Peak Tensile Stress Research Articles

Comparison of residual stress distributions in conventional and stationary shoulder high-strength aluminum alloy friction stir welds

The residual stresses generated in stationary shoulder friction stir welds (SSFSWs) produced in a typical high strength aluminum alloy (AA7010) in 6.3mm thick plate has been mapped over full weld cross sections, using the contour method, and compared to those introduced by conventional friction stir welding (FSW) for welding speeds ranging from 100 to 400mm/min. Compared to in conventional FSW, as a consequence of the material flow being affected by only a rotating probe, the SSFSW process produced a narrower and more uniform weld nugget and heat affected zone profile through the plate thickness. For both processes, ‘M’ shaped residual stress distributions were determined. However, the peak stresses measured in the SSFSWs were slightly lower than those found in the conventional FSWs and the width of the tensile region was appreciably reduced when using a stationary shoulder welding tool. This is shown to be resulting from a more focused temperature distribution obtained from using only a rotating probe to generate heat in the SSFSW process. In both processes, increasing the welding speed led to a narrower residual stress profile, but higher peak tensile residual stresses.

Effects of pre-tension on fatigue behavior of rolled magnesium alloy

The effects of pre-tension on the fatigue properties of a rolled Mg-3Al-1Zn (AZ31) alloy were investigated by imposing 2%, 5%, and 10% tensile strains along the rolling direction before low-cycle fatigue tests. The investigation showed that the applied pre-tension increased the dislocation density and introduced a small number of twins into the material. Although the as-rolled sample and all the pre-tensioned samples exhibited asymmetric hysteresis loops owing to the alternation of twinning and detwinning during each cycle, the tensile and compressive peak stresses increased with an increase in the amount of pre-tension during the first cycle. With an increase in the number of cycles, however, the flow stress, mean stress, plastic strain energy density, and hysteresis loop of the samples became similar irrespective of the amount of pre-tension, which is attributed to cyclic strain softening or hardening behavior depending on the initial dislocation density. Despite the relatively large pre-deformation, the pre-tensioned samples exhibited a fatigue life equivalent to that of the as-rolled sample because of an early extinction of the difference in the cyclic stress-strain response caused by pre-tension. This result demonstrates that the pre-tension process can improve the yield strength of rolled Mg alloys without a loss of their low-cycle fatigue resistance.

Generation of different types of surface acoustic waves by shock wave-Stone interaction

The generation of different types of surface acoustic waves (SAWs) in lithotripsy is investigated by numerical simulations using COMSOL, considering either the case of a weakly focused shock wave as an incident plane wave (in the focal plane) in shock wave lithotripsy (SWL) or a spherically divergent shock wave produced by a spark discharge in Nano Pulse Lithotripsy (NPL). The interaction of these two types of shock waves with an artificial kidney stone immersed in water may generate three different types of SAWs: namely, Scholte wave, acoustic surface evanescent wave (ASEW), and leaky Rayleigh wave (LRW). In particular, we examined the generation of SAWs on a flat, cylindrical, or spherical surface under different incident angles. It was found that the geometry and acoustic properties of the stone could significantly influence the types of SAWs generated and the resultant peak tensile stress produced at the stone-water boundary. Comparison with experimental measurements (i.e., Schlieren and photoelastic imaging) and stone fracture will also be made. [Work supported by NIH through 5R37DK052985-20.]

Microstructure damage evolution associated with cyclic deformation for extruded AZ31B magnesium alloy

