The effect of bridging on fatigue crack growth behavior in Aramid Patched Aluminum Alloy(APAL)

A new hybrid composite (APAL: Aramid Patched Aluminum Alloy), consisting of a 2024-T3 aluminum alloy plate sandwiched between two aramid/epoxy laminate (HK 285/RS 1222), was developed. Fatigue crack growth behavior was examined at stress ratios of R=0.2, 0.5 using the aluminum alloy and two kinds of the APAL with different fiber orientation (0°/90° and 45° for crack direction). The APAL showed superior fatigue crack growth resistance, which may be attributed to the crack bridging effect imposed by the intact fibers in the crack wake. The magnitude of crack bridging was estimated quantitatively and determined by a new technique on basis of compliances of the 2024-T3 aluminum alloy and the APAL specimens. The crack growth rates of the APAL specimens were reduced significantly as comparison to the monolithic aluminum alloy and were not adequately correlated with the conventional stress intensity factor range(ΔK). It was found that the crack growth rate was successfully correlated with the effective stress intensity factor range (ΔK _eff =K _br -K _ct ) allowing for the crack closure and the crack bridging. The relation between da/dN and theΔK _eff was plotted within a narrow scatter band regardless of kind of stress ratio (R=0.2, 0.5) and material (2024-T3 aluminum alloy, APAL 0°/90° and APAL±45°). The result equation was as follow:da/dN=6.45×10⁻⁷(ΔK _eff )^2.4.     

Influences of equal biaxial tensile loads on the stress fields near the mixed mode crack

A hybrid method for photoelasticity is introduced and applied to the plane problems of isotropic polycarbonate plates with a central crack under uniaxial and equal biaxial tensile loads. Also, the influences of equal biaxial tensile loads on the isochromatic fringes, stress fields and stress intensity factors near the mixed mode crack-tip have been investigated. The results show that, when an equal lateral tensile load is added to the specimen under uniaxial tensile load, the asymmetric isochromatic fringes about the line of crack gradually become symmetric, and the slope of the isochromatic fringe loop near the crack-tip is inclined towards the crack surface according to the increasing of the inclined angle of crack. Furthermore, the shapes of distribution of all stress components are changed from asymmetric to symmetric. In the equal biaxial tensile load condition against the uniaxial tensile load condition, the values of stress intensity factors are changed little, and only the region of compressive stress of σ _x /σ _O is changed when β = 0°, but the values of K _I /K ₀ are increased and those of K _II /K ₀ become almost zero, namely, we have the mode I condition when β = 15°∼45°. This paper was recommended for publication in revised form by Associate Editor Chongdu Cho Dong-Chul Shin received the B.S., M.S. and Ph.D. degrees in Mechanical Engineering from Yeungnam University in 1995, 1997 and 2001, respectively. Dr. Shin studied at the University of Tokyo, Japan, for three years (from April, 2005 to January, 2008) as a Post-Doctoral fellow (supported by Korea Research Foundation (KRF) and Japan Society for the Promotion of Science (JSPS)). Dr. Shin is currently a Research Professor at the School of Mechanical Engineering at Pusan National University, Korea. His research interests include the static and dynamic fracture mechanics, stress analysis, and fracture criteria of piezoelectric ceramics, etc. Jai-Sug Hawong received a B.S. in Mechanical Engineering from Yeungnam University in 1974. Then he received his M.S. and Ph.D. degrees from Yeungnam University in Korea in 1976 and from Kanto Gakuin University in Japan in 1990, respectively. Prof. Hawong is currently a professor at the School of Mechanical Engineering at Yeungnam University, in Gyeongsan city, Korea. He is currently serving as vise-president of Korea Society Mechanical Engineering. His research interests are in the areas of static and dynamic fracture mechanics, stress analysis, experimental mechanics for stress analysis and composite material etc.     

