MECHANICS DESIGN MODELS FOR ADVANCED PRESSURE VESSELS:AUTOFRETTAGE WITH HIGHER STRENGTH STEEL; STEEL LINER - COMPOSITE JACKET CONFIGURATIONS; ALTERNATIVE THERMAL BARRIER COATINGS

摘要

Solid mechanics models are described of mechanical and thermal stresses in 1000-1400 MPa yield strength, autofrettaged, steel pressure vessels. Modeling results describe idealized advanced vessel configurations with improved resistance to mechanical damage from internal pressure and thermal damage from transient internal heating, [i] Calculations of autofrettage hoop residual stresses are based on the classic Hill elastic-plastic results for thick-wall tubes, with modifications to account for the Bauschinger-reduced compressive strength of the tube steel near the bore, [ii] Stresses in metal liner - composite jacket tubes are calculated using the Parker layered-tube model, which gives applied and residual elastic stresses for two-layer tubes with specified properties and interference between layers, [iii] Transient thermal stresses in bore barrier coatings are calculated using the finite difference methods of Witherell, describing one-dimensional, convection-conduction heat flow, focusing on near-bore temperatures using time-dependent combustion gas temperatures and convection coefficient data from interior ballistic codes. Temperatures are obtained for various thicknesses of metallic and ceramic coatings on steel substrate, using temperature-dependent conductivity and diffusivity data for the coatings and substrate. In-situ verification of calculated temperature profiles is done by comparing with metallographic observation of depths of the steel phase transformation and the known characteristic transformation temperature. When the transient shear stress near the interface exceeds the reduced elevated-temperature strength of the interface, coating segments are modeled to be lost byshear failure, which in turn would lead to rapid hot-gas erosion of the steel substrate.Results of the model calculations are used to identify potential improvements in advanced pressure vessels, using idealized configurations as examples, [i] Autofrettage of higher strength steel vessels shows significant increase in both yield pressure and fatigue life, but poorer resistance to both hydrogen cracking and yield-before-break final failure, compared to traditional lower strength designs of equivalent weight, [ii] Vessels with steel liner and either high strength carbon/epoxy or unidirectional Al_2O_3/A1 jacket and high liner-jacket interference show similar fatigue life to that of all-steel designs of equivalent weight. However radial compressive crushing of composite materials in transverse orientation limits composite jacketed vessels to lower applied pressure than all-steel designs, [iii] Metal thermal barrier coatings generally suffer from compressive yielding at elevated temperatures near the bore, leading to tensile residual stress, cracking, and erosion failure. The higher hot strength of a Si_3N_4 ceramic provides significant improvement in yielding and cracking resistance and thus erosion resistance, compared with metal coatings subjected to the same thermal conditions.