CFRP composites

Fig. 28.21. Modulus anisotropies for aligned GFRP and CFRP composites.

Figure 29. Comparison of the fatigue behavior of 3D carbon-matrix (CFRC) and epoxy-matrix (CFRP) composites (53).

Figure 1 Micrograph (optical microscopy) showing breaking of airbon fibers, metal fibers, and matrix on a CFRP composite subject to a 5J impact.

Due to their low electrical conductivity, high frequencies are required for eddy current examination of composites. For instance, a 1.0 mm thick CFRP composite requires a skin depth frequency varying from 12.6 to 50,6 MHz [104], Prakash estimates that eddy current... 

Another transducer resonance system is the Fokker Stack Tester, developed to examine ply orientation and stacking sequence in the different layers of CFRP composites. This computer-controlled system displays the signal output in a graphical format. It consists of passing a probe into a rivet or fastener hole and scanning the material surface by a focused laser beam. The intensity of the reflected light is correlated to the orientation of each ply and can provide information on the ply number, the thickness of each ply, and fiber orientation. 

Ata A, Soutis C. Subcritical damage mechanisms of bolted joints in CFRP composite. Composites Part B Engineering 2013 54(1) 20—27. 

Takeda N, Tohdoh M, Takahashi K. Interlaminar fracture toughness degradation of radiation-damaged GFRP and CFRP composites. Adv Compos Mater 1995 4 343-54. http //dx.doi.org/10.1163/156855195X00195. 

Cartie DDR, Irving PE. Effect of resin and fibre properties on impact and compression after impact performance of cfrp. Composites Part A 2002 33 483-93. 

Figure 16.1 Fiber optics essentials, (a) Total internal reflection principle (b) optical fiber embedded in a carbon fiber-reinforced polymer (CFRP) composite [2] (c) single-mode and multi-mode fibers [3].

The electrical SHM of composites refies on the material itself to act as the sensor. Carbon fibers are electrically conductive the epoxy resin is an insulator. The CFRP composite is somehow conductive because the densely packed carbon fibers may touch each other. As damage (e.g., cracks and delamination) takes place in the composite, the electric conductivity is expected to change. The glass fiber-reinforced polymer (GFRP) composite is a nonconductive insulator with certain dielectric properties. Damage in GFRP composites creates microcracks and even... 

Figure 16.8 Experimental frequency tuning results, (a) Maximum So response at around 300 kHz due to Aq rejection in a 1 mm aluminum plate (b) similar rejection of Aq mode in a CFRP composite plate.

Kang et al. [28] report a space-qualified FBG system that uses FBG sensors to monitor the strains in a filament-wound CFRP tank during pressure testing. Mizutani et al. [27] describes a space-qualified on-board FBG system used to monitor the strain on a CFRP composite LH2 tank installed on a reusable launch vehicle (RLV) test article. The FBG sensors were installed on the CFRP composite tank with UV-cured polyurethane adhesive that showed good performance at cryogenic temperatures. The system (which weighs less than 2 kg) was installed, flown, and tested on the RLV typical recorded data are shown in Figure 16.15. 

Figure 16.31 Hybrid PZT/FBG system for the detection of impact delaminalion damage in a CFRP composite plate [67].

Figure 16.32 Detection of impact delamination damage in a CFRP composite plate using a hybrid PZT/FBG system, (a) Reconstructed tomographic image (b) X-ray image [67].

Figure 16.37 Modehng of electromechanical impedance spectroscopy for delamination detection in CFRP composites, (a) Experimental setup (b) analytical model [18].

When a unidirectional CFRP composite specimen is stretched, its longitudinal resistance decreases. This phenomenon (which may be due to both a transverse Poisson effect and carbon fiber piezo-resistivity) is reversible and the resistance returns to its initial value as the load is released. However, if damage occurs, irreversible resistance changes... 

Figure 16.38 Variation of longitudinal electrical resistance during tension—tension fatigue testing of a unidirectional CFRP composite specimen. The cyclic change of resistance is due to reversible cyclic strain the permanent increased in resistance around cycle 218,281 may he due to permanent damage, e.g., some fiber fracture [89].

Iwasaki and Todoroki [92] use a response surface technique to process the electrical resistance measurements. A large number of cross-ply and quasi-isotropic specimens were tested such that statistical data processing could be applied. Copper-foil electrodes mounted on one side of the CFRP specimens during prepreg layup were co-cured with the specimen (Figure 16.39). Impact-induced matrix cracking and delaminations were detected. Probability of location estimation and error bands were computed [92]. The extension of this method to woven CFRP composites is described in Hirano and Todoroki [93]. 

Eddy currents (EC) techniques are well developed for damage detection in metallic stmctures. Recent studies have shown that they can be also used in CFRP composites [101]. CFRP permeability and conductivity influence the EC penetration depth. EC methods permit the determination of textural parameters like layer orientation and gap sizes between roving, the detection of foreign polymer particles fuzzy balls and visualization of undulations can be also done with this method. The detection of a 4.48° fiber misalignment was shown to be possible other textural features can be also automatically found and classified. This technique holds the promise of in situ sensing through MWM-array eddy current technology [102]. 

Okabe Y, Yashiro S, Kosaka T, Takeda N. Detection of transverse cracks in CFRP composites using embedded fiber Bragg grating sensors. Smart Mater Struct 2000 9(6) 832-8. http //dx.doi.Org/10.1088/0964-1726/9/6/313. 

Koplev A, Lystrup A, Vorm T (1983) The cutting process, chips and cutting forces in machining CFRP. Composites 14(4) 371-376... 

Today in construction, only approximately 15% of the potential strength of CFRP strips is used. One important reason for this poor efficiency is that many strengthening tasks are controlled by stiffness, not strength. The focus of future developments should therefore be on better exploitation of the potential offered by CFRP composites and on a change to thermoplastic... 

Apicella, F. and M. Imbrogno (1999). Fire performance of CFRP-composites used for repairing and strengthening concrete. Fifth ASCE Materials Engineering Conference, New York, pp. 260-266. 

Godbehere AP, Mills AR, Irving P. Non-ciimped fabrics versus pre-preg CFRP composites - a comparison of mechanical performance. In Proceedings 6th international conference on fibre reinforced composites, FRC 94. University of Newcastle upon Tyne, Institute of Materials ... 

Cantwell WJ, Morton J. Comparison of the low and high velocity impact response of CFRP. Composites 1989 20(6) 545-51. 

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