Unidirectional CFRP Laminates Research Articles

一方向ＣＦＲＰの面外引張強度に及ぼす試験片形状の影響

Reliable out-of-plane tensile test methods for adequate measurement of out-of-plane mechanical properties (e.g. strength) of laminated composites remain to be established. The present study focuses on the flatwise tension test using cylindrical column and spool column specimens for the measurement of out-of-plane tensile strength. Finite element analyses and experiments of unidirectional CFRP laminates were carried out in order to investigate the effect of specimen configuration on the out-of-plane tensile strength. A suitable guideline for evaluation of the out-of-plane strength was suggested based on the experimental and analytical results.

Tension–Compression Asymmetry in the Off-Axis Nonlinear Rate-Dependent Behavior of a Unidirectional Carbon/Epoxy Laminate at High Temperature and Incorporation into Viscoplasticity Modeling

Off-axis compressive deformation behavior of a unidirectional CFRP laminate at high temperature and its strain-rate dependence in a quasi-static range are examined for various fiber orientations. By comparing the off-axis compressive and tensile behaviors at an equal strain rate, the effect of different loading modes on the flow stress level, rate-dependence and nonlinearity of the off-axis inelastic deformation is elucidated. The experimental results indicate that the compressive flow stress levels for relatively larger off-axis angles of 30°,45° and 90° are about 50 percent larger than in tension for the same fiber orientations, respectively. The nonlinear deformations under off-axis tensile and compressive loading conditions exhibit significant strain-rate dependence. Similar features are observed in the fiber-orientation dependence of the off-axis flow stress levels under tension and compression and in the off-axis flow stress differential in tension and compression, regardless of the strain rate. A phenomenological theory of viscoplasticity is then developed which can describe the tension–compression asymmetry as well as the rate dependence, nonlinearity and fiber orientation dependence of the off-axis tensile and compressive behaviors of unidirectional composites in a unified manner. It is demonstrated by comparing with experimental results that the proposed viscoplastic constitutive model can be applied with reasonable accuracy to predict the different, nonlinear and rate-dependent behaviors of the unidirectional composite under off-axis tensile and compressive loading conditions.

一方向ＣＦＲＰの高温における非主軸引張・圧縮非線形挙動のひずみ速度依存性と粘塑性モデルの定式化

Off-axis compressive deformation behavior of a unidirectional CFRP laminate at high temperature and its strain-rate dependence in a quasi-static range are examined for various fiber orientations. By comparing the off-axis tensile and compressive behaviors at an equal loading rate, the effect of loading direction on the flow stress level, nonlinearity and rate-dependence of off-axis deformation is elucidated. The experimental results show that the compressive flow stresses for relatively larger off-axis angles 30, 45, 90° have about 50 percent larger values than in tension for the same fiber orientations, respectively. The off-axis flow stresses in tension and compression significantly depend on the rate of loading, while they exhibit similar features in fiber orientation dependence and tension-compression asymmetry of off-axis nonlinear behavior regardless of the rate of loading. Then, formulation of a phenomenological viscoplasticity model that can describe the nonlinear rate-dependent behavior as well as its tension-compression asymmetry of unidirectional composites under off-axis loading in a unified manner is attempted. It is demonstrated that the proposed viscoplasticity model succeeds in adequately predicting the fiber orientation dependence of off-axis non-linear behavior, deformation asymmetry in off-axis tension and compression, and the strain rate dependence.

HOMOGENIZED CREEP BEHAVIOR OF CFRP LAMINATES AT HIGH TEMPERATURE

In this study, creep behavior of a CFRP laminate subjected to a constant stress is analyzed based on the time-dependent homogenization theory developed by the present authors. The laminate is a unidirectional carbon fiber/epoxy laminate T800H/#3631 manufactured by Toray Industries, Inc. Two kinds of creep analyses are performed. First, 45° off-axis creep deformation of the laminate at high temperature (100°C) is analyzed with three kinds of creep stress levels, respectively. It is shown that the present theory accurately predicts macroscopic creep behavior of the unidirectional CFRP laminate observed in experiments. Then, high temperature creep deformations at a constant creep stress are simulated with seven kinds of off-axis angles, i.e., θ = 0°, 10°, 30°, 45°, 60°, 75°, 90°. It is shown that the laminate has marked in-plane anisotropy with respect to the creep behavior.

