PREDICTION OF ELASTICITY CHARACTERISTICS FOR THE UNIDIRECTIONAL POLYMER COMPOSITE MATERIALS AT HIGH TEMPERATURES

The scope of the investigations of the paper includes the efficiency in the prediction of strength characteristics of the ablating polymer unidirectional carbon-fiber-reinforced plastics at elevated temperatures based on the properties of the unreinforced matrix and a bundle of fibers in the process of thermal oxidative breakdown at different types of stress state. The characteristics of elasticity and parameters of strength of the transverse-isotropic carbon-fiber-reinforced plastic, as well as of its components (epoxy matrix and carbon fibers of T700 type), have been determined in compliance with the regulatory documents of Ukraine at low heating rates with the subsequent exposure of the specimens at the fixed temperature, when the inertia effects can be neglected. It is illustrated that the variation in densities of the ablating unreinforced matrix and the bundle of fibers at elevated temperatures is rather well described via the models of multiphase media employing the modified integral exponential function. The authors analyze the possibility to evaluate the critical stresses of the transverse-isotropic composite at elevated temperatures based on the mechanics of multiphase media considering the thermal and mechanical characteristics of the ablating polymer matrix and the bundle of carbon fibers. It is assumed that the task of heat and mass transfer can be solved separately from the coupled tasks of thermal mechanics of the ablating materials since mechanical stresses have no effect on its parameters. It is found that the calculation of critical stresses using the hypothesis for the total hermetic sealing between the phases or in the assumption on the low pressure in pores determines the lower and upper lines of variation of the composite strength at different types of stress state. In particular, the calculated strength of the unidirectional composite under tension in the transverse direction and in shear along and across the fibers is in good correlation with the experimental data obtained by the authors and other researches.

Failure criteria of unidirectional carbon fiber reinforced polymer composites informed by a computational micromechanics model

Kink-band failure is a representative failure mode of a unidirectional carbon fiber reinforced plastics (CFRP) under axial compressive loading. Unidirectional CFRP has initial fiber waviness, and it may trigger kink-band failure. However, initiation of kink-band is not clarified yet. In this paper, three-dimensional finite element model with an actual fiber waviness was constructed by means of X-ray computed tomography (XCT) imaging. Simulation on compression was carried out using the three-dimensional model. A small unidirectional CFRP cylinder was fabricated and scanned the internal fiber structure using an XCT system. The three-dimensional model of the unidirectional CFRP was developed from the XCT images by tracking each fiber positions along the longitudinal direction. Numerical simulation on compression was performed using the constructed three-dimensional finite element model. With increase of compressive loading, matrix deformation was increased locally at some volumes inside the unidirectional CFRP due to fiber random waviness. Matrix started to yield at the volumes, which was considered as an initiation of kink-band formation. The applied compressive load started to decrease after the matrix yielded showing snap-through behavior in the load-displacement relation. The yielded volume of the matrix expanded through the cross-section with rotation of carbon fibers, and kinkband was gradually formed. The numerical simulation revealed initiation and formation of kink-band in a unidirectional CFRP by the random fiber waviness model.

Cross-sectional X-ray computed tomography of unidirectional carbon fiber reinforced plastic (CFRP) composites was performed at a synchrotron radiation facility. Fiber locations in each tomogram were identified and connected longitudinally via updated digital image correlation, from which a CFRP numerical model was automatically developed. Fiber-tracking accuracy was important for obtaining exact duplicates; thus, the performance was experimentally analyzed with two types of unidirectional CFRPs formed with circular or kidney-shaped cross-sectional carbon fibers. The accuracy improved and then converged to a certain value with decreasing distance between each tomogram. Better accuracy was obtained for the circular fibers. Average error in the estimated fiber locations was less than in the fiber diameter. However, errors caused overlapped fibers. The experimental results thus indicated that fiber-tracking accuracy had to be improved to develop duplicates without overlapped fibers.

LSWNet: A physics-informed neural network for ultrasonic wavefield prediction and elastic constant inversion in unidirectional CFRP.

The effect of fatigue life was studied at an elevated temperature of 80 °C of a centre pre-cracked specimen of aluminium alloy 6061-T6, repaired with a unidirectional carbon fibre-reinforced plastic (CFRP) patch. The patch was bonded only on one face of the aluminium alloy skin and tested under tension–tension fatigue load with a stress ratio of 0.1 and a frequency of 10 Hz. The performance of the two kinds of CFRP patches was studied with three-ply and five-ply patches. The fatigue life of pre-cracked bare specimen was improved substantially by bonding the CFRP patches. The performance of five-ply patch was significantly better than the three-ply patch. At low applied loads, the repaired specimen of both kinds failed with the growth of the crack tips all the way to the specimen edges. At high loads, the initial growth of fatigue crack was observed for few millimetres and the specimen failed due to the separation of the patch. At low loads and elevated temperature, the fatigue life was increased substantially over those of tests conducted at room temperature. However, at high loads, the failure life at elevated temperature was decreased significantly.

