Quasi-isotropic Carbon Fiber Reinforced Plastic Laminates Research Articles

Evaluation of matrix crack growth in interlaminar toughened quasi-isotropic carbon-fiber reinforced plastic laminates up to the very-high cycle regime by ultrasonic fatigue testing

It is time-consuming to conduct conventional hydraulic fatigue testing up to the very-high cycle fatigue (VHCF, N (number of cycles) ≥ 108) regime. Ultrasonic fatigue testing has been proposed as an accelerated alternative method. In this study, ultrasonic fatigue testing was conducted on interlaminar toughened quasi-isotropic carbon-fiber reinforced plastic (CFRP) laminates to evaluate the characteristics and mechanism of fatigue up to the VHCF regime. Hydraulic fatigue tests were conducted at the same stress ratio as a comparison. The resulting S–N diagrams showed a lower fatigue strength and different fatigue mechanisms for specimens subjected to ultrasonic fatigue tests compared with those subjected to hydraulic fatigue tests. The observed growth of the matrix crack density in each laminate layer was used to perform a shear-lag analysis to calculate the energy release rate considering off-axis matrix cracks. The results suggested different damage growth behaviors for specimens subjected to ultrasonic and conventional fatigue tests.

Effect of span/thickness ratio on fatigue properties of thick quasi-isotropic CFRP laminates subjected to three-point bending loading

In this study, three-point bending fatigue tests were conducted to investigate damage growth behavior of thick quasi-isotropic carbon fiber reinforced plastic (CFRP) laminates. The span/thickness ratio L/h was varied for both static tests and fatigue tests to determine failure mechanisms. In the static tests, the failure mode changed from delamination to compressive buckling as span/thickness ratio increases. The apparent interlaminar shear strength was higher for the specimens with lower span/thickness ratios, and the shear failure was likely to be caused by oblique crack in 90° plies. In the fatigue tests, the specimens with span/thickness ratio L/h=7 caused delamination regardless of stress level. For the specimens with span/thickness ratio L/h =15, three types of failure modes were obtained depending on stress level; delamination near the neutral plane, buckling at compressive area and matrix crack at tensile area. During the fatigue tests, stiffness of specimens with different span/thickness ratios which fail in shear was compared using digital image correlation (DIC) method. The specimens with span/thickness ratio 4 suddenly decreased in stiffness without increasing normal strain over cycles. Both specimens with span/thickness ratio 4 and 11 finally failed by delamination but they showed different failure process in how normal or shear strain developed.

Inverse estimation method for internal defects based on surface stress of carbon-fiber-reinforced plastics using machine learning

Carbon-fiber-reinforced plastic (CFRP) is a composite material whose base material is plastic and reinforcement material is carbon fibers. CFRP is widely used in various fields for laminating prepregs. The laminated plate tends to sustain damage, such as delamination, fiber breakage, and base material breakage; hence, we must conduct high-precision and efficient nondestructive testing (NDT). Examples of NDT are ultrasonic examination, X-ray tomography, and infrared stress analysis. With most NDT methods, it is difficult to easily obtain detailed correct information of defects, such as their depth, position, and size. To solve this problem, we develop a machine-learning-aided inverse analysis model that predicts the spatial information of defects from the sum of the principal stresses on the surface calculated from the temperature change measured by infrared analysis, and we propose it as an alternative method to the existing damage analysis. Applying the proposed method to the simulated stress distributions of quasi-isotropic CFRP laminates with defects, the results showed over 99% success to recognize the detail information of defects. Additionally, we examine the properties of the dataset using a forward analysis model and a variational autoencoder. Our method with a convolutional neural network enables us to successfully estimate the information of defects at high speed. Experimental data can be applicable as well as the simulation results to our proposed method, and we believe our method will be a powerful supporting tool for the current NDT for CFRPs.

An experimental study on impact behavior of quasi-isotropic CFRP laminates

Abstract This work was carried out to bring out the differences in drop weight low velocity impact behavior of two quasi-isotropic (QI) laminates. These laminates were made of AS4/914 carbon fiber reinforced plastic (CFRP). The layup sequence chosen was [0/45/-45/90]2S and [0//90/45/-45]2S. Two types of impactors were employed for the impact tests. One is a hemispherical-end and the other is of a conical-end. Coupon level specimens were supported on all four sides during the impact test. The impact energy was calculated for the laminates and the difference, as a comparison, in each case is presented. It has been observed that the layup sequence has a significant effect on the impact resistance of the laminates under hemispherical impactor. In the case of conical impactor, both the laminates were perforated while absorbing similar energy levels.

