms were incorporated into the carbon network by substituting carbon atoms of the films, which had also been characterized contributed to produce the CSi sp3 bonds stabilized by the oxygen. Residual stress characterization also demonstrated that, the residual stress of the a-C:H:Si:O films was greatly reduced compared with that of the a-C:H films. Therefore, the fraction of the highly strained CC sp3 bonds, which were more likely to break at elevated temperature, was reduced in the a-C:H:Si:O films. This kind of structure evolution endowed the a-C:H:Si:O films higher hardness and adhesion at high temperature.

Interplay of dislocation substructure and elastic strain evolution in additively manufactured Inconel 625

The unique thermal history of direct metal laser sintering (DMLS) leads to complex microstructures and local elastic residual strains which accumulate to affect the global residual stress and mechanical properties of components. Characterization of residual stress using neutron and x-ray diffraction result in bulk and grain level residual stress measurements; however, the contribution of the microscale residual strain remains largely unaccounted for. High-resolution electron backscatter diffraction (HR-EBSD) has emerged as a promising tool for the characterization of such micron-level elastic strains. This work presents an early effort in analysis of microscale elastic strain in conjunction with subgrain dislocation structures to further the understanding of microstructural evolution during laser additive manufacturing (AM) techniques. Elastic strain in DMLS fabricated IN625 is analyzed using open source cross-correlation software OpenXY while the geometrically necessary dislocation (GND) density is calculated from EBSD data using the Nye tensor. Dislocation structures previously seen in similar materials are shown here to contain low elastic strain gradients, supporting the assertion that these structures occur as strain minimization mechanisms during solidification.

Determination of the influence of thermo-mechanical factors on the residual stresses of cylindrical composite tubes: Experimental and computational analyses

The well-established benefits of compressive residual stresses over the fatigue lifetime constantly motivate engineers to scrutinize these trapped stresses in mechanical/civil structures. This research deals with characterization of thermal residual stresses in fiber reinforced polymer (FRP) tubes. Three important thermo-mechanical parameters, namely “cooling temperature”, “CNT reinforcement” and “tube size” are taken into account with the aim of achieving optimum design in manufacturing of FRP tubes. The work includes experimental evaluation of residual stresses coupled with developing a MATLAB programming code and performing a thorough finite element (FE) analysis. After fabricating several hybrid FRP tubes with respect to the mentioned parameters, we measured residual stresses conducting “slitting” semi-destructive method. The impact of either of parameters is then examined and some key points are suggested regarding controlling residual stress in FRP structures. The outcomes of this work suggest that slitting method can practically provide access to the in-depth distribution and through thickness of trapped stresses in the FRP laminates with a good accuracy. It is also underscored that studied parameters could play a crucial role in creation of thermal residual stresses.

Residual Stress and Microstructure Characterization of 34CrMo4 Steel Modified by Shot Peening.

34CrMo4 steel is widely used for drill stem in oil exploration, because of its excellent properties, such as favorable hardenability, shock absorption, less tendency of temper brittleness, and eminent wear resistance. In this study, the main works are residual stress test and microstructure characterization of 34CrMo4 steel upon various shot peening treatments. The residual stress distribution with effect depth was studied upon the shot peening. Face-to-face paste sample preparation method is required for continuous observation for microstructure evolution of shot-peened specimen from the treat surface to matrix. Grain refinement, lath structure, and precipitates are clearly observed in the gradient deformation layer.

A Current Status of the Residual Stress Characterization by the Crystal Lattice Strain (Diffraction) Method

Peening and burnishing are the well-known cold working mechanical processes in many engineering applications. They induce compressive residual stresses on the surface and subsurface of the components, which enhance the functional performance such as fatigue and corrosion. Different methods to evaluate the surface compressive residual stress (CRS) are destructive (hole drilling, slitting, contour, layer exfoliation, crack compliance, etc.) and non-destructive (X-ray, neutron, and synchrotron diffractions, magnetic and ultrasonic methods, etc.). The non-destructive methods are becoming more popular in recent times because of their high reliability, accuracy, cost, and most importantly, re-use of the sample for further testing. The non-destructive diffraction methods evaluate the residual stresses by measuring the induced crystal lattice strain, based on the Bragg law principle. This review focuses on the surface compressive residual stress characterization of peened and burnished components by different non-destructive diffraction techniques. Additionally, the advantages and limitations of each diffraction method are described. Also, the basic principle and theoretical formulation of non-destructive diffraction techniques in determining residual stresses are discussed. A comparison of the compressive residual stress determined by destructive and nondestructive methods for peened Al 7075 alloy is presented. This review may be a guide for new researchers to identify the most suitable non-destructive diffraction technique according to their requirements.