Fatigue damage evolution of extruded AZ31B magnesium (Mg) alloy is investigated under strain-controlled tension-compression loading along the extrusion direction at various strain amplitudes, and the different cyclic deformation behaviors are observed. At the strain amplitude of 2%, the tensile peak stress displays significant cyclic softening, whereas the compressive peak stress shows consistent cyclic hardening. At 1%, moderate cyclic hardening is observed at both the tensile peak and compressive peak stresses. At 0.5%, the tensile peak stress presents stable cyclic hardening, whereas the compressive peak stress almost keeps constant. The microstructure morphologies associated with the cyclic deformation are analyzed by scanning electronic microscope (SEM). The degree of deformation twins is evaluated by analyzing X-ray diffraction (XRD) using a normalized parameter λ. The results show the fatigue crack initiation modes and its propagation modes are dependent on the strain amplitude. At 2%, grain boundary (GB) cracking and triple joint cracking are detected after 1st loading cycle. At 1%, fatigue crack initiates at grain boundary (GB cracking), twin boundary (TB cracking) and triple joint of three neighboring grains. Both grain boundary induced (GB-induced) intergranular and persistent slip band induced (PSB-induced) transgranular propagation modes play an important role in the early-stage crack growth. At 0.5%, crack initiation modes are similar to that at 1%, but GB-induced intergranular propagation mode dominates the early-stage crack growth. The effects of the microstructure (texture, grain size and uniformity) on the fatigue damage behavior are discussed.

Strain-hardening behavior and microstructure development in polycrystalline as-cast Mg-Zn-Y alloys with LPSO phase subjected to cyclic loading

The strain-hardening behavior and microstructural development of polycrystalline as-cast Mg-Zn-Y alloys with various volume fractions of the long-period stacking ordered (LPSO) phase subjected to cyclic loading were experimentally evaluated. For all alloys, cyclic loading tests with a constant strain amplitude of 0.5% for up to 100 cycles showed asymmetric cyclic hardening behavior. That is, the absolute value of the compressive peak stress significantly increased during cyclic loading while the tensile peak stress slightly decreased. With increasing volume fraction of the LPSO phase, the stress amplitude significantly increased. Cyclic loading tests after compressive preloading up to 200 or 250MPa resulted in a significant increase in the stress amplitude, while a number of kink bands developed during preloading. For the cyclic hardening behavior, the contribution of the increase in kinematic hardening was significant in the alloys with a higher volume fraction of the LPSO phase. Transmission electron microscopy observation of the cyclically deformed Mg85Zn6Y9 alloy indicated the formation of a deformation-induced band, where the crystal structure was transformed from 18R-LPSO to hcp-Mg with the exclusion of solute elements.

Stress-Distribution around Pin-Loaded Hole for Orthotropic Laminates

Stresses were calculated for orthotropic laminate plate loaded by a frictionless pin in a circular hole of the same diameter. These calculations were based on finite-element analysis for five laminates; 00, [±450]s, [00/900]s,[00/±450]s, and quasi-isotropic [00/±450/900]s. stress distribution, based on nominal bearing stress, were determined for wide ranges of the ratios of width to diameter and edge distance to diameter. Orthotropic had a significant influence on both the magnitude and location of the maximum tensile stress concentration on the boundary of the hole. The laminates with 00 plies developed the peak tensile stress near the ends of the pin-hole contact arc. But the ±450 laminates had peaks where ply fiber were tangent to the hole. The finite width and edge distances strongly influenced the tensile stress concentration. In contrast, the finite widths and edge distances had little effect on bearing stress concentration. For the practical range w/d = 2, the peak tensile stresses were as much as 50 percent larger than the infinite-laminate value. For e/d=1, these stresses were greater 60 percent than infinite-laminate value. In contrast, the finite width and edge distance had little effect on bearing stress concentrations.

Residual stress state in an induction hardened steel bar determined by synchrotron- and neutron diffraction compared to results from lab-XRD

Induction hardening is a relatively rapid heat treatment method to increase mechanical properties of steel components. However, results from FE-simulation of the induction hardening process show that a tensile stress peak will build up in the transition zone in order to balance the high compressive stresses close to the surface. This tensile stress peak is located in the transition zone between the hardened zone and the core material. The main objective with this investigation has been to non-destructively validate the residual stress state throughout an induction hardened component. Thereby, allowing to experimentally confirming the existence and magnitude of the tensile stress peak arising from rapid heat treatment. For this purpose a cylindrical steel bar of grade C45 was induction hardened and characterised regarding the microstructure, hardness, hardening depth and residual stresses. This investigation shows that a combined measurement with synchrotron/neutron diffraction is well suited to non-destructively measure the strains through the steel bar of a diameter of 20mm and thereby making it possible to calculate the residual stress profile. The result verified the high compressive stresses at the surface which rapidly changes to tensile stresses in the transition zone resulting in a large tensile stress peak. Measured stresses by conventional lab-XRD showed however that at depths below 1.5mm the stresses were lower compared to the synchrotron and neutron data. This is believed to be an effect of stress relaxation from the layer removal. The FE-simulation predicts the depth of the tensile stress peak well but exaggerates the magnitude compared to the measured results by synchrotron/neutron measurements. This is an important knowledge when designing the component and the heat treatment process since this tensile stress peak will have great impact on the mechanical properties of the final component.

