An experimental study of the edge chamfer trigger mechanism
An experimental investigation was conducted to study the behavior of an edge chamfer on the resulting crushing response of flat laminated composite strips. The effects of variables such as chamfer angle, stacking sequence and material system were addressed. In the present study the material systems used was Newport NB 321/7781 fiberglass/epoxy and Toray T700G-12K-PW/3900 carbon-fiber/epoxy. Chamfer angles of 30°, 45° and 60° were used in the present study to simulate the trigger mechanism. Laminate stacking sequences of [0º]N, [±45º]N (N=4,8 and 12), were studied using Newport NB 321/7781 fiberglass/epoxy and Toray T700G-12K-PW/3900 carbon-fiber/epoxy. In addition Newport NB 321/7781 fiberglass/epoxy specimens were fabricated with a [0Nº/±45ºN]S stacking sequence and tested. Monotonic and cyclic load-unload tests were conducted on an MTS electromechanical machine at a quasi-static stroke rate of 10-3 in/s. Monotonic tests were also conducted at stroke rates of 10-1 in/s, but restricted to the specimens with 45° chamfer specimen. Failure modes observed in [0º]N, [±45º]N, [0Nº/±45ºN]S specimens included delamination induced lamina/ sub-laminate bending mode, laminate bending mode, brittle fracture mode, catastrophic failure mode, and mixed mode failures. Lamina/sub-laminate bending mode and brittle fracture mode were observed to be the most dominant failure modes, with corresponding high energy absorption. The amount of energy absorption through delamination in cyclic load-unload test was investigated using image analysis and geometry calculations. With an increase in chamfer angle, the peak load, in the load-displacement curve, increased for the 12 and 8 ply laminates, while no significant differences were observed in the 4 viii ply laminates. The load eccentricity due to chamfering forced a global bending dominated failure mode in 4 ply laminates, which resulted in the observed apparent insensitivity to chamfer angle. Due to the 0° fiber orientation in [0º]N stacking sequence, a higher load carrying capacity was observed in comparison to [±45º]N, [0Nº/±45ºN]S. However, the energy absorption in [±45º]N was higher owing to the crushing mode of failure rather than the delamination induced splaying/bending mode observed in [0º]N laminates. Results from the load-displacement curve showed that the Newport NB 321/7781 fiberglass/epoxy had a higher load carrying capacity than carbon-fiber/epoxy, and this was attributed to the crushing mode being dominant in the fiberglass/epoxy specimens. Although, both fiberglass and carbon-fiber are brittle by nature, there was a shift in the initial load response from linear to non-linear behavior in the load-displacement curve of the [±45º]N stacking sequence. The sustained crush region of the load-displacement curves for the [0º]N laminates exhibited high serrations typically associated with load oscillations in brittle failure. Cyclic load tests revealed the sequence of failure mechanisms in chamfered laminates. While delamination was observed to be the primary driving mechanism, the estimation of energy dissipated due to the same was observed to be small in comparison to the overall energy absorbed.
Thesis (M.S.)--Wichita State University, College of Engineering, Dept. of Aerospace Engineering.