Composite Materials Research Progress

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214 W. Van Paepegem, I. De Baere, E. Lamkanfi et al. Recently, De Baere et al. [16,17] showed that resistance measurement also works very well for monitoring damage in carbon fabric reinforced thermoplastics under tension-tension fatigue loading. Current injection has been done with an innovative technique. Behind the tabs, in the strain-free area of the specimens, the current is injected by means of two rivets at both ends of the specimen, as shown in Figure 5. Figure 5. Use of rivets for electrical resistance measurement in carbon fibre composites [17]. Figure 6 shows the evolution of relative resistance change ρ and axial fatigue stress σxx during fatigue cycles 4025 till 4030. Figure 6. Correspondence between applied sine wave of stress σ xx and measured resistance in a tensiontension fatigue test of a carbon fabric/PPS composite [17]. 2.2. Bending Fatigue Uni-axial fatigue experiments in tension/compression are most often used in fatigue research [18-20] and accepted as a standard fatigue test, while bending fatigue experiments are scarcely used to study the fatigue behaviour of composites [21-23]. Bending fatigue tests differ in several aspects:
<strong>Research</strong> Directions in the Fatigue Testing of Polymer <strong>Composite</strong>s 215 • the bending moment is (piecewise) linear along the length of the specimen (3-point bending, 4-point bending, cantilever beam bending). Hence stresses, strains and damage distribution vary along the gauge length of the specimen. On the contrary, with tension/compression fatigue experiments, the stresses, strains and damage are assumed to be equal in each cross-section of the specimen, • due to the continuous stress redistribution, the neutral fibre (as defined in the classic beam theory) is moving in the cross-section because of changing damage distributions. Once a small area inside the composite material has moved for example from the compressive side to the tensile side, the damage behaviour of that area is altered considerably, • the finite element implementation of related damage models gives rise to several complications, because each material point is loaded with a different stress, strain and possibly stress ratio, so that damage growth can be different for each material point. In tension/compression fatigue tests, the stress- or strain-amplitude is constant during fatigue life and differential equations describing decrease of stiffness or strength, can often be simply integrated over the considered number of loading cycles, • smaller forces and larger displacements in bending allow a more slender design of the fatigue testing facility. Basically, three types of bending fatigue tests can be distinguished: (i) three-point bending [24,25], (ii) four-point bending [26], and (iii) cantilever bending [22,27-30]. The success of these tests for fatigue of polymer composites is quite limited, because the interpretation of the results is more difficult and in case of stiffness degradation, stress redistribution across the specimen height comes into play. Moreover, as long as the bending stiffness of the laminate is high enough (e.g. sandwich composites), the deflections are small and linear beam theory still applies, but once that the bending stiffness of the composite decreases (e.g. thin laminates), the deflections are large and geometric nonlinearities and friction at the roller supports affect the fatigue results. The authors designed a test set-up for cantilever bending fatigue tests as depicted in Figure 7. The power of the motor is transmitted by a V-belt to a second shaft. The second shaft bears a mechanism with crank and connecting rod, which imposes an alternating displacement on the hinge (point C in Figure 7) that connects the connecting rod with the lower clamp of the composite specimen. At the upper end the specimen is clamped (point A in Figure 7). Hence the sample is loaded as a composite cantilever beam. A full Wheatstone bridge on the connecting rod is used to measure the force acting on the composite specimen. Due to the (bending) stiffness degradation of the specimen during fatigue life, the measured force will gradually decrease as the amplitude umax of the prescribed displacement remains constant. In order to record the out-of-plane displacement profile, it was necessary to develop a mechanism to hold the specimen fixed in this state, because recording the profile while the test keeps running at a frequency of 2.2 Hz, gives rise to some practical problems. A rotary digital encoder was attached to the second shaft. Its angular position (relative to a certain reference angle) is directly related with the loading path of the
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COMPOSITE MATERIALS RESEARCH PROGRE
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Copyright © 2008 by Nova Science P
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vi Contents Chapter 9 Recent Advanc
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viii Lucas P. Durand scale transiti
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x Lucas P. Durand machine. In paral
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In: Composite Materials Research Pr
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Multi-scale Analysis of Fiber-Reinf
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T300 fibers (Soden et al., 1998; Ag
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1 I L = L : r v Multi-scale Analysi
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I E ⎡0 ⎢ ⎢0 ⎢ ⎢ ⎢ ⎢0
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Applied macroscopic load Correspond
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In: Composite Materials Research Pr
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Optimization of Laminated Composite
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⎧σ x ⎫ ⎡ mE ⎪ ⎪ ⎢ x
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and γ 2 γ 0 Optimization of Lamin
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4 x 10 5 4 3 2 1 Compliance (Nmm) 0
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3.5 3 2.5 2 1.5 1 Optimization of L
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110 W.H. Zhong, R.G. Maguire, S.S.
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112 Reinforcement: Advanced materia
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130 Yuanxin Zhou, Hassan Mahfuz, Vi
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140 Heat Flow (W/g) Heat Flow (W/g)
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144 Material Yuanxin Zhou, Hassan M
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146 SEM Analysis Yuanxin Zhou, Hass
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150 Governing Equation For the fibe
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152 ⎧ UU = ⎨ ⎩ ⎧ UD = ⎨
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156 Stress (MPa) 120 80 40 0 Yuanxi
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Page 178 and 179: 164 Yuanxin Zhou, Hassan Mahfuz, Vi
Page 180 and 181: 166 Giangiacomo Minak and Andrea Zu
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Page 202 and 203: 188 A B E (MPa) D 250000 200000 150
Page 204 and 205: 190 D D 1.0 0.9 0.8 0.7 0.6 0.5 0.4
Page 218 and 219: 204 Conclusion Giangiacomo Minak an
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Page 225 and 226: Research Directions in the Fatigue
Page 227: Research Directions in the Fatigue
Page 241 and 242: Bending force [N] Research Directio
Page 253 and 254: Damage Variables in Impact Testing
Page 263 and 264: F peak [kN] 16 14 12 10 8 6 4 2 0 D
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Eletromechanical Field Concentratio
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MV/m. Eletromechanical Field Concen
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276 S.C. Tjong developed in the pas
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278 S.C. Tjong ultrasonic waves gen
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280 S.C. Tjong applications. Goujon
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282 S.C. Tjong nanoparticles were p
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284 S.C. Tjong where DL is lattice
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286 S.C. Tjong stress is temperatur
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288 S.C. Tjong threshold stress res
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290 S.C. Tjong NC Al, and the NC Al
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292 S.C. Tjong (a) (b) Figure 19. T
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294 S.C. Tjong Apart from forming u
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296 S.C. Tjong [29] Saravanan, M.;
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298 braids, 115 broadband, 211 bubb
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300 endothermic, 139 endurance, 32
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302 isostatic pressing, 279, 288 It
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304 piezoelectricity, 261 pitch, 12
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306 67, 69, 70, 71, 72, 73, 84, 94,
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