Non-uniform Residual Stress Fields Research Articles

Modulation of localized phonon thermal transport at GaN/AlxGa1-xN heterointerface: Polar surface, doping, and compressive Strain

The phonon transport processes within a few atomic layers near heterointerfaces play a crucial role in thermoelectric applications such as nitride-based high electron mobility transistors (HEMTs) and light-emitting diodes (LEDs). However, a systematic investigation of their localized transport characteristics remains unexplored. This article employs the Interfacial Phonons method based on first-principles calculations to extract interface phonon information for GaN/AlxGa1-xN heterostructures. Molecular dynamics simulations are utilized to compare the phonon behaviors between Ga-face and N-face GaN-based heterointerfaces. The research extensively investigates the dependence of interfacial thermal transport on the concentration of Al atoms under ordered substitutional doping and the strength of uniaxial compressive strain. The research revealed that both Ga-face and N-face GaN/AlxGa1-xN heterointerfaces exhibit phonon branch drift. The differences in charge distribution at the polar surfaces lead to varying degrees of Brillouin zone distortion. The vibrational modes of the atoms at the N-face GaN interface are more compact and localized than those of Ga-face GaN, resulting in a stronger degree of localization for the optical phonon branches. When varying the doping concentration of Al atoms, some phonon branches at the GaN interface appear outside the band structure of the intrinsic bulk and surface states, exhibiting strong localized characteristics. The introduction of Al atoms at the heterointerface induces additional phonon scattering, and the lattice vibration mismatch between Al and Ga atoms results in a non-uniform residual stress field at the interface, further enhancing phonon-interface scattering. Moderate compressive strain-induced lattice distortion promotes the overlapping of phonon modes at the interface, allowing some localized phonons to serve as interface transport modes. This optimization enhances phonon transport at the heterointerface. This research provides valuable insights into a comprehensive understanding of interfacial thermal transport in GaN-based semiconductors. Moreover, it offers useful theoretical guidance for thermal management through the manipulation of phonon wave properties.

Assessment of non-uniform residual stress field of the thermal sprayed stainless steel coatings on aluminium substrates by the integral hole drilling method

Thermal spray is one of the most used techniques to produce coatings on structural materials. Such coatings are used as protection against high temperatures, corrosion, erosion and wear. The combined action of high pressures, temperatures and spraying conditions give rise to non-uniform residual stresses. The latter plays an important role in coating design and process parameters optimization. The present work highlights the influence of coatings thickness on the evolution of residual stresses in layered materials. Therefore, thick stainless steel coatings (ASTM 301) of different thicknesses are manufactured by wire arc spraying on aluminium alloy substrates (ASTM 2017A). For a better bond strength, a Ni–Al bond coat is first deposited. Furthermore, a numerically supported hole drilling strain gage method for residual stress field evaluation is proposed. Required calibration coefficients, for the strain–stress transformation formalism based on the integral method, are computed through finite element calculations using Abaqus software. The results indicate that the maximum residual stresses, for all thicknesses, are tensile and range from 140 to 275 MPa. The bond coat does not seem to affect the stress field. Also, it was found that the mean equivalent Von-Mises stress decreases with increasing coating thickness; hence reducing the interfacial adhesion energy of the sprayed materials.

Evaluation of stress intensity factor for a surface cracked butt welded joint based on real welding residual stress

This paper examines the evaluation of stress intensity factor (SIF) for surface cracks in a non-uniform residual stress field. A technique that based on utilizing the crack face traction (CFT) integral implemented in the interaction integral method using an open source code for models with a particular crack-block mesh density was presented. 3-D flat plate models with different crack-block mesh densities were employed under different loading conditions to examine the accuracy, conditions and easiness to use of the present technique. The influence of the crack-block mesh density on the calculated SIF solutions due to the application of non-uniform tractions over the crack face was discussed. A considerable improvement of SIF solutions calculated by crack face tractions was observed when the crack-block mesh density becomes fine enough. The effectiveness of the present technique was then validated for different 3-D numerical examples of welded joints under different loading conditions. The present technique was thereafter employed to evaluate SIF for a surface crack in a butt welded joint using real welding residual stress. The SIF solution calculated due to the application of welding residual stress was verified with that given by mechanical loading (without welding residual stress).

Measurement of Highly Non-Uniform Residual Stress Fields with Reduced Plastic Error

Experimental validations of a recently proposed method for residual stress measurement are presented. The method, specifically designed for highly non-uniform in-plane residual stress fields is applied in this work to measure the residual stresses resulting from four-point bend of an Aluminium 7075-T6 bar. The benefit of the method is that it can reconstruct stress fields without any assumptions of in-plane uniformity. The method uses two cuts, propagated from both side surfaces and collection of full-field relaxation displacement fields from side surfaces using 2D high-resolution digital image correlation (DIC). The reconstructed residual stress agrees well with that predicted by FE modelling and neutron diffraction measurement. In addition, it is shown that induced plastic strain during the relaxation process, which reduces accuracy of the method, is strongly influenced by the direction of the propagation of the cut. This implies that it is possible to reduce substantially plastic strain during cutting if the orientation and propagation of the cut is carefully chosen.

