Fracture mechanics of glass fibre reinforced polyester composites (gfrp) subjected to impact load

Thesis Abstract

Glass mat reinforcement, which can be easily shattered, is widely used across the world in military, automobile, civil, railway and electronic engineering among others. This research investigated the fracture mechanics of reinforced polyester composites on exposure to sudden impact force, using experimental and analytical methods based on impact and Linear Elastic Fracture Mechanics (LEFM) test procedures, to study the stress distribution around crack tip and zone. Plies of randomly varied combination of E-glass fibre in forms of woven roving, hard and soft mat was used as reinforcement in polyester resin matrix to manufacture test specimens. Fourteen (14) test samples with geometry 210 mm x 150 mm were fabricated using hand lay-up method. Hence, were cut and tested in accordance with ASTM standards for composite polymeric material tests under mode I and Charpy impact test conditions, using the compact tension and the Charpy impact test specimens respectively. From the experiment, the fibre volume fraction, the mode I, KI and mode II, KIIstress intensity factors, critical stress, Æ•C,shear stress, Æ–ntthe impact energy, E and impact strength, U were determined for each specimen. The mode I fracture toughness, KIC was found to be 4.97 MPa.m1/2 at a critical stress of 13.53MPa while the mode II fracture toughness, KIIC was 1.31 MPa.m1/2 at a shear stress, Æ–ntof 3.71MPa and also, the effective thickness was found to be in the range of 80-100mm atfibre volume fraction, Vf of within 0.35-0.50. From the results, the specimens containing woven roving reinforcement were found to possess higher fracture toughness and resistance to both fracture and impact damage. This was largely found to be as a result of fibre bridging and crack arrest mechanisms. This mechanism prevented crack growth direcption in specimens containing woven roving not to propagate along the original direction, but change the direction to an inclined path till failure with the exception of those containing soft and hard mat in which the crack grew in the original crack direction as the stress intensity increased. During the impact test, fibre stacking sequence played a vital role, thereby making specimens containing woven roving to resist impact damage and failure, and this resulted in fibre pull-out during fracture. The increase in fibre volume fraction was also found to improve the impact strength of the laminates. From the experiments, ways of improving composite performance were recommended, to ensure optimum impact and fracture resistance.