Analysis of a crack approaching a circular hole in cross-ply laminates under biaxial loading

The problem of a crack approaching a circular hole in cross-ply laminates under uniaxial and biaxial loading is investigated in this paper. The effects of material orthotropy, geometry [R/d and a/d], and loading conditions on crack tip singularity are investigated. The stress intensity factors are obtained by the modified mapping collocation method. The present results for an isotropic infinite plate show good agreement with existing solutions. The results for cross-ply laminates show that the stress intensity factors strongly depend on material orthotropy, geometry, and loading condition. The stress intensity factors for cross-ply laminates exist between those for θ=0° and those for θ=90° in the whole range of crack length and decrease as the percentage of 0° plies increases. In the range of small crack length the stress intensity factors for biaxial tension are higher than those for uniaxial tension. In the range of large crack length the stress intensity factors for uniaxial tension are higher than those for biaxial tension.     

Investigation of constantly and transiently propagating cracks in functionally graded materials by dynamic photoelasticity

In this study, the stress intensity factors (SIFs) for steady and transient propagation of cracks in transparent homogeneous functionally graded materials were analyzed by using the photoelasticity technique. The fracture analysis was carried out for the cracks propagating from a region with high elasticity towards low elasticity, as well as the cracks propagating from a region with low elasticity towards high elasticity. The analysis includes cracks propagating (1) at an almost steady speed, and (2) with the rapid increase, followed by a decrease in speed. For cracks with almost constant velocity, the SIFs were greater when a crack started from a high elasticity region, as compared to the cracks which initiated from a low elasticity region. For cracks propagating with rapid acceleration and deceleration, when the strain energy accumulated in the material due to an increase in load or stress was released at the moment of crack propagation, the SIF was momentarily lowered by approximately 45 %–50 % of the static SIF(before crack initiation), which subsequently increases by approximately 30 % eventually, the crack acceleration approaches nearly zero; the SIF decreases and increases respectively as the crack propagates in a material with decreasing and increasing modulus of elasticity.

     

The role of casting porosity in fatigue properties of Al−Si 319 lost foam cast alloy

The tensile, fracture toughness and fatigue properties of Al−Si 319 lost-foam-cast alloy were determined at room temperature. The fatigue properties of this alloy were also determined at 150°C. Fatigue cracks were always initiated at the largest casting pore. Initial pore sizes were measured using a scanning electron microscope. Surface replication showed that majority of the fatigue life was spent in fatigue crack propagation and permitted the estimation of the constants in the Paris power law and the threshold stress intensity factor (ΔK _th ). The role of internal casting porosity was quantified using a linear elastic fracture mechanics (LEFM) model for fatigue crack growth. The predicted lives agreed with the measured values within a factor of two.     

Approximation method for the calculation of stress intensity factors for the semi-elliptical surface flaws on thin-walled cylinder

A simple approximation method for the stress intensity factor at the tip of the axial semi-elliptical cracks on the cylindrical vessel is developed. The approximation methods, incorporated in VINTIN (Vessel INTegrity analysis-INner flaws), utilizes the influence coefficients to calculate the stress intensity factor at the crack tip. This method has been compared with other solution methods including 3-D finite element analysis for internal pressure, cooldown, and pressurized thermal shock loading conditions. For these, 3-D finite-element analyses are performed to obtain the stress intensity factors for various surface cracks witht/R=0.1. The approximation solutions are within ±2.5% of the those of finite element analysis using symmetric model of one-forth of a vessel under pressure loading, and 1–3% higher under pressurized thermal shock condition. The analysis results confirm that the approximation method provides sufficiently accurate stress intensity factor values for the axial semi-elliptical flaws on the surface of the reactor pressure vessel.     

Deformations of a simplified flip chip structure under thermal testing inspected using a real-time Moiré technique

Deformations of a Si-epoxy-FR4 (simplified flip chip) structure under thermal testing were inspected with a real-time Moiré technique. Specimens without cracks and specimens with a crack at the silicon-epoxy interface were prepared. The measurement results showed that the maximum deformation appeared at the edge. When the specimen was cooled to 20 °C, there was residual plastic deformation in the specimen. The creep effect was more dominant in the FR4-epoxy interface. Upon cooling to 20 °C, the specimen experienced partial strain recovery. To characterize the behavior of the interfacial crack, stress intensity factors K_I and K_II, and the strain energy release rate G in the vicinity of the crack tip were calculated using the measured deformations to conduct a quantitative study. It was observed that a sharp strain gradient occurred at the crack tip. K_I and K_II were dependent on temperature, and G was dominated by K_I for the interfacial crack in the specimen.     