Experimental observation and FEM analysis of initial fractures in quasi-isotropic CFRP laminates subjected to compressive loading

In this study, the mechanisms governing compressive strength in quasi-isotropic carbon-fiber reinforced-plastic laminates were clarified to gain an understanding of the results of non-hole compression tests. Synchronized observations of both sides of specimens provided insights into the locations of initial fractures in the width directions of the specimens. We also observed the areas surrounding the initial fractures in detail and found that the innermost 0° plies failed under the action of out-of-plane shear deformation. A finite element model was also used to elucidate the factors governing initial fracture. Analytical and experimental results suggest that the occurrence of initial fracture in quasi-isotropic laminates is dominated by out-of-plane shear stress. Based on the results of our analysis, a reasonable sequence of fractures was inferred.

Compressive fracture aspect of thick quasi-isotropic carbon fiber reinforced plastic laminates

The compressive fracture aspect for the thick quasi-isotropic carbon fiber reinforced plastic (CFRP) laminates was investigated using a high-speed camera to clarify the progress of the brooming fracture. The compressive fracture types of the laminates could be classified into two groups, i.e., the fractures initiating from the innermost 0∘ ply and those initiating from the fixed sections of the specimen. The standard deviation of the compressive strength for the innermost 0∘ ply-initiated fracture type was larger than that for the fixed-section fracture type, although the average compressive strengths for each fracture mode were similar. The initial fracture in the innermost 0∘ ply was caused by the microbuckling of the fibers. The numerical results demonstrated a considerable out-of-plane shear stress around the free edges of the specimen, especially for the innermost 0∘ ply. Such a shear stress is thought to cause the microbuckling at the innermost 0∘ ply, with the crack subsequently propagating toward the outer plies of the specimen. The compressive strength of the innermost 0∘ ply was an important factor affecting the fracture behavior of the thick quasi-isotropic CFRP laminates.

Lightweight and anti-corrosive fiber reinforced thermoplastic rivet

A lightweight anti-corrosive rivet is proposed using a fiber reinforced thermoplastic (FRTP) for the joining of carbon fiber reinforced plastic (CFRP). The FRTP rivet was fabricated via the braiding technique, where Glass fiber/PA66 commingled yarns were wrapped around straightly aligned Carbon fiber/PA66 commingled yarns. Using a pultrusion method, the braided yarn was consolidated into a FRTP rod, which was then cut to an appropriate length for use as a FRTP rivet. Quasi-isotropic CFRP laminates were joined by this rivet through a thermal forming process. Single-lap joint specimens were subjected to tensile tests, and compared with a bolted and blind-riveted joint. The joint strength increased with increasing fiber volume fraction of the FRTP rivet, which had a higher specific joint strength than the other metallic joints. The results revealed that, for the same joint strength, FRTP-rivet replacement of the metallic joint yields a reduction in the weight of the joint.

Nondestructive Evaluation of Porosity Content in the Curved Corner Section of Composite Laminates Using Focused Ultrasonic Waves

The feasibility of utilizing focused ultrasonic waves for the nondestructive evaluation of porosity content in curved corner sections of carbon fiber reinforced plastic (CFRP) laminate structures is investigated numerically as well as experimentally. For this purpose, two-dimensional (2D) finite element simulations are carried out to clarify the wave propagation behavior and the reflection characteristics when the nonfocused or focused ultrasonic wave impinges on the corner section of unidirectional and quasi-isotropic CFRP laminates from the inner side via water. The corresponding reflection measurements are carried out for the CFRP corner specimens in the pulse-echo mode using nonfocusing, point-focusing, and line-focusing transducers. The numerical simulations and the experiments show that the use of focused ultrasonic waves is effective in obtaining clearly distinguishable surface and bottom echoes from the curved corner section of CFRP laminates. The influence of the porosity content on the reflection waveforms obtained with different types of transducers is demonstrated experimentally. The experimental results indicate that the porosity content of the CFRP corner section can be evaluated based on the amplitude ratio of the surface and bottom echoes obtained with focusing transducers, if the calibration relation is appropriately established for different ply stacking sequences.

Fully-Modeled Unit Cell Analysis for Macro/Micro Elastic-Viscoplastic Behavior of Quasi-Isotropic CFRP Laminates

A fully-modeled unit cell analysis is performed to investigate the macroscopic and microscopic elastic-viscoplastic behaviors of a quasi-isotropic carbon fiber-reinforced plastic (CFRP) laminate. To this end, a quasi-isotropic CFRP laminate and its microstructure composed of carbon fibers and a matrix material are considered three-dimensionally. Then, a hexagonal prism-shaped unit cell fully modeled with fibers and a matrix including interlaminar areas is defined. For this quasi-isotropic laminate, a homogenization theory for nonlinear time-dependent composites with point-symmetric internal structures is applied, enabling us to analyze both the macroscopic and microscopic elastic-viscoplastic behaviors of the laminate. The substructure method is introduced into the theory to reduce computational costs. The present method is then applied to the elastic-viscoplastic analysis of a quasi-isotropic carbon fiber/epoxy laminate subjected to an in-plane uniaxial tensile load, to investigate the macroscopic elastic-viscoplastic behavior of the laminate and the microscopic stress and strain distributions in them.

