conditioning and the post-conditioning mechanical testing in one procedure. This eliminates all the problems introduced by the conventional environmental-mechanical test method as mentioned above. A schematic diagram of this HPHT in-situ test system is shown in Fig. 36.1. This in-situ mechanical testing system has the capability of accommodating tensile tests per ASTM D638 test standard, among others, with the feature of being submerged in water or other fluids at elevated temperatures. This is of interest because strength and stiffness of composite materials vary with temperature and moisture content due to hygrothermal degradation [3–5]. Yet, in the above publications in-situ mechanical tests were conducted with load and displacement measurements only, and the in-situ strain measurement technique was not established. A widely used method for strain measurement in the field of experimental stress analysis is the resistance strain gage [6]. It consists of a strain gage that changes resistance when subject to mechanical strain. However, resistance change is not practical to measure; thus, an electrical circuit is used to generate a measurable voltage output as a result of mechanical strain. This electrical circuit is called the Wheatstone bridge sensing circuit. The Wheatstone bridge is largely used in strain gage technology due to its ability to detect small changes in resistance, produce zero-output voltage when the test part is at rest, and compensate for temperature-induced resistance changes in the strain gage circuit [7]. Following, Yuan and Sequera [8] presented an in-situ tensile strain measurement method based on traditional resistive strain gage technology based on the Wheatstone bridge sensing circuit. Selected strain gages were mounted on specimens and in-situ tensile strain measurements were conducted in a submerged wet condition at an elevated temperature. A flexible water-resistant sealing material was used on one side of the specimen to seal the strain gage completely and protect it from the wet condition, yet the opposite side of the specimen, the side with no strain gage, was exposed to hot-wet conditions to allow for diffusion of fluid into the sample. It was found that the in-situ strain gage measurement technique in de-ionized water with properly selected strain gages and bonding adhesives is a valid test method for successfully measuring the in-situ tensile stress-strain curves and the elastic moduli of the polymer and composites in hot-wet condition up to 93 C(200 F). The strength of the backing film of the strain gage and the gage-to-specimen bonding strength in hot-wet environment were the major limits to the in-situ strain measurement technique presented in that study. This study introduces an HPHT in-situ extensometer used in combination with the HPHT in-situ thermomechanical test system to enable the technique of in-situ strain measurements in hot-wet environment under a high pressure and high temperature, simulating conditions found in oilfield applications. Then, this study presents performance of such technique at typical oilfield HTHP conditions. The objective is to obtain the in-situ tensile stress-strain curves and determine the elastic moduli of a selected laminated composite material in a water-based fluid condition at elevated temperature under a high pressure. Fig. 36.1 Schematic illustration of the HPHT in-situ thermomechanical testing system with the associated hydraulic, gas pressure and control systems 296 D. Sequera et al.
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