A Predictive Approach To Strength Of Symmetric Composite Laminates

As a new type of materials, composite has an encouraging prospect of practical applications, especially in aeronautic and aerospace industries because of fairly excellent through-thickness properties and lower cost of manufacture, which is a useful multiphase material made of not less than two materials, and at the same time it has unique predominant performance, which other traditional unitary materials don't possess. With increasing developments of composite materials, however, it is no avoidance that there are some local damages and fractures. Compared with metal materials, the composite materials own more complex fracture models, i.e. the fracture model of the former appears that there are only some isolate cracks, when they are under static or periodic loading, whereas the latter's rupture is combined with many fracture and their interaction, such as matrix cracking, interface non-glue, fiber failure and fatigue delaminations and so on. Meanwhile, it also depends on other parameters: properties of fiber or resin, lay-up configurations, solidifying temperature, environmental temperature, working conditions. It is no doubt that all damages will more or less affect in-plane mechanical performances of composite materials, in particular ultimate strength. In order to research damages' effects on in-plane mechanical performances of composite laminates, the equivalent constraint model is established by Zhang to analyze it, which can constitute the relations between stress and strain of the considered lamina under damages and the ones between its stiffness and damages. Based upon the ECM model, a new analytic model for predicting the ultimate strength of general symmetric laminates is studied: it is reckoned that the primary load-bearing lamina controls the final failure of the laminates. Thus composite laminates will be thoroughly destroyed if the fiber of the primary load-bearing lamina is fractured. As a result, the laminates will not bear additional loads any more. Then the current stress corresponds to the ultimate strength of the composite laminates. In that case, the key is to attain the stress field of the primary load-bearing lamina through the ECM and the classical laminate theory.Applying this new model for predicting strength, we investigate different lamina properties, lay-up configurations and combined loading conditions' effects on the strength and draw stress-strain behaviors and failure envelopes of composite laminates and comparative figures between the predicted values and experimental ones for some arrangements of the laminates. It is obviously clear that different conditions may reach entirely different results. If we compare the predicted values with experimental datum, we can draw a useful conclusion that it is reasonable and effective to predict the strength of the general symmetric laminates containing multi-layer cracks through the ECM model. The model can well stimulate matrix cracking and stiffness degradation of every lamina in laminates. Compared to other computational models for predicting the strength, the method proposed by the author in this paper is more compact and accurate. In addition, its physical concept is legible and it can greatly simplify the difficulty and the process for computing it.