Uniform Tensile Stress Research Articles

Evaluation of Fracture Strength of TiN Thin Film on Cemented Carbide

The purpose of this study is to absolutely evaluate the fracture strength of ceramic films deposited on the hard substrate by considering the residual stress on films, and to clarify the effect of the deposition condition on the fracture strength of ceramic films. TiN films were deposited onto two kinds of WC-Co substrates with different hardness using dc magnetron sputtering using various bias voltages VB. Sphere indentation tests were carried out to obtain the micro fracture strength σf, m for ring crack initiation on TiN films using sphere indenters of varying diameter 2R. The main conclusions are the following. (1) σf, m depend on 2R, but are independent of the hardness of a substrate. Based on the probabilistic theory assuming a Weibull distribution, the relationship between the mean value of σf, m and 2R can be predicted. (2) σf, m increase as VB is increased. (3) Based on the probabilistic theory, the residual stress σR on TiN films and the fracture strength σf of TiN films under uniform tensile stress condition for various VB were estimated from the distribution characteristics of σf, m. Variation tendency in σR is consistent with that by X-ray stress measurement. But, variation tendency in σf is opposite to that in σf, m.

Evaluation of Local Stress for Stress Corrosion Crack Initiation by Three-Dimensional Polycrystal Model

In order to understand the initiation behavior of microstructurally small cracks in a stress corrosion cracking condition, it is important to know the tensile normal stress acting on the grain boundary (normal G. B. stress). The local stress in a polycrystalline body is greatly influenced by deformation constraint which is caused by anisotropic and/or inhomogeneous property of each grain. In present study, the local normal G. B. stress on bi- and tri-crystal bodies and a three-dimensional polycrystalline body consisting of 100 grains were evaluated by the finite element method under a remote uniform tensile stress condition. The polycrystalline body was generated by using a Monte Carlo procedure and random orientations were assigned to each grain. It was revealed that the local normal G. B. stress on the polycrystalline body is inhomogeneous under uniform applied stress. The stress tends to be large near the triple points due to the deformation constraint caused by adjacent grains, even though the grain boundary inclination to the load axis has large influence. It was also shown that particular high stress was not observed at corners of the polycrystalline body.

On the contemporary SINTAP approach and its application to the evaluation of the serviceability of welded joints of steam pipelines of thermal power plants

The SINTAP approach is used for the evaluation of the serviceability of a welded joint after ∼19·104 h of its operation in the steam pipeline of a thermal power plant. The characteristics of strength and crack resistance required for calculations are determined on specimens cut out from the corresponding zones of the investigated welded joint. It is assumed that the welded joint is loaded by a uniform tensile stress, that residual stresses are also present in the metal of the weld, and that a semielliptic crack propagates in the base metal and the metal of the weld from the external surface of the pipeline. The theoretically established parameters of the critical state of a pipe element with welded joint are in good agreement with the expert conclusions concerning the causes of failure of the pipeline.

Influence of electromechanical interaction on the morphological instability of an elastic conducting halfspace

The surface evolution of an elastic conducting material subject to a farfield uniform tensile stress in an electric field was analyzed by using the linear perturbation theory. A dispersion relation describing morphological evolution of the elastic material was derived, from which the critical wave number of surface perturbations was determined numerically. It turns out that the electrical surface stress on the surface of the solid can enhance or inhibit the morphological instability of the solid surface. Such morphological evolution is important for electric force microscopy and applications of nanoelectromechanical devices.

Residual stress field and reduction of stress intensity factors in cold-worked holes

Closed-form and semi-analytical solutions are obtained for the residual stress distributions in a plate caused by pressure acting on a central circular hole, representing the cold-work process. The material is elastic–perfectly plastic. Both Tresca and von Mises yield criteria are used and the corresponding residual stress distributions are compared. The relation between the dimension of the plastic zone and the value of internal pressure is presented. The relation between the magnitude of the residual stresses and the remote uniform tensile stress required to open symmetrical radial cracks is also presented. The reduction of the stress intensity factors of cracked open and riveted holes as a function of the internal pressure applied (or mandrel radial displacement) is investigated using numerical models for both an elastic–perfectly plastic material and for an Al 2024-T3 Alclad aluminum alloy.

