Unidirectional Fiber-reinforced Plastic Research Articles

Computational Analysis of the Micromechanical Stress Field in Undamaged and Damaged Unidirectional Fiber-Reinforced Plastics Using a Modified Principal Component Analysis

This study investigated the internal stress distribution of unidirectional fiber-reinforced plastics (UD-FRP) at the micro level using principal component analysis (PCA). The composite material was simulated using a representative volume element model together with the embedded cell approach. Two fundamental quasi-static load cases, transverse and longitudinal tensile deformation, were considered. The experimental results show that mechanical failure occurred at 2.15 ± 0.06% transverse tensile strain and at 1.52 ± 0.07% longitudinal tensile strain. Furthermore, the undamaged state and a combination of matrix and interface damage, as well as fiber breakage, were simulated. From the simulations, the octahedral shear stress and octahedral normal stress were computed at the integration points of the matrix elements, constituting what is known as the octahedral stress field. A modification on the PCA to obtain mesh-independent eigenvalues is presented and was used to investigate the effects of damage events on the octahedral stress field. The results indicate that each damage mechanism had a distinct signature in the redistribution of the stress field, characterized by specific changes in the eigenvalues and orientation of the principal component (θ1). Furthermore, the PCA suggests that the accumulation of matrix damage began to be relevant at the 1% strain, while fiber breakage began at an average longitudinal strain of 0.98 ± 0.12%. Additionally, it is shown that the first principal component served as an indicator of the predominant stress state of the stress field. This investigation suggests that the PCA can provide valuable insights regarding the complex damage mechanisms of UD-FRP that may not be captured by conventional mechanical analysis.

Interaction of multiple micro-defects on the strength and failure mechanism of UD composites by computational micromechanics

The mechanical properties of unidirectional fiber-reinforced plastic (UD-FRP) are affected by a variety of micro-defects, such as random fiber arrangement, fiber misalignment and micro-voids. This study aims to investigate how these multiple micro-defects interact with each other and how they affect the strength and failure mechanism of UD-FRP by means of computational micromechanics. The composite behavior was simulated by the finite element analysis of a representative volume element of the composite microstructure in which the random distribution of fibers, the micro-voids, and the fiber misalignment are explicitly included. Both matrix and interface failure were considered for the loadings of transverse tension/compression, longitudinal compression, transverse/longitudinal shear, and their combination. It was found that these three micro-defects significantly weakened the compressive strength of UD-FRP along the longitudinal direction. Especially, the fiber misalignment magnified the effect of fiber arrangement, while the micro-voids reduce the effect. Besides, the fiber arrangement and micro-voids significantly weakened the tensile and compressive strength of UD-FRP along the transverse direction, but their interaction effect was not obvious. Moreover, transverse and longitudinal shear strength are significantly affected by micro-voids, but only longitudinal shear is affected by geometric fiber arrangement, and this effect is also weakened by micro-voids. Finally, the damage envelope under the combined longitudinal compression and transverse loads was obtained and compared with the Tsai-Wu failure criterion. The results showed that the Tsai-Wu criteria can provide an effective estimation for the failure locus under this biaxial loading condition.

Failure analysis of unidirectional FRP with fiber clusters under transverse pure shear

In nearly all the failure criteria for the FRP (fiber reinforced plastic), the fundamental strength of the FRP under uniaxial tensile, compressive and pure shear stress were utilized to construct appropriate mathematical function relationship with the multiaxial stress. Among them, the strength of the FRP under transverse pure shear stress was influenced by various factors and could not be directly obtained from tests. In this paper, transverse pure shear failure behavior of FRP was studied using finite element analysis based on RVE models, the RSE and HC algorithm were utilized to generate the FRP with different fiber distribution, mainly refers the different fiber clusters which was commonly experimentally observed. The bilinear cohesive model and extended linear Drucker Prager model were used to characterize the mechanical behavior of the interface and matrix respectively. Then, constraint of periodic boundary condition was applied for the RVE models. Difference on failure fracture behavior of FRP with different fiber distribution were identified and assessed under transverse pure shear stress, while the connection between stress triaxiality and initial damage equivalent plastic strain of the matrix was also altered. On this basis, the distinction of the maximum transverse pure shear stress, the crack resistance strength and the transverse shear strength was thoroughly discussed and identified, which provided a theoretic reference for the failure analysis of the FRP.

