Abstract:
Wave dispersion and fracture are phenomena that involve. mechanisms triggered at the scale of a material's internal structure. Experiments were performed with a candidate material to judge the hypothesis that wave dispersion measurements can be used to nondestructively predict fracture strength. Wood-based particleboard was chosen as the candidate material because the entire acoustical branch of the longitudinal wave dispersion curve was accessible with ultrasonics. Wave dispersion and fracture were studied in detail for the weak direction of particleboard, through the panel thickness. The prediction of a relationship between wave dispersion and fracture strength is inherent to nonlocal theory. Particleboard has a complex internal structure, rendering it a difficult material to model. Therefore, it is a good test of the generality of nonlocal theory. Wave dispersion data from particleboard was converted into nonlocal moduli using a fast Fourier transform calculation. The variable frequency/variable path-length, continuous wave method was used to measure dispersion. Dispersion in particleboard occurred at frequencies between 400 and 900 kHz. The dominating mechanism was that of micro-geometric dispersion (i.e. decreasing phase velocity with increasing frequency). Frequencies above 900 kHz were effectively "stopped" by particleboard. The dispersion response of particleboard was found to depend on board specific gravity, resin content and particle size. Mode I load-displacement curves of cracked samples of particleboard exhibited initial linear behavior, followed by nonlinear response. Compliance calibration and the J[subscript I]-Integral were used for linear and nonlinear characterization, respectively. The nonlinear behavior was due to the growth of a local damage zone at the tip of the crack. The damage zone contained permanent deformation. As a result, the strict validity of the J[subscript I]-integral approach may have been violated. The failure energy of particleboard appears to be load history dependent. The nonlocal moduli were related to stress concentration or redistribution for relatively weak or strong materials, respectively. Wave dispersion was found to have a very high correlation with the critical strain energy release rate at the onset of nonlinear behavior.
