Experimental Specimen Selection for Stress-Displacement Behavior and Crack Initiation in Fiber Reinforced Composites

Abstract

With the increasing use of composite materials in multiple industries, especially aerospace, better understanding of damage mechanisms is needed. Damage in composites can be categorized into four types: fiber tension, fiber compression, matrix tension, and matrix compression. Experimental methods for classifying damage and propagation have been thoroughly studied for the first three categories but matrix compression has received little attention. A previous study showed that compact compression (CC) specimens, modified from standard ASTM compact tension specimens, can be used to determine the behavior of matrix compression damage in carbon fiber reinforced polymers (CFRP), however CC specimens are not as effective as needed at identifying initiation conditions [1]. This paper presents a specimen to determine the strain energy dissipation rate at crack initiation and the primary failure mechanism of a selected CFRP that is small, has simple geometry, and requires a simple loading fixture. The simple geometry of the UC specimens allows for the stress-displacement behavior to be measured in a more direct manner than the CC specimens providing an opportunity for examination and classification of the material response. Small 15 mm × 15 mm × 3 mm rectangular cuboid uniform compression (UC) specimens were manufactured and tested to compare experimental results with previously tested CC specimen results. UC specimens were loaded in compression until fracture using two flat plates on the thickness face with fibers oriented at 90° from the loading face. The results indicate CC and UC specimen agreement between the strain energy dissipation rate at crack initiation for comparable crack angles, with a range of values between 33.6 kJ/m2 and 45.7 kJ/m2. The primary failure mechanism for both specimens was observed to be shear cracks through the thickness of the laminate with an angle between 47° and 73° measured from the plane normal to the loading direction. UC specimen results also indicate an inverse relationship between the strain energy dissipation and the fracture angle. The stress-displacement results suggest behavior can be split into three distinct response regions: elastic, plastic, and visible damage progression. These results indicate that small, simple UC specimens can be used to directly measure the stress-displacement behavior, determine the strain energy dissipation rate at crack initiation, and determine the primary failure mechanism under compressive loads. Further studies need to be conducted to fully understand the relationship between crack angle and strain energy dissipation at crack initiation.