Abstract
This research investigates the stress mechanics of reinforced polyester composites when exposure to a sudden impact force, using experimental and analytical methods based on impact and Linear Elastic Fracture Mechanics (LEFM) test procedures. In this study, the stress distribution around crack tip and zone for 14 test samples with geometry 210 mm x 150 mm were investigated. KII stress intensity factors, critical stress, σC, shear stress, τnt the impact energy, E and impact strength, U were determined for each specimen. The mode I fracture toughness, KIC was found to be to be 4.97 MPa.m1/2 at a critical stress of 13.53 MPa while the mode II fracture toughness, KIIC was 1.31 MPa.m1/2 at a shear stress, τnt of 3.71 MPa. The effective thickness was found to be in the range of 80-100mm at fibre volume fraction, Vf of within 0.35-0.50. This was largely found to be as result of fibre bridging and crack arrest mechanisms. This mechanism prevented crack growth direction 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 stress could be seen to concentrated at the crack tip and around the loading pins on a smaller level, compared to the level at the crack tip.
Highlights
In the analysis of impact damage and fracture of composites, the review of the sources, nature and curvature of impactor and how these fracture initiation takes place, propagate from a microstructural scale at the sites of flaws, and how the coalescence of these flaws leads to a visible separation process that manifests on a macrostructural level before resulting in a catastrophic failure are vital
The load – displacement plot was obtained for all specimens from experimental data, and this provided the platform for the fracture mechanics analysis on each specimen, containing varied fibre volume fraction and form, using the compact tension specimen, as shown above under tensile load condition on the universal testing machine
It be shown that specimen I which contained 12 plies of soft mat reinforcement required the least stress magnitude to grow the crack, while specimen C which contained 2 plies of woven roving and one ply of hat mat experienced the highest critical stress. It was observed from the stress intensity and critical stress versus crack length curve, that the crack tip is where the maximum stress exists and, the location of the minimum stress intensity factor
Summary
In the analysis of impact damage and fracture of composites, the review of the sources, nature and curvature of impactor and how these fracture initiation takes place, propagate from a microstructural scale at the sites of flaws, and how the coalescence of these flaws leads to a visible separation process that manifests on a macrostructural level before resulting in a catastrophic failure are vital. According to Làszló and George [1], the mechanical and thermal behaviours of a structure depend on the properties of the fibres and the matrix and on the amount and orientations of the fibres. In the structural analysis of composites, the design steps from the micromechanics (which takes into account the fibre and matrix properties) through macromechanics (which treats the properties of the composite) are taken into consideration.
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