Residual Strain Measurements Research Articles

Measurement of residual strain developed during solidification and adhesion process of paraffin droplets

Measurement of residual strain developed during solidification and adhesion process of paraffin droplets

Measurement of Residual Stresses in Linear Friction Welded In-Service Inconel 718 Superalloy

Measurement of residual strains by neutron diffraction of linear friction welded Inconel® 718 (IN 718) superalloy acquired from a mid-service aero-engine disk was undertaken in this study. Residual strain and stress throughout the various weld regions including the heat affected zone (HAZ), thermomechanical affected zone (TMAZ) and dynamically recrystallized zone (DRX) were characterized. The residual stresses were observed to increase from the base material to the weld interface, with a peak stress at the weld interface in all orthogonal directions. The trends for residual stress across the weld are in agreement with other work published in literature for solid state welding of aerospace alloys, where high residual stresses were commonly reported at the weld interface.

Residual stress mapping in Inconel 625 fabricated through additive manufacturing: Method for neutron diffraction measurements to validate thermomechanical model predictions

The rapid solidification and subsequent thermal cycles that material is subjected to during additive manufacturing (AM) of a component result in a buildup of residual stresses, which lead to part distortion, and negatively impact the component's mechanical properties. We present a method for using neutron diffraction to validate thermomechanical models developed to predict the residual stresses in Inconel 625 walls fabricated by laser-based directed energy deposition. Residual stress calculations from neutron diffraction measurements depend strongly on the determination of stress-free lattice spacings. After measurement of stressed lattice spacings in Inconel 625 walls, reference samples were obtained by extracting thin slices from the walls and cutting comb-type slits into these slices. Reference lattice spacings were measured in these slices, as well as equivalent slices that were also subjected to stress-relieving heat treatment. These heat treatments changed the reference lattice spacings, and therefore affected residual strain measurements. Further, this study shows the importance of using location-dependent reference lattice spacing, as during AM, the thermal history, and therefore elemental composition and stress-free lattice spacing, vary with position. Residual stresses measured by neutron diffraction along the build direction using comb-type reference samples without heat treatment were in good agreement with thermomechanical modeling predictions.

Full-field measurement of residual strains in cold bent steel plates

With the recent American Association of State Highway and Transportation Officials bridge design code acceptance of cold bent fracture-critical and nonfracture-critical plates and the increasing use of cold bending by the bridge industry, there is a need to better understand and predict residual strains in cold bent steel. However, measuring residual strains is a challenging task since the bending process typically interferes with conventional instrumentation. This paper introduces three-dimensional Digital Image Correlation (DIC) as a new, non-destructive approach to measuring residual strains in cold bent steel that captures full-field strain data with high accuracy. DIC was used to measure residual strains in 18 cold bent 12.7mm thick ASTM A36 steel plates, bent using a press-brake to a constant radius of 102mm with varying angles (10, 20, and 30 degrees) and plate widths (76.2mm and 203mm). The measured residual strains are compared to analytical predictions and both two- and three-dimensional finite element models. A parametric study, using the validated finite element models, is completed for additional widths (12.7mm and 140mm) and thicknesses (6.35mm and 19.1mm) to investigate the effect of these parameters on circumferential strains, contact pressure, strain state, and to make modeling recommendations. Results are used to develop a means of predicting peak residual strains based on the final plate dimensions - a useful quality control measure.

The Residual Strain Measurement of Thin Conductive Metal Wire after Electrical Failure with SEM Moiré

In this study, the residual strain of a thin conductive metal wire on a polymer substrate after electrical failure is measured with SEM moire. Focused ion beam (FIB) milling is applied to fabricate micron moire gratings on the surfaces of constantan wires and the random phase shifting technique is used to process moire fringes. The virtual strain method is briefly introduced and used to calculate the real strain of specimens. In order to study the influence of a defect on the electrical failure of the constantan wire, experiments were conducted on two specimens, one with a crack, while the other one without any crack. By comparing the results, we found that the defect makes the critical beam current of electrical failure decrease. In addition, the specimens were subjected to compression after electrical failure, in agreement with the observed crack closure of the specimen. The successful results demonstrate that the moire method is effective to characterize the full-field deformation of constantan wires on the polymer membrane, and has a good potential for further application to the deformation measurement of thin films.

