A comparative study of the interlaminar shear strength of C/C composites

The most common method to evaluate the interlaminar shear strength (ILSS) of composites is by flexure loaded short beam (SB) specimens, according to ASTM D-2344. This method often fails to give pure shear strength when the flexure to shear ratio is low. In addition, the flexure loading is practically not applicable when the beam is very thin. A double notch (DN) specimen, ASTM D-3846 type loaded either in tension or in compression, guaranties pure sheer if specimen's buckling is avoided and the thickness allows notching. Both types of tests could be improved if sandwich specimens (San-SB and San-DN) are replacing the simple beam, using fiberglass/epoxy facesheets or another strengthening material. Such specimens also enable a successive testing in both methods on different specimen's locations, starting with the DN, since no stress field is formed in it except in between the notches. This interlaminar shear testing mode was experienced with two types of 2.5D C/C composites which strength values were evaluated and compared with two types of 2D C/Cs', as well as one 3D C/C, using all or only part of the specimen types. An attempt to get a general perspective of the shear results as related to the testing method was done by using two types of carbon/polymer composites (RTM epoxy and a phenolic matrices).