Research On Interfacial Failure Behavior And Interfacial Toughness Enhancement Of Foam Core Sandwich Composites

The face/core interface plays an important role in structural performance of foam core sandwich composites. The overall load capacity of sandwich composites is often limited not by the strength of face material but by the strength and toughness of face/core interface. In this paper, the interfacial failure modes, interfacial toughness enhancement, interfacial creep fracture behavior as well as the interfacial crack kinking path in foam core sandwich composites are investigated experimenally and numerically under the framework of973Project (No.2006CB601205) and National Science Foundation in China (No.10672027, No.90816025).Related studies of this paper are summarized as follows:1. A double cantilever beam (DCB) test scheme is designed to investigate interfacial frature behavior of foam core sandwich composites. DCB specimens with artificial interfacial crack are manufactured using the vacuum assisted resin injection (VARI) process, and interfacial failure modes of DCB specimens with various foam core are experimentally examined. Two different failure modes of interfacial crack are observed during the test, and their characteristics are also obtained. Analysis is conducted on the mechanism of interfacial failure modes, and the result indicates that interfacial failure modes are mainly dominated by face/core modulus ratio of DCB specimens.2. According to the analysis results of meso-mechanism in chopped fiber toughening and the meso characteristics of foam core surface, an interfacial toughness enhancement method using relatively long chopped glass fibers is proposed. DCB specimens with and without interfacial enhancement are manufactured using VARI process. The interfacial load capacity and interfacial fracture toughness of specimens with and without interfacial enhancement are tested, and the results indicate that the interfacial toughness enhancement technique proposed by this paper can significantly promote the interfacial toughness as well as the interfacial load capacity of the foam core sandwich composites. In addition, the interfacial fracture toughness of specimens with various core thickness is also tested. It is found that the interfacial fracture toughness decreases with increasing core thickness, however, when core thickness is larger than20mm, the interfacial fracture toughness becomes a constant value. The mechanism of this phenomenon is discussed subsequently. 3. Based on digital image correlation (DIC) technique, a precise measurement system is designed to investigate the interfacial creep fracture behavior of foam core sandwich composites. Creep displacement fields near interfacial crack tip of DCB specimens are measured by this system, and creep responses of interfacial fracture parameters are extracted from the creep displacement fields. The creep test results show that creep responses of interfacial fracture parameters present the characteristics of standard solid creep model.'At last, the influences of resin materials on interfacial creep fracture behavior are discussed in detail.4. For the sake of handling cracks in numerical simulation, material point method (MPM) algorithm with cracks is developed in which discontinuity was introduced into MPM by visibility rule. The methodology is validated by typical examples, and the results demonstrate the accuracy and efficiency of the algorithm. After that, the MPM algorithm with cracks proposed in this paper is employed to simulate interfacial crack kinking angles and crack kinking paths in DCB specimens with various foam core, and good accordance is found between numerical results and experimental observations.The method and conclusions presented in this paper would benefit the interface design and interface safety assessment of foam core sandwich composites.