The rate of decrease of the WFRP modulus and that of the LDPE matrix are similar and larger than that of spruce, indicating that the addition of wood fiber does not significantly alter the relaxation, although it does increase the modulus at a given time.
The viscoelastic nature of the WFRP may be expected to cause related effects in the performance of screw-fastened joints, particularly with regard to the clamping force relaxation.
Another measure of the fastener's ability to resist stripping of the WFRP threads is the nominal ultimate shear stress in the composite at the stripping torque.
Thus, the loss of thread friction due to clamping force relaxation was partially offset by interlocking of the WFRP matrix and the screw threads.
The present data for the wood screw in WFRP interpolated to an equivalent initial clamping force and time show a relaxation to about 53% of the initial value.
However, in the clamping force relaxation experiment at different initial load levels neither the strain nor the stress were held constant in the WFRP as relaxation occurred.
The clamping force relaxation of spruce is compared with that of WFRP in Fig.
The effect of retightening the wood screw in WFRP 1 h after installation is shown in Fig.
The main reason was the early creep rupture in the WFRP, which failed within 6 hours.
A power law relation based on 24-h creep data at 20% ultimate flexural strength, predicted the creep of WFRP after 260 days to within [+ or -] 10%.
The pullout force of a wood screw in WFRP was 20% greater than in spruce.