We have observed in our previous works [Miyano, Y., Nakada, M., Kudoh, H. and Muki, R. (1999). Prediction of Tensile Fatigue Life under Temperature Environment for Unidirectional CFRP, Advanced Composite Materials, 8: 235–246; Miyano, Y., Nakada, M., Kudoh, H. and Muki, R. (1999). Determination of Tensile Fatigue Life of Unidirectional CFRP Specimen by Strand Tenting, Mechanics of Time- Dependent Materials, 4: 127–137] that the tensile strength along the longitudinal direction of unidirectional carbon fiber reinforced plastics (CFRP) is highly time–temperature dependent. In this paper, we propose a prediction scheme of the master curve for the tensile strength of CFRP under constant strain rate (CSR) loading; we extend Rosen’s analysis [Rosen,B.W. (1964). Tensile Failure of Fibrous Composites, AIAA J., 2: 1985–1994] for tensile failure of unidirectional fibrous composites with elastic matrix to CFRP with viscoelastic matrix by making two modifications. First, we replace the ineffective length with the recovery length over which the interfacial shear stress is uniform. Second, we substitute the value of shear relaxation modulus at the time of failure for shear modulus in Rosen’s formula after the first modification. The shear relaxation modulus of polymer in the CFRP was estimated from the tensile storage modulus of CFRP obtained from the dynamic double cantilever beam under various frequencies and temperatures. The tensile test for unidirectional CFRP was carried out at various loading rates and temperatures; a smooth curve was drawn to represent inherently scattered strength data for each temperature. The master curve for CFRP strength together with the shift factors was obtained. The predicted master curve for tensile strength using an assumed value 7.5 for Weibull shape parameter of fiber strength captured the test data for CFRP (T400 and Epikote 828) adequately for small to medium time but poorly for large time.

For most multidomain proteins the thermal unfolding transitions are accompanied by an irreversible step, often related to aggregation at elevated temperatures. As a consequence the analysis of thermostabilities in terms of equilibrium thermodynamics is not applicable, at least not if the irreversible process is fast with respect the structural unfolding transition. In a comparative study we investigated aggregation effects and unfolding kinetics for five homologous alpha-amylases, all from mesophilic sources but with rather different thermostabilities. The results indicate that for all enzymes the irreversible process is fast and the precedent unfolding transition is the rate-limiting step. In this case the kinetic barrier toward unfolding, as measured by unfolding rates as function of temperature, is the key feature in thermostability. The investigated enzymes exhibit activation energies (E(a)) between 208 and 364 kJmol(-1) and pronounced differences in the corresponding unfolding rates. The most thermostable alpha-amylase from Bacillus licheniformis (apparent transition temperature, T(1/2) approximately 100 degrees C) shows an unfolding rate which is four orders of magnitude smaller as compared with the alpha-amylase from pig pancreas (T(1/2) approximately 65 degrees C). Even with respect to two other alpha-amylases from Bacillus species (T(1/2) approximately 86 degrees C) the difference in unfolding rates is still two orders of magnitude.