Time-dependent out-of-plane deformation of UD-CFRP in humid environment

Time-dependent out-of-plane deformation of UD-CFRP (unidirectional CFRP laminate) caused by subjection to a humid environment was examined and analyzed. The UD-CFRP plate showed unpredictable geometrical variation with time in a humid environment, like asymmetric materials. The unpredictability was caused by non-uniform fiber distribution in the thickness direction of the specimen. A three-dimensional diffusion-stress coupling analysis considering the non-uniform fiber distribution was conducted based on finite element analysis. The analytical results showed very good agreement with the experimental results. Furthermore, the relationship between the non-uniform fiber distribution and the out-of-plane deformation with time was obtained quantitatively.

The effect of shear on the rigidity in three-point bending of unidirectional CFRP laminates made of T800H/3900-2

Abstract Three-point bending tests in the elastic domain were carried out on unidirectional laminates made of T800H/3900-2 laminae, holding the fibre direction coincident with the longitudinal axis of the beam. The slenderness ratio was varied in the range 10–65, in order to highlight the contribution of the shear deformation to the structural response. Local rigidity tests were also performed, to ascertain the influence of the indentation at the loading points on the rigidities measured. From the results obtained, the effect of the local deformation on the apparent flexural rigidity was the higher, as the slenderness ratio was reduced, with errors as large as 18% for the shorter spans adopted. Using Timoshenko’s beam theory and finite element analyses, a prediction of the rigidity in flexure was attempted. The theoretical results and the experimental data showed an excellent correlation, provided an unexpectedly low value of the through-thickness shear modulus was assumed in the calculations. Possible reasons for this occurrence were pointed out, involving the role played by the epoxy/polyamide interleaves in the behaviour of the material, as well as the actual shear correction factor to be used to take into account the shear effect.

Off-Axis Tensile Creep Rupture Behavior of a Unidirectional CFRP Laminate at High Temperatures

Creep rupture behavior of a unidirectional carbon/epoxy T800H/Epoxy Laminate under constant off-axis loading conditions at different temperatures of 60, 100 and 130°C is examined. Tensile creep rupture tests are performed on plain coupon specimens with different fiber orientations θ=0, 10, 30, 45 and 90°. Creep rupture of specimens takes place predominantly along reinforcing fibers is a brittle manner, regardless of the fiber orientation. The log-log plots of the creep rupture data can approximately be described using straight lines with negative slopes over the range of rupture time up to 10h, regardless of the fiber orientation. Then, two kinds of simple phenomenological models are developed for predicting the time-to-failure of unidirectional composites. Validities of those models are evaluated by comparing with experimental results. It is demonstrated that the proposed models succeed in adequately predicting the creep rupture lives of the unidirectional composite at different temperatures.

1226 CNF強化CFRP積層板の層間強度クライテリオンの評価(J10-3 マイクロ・ナノ材料システムの力学と強度・機能評価(2) 力学特性評価,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)

In the present paper, criterion of interlaminar strength for unidirectional CFRP laminates was discussed. Generally, interlaminar shear strength (ILSS) of the CFRP laminates is evaluated by 3-point bending test, but the criterion of the fracture depends on not only interlaminar shear stress but also interlaminar normal stress in the specimens. In order to obtain the limit curve which is given by the relation between normal stress and shear stress, two types of ILSS test have been employed in the present study. The one is a 'compression type ILSS test' by a simple 3-point bending. The another one is a 'tension type ILSS test' applying the shear tensile loading using a shear pin. Employing 2-dimensional finite element analysis the criterion of the CFRP laminates and CFRP toughened by carbon nanofiber were investigated in detail using a modified 'Tsai-Wu' criterion.

PS23 一方向CFPR積層板のマルチスケールクリープシミュレーション(ポスターセッション)

PS23 一方向CFPR積層板のマルチスケールクリープシミュレーション(ポスターセッション)

Off-Axis Compressive Strength of a Unidirectional CFRP Laminate at High Temperature and Its Modeling

The off-axis compressive strength and its rate dependence of a unidirectional carbon/epoxy laminate are examined for various fiber orientations at high temperature. Static compression tests are carried out at two different strain rates of 0.1 and 10%/min for each fiber orientation. The fiber-orientation and strain-rate dependence of off-axis compressive strengths and associated failure modes are first discussed. The off-axis compressive strengths are compared with the off-axis tensile strengths at the same strain rates and test temperature, and differences between the ultimate off-axis strengths in tension and compression are quantified. Then, the Hoffman failure criterion that distinguishes between the off-axis strengths in tension and compression is tested for the accuracy of prediction of the different off-axis tensile and compressive strengths and the associated failure envelope at different strain rates. It is demonstrated that while the fiber orientation dependence of the off-axis tensile and compressive strengths are predicted with reasonable accuracy, the failure envelope associated with off-axis compression is poorly described using the Hoffman failure criterion. This observation suggests that consideration of the influence of the transverse compressive stress on the in-plane shear strength along fibers is essential for accurate prediction of failure envelope. Finally, a modified form of the Hoffman failure criterion that meets the requirement is proposed, and it is demonstrated to have an improved predictive capability compared with the original form.

