Singh, Chandra Veer (2008-12). Multiscale modeling of damage in multidirectional composite laminates. Doctoral Dissertation.
The problem of damage accumulation in laminated composite materials hasreceived much attention due to their widespread application in the aerospace, automotive,civil, and sports industries. In the aerospace industry, composites are usedto make light weight and efficient structural components. In the Boeing 787, forexample, more than 50% of the structure is made of composite materials. Althoughthere have been significant developments in analyzing cross-ply laminates, none ofthe present approaches provides reasonable predictions for multidirectional laminatesin which intralaminar cracks may form in multiple orientations. Nevertheless, theprediction of damage accumulation and its effect on structural performance is a verydifficult problem due to complexity of the cracking processes.This study presents a synergistic damage mechanics (SDM) methodology to analyzedamage behavior in multidirectional composite laminates with intralaminarcracks in plies of multiple orientations. SDM combines the strengths of micro-damagemechanics (MDM) and continuum damage mechanics (CDM) in predicting the stiffness degradation due to these cracks. The micromechanics is performed on a representativeunit cell using a three-dimensional finite element analysis to calculate thecrack opening displacement accounting for the influence of the surrounding plies, theso-called constraint effect. This information is then incorporated in the CDM formulationdealing with laminates containing cracks in different ply orientations through a `constraint parameter'. Following CDM, a separate damage mode is defined for eachtype of crack and the expressions for engineering moduli of the damaged laminateare then derived in terms of crack density and the constraint parameter. The SDMmethodology is implemented for [0m/??n/0m/2]s laminates containing cracks in ??plies. It is then extended to [0m/??n/90r]s and [0m/90r/??n]s laminates with cracksadditionally in the 90?-plies. The predictions agree well with published experimentaldata as well as independent FE computations. Limited parametric studies areperformed to show usability of SDM for more general laminates.To predict the initiation and growth of intralaminar cracks, an energy basedmodel is proposed in which these cracks initiate and multiply when the work requiredto form new set of cracks exceeds a laminate dependent critical energy release rate.The approach requires determination of average crack opening and sliding displacementsat varying crack spacing. This task is performed through a suitable 3-D FEanalysis. 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 determinedby fitting the damage model with the experimental data for a reference laminate.The predictions from the model for [0/? ?4/01/2]s and [0/90/ ? 45]s laminates showremarkable agreement with the experimental results.The methodology and the results covered in this dissertation will be of interest tomechanics of materials researchers as well as to engineers in industry where compositematerials for structural applications are of interest.