경량화 소재 중 CFRP(Carbon Fiber Reinforced Plastic)는 카본 섬유를 이용한 섬유구조물이다. 카본과 플라스틱의 특성을 갖는 복합소재이다. 섬유구조는 섬유방향으로 큰 강도를 갖는다. 이러한 경량 소재인 CFRP로 가장 많이 사용되는 것은 직조된 CFRP이다. 직조된 CFRP는 단방향 CFRP에 비하여 구조가 안정적이기 때문이다. 단직조된 CFRP는 고가이다. 따라서 본 연구는 단방향 CFRP의 섬유 구조 특성을 파악하고자 하였다. 본 연구에서는 적층각도 [0/X/-X/0]로 X를 변수로 갖는다. X의 각도 위상이 반전되어 적층된 단방향 CFRP이다. 이러한 단방향 CFRP를 이용하여 중앙 크랙을 갖는 두께 2 mm 판재 형태의 해석 모델을 이용하였다. 해석에서는 핀으로 연결된 상부와 하부에서 하중이 가해지고 있으며 중앙 크랙부분에서 파단을 연구한다. 해석 모델은 CATIA를 이용한 3D Surface 모델로 설계하였다. CFRP 적층을 위해, ANSYS프로그램에서 ACP를 이용한 적층 방향을 결정하여 2개의 적층들을 갖는 해석 모델을 만들었다. 이후 구조해석을 진행하였다. Carbon fiber reinforced plastic (CFRP), one of lightweight materials, is the fiber structure using carbon fiber. It is the composite material that has the characteristics of carbon and plastic. As for the fiber structure, it has the great strength due to fiber direction. CFRP for woven type is used mostly as such a CFRP with lightweight. Woven type is more stable when compared with unidirectional type. On the other hand, woven type is highly priced. Therefore, this study aims to analyze the fiber structure of unidirectional CFRP. In this study, as the stacking angle [0/X/-X/0], X is the variable. This is unidirectional CFRP in which the angle phase of X has been reversed and stacked. By using such a unidirectional CFRP, the analysis model which had a crack at the center as the form of panel with the thickness of 2 mm was used. On analysis, the load is applied on the upper and lower parts being connected with a pin. The damage in the area near center crack was investigated. As for the analysis model, 3D surface model was designed by using CATIA. For CFRP stacking, the stacking direction was determined by using ACP in ANSYS program and the analysis model with two stacks was made. Afterwards, the structural analysis was carried out.

Experimental study on micro-drilling of unidirectional carbon fiber reinforced plastic (UD-CFRP) composite using nano-solid lubrication

Relationships between behavioral and physiological performance under elevated CO₂ in marine fishes

Unidirectional carbon fiber-reinforced polymer (CFRP) may exhibit significant mechanical softening in the transverse direction at an elevated temperature. While significant transverse compressive stress exists on CFRP due to the clamping force from anchorage, a CFRP cable may exhibit anchorage failure when suffering an accidental fire disaster. The high-temperature resistance of a CFRP cable anchorage is critical, and clarifying the performance deterioration and failure mechanism of a CFRP cable anchorage system at elevated temperature is fundamental for clarifying its fire resistance. This paper reviews the current research status of the high-temperature resistance of CFRP cable anchorage systems from two aspects, including the high-temperature resistance of the comprising materials and the anchorage system. The reviews on the high-temperature properties of the comprising materials are summarized from two aspects. Firstly, the mechanical performance degradation of bonding epoxy resin at elevated temperatures and the effect of a filler on its mechanical-thermal properties are analyzed. Secondly, the mechanical performances of CFRP composites at elevated temperatures are summarized, with consideration of the stress state of the CFRP cable under the constraint of an anchorage device. The reviews on the high-temperature resistance of the anchorage system also include two aspects. Firstly, the temperature field solution method for the anchorage system is summarized and discussed. Secondly, the current research status of the anchorage performance at elevated temperatures is also summarized and discussed. Based on these reviews, the research shortage of the high-temperature resistance of CFRP cable anchorage systems is summarized, and further research is recommended.

An approach to design high-performance unidirectional CFRPs based on a new sensitivity analysis model

The objective of this study is to clarify the fracture mechanism of unidirectional CFRP (Carbon Fiber Reinforced Plastics) under static tensile loading. The advantages of CFRP are higher specific stiffness and strength than the metal material. The use of CFRP is increasing in not only the aerospace and rapid transit railway industries but also the sports, leisure and automotive industries. The tensile fracture mechanism of unidirectional CFRP has not been experimentally made clear because the fracture speed of unidirectional CFRP is quite high. We selected the intermediate modulus and high strength unidirectional CFRP laminate which is a typical material used in the aerospace field. The fracture process under static tensile loading was captured by a conventional high-speed camera and a new type High-Speed Video Camera HPV-1. It was found that the duration of fracture is 200 microseconds or less, then images taken by a conventional camera doesn't have enough temporal-resolution. On the other hand, results obtained by HPV-1 have higher quality where the fracture process can be clearly observed.

A compression test of a unidirectional carbon fiber reinforced plastic (CFRP) rectangular coupon was performed in an X-ray computed tomography (CT) system. Internal compressive failure of the CFRP was observed under loading. Two-dimensional fiber microbuckling developed non-uniformly in the specimen, and enlarged locally at one side. Fiber failure then initiated at the side edge and propagated through the whole cross section. A two-dimensional kink-band developed as a result of two-dimensional fiber microbuckling. The out-of-plane and in-plane band widths and band angles were almost the same. The coupon specimen twisted slightly owing to the two-dimensional kink-band, which resulted, macroscopically, in a transverse and through-thickness shear failure mode. The scenario of kink-band failure in a unidirectional CFRP coupon was revealed by X-ray CT imaging.