Modelling on stress redistribution around broken fibers for fiber reinforced plastic with poor fiber distribution uniformity

AbstractIn order to calculate stress redistribution after a fiber breaking for unidirectional fiber reinforced plastic (FRP) with practically poor fiber distribution uniformity, a new and efficient FE model is developed with high computation efficiency. Based on the proposed FE model, the stress concentration factor (SCF) of intact fibers around a broken fiber is investigated for unidirectional FRP. It is observed that the SCF depends heavily on the local fiber volume ratio (FVR), and a modified analytical SCF model is proposed which takes into account the variation of local FVR. It is proved that the modified analytical SCF model can provide SCF values with much better accuracy for FRP with poor fiber distribution uniformity. The modified analytical SCF model would be very useful for accurate prediction of progressive fiber breaking in FRP.

Progressive failure characteristics of unidirectional FRP with fiber clustering

For practical fiber reinforced plastic (FRP) structures, fibers are randomly packed in matrix, and some degree of clustering is often observed because of imperfect control of manufacturing process. In this work, a new method to generate random fiber distribution including fiber clustering is proposed according to fiber clustering characteristics of typical micro-image of FRP cross section. Irregularity and nearest neighbor fiber characteristics of random fiber distribution with fiber clustering are analyzed. Two-dimensional geometrically non-linear finite element (FE) modeling on representative volume element (RVE) is conducted to investigate transversely progressive tensile and compressive failure behavior of FRP. Extended linear Drucker-Prager model and cohesive zone model are employed to represent matrix deformation and interfacial debonding. Ultimate strength, fracture strain and crack characteristics of FRP with different degree of fiber clustering are derived and compared. It is shown that fiber clustering tends to result in significant decline and uncertainty of fracture strain and ultimate strength. The characteristics of fiber clustering can lead to two crack modes which affects the macro-scale mechanical properties of FRP.

Transverse mechanical properties of unidirectional FRP including resin-rich areas

Fiber reinforced plastic (FRP) is known to possess advantages of high strength and low density, and it is extensively used in engineering structures. However, most FRP have a certain level of microstructural uncertainty, and resin-rich areas in FRP can exist as inter-lamina resin layers or intra-lamina resin pockets. This work proposes a new algorithm to generate non-uniform random fiber distribution which contains resin-rich areas. The transverse tensile and compressive mechanical behavior of unidirectional FRP including resin-rich areas are studied using numerical simulation. The representative volume element (RVE) of the FRP with resin-rich areas is constructed. Matrix plastic deformation and interfacial debonding are described by extended Drucker-Prager model and cohesive zone model. Detrimental effect of resin-rich areas on the mean values and uncertainty of transversely ultimate strength and fracture strain is quantified. A new observation is that the resin-rich areas would result in large uncertainty on the ultimate strength of FRP, while its influence on the mean values of ultimate strength and fracture strain is limited. The large uncertainty on the tensile strength is attributed to that different morphology of resin-rich areas would lead to different cracks. The size of the resin-rich areas is also found to have an effect on transverse tensile and compressive mechanical properties.

Numerical life prediction of unidirectional fiber composites under block loading conditions using a progressive fatigue damage model

Practical applied components made of Fiber-Reinforced Plastics (FRPs) are usually subjected to cyclic loading with variable load amplitudes and arbitrary load orientation in the course of their service life. For a cost-effective design process of composite structures, it is essential to use computationally efficient and detailed fatigue damage design tools. This contribution focuses on the extension and application of a progressive Finite-Element-based Fatigue Damage Model (FDM) for unidirectional FRPs for lifetime prediction under different block loading conditions. The employed FDM relies on a layer- and energy-based approach, and it is capable of including basic fatigue phenomena such as load sequence effects and stress redistributions. First, the damage evolution laws on which the FDM is based are extended to block loading conditions with arbitrary load sequences. The effects of passive damage, which occur in this context under alternating cyclical tensile and compressive loading, are also taken into account. Secondly, appropriate block loading patterns are defined for the numerical prediction of the lifetime. Finally, the calculation results are presented and critically evaluated based on experimental findings from literature. The comparison of simulations with experiments demonstrates the high predictive capacity of the presented FDM.

