Fatigue behavior of continuous-discontinuous sheet molding compounds

Sheet molding compound (SMC) composites have been well-established as nonstructural and semi-structural components in vehicles due to their high lightweight potential. Over the past decade, increasing need for further vehicle weight reduction has fueled endeavors to further improve mechanical properties of SMC and thereby to expand their suitability as structural components. One promising approach to achieve this goal is the combination of discontinuous glass fiber SMC with local continuous carbon fiber reinforcement, which is currently being investigated by the German-Canadian research training group GRK 2078.
Such hybrid continuous-discontinuous composites enable the production of components with excellent mechanical properties while maintaining the advantages of the SMC process including geometric flexibility, short cycle times and cost efficiency. However, several factors prevent the exploitation of the composite’s full potential. Major obstacles for a safe and efficient application of continuous-discontinuous SMC are the lack of knowledge regarding their fatigue behavior and about uncharted effects of hybridization on damage behavior under cyclic loading
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conditions.
This thesis describes a systematical analysis of the fatigue behavior of continuous-discontinuous SMC both under cyclic tensile and bending loads at different temperatures and frequencies. The hybrid composite, which consists of a discontinuous glass fiber SMC core and unidirectional carbon fiber SMC face plies, shows significantly higher fatigue resistance compared to discontinuous SMC without continuous reinforcement. The effect of hybridization is more pronounced under cyclic loading than under monotonic loading, which is a result of distinct damage mechanisms acting at distinct applied stresses. The mechanical behavior of continuous-discontinuous SMC is dominated by the continuous plies at high stresses and by the discontinuous ply at low stresses. The effect of hybridization is particularly distinctive under cyclic bending load on account of the composite’s sandwich-like structure. In addition, relative stiffness degradation of continuous-discontinuous SMC is less pronounced over a larger loading period compared to discontinuous SMC. While damage evolution within the discontinuous SMC ply remains largely unaffected by hybridization, the continuous SMC plies are enabled to withstand significantly higher cyclic loads than continuous SMC specimens that are not a part of a hybrid composite. When using a cross-ply instead of a unidirectional continuous reinforcement, early initiation of cracks in the 90° ply at comparatively low stresses leads to rapidly growing delaminations and large cracks in the discontinuous SMC ply. Consequently, the effect of hybridization under cyclic loading is less pronounced. While continuous-discontinuous SMC is largely insensitive to a change in frequency, enhanced temperature leads to early failure of the unidirectional ply on the compression loaded side, which results in
a significant decrease of fatigue life.