Cruciform Specimen Design for Biaxial Tensile Testing of SMC

Abstract

This paper presents an investigation of different cruciform specimen designs for the characterization of sheet molding compound (SMC) under biaxial loading. The considered material is a discontinuous glass fiber reinforced thermoset. We define various (material-specific) requirements for an optimal specimen design. One key challenge represents the achievement of a high strain level in the center region of the cruciform specimen in order to observe damage, at the same time prevention of premature failure in the clamped specimen arms. Starting from the ISO norm for sheet metals, we introduce design variations, including two concepts to reinforce the specimens' arms. An experimental evaluation includes two different loading scenarios, uniaxial tension and equi-biaxial tension. The best fit in terms of the defined optimality criteria, is a specimen manufactured in a layup with unidirectional reinforcing outer layers where a gentle milling process exposed the pure SMC in the center region of the specimen. This specimen performed superior for all considered loading conditions, for instance, in the uniaxial loading scenario, the average strain in the center region reached 87% of the failure strain in a uniaxial tensile bone specimen.

Anisotropic Stiffness Degradation in Biaxial Tensile Testing of SMC

“…With the cruciform specimen for biaxial tensile testing developed in [5], anisotropic stiffness degradation as one phenomenon of anisotropic damage is observable. This result confirms the suitability of this specimen to identify anisotropic damage.…”

“…The best performing specimen is reinforced by unidirectional carbon reinforced tapes on both sides of the arms. Details about the design and the manufacturing can be found in [5], [7] and [8]. The present work shows investigations with this specimen where anisotropic stiffness degradation is observable.…”

“…The focus of the present work lies on biaxial tensile testing of SMC. In [5] and [6] different specimen designs are investigated and compared with respect to different optimality criteria, where the most important criterion is the highest strain in the area of interest which is a measure for the likelyhood to observe damage in this area. The detailed characterization of damage also under multiaxial stress states, is indispensable for the understanding of the damage phenomena and serve to validate damage models.…”

“…Biaxial stress and strain states cover a wide range of application loads. Details about the design and the manufacturing can be found in [5], [7] and [8]. The cruciform specimen design is the basic prerequisite for suitable experiments.…”

Investigations of Cruciform Specimen Designs for Biaxial Tensile Testing of SMC

“…For example [3][4][5][6] conducted research to find specimen shapes for fiber reinforced polymers. Serna Moreno et al ( [7]) compared specimens for chopped glass-reinforced polyester and presented a specimen that was suitable to achieve failure in the center area in different loading cases but pointed out that there are still problems such as stress concentrations outside the center area.…”