The effects of severe thermal and pressure transient pulses on the interior of coated tubes with known defects (cracks and blisters) have been analyzed using finite-element methods. For the modeling, both axisymmetric and three-dimensional (3-D) meshes were developed and used to assess the transient thermal-and stress-states and the propensity for fracture related damage. For all calculations, temperature dependent thermophysical and elastic properties were used during the analysis. The model also utilized uniform heating and pressure across the ID surface imposed via convective coefficients and a piece-wise linear pressure function. Results indicated that both had a significant influence on the maximum circumferential (hoop) stresses and temperatures and that the compressive thermal stresses help to offset the tensile stresses generated by the pressure. Calculations also looked into the influence of these factors when a cracks and/or blister defect was introduced at the interface of the coating and substrate with and without pressure.
The effects of severe thermal- and pressure-transients on coated substrates with indentation-induced, blister defects were analyzed using experimental and finite-element methods. An explicit FEA approach was first used to assess the transient thermal- and stress-states and the propensity for fracture related damage and evolution, under uniform convection and pressure transients across the surface; cohesive zone properties were evaluated in a previous study before being applied in an implicit indentation simulation. The indentation simulation results then served as the initial conditions for explicit modeling of interfacial flaw evolution due to thermal and pressure transients. Various conditions were analyzed including thermal and gun tube boundary conditions, and the effects of coating thermal capacitance. Given the need for robust coatings, the experimental and modeling procedures explored by this study will have important ramifications for coated tube design.
Although there are several techniques available for the evaluation of various interfacial cohesive zone properties of coatings, each has difficulties and limitations. For instance, the four-point bend method is often plagued by excessive deformation and plasticity without any coating delamination, and the button test is limited by the constant stress distribution assumption. Given such issues, a new hybrid numerical/experimental technique has been developed that is based on ball indentations, which can usually induce delamination regardless of the materials used. Using this method, indentations were first made on coated samples (nickel-on-steel and aluminum-on-aluminum for the current study) to intentionally create localized, circular delaminations, the initiation and dimensions of which were functions of the applied loads. Numerical models using finite element analysis were then used with the known indentation loads to inversely evaluate the cohesive zone properties, which reproduced the experimental results. The technique was validated based on the successful prediction of the indentation results evaluated using properties from four-point bend results.
A 3-D finite-element model was used to simulate the severe and localized thermal/pressure transients and the resulting stresses experienced by a rifled ceramic-barrel with a steel outer-liner; the focus of the simulations was on the influence of non-traditional rifling geometries on the thermoelastic- and pressure-stresses generated during a single firing event. In order to minimize computational requirements, a twisted segment of the barrel length based on rotational symmetry was used. Using this simplification, the model utilized uniform heating and pressure across the ID surface via a time-dependent convective coefficient and pressure generated by the propellant gasses. Results indicated that the unique rifling geometries had only a limited influence on the maximum circumferential (hoop) stresses and temperatures when compared with more traditional rifling configurations because of the compressive thermal stresses developed at the heated (and rifled) surface.
The effects of severe thermal and pressure transient pulses on the interior of coated tubes have been analyzed using finite-element methods. For the modeling, an axisymmetric mesh was developed and used to assess the transient, thermal- and stress-states and the propensity for fracture related damage. For all calculations, temperature dependent thermophysical and elastic properties were used during the analysis. The model also utilized uniform heating and pressure across the ID surface imposed via convective coefficients and a piece-wise linear pressure function over time. Results for the strictly elastic analysis indicated that both had a significant influence on the maximum circumferential (hoop) stresses and temperatures and that the compressive thermal-stresses help to offset any tensile components generated by the internal pressure on the ID. Preliminary calculations also investigated the influence of these factors when a crack was introduced at the interface of the coating and substrate.
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