Effect of structure of aging metastable austenitic steels on the signal from an eddy-current transducer

Eddy-current parameterf ₀ of the N26T3 steel has been studied as a function of both the aging temperatureT _ag=20–800°C and the time τ of exposure to a constant temperature of 550 and 600°C up to 6h. In the initial state, the steel had two phases: (1) cooling-induced martensite+austenite (α+γ) or (2) strain-induced martensite+austenite (α′+γ). The parameterf ₀ drops monotonically as τ increases, and this drop is the faster, the higherT _ag. The parameterf ₀ changes nonmonotonically with the aging temperature. In addition to the initial two-phase structures, the one-phase γ structure has also been studied. The parameterf ₀ grows monotonically with the plastic cold strain and changes nonmonotonically with the aging temperature (20–800°C). Observed changes inf ₀ have been explained.     

Fatigue cracks at notches

The failure of a component or specimen due to a fatigue crack growing from a notch is considered. Previous methods of analysis involving stress and strain concentration factors are shown to be inadequate. By defining equivalent cracks in notched and un-notched situations as cracks with equal growth rates, the concept of notch contribution to crack length is introduced. Theoretical notch contributions are obtained for a variety of central and edge elliptical notches via stress intensity factor solutions. These results when extended to a very wide range of general notch shapes can be reduced to a useful and simple design rule where e is the contribution to a crack of length l growing from a notch of depth D and root radius . This rule combines the size and shape effects long known to affect fatigue behaviour and defines the extent of the notch field as 0·13√(D).The fatigue crack propagation lives of a wide variety of notches were estimated by this rule and comparisons with experimental values revealed very small errors normally well within the scatter of fatigue lives.The design rule is extended to enable the conventional stress intensity factor method to be employed. A fatigue concentration factor is proposed which takes into account the presence of a fatigue crack which all previous methods have ignored.     

Dynamic mixed mode crack propagation behavior of structural bonded joints

The stress field around the dynamically propagating interface crack tip under a remote mixed mode loading condition has been studied with the aid of dynamic photoelastic method. The variation of stress field around the dynamic interface crack tip is photographed by using the Cranz-Shardin type camera having 10⁶ fps rate. The dynamically propagating crack velocities and the shapes of isochromatic fringe loops are characterized for varying mixed load conditions in double cantilever beam (DCB) specimens. The dynamic interface crack tip complex stress intensity factors,K ₁ andK ₂, determined by a hybrid-experimental method are found to increase as the load mixture ratio of y/x (vertical/horizontal) values. Furthermore, it is found that the dynamically propagating interface crack velocities are highly dependent upon the varying mixed mode loading conditions and that the velocities are significantly small compared to those under the mode I impact loading conditions obtained by Shukla (Singh & Shukla, 1996a, b) and Rosakis (Rosakis et al., 1998) in the USA.     

A study on the initial crack curving angle of isotropic/orthotropic bimaterial

In this paper, when the initial propagation angle of a branched crack is calculated from the maximum tangential stress criterion (MTSC) and the minimum strain energy density criterion (MSEDC), it is essential that you use stress components in which higher order terms are considered and stress components at the position in a distance 0.005 mm from the crack tip ( =γ . When an interfacial crack propagates along the interface at a constant velocity, the initial propagation angles of the branched crack are similar to the mode mixities (phase angle) and the theoretical values obtained from MTSC and MSEDC. The initial propagation angle of the branched crack depends considerably on the stress intensity factor K{IN2}.     

Effect of elastic modulus mismatch on the contact crack initiation in hard ceramic coating layer

Effect of elastic modulus mismatch on the contact crack initiation is investigated to find major parameters in designing desirable surface-coated system. Silicon nitride coated soft materials with various elastic modulus mismatch,E _c /E _s = 1.06 — 356 are prepared for the analysis. Hertzian contact test is conducted for producing contact cracks and the acoustic emission detecting technique for measuring the critical load of crack initiation. The implication is that coating thickness and material strength are controllable parameters to prevent the initiation of contact cracks resulted from the elastic modulus mismatch in the hard ceramic coating layer on the soft materials.     

