PSI - Issue 19

82 Sara Eliasson et al. / Procedia Structural Integrity 19 (2019) 81–89 Sara Eliasson / Structural Integrity Procedia 116 (2019) 000 – 000 a major contribution to the field of research of predicting fatigue failure in composite materials. However, there is still a lack of understanding and there are limited predictive macroscopical fatigue life assessment methods for composite materials and structures in comparison with metallic vehicle structures (Vassilopoulos, 2010). A fundamental first step in understanding and characterizing a material is by experimental testing. Hence, a systematic approach for fatigue testing and material characterization is essential and typically requires “trial and error” in e.g. test set up, specimen geometry and dimensions for achieving satisfying experimental results with adequate failure modes and locations. Llobet et. al. (2017) highlights challenges such as tab geometry and material, load introduction, shear forces, etc. A test methodology to overcome some of these different challenges by using parallel-side-coupon specimens was developed. Brunbauer et. al. (2014), (2015a) and (2015b) carried out an extensive fatigue testing program on Carbon Fiber Reinforced Polymers (CFRP) with epoxy. They recommend careful choice of the net cross-section of the test specimen with respect to the materials ultimate strength and applied cyclic load level. To overcome the problems with tab failure and splitting parallel to fibers in tab region, Meziere et. al. (2005) developed a test set up for large strain cyclic loading on CFRP in the fiber direction. Kawai (2001), (2004) and (2016) reports a comprehensive fatigue testing program of CFRP composites with detailed descriptions of specimen geometry and configuration while avoiding end tab failure and reporting acceptable failure modes for fatigue testing in the fiber direction. Tabs are recommended by the ASTM D3039/D3039M-17 (2017) standard to present a softer interface, gentler load introduction and prevent premature failure. But tabs will introduce stress concentrations leading to frequent problems in fatigue testing and the magnitude of the stress concentrations will be dependent on tab geometry, choice of adhesive, specimen properties etc. It has also been reported that tabs shorter than 60 mm (Kulakov, et al., 2004) are unreasonable. De Baere et. al. (2009), however concludes that tabs should be mounted inside grips with no tapering even though this is not in alignment with the FE results presented, nor the stress concentration factor for the different tab designs. De Baere et. al. (2011a) and (2011b) continues investigating the choice of test specimen and presented an extensive test program to obtain an optimized specimen geometry for a parallel-side-coupon specimen. Korkiakoski et.al. (2016) investigated how the choice of specimen type influenced the fatigue life. They optimized a specimen for fatigue testing of unidirectional and quasi-unidirectional GFRP laminates with failure frequently occurring in the gauge length for uniaxial tension-tension fatigue testing. Zangenberg (2013) reported that dog-bone specimen promotes the failure in the gauge length and there is a significant difference in fatigue life results comparing specimen types. However, there is a risk of longitudinal splitting along the fibers if the gradual curvature is too large (Nijssen, 2010) and therefore the parallel-side-coupon specimen sometimes is preferred. Bailey et. al (2015) focusses on the gripping and clamping forces and their effect on uncertainties in load application during fatigue testing. Pagano et. al. (2018) concluded that the gripping pressure and thickness of the specimen influence the results of uniaxial tensile static testing in the fiber direction and not the choice of tab material. The literature reveals that comprehensive research has been focusing on CFRP composite fatigue and ultimate strength testing and material characterization. However, most of the studies lack information about detailed procedure for defining optimal specimen configuration suitable for fatigue testing. This motivates the current study on developing and in detail documenting procedures for specimen design, manufacturing and fatigue testing of Unidirectional (UD) CFRP composite materials and could serve as a guideline for the research community 2. Test specimens The objective is to find a functional fatigue testing procedure of UD CFRP materials with respect to specimen geometry, tab configuration, adhesive, clamping force and temperature evolution.

2.1. Specimen geometry

In the current testing program a parallel-side-coupon specimen has been used, see Figure 1. The test specimens were manufactured according to the ASTM D3039/D3039M-17 (ASTM International, 2017), with a UD carbon/epoxy material. Fatigue specimens are altered according to the developed procedure and static specimens generate good results with tapered glass fiber tabs as recommended in the ASTM standard.