SIZE EFFECT AND FRACTURE CHARACTERISTICS OF COMPOSITE LAMINATES

Measurements of the size effect on the nominal strength of notched spe cimens of fiber composite laminates are reported. Tests were conducted on graphite/epoxy crossply and quasi-isotropic laminates. The specime ns were rectangular strips of widths 6.4, 12.7, 25.4 and 50.8 mm (0.25 , 0.50, 1.00 nod 2.00 irt.) geometrically similar in two dimensions. T he gage lengths were 25, 51, 102 and 203 mm (1.0, 2.0, 4.0 mid 8.0 in. ). One set of specimens had double-edge notches and a [0/92(2)](s) cro ssply layup, and another set had a single-sided edge notch and a [0/+/ -45/90](s) quasi-isotropic layup. It has been found that there is a si gnificant size effect on the nominal strength. It approximately agrees with the size effect law proposed by Bazant, according to which the c urve of the logarithm of the nominal strength versus the logarithm of size represents a smooth transition from a horizontal asymptote, corre sponding to the strength criterion (plastic limit analysis), to art in clined asymptote of -0.5 slope, corresponding to linear elastic fractu re mechanics. Optimum fits of the lest results by the size Effect law are obtained, nod the size effect law parameters are ther? used to ide ntify the material fracture characteristics, particularly the fracture energy and the effective length of the fracture process zone. Finally the R-curves are also identified on the basis of the maximum load dat a. The results show that in design situations with notches or large in itial traction-free cracks the size effect or the nominal strength of fiber composite laminates must be taken into account.