Stress-induced surface instability of an elastic layer

The surface evolution of an elastic layer subjected to a far-field uniform tensile stress is analyzed by using a linear perturbation theory. The atomic transport mechanism controlling the surface evolution is the gradient of chemical potential associated with elastic strain energy and surface curvature. The critical spatial frequency of unstable surface perturbations is a function of the thickness of the elastic layer, surface energy, Young’s modulus of the elastic layer, and the applied tensile stress. For thin films, the critical spatial frequency is inversely proportional to the square root of the film thickness and proportional to the applied stress, while it is proportional to the square of the applied stress and independent of the layer thickness for thick films.

Slow Crack Growth during Static and Dynamic Loading in Silicate Glasses

Analytical solutions for stable crack growth in silicate glasses are provided in this paper for two important cases of loading, namely (i) constant stress and (ii) constant stress rate. The former simulates real life glass components subjected to uniform tensile stress while the latter represents strength measurement test for glass specimens. In both cases it is desirable to envision the onset and progression of slow crack growth over time. In this manner the state of initial crack and associated strength can be monitored as a function of applied stress and stress duration. In addition, the progressive crack history provided by analytical solutions helps estimate both the threshold and residual strengths of glass articles which, in turn, are needed to quantify their long term mechanical reliability.The analytical solutions obtained by integration of the power law model involve two system constants which are readily evaluated by imposing certain conditions on crack length at threshold stress. These solutions show that the initial crack velocity is extremely small but increases exponentially with crack length. Hence the onset of fast fracture can be predicted from crack kinetics in the stable region. Thereafter, the crack growth is extremely rapid, unstable, difficult to model, and not of practical interest.The analytical solutions can also be used to design proof test conditions which will ensure both the initial quality and long term integrity of glass articles under specified loading and environment.

Mode Mixity for Circular Hollow Section X Joints With Weld Toe Cracks

This paper describes the mode mixity of stress-intensity factors for surface cracks at weld toes located at the saddle point in circular hollow section X joints. The remote loading applies a uniform tensile stress at the end of the brace along its axis. The three-dimensional finite element models employ mesh tieing between a topologically continuous, global mesh and a separate, local crack-front mesh. Analyses of a simple plate model that approximates key features of toe cracks at the brace-chord intersection verify the negligible effects of the recommended mesh-tieing scheme on stress intensity factors. The linear-elastic analyses compute the mixed-mode stress intensity factors along the crack front using an interaction-integral approach. The mixed-mode stress intensity factors indicate that the crack front experiences predominantly mode I loading, with KIII→0 near the deepest point on the front (ϕ=π∕2). The total crack driving force, described by the J integral, reaches a maximum value at the deepest point of the crack for the crack aspect ratio a∕c=0.25 considered here. The mode-mixity angle, ψ=tan−1(KII∕KI), at ϕ=π∕2 is compared for a range of practical X-joint configurations and crack-depth ratios. The present study demonstrates that the mode-mixity angle ψ increases with increasing brace-to-chord diameter ratio (β) and decreasing chord radius to wall thickness ratio (γ). Values of the nondimensional stress intensity factors (FI=KI∕σ¯brπa and FII=KII∕σ¯brπa), however, show an opposite trend, with higher crack driving forces for small β and large γ ratios. The variations in the brace-to-chord wall thickness ratio (τ) and the crack depth ratio (a∕t0) do not generate significant effects on the mode mixity.

Stresses and cracking in freezing spherical foods: a numerical model

Thermal stresses during the freezing of a spherical food were simulated with an elastic model, using realistic thermal and mechanical properties. It was found that to explain observed cracking patterns, both expansion due to phase change and thermal contraction subsequent to it must be taken into account. During freezing, the unfrozen core is always under uniform isotropic tensile stress, but that is unlikely to explain the cracking pattern. Cracking during cryogenic freezing is more likely due to tensile tangential stresses caused by thermal contraction, with vitrification at the surface possibly playing a role in crack initiation. The calculations suggest that once a crack forms at the surface it will propagate towards the centre, as has been observed experimentally.

A bending-to-stretching analysis of the blister test in the presence of tensile residual stress

The adhesion of films and coatings to rigid substrates is often measured using blister geometries, which are loaded either by an applied pressure or a central shaft. The measurement will be affected if there are residual stresses that make a contribution to the energy release rate. This effect is investigated using analytical solutions based on the principle of virtual displacements. A geometrically nonlinear finite element analysis is conducted for comparison. Furthermore, the relationships among strain energy release rate, load, deflection, and fracture radius are discussed in detail. Both analytical solutions and numerical results reveal that uniform tensile residual stresses reduce a specimen’s deflection if it experiences plate behavior under small loads. However, this effect becomes negligible when membrane stresses induced by the loading become dominant.

