Session: 12-16-01: Multiscale Models and Experimental Techniques for Composite Materials and Structures Count

Paper Number: 112524

112524 - The Failure Mechanism in Cfrp Cross-Ply Curved Composite Laminates 

Composite materials have been widely used in the aerospace and wind energy industries due to their high specific stiffness and strength. The complex shaped composite structures like curved parts are utilized in the main load-carrying structures of airplanes and wind turbines, such as ribs and spars. High interlaminar tensile and shear stresses arise in the curved region of these parts and these interlaminar stresses lead to delamination-type failure. As well as the delamination type of failure, matrix cracks occur due to the high tangential stress resulting from the bending of these structures.  

In this study, the cross-ply curved composite laminates having a stacking sequence of [(90/0)_4,/90]_s are investigated experimentally and numerically. The in-house text fixture is designed to apply pure shear load to one arm of the curved composite laminates while the other arm is fixed. The high-speed camera is utilized to observe the delamination occurring dynamically in the curved region of the specimens. The digital image correlation (DIC) method is used in several experiments to obtain the strain field over the curved region of the specimens. It is observed that the specimens lose their load-carrying capacity with three different manners of failure. In two of these failure types, as the high-speed camera observation reveals, the first delamination initiates at the 3^rd interface of the curved beam and the other delaminations occur in the subsequent interfaces in an orderly manner. These failure types, however, differentiate in their load drop type: one load drop where four or five delaminations occur or multiple load drops with one or two delaminations. In the other failure type, the failure initiates in the outer region of the curved beam. The crack tip speed history obtained from the high-speed camera images reveals that the crack tip exceeds the shear wave speed (1831 m/s) and reaches the intersonic speed regime with a speed of 1863 m/s in the single load drop case, whereas the crack tip travels in the sub-Rayleigh wave speed regime in multiple load drop case. The strain concentration regions are seen on the inner side of the curved region. Fractography of post-mortem specimens is conducted using a digital microscope. In the curved region, transverse matrix cracks, delamination, meandering crack path and fiber breakage are observed in the micrographs. In both arms, transverse matrix cracks are seen and the crack propagates as delamination.

The numerical analysis of cross-ply curved composite laminates using ABAQUS is conducted to simulate the failure types observed in the experiments. The explicit analysis is chosen since the failure occurs dynamically. Cohesive Zone Modelling (CZM) is used to predict the interlaminar (delamination) failure and LaRC05 failure criterion is implemented to ABAQUS via user subroutine to predict the intralaminar (matrix failure) failure mode. Analysis time is determined concerning the first mode frequency obtained from the modal analysis, and the loading rate of 1 m/min is chosen to discard dynamic effects during the analysis. The study with different strength and toughness parameters of materials is carried out to realize the effect of these parameters on the failure types.

Presenting Author: Ahmet Çevik Middle East Technical University

Presenting Author Biography: I obtained my B.Sc. and M.Sc. in Aerospace Engineering at Middle East Technical University, Turkey. During my master, I wrote a thesis about the failure mechanism of curved composite laminates under the supervision of Prof. Demirkan Çöker. I now continue to a Doctorate program with my supervisor at the same university and department as well as being a Graduated Research Assistant. My research interest includes experimental mechanics of solid, composite material, fracture mechanics, fractography and fatigue of materials.

Authors:

Ahmet Çevik Middle East Technical University
Demirkan Çöker Middle East Technical University