Evaluation of crack tip fields and fracture parameters in functionally graded materials

Functionally graded materials (FGMs) are non-homogeneous materials with spatially varying properties engineered for specific applications. FGMs can be used to great advantage for eliminating interfacial weakness present in discretely layered materials, in relaxing thermal stresses, and enhancing load-bearing capacity of structures at critical locations. The aim of this dissertation is to address fracture mechanics of such materials under quasi-static and dynamic loading conditions.; Functionally graded beams made of glass-filled epoxy with edge cracks parallel to the direction of the elastic gradient are studied. Situations where cracks are located on the compliant and the stiff sides of the beams are separately examined by mapping crack tip deformations using optical interferometry. A methodology for extracting fracture parameters in FGMs based on locally homogeneous material descriptions is advanced. Additional non-singular terms are proposed to supplement K-dominant description of FGMs to account for far-field effects. Excellent agreement is found between the proposed methodology and companion numerical evaluation. Optical measurements near quasi-statically growing cracks in FGMs are also undertaken. Crack growth resistance behavior is explained using crack initiation toughness variation as a function of filler volume fraction. Under quasi-static loading, FGMs with cracks on the compliant side of the gradient show better resistance to fracture than their homogeneous counterparts or FGMs with the opposite configuration.; Fracture of FGMs with identical properties, subjected to low velocity impact resulting in crack speeds exceeding 500 m/s, is recorded every 5 mus. Pre-initiation stress intensity factors are determined using dynamic equivalent of the proposed FGM stationary fields, while crack tip fields for steadily growing cracks in FGMs are used for post-initiation. Companion finite element simulations agree well with the measurements. Stress intensity factor histories and crack growth resistance show continuous increase in KId( t) when the crack grows into the stiffer region, while a decreasing trend is observed for the opposite case. Dynamic response of different elastic gradient profiles establishes delayed crack initiation and elastic shielding of cracks for cases where these are on the stiff side of the gradient, relative to homogeneous beams and FGMs with the opposite configuration. The fracture behaviors are explained by independent fracture tests, fractured surface micrographs, and stress wave monitoring through transient finite element analyses.