Coupling inverse fin method with infrared thermography to determine the effective thermal conductivity of extruded thermoplastic foams

An inverse method for determining the in-plane effective thermal conductivity of porous thermoplastics was implemented by coupling infrared thermography experiments and numerical simulation of heat transfer in straight fins having temperature-dependent convective heat transfer coefficient. The microstructure heterogeneity of extruded polyethylene foam, in which pores are filled with air with different levels of open and closed porosity, was taken into account. The obtained effective thermal conductivity values were compared with previous results obtained using a numerical solution based on periodic homogenization techniques (NSHT) and the transient plane source technique (TPS) to verify the accuracy of the proposed method. The results show that the suggested method is in good agreement with both NSHT and TPS. Moreover, it is also appropriate for structural materials such as unidirectional fiber-reinforced plastic composites, where heat transfer is very different according to the fiber direction (parallel or transverse to the fibers).

Determining the fiber/matrix interfacial shear strength under cryogenic conditions by statistical inversion

AbstractThe fiber/matrix interfacial shear strength (IFSS) in fiber‐reinforced plastics (FRP) under cryogenic condition is critical to the strength of FRP vessels while used in liquid fuel tanks. However, a direct in situ characterization of IFSS in FRPs under cryogenic condition is difficult to be achieved by combining mechanical testing facility and cryogenic oven at microscale. In this article, a new approach is proposed to identify the cryogenic IFSS of FRPs by statistical inversion by using the results of tensile experiments at cryogenic temperatures and morphological characterization of FRPs at room temperature. The failure of unidirectional FRPs is characterized by cryogenic mechanical testing at macroscale, and microscopic morphological analysis is conducted to summarize the statistical characterization of material failures. Based on the mechanical behavior and the statistical distribution of failure modes in the FRPs, statistical inversion technique is employed to estimate the cryogenic IFSS. This method can avoid the difficulties in conducting in situ cryogenic mechanical tests at microscale and achieve a reliable evaluation of cryogenic interfacial properties in FRPs, which improving the evaluation ability of the mechanical performance of FRPs under cryogenic conditions.

Mechanical Properties of Thermoplastic-Based Hybrid Laminates with Regard to Layer Structure and Metal Volume Content

Multi layered lightweight material compounds such as hybrid laminates are composed of different layers of materials like metals and unidirectional fibre-reinforced plastics and offer high specific strength. They can be individually tailored for applications like outer cover panels for aircraft and vehicles. Many characteristics especially layer structure, volume contents of the embedded materials as well as layer surface adhesion determine the performance of a hybrid laminate. In this study, the influence of layer structure and metal volume content are evaluated with regard to the mechanical properties of the recyclable hybrid laminate CAPAAL (carbon fibre-reinforced plastics/aluminium foil laminate), which consists of the aluminium alloy AA6082 and a graded structure of glass and carbon fibre-reinforced polyamide 6. Hybrid laminates with different ratios of the fibre-reinforced plastic and numbers of aluminium layers were manufactured by thermal pressing. The consolidation quality of in total four laminate structure variations, including 2/1 and 3/2 metal-to-fibre-reinforced plastic layer structures with fibre orientation variation, were investigated by light microscopy through cross-sections and further on computed tomography. For determination and evaluation of the mechanical properties metrologically instrumented quasi-static tensile and three-point bending tests, as well as tension-tension fatigue tests for the establishment of S-N curves were performed. The results were correlated to the microstructural observations, revealing significant influence by the consolidation quality. The layer structure proved to have a proportional impact on the increase of quasi-static tensile and flexure strength with a decrease in metal volume content. Orienting some of the fibre-reinforced plastic layers in ±45° leads to a more evenly distributed fibre alignment, which results in a higher consolidation quality and less anisotropic bending properties. Fatigue results showed a more complex behaviour where not only the metal volume content seems to determine the fatigue loading capability, but also the number of metal-fibre-reinforced plastic interfaces, hinting at the importance of stress distribution between layers and its longevity over fatigue life.

An algorithm for the generation of three-dimensional statistically Representative Volume Elements of unidirectional fibre-reinforced plastics: Focusing on the fibres waviness

An algorithm for the generation of periodic three-dimensional statistically Representative Volume Elements (RVEs) of unidirectional (UD) fibre-reinforced plastics (FRPs) is reported. Fibres are modelled as Bézier curves, which control points are moved in a semi-random fashion, in order to introduce waviness along the fibres. Spatial descriptors, able to statistically characterise the fibre waviness, are proposed, and their probability distributions are optimized for possible matching with experimental measurements. The developed procedure is able to generate the fibre geometry in an effective and fast way. This represents the first step in the generation of high-fidelity micromechanical models for UD FRPs.

