Cracks in functionally graded materials

Abstract

The weight function method is described to analyze the crack growth behavior in functionally graded materials and in particular materials with a rising crack growth resistance curve. Further, failure of graded thermal barrier coatings (TBCs) under cyclic surface heating by laser irradiation is modeled on the basis of fracture mechanics. The damage of both graded and non-graded TBCs is found to develop in several distinct stages: vertical cracking → delamination → blistering → spalling. This sequence can be understood as an effect of progressive shrinkage due to sintering and high-temperature creep during thermal cycling, which increases the energy-release rate for vertical cracks which subsequently turn into delamination cracks. The results of finite element modeling, taking into account the TBC damage mechanisms, are compatible with experimental data. An increase of interface fracture toughness due to grading and a decrease due to ageing have been measured in a four-point bending test modified by a stiffening layer. Correlation with the damage observed in cyclic heating is discussed. It is explained in which way grading is able to reduce the damage.

Characterization of Fracture Process in Ceramic-Metal Functionally Graded Material under Three-Point-Bending

“…In a ceramic-metal FGM plate, surface cracks emanating at the ceramic side behave in several ways depending on the compositional gradation and loading condition. Multiple cracks, crack arrest, crack bowing, crack branching and spallation are well observed in ceramic-metal FGMs under thermal shocks or thermal fatigue (4) - (7) . In order to elucidate the fracture process and to evaluate the strength of FGMs, the following three issues should be considered.…”

Transient Stress Intensity Factors of Functionally Graded Hollow Cylinders with Internal Circumferential Cracks

“…Bahr et al (Bahr et al 2003) applied the weight function method to determine SIFs in FG structures with residual stresses. Shi et al (Shi et al 2014) proposed basic weight function equations for two-dimensional FG cracked structures using Betti's reciprocal theorem.…”

On an Antiplane Crack Problem for Functionally Graded Elastic Materials

“…They are also used as interfacial layers in order to reduce the stress arising from the material property discontinuities occurring, for example, in biomedical applications such as orthopaedic implants for hip and knee joint replacement (Pompe et al [20]). Applications 70 D. L. Clements [2] involving FGMs comprising metal-ceramic combinations are advantageously employed in coatings or interfacial layers to reduce thermal stresses in layered materials in severe temperature environments (Zhang et al [23], Noda [17]). In these applications, consideration of the fracture of FGMs is of importance.…”

“…Some of the earlier studies include the work of Clements et al [8] and Erdogan [10] while other examples include papers by Erdogan and Ozturk [11], Konda and Erdogan [16], Chen and Erdogan [4] and Jin and Batra [15]. More recent studies include papers by Chan et al [3], Dag and Erdogan [9], Noda and Wang [18], Bohr [2], Clements and Ang [7] and Chen et al [5].…”