Strain In Composite Materials Research Articles

Numerical analysis and FBG experimental verification of transient heat and thermal-mechanical response of electrically conductive composite

This paper presents experimental and numerical investigations into transient heat and thermal–mechanical responses of electrically conductive composite under the operating environment of solar-powered aircraft in range of ±120 °C. Four kinds of the flexible electrically conductive composites were manufactured and tested under quasi-static tensile and temperature environmental conditions, respectively. The Fiber Bragg Grating (FBG) technology was used to monitor the in-situ principal strain in composite materials at thermal steady-state condition. Based on its realistic geometry and periodic arrays, a representative volume elements (RVE) of the composite was constructed to investigate the thermal effect on the adhesive layer. The results show that the temperature effect mainly affects the interface of materials with different thermal expansion coefficients, and the more serious risk occurs at the initial process of transition from the moment of expansion to contraction. In addition, the plastic deformation range is one order of magnitude larger than the elastic deformation range and two orders of magnitude larger than the temperature deformation range in the case of 124° rotating thermal coupling analysis. It suggests that the results could provide a reliable reference for the design and lifetime evaluation of the flexible electrically conductive composites.

Large strain detection of SRM composite shell based on fiber Bragg grating sensor

There may be more than 2% strain of carbon fiber composite material on solid rocket motor (SRM) in some extreme cases. A surface-bonded silica fiber Bragg grating (FBG) strain sensor coated by polymer is designed to detect the large strain of composite material. The strain transfer relation of the FBG large strain sensor is deduced, and the strain transfer mechanism is verified by finite element simulation. To calibrate the sensors, the tensile test is done by using the carbon fiber composite plate specimen attached to the designed strain sensor. The results show that the designed sensor can detect the strain more than 3%, the strain sensitivity is 0.0762 pm/με, the resolution is 13.13με, and the fitting degree of the wavelength-strain curve fitting function is 0.9988. The accuracy and linearity of the sensor can meet the engineering requirements.

Nonuniform strain measurement in composite material based on optical frequency domain reflection

Traditional electrical sensor or traditional fiber Bragg grating sensing technology is not applicable to the measurement of nonuniform strain in composite material. Therefore, the distributed nonuniform strain in the lap plate position of composite interlining material is measured using a single fiber with optical frequency domain reflection technology in this study. The experimental results show consistency with the experiment phenomena, and the measurement accuracy could be increased to the submillimeter level.

A new method to measure critical strain in composite materials – Combining the Euler–Fresnel spiral with acoustic emission to assess crack positions

Abstract Critical strain is the strain level corresponding to the very first micro-crack in a composite layer. Current detection methods for such micro-cracks are limited to strain gage measurements, digital image correlation techniques or the Bergen ellipse [9] . With the Bergen ellipse a well-defined but non-linearly increasing strain can be applied to a strip of composite material to induce micro-cracks. In this paper a combined approach is described to measure the critical strain at which the first micro-crack occurs. The combined approach consists of 2 elements. First the Euler–Fresnel spiral is introduced as an elegant alternative to the Bergen ellipse to introduce a well-defined and linearly increasing strain to a strip of composite material secondly, since micro-cracks, caused by exceeding the critical strain values, are difficult to detect, multiple accelerometers on the composite strip are used to detect the micro-cracks by measuring the acoustic emission of the crack initiations. By measuring the time of arrival of the acoustic emission waves at the two ends of the strip, the positions of the micro-crack initiations can be determined. Using the geometrically defined relationship between strain and position on the Euler–Fresnel spiral, the strain-levels at crack initiations can be estimated. An important advantage of the approach described in this paper is that all micro-cracks are detected instantaneously during the measurement procedure. This includes the subsurface cracks that cannot be detected by the penetrant method. In this paper the proposed measuring method for critical strain in composites was successfully demonstrated.

Residual strain-induced birefringent FBGs for multi-axial strain monitoring of CFRP composite laminates

Several solutions to measure multi-axial strain in composite materials using fibre Bragg gratings have already been presented in the literature. Most of them use Bragg sensors written in highly birefringent fibres. Unless they exhibit a higher transverse strain sensitivity, a major drawback of those highly birefringent Bragg sensors is the necessity to orient them before embedding them into the composite material, because otherwise an underestimation of the present strain field can occur. Therefore, in this paper we describe how the residual strain, which is built up during the cure cycle of a composite laminate, can in-situ create a highly birefringent fibre Bragg sensor making the need of extra manipulations before embedding redundant. Consequently, in the paper this Bragg sensor is used to quantitatively measure multi-axial strain fields of cross-ply laminates loaded along their three principal mechanical axes. Good similarities were found for the measured strains and the reference strains.

Advances in Strain Gauge Measurement on Composite Materials

Abstract: This article gives an overview on the application of strain gauge techniques to the analysis of the strains in composite materials. The orthotropic behaviour of the composite influences the performance of strain gauges that are calibrated for use on isotropic materials. The article considers therefore the typical topics of the strain gauge technology applied to composites with particular reference to the compensation of thermal output, the measurement of the coefficients of thermal expansion, the determination of the strain and stress state, the influence of the misalignment error, the reinforcement effect, the determination of the stress intensification factor, the analysis of residual stresses by the hole drilling method and the effect of transverse sensitivity on the measurement of strains along the fibres.

