Material Residual Stresses Research Articles

Strain Scanning in Australia: Instrumentation and Recent Experiments

Neutron scattering using diffraction techniques is now recognised as the most precise and reliable method of mapping sub-surface residual stresses in materials and industrial devices. It was therefore decided to build the dedicated strain scanner KOWARI among the first suite of instruments for the new Australian Replacement Research Reactor OPAL. In order to support the new instrument and familiarize the Australian user community with the technique a „Neutrons for Engineering“ project has been established to provide a fully integrated residual stress service. To make use of the neutrons at Australias HIFAR reactor the existing three-axis spectrometer has been refurbished and modified to allow strain measurements. The Australian Strain Scanner (TASS) will operate until the new strain scanner becomes operational in 2006. Apart from giving an overview about the technique and properties of the new strain scanner we would like to present and discuss two case studies: a) residual-stresses obtained from a break-disc of a car and b) residual-stresses within a flash-butt welded plate cut out of a railway.

A technique for measuring stresses in small spatial regions using cube-corner indentation: Application to tempered glass plates

Vickers indentation cracks have been used to estimate residual stress in materials; however, a high threshold load for cracking limits the smallest spatial region for stress measurement. Cube-corner indenters have a lower included angle, and their sharpness leads to lowered cracking thresholds enabling stress measurement in small spatial regions. Cube-corner indentations on tempered glass plate and on annealed soda-lime-silica glass revealed that crack surface traces on the tempered material were significantly smaller. Cracks were found to be quarter-penny shaped as opposed to half-penny/radial for Vickers indentation. Using an appropriate stress-intensity factor and a stress-intensity factor superposition approach, surface stresses in the tempered plate were calculated. The stresses were in good agreement with those determined using well-established Vickers indentation approach; however, the spatial region sampled is 6–10 times smaller. An estimate of the smallest spatial region at which a particular stress may be measured using this technique is presented.

Residual stress diffractometer KOWARI at the Australian research reactor OPAL: Status of the project

Neutron scattering using diffraction techniques is now recognized as the most precise and reliable method of mapping sub-surface residual stresses in materials or even components, which are not only of academic but also of industrial-economic relevance. The great potential of neutrons in the field of residual stresses was recognized by ANSTO and its external Beam Instrument Advisory Group for the new research reactor OPAL. The recommendation was to build the dedicated strain scanner KOWARI among the first suite of instruments available to users. We give an update on the overall project and present the current status of the diffractometer. It is anticipated that the instrument will be commissioned in mid 2006 and available to users at the end of the OPAL project.

Development of Instrument for Residual Stress Measurement in a 50 mm Inner Diameter Power Generation Pipes

With recent issues related to stress corrosion cracking (SCC) and thermal fatigue of power plant components, including nuclear steam generator tubing, condenser tubing, boiler tubes steam turbine blades, the residual stress in materials plays an important role in determining those susceptible for failure caused by fatigue or SCC. These residual stresses, combined with the applied stresses inherited from manufacturing and assembly can be tensile or compressive. The SCC can act in a corrosive environment only when the stresses in the material are tensile, thus leading to cracking of the component and the structure. Thermal fatigue can be originated from thermal gradients caused during the operation of power plants. Proto Manufacturing Limited, leader in the residual stress measurement and equipment development tailored for these applications has developed instruments capable of measuring residuals tresses in confined areas such as inner diameter of tubes as small as 50 mm in diameter, non-destructively in the direction of interest, axial direction. In this paper interesting examples of new x-ray diffraction instrument and measurement validation are presented along with application inside a 50 mm I.D.

3-D FEM simulation of laser shock processing

Laser shock processing is an innovative surface treatment technique similar to shot peening. It can impart compressive residual stresses in material for improving fatigue, corrosion and wear resistance of metals. FEM simulation is an effective method to predict mechanical effects induced in the material treated by laser shock processing. An analysis procedure including dynamic analysis performed by LS-DYNA and static analysis performed by ANSYS is presented in detail to attain the simulation of the single and multiple laser shock processing to predict the residual stress field and the surface deformation. History of the energies during dynamic analysis is analyzed and validated by the theoretical calculation. The predicted residual stress field for single laser shock processing is well correlated with the available experimental data and a homogeneous depression with little roughness modification in the action zone of the shock pressure is induced on the treated surface according to the simulation of surface deformation. Simulation of multiple laser shocks is also performed, which indicates that compressive residual stresses and plastically affected depth can be extensively increased and gradually reach the saturated state with the increase of laser shock number.

