scispace - formally typeset
Search or ask a question

Showing papers on "Stress concentration published in 2017"


Journal ArticleDOI
TL;DR: In this article, the effects of building orientation and post-fabrication heat treatment on fully-reversed strain-controlled fatigue behavior of 17-4 precipitation hardening (PH) stainless steel (SS) fabricated via Selective Laser Melting (SLM) was investigated.

390 citations


Journal ArticleDOI
TL;DR: In this paper, the influence of base plate heating and post-process stress-relief on part properties like process-incited defects, which are critical for fatigue loading, for AlSi12 alloy was investigated.

122 citations


Journal ArticleDOI
TL;DR: In this paper, the authors investigated the role of crack closure and residual stress on crack retardation following an overload and concluded that the residual stress effect is relatively short-lived, whilst the closure effect that is dominant at low values of R causes longer range retardation.
Abstract: The introduction of an overload or underload within a constant amplitude loading fatigue test leads to a retardation or acceleration of the Fatigue Crack Growth Rate (FCGR). The understanding of the causes of these effects is essential in the context of variable amplitude fatigue loading, since in principle any loading history can be represented as a sequence of overloads and underloads. In the case of overload, along with some other minor causes, the residual stress changes at the crack tip and crack closure behind the tip can be thought of as the main factors that affect the fatigue crack growth rate. Whilst this has been recognised and accepted for many decades, controversy persists regarding the relative significance and presence of these two effects, and consensus is yet to emerge. The effect of crack closure, when the baseline loading ratio is high enough, can be inhibited so that the main cause of retardation becomes doubtless the residual stress present ahead the crack tip. In the present paper we report our attempt to deconvolve the contributions of crack closure and residual stress on crack retardation following an overload. To accomplish this task we analyse the results of fatigue tests run at two baseline load ratios, namely R=0.1 and R=0.7. At the load ratio of R=0.7 the crack closure effect is not operative, as confirmed by Digital Image Correlation analysis of the crack flanks close to the tip, and post mortem fractographic analysis of crack surfaces. Therefore, for R=0.7 the compressive residual stress region created by the overload ahead of the crack tip is the sole mechanism causing crack retardation. Therefore, for R=0.7 the focus must be placed entirely on the strain field around the crack tip. To this end, line profiles along the crack bisector of elastic strain in the crack opening direction were collected at several stages of crack propagation past the overload using in situ Synchrotron X-ray Powder Diffraction (SXRPD) technique. By performing comparison between the two loading conditions (R=0.7 and R=0.1), information was extracted regarding the role of residual stress alone, and then, by subtracting this effect for the R=0.1 sample, for crack closure alone. To enable this analysis, we propose a introducing the concept of equivalent effective stress intensity factor range, ∆ K eq , eff proposed by Walker. Afterwards, the SIF range reduction ratio, β , which represents the “knock down” factor with respect to the steady state growth was assessed. It is in terms of these newly introduced parameters that the magnitude and extent of the overload-induced crack growth rate retardation can be plotted, fitted and decomposed into closure and residual stress effects, respectively. It is concluded that although the residual stress effect is present at all values of the load ratio R, its effect is relatively short-lived, whilst the closure effect that is dominant at low values of R causes longer range retardation.

108 citations


Journal ArticleDOI
TL;DR: In this article, a transition layer reinforced by oxidized multiwall carbon nanotubes (OCNTs) was built to synergistically improve interfacial and fatigue-resistant performance of carbon fiber/epoxy composites.
Abstract: To synergistically improve interfacial and fatigue-resistant performance of carbon fiber/epoxy composites, a transition layer reinforced by oxidized multiwall carbon nanotubes (OCNTs) was built. OCNTs were integrated onto carbon fibers using a continuous electrophoretic deposition method. Results of static and fatigue tests showed that compared with composites without OCNTs, the hierarchical composites not only showed increases of 33.3% in interfacial shear strength, 10.5% in interlaminar shear strength and 9.5% in flexural strength but also acquired 4.5% improvement in residual bending strength retention after fatigue tests. The transition layer detected by energy dispersive X-ray spectroscopy and atomic force microscope in force mode might be responsible for the above improvements. Combined with scanning electron microscopy analysis and ultrasonic C-scan detection, the functions of modified interfacial microstructure were discussed. The enhanced interface could help to reduce stress concentration and lead destructive cracks to spread along multiple paths, enhancing the damage resistance.

