Stress-free Samples Research Articles

An indentation method for measuring welding residual stress: Estimation of stress-free indentation curve using BP neural network prediction model

The measurement of welding residual stress (WRS) is necessary to formulate the stress control strategies, in order to ensure the reliability of engineering structures during their service life. The instrumented indentation technique can detect surface residual stress nondestructively with an acceptable accuracy, making it a desirable option for in-situ applications. However, it is difficult to meet the requirement for stress-free samples, particularly in the case of welded joints with non-uniform mechanical properties. In this work, the impact of stress-free indentation curves on determining WRS is furtherly studied based on the previously proposed indentation energy difference method. The WRS in a mismatched welded plate is measured using single reference method and distributed reference method, respectively. The study suggests that using the single reference method can lead to significant errors in determining the sign and magnitude of WRS in the weld zone. A prediction model combining the continuous spherical indentation (CSI) method and Back Propagation (BP) neural network is proposed to obtain the stress-free indentation curves of local positions nondestructively. And the CSI-BP method is verified through reverse finite element analysis and experiments.

Determination of biaxial residual stresses via indentation energy difference method

<p indent="0mm">In this study, a novel method is proposed to estimate biaxial residual stress (RS) by using the indentation energy difference between the stressed and the stress-free samples. A Knoop indenter is applied along the directions of principal stresses, and the indentation energy obtained from the load-depth (<italic>F</italic>-<italic>h</italic>) curves is used as the calculation parameter without observing the indentation morphology. A work-energy transformation model is constructed to explain the change in the <italic>F</italic>-<italic>h</italic> curve with the RS in terms of energy. Then, the finite element analysis of different power-law hardening materials under different RS combinations is implemented to study the energy evolution. Consequently, the formula for calculating the indentation energy difference under a uniaxial RS is established. Meanwhile, the indentation energy difference of the Knoop indenter exhibits superposition and direction characteristics, which can be used to convert the indentation energy difference under a biaxial RS into the sum of that under two equivalent uniaxial stresses. Through this conversion, the two stress components can be calculated from the two indentation energy difference formulas. An indentation testing device is developed to verify the proposed method. The results obtained using the proposed method agree well with those obtained using the indentation strain-gage method.

Revealing the residual stress distribution in laser welded Eurofer97 steel by neutron diffraction and Bragg edge imaging

• Through-thickness residual stress distribution in three dimensions is evaluated. • High-resolution residual strain is mapped by neutron Bragg edge imaging. • Location-dependant reference lattice spacing is first applied on neutron imaging. • Correlation between microstructure, residual stress and micro-hardness is studied. Eurofer97 steel is a primary structural material for applications in fusion reactors. Laser welding is a promising technique to join Eurofer97 plasma-facing components and overcome remote handling and maintenance challenges. The interaction of the induced residual stress and the heterogeneous microstructure degrades the mechanical performance of such fusion components. The present study investigates the distribution of residual stress in as-welded and post-heat treated Eurofer97 joints. The mechanistic connections between microstructure, material properties, and residual stress are also studied. Neutron diffraction is used to study the through-thickness residual stress distribution in three directions, and neutron Bragg edge imaging (NBEI) is applied to study the residual strain in high spatial resolution. The microstructures and micro-hardness are characterised by electron backscatter diffraction and nanoindentation, respectively. The M-shaped residual stress distribution through the thickness of the as-welded weldment is observed by neutron diffraction line scans over a region of 1.41 × 10 mm 2 . These profiles are cross-validated over a larger area (∼56 × 40 mm 2 ) with the higher spatial resolution by NBEI. The micro-hardness value in the fusion zone of the as-welded sample almost doubles from 2.75 ± 0.09 GPa to 5.06 ± 0.29 GPa due to a combination of residual stress and cooling-induced martensite. Conventional post weld heat treatment (PWHT) is shown to release ∼ 90% of the residual stress but not fully restore the microstructure. By comparing its hardness with that of stress-free samples, it is found that the microstructure is the primary contribution to the hardening. This study provides insight into the prediction of structural integrity for critical structural components of fusion reactors.