Residual stresses in welded I-shaped sections fabricated from Q460GJ structural steel plates

A new type of high-performance structural steel named by GJ steel, has been widely used in many large scale structures like National Olympic Stadium (Birds Nest), new CCTV Headquarters and Canton Tower in China. However, previous studies on 460MPa GJ steel are very limited. In this study, the residual stresses on eight full scale I-shaped sections welded by flame-cut Q460GJ (“Q” means yield strength) steel plates including five doubly symmetric sections and three singly symmetric sections, were investigated by experimental tests. By using sectioning method, it was found that the deformation of curved strips was one part of circular curve. The results from the circular curve correction method are generally identical to the results from a new curve correction method named by “straightening measurement”. Then the residual stress distributions on Q460GJ welded I-shaped sections were obtained. It was concluded that residual stresses were related to sectional dimensions and flanges and webs were influenced with each other. Compared with the existing residual stress distribution models, the residual stresses on sectional flanges obtained in this study are much smaller, while no significant difference is found for sectional webs. It is also found that the flange tips are all under compression rather than tension, and the average value of the peak tensile residual stresses on flanges is only about 0.5fy. Finally, the residual stress levels at typical locations of I-shaped sections were summarized and suggested.

Peak Stress in the Annulus Fibrosus Under Cyclic Biaxial Tensile Loading.

Numerous in vitro studies have examined the initiation and propagation of fatigue injury pathways in the annulus fibrosus (AF) using isolated motion segments; however, the cycle-varying changes to the AF under cyclic biaxial tensile loading conditions have yet to be examined. Therefore, the primary objective of this study was to characterize the cycle-varying changes in peak tensile stress in multilayer AF tissue samples within a range of physiologically relevant loading conditions at subacute magnitudes of tissue stretch up to 100 loading cycles. A secondary aim was to examine whether the stress-relaxation response would be different across loading axes (axial and circumferential) and whether this response would vary across regions of the intervertebral disk (IVD) (anterior and posterior-lateral). The results from the study demonstrate that several significant interactions emerged between independent factors that were examined in the study. Specifically, a three-way interaction between the radial location, magnitude of peak tissue stretch, and cycle rate (p = 0.0053) emerged. Significant two-way interactions between the magnitude of tissue stretch and cycle number (p < 0.0001) and the magnitude of tissue stretch and loading axis (p < 0.0001) were also observed. These findings are discussed in the context of known mechanisms for structural damage, which have been linked to fatigue loading in the IVD (e.g., cleft formation, radial tearing, increased neutral zone, disk bulging, and loss of intradiscal pressure).

Test study on residual stress distribution of hybrid steel u-rib stiffened plates