107 citations


Journal ArticleDOI
TL;DR: In this article, the rib-to-deck (RD) welded connections are the most sensitive locations to encounter the fatigue failure in orthotropic steel decks (OSDs), and numbers of fatigue cracks arising from these areas have been found in existing OSD bridges.

90 citations


Journal ArticleDOI
TL;DR: In this paper, the effect of laser power on crack sensitivity of the weld was investigated, and the results show that full penetrated welds are obtained when the laser power of 7.2 W is used, many cracks are observed in the weld.
Abstract: Dissimilar metals of NiTiNb shape memory alloy and Ti6Al4V alloy with a same thickness of 0.2 mm were joined by micro laser welding. The effect of laser power on crack sensitivity of the weld was investigated. The results show that full penetrated welds are obtained when the laser power of 7.2 W is used, many cracks are observed in the weld. With increasing the laser power to 12 W, the number of all cracks and cracking width first increase and then decrease. By XRD analysis, three different kinds of Ti2Ni, NbNi3 and AlNbTi2 intermetallic compounds are found in the weld. According to the formation enthalpy and binary phase diagram, brittle Ti2Ni phase with more contents is existed in the weld due to final solidification, and which is the main reason of crack formation along with large stress concentration. Moreover, the welding cracks like the weld center longitudinal solidification cracks, weld metal toe transversal liquid cracks, heat-affected-zone hot cracks and crater cracks are classified in the laser welded joints. A brittle cleavage fracture with cleavage planes and river patterns in the joints is presented from the fracture surface.

88 citations


Journal ArticleDOI
TL;DR: In this paper, the corrosion fatigue crack initiation and initial propagation mechanism of E690 steel in simulated seawater were studied by stress-controlled fatigue tests and a series of subsequent characterizations on the fracture surface, microstructure and secondary cracks.
Abstract: In the present paper, the corrosion fatigue crack initiation and initial propagation mechanism of E690 steel in simulated seawater were studied by stress-controlled fatigue tests and a series of subsequent characterizations on the fracture surface, microstructure and secondary cracks. Results show that the corrosion fatigue crack initiation and initial propagation mechanism evolves with elevated peak stress level in simulated seawater. When peak stress is far below the proof stress, cracks preferentially initiate at the parent austenite grain boundaries (PAGBs) with 68.4% probability and at the ferrite lath boundaries (FLBs) with 31.6% probability. Meanwhile, the cracks also preferentially propagate along the PAGBs and FLBs. Upon the peak stress close to or above the proof stress, cracks turn to initiate from the emerging corrosion pits and propagate without zigzag detour but by splitting the ferrite laths which transversely block its propagation way.

87 citations


Journal ArticleDOI
TL;DR: In this paper, the authors used non-equilibrium molecular dynamics simulations to characterize the ductile tensile failure of a model body-centered cubic metal, tantalum, over six orders of magnitude in strain rate.

85 citations


Journal ArticleDOI
TL;DR: In this article, the influence of pore aspect ratio on strength and Young's modulus of porous sandstones has been investigated in two-dimensional numerical simulations (RFPA 2D ).