A Modified Kinematic-Hardening Viscoplasticity Model for Off-Axis Creep Behavior of Unidirectional CFRPs at High Temperature

A macromechanics constitutive model to describe the anisotropic creep behavior of unidirectional composites under off-axis loading conditions is developed with a particular emphasis on accurate prediction of temporal creep softening due to stress variation. A viscoplasticity model that takes account of a combined isotropic and kinematic hardening is adopted as a base for this formulation, and the evolution equation of the kinematic hardening variable is elaborated to enhance the accuracy of prediction of the transient creep softening due to stress variation. Validity of the modified kinematic-hardening viscoplasticity model is evaluated by comparing with the experimental results on unidirectional T800H/3631 carbon/epoxy composites. It is demonstrated that the proposed model can adequately describe the off-axis creep behavior of the unidirectional CFRP laminate under constant and variable stress conditions.

Tensile strength of unidirectional CFRP laminate under high strain rate

The tensile strength of unidirectional carbon fiber reinforced plastics under a high strain rate was experimentally investigated. A high-strain-rate test was performed using the tension-type split Hopkinson bar technique. In order to obtain the tensile stress–strain relations, a special fixture was used for the impact tensile specimen. The experimental results demonstrated that the tensile modulus and strength in the longitudinal direction are independent of the strain rate. In contrast, the tensile properties in the transverse direction and the shear properties increase with the strain rate. Moreover, it was observed that the strain-rate dependence of the shear strength is much stronger than that of the transverse strength. The tensile strength of off-axis specimens was measured using an oblique tab, and the experimental results were compared with the tensile strength predicted based on the Tsai–Hill failure criterion. It was concluded that the tensile strength can be characterized quite well using the above failure criterion under dynamic loading conditions.

537 一方向CFRP積層板のマルチスケール高温クリープ解析(OS7-1 微視構造を有する材料の変形と破壊,OS7 微視構造を有する材料の変形と破壊)

537 一方向CFRP積層板のマルチスケール高温クリープ解析(OS7-1 微視構造を有する材料の変形と破壊,OS7 微視構造を有する材料の変形と破壊)

OS16-2-2 Stress analysis of unidirectional CFRP laminate by digital image correlation and the 2-D nonlinear intelligent hybrid method

OS16-2-2 Stress analysis of unidirectional CFRP laminate by digital image correlation and the 2-D nonlinear intelligent hybrid method

Off-axis tensile creep rupture of unidirectional CFRP laminates at elevated temperature

Abstract Off-axis tensile creep fracture behavior of unidirectional carbon/epoxy T800H/Epoxy laminates is studied at 100 °C under constant load conditions. Off-axis creep rupture data on plain coupon specimens are obtained within the time range up to 10 h for four kinds of off-axis fiber orientations. The log–log plots of the creep stress against the rupture time can approximately be described by straight lines with negative slopes over the range of creep life for all the fiber orientations. The static tensile strengths extrapolated from those straight lines almost agree with the experimental results for respective fiber orientations. The creep rupture data normalized with respect to the static strength approximately falls on a single creep rupture curve. These observations suggest that the fiber orientation dependence of creep rupture strength is similar to that of static tensile strength. The creep fracture occurs along reinforcing fibers in a brittle manner without accompanying the appreciable secondary and tertiary creep stages, regardless of the fiber orientations, and thus the off-axis creep deformation prior to fracture is characterized by the primary creep stage for all the fiber orientations. Then, a phenomenological model for the creep deformation and rupture behaviors of the unidirectional composite is developed, with a view to making preliminary predictions of creep life from a limited amount of data. It is demonstrated that the proposed model can moderately describe the observed features of the creep deformation and creep rupture behaviors under off-axis loading conditions.