A current status on the residual stress characterization by the crystal lattice strain (diffraction) method

Peening and burnishing are the well-known cold working mechanical processes in many engineering applications. They induce compressive residual stresses on the surface and subsurface of the components, which enhance the functional performance such as fatigue and corrosion. Different methods to evaluate the surface compressive residual stress (CRS) are destructive (hole drilling, slitting, contour, layer exfoliation, crack compliance, etc.) and non-destructive (X-ray, neutron, and synchrotron diffractions, magnetic and ultrasonic methods, etc.). The non-destructive methods are becoming more popular in recent times because of their high reliabi­lity, accuracy, cost, and most importantly, re-use of the sample for further testing. The non-destructive diffraction methods evaluate the residual stresses by measuring the induced crystal lattice strain, based on the Bragg law principle. This review focuses on the surface compressive residual stress characterization of peened and burnished components by different non-destructive diffraction techniques. Additionally, the advantages and limitations of each diffraction method are described. Also, the basic principle and theoretical formulation of non-destructive diffraction techniques in determining residual stresses are discussed. A comparison of the compressive residual stress determined by destructive and non-destructive methods for peened Al 7075 alloy is presented. This review may be a guide for new researchers to identify the most suitable non-destructive diffraction technique according to their requirements.

An optimization of part distortion for a structural aircraft wing rib: an industrial workflow approach

Inherent residual stresses in the raw material of large monolithic structural components cause distortion when parts are machined. This is a frequent problem in the manufacturing life cycle of aircraft parts that has resulted in recurring concession, rework or scrap, which cost millions of euros to the aerospace industry. In this study an industrial workflow was implemented to predict and optimize part distortion after the machining of a representative structural aircraft-component. This was carried out by means of the following steps: i) characterization of inherent residual stresses of a material using a modified layer removal method; ii) generation of residual stress profiles using part-distortion measurements; and iii) optimization of distortion using Finite Element Analysis to derive an optimal part location. The workflow was validated with three experiments from two aluminum 7050-T7451 specimens with different residual stress states. A prediction of distortion with a Finite Element model resulted in 0.44mm from the higher residual stresses while from the lower residual stresses the maximum distortion was 0.36mm. The experimental testing was carried out in a CNC center to validate the numerical results. Good agreement from the numerical results in regards experimental tests led to propose the optimal part location which decreased the distortion up to 0.12mm.

Innovative rotary swaged Al/Cu laminated wire conductors: characterisation of structure and residual stress

This study presents innovative, uniquely sequenced Al/Cu laminated composite wires having perspective usage for various applications, from transportation to electrotechnics. The composites were processed via the industrially applicable technology of rotary swaging, which was performed at 20 °C and 250 °C to provide the possibility to compare the effects of different swaging conditions on structure characteristics of the wires. The work-pieces were gradually swaged down to the final wire diameters of 5 mm, the total swaging degree for each swaged-piece was 3.6. After swaging, both the aluminium components of the swaged wires exhibited recrystallized ultra-fine-grained (UFG) structures with minor presence of residual stress, certain presence of stress was observed within the refined Cu structures. Microhardness measurements revealed occurring work hardening, especially for the 20 °C swaged composite.

