Viscoelastic relaxation in bolted thermoplastic composite joints
Results from a research program to investigate the long term effects of
through-the-thickness fastener clamp-up force (preload) relaxation on the
strength of mechanically fastened joints for two graphite/thermoplastic
composite materials (Dupont's IM6/KIII and ICI-Fiberite's IM8/APC(HTA)) are
summarized and compared with analytical methods. An experimental program
was conducted in which 56 mechanically fastened single-shear joints were
tested. Phase I static tests established joint bearing strength as a function of
clamp-up force for two types of fasteners (protruding head and countersink) with
no relaxation of preload. Phase II testing monitored short-term fastener preload
relaxation (up to 1 ,000 hours), with special bolt force sensor washers. Inservice
parameters included were temperature, in-plane loads, and torque. The
jOints were tested to failure at the end of the relaxation time period to determine
any subsequent effect on joint strength.
Phase I test results indicated that joint bearing strength increased by as
much as twenty-eight percent over the clamp-up force range of a Ibs (fingertight)
to 3,500 Ibs for both materials. Fastener head type, material, and
temperature also affected the resultant bearing strength. For Phase II, fastener
clamp-up force at room temperature (78°F) relaxed an average of six percent
from the initial value during the short-term test period. The relaxation was
projected to be as high as fourteen and sixteen percent at 100,000 hours for
HTA and Kill, respectively. The elevated temperature condition (250°F)
significantly increased the relaxation rate with the projected 100,000 hour
relaxation amount being as high as thirty-seven percent for HTA and sixty
percent for Kill. Comparison of the Phase II bearing strengths to the Phase I
results indicated that portions of the data correlated well, while others did not. It
was concluded that relaxation of the clamp-up force over the short-term time
period did not significantly lower the bearing strength of either material,
however an extended exposure to 250°F could affect the bearing strength.