EM388F Term Paper: Fracture of Orthotropic Materials under Mixed Mode Loading

The objective of this paper is to analyze the fracture of orthotropic materials, with emphasis on wood.

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(i) Describe the fracture phenomenon
Wood is usually considered as a cylindrically orthotropic material, with the principal axes of orthotropy (R,T,L) given by the radial, tangential and longitudinal directions. There are large differences in stiffness and strength between these directions. Moreover, the fracture toughness is highly dependent on both the crack propagation direction and the crack plane orientation. Due to large variations in fracture toughness depending on the orientation cracks usually propagate in the direction along the fibers. Even when cross-fiber notches are loaded in longitudinal tension, cracking occurs along the fibers (perpendicularly to the notch).

(ii) Explain how fracture mechanics analysis can be performed
Cracks in wood generally grow along the fibers, irrespective of both the original orientation of the crack and the mode mixity. Therefore, mixed mode fracture criteria derived for homogeneous materials cannot be expected to be directly applied to wood.
The following approaches for the fracture criteria within the framework of linear elastic fracture mechanics (LEFM) are presented:
- Critical Energy Release Rate
- Critical Strain Energy Density
- Critical In-Plane Maximum Principal Stress
These criteria are compared to experimental results of mixed mode I/II fracture in Picea abies.

(iii) Cite at least 1 journal article
[1] Jernkvist LO. Fracture of Wood under Mixed Mode Loading I: Derivation of Fracture Criteria. Eng Fract Mech 2001; 68: 549-563.
[2] Jernkvist LO. Fracture of Wood under Mixed Mode Loading II. Experimental Investigation of Picea abies. Eng Fract Mech 2001; 68: 565-576.