Determination of the shear characteristics of polymer composite materials

Layered polymer composite materials (LPCM) based on carbon fiber and epoxy are widely used in the construction of commercial aircraft both in manufacturing lightly loaded and main power elements of aircraft airframes. The level of mechanical characteristics for all the composite constructions must be confirmed by experimental data of coupon testing. One of the most important characteristics of a layered composite material is the ability to perceive shear loads. Standard test methods ASTM D5379 and ASTM D7078 are used to determine the shear characteristics. These test methods enable determination of the shear properties of composite materials based on a polymer matrix reinforced with a high-modulus fiber. Comparative tests were carried out according to both methods. The samples were made from the wall and the tehnological zones of the spar to determine the values of destructive stresses and shear modulus. The results showed that the shear stresses in the material obtained by ASTMD7078 significantly (1.8 times) exceed those determined by ASTM D5379. At the same time, the modulus of elasticity differs slightly (by 1.1 times). This is due to the sensitivity of the test results to the method of loading and the quality of the specimens. When stretched according to ASTMD7078, the load is transmitted through the front surfaces of the coupons, which are obtained with sufficiently high quality from the tooling or vacuum bag. When compressed by ASTM D5379, the parallelism of the faces through which the load is distributed becomes critical. Moreover, the thickness of the sample also affected the test results obtained according to ASTM D5379, the greater the thickness, the greater the shear modulus. Proceeding from the results obtained, the recommendations on the choice of the method to be used for determining the shear characteristics of LPCM are formulated.