Bridging micro-to-macro scale damage in UD-FRP laminates under tensile loading

A unified numerical framework for progressive damage analysis of a unidirectional fiber reinforced plastic (UD-FRP) laminate has been developed by explicitly accounting for damage progression at the micro-level. At the micro-level, a three dimensional repeating unit cell (3D-RUC) comprised of polymer matrix with fibers and interfaces has been modeled allowing for plasticity based matrix damage, maximum stress based fiber failure, and a cohesive traction-separation criterion for interface failure. Coupling of macro- and micro- levels has been accomplished using non-linear homogenization. In other words, the homogenized stress–strain curve obtained at the micro-level is utilized to predict the constitutive behavior at the macro-level. Typical material characterization tests have been conducted to estimate constituent behavior. Predictions from the current analysis are found to match well with the response of UD-FRP plates of different fiber orientations with a central hole under tensile loading.

Micro-scale progressive damage development in polymer composites under longitudinal loading

A detailed investigation of micro-scale damage development in uni-directional polymer composites under longitudinal loading (loading along the fiber-direction) has been presented in the current work. Three major damage modes have been considered, namely, matrix damage, fiber–matrix interface debonding and multiple fiber fragmentation. The development and progression of these damage modes and their effect on the overall stress–strain response of the uni-directional fiber reinforced plastic (UD-FRP) is of interest. From the study, it has been demonstrated that the dimensions of the three dimensional repeating unit cell (3D-RUC) needed to generate repeatable results are inherently dependent on the fiber volume fractions. The length of 3D-RUC is longer for a higher fiber volume fractions. In addition, the physics of fiber fragmentation is inherently linked to the proximity of other fibers triggering failure mechanics different from the single fiber fragmentation behavior. Stochastic fiber fragmentation characteristics have also been studied by quantifying the effect of Weibull parameters on overall response of the micro-structure.

Conventional Orthogonal Cutting Machining on Unidirectional Fibre Reinforced Plastics

The results of orthogonal cutting tests on unidirectional carbon and glass fibre reinforced plastics are presented. The specimens were under shape of rectangular plates, circular disks and cylinders with different fibre architectures and a milling machine, a lathe machine and a five-axis high-speed vertical machining centre, were used for the experimental tests. The cutting speed was varied. During the tests, performed at low cutting speed, avoiding thermal effects, and high speed, to investigate about the effect of the cutting velocity on the cut quality, the fibre orientation respect to the cutting direction, the tool rake angle and the depth of cut were varied to investigate their influence on the phenomenon. A high speed steel tool in different geometries, was used.The mechanisms of chip formation and the cutting quality were investigated. A tentative to correlate the mechanisms of chip formation and cutting forces signals was done. Since the anisotropy, the mechanisms of chip formation consists of different failure modes occurring simultaneously and their identification, on the basis of the cutting force evolution, is very complex. Only in particular conditions, the features of cutting forces allow a precise identification of the chip development and detachment.The results indicated that the fibre orientation respect to the cutting direction determines the mechanisms of chip formation and influences the cutting quality. It was noted that for fibre orientation higher than 60°, the quality of the surface was revealed unacceptable. These conclusions were obtained independently of the particular shape of specimen tested and of the speed adopted.

Micromechanical damage analysis in laminated composites with randomly distributed fibers

Damage mechanisms in uni-directional fiber-reinforced plastics have been studied at the micro-scale by developing three-dimensional-repeating unit cells with randomly distributed fibers. Three damage mechanisms have been considered, viz., matrix damage, fiber failure and fiber-matrix interface debonding. The development of these damage modes and their effect on the overall stress–strain response of the micro-structure due to varying fiber volume fractions, loading conditions (longitudinal, transverse and combined transverse tension and shear), spatial distribution of fibers and fiber strength distributions (deterministic and Weibull) are of interest. A numerical framework has been developed that allows for conducting such studies for the chosen parameters. Microscopic images of real micro-structure of uni-directional fiber-reinforced plastics have been used to develop the random micro-structure for the three-dimensional-repeating unit cell. In the longitudinal loading scenario, fibers with Weibull strength distribution are closer to reality. While the three-dimensional-repeating unit cell under longitudinal loading fails in predominantly fiber failure mode, debonding and matrix damage play a major role in determining the average response of the micro-structure under transverse and combined transverse and shear loading.

