Fiber bridging concepts applied to short fiber composites

Sammanfattning: Polymer composite materials are in wide-spread use in the transportation industry. In aerospace industry the use these materials are established while in automotive industry the interest is increasing. The attention of automotive industry is to a great deal focused on various kinds of molded composites such as glass mat reinforced thermoplastics (GMT) and sheet molding compound (SMC). Their interest is to a large extent driven by the possibility to manufacture components of complex geometry in a cost- efficient process with these materials. An increasing number of car and truck manufacturers are using SMC for external panels such as trunk covers, hoods, roofs and spoilers. A property of obvious importance for an external car- or truck-panel is its capacity to withstand impact. In this context, improved understanding of crack growth and toughening mechanisms of the material is of great interest. A major part of the work presented in this thesis is driven by an interest to increase the understanding of how material composition and microstructure of short fiber composites influence their overall fracture behaviour. In materials such as metals and unreinforced polymers, linear elastic fracture mechanics (LEFM) is widely used, often with great success, both in design and in development of new materials. Unfortunately, problems arise when LEFM is applied to short fiber composites. This is due to the large process zone that develops ahead of a crack in these materials. The fundamental assumption of LEFM, that the damage zone at the tip of the crack is small compared to crack length, is often violated in experiments. The presented thesis considers a different approach, in which the damage ahead of a crack tip is described by a bridging-law. By considering the bridging-law as the major failure property of the material, a coupling between mechanisms acting on a microscale and the macroscopic failure behaviour can be established. No such information can be obtained using a LEFM approach where the material behaviour is described in terms of a single value, the fracture toughness. Bridging-laws for three different short fiber composites are experimentally determined and presented in the first paper of the thesis. A matter of key importance for future work in this field is that there are methods available for experimental verification of the suggested fiber bridging approach. Optical strain field measurement methods are therefore very useful. The thesis contains a pilot study to evaluate the use of two recently developed optical methods , Stereoscopic Digital Speckle Photography (Stereo-DSP) and combined DSP-DSPI (Digital Speckle Pattern Interferometry), for measurements of fracture behaviour of notched short fiber composites. We found Stereo-DSP to be a versatile technique that can be used when knowledge of overall displacement fields is required. The combined technique can with advantage be used when detailed information about large deformation at small areas is of interest, e.g. the complex fiber bridging interaction at the crack tip of a short fiber composite. The last paper in the thesis presents a study where the influence of fiber surface treatment on transverse cracking in cross-ply laminates was investigated. In the case of tubes and pressure vessels, the formation of transverse cracks ultimately leads to leakage since cracks connect and form a path through the wall. In the presented study, our ambition was to investigate the influence of film former polymer on transverse cracking properties of cross-ply laminates.Both onset of transverse cracking and tendency for multiple crack development were strongly affected by the different film formers. The strong film former effect was proposed to be due to a combination of improved interfacial adhesion and the plasticizing effect from the film former on the interphase region. The thesis is composed by the following papers: Patrik Fernberg, Lars Berglund, Bridging law and toughness characterisation of CSM and SMC composite, to be submitted. Angelica Andersson, Patrik Fernberg, Mikael Sjödahl, Optical methods to study fracture of notched glass mat composites. Proceedings of the International Conference on trends in Optical Nondestructive Testing, Lugano, Switzerland, May 3-6, 2000 (in press). Patrik Fernberg, Lars Berglund, Effects of glass fiber size composition (film former type) on transverse cracking in cross-ply laminates. Accepted for publication in Composites, Part A.