Solution of the moiré hole drilling method using a finite-element-method-based approach

The moiré hole drilling method in a biaxially loaded infinite plate in plane stress is an inverse problem that exhibits a dual nature: the first problem results from first drilling the circular hole and then applying the biaxial loads, while the other problem arises from doing the opposite, i.e., first applying the biaxial load and then drilling the circular hole. The first problem is hardly ever addressed in the literature but implies that either separation of stresses or material property identification may be achieved from interpreting the moiré signature around the hole. The second is the well-known problem of determination of residual stresses from interpreting the moiré fringe orders around the hole. This paper addresses these inverse problem solutions using the finite element method as the means to model the plate with a hole, rather than the typical approach using the Kirsch solution, and a least-squares optimization approach to resolve for the quantities of interest. To test the viability of the proposed method three numerical simulations and one experimental result in a finite width plate are used to illustrate the techniques. The results are found to be in excellent agreement. The simulations employ noisy data to test the robustness of this approach. The finite-element-method-based inverse problem approach employed in this paper has the potential for use in applications where the specimen shape and boundary conditions do not conform to symmetric or well-used shapes. Also, it is a first step in testing similar procedures in three-dimensional samples to assess the residual stresses in materials.

Application of EDM Hole-Drilling Method to the Measurement of Residual Stress in Tool and Carbon Steels

This study adopts the application of the electrodischarge machining (EDM) hole-drilling method to the measurement of residual stress in AISI D2 cold work tool steel, AISI H13 hot work tool steel, and AISI 1045 medium carbon steel. A calibration procedure based on the thermal conductivity of the material is conducted to compensate for the additional compressive stress induced in the workpiece by the EDM hole-drilling operation. Since the formation of this white layer influences the magnitude of the induced stress, the scanning electron microscopy, transmission electron microscopy, and nanoindentation techniques are used to examine the microstructure and hardness of the white layer resolidified on the EDMed surface. The experimental results reveal that combination of the hole-drilling strain-gage method (ASTM standard E837) with an EDM drilling process provides the effective means of determining the residual stress in materials with high hardness and good wear resistance.

Residual stress measurement with focused acoustic waves and direct comparison with X-ray diffraction stress measurements

The technique of measuring small changes in acoustic wave velocity due to external or internal stress has been used for quantitative determination of residual stress in materials during the last decade. Application of similar methodology with focused acoustic waves leads to residual stress measurement with spatial resolution of a few millimeters to a few microns. The high spatial resolution residual stress measurement required development of new methodologies in both the design of acoustic lenses and the instrumentation for acoustic wave velocity determination. This paper presents two new methodologies developed for the measurement of residual stress with spatial resolution of a few millimeters. The design of new type of acoustic lens for achieving higher spatial resolution in residual stress measurement is introduced. Development of instrumentation for high precision local surface wave velocity measurement will be presented. Residual stresses measured around a crack tip in a sample of Ti–6A1–4V using a focused beam will be compared with X-ray diffraction measurements performed on the same region of the sample. Results of residual stress measurements along a direction perpendicular to the electron beam weld in a sample of Ti–6A1–4V, determined using focused acoustic waves and X-ray diffraction technique, are also presented. The spatial resolution and penetration depth of X-rays and focused acoustic beams with reference to residual stress measurements are discussed.

In Situ X‐Ray Diffraction Study of Electric‐Field‐Induced Domain Switching and Phase Transition in PZT‐5H

In situ X‐ray experiments were conducted to examine the electric‐field‐induced phase changes in PZT‐5H materials. The X‐ray diffraction profiles at different electric‐field levels were analyzed by peak fitting and were used to identify the occurrence of non‐180° domain switching and phase transition. We found that in depolarized samples, there exists a threshold electric field for the phase changes, whereas in polarized samples, no such threshold exists. The profound difference in the diffraction profile changes under positive and negative electric fields in polarized samples is responsible for the asymmetry of piezoelectric effects. Peak fitting results show phase transition as well as domain switching at different electric‐field levels. These observations further indicate the importance of residual stresses in materials' behavior.

Ion irradiation effects on the mechanical stability of compressed metallic thin films

Residual stresses in materials and components such as thin films in microelectromechanical systems are developed during the production process and may drastically limit their functionality and lifetime; their minimization is thus of utmost technical importance. Ion beam irradiation in polycrystalline Mo thin films sputter-deposited on (100) silicon wafer at room temperature has been used to reduce the residual stresses present in the layer and thus to improve the mechanical stability of the film/substrate set. Although a strong reduction of residual stresses in the whole film is evidenced, irradiation affects the film-substrate bond strength; defect creation at the film-substrate interface due to ion implantation induces thin-film delamination.

