Composite structures under impact loading

General background Advanced composite materials are being used increasingly in a variety of sectors such as maritime, offshore, aviation, defense, transportation, construction, infrastructure (roads, railways, power grids etc.) and sports. This development has come as a result of the many positive characteristics these materials have compared to conventional materials; such as a favorable combination of low weight and good mechanical properties, low corrosion and good resistance to many chemicals. The challenge in taking these materials into use has been that they, as opposed to conventional materials such as metals, have a relatively short elongation at break with very little or no plastic deformation. They are what we call brittle! Therefore, damage and cracks in these materials is a concern: Much research and work have been performed to become familiar with these materials in order to put them into safe use. But for optimal design of structures and to minimize/optimize costs of service and maintenance, still much remains to be improved. This applies first and foremost to the models in use in numerical tools for predicting the extent and severity of damage in different situations. As the numerical models become more complex, ever more sophisticated material parameters are need and must be determined. New experimental techniques must be developed in order to determine these. A constraint in practical use is to understand what is good enough: Determination of material parameters and numerical modeling must be conducted in a time-/cost window which is acceptable. The aim of the COMPACT project Impact damage in composite structures is a question that still causes concern in many applications. Unlike ductile materials such as metals that can absorb large amounts of energy via plasticity without loss of stiffness and strength, composites which are relatively brittle, mainly absorbs energy by elastic deformation and irreversible damage mechanisms. These damage mechanisms degrade the material and weaken structures stiffness and strength. Damages are divided into cracks in the polymer matrix, damage/breakage of fiber, and delamination between fiber and matrix and between different lamina layers through the thickness of the composite. The aim of COMPACT project has been to develop new models and methods to predict the effect and extent of damage in composite structures subjected to impact loads. Through our research, we have developed generic competence, experimental technologies and numerical methods for design of composite structures. Implementation and results First we implemented several different failure criteria described in literature into numerical models and performed simulations comparing the predictability of these models with experimental results. This gave us insight into strength and weakness of these models, and effects which are important to capture. We started with a simplified situation with slow deformation rates and where delamination can be neglected. The results indicated that existing models provide fairly good predictions in situations where tension and compression dominated, but situations dominated by shear deformation were more challenging. We then went on to study deformation at higher rates, up to what is called low-speed impact, in addition to deformation situations where delamination of the structure is important. One challenge with composite materials compared to other materials, is that material properties are built into the product during the manufacturing process. This means that the material properties in a flat laminate are not necessarily the same as that of a tube. Within the project we have develop characterization methods for determining material properties of pipes and other (mainly) tubular products. In particular a test program was carried out in order to obtain a better understanding of the delamination process and to obtain reliable material data needed to describe the process numerically. Part of this work has been performed in close collaboration with DTU Risø in Denmark. They have developed a unique experimental setup that we have used. We have developed a new way of analyzing these experimental test results which has enabled us to develop further a numerical model that describes the delamination process. Finally we have tested our numerical models and our experimental methods for obtaining reliable material data needed as input, in situations and on products relevant to the industry partners who have participated in the project, such as pressure pipes for Flowtite, gas containers for Ragasco and pressure vessels for Nammo. Comparison of the experimental results with predictions from numerical modeling shows in many cases satisfactory correspondence. Our work has shown that we are on the right track to model impact and damage, but there are still challenges that must be solved before one can obtain robust predictions based solely on numerical modeling.

The research project shall establish generic competence, experimental technologies, and numerical methods for the design of safe, robust and cost-efficient composite structures. The developed methodologies shall be applicable for design of structures unde r loading ranging from quasi-static to impact loads. Preliminary design through reliable simulation tools will shorten the development and test phase, and reduce unexpected failure occurrences and the number of late expensive modifications. This knowledge shall be implemented in the industrial companies and used for design, improvement and optimisation of any relevant industrial product. Better conceptual and preliminary design methodologies are essential to ensure a competitive Norwegian industry. The de velopment of design tools and methodologies requires knowledge on material behaviour and material modelling, structural behaviour of elementary structures and accurate and robust solution techniques. The research methodology in this project will be based on an integrated use of theoretical work, numerical simulations and high precision physical. Four work packages have been defined within the project: WP 1 Mechanical characterisation of composite materials, WP 2 Modelling of composite materials, WP 3 Test ing of composite structures and WP 4 Modelling of composite structures. The project will address major shortcomings of models and analysis techniques reported in the literature. The project will yield new, more reliable and more complete experimental data of failure envelopes, especially for certain ratios of biaxial stress. Furthermore, numerical models including state-of-the-art criteria for prediction of delamination resulting from impact loading will be established and checked against experimental res ults from advanced tests on coupon and structural level, and a recommended design practice will be established.