Stress relaxation in asymmetric bistable composites: Experiments and simulations

In the last years, bistable composite structures are finding interest in several aeronautical applications such as power harvesting devices or morphing applications on very small aircraft/drones, not needing servo-activated control systems. Residual stresses, developed upon cooling after curing, leads to warped the composite laminates. Several batches of unsymmetrical and unbalanced [0°/90°] laminates were cured in an autoclave according to a standard temperature cycle, following the pre-preg supplier suggested curing cycle. In order to increase the thermal stresses (and hence the bistability phenomenon), these laminates were removed from the autoclave immediately after the curing reaction and rapidly cooled down at room temperature not applying the indicated cool rate between 2 and 5 °C min−1. During storage at room temperature, thermal stresses changed over time, indicating that asymptotic stress relaxation occurs. The first part of this work looks at residual stress characterization of bistable composite plates measuring the changes of shape observed during room temperature annealing. Rectangular plates were produced and the bistable geometric shapes were accurately assessed using a laser scanner system over several days, in order to monitor the curvature changes due to stress relaxation. Then phenomenological viscoelastic predicting models were proposed for a quick estimate for the strain/stress relaxation phenomenon. The loss of bistability was demonstrated with the help of numerical simulation and experimental testing. The final goal was to gain a better knowledge of the relation between processing and final shape of bistable laminates, in order to make them suitable for application on small air vehicles.

Characterization of residual stress in laser melting deposited CoCrFeMnNi high entropy alloy by neutron diffraction

The dynamic thermal history during the laser melting deposition (LMD) process can induce significant residual stress that can lead to distortion and deteriorate functionality. The residual stress distribution in a bulk CoCrFeMnNi high entropy alloy with a single FCC phase and the dendritic structure fabricated by LMD process was studied using neutron diffraction and finite element modelling to reveal the effects of the thermal constraints, thermal cycles and temperature gradient along the deposition direction. Residual stress was found to decrease as the thermal gradient decreases along the building direction in the LMD process.

Additive/subtractive hybrid manufacturing of 316L stainless steel powder: Densification, microhardness and residual stress

316L stainless steel specimens were manufactured by additive/subtractive hybrid manufacturing (ASHM). The densification level, mi-crostructure, microhardness and residual stress characterization in the different zones of the part were investigated. The density was determined by the Archimedes method, and the density measurement was divided into three regions, namely, the bottom, middle and top zones. The results show that the middle zone has a much smoother melting surface and that a relative density of nearly 100 % was achieved for the part in this study. The hardness profiles at room temperature, along the width and height directions of the cross-section of the top and bottom zones, were also studied. The residual stress was evaluated by X-ray diffraction (XRD) for the 316L SS specimen fabricated by the ASHM process, and it was compared with a specimen manufactured by the additive laser directed energy deposition (DED) process. The results show that the top and bottom zones exhibit tensile stress, and compressive stress occurs in the middle area. Moreover, the residual stress of ASHM shows a slightly smaller trend than that of the simplex DED due to the stress relaxation of the subsequent subtractive milling. These results may offer guidance for ASHM-fabricated 316L parts.

Residual stresses in welded I section steel members

Residual stresses have major influence on the stability of steel members. It is well-known that, due to the differences in manufacturing procedures, the residual stresses for welded and hot – rolled members are rather different. In the same way, despite several studies on residual stresses distribution and magnitude for hot-rolled members were already done, for prismatic and tapered welded I section steel members the investigation is still very scarce. In the current work, an experimental programme on the buckling resistance of tapered members, with different tapering ratios and section heights, was conducted and residual stress measurements were carried out using the sectioning method. Numerical simulations of the heat input during welding were also performed in order to predict the residual stress fields. A statistical analysis of the residual stresses distribution for welded I section members was performed, using the experimental and numerical results, as well as literature data. Systematized information on residual stresses distribution and magnitude was generated and some guidelines for the statistical characterization of residual stresses in welded members were proposed.

In situ characterization of residual stress in glass-to-metal seal

This study presents a new approach based on fiber Bragg grating (FBG) sensors toward measuring instantaneous and continuous changes of residual strain and stress in glass during glass-to-metal (GTM) sealing process. The GTM seal consists of three components which include stainless steel housing, borosilicate sealing glass and Kovar pin. The temperature cycle of sealing process was from room temperature (20 °C) to 880 °C, and then back to 20 °C. It was found that residual strain in glass increased rapidly at the glass transition region and gradually reached an asymptotic value with the decrease of temperature. The axial residual stress measured by the FBG sensor was 194.35 MPa at 20 °C. In addition, finite element modeling (FEM) was conducted to provide theoretical support. Stress modeling revealed that axial residual stress at the location of sensor was 145.47 MPa. The experimental determination of the residual stress is highly consistent with the theoretical calculation.

