Slow crack growth in fatigue and creep of polyethylene pipe resins; and adhesion of styrenic triblock copolymers

Shear and peel mode fracture of adhesives, and slow fracture of polyethylene pipe resins used for natural gas piping were evaluated. Shear mode adhesive strength of styrenic triblock copolymers bonded to polypropylene and polystyrene was examined as a function of midblock structure, polystyrene content, and molecular weight in Chapter 1. Shear strength was related to the nanoscale block copolymer morphology. Adhesive strength increased as the adhering phase of the triblock, styrene when bonded to polystyrene and the rubber midblock when bonded to polypropylene, became more continuous.; The constrained blister test (CBT), which is novel method for measuring peel strength, was evaluated for measuring adhesion in Chapter 2. The CBT mimics the slow adhesive fracture mode in failure of paints, adhesives, and coatings. For an electrical tape - polystyrene model system, peel energy was evaluated as a function of peel rate, and a rate-independent adhesion energy was obtained by extrapolation to zero crack growth rate.; The kinetics and mechanism of slow stepwise (discontinuous) crack growth in polyethylene pipe resins were examined in Chapters 3–6. A quantitative relation between fatigue and creep was developed that enabled an extrapolation to room temperature creep crack growth kinetics to be made from fatigue and creep tests at ambient and elevated temperatures. Crack growth in high density polyethylene (Chapters 3 and 4) and a medium density polyethylene pipe material (Chapter 5) was compared. A model relating crack growth rate to stress intensity factor parameters and applied strain rate was proposed by considering the total crack growth rate to consist of contributions of fatigue and creep loading components. The creep contribution depended on stress intensity factor parameters and the fatigue component depended on strain rate only.; Slow crack growth resistance was related to the morphology of the damage zone ahead of the crack tip. In highly creep resistant medium density pipe resins, the damage zone consisted of a wedge-shaped main craze with subsidiary shear crazes emanating from the crack tip. In high density polyethylene, which has poor creep resistance, the zone consisted only of a wedge-shaped main craze.