Yasser M. Shabana

Geometry effects of substrate and coating layers on the thermal stress response of FGM structure

SummaryDue to transient temperature change, the plane strain elastic-plastic problem for a functionally graded material (FGM) bonded to a homogeneous coating layer and a metal substrate is considered by the use of the finite element method (FEM). The substrate and the coating are assumed to be aluminum and partially stabilized zirconia, respectively. The FGM layer is a particulate composite of aluminum and partially stabilized zirconia with volume fractions continuously varying through the thickness. Generally in high temperature applications, the FGM system is sandwiched between a substrate layer and a coating layer. The coating layer increases the protection from heat but decreases the thermal shock resistance while the substrate layer increases the rigidity of the structure and decreases strength-related properties at high temperature. In order to compromise the thickness of both the coating and substrate layers, different values of the substrate and coating thickness are studied in order to evaluate their effects on the thermal stress response of the FGM structure. Since the main objective of the FGMs is using them in different applications with severe thermal loading conditions, the thermal stresses may be so high that some reinforcements may be fractured and/or debonded from the matrix giving a weakening effect instead of a reinforcing one. Hence, the behaviors of the reinforcements and the matrix are essential to be studied. In this regard, microscopic constitutive equations along with the temperature-dependent properties of the constituent materials are considered to enable us obtaining more realistic results of thermal stresses. Since the FGM structures are fabricated at high temperatures, thermal residual stresses are produced. In order to find out the importance of the consideration of the residual stresses arising from the fabrication process, the FGM structure with stress-free conditions is heated to the operating temperature, and its thermal stress response is compared with that one where the residual stresses are taken into account. Also, several functional forms of gradation of the constituents in the FGM layer are examined to reach the optimum profile giving the minimum stress level for the FGM structure under thermo-elasto-plastic behavior.

Incremental constitutive equation for discontinuously reinforced composites considering reinforcement damage and thermoelastoplasticity

Abstract The particle and short-fiber reinforced composites are used in different engineering applications. Therefore, in the analysis of the composite materials not only the mechanical loading but also the thermal loading should be studied. Due to loading conditions, some reinforcement may be cracked and/or debonded from the matrix. Consequently, the load carrying capacity of the reinforcement depends on whether the reinforcement is intact, cracked or debonded from the matrix. The constitutive equation of composite material which take into account the damage process and the change in temperature is necessary in order to solve these phenomena. In this paper, an incremental constitutive equation of the particle and short-fiber reinforced composites with progressive cracking damage of the reinforcement has been developed considering the temperature change and elastoplasticity. By modifying the load carrying capacity of the damaged reinforcement, the present constitutive equation can describe the cracking damage and the debonding damage of the reinforcement as well as the perfect composite. To check the applicability of the present constitutive equation, the results are compared with those of some other approaches. Moreover, the coefficient of thermal expansion and the stress–strain relation are evaluated at different volume fractions and aspect ratios of the reinforcement as well as at different temperatures.

Minimizing Stresses of Layer Composites by Controlling the Interface Geometry

One of the most critical issues of layer composite structures is the induced high stresses at the interfaces due to the property mismatch of the layers. In the previous attempts to reduce these stresses, the interfaces are considered as flat surfaces perpendicular to the lay-up direction of the layers. The objective of this article is to minimize these stresses by proposing other forms of the interface geometry, such as curved surface and flat surface inclined on the lay-up direction of the layers. It is found that the induced highest stress can be reduced greatly by considering the proposed interface geometries.