Residual stress characterization in friction stir welds of alloy 625

Alloy 625 (UNS N06625) welded sheets were evaluated in the present study. Friction stir welding (FSW) was performed in a range of tool rotational speed from 1200 to 200rpm, welding speed from 1.0 to 1.5mm/s and constant axial force. Residual stress states were investigated by X-ray diffraction (XRD). Besides, microstructural features were analyzed by optical microscopy (OM), and scanning electron microscopy (SEM). The FSW application was effective in the Ni-based alloy, promoting different levels of grain refinement, microstructural characteristics, and enhanced microhardness. Results also showed that distinct process parameters led to changes in the joints and distinguishable residual stress distributions. In general, as the tool rotational speed decreased, the grain refinement increased, more homogeneous microstructures and microhardness profiles, and lower residual stresses were achieved within the stirred zones.

Repeatability and stability study of residual magnetic field for domain wall characterization

Residual magnetic field (RMF) for metal material has a relationship with material stress status. Bridging the relationship between macro parameters and material microstructure is an essential step to deal with the characterization of residual stress. This paper applies magneto-optical Kerr microscopy (MOKE) with the help of magneto-optical indicator film (MOIF) to observe and analyze variations of magnetic domain pattern under different stress conditions of high permeability grain oriented electrical steel (HGO steel). Experiment about repeatability is carried under cyclic stress applications and the stability study is proceeded during the relaxation time after release of stress. By using two domain texture features, mean value (MV) of domain wall image intensity linked to domain rotation to characterize RMF amplitude and the angular second moment (ASM) related with domain wall displacement to express the RMF distribution are used for investigation of repeatability and stability. Results show that domain wall displacement is repeatable and stable both under cyclic stress round and during relaxation time after release of stress. It has been found that the situation of domain rotation is varied in different locations, and broad texture area of domain wall images has better repeatability and stability when compared with area with narrow texture of domain wall images. The results provide substantial potential to evaluate the residual stress from the view of characterization of micro-structure.

Characterization of residual stresses and microstructural features in an Inconel 718 forged compressor disc

Abstract Residual stress plays an important part in fabricating commercial aero engine Inconel 718 components for their fatigue properties, reliability and durability. Due to the limitation of Chinese neutron diffraction instrument and lack of test practice and specifications, there is little systematic research on the residual stress of forged compressor disc. X-ray diffraction and neutron diffraction methods were used to measure the residual stress of Inconel 718 forged discs at the surface and in the interior, respectively. Scanning electron microscope and transmission electron microscope were used to characterize the microstructural features. The residual stress state at the disc is in near-surface compression, balanced by tension within the disc core. However, the surface residual stress of disc depends more on the rough machining than on the forging process. Also, the dislocation densities increase with decreasing distance to the surfaces of disc, and the residual stress accelerates dislocation generation and dynamic recrystallization.

A short-wave X-ray diffraction method to evaluate the residual stress of welding joint for 7N01 Al alloy

The residual stress with different degree is inevitable to be introduced during the welding process for high strength 7N01 Al alloys, which will deteriorate the mechanical properties in service. Therefore, a proper characterization method is of vital importance to obtain the detailed residual stress distribution. The conventional X-ray diffraction method can only measure the residual stress of near surface but cannot determine the internal bulk stress with nondestructive way. In the present work, a new Short-Wave X-Ray Diffraction (SWXRD) test methodology was utilized, which can evaluate not only the surface but also the internal residual stress of a crystalline material by applying the monochromatic short-wave X-ray of high-energy. The results show that there is a “M” shaped distribution of residual stress of longitudinal direction (LD) in different affected zone. Furthermore, the characterization of residual stress along weld depth direction demonstrates that the middle part of weld joint is subjected to tensile stress while the rest is subjected to compressive stress.