Methods for assessment of interlaminar tensile strength of composite materials

Abstract

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.

Measurement of Interlaminar Tensile Strength and Elastic Properties of Composites Using Open-Hole Compression Testing and Digital Image Correlation

“…Similarly, typical high-quality IM7/8552 ASTM D 6415 CB specimens were tested in fatigue in [8] and showed large variation in fatigue life and correlated poorly with the power-law least-squares fit commonly used for description of the S-N curves of carbon-epoxy materials. CT scans of the CB specimens tested in [7,8] revealed the presence of individual critical voids at ply interfaces that were correlated with the knockdown and large scatter found in the ASTM D 6415 CB strength and ILT CB fatigue life. It is worth noting that a test method that relies on extensive CT measurement may not be suitable for efficient characterization.…”

“…In [6], Jackson and Martin observed a drastic (up to a factor of four) CB strength reduction in low-quality CB specimens due to large (macroscopic) voids detected based on a cross-section cut using a diamond saw. In [7], the authors of this work measured the ASTM D 6415 CB strength using high-quality 6.6 mm (0.26 inch) thick CB coupons manufactured from Hexcel IM7/8552 unidirectional tape and cured per manufacturer's specifications. ASTM D 6415 ILT strength values ranging between 37.7 MPa (5.5 ksi) and 94.6 MPa (13.7 ksi) were reported with a coefficient of variation higher than 26%.…”

Experimental and numerical analysis of unfolding failure of L-shaped CFRP specimens

“…A three-point bending test [17] is used to find the interlaminar shear strength, while, most of the time, the interlaminar tensile strength is obtained by a four-point bending test [18], initially proposed by Jackson [19]. Nevertheless, it is possible to use the three-point bending test with a stacking sequence along the axial direction of the test [20] or use the tensile test in the interlaminar direction [21,22] to obtain the interlaminar tensile strength.…”

Structures Perspective for Strength and Fatigue Prognosis in Composites with Manufacturing Irregularities

“…[1][2][3][4]. Such basic material properties, characterizing three-dimensional (3D) material behavior under mechanical loads, are essential for structural analysis predicting failure of multidirectional composite laminates.…”

“…The authors of this work have recently developed a method that uses nondestructive inspection and finite element (FE) modeling capturing the effects of defects at the material scale (Refs. [19][20][21]. The objective of this work is to expand the method to predicting failure at the structural scale.…”