Proppants are a specific application of granular materials used in oil/gas well stimulation. Employment of hard and soft particle mixtures is one of the many approaches availed by the industry to improve fracture resistance and the stability of the granular pack in the hydraulic fracture. Current industrial practices of proppant characterization involve long term and expensive conductivity tests. However, the mechanics governing the proppant pack response, in particular the effects due to material, shape and size of particles on the pack porosity, stiffness and particle fragmentation are not understood clearly.The present research embodies analytical and experimental approach to model hard (ceramic) and soft (walnut shell and/or pure aluminum) proppant mixtures by taking into account polydispersity in size, shape and material type of individual particles.The hydraulic fracture condition is represented through confined compression and flowback loads. The particle interactions clearly illustrate changes in pore space as a function of pressure, mixture composition and friction. Single particle compression tests on individual particles are carried out to obtain mechanical properties which are incorporated into the finite element models and are further correlated with the compression/crush response of the mixture. The proppant pack stiffness and particle fragmentation depends strongly on the mixture composition as illustrated in the models and experiments. The flowback models demonstrated that the formation of a stable arch is essential to pack stability. Additional variables that enhance flowback resistance are identified as; addition of softer particles to a pack, softer rock surfaces and higher interparticle friction. The computational studies also led to the discovery of better, and more iv efficient pack compositions such as -short and thin pure Al needles/ceramic and the pistachio shells/ceramic mixtures. These analytical results have generated great interest and are engaged in the design of experiments to formulate future proppant pack mixtures at Baker Hughes Pressure Pumping, Tomball, TX. v

In this article, the computational and experimental techniques developed to understand and characterize the mechanical response of polymer coated walnut shell proppants is reported. Microscopy and single particle compression are employed to study cellular microstructure, elucidate influence of particle shape, and study cellular constitutive behavior. Computational models are created to simulate single-coated and uncoated particles under compression. The results reveal significance of the particle shape and constitutive material properties on its mechanical response. Polymer coating provides a marginal improvement in load transfer while stress and strain fields show a more gradual transition.

Impact damage is of critical concern in structures using laminated fibre reinforced composites. Thermoplastic polycarbonate (PC) offers high impact resistance and is often used in a variety of applications, including the windshield and canopy of aircraft, pressure windows and automotive parts among others. Impact damage to structures utilizing composites has been an area of constant concern. In the current study, S2-glass/epoxy composite laminates bonded to PC (referred to as ‘S2-glass/epoxy-PC’ or ‘Laminate-PC’) have been investigated for their high strain rate (HSR) impact response. A compression Split Hopkinson Pressure Bar (SHPB) with 19.05 mm (0.75”) incident and transmission bars with dynamic recovery technique was used to test four, six and eight layers S2-glass/epoxy-PC samples. Using dynamic recovery technique on the SHPB, the sample was subjected to a controlled single compressive pulse, providing information on damage evolution. The samples were tested under HSR impact in three directions (with respect to the side facing the incident bar of the SHPB). In the first, the PC faced the incident bar, in the second, the S2-glass/epoxy laminate faced the same, whereas in the third, the loading was in the in-plane direction of the laminate. The HSR impact tests were performed by considering three strain rates - 109, 327 and 544/s - for all the experiments. The results of the investigation showed that providing a PC facing to S2-glass/epoxy laminate is an effective way of obtaining deformation in the PC facing which absorbs a large portion of the incident energy, thereby minimizing any delamination threat to the composite backing. PC acts as a sacrificial layer that can be readily replaced in the application.

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Fiber reinforced polymer matrix composites are being increasingly used in many applications including aerospace, armored vehicle, marine and sporting industries. They provide the advantage of high specific stiffness and specific strength as compared to conventional metallic materials. However, they are well known by their vulnerability to threats like impact loading that occurs normally in the transverse direction. Hence, there is a need to improve the damage resistance in the transverse direction at minimum weight penalty. In this current investigations, a two component material system made of fiber reinforced laminates bonded by a polycarbonate facesheet was studied. Laminates made of both unidirectional graphite/epoxy prepregs and S2-glass twill weave woven fabric/SC-15 resin systems were used in the study. Specimens of size 304.8 × 304.8 mm were made using different thickness ratios of facesheet and the backing laminate. Three different thicknesses of s2-glass laminates (3.17, 4.31 and 5.71 mm) were used to bond to two polycarbonate sheets of thickness 2.54 and 3.17 mm each. Similarly, three different thickness of graphite/epoxy laminates (2.05, 3.75 and 7.62 mm) were used to bond to polycarbonate facesheet of thickness 1.00, 3.175, and 4.30 mm. The ballistic tests are performed with a space test setup using fragment simulating projectiles(FSP). The ensuing damage was evaluated using ultrasonic C-scan technique using pule-echo immersion method.