Experimental and numerical analysis of Pitting Corrosion in CUSTOM 450 Stainless Steel

The present study aimed at the propagation of the pit in CUSTOM 450 Stainless Steel blades after the initiation stage. The depth of pits was measured by Eddy Current and the relationship between pitting corrosion time and pit depth was determined based on simulation and compared to the experimental results. Moreover, the electrolyte potential and the local strain distribution around the pit were displayed at different potentials and times. The pit growth increased rapidly for a certain period and then declined. For stressed and stress-free samples, the local strain distribution and pit depth growth were compared at 350 mVSCE potential. Based on the results, the presence of stress in the specimens accelerated the pitting corrosion growth in CUSTOM 450 stainless steel. To calculate the stresses on the blade surface, the model of the blade was constructed and two adjacent pits were created at the location of the fractured surface. The results show that the pits are turned into stress cracks after about 4 months corresponding to the result of the other references. To make a connection between accelerated pitting corrosion tests and the operating conditions of the gas turbine, the comparison of pitting time (simulation) was made for the stressed sample at 350 mVSCE potential and sample with real operating conditions.350 mVSCE potential and sample with real operating conditions.

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.

Enhanced age hardening response and precipitation evolution of elastic stress aged Mg–Zn alloys

How to improve the age hardening response of Mg alloys in economical and effective ways is an important and interesting issue. The age hardening response and precipitation evolution of Mg–Zn alloys with and without external elastic stress are investigated. Microstructural observations of stress-free aged samples show that the low hardness is mainly due to the low number density and relatively small aspect ratio of β1′ precipitates, while a large number of nano-scale strengthening β1′ precipitates appear in the α-Mg matrix at the early stage of stress aging. In addition, the aspect ratio of β1′ precipitates in the stress aged samples is evidently higher than that of the stress-free aged samples at the same peak aged stage. Abundant dislocations introduced by the applied stress act as nucleation sites of precipitates and rapid diffusion path of solutes, which are mainly responsible for the remarkable increment in hardness and promoted kinetics. Precipitate-free zones ( PFZs) are observed in both samples with and without stress. In the stress aged samples, the formation of PFZs is mainly controlled by the motion of dislocations, which is different from the solute-vacancy depletion mechanism in the stress-free samples. Results imply that the external stress plays a similar role to that of commonly used methods in reducing the size, increasing the density, and promoting the precipitation kinetics, but has a negligible stress-orienting effect on the precipitate evolution.

4H-SiC MOSFET Source and Body Laser Annealing Process

This work describes the development of a new post-implant crystal recovery technique in 4H-SiC using XeCl (l=308 nm) multiple laser pulses in the ns regime. Characterization was carried out through micro-Raman spectroscopy, Photoluminescence (PL), Transmission Electron Microscopy (TEM) and outcomes were than compared with 1h thermally annealed at 1650-1770-1750 °C P implanted samples (source implant) and P and Al implanted samples for 30 minutes at 1650 °C (source and body implants). Experimental results demonstrate that laser annealing enables crystal recovery in the energy density range between 0.50 and 0.60 J/cm2. Unlike the results obtained with thermal annealing where stress up to 172 Mpa and high carbon vacancies (Vc) concentration is recorded, laser annealing provides almost stress free samples and much less defective crystal avoiding intra-bandgap carrier recombination. Implant was almost preserved except for step-bouncing and surface oxidation phenomena leading to surface roughening. However, the results of this work gives way to laser annealing process practicability for lattice damage recovery and dopant activation.

NALYTICAL STUDIES CONCERNING RESISTANCE OF STRESSED ROCKS TO DISINTEGRATION

Purpose. State-of-the art review concerning the effect of stress-strain state of the rock upon its resistance to mechanical breaking is represented. Methods. It is highlighted that while calculating efficient breaking modes of complex stressed rock and parameters of rock-breaking machine organs, it is required to consider the degree of natural stress of the near-face rock under breaking. Results. Necessity to take into account the rock stress degree has been substantiated as the index of breaking strength of stress-free rock samples differs considerably from the one obtained with the consideration of stress state of the natural rock mass. The simple strock in terms of rheology (i.e. linear-elastic one) has been analyzed. Scientific novelty. In this context, it is taken into consideration that simultaneous availability of all the components of stress tensor due to natural and external forces does not make the rock to be plastic. It means that the rock is within the frameworks of elastic deformations. In this context, potential deformation energy accumulated within the rock volume unit is specified as the basis to determine the reduced stress in terms of which the ultimate elastic state of that volume takes place. Practical importance The amount of cone feed to the bottomhole of stressed rocks is limited by the value of the safe penetration rate, depending on the degree of rock tension.