Contrast enhancement of ultrasonic imaging of internal stresses in materials

The ultrasonic methods, which detect applied or residual stress in materials, are based on nonlinear interaction of a small dynamic disturbance (acoustic waves) with the pre-deformed state of the solid. This weak phenomenon (acoustoelasticity) leads to a dependence of acoustic wave velocities on the initial stress, and a stress-induced anisotropy in the acoustical properties of the material. In anisotropic media, the transversal wave velocity depends on its polarization. The amplitude of the conical polarized shear wave, propagating through a plate specimen, is sensitive to pre-stress due to acoustoelastic birefringence. The resulting scan image is created by variations of the amplitude. The previous description is a basic principle of the approach used for stress mapping in Al-alloys by time-resolved acoustic microscopy. Disk specimens with central stress concentrators are loaded step by step. The acoustic scans are created during each loading step. Thermal stress detection is also shown on specimens with an Invar core. The original image processing procedure has been developed to improve edge detection of obtained stress maps. The acoustic images are compared with theoretically predicted isocline contours. The inherent material anisotropy and the structural inhomogeneities influence significantly the acoustoelastic measurements. Advantages and limitations of the nondestructive technique are summarized on the basis of presented experimental results.

The electronic laser interferometry and laser heating method for residual stress determination

Residual stress is one of the causes which makes defects in engineering components and materials. These residual stresses can occur in many engineering structures and can sometimes lead to premature failures. There are commonly used methods by which residual stresses are currently measured. But these methods have a little damage and other problems; therefore, a new experimental technique has been devised to measure residual stress in materials with a combination of electronic laser interferometry, laser heating and finite element method. The electronic laser interferometer measures in-plane deformations while the laser heating and cooling provides for very localized stress relief. FEM is used for determining the heat temperature and other parameters. The residual stresses are determined by the amount of strain that is measured subsequent to the heat-up and cool-down of the region being interrogated. A simple model is presented to provide a description of the method. In this paper, the ambiguity problem for the fringe patterns has solved by a phase shifting method.

Surface Rayleigh Waves in the Determination of Near-Surface Stresses of Structural Elements

A supersonic nondestructive method for determining stresses in near-surface layers of solids is stated on the basis of the acoustoelastic theory of surface Raylegh waves. Examples are presented of how this method is used to determine the operating and residual stresses in materials and structural elements. Features of the mutual use of surface and volume waves to obtain additional information on the stress distribution over the cross section of a specimen are indicated

A Study of Process-Induced Residual Stress in PBGA Packages

Process-induced residual stresses can play a significant role in the reliability of electronic components and packages. In this paper, a practical method is developed to determine residual stresses for electronic packaging. In this method, blind holes are drilled into the specimens and relationships are established, between the released surface displacements and the corresponding residual stress, by introducing a set of calibration coefficients. A multilayer 3D-FEM model is established to determine the relevant calibration coefficients. This methodology, which combines moire´ interferometry and the incremental hole drilling method, can provide an accurate determination of residual stresses in materials and structures by precisely controlled incremental blind-hole drilling and an accurate determination of the surface in-plane displacement fields in the hole drilling region. The methodology is implemented by investigating the residual stress in the Plastic Ball Grid Array (PBGA) packages. The tensile residual stresses are determined in both the plastic molding compound and the glass/epoxy laminate chip carrier. The method is accurate, simple, convenient, and practical. More applications, in residual stress determinations and in process evaluations in electronic packaging, are anticipated. [S1043-7398(00)00103-1]

Finite element simulation of laser shock peening

Laser shock peening is a technique similar to shot peening that imparts compressive residual stresses in materials for improved fatigue resistance. During a laser shock peening event, pressures well above the dynamic yield strength of the material are imparted on the target in a fraction of a microsecond. The severity of the loading causes local plastic deformation which ultimately results in the development of the favorable residual stresses. Finite element analysis techniques have been applied to predict the residual stress induced from laser shock peening. The analytical development, including the loading history and the elastic–plastic constitutive model, is discussed. Analysis results are presented and correlated with experimental data.

