Session: 01-06-03: Impact, Fatigue, Damage and Fracture of Composite Structures

Paper Number: 110813

110813 - Analysis of Ply Transition Regions in Localized Hybrid Metal/cfrp Composite Laminates Under Tensile Loading 

Hybridization of carbon fiber reinforced polymer (CFRP) composite laminates with metal foils in fiber metal laminates (FMLs) have been shown to increase the bearing strength. Weight optimal FMLs that use local hybridization of CFRPs with metal foils at bolted joints to increase bearing strength, creates a transition region in between the full monolithic CFRP laminate region and the FML section. Designing a hybrid laminate of constant thickness by replacing select CFRP plies with metal plies, determining which plies are favorable to replace to increase bearing strength and designing a ply drop pattern for optimizing strength of the transition region.

The failure mechanics of ply transition zones in hybrid metal/CFRP laminates are far less studied in comparison to the region at which the bearing load is applied. Previous investigations have investigated the effect of introduction of  metal plies into CFRP laminates to increase the bearing, tensile, and bending strengths in these types of laminates. It is apparent from these studies that more work on hybrid laminates with transition zones is still needed to understand this region’s influence on the failure mechanisms of localized hybrid composite laminates.

This work presents the results of a numerical analysis of transition regions in hybrid laminates. Detailed numerical models of different transition region designs are  investigated to understand the influence different laminate configurations have on the damage/failure mechanisms that occur within the transition region under bearing load conditions.

The finite element analysis (using ABAQUS™) of the laminate ply transition regions are performed using 3D layerwise solid element model. The progressive failure analysis includes material failure models for inter/intra-ply failure. The cohesive zone method (CZM) is utilized to model the interlaminar failure in the resin region between plies. Finite thickness cohesive elements are implemented using a bilinear traction-separation law with energy based damage evolution. An energy-based continuum damage mechanics (CDM) model with smeared crack damage representation and Hashin’s criteria for damage initiation is used to model for intra-ply damage in fiber tension, fiber compression, matrix tension and matrix compression modes. Metal plasticity is modeled using a bilinear stress train curve and the effects of cure induced thermal residual stresses are included. Effect of model fidelity is explored by comparing the solid element model,  with a 3D layerwise continuum shell model.

The results indicate that the load carrying capacity of transition regions between the FML and CFRP composite laminates is heavily influenced by the transition region configuration (stagger pattern and the plies replaced). Under axial loads, traverse cracking of the interface occurs first at material junctions of high stiffness mismatch; interlaminar stress transfer arise as a result of stiffness mismatch throughout the transition region. The configuration in which the metal foils are implemented within the laminate is integral in global strength performance. Staggered patterns of metal foil drops progressing from outside to inside or vice-versa) is investigated. Varying the distance between metal ply drop off points is investigated. The transition zone joint strength is highest for a stagger pattern with longest metal ply at center (16% higher, compared to the opposite stagger design. The inclusion of thermal residual stresses decreases the transition zone strength by 17%; due to the CTE mismatch at the butt-joint region where CFRP and metal plies meet and interface damage develops during the shear stresses developed during the cool down from curing.  The failure of this region from cure thermal stresses means that butt-joint regions with high stiffness mismatch exhibit an initial transverse crack that leads to early onset of delamination cracking on application of mechanical loads.

The stiffness and CTE mismatch between materials, stress transfer across interfaces due to inter/intra-ply failure and the effects of altering the laminate configuration lead to highly complex interactions and mixed-mode failure in the transition region.  The computational investigation provides insight on the damage initiation and failure of transition regions that can help in optimization of the configuration of transition regions between the FML and CFRP laminate.  

Presenting Author: Rommel Pineda San Diego State University

Presenting Author Biography: Rommel Pineda is a Master's student studying Aerospace Engineering: Structural Mechanics at San Diego State University. Rommel has been investigating localized hybrid metal/CFRP composites since his second year as a graduate student. Rommel is currently working at NAVAIR as an Aerospace Engineer for the E-2/C-2 program.

Authors:

Rommel Pineda San Diego State University
Satchi Venkataraman San Diego State University