Multiscale modeling of damage in multidirectional composite laminates

The problem of damage accumulation in laminated composite materials has received much attention due to their widespread application in the aerospace, automotive, civil, and sports industries. In the aerospace industry, composites are used to make light weight and efficient structural components. In the Boeing 787, for example, more than 50% of the structure is made of composite materials. Although there have been significant developments in analyzing cross-ply laminates, none of the present approaches provides reasonable predictions for multidirectional laminates in which intralaminar cracks may form in multiple orientations. Nevertheless, the prediction of damage accumulation and its effect on structural performance is a very difficult problem due to complexity of the cracking processes.;This study presents a synergistic damage mechanics (SDM) methodology to analyze damage behavior in multidirectional composite laminates with intralaminar cracks in plies of multiple orientations. SDM combines the strengths of micro-damage mechanics (MDM) and continuum damage mechanics (CDM) in predicting the stiffness degradation due to these cracks. The micromechanics is performed on a representative unit cell using a three-dimensional finite element analysis to calculate the crack opening displacement accounting for the influence of the surrounding plies, the so-called constraint effect. This information is then incorporated in the CDM formulation dealing with laminates containing cracks in different ply orientations through a 'constraint parameter'. Following CDM, a separate damage mode is defined for each type of crack and the expressions for engineering moduli of the damaged laminate are then derived in terms of crack density and the constraint parameter. The SDM methodology is implemented for [0 m/+/- thetan/0 m/2]s laminates containing cracks in +/-theta plies. It is then extended to [0m /+/- thetan/90 r]s and [0m/90 r/+/- thetan] s laminates with cracks additionally in the 90°-plies. The predictions agree well with published experimental data as well as independent FE computations. Limited parametric studies are performed to show usability of SDM for more general laminates.;To predict the initiation and growth of intralaminar cracks, an energy based model is proposed in which these cracks initiate and multiply when the work required to form new set of cracks exceeds a laminate dependent critical energy release rate. The approach requires determination of average crack opening and sliding displacements at varying crack spacing. This task is performed through a suitable 3-D FE analysis. In case of off-axis ply cracking, a mixed mode fracture criterion is utilized, where the critical energy release rates in normal and shear modes are determined by fitting the damage model with the experimental data for a reference laminate. The predictions from the model for [0/+/-theta4/01/2]s and [0/90/∓45]s laminates show remarkable agreement with the experimental results.;The methodology and the results covered in this dissertation will be of interest to mechanics of materials researchers as well as to engineers in industry where composite materials for structural applications are of interest.