Orthotropic steel bridges were found to experience early fatigue failures of several welded connections in the steel deck plate. The welding process inevitably produces welding residual stresses and deformations that should be evaluated and considered to better understand the fatigue life of steel bridges. Aiming to study the residual stress distribution of hybrid steel u-rib stiffened plates, the blind hole method was applied. Various parameters, including the plate widths, the ratio of the u-rib thickness to the motherboard thickness, and the steel strength of the combination of the u-rib with the motherboard, were considered, and nine specimens were subjected to experimental tests. The results indicate that strain release factors have great impact on the calculated residual stress: as the value of strain release factor increases, the calculated residual stress in absolute term increases accordingly. However, due to experimental errors, as the ratio of the motherboard to the u-rib thickness varies from 1:2 to 1:2.5 and the u-rib spacing increases from 120mm to 160mm, the residual stress value and the distribution change are not obvious; when different steel strengths were used in the specimens, as the material yield strength of the motherboard (u-rib) increases, the residual tensile stress peaks of the motherboard (u-rib) increase, while the residual tensile stress distribution of the u-rib (motherboard) remains essentially the same. Based on the comparison between the cutting method and the blind hole method, the values from residual stress test points appear to be similar; from the perspective of measuring points' distribution and the directions of the tested residual stress, the cutting method cannot test the weld location, and the residual stress at the measuring point can only be obtained along the cutting direction because the blind hole method can test the weld location residual stress, and the residual stress of the measuring point can be obtained in any direction.

Finite element study of 3D simulated welding effect in aluminium plates

The effects of metal inert gas (MIG) welding and friction stir welding (FSW) on the development of the heat-affected zone (HAZ), residual stress and distortion fields in aluminium plates were investigated in this paper. The FSW and MIG butt welding process of two aluminium alloy 6061-T6 plates were simulated in a three-dimensional, two-step thermo-mechanical analysis using the finite element code ANSYS. The element birth and death technique was implemented in the MIG simulation to model the addition of weld metal to the workpiece and the tool plunging force and fixture release were considered in the FSW simulation. The finite element thermal results revealed that the FSW process resulted in significantly lower temperatures in comparison with MIG welding. The mechanical results indicated that in comparison with MIG welding, the FSW process resulted in up to 40% and 75% less peak tensile and compressive residual stresses, respectively, as well as a reduction of 40%– 60% in HAZ area and considerably smaller distortions.

An integrative modeling approach for the efficient estimation of cross sectional tibial stresses during locomotion

The purpose of this research was to utilize a series of models to estimate the stress in a cross section of the tibia, located 62% from the proximal end, during walking. Twenty-eight male, active duty soldiers walked on an instrumented treadmill while external force data and kinematics were recorded. A rigid body model was used to estimate joint moments and reaction forces. A musculoskeletal model was used to gather muscle length, muscle velocity, moment arm and orientation information. Optimization procedures were used to estimate muscle forces and finally internal bone forces and moments were applied to an inhomogeneous, subject specific bone model obtained from CT scans to estimate stress in the bone cross section. Validity was assessed by comparison to stresses calculated from strain gage data in the literature and sensitivity was investigated using two simplified versions of the bone model-a homogeneous model and an ellipse approximation. Peak compressive stress occurred on the posterior aspect of the cross section (−47.5±14.9MPa). Peak tensile stress occurred on the anterior aspect (27.0±11.7MPa) while the location of peak shear was variable between subjects (7.2±2.4MPa). Peak compressive, tensile and shear stresses were within 0.52MPa, 0.36MPa and 3.02MPa respectively of those calculated from the converted strain gage data. Peak values from a inhomogeneous model of the bone correlated well with homogeneous model (normal: 0.99; shear: 0.94) as did the normal ellipse model (r=0.89–0.96). However, the relationship between shear stress in the inhomogeneous model and ellipse model was less accurate (r=0.64). The procedures detailed in this paper provide a non-invasive and relatively quick method of estimating cross sectional stress that holds promise for assessing injury and osteogenic stimulus in bone during normal physical activity.

Biomechanical Prediction of Crack Angle Effect on Local Tensile Stress of Articular Cartilage

Purpose: Articular cartilage is an important load-bearing structure in synovial joint. Posttraumatic osteoarthritis may occur after cracking insult to cartilage, while not all insult to cartilage could progress into osteoarthritis. The initiating mechanism of osteoarthritis is still not well understood. Tensile stress is considered to have strong relationships with cartilage degradation. We hypothesize that cracks on articular cartilage under compression will induce tensile stress around the crack region, which may lead to chondrocytes change and cartilage degeneration. Methods: In this study, using a 2D finite element model and animal validation study, the relationship between crack angle and local tensile stress was investigated. Results: Under pure compressive load, 45 degrees oblique crack on cartilage could induce the highest peak tensile stress (3.9 times to compressive pressure) while no tensile stress was found in perpendicular crack cartilage. Animal study also confirmed finite element prediction that oblique crack may produce type I collagen around crack tip region. Conclusions: This finding implies that crack angle could be an important factor contributing to initiate osteoarthritis after a traumatic injury. New strategies for prevention and early treatment of osteoarthritis might benefit from this study.