Influence of stress aging process on variants of nano-Ni4Ti3 precipitates and martensitic transformation temperatures in NiTi shape memory alloy

In this study, the effect of a stress aging process on the microstructure and martensitic phase transformation of NiTi shape memory alloy has been investigated. NiTi samples were aged at 450 °C for 1 h and 5 h under different levels of external tensile stress of 15, 60 and 150 MPa. Transmission electron microscopy (TEM) was used to characterize different variants and morphology of precipitates. The results show that application of all stress levels restricts the formation of precipitates variants in the microstructure after 1 h stress aging process. However, all variants can be detected by prolonging aging time to 5 h at 15 MPa stress level and the variants formation is again restricted by increasing the stress level. Moreover, the stress aging process resulted in changing the shape of precipitates in comparison with that of the stress-free aged samples. Coffee-bean shaped morphologies were detected for precipitates in all stress levels. According to the Differential Scanning Calorimetry (DSC) results, the martensite start temperature (Ms) on cooling shifts to higher temperatures with increasing the tensile stress during the aging process. This can be related to the change of austenite to martensite interface energy due to the different volume fractions and variants of precipitates.

Propagation of threading dislocations in heteroepitaxial diamond films with (111) orientation and their role in the formation of intrinsic stress

In the present study, systematic correlations were revealed between the propagation direction of threading dislocations, the off-axis growth conditions, and the stress state of heteroepitaxial diamond on Ir/YSZ/Si(111). Measurements of the strain tensor ε⃡ by X-ray diffraction and the subsequent calculation of the tensor of intrinsic stress σ⃡ showed stress-free samples as well as symmetric biaxial stress states for on-axis samples. Transmission electron microscopy (TEM) lamellas were prepared for plan-view studies along the [1¯1¯1¯] direction and for cross-section investigations along the [112¯] and [11¯0] zone axes. For samples grown on-axis with parameters which avoid the formation of intrinsic stress, the majority of dislocations have line vectors clearly aligned along [111]. A sudden change to conditions that promote stress formation is correlated with an abrupt bending of the dislocations away from [111]. This behaviour is in nice agreement with the predictions of a model that attributes formation of intrinsic stress to an effective climb of dislocations. Further growth experiments under off-axis conditions revealed the generation of stress states with pronounced in-plane anisotropy of several Gigapascal. Their formation is attributed to the combined action of two basic processes, i.e., the step flow driven dislocation tilting and the temperature dependent effective climb of dislocations. Again, our interpretation is supported by the dislocation propagation derived from TEM observations.

Descriptions of crack growth behaviors in glass–ZrO2 bilayers under thermal residual stresses

ObjectivesThis study was intended to separate residual stresses arising from the mismatch in coefficients of thermal expansion between glass and zirconia (ZrO2) from those stresses arising solely from the cooling process. Slow crack growth experimentes were undertaken to demonstrate how cracks grow in different residual stress fields. MethodsAluminosilicate glass discs were sintered onto ZrO2 to form glass–ZrO2 bilayers. Glass discs were allowed to bond to the ZrO2 substrate during sintering or prevented from bonding by means of coating the ZrO2 with a thin boron nitrade coating. Residual stress gradients on “bonded” and “unbonded” bilayers were assessed using birefringence measurements. Unbonded glass discs were further tested under biaxial flexure in dynamic fatigue conditions in order to evaluate the effect of residual stress on the slow crack growth behavior. ResultsWhen fast-ccoling was induced, residual tensile stresses on the glass increased significantly on the side toward the ZrO2 substrate. By allowing the bond between glass and ZrO2, those tensile stresses observed in unbonded specimens are overwhelmed by the contraction mismatch stresses between the ZrO2 substrate and the glassy overlayer. Specimens containing residual tensile stresses on the bending surface showed a time-dependent strength increase in relation to stress-free annealed samples in the dynamic biaxial bending test, with this effect being dependent on the magnitude of the residual tensile stress. The phenomenon observed is explained here on the basis of the water toughening effect, in which water diffuses into the glass promoting local swelling. An additional residual tensile stress at the crack tip adds an applied-stress-independent (Kres) term to the total tip stress intensity factor (Ktip), increasing the stress-enhanced diffusion and the shielding of the crack tip through swelling of the crack faces. SignificanceResidual stresses in the glass influence the crack growth behavior of veneered-ZrO2 bilayered dental prostheses. The role of water in crack growth might be of higher complexity when residual stresses are present in the glass layer.