84 citations


Journal ArticleDOI
TL;DR: In this article, the fracture and fatigue crack growth properties of Ti-6Al-4V produced by the Wire + Arc Additive Manufacture (WAAM®) process were investigated.
Abstract: This paper presents an experimental investigation of the fracture and fatigue crack growth properties of Ti-6Al-4V produced by the Wire + Arc Additive Manufacture (WAAM®) process. First, fracture toughness was measured for two different orientations with respect to the build direction; the effect of wire oxygen content and build strategy were also evaluated in the light of microstructure examination. Second, fatigue crack growth rates were measured for fully additive manufactured samples, as well as for samples containing an interface between WAAM® and wrought materials. The latter category covers five different scenarios of crack location and orientation with respect to the interface. Fatigue crack growth rates are compared with that of the wrought or WAAM® alone conditions. Crack growth trajectory of these tests is discussed in relation to the microstructure characteristics.

82 citations


Journal ArticleDOI
Rongqiao Wang1, Li Da1, Dianyin Hu1, Fanchao Meng2, Liu Hui1, Ma Qihang1 
TL;DR: In this paper, the statistical size effect of such stress concentrators was investigated by LCF experimentations and theoretical predictions for titanium alloy plate specimens with a central circular hole (CHP).

Journal ArticleDOI
TL;DR: In this paper, a modification to the Fatemi-Socie (FS) critical plane damage parameter has been shown to provide satisfactory fatigue life correlations for a variety of materials and loading conditions.

Journal ArticleDOI
TL;DR: In this paper, the effects of cyclic junction temperature variations of low amplitudes in power modules, to help the capturing of module reliability characteristics and the derivation of lifetime models in the future, are presented.
Abstract: This paper presents a series of experiment results on the ageing effects of cyclic junction temperature variations ( $\Delta T_{j}$ ) of low amplitudes in power modules, to help the capturing of module reliability characteristics and the derivation of lifetime models in the future. Power cycling tests, for nonaged and aged modules, are designed to illustrate the failure mechanisms. Insulated gate bipolar transistor (IGBT) modules in actual converters are usually operated in a $\Delta T_{j}$ range up to 40 °C; therefore, tests are carried out to observe the effects of such narrow $\Delta T_{j}$ stress cycles on the module lifetime. It is found that the relatively minor stress cycles may not be able to directly initiate a crack but can contribute to the development of damage in the die attach solder layer due to stress concentration. Finite element analysis modeling is utilized to verify the stress concentration effect. The experiment results show that the effects of the narrow $\Delta T_{j}$ stress cycles are affected by the ageing status of the module and the stress level itself.

Journal ArticleDOI
TL;DR: In this article, the authors present stress concentration factors at the edges of 3D voids in uniaxial compression and quantitatively assess the proximity of a void to the two hypothesised modes of failure.

Journal ArticleDOI
TL;DR: In this article, the microstructural evolution of resistance spot welded 1000MPa dual phase steel under two different welding conditions, and their relation to the mechanical performance and failure mechanisms are reported.

Journal ArticleDOI
TL;DR: In this paper, the effect of stress ratio on the very high cycle fatigue (VHCF) behaviors of titanium alloy (Ti-8Al-1Mo-1V) was investigated.

Journal ArticleDOI
TL;DR: In this article, the authors examined experimentally the creep behavior of an immature carbonate-rich Posidonia shale, subjected to constant stress conditions at temperatures between 50 and 200 ÂC and confining pressures of 50-200 ÂMPa, simulating elevated in situ depth conditions.
Abstract: The economic production of gas and oil from shales requires repeated hydraulic fracturing operations to stimulate these tight reservoir rocks. Besides simple depletion, the often observed decay of production rate with time may arise from creep-induced fracture closure. We examined experimentally the creep behavior of an immature carbonate-rich Posidonia shale, subjected to constant stress conditions at temperatures between 50 and 200 °C and confining pressures of 50–200 MPa, simulating elevated in situ depth conditions. Samples showed transient creep in the semibrittle regime with high deformation rates at high differential stress, high temperature and low confinement. Strain was mainly accommodated by deformation of the weak organic matter and phyllosilicates and by pore space reduction. The primary decelerating creep phase observed at relatively low stress can be described by an empirical power law relation between strain and time, where the fitted parameters vary with temperature, pressure and stress. Our results suggest that healing of hydraulic fractures at low stresses by creep-induced proppant embedment is unlikely within a creep period of several years. At higher differential stress, as may be expected in situ at contact areas due to stress concentrations, the shale showed secondary creep, followed by tertiary creep until failure. In this regime, microcrack propagation and coalescence may be assisted by stress corrosion. Secondary creep rates were also described by a power law, predicting faster fracture closure rates than for primary creep, likely contributing to production rate decline. Comparison of our data with published primary creep data on other shales suggests that the long-term creep behavior of shales can be correlated with their brittleness estimated from composition. Low creep strain is supported by a high fraction of strong minerals that can build up a load-bearing framework.