Compressive, shear, and fracture behavior of CNT reinforced Al matrix composites manufactured by severe plastic deformation

In this work, carbon nanotubes (CNTs) reinforced aluminum (Al) matrix composites are synthesized using Bc equal-channel angular extrusion (ECAP) route and their mechanical behavior is examined under compression and shear deformation. The results show that at room temperature, eight ECAP passes are necessary to achieve the density of the composite where the effect of CNTs in enhancing the mechanical properties become significant. Samples of pure Al are also processed under the same ECAP conditions, and their properties are further examined to facilitate the comparison. The well-densified composites with only 2vol.% of CNTs exhibit an approximately 30% increase in yield strength compared to the pure Al samples. Microstructure data in terms of porosity volume fraction, crystallite size, and dislocation density, along with the residual lattice strain measurements, are used to explain the observed improvements in strength. As measured by X-ray diffraction (XRD), higher levels of dislocation density, smaller crystallite sizes, and larger residual lattice strains are present in Al-CNT than in pure Al samples. Finally, fractographic analysis using scanning electron microscopy is performed revealing that the fracture surfaces of the composite exhibit a more brittle behavior than pure Al samples.

Residual Strain Measurement Using Wire EDM and DIC in Aluminum

In this paper, a technique that combines digital image correlation and wire electrical discharge machining for the measurement of the residual strain is presented. The significant potential benefits of this work include noncontact, high-speed, and online measurement. A ring-core-based groove in the surface of the specimen is cut to release the residual stresses present inside the core. The deformation due to these stresses can be measured by digital image correlation to measure the relieved strains. Moreover, the sizes of the reference areas on the specimens and the tracks of these areas after wire electrical discharge machining are investigated. The proposed method is applied to six different specimens to characterize their relieved strains.

Material Properties of Cold-Formed and Hot-Finished Elliptical Hollow Sections

This paper presents the material properties of both cold-formed and hot-finished elliptical hollow sections. The material properties of the elliptical hollow sections caused by different formation processes are investigated. Tensile coupon specimens extracted from different locations across the cross-sections were tested. The mechanical properties of the cold-formed and hot-finished elliptical hollow sections, such as the initial Young's modulus, 0.2% proof stress (yield stress), ultimate tensile strength, strains at ultimate strength and fracture as well as Ramberg-Osgood parameter that describe the shape of stress-strain curve, were obtained. The mechanical properties of the cold-formed elliptical hollow section obtained from the coupon tests were compared with those of the hot-finished elliptical hollow section. In addition, residual strains in the cold-formed and hot-finished elliptical hollow sections were also measured. The strain measurements involved both the longitudinal and transverse directions across the cross-sections, and the residual strain distributions in the cold-formed and hot-finished elliptical hollow sections were plotted. In this study, the test data obtained from the tensile coupon tests and the residual strain measurements for both cold-formed and hot-finished elliptical hollow sections are valuable information for future analytical, numerical and theoretical studies on elliptical hollow sections.