Fatigue crack growth in residual stress fields

A fatigue crack growth (FCG) model for specimens with well-characterized residual stress fields has been studied using experimental analysis and finite element (FE) modeling. The residual stress field was obtained using four point bending tests performed on 7050-T7451 aluminum alloy rectangular specimens and consecutively modeled using the FE method. The experimentally obtained residual stress fields were characterized using a digital image correlation technique and a slitting method, and a good agreement between the experimental residual stress fields and the stress field in the FE model was obtained. The FE FCG models were developed using a linear elastic model, a linear elastic model with crack closure and an elastic–plastic model with crack closure. The crack growth in the FE FCG model was predicted using Paris–Erdogan data obtained from the residual stress free samples, using the Harter T-method for interpolating between different baseline crack growth curves, and using the effective stress intensity factor range and stress ratio. The elastic–plastic model with crack closure effects provides results close to the experimental data for the FCG with positive applied stress ratios reproducing the FCG deceleration in the compressive zone of the residual stress field. However, in the case of a negative stress ratio all models with crack closure effects strongly underestimate the FCG rates, in which case a linear elastic model provides the best fit with the experimental data. The results demonstrate that the negative part of the stress cycle with a fully closed crack contributes to the driving force for the FCG and thus should be accounted for in the fatigue life estimates.

In situ neutron diffraction analysis of stress-free d-spacing during solution heat treatment of modified 319 Al alloy engine blocks

Aluminium alloy engine blocks have successfully replaced ferrous materials in order to maximise weight savings and improve vehicle fuel efficiency. However, the development of an optimal heat treatment process is required to improve engine block casting integrity and prevent potential problems such as in-service cylinder distortion. Optimisation of heat treatment parameters requires an in-depth study to determine how residual stresses are relieved with time during solution heat treatment. In order to perform this analysis, however, in situ neutron diffraction must first be carried out on stress-free samples of the same composition and processing history as the engine blocks to account for factors such as thermal expansion and changes in lattice parameter due to dissolution of secondary phases. The results from this study suggest that thermal expansion caused the largest change in d0 spacing, while prolonged exposure at the solutionising temperature resulted in relatively small changes in {311} and {331} d0 spacing due to phase dissolution.

Structural and optical investigation of non-polar (1-100) GaN grown by the ammonothermal method

We studied the structural and optical properties of state-of-the-art non-polar bulk GaN grown by the ammonothermal method. The investigated samples have an extremely low dislocation density (DD) of less than 5 × 104 cm−2, which results in very narrow high-resolution x-ray rocking curves. The a and c lattice parameters of these stress-free GaN samples were precisely determined by using an x-ray diffraction technique based on the modified Bond method. The obtained values are compared to the lattice parameters of free-standing GaN from different methods and sources. The observed differences are discussed in terms of free-electron concentrations, point defects, and DD. Micro Raman spectroscopy revealed a very narrow phonon linewidth and negligible built-in strain in accordance with the high-resolution x-ray diffraction data. The optical transitions were investigated by cathodoluminescence measurements. The analysis of the experimental data clearly demonstrates the excellent crystalline perfection of ammonothermal GaN material and its potential for fabrication of non-polar substrates for homoepitaxial growth of GaN based device structures.

Microstructures and mechanical properties of age-formed 7050 aluminum alloy

The effects of age-forming on microstructures and mechanical properties of 7050 Al alloy were investigated in this work. The alloy was subjected to age-forming as well as stress-free ageing at 160 °C for 6, 12, 18 and 24 h, and its microstructures were characterized by electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). It was shown that creep might lead to grain elongation during age-forming, and the applied stress induces the coarsening of precipitates in 7050 Al alloy. The texture in the alloy was also influenced by age-forming. Consequently, the differences in microstructures result in differences in mechanical properties of age-forming versus traditional stress-free ageing. The ultimate tensile strength of age-formed samples were slightly lower than that of stress-free aged samples, while the yield strength of age-formed samples were apparently lower than that of stress-free aged samples. Specifically, the elongation of samples age-formed displays apparently decrease.

Effects of glass-forming histories on the crystallization kinetic of the 20Li 2O–80TeO 2 glass

