The macroscopic properties of materials are strongly determined by the microstructure of the materials. This holds in particular for ﬁber reinforced composites where ﬁber distribution and orientation are crucial for the reinforcement to serve it’s purpose. This essential microstructural information can be obtained from high-resolution images using non-destructive testing methods.
The mixing of ﬁbers and polymers to form composites opens new ways for designing lightweight materials substituting traditionally used steel alloys or iron sheets in aviation and car industry. Composite materials are traditionally regarded as materials that can save energy in large structures and components associated with transportation. Some of the most interesting applications of ﬁber reinforced composite materials are those where they are used to protect lives or property by absorbing the energy of projectiles in impacts or crashes. Due to the increasing use of ﬁber reinforced composites as safety-relevant parts, an enhanced understanding of the connection between the spatial distribution of microscopic ﬁbers, controlling the macroscopic performance, has become a major issue (compare the twofigures).
Knowledge about the components as well as the production process of ﬁber reinforced composites provides broad qualitative information about the microstructure, for instance about ﬁber volume fraction, preferred ﬁber direction or layers within the microstructure. However, quantitative image analysis is needed in order to go beyond mere qualitative descriptions of composite materials. Essentially, there are two strategies for assembling a composite – stacking layers of different materials yielding a laminate or inserting particles into a matrix to get a particle-reinforced composite. Fiber-reinforced SMC falls into the ﬁrst class as it is produced by compression molding (compare third figure from top).
Industrially, SMC is processed in three steps: First, up to a dozen liquid and solid raw materials are mixed to form a homogeneous resin-ﬁller paste. Reinforcement ﬁbers are incorporated, resulting in a semi-ﬁnished SMC panel exhibiting a rather low viscosity which allows the ﬁbers to be effectively wetted with a coating ﬁlm. Second, the SMC is stored for a while to give the paste time to thicken, or mature. During this process the SMC cures, resulting in an increase of the dynamic viscosity. Finally, the semi-ﬁnished SMC panel is formed into a component. The SMC starts to ﬂow due to the pressure, eventually ﬁlling the cavities of the mold and at the same time dragging along the ﬁbers in the resulting ﬂow ﬁeld. In our research project we have investigated in detail the fiber orientation in varying depths of glass- and carbon fiber-reinforced SMC materials using several non-destructive testing methods (CT, X-ray, SAM and Infrared).
Non-destructive characterization of fiber orientation in reinforced SMC as input for simulation based design
K. Schladitz, A. Büter, M. Godehardt, O. Wirjadi, J. Fleckenstein, T. Gerster, U. Hassler, K. Jaschek, M. Maisl, U. Maisl, S. Mohr, U. Netzelmann, T. Potyra, M. O. Steinhauser
Composite Structures 2017, 160, 195-203