Thermal Expansion Coefficients Of Fiber Research Articles

Residual stresses in thick composites considering through-thickness temperature gradients during manufacturing

In the composite materials of wind turbine rotor blades, residual stresses form during manufacturing due to the different thermo-chemo-mechanical behaviors of the fibers and the matrix. These residual stresses decrease the ultimate and fatigue strength of the blade because the failure stress and cycles to failure are reached at lower levels of external loading. It is thus of utmost importance to consider residual stresses in the design stage to obtain reliable blade designs.This work deals with the prediction of residual stresses in a thick composite representative for a spar cap in a wind turbine rotor blade. A model is formulated for the thermo-chemo-mechanically coupled problem. An emphasis is given on the non-uniform and time-variable temperature distribution across the thickness of the composite. The different sources of residual stresses (e.g., chemical shrinkage, non-uniform distribution of different thermal expansion coefficients of fibers and matrix, exotherm, etc.) are analyzed, quantified, and compared. The impact of the time-dependent temperature distribution on the formation of residual stresses is highlighted by means of a representative numerical example.

Comparison of stress-based failure criteria for prediction of curing induced damage in 3D woven composites

Several stress-based failure criteria (von Mises, dilatational strain energy density, parabolic stress and Drucker-Prager) are implemented in a numerical model of a 3D woven composite to predict initiation of damage due to cooling after curing. It is assumed that the composite is completely cured at elevated temperature and the residual stresses arise due to difference in the thermal expansion coefficients of fibers and matrix. The stresses are found by finite element analysis on the mesoscale while the effective thermoelastic properties of fiber tows are determined by micromechanical modeling. The matrix is modeled as an isotropic material with temperature dependent elastic properties and thermal expansion coefficient.Comparison of numerical simulation results with the microcomputed tomography data obtained for a one-by-one orthogonally reinforced carbon/epoxy composite shows that the parabolic stress and the dilatational strain energy criteria provide the most accurate predictions of cure-induced damage. However, the accuracy of the parabolic failure criterion is dependent on the choice of the mechanical tests used to determine the values of its two material parameters.

Stochastic homogenization analysis for thermal expansion coefficients of fiber reinforced composites using the equivalent inclusion method with perturbation-based approach

This paper describes a perturbation-based stochastic homogenization analysis method for a thermal expansion coefficient of a fiber reinforced composite material using the equivalent inclusion method. The proposed method is formulated for evaluation of the probabilistic characteristics of the homogenized thermal expansion coefficients considering uncertainties in material properties and geometry of a component material. As a numerical example, the probabilistic characteristics of the homogenized thermal expansion coefficients of a unidirectional fiber reinforced plastic are analyzed. With comparison between the results of the proposed method and the results of the Monte-Carlo simulation, validity, accuracy and effectiveness of the proposed method are investigated.

Thermal stress development in fibrous composites

In this work, the thermal stress development in anisotropic fiber-reinforced polymer composites is investigated for temperatures below the glass transition temperature of the resin. By applying two independent experimental methodologies, it was found that the initial thermal (residual) strain in the reinforcing fibers is compressive of about − 0.04% at ambient temperatures. This is due to the mismatch of the thermal expansion coefficient between the polymer matrix and fiber, as the material is cooled down from the processing temperature. However, on reheating the composites the compressive stress in the fiber gradually diminishes and becomes zero at 50 °C. Further heating to 100 °C introduces tensile strains in the fiber of maximum of 0.13%. The conformity of these results to analytical models that relate the composite thermal strain to the thermal expansion coefficients of fiber and resin, as well as, the fiber volume fraction, is examined. Finally, the possibility of tailoring the sign (positive, negative or, even, zero) of the composite thermal expansion coefficient of certain advanced composites by simply varying the thermal expansion of the polymer matrix, is discussed.

Damage tolerant, translucent oxide fiber/glass matrix composites

Starting out from damage-tolerant oxidic composites with carbon (C)-coated fibers, the main part of our investigations was concentrated on Nextel 440-fibers/glass matrix composites. The matrix glasses were chosen in consideration of suitable thermal expansion coefficients and optical refractive indices. The fiber pull-out is enabled by a CVD coating with boron nitride (BN). In comparison with a single BN layer, a BN/TiO 2-double layer yields better mechanical and optical properties. All composites are made by means of a traditional lab method including hot pressing. Varying differences of the thermal expansion coefficients of fibers and matrix glasses (minus, zero, plus) allow the preparation of composites presenting various mechanical properties and also different damage tolerances. A composite, which is both damage-tolerant and translucent at one and the same time requires an identical thermal expansion coefficient and, in addition, an identical optical refractive index of the fibers and the matrix. Bubbles must be avoided in any case.

