Search

Delamination or interlayer cracking is one of the most important imperfections in composite materials. The existence of this defect in a structure reduces the strength and, as a result, disables the structure. To analyze the effective factors in interlayer separation, it is necessary to analyze the effective loading parameters. In this paper, the effect of the change in loading rate on the failure mechanism in I failure mode was analyzed using an acoustic emission for unidirectional samples made of glass fiber/epoxy resin. At first composite, samples were made according to standard and placed at different rates of displacement under loading. Force data, displacement and crack growth rate for different loading rates were used to calculate the exact strain energy release rate. In addition to the extensometer, the Dino camera was used. In this paper, a high-reliability method was proposed to evaluate the separation between the layered composites using acoustic emission method. By comparing mechanical data and acoustic emission signals, the mechanical behavior obtained for each loading rate was determined so that the mechanical behavior of the composite material varied with the change in loading rate. The results show that, with increasing loading rates, the resin lost its elastic properties, and the specimen exhibited a more rigid behavior and is quite rigorous so that the fracture failure process is changed. The failure processes and crack growth rate was validated by use of acoustic emission signals. There was good agreement between the fracture toughness of accretion of acoustic emission signals with the experimental values. [ABSTRACT FROM AUTHOR]

In this paper, the Finite Cell Method (FCM) is used to predict the ductile damage and crack evolution in ductile materials under small strains and nonlinear isotropic hardening conditions. In the first step, a fully coupled elastic-plastic-damage model based on modified Lemaitre ductile damage model was developed and implemented into FCM implicit codes. Also, the effect of micro-crack closure, which may dramatically decrease the rate of damage growth under compression, was incorporated and its computational implementation discussed. The FCM is the result of combining the p-version finite element and fictitious domain methods, and has been shown to be effective in solving problems with complicated geometries for which the meshing procedure can be quite expensive. It, therefore, combines fast and simple mesh generation with a high convergence rate inherited from p-FEM. The performance of the FCM and damage model was verified by means of numerical examples and the results were compared with experimental observation. The results showed that modified Lemaitre damage model can be used as a quick and accurate tool to predict ductile damage and fracture in metal forming processes. [ABSTRACT FROM AUTHOR]

In this paper, the new theory is proposed to investigate the fracture behavior of cracked composite materials. Based on this theory, crack is created and distributes in the isotropic matrix. Therefore, contrary to the previous fracture theories of composite materials presuming that crack growth occurs in anisotropic homogenous material, the new theory assumes that crack growth occurs in the isotropic matrix. It states that the isotropic matrix is affected by fibers in the composite structure of the material. In this approach, fibers are considered as isotropic matrix reinforcements and the reinforcement effects are defined as coefficients in stress state of the isotropic matrix. The coefficients are called reinforcement factors and derived via three different approaches to study the arbitrary crack in 2D materials. The reinforcing effects of fibers are obtained when tension across and along fibers and shear loadings exerted on the body. The three methods demonstrate that the reinforcement factors depend on elastic properties, crack growth location and the crack and fiber orientations. However, the method, derived from the micro-mechanic approach, displays their dependence on the fiber volume fraction. Comparing the results of these cofficients with the existing fracture theories illustrates the efficiency and ability of the reinforcement factors in investigation and explanation of the fracture behavior of orthotropic materials. [ABSTRACT FROM AUTHOR]

One of the applications of composite materials in the oil and gas industry is to repair worn metal pipelines. Calculating the strain energy release rate of the first failure mode is an important criterion for testing the bond strength and predicting the failure of these types of structures. In this paper, the rate of strain energy release during crack growth in bonding a composite patch to a steel substrate is investigated. In this regard, using the theory of elastic beam first, a new method is proposed to calculate the thickness of the metal and composite for Unlike Double Cantilever Beam (UDCB). This is due to the fact that the standard for experimental test procedure of strain energy release rate (ASTM-D5528) is for symmetric double cantilever beams. In this study, samples are fabricated from composite consisting of unidirectional fiberglass/ epoxy resin with harder in the upper and steel in the lower half of the beam. After sample fabrication, the strain energy release rate of UDCB and Asymmetric Unlike Double Cantilever Beam (AUDCB) are calculated experimentally. In addition, for the separation of first and second failure modes in symmetric and asymmetric samples, finite element simulation based on the virtual crack closure technique is presented. This analysis is to qualify the accuracy of the proposed equation for the thickness of unlike beams to achieve the first failure pure mode of symmetric samples. Also, it calculates the contribution of the first and second modes of failure in the strain energy release rate of AUDCB samples. [ABSTRACT FROM AUTHOR]

