Rate sensitivity of the interlaminar fracture toughness of laminated composites

Thesis (M.S.)--Wichita State University, College of Engineering, Dept. of Aerospace Engineering.The rate sensitivities of mode-I and mode-II fracture toughness, in the form of GIc and GIIc of Toray T800S/3900 carbon unitape/epoxy, T700G/3900 Plain weave carbon fabric/epoxy, and Newport NB321/7781 fiberglass/epoxy materials were investigated experimentally. Static and dynamic tests, with stroke rates ranging from 8.33×10-4in/s to 100in/s, were conducted on double cantilever beam and end notch flexure specimens. Both [0°] N and [±45°] N laminates were studied in this investigation. The rate sensitivities were characterized in terms of the initiation fracture toughness and crack growth resistance. The average mode-I initiation fracture toughness, GIc of NB321/7781 [0°] laminate was observed to decrease with stroke rate from 6.5lbf-in/in2 to 4lbf-in/in2 and further increased to 6lbf-in/in2 over a range of 5 decades of crack opening displacement rates. The resistance tended to be constant with crack length for the low rate tests. At higher rates, the resistance decreased gradually. The tests data indicated ductile stable and brittle unstable behavior with a transition stick-slip behavior. The average mode-II fracture toughness of the NB321/7781 [0°] laminate was observed to increase from 12lbf-in/in2 to 17lbf-in/in2 over the entire range of 5 decades of shear displacement rates. The fracture behavior was observed to be brittle stable at all rates. The NB321/7781 [±45°] laminate’s opening mode initiation toughness was observed to decrease from 9lbf-in/in2 to 3lbf-in/in2 and further increased to 4lbf-in/in2 over a range of 5 decades of crack opening displacement rates. The delamination resistance increased with crack length for the low rate tests. At higher rates, the resistance decreased gradually. The fracture behavior transitioned from ductile stable to brittle unstable. The opening mode initiation toughness of T700G/3900 [0°] laminate decreased from 3.5lbf-in/in2 to 2lbf-in/in2 and further increased to 3.2lbf-in/in2 over a range of 5 decades of viii crack opening displacement rates. The delamination resistance indicated constant pattern irrespective of the stroke rates. The fracture behavior at low rates indicated transition stick-slip behavior, whereas brittle unstable behavior was evident at higher rates. The mode-II fracture toughness tended to be constant at 4.5lbf-in/in2 over a range of 5 decades of shear displacement rates. Brittle stable fracture behavior was observed at all rates. The opening mode initiation toughness of T700G/3900 [±45°] laminate was observed to decrease from 2.75lbf-in/in2 to 1.8lbf-in/in2 and further increased to 2.8lbf-in/in2 over a range of 5 decades of crack opening displacement rates. The delamination resistance tended to increase with crack length at lower rates, whereas it tended to decrease at higher rates. The fracture behavior indicated transition stick-slip behavior at lower rates, whereas brittle unstable behavior was evident at the highest rate tested. The T800S/3900 [0°] laminate’s opening mode initiation toughness was observed to steadily decrease from 5lbf-in/in2 to 2.5lbf-in/in2 over a range of 5 decades of crack opening displacement rates. The delamination resistance was observed to increase with crack length at lower rates, whereas it tended to decrease gradually at higher rates. Brittle stable fracture behavior was observed at all rates. Fiber bridging was prominent with an increase in test rates. The mode-II fracture toughness increased steadily from 9lbf-in/in2 to 20lbf-in/in2 over a range of 5 decades of shear displacement rates. Brittle stable fracture behavior was observed at all rates