Suppression of Delamination Crack Propagation in Laminated Composites by Using Thin SMA Plates

The use of thin actuator plates bonded to surfaces of laminated composites to suppress the growth of delamination cracks is investigated. A composite beam analysis was performed to estimate the effect of actuation strains on the strain energy release rate of delamination cracks. When compared at the same value of actuation strain, the increase in the apparent crack growth resistance is larger for smaller FRP thickness and for smaller value of the critical strain energy release rate of FRP without actuators, Gcr, under both mode I and II loadings. Mode I delamination suppression tests of unidirectional CFRP laminates were conducted by using thin SMA actuators. The value of the apparent crack growth resistance, Glapp, under actuation strains of about 0.1% was 2.5 times larger than fracture toughness of CFRP and was also about 1.5 times as large as that just before actuation.

An investigation of non-linear elastic behavior of CFRP laminates and strain measurement using Lamb waves

This paper investigates the non-linear elastic behavior of unidirectional and cross-ply CFRP laminates and proposes a new method to measure tensile strain using Lamb waves. Young’s modulus was measured as a function of strain in situ using Lamb wave velocity during a tensile test. The stiffening effect of the carbon fibers on [0] 8 specimens and the softening effect of the epoxy matrix on [90] 8 specimens were accurately evaluated. Young’s modulus of the 0° ply was obtained as a quadratic function of strain. Using the function and the rule of mixture, the dependence of Young’s modulus on strain was accurately predicted for cross-ply laminates. Based on the results, the tensile strain was quantitatively correlated with the corresponding arrival time of the Lamb waves. The strains obtained from the proposed method agreed well with those from the strain gauge. Finally, the effect of transverse cracks on the in situ Young’s modulus of the cross-ply laminate under a tensile load was investigated. This method clearly detected even a small decrease in the Young’s modulus due to the transverse cracks in stiffening cross-ply laminate.

Application of a phenomenological viscoplasticity model to the stress relaxation behavior of unidirectional and angle-ply CFRP laminates at high temperature

Abstract High-temperature stress relaxation behavior of symmetric angle-ply laminates made of unidirectional T800H/3631 carbon/epoxy composite is examined at relatively high strain levels. Stress relaxation tests are performed at 100 °C for 5 h on plain coupon specimens of three types of angle-ply laminates [±30] 3S , [±45] 3S and [±60] 3S under displacement controlled conditions. For each ply orientation, the stress relaxation tests are carried out at three different strain levels. Stress relaxation phenomena clearly appear in all kinds of angle-ply laminate. The average stress in the angle-ply laminates rapidly relaxes to approach a steady-state value associated with the strain sustained. The rate of stress relaxation in the angle-ply laminates substantially diminishes in the tested time range. Stress relaxation simulation of the angle-ply laminates is performed using the classical laminated plate theory and a phenomenological viscoplasticity model for individual plies. Material constants involved in the ply viscoplasticity model are identified on the basis of the off-axis creep behavior for a unidirectional laminate based on the same kind of carbon/epoxy prepregs. Excellent agreements between the predicted and observed results are obtained by taking into account a local recovery of total strain in the gauge-length part of specimens as well as a fiber rotation induced by deformation. The successful application of the ply viscoplasticity model confirms that the stress relaxation behavior is consistent with the creep behavior in the same composite system.

318 スプリットホプキンソン棒法による動的破壊靭性試験への境界要素法の適用

In the present study, boundary element method is applied to the evaluation of interlaminar fracture toughness of CFRP composite. The dynamic fracture toughness of CFRP was measured by split Hopkinson bar method, where the dynamic three point bending tests were carried out using short-beam-type specimens with end notch. In order to evaluate the dynamic deformation of CFRP laminates, Laplace-transformed BEM scheme was employed for the numerical computation, and the extrapolation scheme was applied to obtain the stress intensity factor (SIF) of Mode II in the unidirectional CFRP laminates. The basic formulation of BEM analysis for the anisotropic body of CFRP is introduced, and the experimental results of dynamic fracture toughness obtained from SHPB test and BEM analysis are discussed.

124 FBG センサを用いた複合材構造物の温度補償ひずみ計測

Although FBG sensors are useful for the strain measurement, they are influenced by the temperature change. In the present research, the center wavelength shift of an FBG sensor embedded in an unidirectional CFRP laminate was measured under the cryogenic temperature, and the relation between the temperature change and the wavelength shift was determined. Then, the accurate strain applied to the laminate was obtained by subtraction of the wavelength shift caused by the temperature change. The result showed that the temperature-compensated strain measurement could be carried out appropriately.