Residual Stress Concentration Research Articles

Measurement of surface residual stresses and testing mechanical properties of high-strength steel butt joints obtained by hybrid laser/gas metal arc welding

Hybrid laser/gas metal arc welding was applied to join the thick high-strength steel in square butt joints with narrow gaps. An X-ray diffraction technique was used to measure the surface residual stress distribution of high-strength steel by a hybrid laser/gas metal arc welding in a butt joint configuration. The influence of welding parameters, including welding speed and wire feeding rate, on the residual stress distribution and mechanical properties of the hybrid weld was experimentally studied. The obtained results show that there exist high compressive transverse and longitudinal residual stresses along the centerline of the weld. However, high tensile transverse and longitudinal stresses are located in the nearby heat-affected zone. An increase in the welding speed definitely decreases the residual stress concentration. A good weld quality of high-strength steel butt welds without cracking or defects can be achieved by a hybrid laser/gas metal arc welding technique.

Effects of sintering velocity on the microstructure and mechanical properties of hot-pressed ZrB2–SiC–ZrO2f ceramics

The effects of cooling and heating velocity on the microstructure and mechanical properties of the hot-pressed ZrB2–20vol.% SiC ceramics added with 15vol.% Zirconia fiber were investigated in details. It was indicated that the appropriate cooling velocity could reduce the residual stress concentration, and the proper heating velocity could allowed the green compacts to get densified, which was favorable to enhance the flexural strength and fracture toughness to 1117±98MPa and 7.1±0.5MPam1/2, respectively. Due to the introduction of the Zirconia fiber, the observed toughening mechanisms were attributed to the fiber debonding, fiber pull-out and phase transformation toughening. The results presented here point out a promising way for improving the mechanical properties of ZrB2-based ultra-high temperature ceramics.

Laser-arc hybrid welding of wrought to selective laser molten stainless steel

Selective laser melting (SLM) is a successful tool-free powder additive technology. The success of this manufacturing process results from the possibility to create complex shape parts, with intrinsic engineered features and good mechanical properties. Joining SLM steel to similar or dissimilar steel can overcome some limitations of the product design like small dimension, undercut profile, and residual stress concentration. In this way, the range of applications of the SLM process can be broadened. In this paper, the hybrid laser welding of selective laser molten stainless steel was investigated. A high-power fiber laser was coupled to an electric arc and austenitic stainless steel wrought and SLM parts were welded together. The power and speed parameters were investigated. The joints were analyzed in terms of weld bead profile, microstructure, microhardness, and tensile test. The efficiency of the welding process was evaluated through the line energy input versus the weld molten area.

Increasing the strength and toughness of a carbon fiber/silicon carbide composite by heat treatment

The effect of heat treatment on the strengthening and toughening of a carbon fiber/silicon carbide composite (C/SiC) with a thin pyrolytic carbon (PyC) interphase was investigated. Tensile strength and modulus were measured using tensile tests, and toughness was obtained by calculating the area under the stress–strain curves. Results show that with increasing heat treatment temperature both the strength and toughness of the C/SiC composite increased, but the modulus decreased. After heat treatment at 1900°C the tensile strength and toughness increased by a maximum of 42% and 252%, respectively, and the modulus decreased by 48%. X-ray diffraction analysis and microstructural observation confirmed that the heat treatment mainly increased the graphitization of the amorphous PyC interphase, and this was responsible for the property changes observed because it decreased the interfacial sliding resistance associated with long fiber pull-out, relieved the thermal residual stress and lower stress concentrations on the fibers to uniformly share the load for improving the strength and toughness.

Design and Application of MMM System for Welding Residual Stress Test

In order to apply metal magnetic memory (MMM) technology to welding residual stress testing, the MMM hardware and software system is designed. The software subsystem is based on object-oriented programming and the hardware subsystem is portable. The MMM system can test the welded joint residual stress distribution, residual stress concentration zones and defects at workshop. Moreover, the MMM testing system can output data to a computer not only for automatic evaluation but also for further analysis and evaluation with expert intervention. According to the trend of the MMM characteristic, the welding residual stress concentration and defects can be detected and evaluated. The field X-ray examination result is consistent with the result of the MMM testing system, which has proved the accuracy and engineering practicality of the MMM testing system.

