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In this paper, the equivalent layup method to estimate the delamination initiation of asymmetric double cantilever beam (ADCB) is presented. A relation for critical strain energy release rates (SERR) of unidirectional (UD) and multidirectional (MD) laminates, as a criterion for crack growth initiation, was presented. For finite element analyses, the ADCB specimens were modeled in ABAQUS/Standard software and SERR is determined using the virtual crack closure technique (VCCT). According to this method, the ability of estimating delamination initiation without using experimental tests and finite element modeling of MD specimens is provided. This method reduces the volume of calculations FEM and experimental tests significantly. Accuracy of the proposed method has been verified by experimental data. Also, the total strain energy release rate analyzed by this method is compared with the FEM and theoretical results. Results show that SERR of MD lay-ups can be predicted by measuring SERR of the unidirectional specimens using FEM method with an error less than 10%.

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In the present study, using a precise shear lag parameter in an extended shear lag model, by considering the effects of out of plane shear stresses, the stress fields distribution as well as strain fields and displacement distributions will be obtained for a typical [0m/90n]s cross ply composite laminate containing a specified matrix cracking density. Then, the stiffness degradation due to existence of matrix cracking in these cross-ply composite laminates will be evaluated and specific damage parameters, which affect the stiffness matrix of composite ply, will be defined. Furthermore, using the concept of fracture mechanics by applying two different criteria including the maximum stress and strain energy release rate, the matrix cracking initiation and evolution as well as induced delamination propagation will be studied. Finally, a closed form relation will be presented which predicts the evolution of matrix cracking under uniaxial loading conditions in cross-ply composite laminates. At last, the obtained results by present study will be compared with available semi-analytical and experimental results. The obtained results reveal that the proposed closed form relations by the authors have a less difference with experimental results in comparison with the previous semi analytic results.

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In this paper, the variations of the stress intensity factor and energy release rate have been investigated based on the displacement correlation and modified crack closure integral methods for external surface cracks in the autofrettaged functionally graded cylinder (FGC). Mechanical properties vary in the radial direction according to the desired function. Isotropic material behavior and bilinear elastoplastic stress-strain relationship are considered for the FGC. Autofrettage process induces the tensile residual stresses in the outer parts of the cylinder wall, which causing the undesirable effects on the external surface cracks. Many variables affect the distribution of tensile residual stresses. Effects of autofrettage ratio, volume fraction of material and cylinder thickness on the residual stress changes and addition, changes in the size and direction of surface cracks on the stress intensity factor and energy release rate are studied. The results show that the volume fraction has the greatest effects on both crack parameters. The axial cracks are critical in compared with circumferential and angled cracks. The principle of superposition can be used to determine the combinational effects of the residual stresses and applied loads on the behavior of cracks in the graded materials.

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In this paper, the strain energy release rate of first mode of failure in the adhesive bonding of two composite plates composed of unidirectional glass fiber is calculated using double cantilever beam specimen. Araldite 2011 adhesive connection which is widely used in the aerospace industry has been employed. Strain energy release rate is calculated by the modified beam method, compliance calibration method and modified compliance calibration method from experimental results. For modeling crack growth in adhesive bonding of two composite plates, the Extended Finite Element Method has been employed. Average value of critical strain energy release rate calculated by the modified compliance calibration method is considered as software input. After comparing force - displacement curve obtained from experimental data and numerical solution that represents good precision of the Extended Finite Element Method in calculating the maximum force and corresponding displacement and also linear part of force-displacement curve, strain energy release rate - force curve, stress intensity factor – force curve, strain energy release rate – displacement of the load effective point and failure stress - stress intensity factor curve are evaluated.

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The unidirectional composite DCB specimen is considered as two finite length Timoshenko beams, attached together along a common edge except at the initial delamination length. Because of symmetry, only one half of the specimen is considered, which is partly free and partly resting on an elastic foundation. The problem is analytically solved by considering Timoshenko beam resting on Winkler and Pasternak elastic foundations and fracture toughness is generally derived. In the prior researches on this specimen using Timoshenko beam theory, the effect of the ligament length on the energy release rate was ignored. This research presents the solution for finite ligament length. Besides, the effect of ligament length on energy release rate and its minimum value that makes the energy release rate independent of the ligament length, is presented. For the special case when the ligament is large compared with the beam thickness, a closed form solution is derived for Timoshenko beam resting on Winkler elastic foundation. The analytical results are compared to prior researches on this subject and a good agreement is observed. The fracture toughness and compliance obtained by Timoshenko beam resting on Winkler elastic foundation predicts more accurate results with respect to experimental results.

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Dual laminate pipes made of thermoset polymer composite structure and thermoplastic liner are the only alternative in pipelines conveying high temperature corrosive fluids. Investigating the bonding between thermoset composite and thermoplastic liner is very important in these pipelines. Calculating the strain energy release rate of first mode of failure is very important criteria in bonding strength and failure of doulas pips. ASTM-D5528 is the standard for experimental test procedure of strain energy release rate of symmetric double cantilever beam. In this study, using the classical laminates theory, the general equation for determination the laminates thicknesses in unlike double cantilever beam is presented, for the first time. To study the validity of the equation, in unlike double cantilever beam samples consists of laminates with different thicknesses are manufactured for the experimental tests. Upper, lower and bonding regions consist of composite made of unidirectional fiberglass/Vinylester resin, PVCU and epoxy or Vinylester primers, respectively. The samples of this study are manufactured base on the practical case studies of chemical fluid pipelines with chlor-alkali process like Arvand Petrochemical units. The main aim of this work is to help manufacturers of these unites equipment to have practical guideline. To qualify the efficiency of the proposed equation, finite element simulation base on the virtual crack closure technique is presented. Good agreement is achieved in comparing the numerical and experimental results that shows the efficiency and accuracy of the proposed equation.