Compact sensor combining digital speckle pattern interferometry and the hole-drilling technique to measure nonuniform residual stress fields

A portable device to essentially measure residual stress fields outside an optical bench is presented. This system combines the hole-drilling technique with digital speckle pattern interferometry. A novel feature of this device is its high degree of compaction since only one base supports simultaneously the measurement module and the hole-drilling device. A new version of the American society for testing and materials standard E837 for the measurement of residual stresses has been improved including a computation method for nonuniform residual stresses. According to this standard, a hole with a maximum depth of 1.0 mm should be introduced into the material to assess the stress distribution along the hole’s depth. The discretization of the stress distribution is performed in 20 equal steps of 0.05 mm, getting the deformations generated for stress relief in every drilling step. A description of the compact device showing the solution for a fast and easy interchanging process between modules is also presented. The proposed system was compared with a traditional method using strain gages, and a good agreement was shown between stress distributions measured with both methods. Finally, the portable device was used to evaluate the residual stress distribution in a sample with a rod welded by friction hydro pillar processing.

Genetic algorithm optimization of the hole-drilling method for non-uniform residual stress fields

The hole-drilling method, coupled with the influence functions method, is a versatile technique for measuring residual stresses. The latter determines a relationship between residual stress and measured relaxed strains for any experimental configuration and material. However, several parameters which describe the functional basis for the residual stress must be defined. This paper presents a procedure, based on the least squares method and genetic algorithms, optimally to choose these parameters to reduce the effects of noise and uncertainty in experimental measurements. The performance of the method was tested against finite element simulations for typical applications such as shot peening, showing that the method is flexible and accurate and allows the reconstruction of residual stress at deeper positions than other techniques.

X-Ray Diffraction Analysis of Nonuniform Residual Stress Fields σii(τ) under Difficult Conditions

Thin hard coatings for wear protection usually do not consist of single layers but of stacks of alternating sublayers which have to meet different demands. With respect to X-ray residual stress analysis (XSA) such multilayer systems pose a series of challenges. In addition to those problems which generally arise in thin film diffraction like small layer thickness or strong texture, neighbouring sublayers with similar chemical composition may superimpose each other, or sublayers of identical structure and composition, which contribute to the same diffraction line, are separated by other sublayers. Starting from a formalism that yields the X-ray penetration depth τ in multilayer systems of arbitrary sublayer sequences, we show how a combination of measurements using ‘conventional’ photon sources available at any X-ray lab and synchrotron radiation allowing for wavelength tuning near the TiKβ absorption edge, can be used to evaluate the residual stress state in the top sublayer stack of a hard coating multilayer system deposited by chemical vapour deposition.

Metrological justification of reflection hologram interferometry with respect to residual stresses determination by means of blind hole drilling

A description of practical implementing combined technique for residual stress determination in plane thick-walled structures is presented. The main features, which are inherent in a procedure of reconstruction and quantitative interpretation of fringe patterns arising as a result of blind hole drilling in residual stress field, are discussed in detail. Two plane aluminium specimens welded from two equal parts having dimensions 130×80 mm in-plane and thickness of 18 and 12 mm are the objects of investigation. Weld seam is performed along the shortest side of both specimens. A wide set of actual fringe patterns related to both uniform and non-uniform residual stress field is reconstructed. Most of typical fringe patterns are presented as illustrations. A whole set of available interferograms shows that a residual stress field in both thick-walled plane specimens investigated consists of both a uniform and non-uniform part. It is shown that a form of both non-uniform and uniform stress fields remains practically the same along the weld line. Moreover, non-uniform residual stress field at the proximity of weld seam is of similar form for both specimens. Emphasis is made in a choice of the transition model proceeding from a comparison of actual interferograms and reference fringe patterns. For the first time reference fringe patterns are constructed by using data obtained from corresponding actual fringe patterns, which reveal a non-coincidence between specimen's symmetry cross-sections and principal strain directions. It is shown that relations developed for a probe hole drilled in uniform 2-D stress field can be used for deriving residual stress components when fringe patterns obtained reveal a presence of local strain gradients. A good agreement between actual and reference fringe patterns is achieved in a case of local strain gradients existing. Comparing two types of interferograms related to uniform residual stress fields demonstrates their full coincidence. It is shown that the whole set of obtained fringe patterns consists of three main groups. The main evidence, which reveals an interferogram belonging to each specific class, is a typical form of fringes configuration. The main principles of creating the general fringe patterns catalogue are established and initial draft of this catalogue is developed. Distributions of residual stress components are constructed for different surfaces of both specimens under study. Quantitative results have a form that is typical for welded joints of structures bounded by plane surfaces. It is shown that fine nuances inherent in residual stress distributions over different specimen faces can be reliably derived from recorded fringe patterns of any type.

