Composite Materials Research Progress

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182 Giangiacomo Minak and Andrea Zucchelli From figures 6 to 10 it is possible to observe that the five types of laminates have different behaviours from the AE point of view. A preliminary observation can be done considering the AE domain that can be used to identify the Free Failure Domain (FFD), i.e. the strain domain over which no failures are detected. Considering the ratio between ΩAE and the strain at rupture it can be seen that the percentages of the FFD over the all strain domain are the following: 11% in the case of UD laminates, 0.5% in the case of AP laminates, 1% in the case of QI1 laminates, 30% in the case of QI2 laminates and 1.4% in the case of QI3 laminates. The estimated percentage values of FFD indicate the different attitude to the damage onset of each type of laminate, and in particular it is interesting to note that the QI2 laminate type is the one that has the greater capability to be strained without significant damage. On the contrary the AP laminate types are the most sensitive to the applied strain and reveal an early damage onset, probably due to the high matrix percentage content and to fibre orientation (±45°). Considering the diagram of AE event cumulative counts, figures 6A to 10A, it can be observed that only in the case of AP laminates the slope of the diagram is quite constant during the all test. For the other laminate types, UD and QI, the cumulative counts reveal an initial trend with low slope values that progressively increase during the test. Such behaviour can be interpreted considering the different damage attitude of the laminates and their structure. In the case of AP laminates the mechanical behaviour was dominated by the matrix deformation and cracking. The effect of fibres in AP laminates did not influence the material behaviour and, on the contrary, as observed during experiments at the early stage of tests, fibres promoted matrix breakage and spalling. Such interpretation of AP laminates behaviour is also supported by the AE energy diagram, figure 7B, where it is noticeable the presence of AE events with an energy content (the maximum AE event energy is about 4.0⋅10 -4 J) that is typical of composite laminate matrix failures [47]. Different behaviour was observed in the case of UD, QI1 and QI2 laminates. Considering, for example, diagrams of figures 6A, 8A and 9A, for the UD, QI1 and QI2 laminates respectively, it is possible to note the presence of a strain domain where the cumulative count rate is quite low. For these laminates, during the initial test stage no significant failures can be detected and considering also the energy diagrams, figures 6B, 8B and 9B it is possible to assume that the sources of AE event are mainly due to matrix cracks onset. Comparing in particular the cumulative counts and energy diagrams of QI1 and QI2 laminates it is interesting to note that in the case of QI2 the maximum number of cumulative counts (∼ 3⋅10 5 counts) is lower than the one of QI1 laminate (∼ 2⋅10 6 counts), but, at the same time, the maximum AE event energy of QI2 (∼ 1.2⋅10 -3 J) is comparable to the one of QI1 laminate (∼ 2.2⋅10 -3 J). This behaviour can be understood considering the different delamination strength of the two laminates. In fact, as reported in [47, 48], delamination is a possible failure mechanism for laminates of type QI1, and, on the contrary, it is not a typical failure for laminates of type QI2. So in the case of QI1 the maximum number of counts is greater than in the case of QI2 thanks to the contribution of events caused by inter-laminar fractures and delamination. Nevertheless the maximum AE energies for the QI1 and QI2 laminates are comparable because the final crisis of both materials is characterized by a fibre breaking process that determines the release of an AE event with an high energy content. The behaviour of QI3 laminate is quite different if compared to QI1 and QI2. In particular the cumulative counts trend, figure 10A, shows a consistent release of AE events at the early test stage, but the total number of cumulative
Damage Evaluation and Residual Strength Prediction of CFRP Laminates … 183 counts over all test (∼ 1.4⋅10 4 counts) is lower than in the case of QI1 and QI2. Considering the AE energy diagram of QI3, figure 10B, it is possible to observe that events with a high energy content happens also in the early-middle stage of the test, differently than in the case of QI1 and QI2 where events with a high energy content appear only at the final test stage. All these facts are related to the QI3 ply composition: the matrix volumes of each ply influences the dominant material failure mode, the matrix cracking, as in the case of AP. All the previous considerations can be effectively summarized and completed considering the diagrams of the sentry functions f. In the case of UD laminate the construction of the function f reveals the initial material damage, figure 6C, that is highlighted by the sub-function PI,1 and PII,1. This initial damage can be related to some material internal adjustment and to the onset of some internal cracks but, as revealed by the following PI,2, such damages do not compromise the material capability of storing strain energy. The following PIV,1 sub-function reveals that the internal cracks propagate and that the material storing energy capability is progressively reduced until a sequence of drops due to splitting (PII,2, PII,3 and PII4) mixed with some strain energy storing phases (PI,3, PI,4, PI,5) that prelude to the first and most critical drop PII,5. This drop is mainly related to the free edge delamination and after this a new but small strain energy storing phase starts: PI,6. During this phase all the laminate plies work as springs in parallel and go on storing strain energy. After this phase next drops, PII,6 and PII,7, mixed with a slowly increasing part of f, PI,7, and two constant trends for f, PIII,1 and PIII,2, prelude the final crisis of the laminate. Considering the diagram in figure 7C of the AP laminate it can be observed that the structure of the sentry function is simpler if compared to the UD case. In fact only three subdomains of strain energy storing phases, PI,1, PI,2 and PII,6, and three drops, PI,1, PI,2 and PI,3, characterize the structure of the sentry function for the AP laminate reported in figure 10C. The smooth trend of the sub-function of type I reveal the modest capability of the material to store the strain energy and this fact is due to the high matrix volume percentage in each ply and to the fibre orientation. The initial trend of the sentry function of QI1 is characterized by a first material crisis, PIV,1, followed by a drop, PII,1. Such behaviour is due to an initial material adjustment and to some inter-laminar cracks onset that will contribute to delamination process. The next phase is characterized by two strain energy storing phases, PI,1 and PI,2, respectively followed by a sudden drop, PI,2, and a decreasing sub-function, PIV,2. In particular the sudden drop and the decreasing sub-function indicate the onset of the material crisis due to delamination and transversal cracks. After the sub-function PIV,2 the sentry function is characterized by a complex combination of sudden drops and constant sub-functions. This behaviour indicates that the material integrity is compromised and that each ply is progressively damaged until the final crisis. In this way it is interesting to notice that after the PIV,2 there are four sudden drops indicating the important crisis of each basic ply type (0°, +45°, -45°, 90°) that constitutes the original laminate QI1. In the case of QI2 the sentry function has a different trend with respect to the case of QI1. In fact at the test beginning damage is not appreciable and a sub-function of type I, PI,1, indicates a strain energy storing phase. After the first drop, PII,1, a consistent material damage indicates the reduced strain energy storing capability. Such material damage is mainly due to a global laminate loss of strength: the absence of delamination contributes to the cracks distribution and growth inside all the deformed material volume and this creates the condition
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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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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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Page 154 and 155: 140 Heat Flow (W/g) Heat Flow (W/g)
Page 158 and 159: 144 Material Yuanxin Zhou, Hassan M
Page 160 and 161: 146 SEM Analysis Yuanxin Zhou, Hass
Page 164 and 165: 150 Governing Equation For the fibe
Page 166 and 167: 152 ⎧ UU = ⎨ ⎩ ⎧ UD = ⎨
Page 170 and 171: 156 Stress (MPa) 120 80 40 0 Yuanxi
Page 180 and 181: 166 Giangiacomo Minak and Andrea Zu
Page 184 and 185: 170 ⎟ ⎞ ⎠ s ⎜ ⎛ ⎝ = a E
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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Research Directions in the Fatigue
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Research Directions in the Fatigue
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Damage Variables in Impact Testing
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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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