1809 引張荷重を受ける擬似等方積層CFRP単純重ね合せ接着継手の応力解析と強度(G04-2 複合材とサンドイッチ材,G04 材料力学部門)

Recently, carbon fiber reinforced plastic (CFRP) material has been used in structural element of transport vehicles. Besides, adhesive bonding is known as better method for joining the CFRP structures The purpose of our work was to examine the characteristics of strength for CFRP composite structures. We modeled single-lap adhesive joints composed of quasi-isotropic CFRP laminates having various lay-up pattern subjected to static tensile load to carry out finite element method analysis, and compared the analysis results of stress distribution at adhesive layer. In addition, failure load of various adhesive joint specimens were measured. As a result, it was found that the stress distributions on adhesive layer and failure load for joints were affected by each lay-up patterns of CFRP adherends.

High-cycle fatigue characteristics of quasi-isotropic CFRP laminates over 10 8 cycles (Initiation and propagation of delamination considering interaction with transverse cracks)

High-cycle fatigue features of over 10 8 cycles, particularly the initiation and propagation of edge delamination considering the effects of transverse cracks, were investigated using quasi-isotropic carbon-fiber-reinforced plastic (CFRP) laminates with a stacking sequence of [45/0/−45/90] s in this study. In the relationship between a transverse crack density and initiation and growth of edge delamination, it was found that fatigue damage growth behavior varied depending on applied stress. It was observed that edge delamination initiated and grew at parts where transverse cracks were dense at ordinary applied stress, whereas it was observed that edge delamination grew before or simultaneously with transverse crack propagation at a low applied stress and high-cycle loading. In addition, the critical transverse crack density where delamination begins growing was calculated to evaluate the interaction between transverse crack and edge delamination growth.

Transverse crack growth behavior considering free-edge effect in quasi-isotropic CFRP laminates under high-cycle fatigue loading

The high-cycle fatigue characteristics focused on the behavior of the transverse crack growth up to 10 8 cycles were investigated using quasi-isotropic carbon fiber reinforced plastic (CFRP) laminates whose stacking sequence was [−45/0/45/90] s . To assess the fatigue behavior in the high-cycle region, fatigue tests were conducted at a frequency of 100 Hz in addition to 5 Hz. In this study, to evaluate quantitative characteristics of the transverse crack growth in the high-cycle region, the energy release rate considering the free-edge effect was calculated. Transverse crack growth behavior was evaluated based on a modified Paris law approach. The results revealed that transverse crack growth was delayed under the test conditions of the applied stress level of σ max/ σ b = 0.2.

1319 ひずみ検知型電位差法による擬似等方CFRP積層板はく離同定のための数値解析(J14-4 知的材料・構造システム(4) 計測・モニタリングIII,ジョイントセッション,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)

Since carbon fiber reinforced plastic (CFRP) laminate has superior specific strength and stiffness as compared with metals, it has been increasingly applying to aerospace structures. It is, however, sensitive to an impact which causes a delamination in the laminate. The delamination strongly reduces the mechanical property of the laminate with difficulty of visual inspection. Simple structural health monitoring method is required to maintain the structural reliability of CFRP structures. Many researchers have been reported strain-sensing system of CFRP in which piezoresistivity of CFRP laminate is utilized. It is, however, not reported the delamination sensing. The delamination releases the residual stress in the laminate which was caused in the fabrication process. The delamination may be detected by measuring electric potential changes due to the release of residual stress. In this paper, FEM analyses are performed to investigate the applicability of the method for delamination detection in the quasi-isotropic CFRP laminates.

High-cycle fatigue characteristics of quasi-isotropic CFRP laminates

High-cycle fatigue characteristics of quasi-isotropic carbon fiber reinforced plastic (CFRP) laminates [−45/0/45/90]s up to 108 cycles were investigated. To assess the fatigue behavior in the high-cycle region, fatigue tests were conducted at a frequency of 100 Hz, since it is difficult to investigate the fatigue characteristics in high-cycle at 5 Hz. Then, the damage behavior of the specimen was observed with a microscope, soft X-ray photography and a 3D ultrasonic inspection system. In this study, to evaluate quantitative characteristics of both transverse crack propagation and delamination growth in the high-cycle region, the energy release rate associated with damage growth in the width direction was calculated. Transverse crack propagation and delamination growth in the width direction were evaluated based on a modified Paris law approach. The results revealed that transverse crack propagation delayed under the test conditions of less than σ max/σ b = 0.3 of the applied stress level.