Journal ArticleDOI
TL;DR: Based on poroelasticity theory and diffusion-seepage mechanism, the dual porosity and permeability evolution model was established by considering fracture gas pressure and pore gas pressure, respectively.

Journal ArticleDOI
TL;DR: In this paper, the effects of double stop-hole technique on the subsequent delay in fatigue crack propagation were studied and a parametric study was conducted on different geometrical parameters of double-stop-hole.

Journal ArticleDOI
TL;DR: In this article, a coupled thermal-mechanical analysis is conducted to evaluate the residual stresses induced by welding processes, which are used as the initial state in the fatigue damage analysis.

Journal ArticleDOI
J.F. Zhang1, X.X. Zhang1, Quanchao Wang1, B.L. Xiao1, Z.Y. Ma1 
TL;DR: In this article, the deformation, damage and failure behaviors of 17.vol.% SiCp/2009Al composite were studied by microscopic finite element (FE) models based on a representative volume element (RVE) and a unit cell.

Journal ArticleDOI
Yu Li1, Wei Li1, Na Min2, Wenqing Liu2, Xuejun Jin1 
TL;DR: In this article, a quenching-partitioning-tempering (QPT) treatment with different initial conditions, cold rolling (CR) and hot rolling (HR), was investigated in ultra-low carbon medium Mn steels for enhanced strength, plasticity and toughness.

Journal ArticleDOI
TL;DR: In this article, the authors explore and analyze the crack initiation and propagation in Ti/Al composite laminates during the in-situ tensile test, and show that the intermetallic compounds could form at the interface of Ti and Al composite materials, and also induce the initiation of microcracks under tensile forces.

Journal ArticleDOI
TL;DR: The influence of resin based and lithium disilicate materials on the stress and strain distributions in adhesive class II mesio-occlusal-distal (MOD) restorations is studied using numerical finite element analysis (FEA).

Journal ArticleDOI
TL;DR: In this paper, the mechanical behavior of a limestone with an initial porosity of 14.7 % is investigated at constant stress and dilatancy associated with micro-fracturing occurs during constant stress steps, ultimately leading to failure.
Abstract: Deformation and failure mode of carbonate rocks depend on the confining pressure. In this study, the mechanical behaviour of a limestone with an initial porosity of 14.7 % is investigated at constant stress. At confining pressures below 55 MPa, dilatancy associated with micro-fracturing occurs during constant stress steps, ultimately leading to failure, similar to creep in other brittle media. At confining pressures higher than 55 MPa, depending on applied differential stress, inelastic compaction occurs, accommodated by crystal plasticity and characterized by constant ultrasonic wave velocities, or dilatancy resulting from nucleation and propagation of cracks due to local stress concentrations associated with dislocation pile-ups, ultimately causing failure. Strain rates during secondary creep preceding dilative brittle failure are sensitive to stress while rates during compactive creep exhibit an insensitivity to stress indicative of the operation of crystal plasticity, in agreement with elastic wave velocity evolution and microstructural observations.

