The effect of span length of flexural testing on properties of short fiber reinforced composite

The aim of this study was to determine the effect of different span lengths of flexural testing on some properties of test specimens made of experimental short fiber reinforced composite resin (FC). Bar shaped specimens with different lengths were made from experimental FC composite with an average fiber length of 3 mm and particulate filler composite (PFC, control: Z250). The specimens (n = 8) were polymerized with a hand light-curing unit for 40 s and dry stored in a room temperature for 24 h before testing. Three-point flexural test for determination of ultimate flexural strength, toughness and flexural modulus of specimens was made with different span lengths (20, 15, 10, 7, 6, 5 mm) with a speed of 1.0 mm/min until fracture. By shortening the span length for specimens made of FC or PFC, the flexural modulus decreased (from ca. 11 to 4 GPa) and flexural toughness increased (from ca. 0.25 to 2.25 MPa). Reduction in flexural strength by shortening the span length was found with PFC (from 170 to 125 MPa) but not with FC, which showed reduction by span lengths from 20 to 7 mm and considerable increase of flexural strength by further shortening the span length from 7 to 5 mm. Shortening of span length of flexural testing showed linear reduction of the measured and calculated flexural properties of PFC and some properties of FC, but flexural strength values for FC were non-linearly related to the span length: the highest values were obtained with the longest and the shortest span lengths. In reporting the flexural values of composites, the span length-specimen dimension ratio, and the length of the reinforcement need to be taken into consideration.