Measurement of Residual Stresses by Load and Depth Sensing Spherical Indentation

AbstractThe finite element method was used to determine whether load and depth sensing indentation with spherical indenters may be useful in the measurement of residual stresses in materials. The spherical indentation process for a wide range of elastic/ideal-plastic materials to which compressive and tensile biaxial stresses were applied was simulated using standard finite element techniques. The elastic moduli and yield stresses of the materials were varied systematically to model the behavior of a wide variety of metals and ceramics. Elastic-ideal-plastic materials were considered in the study with residual stress levels varied from zero up to the yield stress. All three indentation regimes - elastic, elastic-plastic, and plastic - were examined, with emphasis given to the elastic and the early part of the elastic-plastic regimes, where differences in the load-displacement characteristics caused by residual stress were found to have a significant effect. Systematic examination of the relationships among residual stress, contact pressure, and elastic recovery revealed a simple, measurable indentation parameter which correlates well with the residual stress. Using this parameter, an experimental technique is proposed by which residual stresses can be estimated from measurements of the load of an indentation in the elastic-plastic regime, the yield stress, and the elastic modulus of the material, all of which can be determined by load and depth sensing indentation methods. Based on a critical examination of the technique by finite element simulation, the technique appears promising.

Characteristics of residual stress generated by MoSi2 plasma thermal spraying

Summary This paper describes an investigation of plasma thermal spraying of MoSi2 offering a high melting point together with excellent high-temperature oxidation resistance. MoSi2 powder was deposited by low-pressure plasma thermal spraying on the surfaces of blasted SS400 substrate specimens precoated with 50%Ni-50%Cr. The residual stress generated in the specimens was then determined. Compressive stress is generated in the MoSi2 sprayed deposit. Tensile stress is generated in the 50%Ni-50%Cr sprayed deposit except at the SS400 substrate surface. A series of experiments was conducted to clarify the validity of the residual stress distribution obtained. These results suggest the following: 1. When only blasting treatment is performed, compressive stress is generated from the upper surface to 0.3 mm inside. Minor tensile stress is also generated inside. 2. When 50%Ni-50%Cr is deposited after blasting treatment, the residual stress is tensile in the 50%Ni-50%Cr layer and compressive on the substrate upper surface. Tensile stress is found inside. This is due to the fact that the substrate restrains the contraction of the sprayed deposit with a high thermal shrinkage. The validity of this result is also supported by the fact that the compressive stress near the substrate upper surface is greater than when only blasting treatment is performed. 3. When MoSi2 is deposited after blasting treatment, compressive stress is generated in the MoSi2 layer. This is due to the fact that, despite the MoSi2 layer having a higher temperature than the substrate, the linear thermal expansion coefficient of MoSi2 is much smaller than that of the substrate, so that the MoSi2 layer restrains the contraction of the substrate. 4. During estimation of the residual stress in materials deposited by plasma thermal spraying, it is important to ensure that the modulus of elasticity of the sprayed deposit is measured separately for each set of thermal spraying conditions.

On the measurement of residual stress through changes in critical angle

The critical angle method of measuring residual stresses in materials, sometimes referred to as critical angle reflectivity, has been reexamined with an emphasis on improving the sensitivity of the technique by a judicious choice of reference material. The sensitivity of the critical angle to prestress level was found to be dependent upon the ratio of phase velocities of the incident wave to that of the critically refracted wave in an unstressed medium. The sensitivity is shown to increase monotonically as this ratio approaches unity. Some numerical results are tabulated for the expected change in critical angle per MPa of prestress for various reference/prestressed material combinations for various materials of industrial interest.

Editorial

Residual Stresses in material, tensile or compressive, are pent up furies that are released, unpredictably, over a period of time depending much upon temperature, ambient or applied. Tensile residual stresses add-on to applied stresses thereby increasing actual stress-situation in a component could promote premature failure - even more sounder the conjoint action of cyclic loading and ‘the environment’. On the other hand compressive residual stresses would hinder (surface) crack nucleation, somewhat obviating the situation In the material as under tensile residual stresses. In this issue of the IWJ authors of two papers, dedicated to the generic terms related to residual stresses in material ( P-7 & 24 ), describe in worthy details with wealth of references, difficulties both in theoretical modeling and in data accumulation processes. Readers, particularly In R & D works, would find useful references.

Thermal residual stresses in spray atomized and deposited Ni 3Al

Over the past decade, spray atomization and deposition has generated considerable interest in materials field as a viable processing alternative for the synthesis of structural materials, and has been successfully applied to numerous alloy systems. It is well known that most manufacturing and fabrication processes induce some level of residual stresses in materials. The distribution and magnitude of these residual stresses vary drastically from one manufacturing process to another. It is also well established that residual stresses have a significant effect on the mechanical properties of the materials. The undesirable effects caused by the residual stresses typically include diminished fatigue resistance, shape distortion, and accelerated stress corrosion. In addition, the presence, the sign, and the magnitude of the residual stresses affect the yield strength and the fracture toughness of the materials.