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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.