A fatigue damage meso-model for fiber-reinforced composites with stress ratio effect

This work presents a fatigue damage meso-model for fiber-reinforced plastic composites, in which the effect of stress ratios on the off-axis fatigue behavior is taken into account. The non-dimensional effective stress concept is introduced in the continuum damage mechanics method. Damage growths and fatigue failure are studied along axial, transverse and shear directions at meso-scale level. The proposed model is validated through numerical simulations that describe the meso fatigue damage accumulation and the fatigue life for off-axis unidirectional fiber-reinforced plastic composite laminates of arbitrary fiber orientation under different stress ratios. It is shown that the fatigue damage behavior and fatigue life for off-axis unidirectional glass/epoxy and carbon/epoxy composite laminates are adequately described by the proposed fatigue model over the range of different stress ratios.

Effects of fiber orientation angles of fiber-reinforced plastic on sand solid particle erosion behaviors

Solid particle erosion in industrial applications has been a serious problem in many engineering fields. Earlier studies on fiber-reinforced plastic (FRP) composites were mainly focusing on the erosive wear behavior at several different impact angles. However, the effect of fiber orientation on FRP composites has not been thoroughly investigated. Since fiber orientation is one of the important factors in which causing erosive wear damages to FRP composites, in order to understand the virtue of this problem, it is important to investigate the effect of fiber orientation at different impact angles. In this research, the effect of fiber orientation of unidirectional fiber-reinforced plastic composites on erosive wear behavior was studied. Sandblasting-type erosion tests were conducted on the FRP composites with fiber orientation ranging at three impact angles to clarify the relation between fiber orientation and erosive wear behavior. The Dyneema fiber (ductile material) and the carbon fiber (brittle material) were used for the reinforcement fiber in FRP. From the result, it is confirmed that CFRP composites with higher fiber orientation angle erode faster than the composites with lower fiber orientation angle. But the erosion characteristic of DFRP was almost the same regardless of the fiber orientation angle. The damaged surfaces of the FRP composites were then analyzed using scanning electron microscopy and the possible erosion wear mechanisms were investigated.

A continuum damage model to predict the influence of ply waviness on stiffness and strength in ultra-thick unidirectional Fiber-reinforced Plastics

Ply waviness is a commonly observed manufacturing defect of ultra-thick composite materials essentially affecting stiffness, strength, and fatigue behavior of the composite. A specimen's geometry is designed to represent the failure mechanisms in thick wavy laminates, typically observed in spar caps of today's wind turbine blades. A material model is developed to simulate the phenomenological processes in wavy laminates to obtain a strength knock-down. Particular attention is taken on the nonlinear shear behavior and kink band formation. The presented model correlates well with results achieved by experiments. This paper shows a significantly higher influence of compressive compared with tensile loading on the mechanical material behavior of wavy laminates.

Multiple-response optimization of turning machining by the taguchi method and the utility concept using uni-directional glass fiber-reinforced plastic composite and carbide (k10) cutting tool

This paper presents a utility concept for multi-response optimization in turning uni-directional glass fiber-reinforced plastics composite using Carbide (K10) cutting tool. The single response optimization resulted in the non-optimization of other responses. The Taguchi method (Orthogonal L18 array) was employed in the experimental work. The process parameters selected for this study were tool nose radius, tool rake angle, feed rate, cutting speed, depth of cut, and cutting environment. Statistically significant parameters were found to simultaneously minimize surface roughness and maximize the material removal rate by ANOVA. The results were further verified by confirmation experiments.

S133012 マルチワイヤソーによるガラス繊維強化プラスチックのスライシング面生成機構

Anisotropic materials represented by fiber reinforced plastics (FRP) or fiber reinforced metals (FRM) have been used in various fields because of their superior mechanical properties such as high specific strength. The machinability of these materials has been researched in the field of cutting for a long time. However, there is no report concerning the performance in loose abrasive machining. If the high precision machining of FRPs by the multi-wire saw becomes possible, the application to various fields is expected. The main purpose of this study is to clarify the suitable slicing conditions of FRPs by the multi-wire saw. In this report, unidirectional FRP is used as a work material, and the results through the slicing experiment by the multi-wire saw are described. The main conclusions obtained in this study are as follows. The sliced surface roughness deteriorates at the slicing angle of 30°. In slicing by multi-wire saw, which is the same as that of lapping, cracks in same degree as that of the particle size of abrasive grains used occur below the sliced surface, so that fibers are broken and chipped off when cracks reach the interface between the matrix and the fiber.