JEC COMPOSITES MAGAZINE - Issue #35 - September 2007 - 65

KNOW-HOW Testing g…/… Non-destructive evaluation to designers for establishing reliable failure criteria (Fig. 2 & 3). The ability to predict the strength of high-performance composite materials under complex loading conditions (cryogenic and atmospheric) is required for rational design. The information that can be used for predicting strength stems from the micromechanics level. As soon as the main principles are established, the approach can be used separately for tensile (cryogenic) and compressive (atmospheric) loads. Mechanical properties Tensile strength Tensile modulus Flexural strength Flexural modulus Compression strength Compression modulus Interlaminar shear strength Table 2: Mechanical properties of the IM7 carbon fibres Values 2,760 MPa 168 GPa 1,655 MPa 148 GPa 100 MPa 1,770 kg/m3 62% Material Modulus of elasticity, ExMsi Coefficient of thermal expansion, CTE, PPMF IM7/ carbon 40 Boron/ fibres 50 K63712/ carbon 93 PEEK PPS 0.51 0.55 Fig. 3: Elements of the lattice structure -0.2 2.5 -0.6 28.9 Table 3: Modulus of elasticity and coefficient of thermal expansion for the prepreg components Material Modulus of elasticity, ExMsi Coefficient of thermal expansion, CTE, PPMF Fig. 4: Model of the satellite lattice structure IM7/PEEK IM7/PPS IM7/boron /PEEK IM7/boron /PPS 24.2 24.2 27.5 27.5 -0.02 0.06 0.57 0.57 Table 4: Modulus of elasticity and coefficient of thermal expansion for the prepreg laminates based on IM7 carbon fibres Experimental results We determined deflection on the cantilever bar using a Nikon digital camera. The stiffness was determined based on the correlation between the velocity of propagation of ultrasonic waves and the modulus of elasticity. E1=C1 ρ(1-µ12 µ21) K63712/ Material Modulus of elasticity, ExMsi Coefficient of thermal expansion, CTE, PPMF PEEK K63712/ PPS K63712/boron/ K63712/boron/ PEEK PPS 56.0 56.0 56.2 56.2 (1) -0.50 -0.48 0.00 0.00 The temperature gradient was determined using a Radiance infrared camera. Our NDE analysis was performed on IM7 carbon fibres (see mechanical properties in Tables 2 and 3). However, no combination of the IM7 material with boron fibre and PEEK or PPS polymer reached the targeted near-zero CTE. Some results are summarized in Tables 4 and 5. Consequently, we looked for carbon fibres with a maximally negative CTE and improved modulus. Driven by these selection criteria, we identified the pitch carbon fibre Dialead K63712 from Mitsubishi as our leading candidate. According to our model, it can improve the laminate’s tensile modulus and approach a near-zero CET in a certain combination with boron fibres, in the 1020% range of volume fractions. Table 5: Modulus of elasticity and coefficient of thermal expansion for the prepreg laminates based on the K63712 carbon fibres estimated using NDE methods Physical characteristics of the prepregs The typical prepreg tapes manufactured by Phoenixx TPC are 25mm wide and 0.15-mm thick. We estimated that our final laminate would include 10-12 carbon layers and 6-8 boron layers, with a total thickness of 2.0-3.0 mm. The lamination sequences of the prepreg layout would include a cross ply configuration, [0,90]2s, and two quasi-isotropic configurations, [0/+45/-45/90]s/[0/+45/90/45]s, in alternating order. The curing process will be carried out and optimized using a hot hydraulic press, within a pressure range of 20-50 Psi, at curing temperature of 330-385°C. JEC Composites Magazine / No35 September 2007

JEC COMPOSITES MAGAZINE - Issue #35 - September 2007

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