Investigating On Buckling And Post-buckling Behaviour Of CFRP Laminates With Delaminations

The versatile properties, such as light-weight, high strength and stiffness to weight ratio, resistance to corrosion, high energy absorption capacity under impact loading, have caused Carbon Fiber-Reinforced Plastics(CFRP) to attract much attention from the aerospace industry. In laminated composites, delamination is the most common form of damage and can result from manufacturing defects or the effect of events during the service life, such as hard object impacts or slamming. When a composite plate with a delamination is subjected to in-plane compression or a bending load, initial localized buckling occurs in the delamination due to its reduced flexural stiffness, associated other failure features, which results in the whole structure damaged under a lower load condition.A deep understanding of delaminations in composite structures is important for engineering applications and was the focus of this thesis. This study has investigated the buckling, post-buckling behaviour and associated damage mechanisms and damage growth law of delaminated CFRP laminates. The buckling behaviour and delamination growth under four-point bending have been investigated using analytical model, Finite Element method and experimental methodology respectively in this thesis. The main contents are as follow:First, based on the delaminated buckling model, one-dimension analytical model for the buckling behavior of a delaminated composite laminates under pure bending load has been presented. The formula for the critical buckling load of delaminated CFRP laminates has been established.Second, 3D Digital Image Correlation(DIC) method has been used to explore the buckling and post-buckling behaviour of CFRP laminates with impacted under four-point bending. Based on the location of delamination and the buckling model, CFRP composite laminates with an appropriate artificial, idealized delamination exhibit an equivalent response and failure to laminates with delamination damage due to an impact have been established. Date-fields for displacements have been acquired using 3D digital image correlation and used to validate the appropriate artificial, idealised delamination exhibit an equivalent response and failure to laminates with delamination damage by comparing the results from specimens with artificial delaminations and specimens with equivalent impacted delaminations using an image decomposition method. After error analysis and validation, only small differences between two types of specimen occur just prior to ultimate failure. What’s more, based on the 3D digital image correlation method, the effect of delamination size, shape and orientation on buckling behavior and delamination growth has been investigated using artificial delaminations in CFRP under four-point bending. The following conclusions have been drawn: 1. When the delamination size is larger than a specific value, the delaminated buckling occurs and the critical buckling load is decreased with increasing the delamination size. 2. The delamination shape has an important effect on the buckling model and delamination growth. 3. For a constant size and shape in delaminations, the delamination orientation has small effect on the buckling model, delamination growth and the critical buckling load.Finally, this study has also developed a non-linear Finite element Analysis(FEA) incorporating a fibre/matrix constitutive model, a modified fibre/matrix failure criterion and a delamination growth criterion to examine the interaction between the buckling behaviour and delamination growth. A modified fibre/matrix failure criterion based on Dávila and Camanho’s criterion has been proposed to predict the initiation and evolution of the fibre/matrix failure in CFRP laminates. This computational model has been validated to predict the buckling, post-buckling behaviour and delamination growth of composite panels by comparing the results from Finite Element Analysis and experiment using image decomposition method.