Investigation of moisture-assisted fracture in SiO 2 films using a channel cracking technique

The technique of channel cracking was developed to study moisture-assisted crack growth in SiO 2 films, which are used widely in many silicon-based technologies. Films of various residual stresses from tensile to compressive were PECVD deposited, pre-cracked and subjected to an applied uniform tensile stress by bending the silicon substrate using a 4-point beam-bending fixture. The tests were performed in controlled environments: dry (<2% RH), ambient (40–60% RH) and wet (>90% RH). Crack velocities were varied by changing the applied stress and measured by optically monitoring crack growth in situ. It was observed that under the same moisture conditions, films with higher compressive stresses were tougher than those with higher tensile stresses. The presence of moisture at the crack tip was found to reduce the films' resistance to cracking dramatically. The three principal regions of moisture-assisted cracking established by Weiderhorn in a bulk glass system were also observed.

Time-dependent crack behavior in an integrated structure

Devices in modern technologies often have complex architectures, dissimilar materials, and small features. Their long-term reliability relates to inelastic, time-dependent mechanical behavior of such structures. This paper analyzes a three-layer structure consisting of, from top to bottom, an elastic film, a power-law creep underlayer, and a rigid substrate. The layers are bonded. Initially, the film is subject to a uniform biaxial tensile stress. A channel crack is introduced in the elastic film. As the underlayer creeps, the stress field in the film relaxes in the crack wake, but intensifies around the crack tip. We formulate nonlinear diffusion-like equations that evolve the displacement field. When the crack is stationary, the region in which the stress field relaxes increases with time. We identify the length scale of the region as a function of time. The stress intensity factor is proportional to the square-root of the length scale. For the power-law creep underlayer, this newly identified length depends on the film stress, and corrects an error in a previous paper by Huang, Prevost and Suo (Acta Materialia 50, 4137, 2002). When the crack advances, its velocity can reach a steady state. We identify the scaling law for the steady velocity. An extended finite element method (X-FEM) is used to simultaneously evolve the creep strain and crack length. Numerical results are presented for the stress intensity factors of stationary cracks, and the steady velocities of advancing cracks.

A cracked sliding interface between anisotropic bimaterials

A general solution for the stresses and displacements of a cracked sliding interface between anisotropic bimaterials subjected to uniform tensile stress at infinity is given by using the Stroh’s formulation. Horizontal and vertical opening displacements on the interface, stress intensity factors, and energy release rate are expressed in real form, which are valid for any kind of anisotropic materials including the degenerate materials such as isotropic materials. It is observed that stresses exhibit the traditional inverse square root singularities near the crack tips, and the vertical opening displacement and energy release rate are intimately related to a real parameter λ determined by the elastic constants of the anisotropic bimaterials.

Determination of the interface strength between two polymer materials by a new curved interface tensile test: Preliminary experimental results

Recently, the authors have proposed a new experimental method for the determination of adhesion strength between two different materials. A curved interface and special arrangement of materials is used for the tensile test of bimaterial specimens to avoid singular stress fields around corners and edges. The main advantage of the test consists in the fact that the strength is determined under conditions of a uniform tensile stress field normal to the interface in the region where debonding starts. The present paper presents experimental results for two bimaterial systems - PMMA/TPE and PC/TPE (two stiff standard polymers with a thermoplastic elastomer). The expected failure behaviour was observed during the experiments, thus enabling the estimation of adhesion strength by using calculated stress concentration factors. The influence of the radius of curvature is discussed in detail.

Determination of interface strength between two polymer materials by a new curved interface tensile test

A new experimental method for the determination of adhesion strength between two different materials is proposed. A curved interface and special arrangement of materials is used for the tensile test of bimaterial specimens to avoid singular stress fields around corners and edges. Assuming that normal stresses are responsible for debonding at the interface, the adhesion strength can be determined from critical loads at debonding initiation. The stress concentrations at the interface as a function of applied load are derived by the finite-element method. The calculations are made for material constants that are typical for a polymer-polymer bimaterial; however, the method is applicable for all composites of two materials which show elastic behaviour prior to first failure. The main advantage of the test consists in the fact that the strength is determined under conditions of a uniform tensile stress field normal to the interface in the region where debonding starts.