Surface treatment to reduce and redistribute residual-stresses in A7N01 weld by micro-arc oxidation

Micro-arc oxidation (MAO) process is applied to the surface treatment of A7N01 alloy welded joint to improve the residual-stress distribution and its properties. Oxide ceramic coating with a thickness of 13μm is prepared on the surface of the welded joints. The resulting residual stress of welded joint decreases significantly. The transverse residual stress σy on the region of weld metal, with a peak tensile stress of 220MPa, drops to a compressive stress of 30MPa. The treated A7N01 joint has better anti-corrosion property, due to the protection of ceramic coating and the decreasing trends of stress corrosion with lower tensile stress. Both impact toughness and tensile strength of the treated joints are improved. The fatigue test shows that the fatigue performance is improved due to the resulting compressive stress during MAO processing. The SEM observation results show that the fatigue crack originates at the hard phase in sub-surface, not the micro-porous of the coating.

Microstructure and mechanical properties of SiO2-BN ceramic and Invar alloy joints brazed with Ag–Cu–Ti + TiH2 + BN composite filler

Ag–Cu–Ti + TiH2+BN composite filler was prepared to braze SiO2-BN ceramic and Invar alloy. The interfacial microstructure, mechanical properties, and residual stress distribution of the brazed joints were investigated. The results show that a wave-like Fe2Ti–Ni3Ti structure appears in the Invar substrate and a thin TiN–TiB2 reaction layer forms adjacent to the SiO2-BN ceramic. The added BN particles react with Ti to form TiN–TiB fine-particles, which is beneficial to refine the microstructure of the brazing seam and to greatly inhibit the brittle compounds formation. The interfacial microstructure at various brazing temperatures was analyzed, and the mechanism for the interfacial reactions responsible for the bonding was proposed. The maximum shear strength of the joints brazed with the composite filler at 880 °C for 10 min is 39 MPa, which is 30% greater than that brazed with Ag–Cu–Ti alloy. The improvement of the joint strength is attributed to the variation of joint microstructure and the reduction of tensile stresses induced in the SiO2-BN ceramic. The finite element analysis indicates that the peak tensile stress decreases from 230 to 142 MPa due to the addition of BN particles in the ceramic.

Topological design of all‐ceramic dental bridges for enhancing fracture resistance

Layered all-ceramic systems have been increasingly adopted in major dental prostheses. However, ceramics are inherently brittle, and they often subject to premature failure under high occlusion forces especially in the posterior region. This study aimed to develop mechanically sound novel topological designs for all-ceramic dental bridges by minimizing the fracture incidence under given loading conditions. A bi-directional evolutionary structural optimization (BESO) technique is implemented within the extended finite element method (XFEM) framework. Extended finite element method allows modeling crack initiation and propagation inside all-ceramic restoration systems. Following this, BESO searches the optimum distribution of two different ceramic materials, namely porcelain and zirconia, for minimizing fracture incidence. A performance index, as per a ratio of peak tensile stress to material strength, is used as a design objective. In this study, the novel XFEM based BESO topology optimization significantly improved structural strength by minimizing performance index for suppressing fracture incidence in the structures. As expected, the fracture resistance and factor of safety of fixed partial dentures structure increased upon redistributing zirconia and porcelain in the optimal topological configuration. Dental CAD/CAM systems and the emerging 3D printing technology were commercially available to facilitate implementation of such a computational design, exhibiting considerable potential for clinical application in the future. Copyright © 2015 John Wiley & Sons, Ltd.

