Loading Response of Laminated CFRP Composites with Hole cutout: Geometrical Design Effect on Material Strength and Failure Mechanism

Abstract

In recent decades, CFRP laminates have become popular material among design engineers due to their high strength and stiffness-to-weight ratio, corrosion resistance, thermal stability and chemical inertness. It is being exploited in many engineering applications like automobile, aerospace, civil, marine, commodity, sports, etc. As a further matter, CFRP is typically a brittle material and catastrophically fails without sufficient warning. The catastrophic failure of CFRP is usually compensated by a conservative design limit which hinders exploiting outstanding materials’ mechanical properties. The design of the component must be compatible with the material and must be able to resist specific loading condition. Since welding of CFRP components are not possible therefore design of open hole in the component is inevitable; it is made to incorporate pins, rivets or bolts for assembly of the components into the final structure. Moreover, holes or cutouts in the component are also used to lighten the structure and fulfill some functional requirements such as the passage of electrical and fuel lines, accommodation of doors and windows. Thus, understanding material response due to the design of hole/cutout is of utmost importance. The present study demonstrates the effect of hole, hole shape, hole size and stacking sequence on laminated CFRP components under two different loading condition. Additionally, detailed failure mechanism and strength prediction models have also been assessed. The study started through experimental investigation on the effect of stacking sequence, hole size, and hole shape on the tensile strength of CFRP laminate. Reduction in Open Hole Tensile (OHT) strength is observed when stacking sequence is changed from [0]8 to [0/90]2s and when the hole size is increased. However, OHT strength is noticed to get increased when hole shape is changed from circular to square. Laminates containing square shaped hole demonstrate less sensitivity towards tensile strength and show negligible effect of stacking sequences on the normalized strength than the circular hole. A further testing program was performed on CFRP laminates under flexural loading. Flexural and Open Hole Flexural (OHF) test of CFRP as a function of stacking sequences report that the [0]8 laminates have the highest flexural and open hole flexural strength. During the evaluation of strength retention value, [02/902]s laminate is found to be best among all investigated stacking sequences. [02/902]s and [90/0]2s is found best for better isotropy of open hole flexural strength among the laminates having first ply at 0◦ and 90◦, respectively. Further, a comparative analysis has been done on CFRP laminate to understand the design aspect of holes/cutouts under the tensile and flexural loading. Tensile loading is found to be more detrimental than flexural loading for composite laminate with hole. SEM and optical micrographs have been analyzed to perceive the failure mechanisms of laminates under two different loading conditions. Mode I dominated delamination and mode II dominated delamination are found to be prevalent in OHT (open hole tensile) and OHF (open hole flexural) failure, respectively. The results of the testing program and strength prediction models were compared and assessed through deterministic analysis. Longitudinal laminates show some discrepancy between predicted data and experimental data. However, crossply laminates are found to approach congruence with experimental data. In deterministic analysis, the failure pattern of each laminate system has been studied. The microscopic examination of fractured samples has also been done to study failure mechanisms. Both longitudinal and crossply laminates show a brittle fracture, but the splitting in longitudinallaminates accorded a hint of gradual failure because laminates continue to bear load; Nevertheless, laminates suddenly lead to lateral brittle fracture.