Methods for assessment of interlaminar tensile strength of composite materials

Among the mechanical properties of polymer-matrix composite materials, the interlaminar tensile strength is among the most difficult to characterize. ASTM Standard D 6415 uses a curved-beam configuration for measuring interlaminar tensile strength. Not only the manufacturing process to produce curved-beam coupons with uniform radius and thickness could be challenging but also the curved-beam strength data typically exhibits large scatter. One question is whether ASTM D 6415 curved-beam interlaminar tensile strength data are coupon-specific, that is the curved-beam strength is not really a coupon-independent material property, suggesting that ASTM D 6415 is not adequate to measure interlaminar tensile strength. The objective of this work is to develop efficient and accurate methods to capture interlaminar tensile strength of composites. The authors expand a recently developed short-beam method coupled with the digital image correlation full-field deformation measurement technique to measuring the interlaminar tensile strength. The interlaminar tensile strength data are presented for IM7/8552 tape composite system. However, average curved-beam strength value is significantly lower compared to the short-beam test results. Micro-focus CT measurements show that porosity in the radius area is the reason for the low average strength value and the large scatter in the curved-beam strength test data. Once the stress concentration effects of porosity are captured through transfer of CT measurements into three-dimensional finite element model, the short-beam and the curved-beam test results agree. The short-beam method, which measures the interlaminar tensile strength for a pristine material, and the refined curved-beam method which accounts for manufacturing defects, represent more complete interlaminar tensile strength assessment methodology for composite structural designs.