Slip localization and fatigue crack nucleation near a non-metallic inclusion in polycrystalline nickel-based superalloy

Fatigue crack nucleation at a non-metallic agglomerate inclusion has been studied by high spatial resolution Digital Image Correlation (HR-DIC) and high angular resolution Electron Backscatter Diffraction (HR-EBSD). Spatial and temporal characterization and correlation of deformation with underlying microstructures has been performed, with distributions of plastic strain measured from HR-DIC; and residual stress and density of geometrically necessary dislocations (GND) measured from HR-EBSD. Initial residual stress and GND fields, as a consequence of differing thermal expansivities in the metallic and oxide phases, localized around the agglomerate have been quantified using HR-EBSD. The localization of the pre-existing stress and dislocation states appear to lead to early onset of plasticity upon subsequent mechanical loading. Heterogeneous distributions of plastic strain have been observed in the course of the fatigue test by HR-DIC. Crack nucleation via agglomerate/nickel interface decohesion and particle fracture has been demonstrated and this is correlated with the elevation in strain and dislocation density. The measurements of residual stress, strain, and dislocation density provide key information for the mechanisms of fatigue cracking and the development of damage nucleation criteria in these material systems.

Analysis of Internal-Tin <inline-formula> <tex-math notation="TeX">$\hbox{Nb}_{3}\hbox{Sn}$</tex-math></inline-formula> Conductors for ITER Central Solenoid

Japan Atomic Energy Agency (JAEA) is procuring 100% of the ITER Central Solenoid (CS) conductors. The CS conductor is required to maintain the performance under 60000 pulsed electromagnetic cycles. JAEA tested two internal-tin Nb3Sn conductors for the CS at the SULTAN test facility. As a result of destructive examination, the twist pitches of both of the cables satisfied requirements of the ITER Organization (IO). The current sharing temperatures Tcs of each sample were 6.6 and 6.8 K before cyclic operation, and the Tcs values were 6.8 and 6.9 K after 9700 electromagnetic cycles, including three warm-up/cooldowns, respectively. The Tcs performance of both samples satisfied the IO requirement. The ac losses of CSKO1-C and CSKO1-D were approximately half of typical bronze-route CS conductors at 2 and 9 T. The ac loss at 45.1 kA after the cycling was 1.5 times higher than that without the transport current. An almost constant strain of the jacket was observed after the test as a result of the residual strain measurement. Therefore, the deformation of the cable might have been homogeneous along the conductor axis. Because of the higher Tcs of CSKO1-D than CSKO1-C, JAEA started the manufacturing of the CS conductor with the same specification as CSKO1-D.

Applied, residual strain and texture investigations by means of neutron time-of-flight diffraction combined with acoustic emission detection: Application to a gneiss sample from Forsmark, Sweden

The mechanical behaviour of a gneiss sample from the Forsmark drilling site (Sweden) consisting of quartz, plagioclase (oligoclase), microcline and biotite, is characterized by residual strain and crystallographic preferred orientation measurements. Cyclic in situ uniaxial deformation experiments were carried out on a cylindrical sample at four uniaxial stress levels (55, 84, 114 and 141 MPa) and examined using time-of-flight neutrons and simultaneous recording of acoustic emissions. At each stress level applied, a residual strain scan was performed around the sample axis and residual strains were determined. The residual compressional strain detected for quartz was −1.0 × 10−3, the maximum value of residual strain approached 1.2 × 10−3. The onset of acoustic emissions is an indicator of the previous in situ maximum stress. Comparison of acoustic emission data with applied and residual stress values at different load levels is the key to understand the freezing-in mechanism of residual stress and the Kaiser Effect in rock.

Determination and mitigation of the uncertainty of neutron diffraction measurements of residual strain in large-grained polycrystalline material.

For large-grained samples it is advantageous to perform pairs of neutron diffraction measurements at the same spatial location but rotated 180° around the geometric centre of the gauge volume as a means of minimizing the scatter coming from the random positioning of grains within the gauge volume.

