Carbon Fiber Reinforced Plastic Cross-ply Laminates Research Articles

Detection of a single transverse crack in a CFRP cross-ply laminate by visualizing mode conversion of Lamb waves

Detection of transverse crack (TC) is important in establishing reliable structural health monitoring (SHM) and nondestructive inspection (NDI) for warning the potential occurrence of large-scale damage in carbon fiber reinforced plastic (CFRP) structures at an earlier stage. In this research, a diagnostic method that is sensitive to a single TC was established by visualizing laser-induced guided ultrasonic waves in a cross-ply CFRP laminate. First, the phenomenon of TC-induced mode conversion in the laminate was clarified using finite element analysis (FEA) of harmonic oscillations. This FEA method is explicit and precise for investigating the conversion behavior of individual modes in a cracked plate. Then, the conversion phenomenon was also validated by another FEA-based pulse laser ultrasonic simulation. Finally, to observe the mode conversion phenomenon in practice, we conducted an experiment with a laser ultrasonic visualizing inspector (LUVI). The LUVI system could visualize the wave propagation in a two-dimensional (2D) wavefield that was formed on the surface of the CFRP cross-ply laminate with a single TC. By observing the image results of wave propagation, we verified that the wave component of the A0 mode was converted from the S0 mode at a TC even though the converted A0 mode had a much smaller amplitude than the incident S0 mode in the cross-ply CFRP laminate. Thus, those visualization results could be used to detect a single TC in the CFRP cross-ply laminate reliably.

Characterization of piercing damage in CFRP cross-ply laminates after punch shear machining via impact loading

In industrial processes, piercing and trimming are essential because composite structures are usually manufactured in a near-net shape to reduce machining operations. Punching and shear cutting using out-of-plane shear loading are expected to increase productivity. Nevertheless, little is known about the effects of such operations on polymer-matrix composites. This study presents on the characterization of piercing damage in typical carbon fiber reinforced plastic (CFRP) cross-ply laminates [0°2/90°2]s after punching using quasi-static (QS) and drop-weight impact (DWI) loadings. During QS punching, the upper and lower ply interfaces delaminate due to the high shear stress to cut fibers and gradual shear deformation in the middle ply; however, during DWI punching at a low impact velocity, delamination of the lower ply interface can be reduced due to the localization of shear deformation, as compared to that in QS punching. Finally, the damage accumulation process during DWI punching is discussed.

Gas permeability of CFRP cross-ply laminates with thin-ply barrier layers under cryogenic and biaxial loading conditions

Utilization of carbon fiber reinforced plastic (CFRP) composite materials in propellant tanks is expected to drastically reduce the structural weight of space vehicles. However, fuel leakage through the chain of matrix cracks caused by serious thermal residual stress is a considerable problem. To avoid fuel leakage, this study proposes leak barrier layers consisting of thin-ply prepregs. Biaxial tensile tests are performed for specimens with or without thin-ply barrier layers at cryogenic temperature, and the gas leakage properties of damaged CFRP cross-ply laminates are investigated. In addition, the formation of leakage paths at cryogenic temperature is simulated by the finite element method based on the experimental data. Finally, the experimental and numerical results demonstrate that thin-ply barrier layers increase the tolerance against damage and leakage.

Experimental study on shear-dominant fiber failure in CFRP laminates by out-of-plane shear loading

Understanding the shear behavior and resulting fiber failure of fiber-reinforced plastics is required for better prediction of their behavior during the machining process, but knowledge regarding the shear strength of fiber failure is limited. In this study, out-of-plane shear tests were conducted to observe the shear behavior of carbon fiber-reinforced plastic laminates subjected to high shear stress exceeding the shear strength of matrix failure. The longitudinal fibers in carbon fiber-reinforced plastic unidirectional laminates were cut by shear loading without severe internal damage and the maximum shear stress causing progressive fiber breaks was much higher than the shear strength of matrix failure. This result suggested the possibility of out-of-plane shearing as a machining method for fiber-reinforced plastics and shear tests were subsequently performed for carbon fiber-reinforced plastic cross-ply laminates. Delamination was generated by high shear stress to cut the reinforcing fibers, but the size of the remaining damage was small even in the thermoset carbon fiber-reinforced plastic laminates in which delamination likely occurs, without any optimization of the trimming conditions.