Experimental and Numerical Study of Cable Anchorage for Cable-Supported Bridge

In this study, a 1:2 scaled-model experiment of a cable anchorage was performed. The data collected from the experiment were analyzed, and a finite element analysis (FEA) shell model was described and compared with the experimental results to estimate the level of residual stress and to study its influence together with stress concentration. The ultimate capacity and the failure mode were discussed further to clarify the mechanical behavior. The research showed that residual stress was high on some local areas of the anchorage; residual stress and stress concentration would influence local stress distribution but would have little impact on global behavior. FEA was not a perfect substitute for experiments because of the inaccuracy of its stress results in local areas subject to residual stress and stress concentration. Global behavior and ultimate capacity from elastic–plastic FEA were reliable, however. Judging from the plastic strain and the area of yielding region, other than local maximum stress, the failure mode of the cable anchorage was not the local yielding of the diaphragm but the complete yielding on the anchor plate.

3D FEM for Sintering of Solar Cell with Boron Back Surface Field Based on Solidwork Simulation

Numerical simulation was carried out on the sintering process of the solar cell with boron back surface field (B-BSF) using the finite element analysis (FEA) software Solidwork Simulation, to analyze the bend problem and to obtain the distribution of residual stress. The impact was discussed on the thicknesses of silicon material and B-BSF on the bend level of the cell, and the stress state of the solar cells under different bend levels was investigated. The results reveal that the increasing sintering temperature causes increasing bow and residual stress but not remarkable, because B-BSF already has reached the yield stress to cause the plastic deformation. The thinner wafer will make the both bow and residual stress increase. When the thickness of the wafer was lower than 140μm, there would be some damages occur near the electrode which was the location of residual stress concentration. If the thickness of wafer is below 6μm, there may be some local damages occur in the region of the wafer near the electrode.

Numerical and experimental study of thermally induced residual stress in the hybrid laser–GMA welding process

A model based on a double-ellipsoidal volume heat source to simulate the gas metal arc welding (GMAW) heat input and a cylindrical volume heat source to simulate the laser beam heat input was developed to predict the temperature field and thermally induced residual stress in the hybrid laser–gas metal arc (GMA) welding process. Numerical simulation shows that higher residual stress is distributed in the weld bead and surrounding heat-affected zone (HAZ). Effects of the welding speed on the isotherms and residual stress of the welded joint are also studied. It is found that an increase in welding speed can reduce the residual stress concentration in the as-weld specimen. A series of experiments has been performed to verify the developed thermo-mechanical finite element model (FEM), and a qualitative agreement of residual stress distribution and weld geometrical size is shown to exist.

The Use of Cast Steel in Steel Structure

With the improvement of casting and material technology, cast steel is continuously used in practical engineering projects. Domestic and foreign scholars studied the performance of cast steel joints by experiment or the other methods and extended them to beam-column connections in steel frames. This paper mainly introduced the development of beam-column connection forms, the main problems in joint design, advantages and disadvantages of the joints with various forms, material properties and manufacturing process characteristics of cast steel, advantages of cast steel joints and fasteners, research status and achievements at home and abroad. The results show that compared with traditional connection forms, designers have full freedom to choose the shapes and the sizes, and joints made of cast steel have the best adaptability and changeability in different engineering practice, what’s more, they have no problems with welding residual stress and stress concentration. Compared with other materials, cast steel has more advantages such as flexible application, convenience of connections, small volume, low cost. In a word, cast steel has great research value and application potential in connections of steel structure.

Numerical Simulation of Thermal Stress Fields of Steel Casting

Many defects relative to stress occur during the complicate casting process, such as hot tearing, residual stress concentration and distortion. Modeling of casting thermal stress during casting solidification process is of great significance to predict and analyze casting stress defects. Involving too many complex influencing factors, the stress simulation is very difficult and retains a hot spot of macro simulation in foundry engineering. Currently most researchers adopt integrated FDM/FEM method, i.e. using finite difference method (FDM) to calculate solidification and heat transferring, while finite element method (FEM) to simulate stress. Some universal commercial FEA packages are usually adopted. This study has tried two kinds of approaches to simulate casting thermal stress. One is based on ANSYS, a well-known powerful FEA analysis software. Another is to develop an independent own copyrighted casting stress simulation system based on FDM. The routes of these two methods were given respectively. To calibrate the simulation system, a stress frame sample and a real practical casting were simulated and pouring experiment was also carried on. The results of simulation were in agreement with the experiment results and practical cases. It indicates that these two approaches can all meet demands. When adopting FDM method, thermal analysis and stress analysis can use the same FD model, which can avoid the nodes matching between different models and reduce the errors of thermal load transferring. It makes the simulation of fluid-flow field, temperature field and stress field unify into one model. This system takes full advantages of mature FDM technology and can be used to simulate the forming of residual stress and predict the occurrence of hot tearing.

