Variations of Fracture Strength, Crack Velocity and Fracture Surface Marks due to the Design of a Single Edge Notched Tensile Specimen : In the Case of Unsaturated Polyester Resin

Brittle fracture strength and crack propagation velocity change with the design factors of a single edge notched tensile specimen, for example, the length to width ratio (L/W), the location and number of the loading pins, crack length (C_<01>) and its tip radius (ρ). Fracture strength shows a little change with an decrease in L/W when sharp tip radius specimens tensioned by two-bolts at near the left-side edge and it shows the notable change of strength when specimens tensioned by a pin. The specimens having the radius of 0.5mm, tensioned by a pin, show a strong effect of L/W compared with above two. Fracture strength in this case, of course, is larger than the former two cases. Fracture strength of those having a saw-cut tip decreases as the L/W decreases and the effect of L/W seems to be obvious. The measured crack velocity is genarally larger the larger the fracture strength. The crack propagates very slowy (at the order of mm/sec.) for the small L/W specimens of sharp tip radius and it does very quickly (at the order of m/s) for the large L/W ones. The crack velocity of specimens having a saw-cut tip reaches to the maximum of about 600m/s and it decreases as the crack propagates. The fracture surface marks of both end closed mark (like an ellipse) and one end opened mark (like a parabola) are observed on the saw-cut tip specimens and the crack arrest-lines dominates in specimens having a sharp tip radius. Detailed analysis of those marks are given.