Identification of impact force acting on composite laminated plates using the radiated sound measured with microphones

Foreign object impact events are serious threats to composite laminates because impact damage leads to significant degradation of the mechanical properties of the structure. Identification of the location and force history of the impact that was applied to the structure can provide useful information for assessing the structural integrity. This study proposes a method for identifying impact forces acting on CFRP (carbon fiber reinforced plastic) laminated plates on the basis of the sound radiated from the impacted structure. Identification of the impact location and force history is performed using the sound pressure measured with microphones. To devise a method for identifying the impact location from the difference in the arrival times of the sound wave detected with the microphones, the propagation path of the sound wave from the impacted point to the sensor is examined. For the identification of the force history, an experimentally constructed transfer matrix is employed to relate the force history to the corresponding sound pressure. To verify the validity of the proposed method, impact tests are conducted by using a CFRP cross-ply laminate as the specimen, and an impulse hammer as the impactor. The experimental results confirm the validity of the present method for identifying the impact location from the arrival time of the sound wave detected with the microphones. Moreover, the results of force history identification show the feasibility of identifying the force history accurately from the measured sound pressure using the experimental transfer matrix.

A novel method of identifying damage types in carbon fiber-reinforced plastic cross-ply laminates based on acoustic emission detection using a fiber-optic sensor

We applied a highly sensitive phase-shifted fiber-optic Bragg grating (PS-FBG) sensor with a broad bandwidth to acoustic emission (AE) detection in a composite material. Based on AE detection, this research proposed a novel method to identify damage types in carbon fiber-reinforced plastic (CFRP) laminates. Because the PS-FBG sensor measured pure dynamic strain, the detected AE signals had great physical reliability, which made elastic wave theory suitable for analysis of the signal. The analysis clarified the different characteristics of Lamb wave modes in three types of AEs caused by a transverse crack, delamination and fiber breakage. Additionally, the ratios of the amplitudes of the S0 mode to the A0 mode and the peak frequencies quantitatively evaluated the mode characteristics in the AE signals. Simultaneously using the two physical parameters, we identified the three types of damage among the AE signals detected in a three-point bending test. Furthermore, the finite element method-based AE wave propagation simulation agreed well with the identification results. Hence, the proposed identification method with the PS-FBG sensor has great physical reliability for evaluating damage in composite laminates.

The identification of damage types in carbon fiber–reinforced plastic cross-ply laminates using a novel fiber-optic acoustic emission sensor

In this study, a phase-shifted fiber Bragg grating sensor was employed for acoustic emission detection in cross-ply carbon fiber–reinforced plastic laminates. First, we evaluated the response of the phase-shifted fiber Bragg grating sensor to a simulated acoustic emission source using a pencil lead breakage test. The existing modes of Lamb waves were observed, and their relations to the acoustic emission source orientation were investigated using a modal acoustic emission method for discriminating between acoustic emission signals generated by transverse cracking and delamination. As a result, those two types of damage were identified during a tensile test. In the time–frequency analysis of the waveforms detected by the phase-shifted fiber Bragg grating sensor, in addition to the S0 and A0 modes that are usually used in modal acoustic emissions, we found an A1 mode in the high-frequency range of the acoustic emission generated by a transverse crack. Then, we conducted a finite element analysis to examine the inherent mechanism of the A1 mode excitation. The results of the calculation almost agreed with the experimental results and indicated that a phase-shifted fiber Bragg grating sensor has a high sensitivity response to acoustic emission in the carbon fiber–reinforced plastic laminates over a broad bandwidth. Therefore, this novel fiber-optic sensor has been shown to have high potential for monitoring the ultrasonic structural health of composites.

Empirical models for matrix cracking in a CFRP cross-ply laminate under static- and cyclic-fatigue loadings

This paper presents empirical models for predicting matrix crack density in a carbon fiber reinforced plastic (CFRP) cross-ply laminate under static-fatigue and cyclic-fatigue loadings. First, a modified slow crack growth (SCG) law, that covers the whole range of stress ratio R of tension-tension fatigue (0 ≤ R ≤ 1), was proposed. The modified SCG law and three conventional SCG laws were then combined with Weibull's probabilistic failure concept for predicting fatigue matrix crack density in a cross-ply laminate. Matrix crack density was expressed as a function of R, the maximum stress in the transverse ply and the number of cycles. Next, fatigue tests were performed for R of 0, 0.2, 0.4, 0.6, and 1 to determine the applicability of these four models. Finally, constant fatigue life (CFL) diagrams were investigated based on the modified model. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers

High-Velocity Impact Damage in CFRP Unidirectional and Cross-Ply Laminates

This paper aims at characterizing the high-velocity impact damage behavior in carbon fiber reinforced plastic (CFRP) unidirectional (UD) and cross-ply (CP) laminates. First, the surface and internal damages of CFRP plates impacted at a velocity of 200 and 430m/s were observed by using optical microscopy together with radiography. Next, dynamic finite element analysis was performed to simulate the damage process. Cohesive elements were introduced to express the delamination and splitting cracks while the maximum stress fracture criteria were employed to express the intralaminar failure. Finally, the simulations were compared with the experiment results to verify the reasonability of the analysis. The damage process in both laminates was as follows : 1. A crater accompanied by splitting cracks is generated at the impact point on the front surface while splitting cracks due to bending are observed on the back surface. 2. Cone-shaped matrix cracking is observed in the UD laminate while few matrix cracking is generated in the CP laminate. 3. The shape of the delamination in the UD and CP laminates is a galaxy and a circle, respectively. Delamination size is larger in the CP laminate than in the UD laminate, which results in higher energy absorption capability in the CP laminate. Simulations are in qualitatively good agreement with the experiment results.

Fracture behavior in CFRP cross-ply laminates with initially cut fibers

This study qualitatively investigates the effects of initially cut fibers (slits) on fracture behavior in carbon fiber reinforced plastic (CFRP) cross-ply laminates, which had alternate or identical slit angle ± θ in the 0° plies. Damage progress during tensile tests was observed for several geometries of cutting. We also numerically evaluated fracture behavior in laminates with slits by a layer-wise finite-element model with cohesive elements. The simulated damage patterns included matrix cracks along the slits, splits in the 0° layer from the slit tips, and transverse cracks in the 90° layer. Delamination was also generated at the crossing point of ply cracks due to the large shear stress, and then extended to form the triangular region bounded by the slits and splits. The predicted damage extension to the final failure agreed with the observations. A numerical study demonstrated that the damage near the slits produced a stress field similar to that of a penetrating notch.

1516 渦電流探傷法によるCFRP積層板の層間はく離検出(S10-3 非破壊評価とモニタリングIII,21世紀地球環境革命の機械工学:人・マイクロナノ・エネルギー・環境)

Carbon fiber reinforced plastic (CFRP) laminates are very sensitive to an impact which causes delaminations in the laminate. Since delamination reduces the mechanical property of the laminate, nondestructive inspection is required to maintain the structural reliability of the CFRP structure. Eddy current test (ECT) has been studied to be applied to CFRP laminate. Strong electric anisotropy, however, makes application of the method difficult. Since delamination is in-plane damage in the laminate, conventional ECT has a problem for making current-loop which should be impeded by the damage to be detected. In the present paper, rectangular twin probe for electrically anisotropic CFRP laminate is introduced to resolve the problems. The probe is made of two rectangular coils put side-by-side to induce strong uniform electric current. Delamination was made in CFRP cross-ply laminates by means of the indentation test; the applicability of the ECT with rectangular twin probe is examined experimentally.

OS17-3-4 Detection of damage in CFRP laminates using the eddy current method

Carbon fiber reinforced plastic (CFRP) laminates, used at the beginning in aerospace structure, are now widely used in various fields. It is, however, very sensitive to an impact which causes delaminations in the laminate. Since the delamination reduces the mechanical property of the laminate with difficulty of visual inspection, nondestructive inspection (NDI) is required to maintain the structural reliability of the CFRP structure. Eddy current method, which is often used for metallic materials, can be applied to CFRP laminate due to conductivity of carbon fibers. The advantages of the method are short scanning time, low cost and portability. The method has been studied to be applied to the CFRP laminate. Strong electric anisotropy and inhomogenity of the laminates, however, makes simple application of the method difficult. Since delamination is in-plane damage in the thin laminate, conventional eddy current method has problem for making current-loop which should be disturbed by the damage to be detected. In this paper, eddy current method with conventional probe was firstly applied to CFRP fabric and cross-ply laminate to clear the problem of the method. For the fabric laminate, the damage location was successfully detected. For the cross-ply laminates, it was difficult to detect the location. The large scatter of response signal was observed due to the strong anisotropy of cross-ply laminate. To resolve the problem, a new probe for electrically anisotropic CFRP laminate is introduced. The new probe is made of two coils put side-by-side and induces uniform electric current that is stronger than the electric current induced by a conventional probe. It is applied to the cross-ply laminate. The new probe is arranged perpendicular to the scanning direction and move straight along specimens with fiber breakages and a delamination; the applicability of the new probe is shown experimentally.