Maximum twin shear stress factor criterion for sliding mode fracture initiation

Previous researches on the mixed mode fracture initiation criteria were mostly focused on opening mode fracture. In this study, the authors proposed a new criterion for mixed mode sliding fracture initiation, which is the maximum twin shear stress factor criterion. The authors studied a finite width plate with central slant crack, subject to a far-field uniform uniaxial tensile or compressive stress.

CFK-Zugelemente verankern

A new anchoring system for tension elements made of composites is presented in this contribution. The anchoring system reduces the chances for premature failure at the ends of the tension elements which might be caused by stress concentration. The system provides tensile force initiation at the end of the tension element, in order to procedure a uniform tensile stress field. In this respect, the results of the comparative tensile test on CFRP (Carbon Fiber-Reinforced Plastic) laminates as well as finite-element simulations are addressed. Differently designed anchoring systems were tried out by tensile tests on CFRP laminates and compared to the standard anchoring system by experimental and theoretical investigations. Some of the new anchoring systems reveal improvements in comparison to the standard clamping device. The new anchoring system might be utilized to improve the tensile testing procedure for CFRP laminates, the improvement of force initiation at adhesively bonded external CFRP reinforcements as well as for the pre-loading of tension elements for cable-stayed bridges.

701 片側き裂を有する帯板の応力拡大係数

A series of finite element analyses were performed for a single edge cracked plate with finite width subjected to point load at a point on the crack surface. The ratio of crack length to plate width a/W was varied in a range from 0.1 to 0.9. Stress intensity factor K was evaluated by means of the virtual crack extension method. Approximate equations for K were made in the same manner as those for a single edge cracked semi-infinite plate proposed by Hartranft et al. K values of a single edge cracked plate with uniform remote tensile stress were evaluated using these approximate equations for various a/W conditions, and compared with those calculated using the existing formula. It was clarified that the estimation error of these approximate equations was less than 1%.

Measuring the interfacial transverse strength: a novel test for evaluating polymer joints

The paper presents a novel method to determine the adhesion strength of a polymer joint. The experimental arrangement provides a uniform transverse tensile stress at the interface, which does not have concentrations or singularities at the edges. The stress state at the interface is purely tensile and does not contain shear components. A special optimised sample geometry generates this stress state. The connection between the applied load and the interfacial transverse stress is obtained from a three-dimensional finite-element analysis. The first experimental results are presented for a polyethylene sample with a glued interface. The possibilities of improvement and also of extending the application field are discussed.

Oxygen plasma wafer bonding evaluated by the Weibullfracture probability method

In this paper, the oxygen plasma bonding process for fusionbonded silicon wafers has been characterized by a new approach. Themechanical reliability of bonded microstructures was determined usingburst tests and Weibull statistic analyses. The fracture characteristic ofthe bonded system is considered to depend on the stress distribution, thedefect distribution and the fracture surface energy at the bond. UsingWeibull theory, it is possible to extract the Weibull modulus m and themean fracture uniform tensile stress per unit length,ℏσfc, from the measured data. These quantitiesmake it possible to compare the joint defect distribution and the fracturesurface energy at the bonded interface for the processing conditions underobservation. These experiments also demonstrate that it is possible todistinguish between these quantities under certain conditions.The fracture probability for different annealing temperatures has beenevaluated and found to agree with previous results from surface energymeasurements. It is shown that the bond fracture probability increaseswith annealing times in the range of 10-100 h. The saturated bondstrength value is considerably enhanced by oxygen plasma activation priorto bonding. In this study, plasma activations at room temperature and300 °C compare to chemical activations in hot nitric acid annealedat 120 °C and 700 °C, respectively. The tendency to formvoids at elevated temperatures, e.g. 300 °C, is increased by theoxygen plasma treatment.If the surface energy is considered to be homogeneous over the bondedinterface, the Weibull modulus m is an indirect measure of the defectdistribution, low m values indicate a wide spectrum of defect types,whereas a high m value narrows the defect distribution responsible forfracture. The Weibull modulus m is shown to be valuable for evaluationof the bonded interface. It is demonstrated that a more scattered defectdistribution emerges for in situ bonded wafers as compared toex situ, and annealing at 300 °C for 90 h as compared toroom-temperature storage. However, the defect distribution becomesincreasingly more narrow with storage time. These variations may be due toeither changes in microcracks or void configuration or inhomogeneities inthe fracture surface energy over the bond interface.