Peridynamics is a nonlocal version of the continuum mechanics, in which partial differential equations are replaced by integro-differential ones. Due to not using spatial derivatives of the field variables, it can be applied to problems with discontinuities. In the primary studies, peridynamics has been used to simulate crack propagation in brittle materials. With proving the capabilities of peridynamics, the idea of using this theory to simulate crack propagation in quasi-brittle materials and plastic behavior has been proposed. To this end, formulations and models based on peridynamics have been developed. Meanwhile, the high computational cost of peridynamic methods is the main disadvantage of this theory. With the development of peridynamic methods and introduction of hybrid methods based on peridynamics and local theories, the computational cost of peridynamic methods has been reduced to a large extent. This paper introduces peridynamics and the models based on it. To this end, we first review peridynamics, its formulations, and the methods based on it. Then we discuss the modeling of quasi-brittle materials, simulation of plastic behavior and employing the differential operators in this theory.

In a structural analysis, dynamic response of a crack is of significant importance as well as the impacts of elastic waves on stress intensity factors (SIF). In this paper, dynamic analyses of multiple cracks on a half-plane subjected to anti-plane shear stresses are presented. Stress intensity factors are calculated and the interaction of elastic waves with the boundary of plane and the cracks' tips is investigated at different locations. The distribution discontinuous displacement techniques are used, enabling us to solve the crack problems in dynamic fracture mechanics. Integral transformations (Laplace and Fourier) are applied to elastodynamics equations and by using a set of appropriate boundary conditions, the crack problem is solved through discontinuous displacement method. As a result, the stress equations with hypersingularity terms are obtained. Using Chebyshev series expansion and collocation points in Laplace domain, the crack solution is achieved. Finally, different algorithms of numerical Laplace inversion are presented and the stress intensity factors (SIF) are obtained. The presented results are compared with published data and a good agreement is observed. Moreover, it is also demonstrated that the present theoretical study is capable of modelling multiple cracks with different arrangements. [ABSTRACT FROM AUTHOR]

Seismic behavior in concrete weights due to the need to increase the safety of dams during earthquakes has been very much considered in recent years. Most concrete dams are exposed to cracking damage. These cracks are created by various factors such as operation, concrete treatment, volumetric changes in concrete mass, loads, etc., and the possible expansion of these cracks may reduce the efficiency, failure and instability of such dams. The use of fracture mechanics as a new method for estimating the instability and durability of concrete dams has been recommended in world-renowned journals and papers. Due to the sensitivity of the subject of crack-erosion in concrete dams, as well as considering that in the past the behavior of the Gotvand dam was not investigated due to trapping, this study examined the behavior of the Gotvand Regulatory Dam due to trapping Has been. Abkhus software is used to model this research. Parameters that are considered as variables include the length of the variable for the cracks and the crack angle. In the case of the effect of the crack angle, the results indicate that the worst case for the dam section is to leave a negative angle. The horizontal displacement value for the -30 degree angle is 0.031 m and is 0.025 m for cracking at an angle of +30 degrees. Also, the increase in the length of the crack increased the amount of displacement, tension and rebound response in the dam. Maximum reciprocating photo in a model with a length of 200 cm, with a crack angle of +45, is 4% higher than the model with a gap of 30 cm and equal to 1.78E+8 of Newton.

