PSI - Issue 17

Trevor Sabiston et al. / Procedia Structural Integrity 17 (2019) 666–673 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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easier to mould similar to a textile composite due to stitching yarns holding the bundles of unidirectional material in place (Vallons et al., 2011). NCF composites have improved through thickness properties compared to unidirectional layups due to the stitching yarns (Mouritz et al., 1997). Using processes such as resin transfer moulding the manufacturing time and cost of NCF composites is significantly reduced making the process more attractive to the automotive industry (Edgren et al., 2006). Fatigue characterization of the NCF composite is required for its use in automotive structural applications. Typically, fatigue characterization has been conducted under tension-tension loading for NCF materials due to buckling experienced under compressive loads (Carvelli et al., 2010, Vallons et al., 2007, Vallons et al., 2009, Vallons et al., 2011). The tension-tension testing procedure is described in ASTM D3479 using tensile specimens from ASTM D3039. It is desirable to characterize NCF composites under fully reversed tension/compression loading for applications in the design of components subjected to such loadings in their service life. The failure modes of the NCF composite system are also of interest. The temperature dependent response of NCF materials and its effect on fatigue life has not been studied. Charalambous et al., 2015 have conducted testing on carbon/epoxy unstitched uniaxial composite at elevated temperature for use in aerospace structures from -50 to 80 ºC for uniaxial and fatigue loadings. For use in automotive applications the NCF material must be characterized to higher temperatures which are applicable to internal combustion engine applications. The NCF material studied is a [0/90,90/0] 6 carbon/epoxy biaxial NCF with a Tricot-warp stitch pattern. In the present study, four specimen geometries were evaluated for use in fatigue testing under fully reversed loading. A baseline strip geometry along with three dog bone shaped specimens were studied to ensure a uniform strain field in the gauge section and resistance to buckling under compressive loading. Fatigue characterization of the NCF composite was conducted using fully reversed loading at room temperature for the four specimen geometries to evaluate their fatigue performance. Fatigue testing with fully reversed loading was conducted on the chosen geometry at room temperature and 130 ºC. In the NCF composite studied, 12 layers of the biaxial NCF fabric were laid up in the stacking sequence [0/90,90/0] 6 . Flat plaques were moulded using compression moulding. To study the effect of specimen geometry four different specimen geometries were considered. All of the specimens were cut from flat plaques of moulded NCF material with thickness ranging from 4.59-4.80 mm. The baseline test was conducted on a 15 mm wide strip of the NCF material which is similar in geometry to standardized tests such as ATSM D3039 for fatigue loading, for tensile test baseline testing a specimen width of 10 mm is used. The remainder of the test geometries were based on the geometry presented in Dabayeh et al., (1996), which is a dog bone geometry with a varying radius curvature throughout the gauge length. The geometry was modified to center point gauge width of 15 mm, 10 mm, and 6.34 mm were tested all using the same external dimensions which are shown in Fig. 1 for the 6.34mm width. The specimens were cut from the flat plaques using computer numerically controlled (CNC) machining, waterjet cutting was used on some of the 15 mm width modified Waterloo geometry specimens to compare the fatigue results of the different cutting methods. The 15 mm wide strip geometries were produced using CNC and saw cutting to compare the specimen production methods. The purpose of evaluating the geometries is to ensure that the gauge section undergoes a uniform strain field using a commercially available 3D optical strain mapping system, Aramis based on digital image correlation (DIC) and to evaluate if the geometry resists buckling under compressive loading. The tests to evaluate the strain fields developed within the geometries were conducted under uniaxial tension and compression using DIC. The crosshead speed used for both tensile and compressive tests was 1.5 mm/min. The moduli in tension and compression, ultimate tensile stress (UTS), and ultimate compression stress (UCS) were evaluated during these tests. 2. Materials and Test Procedures