Squeeze infiltration processing of nickel coated carbon fiber reinforced Al-2014 composite

Ni-coated fibers are widely used to synthesize carbon fiber reinforces Al based composites. It is observed that during the pressure infiltration process, significant amount of damage takes place in the nickel (Ni) coating due to a difference in the thermal expansion coefficients of fibers and Ni-coating. A modified squeeze infiltration set up is studied in the present work to minimize the damage to the Ni-coating during the infiltration process. This process resulted in significant improvement in retention of the Ni-coating compared to the unmodified process. The solidification pattern in the matrix was established by Energy Dispersive Spectroscopic analysis, which suggests that solidification of primary α-Al started in the interfiber region. The Vickers hardness values for the unreinforced portion of the sample, the matrix regions within the fiber tows and at the fiber matrix interface were found to be 55, 72 and 87, respectively.

Thermal Expansion Coefficient of Unidirectional High-Strength Polyethylene Fiber Reinforced Plastics at Low Temperature

High-strength polyethylene fiber (Dyneema®, DF) has a negative linear thermal expansion coefficient in the direction of the fiber. Thermal expansion coefficients of fiber-reinforced plastics are of important applications in cryogenic use. The purpose of this study is to construct the engineering technology for the thermal strain of DF reinforced plastics (DFRP) for cryogenic engineering. In this study, we investigate the thermal expansion coefficient of unidirectional high-strength DF reinforced plastics (UD-DFRP) in which DFs with different tensile moduli (15–134 GPa) and thermal expansion coefficients are used. Furthermore, using the thermal expansion coefficient and tensile modulus of each DF fiber and the matrix resin, the thermal expansion coefficients of the DFRPs are estimated by the rule of mixtures and are compared with the measured values. The results indicate that the thermal expansion coefficient of UD-DFRP behaves in a similar way to that expected from calculations based on the rule of mixtures. In addition, relationships concerning the difference between the calculated and measured values, the difference in the thermal expansion coefficient of the fiber and the resin, and the surface treatment of the fiber (adhesivity), as well as the diameter of the fiber (the area of the interfacial bonding) are studied. The results show that all of these influence the thermal expansion coefficient of UD-DFRP. The design of negative thermal expansion coefficient of UD-DFRP is established.

Interface debonding from bottom face and frictional transition during pushout testing of a tungsten fiber-epoxy matrix composite

The progress of interface debonding behavior in a tungsten fiber-reinforced epoxy matrix composite, which has an initially debonded interface due to thermal mismatch stress, has been studied by using the thin-specimen pushout test. Load drop behavior just after the maximum load in the load–displacement curve is discussed with respect to the interface debonding and frictional load transfer mechanism. The interface debonding propagates from the initially existing debond crack tip at the bottom face while no interface debonding crack propagation is observed from that located at the top face. This characteristic interface debonding behavior is explained by the interaction of interface shear stresses which originated from the thermal misfit and the applied load. Near the top face, the two shear stresses are opposite while those at the bottom face are added. This creates a condition of interface debonding from the bottom face when the thermal expansion coefficient of fiber is smaller than that of matrix. The load drop observed just after the maximum load in the load–displacement curve is not related with the interface debonding but with the transition of interface frictional sliding resistance from static to dynamic. The maximum pushout load is predicted by the static coefficient of friction and the load after the drop is predicted by dynamic coefficient of friction.

Thermal expansion of organic and inorganic matrix composites: A Review of theoretical and experimental studies

AbstractThis paper reviews theoretical and experimental studies conducted in organic, ceramic, and metal matrix composites containing cylindrical, lamellar, and spheroidal inclusions as reinforcements. Mathematical formulations proposed to predict thermal expansion coefficients of fiber, disc, and sphere reinforced organic and inorganic matrix composites have been reviewed. Experimental studies undertaken to confirm theoretical predictions of thermal expansion coefficients of a variety of reinforcement geometry composites have also been discussed.

Critical fiber size for microcrack suppression in ceramic-fiber/ceramic-matrix composites

In this paper the microcracking of ceramic-fiber/ceramic-matrix composites due to residual stresses is studied. Starting with the model of a single fiber embedded in an infinite matrix, the critical fiber size for suppression of interface and radial matrix cracking is determined. The relationship between critical fiber size and the parameters governing microcracking (such as cooling temperature, thermal expansion coefficients of fiber and matrix, Young's moduli of the constituents, etc.) is established. It is shown that a smaller difference between the thermal expansion coefficients of matrix and fiber leads to a larger critical fiber size. Cooling temperature and the elastic properties of the constituents also affect microfracture.