The reflected caustics method for the evaluation of mode III stress intensity factor

The optical method of reflected caustics, which has been applied to the evaluation of stress intensity factors in cracked plates deformed under mode I and II, is extended in this paper to the evaluation of the same factor in cracked plates subjected to mode III deformation. It is shown that the method of reflected caustics is capable of detecting and evaluating this factor, whereas all the other experimental methods, i.e. photoelasticity, holographic interferometry and especially the method of transmitted caustics are incapable to yield this quantity. Based on the first-order approximation of the elastic solution around the crack tip and on Sneddon's formulas, the theory of formation of the reflected caustics is developed and the characteristic geometric properties of this envelope curve are defined. It is shown that this envelope is again a generalized epicycloid, whose characteristic dimensions are directly related to K_III. Experimental evidence with specimens made of optically isotropic materials (plexiglas), elastically loaded, has corroborated the theoretical results.     

On the Pressure Dependence of Shear Strengths in Sliding, Boundary-Layer Friction

Density functional theory, as implemented in a full-potential linearized augmented plane wave method, flair, is used to calculated the pressure-dependent shear strength S of KCl on a Fe(100) substrate and the results are compared to the experimental dependence given by S = S₀ + aP S = S_{0} + \alpha P , where P is the contact pressure and S ₀ = 65 ± 5 MPa and α = 0.14 ± 0.02. Calculations were performed for a KCl bilayer enclosed between two Fe(100) slabs, where the energy was found to vary harmonically as a function of the separation between the outermost layers. Thus, a simple analytical model was developed for the pressure-dependent shear strength of the film, which includes both linear and quadratic pressure dependence. However, the coefficient of the quadratic term was found to be much smaller than the linear term, leading to the linear shear-strength pressure dependence found experimentally. The calculated values of S ₀ 〈10〉 = 64 ± 9 and S ₀ 〈11〉 = 69 ± 8 MPa are in excellent agreement with experiment, while α 〈10〉 and α 〈11〉 equal 0.05 ± 0.01, somewhat lower than, but within the same range as the experimental values.     

An elastic-plastic stress analysis in silicon carbide fiber reinforced magnesium metal matrix composite beam having rectangular cross section under transverse loading

In this work, an elastic-plastic stress analysis has been conducted for silicon carbide fiber reinforced magnesium metal matrix composite beam. The composite beam has a rectangular cross section. The beam is cantilevered and is loaded by a single force at its free end. In solution, the composite beam is assumed perfectly plastic to simplify the investigation. An analytical solution is presented for the elastic-plastic regions. In order to verify the analytic solution results were compared with the finite element method. An rectangular element with nine nodes has been choosen. Composite plate is meshed into 48 elements and 228 nodes with simply supported and in-plane loading condations. Predictions of the stress distributions of the beam using finite elements were overall in good agreement with analytic values. Stress distributions of the composite beam are calculated with respect to its fiber orientation. Orientation angles of the fiber are chosen as 0°, 30°, 45°, 60° and 90°, The plastic zone expands more at the upper side of the composite beam than at the lower side for 30°, 45° and 60° orientation angles. Residual stress components ofσ _x andτ _xy are also found in the section of the composite beam.     

Fatigue crack propagation behavior in STS304 under mixed-mode loading

The use of fracture mechanics has traditionally concentrated on crack growth under an opening mechanism. However, many service failures occur from cracks subjected to mixed-mode loading. Hence, it is necessary to evaluate the fatigue behavior under mixed-mode loading. Under mixed-mode loading, not only the fatigue crack propagation rate is of importance, but also the crack propagation direction. In modified range 0.3≤a/W≤0.5, the stress intensity factors (SIFs) of mode I and mode II for the compact tension shear (CTS) specimen were calculated by using elastic finite element analysis. The propagation behavior of the fatigue cracks of cold rolled stainless steels (STS304) under mixed-mode conditions was evaluated by using K_I and K_II(SIFs of mode I and mode II). The maximum tangential stress (MTS) criterion and stress intensity factor were applied to predict the crack propagation direction and the propagation behavior of fatigue cracks.     

