Thermomechanical cohesive zone models for analysis of composites failure under thermal gradients and transients
Abstract
A numerical framework to study multi-physics problem involving coupled thermomechanical analyses for cracks is outlined. Using a thermomechanical cohesive zone model (TM-CZM), load transfer behavior is coupled to heat conduction across a crack. Non-linear effects due to coupling between the mechanical and thermal problem occur through the conductance-separation response between crack faces as well as through the temperature dependence of material constants of the CZM. The TM-CZM is implemented in a convenient framework within the finite element method and applied in the study of: (i) interface crack growth; (ii) crack bridging; and (iii) photo-thermal imaging. Interface fracture in a thermal protection system (TPS) under transient monotonic and cyclic thermal loading is studied using the new TM-CZM and an analytical model. TPS includes an oxidation protection coating (OPC) on a carbon-carbon (C-C) composite substrate. The description of the load transfer behavior uses a traction-separation law with an internal residual property variable that determines the extent of damage caused by mechanical separation. Temperature dependence is incorporated, such that the interfacial strength and therefore the tractions decrease with temperature. The description of thermal transport includes an accurate representation of breakdown of interface conductance with increase in separation. The current state of interface failure, the presence of gas entrapped in the crack as well as radiative heat transfer determines the crack conductance. Coupling between thermal-mechanical analyses affects the interface crack initiation and growth behavior. An analytical model is presented for the uncoupled thermal-mechanical problem to calculate temperature fields and energy release rates. The TM-CZM is also applied in the study of bridged delamination cracks in composite laminates loaded under a temperature gradient. A micromechanism based bridging law is used for load transfer coupled to heat conduction across bridged delamination crack. Crack tip energy release rate and crack heat flux are computed to characterize the loading of the structure. Specimen geometries that lead to crack opening through bending deformation and buckling delamination are presented. The influence of critical mechanical and thermal parameters of the bridging zone on the thermomechanical delamination behavior is discussed. The TM-CZM allows the assessment of the effectiveness of the individual physical mechanisms contributing to the thermomechanical failure embedded into the structural analysis.
Degree
Ph.D.
Advisors
Siegmund, Purdue University.
Subject Area
Mechanical engineering|Materials science|Aerospace materials
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