Effect of tool rotational speed on residual stress, microstructure, and tensile properties of friction stir welded 6061-T6 aluminum alloy thick plate

In the present study, the effect of tool rotational speed on residual stress, microstructure, and tensile properties of friction stir welded AA 6061-T6 was investigated. AA 6061-T6 plates with a thickness of 16 mm were friction stir welded using three different tool rotational speeds of 500, 700, and 900 rpm and a constant welding speed of 120 mm/min. The results indicate that the longitudinal residual stress distribution is an M-shaped distribution in the transverse direction; in weld region, the tensile longitudinal stress in the advancing side is larger than that in the retreating side. With increasing rotation speed, the values of the tensile longitudinal residual stresses increased slightly, the peak tensile longitudinal stresses appear at the edge of the shoulder in the AS of the joints, and the maximum value of tensile longitudinal residual stress was 153 MPa with the rotation speed of 900 rpm which reaches up to 55 % of the yield strength of AA 6061-T6. Meanwhile, only the appearance of welded joint with 700 rpm is smooth. The grain size in WNZ grows up with increasing the rotation speed. The microhardness variation is determined by microstructure evolution, and the average microhardness decreases with increasing the rotation speed. The welded joint with the rotation speed of 700 rpm performs the maximum tensile strength of 236 MPa which is 76 % of the base material strength.

Characterization of Low Cycle Fatigue of Ferritic–Martensitic P92 Steel: Effect of Temperature

Isothermal, strain‐controlled low cycle fatigue (LCF) tests of P92 steel are conducted in air at different temperatures ranging from 20 to 650 °C and various strain ranges to clarify the influence of temperature on its LCF behavior. A symmetrical triangular wave form and a constant 1.0 × 10−3 s−1 strain rate are employed for all the tests. Cyclic softening behavior is expressed in two ways: peak tensile stress in terms of cycle numbers and in terms of accumulated inelastic strain. Based on the definition of “inelastic ratio,” correlation between the value of accumulated inelastic strain () at the beginning of the second softening stage and the inelastic ratio can be distinctively divided into three regions. The modified cyclic plasticity model, in which a linear term is added into the isotropic hardening variable, is developed to simulate the stress–strain hysteresis loop and the cyclic softening curve. Additionally, parameters in the modified isotropic hardening variable at different temperatures are correlated to the cyclic softening phenomenon. Performances of the modified model are assessed by comparing predictions with experimental results. Good predictive capability of the modified constitutive model is obtained.

Reduction of Thermomechanical Stress Using Electrically Conductive Adhesives

We compare the thermomechanical stresses in solar cell interconnections based on electrically conductive adhesives (ECA) with soldered joints by using bending experiments and finite element analysis (FEA). Additionally, the influence of an increasing number of busbars is studied. The FEA is validated by measuring the bending of cell strips after cooling down from a single-sided interconnection process. The material parameters are determined by tensile tests, microscopy and nanoindentation. The comparison of ECA and soldering shows that an elastomer with a Young's modulus of below 0.5GPa is capable of reducing the thermomechanical stress effectively resulting in, approximately, a mean tensile stress in the ECA of 5MPa, 110MPa in the ribbon, and a maximum compressive stress in the silicon of 75MPa. Increasing the number of busbars from three to five leads to a reduction in compressive stresses in the silicon and a slight increase of the peak tensile stress in the busbars.

Experimental and analytical investigation into the stress performance of composite anchors for CFRP tendons

This paper presents a study of the internal stress distribution and its stress transfer mechanism of composite anchors for carbon fiber-reinforced polymer (CFRP) tendons. One set of static tensile test for composite anchor was introduced and carried out. Three zones designated as tension zone, holding zone and compression zone respectively were divided to analyze the stress distribution separately based on some assumptions. Test and analysis results show that the tensile stress and its variation on the steel tube surface will reflect the internal stress distribution of composite anchor. Peak tensile stress exists in the tension zone test points indicates the bonding failure occurs between adhesive and CFRP tendons. The radial clamping action of wedge could effectively enhance the anchoring effect. The obvious decrease of stress curve in the compression zone indicates the bonding damage of entire composite anchor occurs.