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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.

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The sandwich panel is a combination of a soft core and two stiff, high-strength facesheets. In many cases, the connection between the facesheet and the core is considered as a critical point that can damages the integrity of the sandwich structure. In this study, the debonding toughness between the facesheet and the core in sandwich beams with grooved cores made of Kevlar 49/polyester facesheets and polyurethane foam core has been measured experimentally. The values ​​of the strain energy release rate obtained at the onset of crack growth for the tested specimens are in the range of 340 (J/Square meters) and increase with the crack growth up to 500 (J/Square meters). One of the innovations of the present study is to investigate the effect of grooving the core of the sandwich panel on the resistance of the structure to the growth of interfacial cracks. The results show that by placing the groove inside the core of the sandwich panel, the interfacial crack stops during growth by hitting each groove and requires higher force to restart its growth. This phenomenon increases the resistance of this type of structure against the growth of cracks in the face/core area. In this research, a model based on cohesive zone theory was used to simulate crack growth in the tested specimens. Comparison of load-displacement curves obtained from the analysis shows that the proposed model has a good ability to predict the behavior of the structure under similar loading conditions.

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Road pavements are one of the most important assets of any country, and tremendous amounts of budgets are allocated for their maintenance every year. Unexpected distresses in asphalt pavement cause many financial losses. Winter maintenance of roads and infrastructures and the study of the effects of anti-icers and deicers on the asphalt pavements have always been of interest to researchers, departments, and agencies in the field of roads and transportation. As a contribution to this task, the present study was conducted to evaluate the effect of Zycotherm on the fracture behavior of asphalt mixtures in the presence of moisture and deicers. In order to achieve the research objectives, PG58-22 bitumen and siliceous materials were used to prepare the asphalt mixture and also Zycotherm was used to modify the asphalt binder. Data were collected by testing on laboratory samples. The asphalt mixture samples were conditioned in the presence of distilled water and solutions of brine, calcium magnesium acetate, and potassium acetate in their normal concentration for 96 hours at 60°C. Then, the fracture toughness of the specimens at low temperatures (K_1c) and the critical strain energy release rate (J_c) at intermediate temperatures were measured by performing a semi-circular bending test (SCB). The results showed that simulation of low-temperature environmental conditions in the vicinity of distilled water and all deicers reduces the fracture toughness of asphalt mixtures compared to the dry sample. Brine solution has the most negative effect among all the deicers and reduces the K_1c parameter by approximately 30%. On the other hand, Zycotherm maintains the fracture toughness of the asphalt mixture at low temperatures in the vicinity of distilled water and deicers at an almost constant level and recovers about 70% of the lost fracture strength of the sample conditioned in the brine solution. The effect of Zycotherm at intermediate temperature is different and causes the softening of bitumen and the reduction of the critical fracture force and the reduction of the critical strain energy release rate. This reduction is 34% and 32% for the dry sample and the specimen in the presence of brine solution, and 23% and 12% for the samples in the presence of calcium magnesium acetate and potassium acetate, respectively, compared to the sample made of neat bitumen. Also, samples in the vicinity of distilled water and potassium acetate solution showed no significant change in their critical strain energy release rate compared to samples in dry conditions. Visual inspection also revealed that calcium magnesium acetate causes additional stress and cracking in the samples. In a general summary and based on the obtained results, Zycotherm has a positive effect on the fracture toughness of the asphalt mixture at low temperatures but reduces the J_c parameter at intermediate temperatures. All specimens have the minimum critical strain energy release rate recommended by ASTM D-8044 at intermediate temperatures. Potassium acetate has no effect on the fracture toughness of asphalt mixtures at low and medium temperatures and can be an appropriate alternative in comparison with other deicers in winter road maintenance.

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The proper method for jointing Carbon fiber reinforced polymers (CFRP) to aluminum, which causes uniform stress distribution, more suitable fatigue performance and weight reduction, is adhesive bonded joint. In adhesive bonding, the interface of adhesives and adherent are sensitive areas for the initiation and propagation of failure. In order to eliminate surface contamination, adherents must be surface treated. In this research, the effect of the functional pattern of laser surface treatment on the strength of aluminum/composite adhesive bonded joint in the mode I fracture has been investigated. At first, laser surface treatments were performed throughout the specimen in order to find the parameters of the laser device that increase the strength of the adhesive bonding by creating a suitable surface quality. After that, the functional pattern of laser surface treatment with the appropriate parameters for ablation and cleaning of the adhesive surface is done. The results show a 15.5% increase in the critical strain energy release rate of the mode I for the all-over laser surface treatment specimen compared to the sanding method. Meanwhile, with the functional pattern of laser surface treatment, the critical strain energy release rate of the mode I has increased by 5.9% and 22.4% compared to all-over laser surface treatment and sanding, respectively. Examining the fracture surface of the specimen shows the delay in crack growth in the specimen of the functional pattern with changes from the adhesive failure to the fiber tearing, which has improved the strength of the adhesive bonding.