ABSTRACT of the Doctoral Thesis

Computational Methods for the Prediction of Emergence and Growth of Delaminations in Laminated Composite Components

by Gerald Wimmer (2009)

The present thesis is concerned with the computational simulation of delamination in fiber reinforced polymer laminates. The delamination process, from the pristine structure to the formation of large delaminated regions, is considered as a two-step process. In the first step an initial delamination emerges in an intact interface and in the second step the delamination grows. The objective of this work is to develop numerically efficient and robust tools for predicting the emergence and growth of delaminations. To demonstrate the application and the capability of the proposed approaches several examples are analyzed.

For the prediction of emergence of delaminations a combination of a strength criterion with an energy release rate criterion is developed. A stress based failure criterion is employed to predict overloaded interface regions and initial delaminations are assumed to emerge there. The propagation of delaminations is analyzed using linear elastic fracture mechanics. The strength/energy approach allows to predict the critical size and position of an initial delamination as well as the load carring capacity of the structure. Furthermore, the sensitivity of the predicted load carring capacity with respect to changes and uncertainties in the material properties is determined.

For the prediction of consecutive growth of delaminations with straight fronts a computationally efficient semi-analytical approach is implemented. It uses the fact that the energy released at delamination growth is proportional to the increase of the structural compliance caused by an increase of the delaminated area. The structural compliance is determined as a function of the delaminated area by an automated procedure employing the finite element method. The semi-analytical approach can handle arbitrary combinations of mechanical, temperature, and moisture loads. A Griffith type growth criterion is used to predict consecutive growth caused by quasi-static loads. The stability of the growth process as well as the non-linear structural response are predicted. For the analysis of delamination growth caused by cyclic loading a Paris type growth law is employed.

For simulation of delaminations with curved fronts a total delamination front criterion is developed. Growth along the entire delamination front is assumed to take place if the growth criterion is satisfied. A set of smooth and continuous delamination fronts is defined and growth is predicted by selecting that shape for which the load required to caused delamination growth is smallest. For computation of the delamination growth load an automated finite element method is employed. A criterion is developed that allows to check whether or not the assumed shape of the delamination front is a good approximation. This criterion can also guide the way towards improvement of the shape.

For verification of the developed approaches emergence of delaminations and growth of existing delaminations in an L-shaped laminate is predicted numerically and compared to results from experimental tests. The test are performed at the Polymer Competence Center Leoben GmbH (PCCL, Leoben, Austria). The test specimens are produced by FACC AG (Ried i.I., Austria). Test specimens without initial delaminations and test specimens with defined initial delaminations are studied. For both test series reasonably good agreement between the numerical predictions and the experimental results is obtained.

revised 090128 (hjb)