In recent years, wind energy experienced faster growth compared with the other renewable energies. The interaction between fluid and structure becomes more important as the wind turbine size and its power production capacity increases. In the present research, the effect of wind speed and blade materials on static deformation of a small size horizontal axis wind turbine blade is investigated. The shaft torque and root flap bending moment values obtained from simulation are in a good agreement with experimental data. Results demonstrated that the deformation of the blade increases as the wind speed grows, although the increase rate has declined in the mean wind speed range because of the occurrence of separation phenomenon on the blade surface. The effect of blade components materials on blade deformation was investigated and the least deformed configurations were introduced. The thickness of the designated blade components has been investigated by means of the maximum strain theory. The final thickness of the skin, spur and root was estimated by 2.1 mm, 2.8 mm and 10 mm respectively which are 30% less than the primary one. [ABSTRACT FROM AUTHOR]

In the present study, in order to evaluate the elastic displacement field and subsequently the fracture parameters within the isotropic homogeneous elastic solids with the edge or interior cracks, the extended finite element method with level set technique was used to avoid the disadvantages associated with the standard finite element method. An over deterministic least squares method was utilized to determine the crack stress intensity factors as well as the coefficients of the higher order terms in the Williams' asymptotic series solution for structures containing crack in various modes of failure by fitting the series solution of displacement fields around the crack tip to a large number of nodal displacements obtained from the extended finite element method. For validating the results, several cracked specimens subjected to pure mode I, pure mode II, and mixed modes I/II loading were performed. Comparison with results available from the literature obtained by other formulations reveals the efficiency and simplicity of the proposed method, and demonstrate the capability of it to accurately capture the crack stress intensity factors and the coefficients of higher order terms. [ABSTRACT FROM AUTHOR]

The investigation of failure modes of plates and behavior of various resistive structures to destructive effects of explosive waves, due to its importance in design of blast resistive structures, has been of interest to researchers for a long time. In this study, three different methods of numerical simulation of blast wave issues were carried out to evaluate and compare with the experimental results. As a consequence, by the means of study of clamped isotropic square plates under shockwave loading from various weight and distance of charges, the couple of ALE and ConWep methods were approved to have 8.54 per cent error in comparison with ALE and ConWep methods individually. Given that in the coupled approach and ConWep method, the equivalent weight of TNT for different types of explosives is needed, the equivalent weight of TNT for C4 was estimated by 1.14, and according to the empirical pressure-time chart and empirical equation for pressure in the air, this coefficient was proved to be right and the pressure and impulse charts for TNT and C4 explosives with the same weights was studied. [ABSTRACT FROM AUTHOR]

Bolt joints play an important role in the industries, so the estimation of fatigue life of bolts is an essential task. The aim of present study is estimation of fatigue life of connection bolts of two flanges in reinforced cylindrical shell with cutout. Two groups of data are needed for mentioned bolt: fatigue properties of bolt and value of stress of bolt due to applying load to structure. So, two paths have been gone. First, the fatigue properties of bolt have been measured in laboratory according to ISO 3800 standard. For this purpose a specific fixture was designed and manufactured which provided testing different bolts. By doing fatigue experiments, the fatigue properties of mentioned bolt such as fatigue limit and Basquin's equation constants (fatigue strength coefficient and fatigue strength exponent) have been measured. Fracture mechanism and fracture surface have been investigated, too. Afterward, in the next step the value of stress in bolt that is subjected to mix loading has been calculated by using of FE modeling. Because of problem complexities, cost of three dimensional analysis of this problem increases, so analysis of the problem has been performed by shell-to-solid sub-modeling technique. At the end, by calculating the nominal stress of bolt from FE modeling and using fatigue properties witch obtained from experiments, life of the mentioned bolt has been estimated. [ABSTRACT FROM AUTHOR]

A vibration absorber is used to reduce vibration of an Euler-Bernoulli beam with elastic supports subjected to a moving oscillator. Dynamic response of the beam under moving harmonic exciter with different moving velocities and different parameters are obtained. The critical velocity of the moving oscillator is determined and absorber parameters are optimized by numerical algorithm and effect of mass and damping on the absorber performance is investigated. When absorber is applied to the beam, effect of crack occurrence on its performance is investigated. Crack is assumed to be open and is modeled by sectional flexibility increase. Two different cases considered for crack severity. In each case, optimal absorber for intact beam is applied and dynamic response of the midpoint of the beam with different velocities of moving exciter is obtained. Results show although crack can increase dynamic deflection of the beam with absorber, dynamic deflection of cracked beam without considering absorber is higher than dynamic deflection of cracked beam with absorber used. It is found that the vibration absorber designed for intact beam keeps its performance in dynamic deflection reduction after crack occurrence and changing in structural dynamic. [ABSTRACT FROM AUTHOR]