Design and characterization of thick InxGa1-xAs metamorphic buffer layers grown by hydride vapor phase epitaxy

Thick InxGa1-xAs metamorphic buffer layers (MBLs) grown by hydride vapor phase epitaxy (HVPE) were studied. Relationships between MBL properties and growth parameters such as grading rate, cap layer thickness, final xInAs, and deposition temperature (TD) were explored. The MBLs were characterized by measurement of in-plane residual strain (妦), surface etch pit density (EPD), and surface roughness. Capping layer thickness had a strong effect on strain relaxation, with thickly capped samples exhibiting the lowest 妦. EPD was higher in samples with thicker caps, reflecting their increased relaxation through dislocation generation. 妦 and EPD were weakly affected by the grading rate, making capping layer thickness the primary structural parameter which controls these properties. MBLs graded in discrete steps had similar properties to MBLs with continuous grading. In samples with identical thickness and 10-step grading style, 妦 increased almost linearly with final xInAs, while total relaxation stayed relatively constant. Relaxation as a function of xInAs could be described by an equilibrium model in which dislocation nucleation is impeded by the energy of the existing dislocation array. EPD was constant from xInAs = 0 to 0.24 then increased exponentially, which is related to the increased dislocation interaction and blocking seen at higher dislocation densities. RMS roughness increased with xInAs above a certain strain rate (0.15%/µm); samples grown below this level possessed large surface hillocks and high roughness values. The elimination of hillocks at higher values of xInAs is attributed to increased density of surface steps and is related to the out-of-plane component of the burgers vector of the dominant type of 60° dislocation. TD did not affect 妦 for samples with a given xInAs. EPD tended to increase with TD, indicating dislocation glide likely is impeded at higher temperatures.

Residual Stress Analysis in Deep Drawn Twinning Induced Plasticity (TWIP) Steels Using Neutron Diffraction Method

In Twinning Induced Plasticity (TWIP) steels, delayed fracture occurs due to residual stresses induced during deep drawing. In order to investigate the relation between residual stresses and delayed fracture, in the present study, residual stresses of deep drawn TWIP steels (22Mn-0.6C and 18Mn-2Al-0.6C steels) were investigated using the finite element method (FEM) and neutron diffraction measurements. In addition, the delayed fracture properties were examined by dipping tests of cup specimens in the boiled water. In the FEM analysis, the hoop direction residual stress was highly tensile at cup edge, and the delayed fracture was initiated by the separation of hoop direction and propagated in an axial direction. According to the neutron diffraction analysis, residual stresses in 18Mn-2Al-0.6C steel were about half the residual stresses in 22Mn-0.6C steel. From the residual strain measurement using electron back-scatter diffraction, formation of deformation twins caused a lot of grain rotation and local strain at the grain boundaries and twin boundaries. These local residual strains induce residual stress at boundaries. Al addition in TWIP steels restrained the formation of deformation twins and dynamic strain aging, resulting in more homogeneous stress and strain distributions in cup specimens. Thus, in Al-added TWIP steels, residual stress of cup specimen considerably decreased, and delayed fracture resistance was remarkably improved by the addition of Al in TWIP steels.

Development and Evaluation for Microdamage of Concrete under Uniaxial Compressive Load

Development of microcracks and microstructures under uniaxial compressive load were studied by the measurement of axial residual strain and the calculation of specific crack area. Two opposite mechanisms could be distinguished for describing the development of microstructures, i.e. densification and micro-crack formation. Based on the experimental results, consistent tendency was found for the development of inside microcracks and overall deformation. Higher load level induced larger residual strain and residual specific crack area after unloading. Significant difference in the development of cracks was found for concrete specimens of different proportions. Specimen with higher water to binder ratio and lower strength developed earlier corresponds to lower initial stress-strength ratio for the beginning of crack propagation, and lower critical stress level for the beginning of unstable development. Effect of uniaxial compressive load on the durability of concrete specimen depends on the relative relationship between the two mechanisms.