304 複合材の熱ひずみによる遮熱コーティングの線膨張係数評価法の誤差感度の検討(OS3-1 薄膜・コーティング材の機械的特性と界面強度特性)

304 複合材の熱ひずみによる遮熱コーティングの線膨張係数評価法の誤差感度の検討(OS3-1 薄膜・コーティング材の機械的特性と界面強度特性)

Effective properties of three-phase electro-magneto-elastic composites

Coupling between the electric field, magnetic field, and strain of composite materials is achieved when electro-elastic (piezoelectric) and magneto-elastic (piezomagnetic) particles are joined by an elastic matrix. Although the matrix is neither piezoelectric nor piezomagnetic, the strain field in the matrix couples the electric field of the piezoelectric phase to the magnetic field of the piezomagnetic phase. This three-phase electro-magneto-elastic composite should have greater ductility and formability than a two-phase composite in which the electric field and the magnetic field are coupled by directly bonding two brittle materials. A finite element analysis (FEA) and micromechanics based averaging of a representative volume element (RVE) are performed in this work to determine the effective dielectric, magnetic, mechanical, and coupled-field properties of an elastic matrix reinforced with piezoelectric and piezomagnetic fibers as functions of the phase volume fractions, the fiber arrangements in the RVE, and the fiber material properties with special emphasis on the poling directions of the piezoelectric and piezomagnetic fibers. The effective magneto-electric moduli of this three-phase composite are found to be less than the effective magneto-electric moduli of a two-phase piezoelectric/piezomagnetic composite, because the elastic matrix is not stiff enough to transfer significant strains between the piezomagnetic and piezoelectric fibers.

Pneumatic strains in woven glass/epoxy composite laminate induced by fluctuating air pressure

Pneumatic strain of composite material is a novel discovery within the field of composite engineering. Ambient air pressure change induces the strain, which is not negligible in comparison with hygrothermal strain and is physically reversible. Previous research has verified that a sudden ambient air pressure change can result in a monotonically increasing or decreasing strain, which always approaches to a plateau value asymptotically. The final strain is proportional to air pressure change. The pneumatic strain can be predicted via the theory based on the Fick's diffusion law. In this work, the same theory was modified to predict the pneumatic strains induced by multi-step air pressure change and fluctuating air pressure. The theoretical predictions matched very well with the experimental data. The theory can be used to describe the pneumatic behavior of composite structures serving in the environment that has fluctuating air pressure such as the composite components of aircraft, pressure vessels and pipes.

Measurement of process-induced strains in composite materials using embedded fiber optic sensors : Lawrence, C.M.; Nelson, D.V.; Spingarn, J.R.; Bennett, T.E. SPIE Smart Structures and Materials Conference, San Diego, CA (United States), 26–29 Feb. 1996, 9 pp. DE96008710. U.S. Government Printing Office (1996)

Measurement of process-induced strains in composite materials using embedded fiber optic sensors : Lawrence, C.M.; Nelson, D.V.; Spingarn, J.R.; Bennett, T.E. SPIE Smart Structures and Materials Conference, San Diego, CA (United States), 26–29 Feb. 1996, 9 pp. DE96008710. U.S. Government Printing Office (1996)

Damage monitoring of carbon-fibre-reinforced plastics using Michelson interferometric fibre-optic sensors

Structures having the ability both to inspect their integrity with sensors and to control damage propagation with actuators are referred to as ` smart structures''. Fibre-optic sensors have the advantage that they are immune to electromagnetic interference and they can measure various physical parameters using one probe. Thus ®bre-optic sensors have been promising sensors for smart structures [1, 2]. Fibrereinforced plastics have been considered prospective smart structural materials because ®bre-optic sensors can be easily embedded in them. Recently, ®brereinforced plastics have been widely used as structural materials for aerospace applications. A smart airplane that would be developed in the future would have the function to inspect its own structural integrity, i.e., to measure strain measurement and to detect damage. The use of ®bre-optic sensors for damage detection is well known. Hofer [3] and other researchers [4±6] have demonstrated that optical ®bre systems are capable of detecting damage in composite materials. They indicated that damage in composite materials could be detected by the fracture of embedded optical ®bres. The intensity of the light propagating through an optical ®bre drops substantially whenever a ®bre is broken. Fukuda et al. [7] have measured the strain of composite materials with an embedded optical interferometer by utilizing the optical interferometric phenomenon. In the present study, the possibility of damage monitoring of composite materials using optical interferometers was investigated. A sudden occurrence of an interference signal with a high amplitude and frequency during tensile loading was found to result from damage of the composite. Optical interferometric sensors proved to be effective not only to measure strain but also to monitor damage of composite materials. Optical interferometric strain sensors utilize the change in optical path length caused by deformation in an optical ®bre. Consider the case where coherent light of wavelength, e, is launched into two single-mode optical ®bres. One ®bre is strained, and the other is maintained in a strain-free state. The strained and strain-free ®bres are called the sensing and the reference ®bres, respectively. The change in optical path length is associated with the phase shift in the interference signal obtained by combining the light in a sensing ®bre with that in a reference ®bre. When the sensing ®bre with a gauge length, L, is strained along the ®bre axis by az, the induced phase shift, AO, in the interference signal can be expressed by Equation 1 [8]

X-Ray Measurement of Plastic Strain by Means of Eshelby/Mori-Tanaka Model and Its Application.

A new method is proposed in this paper for determining plastic strains in composite materials using the X-ray diffraction method. The present method was derived by using both Eshelby's approach and the Mori-Tanaka theory to express the stress state in composite materials instead of the elasticity in single-phase materials which is used in the conventional method of X-ray stress measurement. It was found that the plastic strain can be determined from the slope of the linear relation between lattice strains measured by the X-ray diffraction technique and sin2 ψ using almost the same procedure as that for determining stresses by the conventional X-ray method. The results on ferritic and austenitic dual-phase stainless steel are shown. We discuss the effects of a uniaxial tensile load in a range of plastic deformation on the field of plastic strain as well as on residual macro-, micro-and phase stresses built up in the sample.

A Strain-gage Transducer for Measuring Strains in Composite Materials

A Strain-gage Transducer for Measuring Strains in Composite Materials