Tensile strength of CFRP cross-ply laminates containing transverse cracks

The effect of transverse cracks on the ultimate tensile strength (UTS) was experimentally and analytically studied for carbon fiber reinforced plastic (CFRP) cross-ply laminates. For coupon specimens of various cross-ply stacking sequences, quasi-static tensile tests were carried out and transverse cracks were observed. The experimental results showed that the measured fiber bundle strength of 0° ply was almost constant, which indicated that transverse cracks had little influence on the notch sensibility of the bundle strength. Then, we proposed a new numerical model based on the finite element method to investigate the damage extension to the fracture. This model considered the elastoplastic behavior of epoxy matrix and the fiber breakages. Using this model, we applied a Monte-Carlo method to the damage extension simulation. It was found that the plastic region at the tip of the transverse crack reduced the stress concentration due to the crack, and that the bundle strength was barely affected by transverse cracks.

Gas Leakage Evaluation of CFRP Cross-ply Laminates under Biaxial Loadings

In this study, leak characteristics of CFRP (carbon fiber reinforced plastic) cross-ply laminates are experimentally investigated under biaxial loadings using a specially prepared in-plane biaxial testing system. Permeability through the damaged laminate under biaxial stresses was measured with a leak detection system. In order to evaluate the effect of damage on leakage, ultrasonic C-scan was used to inspect the matrix cracking in the specimens. Experimental results reveal that leakage through the damaged laminate is in correlation with the amount of damage and depends not only on load level but also on biaxial load ratio. Leak analysis, which had been previously developed, was employed and calculations of permeability were in good agreement with the experimental data. The results indicate the validity of the assumption of domination of crack openings on permeability, and the dependency of constants of leak conductance on damage susceptibility in the leak analysis.

Modeling of thermo-mechanical behavior of Ti-Ni shape memory alloy foils embedded in carbon fiber reinforced plastic laminates

Ti-Ni shape memory alloy (SMA) foils are embedded into carbon fiber reinforced plastic (CFRP) cross-ply laminates in order to suppress the occurrence and progress of the transverse cracks in the laminates. When the pre-strained SMA foil is heated, the shape memory effect produces appropriate recovery compressive stress in the direction to suppress transverse cracks. However, Ti-Ni SMA foil has these specific characteristics due to the existence of a rhombohedral phase (R-phase). Thus, in this research, in order to use this SMA foil as an actuator, an extended Brinson model was proposed to clarify complicated behaviors of SMA. Then, for the determination of the parameters in the constitutive equation in this model, experimental tests were performed for the SMA foils. Using this model, several simulations were conducted and compared with experimental results. The agreement indicated that this model could express the thermo-mechanical behavior of the Ti-Ni SMA foil adequately. Finally, the behavior of the SMA foil during the CFRP manufacturing process was calculated on the basis of this model. The calculation results proved that the recovery stress already existed after the manufacturing process and the embedment of 4% pre-strained SMA into CFRP cross-ply laminate had the effect of suppressing the occurrence of transverse cracks.

Evaluation of the damage suppression effect of Ti-Ni shape memory alloy foils embedded in carbon fiber reinforced plastic laminates

In this research, shape memory alloy (SMA) foils were embedded into carbon fiber reinforced plastic (CFRP) cross-ply laminates. When the tensile pre-strained SMA foil is heated, the SMA produces appropriate compressive stress (recovery stress) to suppress the transverse cracks in the laminates. In order to investigate the damage suppression effect, tensile loading–unloading tests were conducted for the CFRP cross-ply laminates. The test results implied that the recovery stress of embedded SMA foil suppressed the occurrence and the progress of the transverse crack. Then, to confirm that the damage suppression effect was caused by the embedded SMA foil, the one-dimensional (1D) shear-lag model considering the behavior of SMA was newly derived. Using this model, the progress of transverse crack density was predicted probabilistically. The simulation results showed the same tendency as the experimental results. As a result, it was proved that the embedment of SMA foils into CFRP laminates is effective to suppress the occurrence and growth of the transverse cracks.

Application of chirped fiber Bragg grating sensors for identification of crack locations in composites

Chirped fiber Bragg grating (FBG) sensors were applied for the identification of crack locations in carbon fiber reinforced plastic (CFRP) laminates since the reflection spectrum from a chirped FBG was expressed as a function of the position along the grating. For the crack identification in the 90° ply of a cross-ply laminate, a chirped FBG sensor, whose grating length and spectrum width were 50 mm and about 5 nm respectively, was embedded into the 0° ply of the CFRP cross-ply laminate, and the reflection spectra were measured after tensile loadings were applied to the laminate. As a result, the spectra had dips corresponding to locations of transverse cracks. This change in the form of the spectrum was also confirmed by a theoretical calculation. Hence, the crack locations could be identified from the form of the spectrum. Furthermore, it was demonstrated that a small-diameter chirped FBG sensor could also be applied for the identification of the crack locations.