Journal ArticleDOI
Xudong Chen1, Jingwu Bu1, Xiangqian Fan, Jun Lu, Lingyu Xu1 
TL;DR: In this paper, the effect of loading frequency on the low cycle fatigue behavior of plain concrete in direct tension was investigated, and a new damage model was proposed based on the stiffness degradation and the secondary strain rate during low-cycle fatigue tests.

Journal ArticleDOI
TL;DR: In this article, the fatigue behavior of additive manufacturing (AM) materials with rough as-built surfaces was investigated using the Short-FALSTAFF (Fighter Aircraft Loading STAndard For Fatigue) load sequence.

Journal ArticleDOI
TL;DR: Focusing on critical sites where the local stress is above the global fracture threshold, this work derives a general expression for the fracture stress as a decreasing function of porosity and disorder and analyses the transition to failure in terms of a coarse-graining length.
Abstract: By means of extensive lattice-element simulations, we investigate stress transmission and its relation with failure properties in increasingly disordered porous systems. We observe a non-Gaussian broadening of stress probability density functions under tensile loading with increasing porosity and disorder, revealing a gradual transition from a state governed by single-pore stress concentration to a state controlled by multipore interactions and metric disorder. This effect is captured by the excess kurtosis of stress distributions and shown to be nicely correlated with the second moment of local porosity fluctuations, which appears thus as a (dis)order parameter for the system. By generating statistical ensembles of porous textures with varying porosity and disorder, we derive a general expression for the fracture stress as a decreasing function of porosity and disorder. Focusing on critical sites where the local stress is above the global fracture threshold, we also analyze the transition to failure in terms of a coarse-graining length. These findings provide a general framework which can also be more generally applied to multiphase and structural heterogeneous materials.

Journal ArticleDOI
TL;DR: In this paper, basalt fiber was used as reinforcement material in ±[55]6 filament wound ring composite for creating the alternative to carbon, kevlar and glass fibers, to contribute to the research studies and literature.
Abstract: Matrix cracking which is the major initial form of damage in fiber reinforced polymer composites plays significant role in determining the fracture toughness. The fast crack propagation in polymer matrix causes to decrease the fracture toughness of fiber reinforced polymer (FRP) composite. In order to retard the fast crack propagation in polymer matrix and provide to increase of the fracture toughness of FRP composite, the polymer matrix of FRP composite is modified by filling the different kinds of nanoparticles. In such a way, the crack propagation leads to retard and dissipate the stress concentration affected to form the fiber cracks along of fibers in composite structure. In this study, basalt fiber was used as reinforcement material in ±[55]6 filament wound ring composite for creating the alternative to carbon, kevlar and glass fibers, to contribute to the research studies and literature. SiO2 nanoparticles that provides to form the effects of fracture toughness mechanism based on the effect of retarding crack propagation were filled into epoxy matrix to increase the mechanical properties and fracture toughness of ±[55]6 filament wound BFR/Epoxy ring composite. The split-disk tensile tests of single edge notched and un-notched ±[55]6 filament wound BFR/Epoxy ring composite specimens were conducted to determine the mechanical properties and mode I fracture toughness. SiO2 nanoparticle addition into epoxy matrix of ±[55]6 filament wound BFR/Epoxy ring composites has given the results of hoop tensile stress within the range of 27.7–30.3%. The fracture toughness of composite ring specimen was specified by ASTM E 399-12E3 by adapting to the directed mode I crack propagation and compared with each other. An effective increase in mode I fracture toughness of 43%–50% was obtained at 4 wt% addition level of SiO2 nanoparticles. The crack branching in epoxy matrix provided by SiO2 nanoparticle, matrix cracking, debonding, delamination and fiber breakage failures has been observed via microscope and SEM analysis.

Journal ArticleDOI
TL;DR: In this paper, the authors examined the influence of various types of small artificial defects on fatigue strength, including circumferential notches, corrosion pits, drilled holes and pre-cracked holes.