Experimental determination of compressive strength of an unidirectional composite lamina: indirect estimate by Using Back-out Factor (BF)

In this paper, the determination of compressive strength of an unidirectional lamina (carbon/epoxy composite), using the Back-Out Factor correction of experimental data, is presented. Since early ’70s, many methods were developed to deduce a compressive characterization of a composite material, each of which has been based on a different way of applying load to specimens. After a detailed examination of these methods, the ASTM D 6641/D 6641M-09 Test Method has been chosen due to its advantages, i.e. load application to the sample (end load combined with shear load), measurement reproducibility and experimental equipment quite simplified. The experimental equipment (according to ASTM D 6641) used for compressive tests is the Wyoming Combined Loading Compression (CLC) Test Fixture. Four different laminates were tested in compressive tests. They were realized by the same unidirectional prepreg, but with different stacking sequences: two cross-ply [0/90]ns and two unidirectional laminates, [0]ns and [90]ns. The last two laminates were used to evaluate the linear elastic properties of unidirectional lamina. The Back-Out Factors, that multiply the laminate measured compressive strength, were determined using the elastic properties previously measured. This indirect analytic method, developed from the classical lamination theory, was used to find out the unidirectional 0° lamina compressive strength. At last, extrapolated data were compared with pre-preg manufacturer datasheet.