Initiation and propagation laws of the glass cracks in specimens subjected to normal loading under the action of symmetric wedges

With more and more applications of glass in advanced fields of science, the demand for glass machining precision has increased greatly. More and more attention is being paid to glass cutting because precise glass parts with various shapes can be obtained at high efficiency and low cost. To improve the machining precision of part surfaces and to facilitate tool design and cutting parameter selection, the initiation and propagation laws of glass cracks in specimens subjected to normal loading by symmetric wedges were investigated. Research results show that initiation and propagation laws are the same with interior symmetric wedge angles of 30°–120°, while the laws are different with interior symmetric wedge angles equal to or more than ≥150°. The relationship between medial crack length and normal loading was also investigated when specimens were indented by symmetrical wedges with interior angles of 30°–120°. __________ Translated from Journal of South China University of Technology (Natural Science Edition), 2004, 32(7) (in Chinese)     

埋头紧固件椭圆形表面裂纹应力强度因子的三维有限元分析

采用裂端为 2 0节点奇异单元的三维有限元模型 ,对工程中常见的 90°、10 0°、12 0°埋头紧固件埋头部分与直杆部分相交处的表面裂纹 ,以及直圆杆表面裂纹的应力强度因子进行了计算分析。给出了圆直杆以及 90°、10 0°、12 0°埋头紧固件椭圆形表面裂纹的应力强度因子拟合公式。研究结果表明 ,本文的应力强度因子计算方法和计算结果是有效的。本文的工作可为埋头紧固件的损伤容限分析提供应力强度因子。     

Effect of fiber orientation on interfacial fracture toughness for adhesively bonded composite plates

In this study, interfacial fracture toughness was investigated experimentally and numerically in laminated composite plates with different fiber reinforcement angles bonded with adhesive. The composite plates are four-layered and the layer sequence is [0º/θ]_s. DCB test was applied to composite plates reinforced with epoxy resin matrix and unidirectional carbon fiber. The experimental sample model for the DCB test was made using the ANSYS finite element package program. In the numerical study, four layered composites were prepared in three dimensions. Under critical displacement value; mode I fracture toughness at the crack tip was calculated using VCC (virtual crack closure) technique. Numerical values consistent with experimental results have presented in graphical forms. At 60o and 75° the greatest fracture toughness was obtained. In addition, numerical results have shown that fiber orientation prevents the uniform distribution of stress on the interface crack tip and causes stress accumulation, especially at the edge of the plate.

     

Asymptotic approach to the constant velocity of hydrogen delamination growth

We consider delamination crack growth controlled by gas diffusion into the crack. If the gas is accumulated inside the delamination, after some incubation period, the crack starts growing under the pressure of the accumulated gas. An important example is given by hydrogen-induced delamination. Hydrogen absorbed by a metal is typically dissolved in the proton form within the lattice. Some of the protons reach the surface of pre-existing or freshly created cracks or delaminations where they recombine with electrons and form molecular hydrogen in the crack cavity. Since the molecular form of hydrogen is thermodynamically more stable, this process leads to accumulation of hydrogen gas inside the delamination crack. Under the excessive hydrogen pressure fracture often takes place even in the absence of any additional external loading. It is especially dangerous if the metal is covered with coating, where because of the dissimilarity of materials, microscopic voids appear more frequently. This leads first to hydrogen precipitation within these coating–metal interface voids and then to their development. This results in the delaminating of the coating. As it is well known, in the absence of aggressive media the cracking resistance of a material is characterized by a unique constant: so that if the stress intensity factor is smaller than this constant, the crack is fixed, and if it is larger, the crack moves in a dynamic regime. But in the presence of aggressive media, particularly hydrogen, the crack development is characterized by a smooth kinetic function v(K_I), which is dependence of the crack velocity on the stress intensity factor, with the lower threshold value of resistance K_scc smaller than the static critical stress intensity factor K_Ic. As numerically shown in the author's earlier work for the internal crack growth, the crack velocity first increases in accordance with the kinetic function, until reaching some value of v_s, and remains at the same level afterwards. Since, as has been obtained by the author in the previous paper, kinetic equations for internal and delamination cracks are essentially identical, the same conclusion can be derived for the latter. In this paper the stability and asymptotic approach to the constant velocity of delamination growth is proved analytically. If v_s is known, the corresponding value of the stress intensity factor, K_s, is obtained by substituting v_s into the kinetic function v(K_I).