Residual stress caused magnetic field abnormal change upon arc welding joints

Residual stress concentration of welding joints is an essential factor to structure safety. It was found that the residual stress could generate a change of magnetic field in arc welding. Things became simple because of the methods based on measuring the distribution of the surface fields and analyzing the relation between the fields and the stress. However, we believe the fact that the magnetizing field caused by electrical current in arc welding has effects can not be ignored. Also, the magnetic field of the Earth is sometimes considered as a critical point in the field's change. In this paper, experiments were carried out to study the role of arc current by comparing the methods of arc welding and laser welding. The magnetic field was measured upon the welding sites. By controlling the input energy, the residual stress was almost the same. The results obtained from these two welding methods varied. Magnetic fields testing from arc welding specimens were changed more sharply than the laser ones. This means that the magnetic change caused by electrical arc current is important to the surface field. Therefore, by analyzing the status of arc current, electro-magnetic methods can be used as non-destructive testing tool of the arc welding joints merely by testing the surface magnetic field without specific magnetization.

Process Induced Stresses in Cavity SOI Wafers

Silicon-on-insulator wafers with buried cavities have been utilized in recent applications ranging from MEMS based resonator devices to double gate MOS devices. The objective of this study is to determine whether the magnitude and orientation of residual stress concentrations generated by the etched silicon cavities, which can introduce reliability issues for these devices, particularly if a large tensile stress results from this processing. The results of this study show that the residual stress state of the bonded wafers may be tensile or compressive and the magnitude and orientation of these stresses depend on both the etched cavity geometry and the oxidation process, which can be verified in the development phase with an infrared polariscope. There is evidence that the stress in the handle wafer prior to bonding is maintained even after high temperature annealing and wafer thinning. Thus, residual stresses may be controllable prior to bonding.

Surface Stress Concentration Analysis of Shot Peened Aluminium Alloys

Fatigue limit for surface crack initiation on shot peened Aluminium Alloys resulted to be determined by surface residual stress and stress concentration, according to the magnitude of the applied stress. The effect of surface roughness considered as micro-notches (dents) has been analysed by using the Vallellano and Navarro’s formulation. It was also determined that the stress gradient associated to those dents dominates over the closure stress effect.

3D finite element modeling of the thermally induced residual stress in the hybrid laser/arc welding of lap joint

In this study, a three-dimensional (3D) finite element model is developed to investigate thermally induced stress field during hybrid laser–gas tungsten arc welding (GTAW) process. In the hybrid welding case, we focus on the GTAW process sharing common molten pool with laser beam and playing an augment role in the hybrid welding system. An experiment-based thermal analysis is performed to obtain the temperature history, which then is applied to the mechanical (stress) analysis. A modified material model is used to consider the influence of face-to-face contact between the top and bottom metal sheets in the thermo-mechanical analysis of welding lap joints. Results show that the normal stress components prevail in the weld zone during hybrid welding process, and maximum thermal stress exceeding the yield point of material exists at the heat affected zone (HAZ) near the weld pool. Increasing the welding speed causes the penetration and width of weld bead to decrease, and the thermal stress concentration at the welded joint is also reduced accordingly. After welding and cooling down, longitudinal tensile stress (SZ) and transverse compressive stress (SX) are retained in the formed weld, and the higher longitudinal compressive stress exists around the weld bead. In addition, a series of experiments are performed to validate the numerical results, and a qualitative agreement is achieved. Compared to the welded joint obtained by GTAW and laser welding alone, the residual stress concentration in the weld joint obtained by hybrid laser–GTAW is the minimal one.

Effects of the machining conditions on the strain hardening and the residual stresses at the roots of screw threads

The strain hardening and the formation of the residual stresses at the roots of threaded joints produced by machining according to American Petroleum Institute (API) standards was investigated. Initially the problem is defined and then an experimental technique was developed for measuring the residual stresses at the roots of fine screw threads. The screw thread was cut on a tubular bar from chromium–nickel based alloyed steel. Residual stresses were measured by the electro-chemical layer removal technique. Experimental work showed that, sever strain hardening and the concentration of residual stresses at the roots of the screw threads took place depending on the machining conditions. Strain hardening in the range of 320–430 HV were found on the screw thread surfaces as compared to the base hardness of 260 HV. Residual stress ranging from 600 to 1450 MPa was developed as compared to the material tensile strength of 850 MPa. The depth for maximum residual stress is around 20 μm.