Optimization of a multi-channel parabolic guide for the material science diffractometer STRESS-SPEC at FRM II

Neutron strain diffractometers usually use slits to define a gauge volume within engineering samples. In this study a multi-channel parabolic neutron guide was developed to be used instead of the primary slit to minimise the loss of intensity and vertical definition of the gauge volume when using slits placed far away from the measurement position in bulky components. The major advantage of a focusing guide is that the maximum flux is not at the exit of the guide as for a slit system but at the focal point relatively far away from the exit of the guide. Monte Carlo simulations were used to optimise the multi-channel parabolic guide with respect to the instrument characteristics of the diffractometer STRESS-SPEC at the FRM II neutron source. Also the simulations are in excellent agreement with experimental measurements using the optimised multi-channel parabolic guide at the neutron diffractometer. In addition the performance of the guide was compared to the standard slit setup at STRESS-SPEC using a single bead weld sample used in earlier round robin tests for residual strain measurements.

Thin, high quality GaInP compositionally graded buffer layers grown at high growth rates for metamorphic III–V solar cell applications

The metamorphic growth of lattice-mismatched materials has allowed optimizing the bandgap combination in multijunction solar cells for the solar spectrum under consideration. Buffer structures are used to accommodate the lattice-mismatch by introducing dislocations and relaxing the material in a controlled way. However, the metamorphic buffers typically involve significant growth time and material usage, which increases the cost of these solar cells. In this work, the thinning of buffer structures with continuously, linearly graded misfit is addressed with the goal of increasing the cost-effectiveness of metamorphic multijunction solar cells. The relaxation dynamics and quality of the buffer layers analyzed were assessed by in-situ stress measurements and ex-situ measurements of residual strain, threading dislocation density and surface roughness. Their ultimate quality has been tested using these buffers as templates for the growth of 1eV Ga0.73In0.27As solar cells. The deleterious effect of thinning the grade layer of these buffer structures from 2 to 1μm was investigated. It is shown that prompting the relaxation of the buffer by using a stepwise misfit jump at the beginning of the grade layer improves the quality of the thinned buffer structure. The residual threading dislocation density of the optimized thin buffers, grown at a high growth rate of 7μm/h, is 3×106cm−2, and solar cells on these buffers exhibit near-ideal carrier collection efficiency and a Voc of 0.62V at 1-sun direct terrestrial spectrum.

Optimising parameters for meaningful measurement of residual strain by neutron diffraction in self-pierce riveted joints

The aim of this work is to examine the uncertainties involved in measuring residual strain and their dependence on both the gauge volume of the neutron beam and the acquisition time in self-pierce riveted (SPR) joints while using the neutron diffraction technique. The main challenge involved dealing with the very small dimensions of SPR joints and developing optimum instrument configuration that allowed faster and/or more accurate stress measurement in SPR while maintaining the same time resolution required for the millimetre scale of the problem. Two different gauge volumes were used (0·125 and 1·0 mm3), and two different measuring directions were chosen in order to examine the rotational accuracy of the sample table. All measurements were conducted in steel‐steel joints. Even though small sizes were involved, meaningful results were obtained and measurement errors were reduced by optimising the instrument parameters.

Neutron Characterization for Additive Manufacturing

Abstract Neutron-based imaging and measurement techniques are an ideal complement to x-ray methods for characterizing additively manufactured materials and components. This article presents examples showing how neutron radiography and residual strain measurements reveal the internal structure and stress distributions in highly engineered turbine blades produced by additive processes. It also discusses the development of neutron characterization tools at ORNL and a recent effort in which they were used to evaluate stress-relief treatments for large-scale metal AM structures.

Anelasticity in austenitic stainless steel

Time-dependent anelastic deformation mechanisms arise in austenitic stainless steel when load is removed during a high-temperature creep test. This phenomenon is investigated by conducting creep tests, with intermittent load removal, on AISI Type 316H austenitic stainless steel at 550 and 650°C, supported by in situ measurement of creep-induced intergranular residual strains by neutron diffraction. All the cyclic tests exhibit anelastic behaviour on unloading and develop substantially lower load-on creep strain rates, reduced ductility and longer rupture times than baseline steady-load creep tests at similar conditions. The mechanisms underlying the observed anelastic behaviour and changes in macroscopic creep properties are discussed with reference to the development of intergranular strains and dislocation behaviour.