ELASTOPLASTIC FRACTURE-MECHANICS OF WOOD USING THE J-INTEGRAL METHOD

Linear-elastic fracture mechanics has been applied extensively to wood. However, wood is not a perfectly linear-elastic material, particularly at elevated temperatures and moisture contents. The applicability of the J-integral method to wood was therefore investigated. Douglas-fir (Pseudotsuga menziesii) and Pacific madrone (Arbutus menziesii) were tested. Opening mode (Mode I) fracture tests at two loading rates were conducted in a test chamber with controlled environment at temperatures from 70 to 140 F and moisture contents from 12% to green. The critical elasto-plastic fracture toughness (J(IC)), critical strain energy release rate (G(IC)), and fracture toughness (K(IC)) were calculated. Species, moisture content, temperature, and loading rate were all found to have a significant effect on the three fracture parameters. The relationship between any two of these parameters was also established by empirical equations, making it possible to estimate J(IC) from existing values of K(IC) and G(IC). Since the degree of plasticity was as high as 60% for Pacific madrone and 40% for Douglas-fir at elevated temperature and high moisture content conditions, a large amount of strain energy capacity might be held in reserve (would not be accounted for) when the linear-elastic fracture mechanics theory is applied. Consequently, the J-integral method is more appropriate to characterize the actual fracture strength of wood.