On the Manipulation of Nanoscale Self-Assembly by Elastic Field

ABSTRACTMorphological and compositional self-assembly can be manipulated by the long-range elastic field. This paper gives a universal formulation that determines the dependence of energetically favored orientation of those self-assembled structures on the elastic interaction. Elasticity anisotropy can lead to symmetry breaking and herringbone structures. A layered substrate can tune the feature size by modulus mismatch, or tune the orientation if the layers have different orientation preference, or guide the self-assembly by embedded structures. A closed-form result is derived for elastically isotropic layers by using Dundurs parameters. The self-assembled structures can also be affected by a nonuniform residual stress field or external force field. Higher order (nonlinear) perturbation theory, coupling between morphology and composition, and other issues are also addressed in the discussion.

A heuristic approach to microcracking and fracture for ceramics with statistical consideration

Microcracking damage and toughening are examined for ceramics. These effects have been found to depend on the material microstructure and macrocrack growth. Isotropic damage, attributed to random distribution of microcrack location, length and orientation can be associated with a disordered microstructure and a non-uniform residual stress field. When the applied stress is the main cause of cracking, the microcrack distribution is no longer random such as a system of quasi-parallel cracks. To highlight the effect of crack interaction, discrete models are advanced where damage is simulated by a distribution of microcracks. The dilute concentration assumption is invoked to simplify the analysis. The two-dimensional discrete model is based on a phenomenological approach that is statistical in character. Interactions of microcracks and with a macrocrack are considered by means of a boundary element technique (A. Brencich, A. Carpinteri, Int. J. Fracture 76 (1996) 373–389; A. Brencich, A. Carpinteri, Eng. Fract. Mech. 59 (1998) 797–814) where both isotropic and anisotropic damage could be treated. Comparisons with other results are made to show that the model can be applied to analyse the fracture behaviour of different materials.

Determination of Nonuniform Residual Stresses Using the Ring-Core Method

This paper considers residual stress analysis using the ring-core method. In particular, the so-called integral equation method is applied to evaluate nonuniform residual stress fields. The proposed method overcomes typical drawbacks of the incremental strain method which lead to incorrect results for strongly varying stress fields. The experimental results obtained with a specimen subjected to a bending load confirm the theoretical predictions.

On the mechanism of two way shape memory effect obtained by stabilised stress induced martensite

Two way shape memory effect (TWME) can be obtained by suitable thermomechanical processing which involve repetitive training routines. J. Perkins found that TWME take place as a result of a macroscopic non-uniform residual stress field, concluding that plastic deformation was necessary to get TWME. K. Enami et al found that complex dislocations arrays are generated by thermomechanical cycling during the training procedure. TWME results obtained by the above training methods are not as good as would be expected, because during the thermal cycles, above and below A[sub f] and M[sub f] respectively, new dislocations are generated which interact with the dislocations that control TWME so giving a loss of TWME. A different training method has been developed by J.M. Guilemany et al based on the stabilization of stress induced martensite variants (SSIM). This method has been derived from the observation made by J. Perkins and R.O. Sponholz who found that retained (not stabilized) martensite acts as a nucleation site of thermal martensite. Thus, during cooling the stabilized martensite would grow or influence the nucleation and growth of the thermal martensite giving TWME. The effect of training temperature, time and stress on TWME obtained by SSIM has been studied.

Measurement of Non-Uniform Residual Stresses Using the Hole-Drilling Method. Part II—Practical Application of the Integral Method

The Integral Method for calculating non-uniform residual stress fields using strain relaxation data from the incremental hole-drilling method is examined in detail. Finite element calculations are described which evaluate the calibration coefficients required for practical applications of the method. These calibration data are tabulated for a range of hole sizes and depths. It is found that the hole drilling method is not well adapted to measuring stresses remote from the surface, and a theoretical depth limit for stress measurements of 0.5 of the mean radius of the strain gauge rosette, rm, is identified. A practical depth limit is in the range 0.3–0.4 rm.

Measurement of Non-Uniform Residual Stresses Using the Hole-Drilling Method. Part I—Stress Calculation Procedures

The Incremental Strain, Average Stress, Power Series, and Integral methods are examined as procedures for determining non-uniform residual stress fields using strain relaxation data from the hole drilling method. Some theoretical shortcomings in the Incremental Strain and Average Stress methods are described. It is shown that these two traditional methods are in fact approximations of the Integral Method. Theoretical estimates of the errors involved are presented for various stress fields. Also, some simple transformations of stress and strain variables are introduced so as to decouple the stress/strain equations and simplify the numerical solution.