In the present work, tellurite 20Li 2O–80TeO 2 glasses were prepared with identical nominal composition under different glass-forming histories to produce a stressed and stress-free samples. X-ray Diffraction (XRD) and Differential Scanning Calorimetry (DSC) techniques were used to study the effects of the glass-forming histories on the thermal and structural properties of these glasses. The γ-TeO 2 (metastable), α-TeO 2 and α-Li 2Te 2O 5 phases were identified during the controlled devitrification in these glasses. The mestastable character of the γ-TeO 2 phase was clearly observed in the glass under stress but this effect is not so clear in the stress-free glass. The γ-TeO 2 and α-TeO 2 phases crystallizes during the initial stages of crystallization in both studied glasses while the α-Li 2Te 2O 5 phase crystallize in the final stages of the crystallization. The activation energies and Avrami exponent were calculated for both studied glasses with different particle size leading to E 3 > E 2 > E 1 for stressed glass and E 3 > E 2 ≈ E 1 for stress-free glass, where E 1, E 2 and E 3 were associated to the γ-TeO 2, α-TeO 2 and α-Li 2Te 2O 5 phases, respectively. The observed distinct n ¯ 1 < n ¯ 2 < n ¯ 3 in both glasses is an indicative that nucleation and growth takes place by more than one mechanism in the early stages of the crystallization.

Automated measurement and statistical modelling of elastic laminae in arteries

Structural features of elastic laminae within arteries can provide vital information for both the mechanobiology and the biomechanics of the wall. In this paper, we propose, test and illustrate a new computer-based scheme for automated analysis of regional distributions of elastic laminae thickness, inter-lamellar distances and fragmentation furcation points (FPs) from standard histological images. Our scheme eliminates potential artefacts produced by tissue cutting, automatically aligns tissue according to physiologic orientations and performs cross-sectional measurements along radial directions. A statistical randomised complete block design and F test were used to assess the potential (non)-uniformity of lamellar thicknesses and separations along both radial and circumferential directions. Illustrative results for both normotensive and hypertensive thoracic porcine aorta revealed marked heterogeneity along the radial direction in nearly stress-free samples. Clearly, regional measurements can provide more detailed information about morphologic changes that cannot be gained by globally averaged evaluations alone. We also found that quantifying FP densities offers new information about potential elastin fragmentation, particularly in response to increased loading due to hypertension.

Evaluation of Residual Stresses Induced by Robotized Hammer Peening by the Contour Method

Welded components suffer from high tensile residual stresses close to the weld beads. These stresses seem to be the origin of premature cracking which could result in a catastrophic rupture during operation and a reduction of the lifespan of these components. In this context, the Hydro-Quebec’s Research Institute (IREQ) developed a technique of residual stresses relaxation by robotized hammer peening which makes it possible to release stresses close to surface and preserve the mechanical and dimensional properties of manufactured components. Robotized hammer peening was used to induce compressive residual stresses on initially stress free samples of austenitic stainless steel 304L. Hammer peening layers from one to nine were performed and the resulting residual stresses were evaluated thanks to the contour technique. Complete 2D residual stress fields on samples cross sections were obtained. The ability of hammer peening to relax residual stresses within welded plates was then quantified on austenitic stainless steel 304L plates welded with a 308 steel and hammer peened. These tests show the efficiency of hammer peening as a method to relax tensile residual stresses and induce compressive ones to a depth of a few millimetres. Process parameters were optimized such as the number of hammer peening layers to be applied to reduce processing time and maximization of the intensity and spatial distribution of the compressive residual stresses.

Modelling the X-ray powder diffraction of nitrogen-expanded austenite using the Debye formula

Stress-free and homogeneous samples of nitrogen-expanded austenite, a defect-rich f.c.c. structure with a high interstitial nitrogen occupancy (between 0.36 and 0.61), have been studied using X-ray powder diffraction and Debye simulations. The simulations confirm the presence of deformation stacking faults in the structure, while twin or growth faulting can be ruled out. Screw dislocations are abundant and the dislocation density increases with the interstitial nitrogen occupancy. Whether the N atoms are clustered or distributed randomly among the octahedral interstices was found to be indistinguishable to X-ray powder diffraction.

Preferential substitution of Fe on physically equivalent Ga sites in GaN

The EPR spectra of Fe 3 + in high-quality thick freestanding hydride vapor phase grown GaN have been studied in the X- and Q-band. A complex resonance pattern with numerous lines of different intensities provided by three different defects is observed for these nearly stress-free iron-doped samples. The dominating part is due to isolated Fe 3 + on Ga sites and can be well described with a spin Hamiltonian (SH) with S = 5 2 considering the two magnetically inequivalent Ga sites with C 3 v symmetry in the wurtzite structure of GaN. Aside from the displacement of their magnetic axis the two Ga sites are physically equivalent. However, contrary to the expectation, we observed different Fe populations of both sites that varied from a ratio 1:4 to a ratio of nearly 1:1 in different crystals. In addition and in contrast to previously published data we determined nearly isotropic g-values and larger values for the axial fine-structure parameters D , a – F and much larger value for the cubic fine-structure parameter a.