Corrosion fatigue cracking of AISI 316L stainless steel screen mesh

Abstract This paper describes the failure of stainless steel screen mesh grade AISI 316L after being in service for only 8 months. The characterization methods included visual examination, optical microscopy, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and metallography. The results showed that the screen mesh failed by pitting corrosion and subsequent corrosion fatigue cracking. Pitting was initiated by the attack of chlorides from PVC powder and service environment as well as the action of excessive wear. Subsequent corrosion fatigue cracking arose from the presence of chlorides along with the residual and cyclic stress concentration at the pits. Failure prevention can be achieved by annealing after wire drawing, periodic surface cleaning, and proper material selection.

Finite Element Analysis of Residual Stress in Diamond/Steel Brazed Joint

In the application of brazed diamond tools, residual stress caused by differences in materials properties is an important factor in obtaining good brazing quality. The residual stress in diamond/steel brazed joint is studied by means of finite element method and Raman spectroscopy in this paper. The comparison between the results from the simulations and the measurements shows that a good agreement has been reached. Tensile stress concentration at the diamond edge near interface was observed. High residual stress concentration may cause cracks in diamond. Actually, cracks in diamond caused by brazing residual stress after were observed. The stress concentration of the joint must be reduced to obtain good brazed joint. The change of brazing atmosphere or to adopt the soft brazing alloy will reduce the stress concentration.

Analysis of Stress Concentration around Inclusions due to Thermally Induced Strain to the Steel Matrix

Abstract A finite element method (FEM) was employed to aid in the thermodynamic analysis of the cooling process of steel containing nonmetallic inclusions in a homogenous, isotropic, single-phase steel matrix. Three different contact techniques available in ANSYS were used in a 2-D model of Al2O3 inclusions in a 1010 steel grade to define different types of inclusion-steel interfaces. Comparisons of these numerical techniques examine the effect of the characteristic of the interface on the residual stress concentration zone around inclusions embedded in a steel matrix with an initially free stress state and their thermal interaction during the cooling process from 1300°C. Results are presented to discuss the significance of the inclusion surface and boundary conditions (axisymmetric and fully constrained) on the stress distribution within the stress concentration zone in the vicinity of the interface.

Theoretical and experimental determination of residual stresses in plane joints

We have developed a mathematical model for determining residual stresses and stress concentration in the neighborhood of a plane joint of dissimilar materials and formulated variational statements of direct and inverse problems. The parameters of the stress–strain state of the object under study measured by nondestructive methods serve as input data for the inverse problem. We carry out a numerical analysis of solutions of the direct and inverse problems of determining stress concentration in the neighborhood of a joint of materials with different moduli. Finally, we discuss the applicability of the developed approach to the monitoring of the stress–strain state of joints in the course of their operation and to the evaluation of their residual life.

X-Ray Diffraction and Photoluminescence Studies of InN Grown by Plasma-Assisted Molecular Beam Epitaxy with Low Free-Carrier Concentration

We report studies of InN grown by plasma-assisted molecular beam epitaxy. GaN templates were first grown on sapphire substrates followed by InN overgrown at 457°C to 487°C. Atomic force microscopy shows the best layers to exhibit step-flow growth mode of the InN, with a root-mean-square roughness of 0.7 nm for the 2 μm × 2 μm scan and 1.4 nm for the 5 μm × 5 μm scan.␣Measurements of the terrace edges indicate a step height of 0.28 nm. Hall measurements at room temperature give mobilities ranging from 1024 cm2/V s to 1904 cm2/V s and the electron concentrations are in the range of 5.9 × 1017 cm−3 to 4.2 × 1018 cm−3. Symmetric and asymmetric reflection x-ray diffraction measurements were performed to obtain lattice constants a␣and c. The corresponding hydrostatic and biaxial stresses are found to range from −0.08 GPa to −0.29 GPa, and −0.05 GPa to −0.32 GPa, respectively. Low-temperature photoluminescence peak energies range from 0.67 eV to 0.70 eV, depending on residual biaxial stress, hydrostatic pressure, and electron concentrations. The electron concentration dependence of the estimated Fermi level is analyzed using Kane’s two-band model and conduction-band renormalization effects.