Key Engineering Materials Vols. 306-308

Abstract: Because the layup of composite laminates influences there properties, the strength of composites depends on layup sequence of CFRP laminates. It is very important to detect ply error before the laminate is cured for both manual procedure and fiber placement procedure. An ultrasonic technique would be very beneficial, which could be used to test the part after and before curing laminates and requires less time than the optical test. Scanners were set out for different measurement modalities for acquiring ultrasonic signals as a function of in-plane azimuthal angle. The first motorized scanner was utilized for making transmission measurements using a pair of normal-incidence shear wave transducers. A scanner was built for the acousto-ultrasonic configuration using contact transducers. And a ply-by-ply vector decomposition model has been utilized for evaluating layup errors in composite laminates fabricated from unidirectional plies. We have compared the test results with model data. It is found that high probability shows between experimentations and the decomposition model in characterizing cured and uncured laminates with defect angles.

Abstract: A nondestructive evaluation (NDE) technique would be very beneficial to assess resininfiltrated absorption effect of wood when wood was resin-treated. In this work, a wood material was nondestructively characterized and a technique was developed to measure ultrasonic velocity in wood using automated data acquisition software. We have proposed a peak-delay measurement method based on the pulse overlap measurement method. Also through transmission mode was performed to compare ultrasonic velocity with the above peak-delay measurement method in an immersion tank. The variation of ultrasonic velocity was measured and found to be somewhat consistent with those in infiltrated area in the course of nature absorption. For mapping out the material property inhomogeneity, through-transmission scans were used based on both amplitude and time-of-flight of the ultrasonic pulse. A peak-delay measurement method well somewhat corresponded to ultrasonic velocities of the pulse overlap method and through-transmission mode and C-scan image signal based on peak-to-peak amplitude.

Abstract: The numerical prediction of the effective mechanical properties of the reinforced braid inserted in automobile power steering hose is addressed. The key role of the reinforced braid layer is to suppress the excessive radial expansion of the hose subject to high pressure and temperature. The reinforced braid layer is in the structure composed of wrap and fill tows inclined to each with the specific helix angle. In order to predict the effective mechanical properties, we construct a 3-D finite element model of the unit cell (or RVE) of the reinforced braid in a periodic pattern, in which the detailed geometry of individual fiber tows is fully modeled. By making use of the superposition method and the 3-D finite element analysis, the effective mechanical properties are predicted. Numerical experiments illustrating the theoretical work are also presented.

Abstract: An analysis of fracture problems near crack tip for linear elastic orthotropic composite plate under bending and twisting is carried out. By solving the boundary value problem of partial differential equations and applying the method of undetermined coefficients, the two group expressions for bending moment, twisting moment and displacement near crack tip are derived. The formulae have certain practical and referential value in related fracture analysis.

Abstract: For measuring fiber content of GMT Sheet, moldings should be divided into specimens. And then by burning this specimen, we can get remaining glass fiber ratio. This remaining glass fiber ratio is regarded as fiber content and this process is called 'burning method'. This method yields high value but it can destroy the molding. So any standard method in fiber content measurements isn't established yet. An experimental method is proposed which can be used to measure the fiber content of compression molded glass mat reinforced thermoplastic sheet by image processing. Therefore to predict the fiber content may be useful for determination of optimum molding conditions, charge pattern etc. so far, the burning method has been used for measuring fiber content of glass fiber-reinforced thermoplastic sheet. Then the burning method is precise but breaks the material sheet. In this study, the soft X-pray photograph intensity of the sheet is measured using the image scanner, and the intensity information is worked out by personal computer. We suggested image processing method which can be used to nondestructively measure GMT-Sheet's fiber content. And fiber content measured by image processing method agree well with the one by the burning method. If GMT-Sheet is thick and fiber content is high, fibers overlapped, then photographing soft x-ray is impossible, so the measurements of exact fiber content is difficult.

Abstract: The purposes of stitching multi-axial warp knitted fabric preform prior to the fabrication of the composite materials by resin-transfer molding technique are to improve the resistance to delamination and to increase the out-of-plane properties of the composite materials for structural integrity. The influence of the through-the-thickness stitching on the elastic properties and behaviors of the multi-axial warp knit fabric composites is studied. An analytical model based on the representative volume is proposed to predict the elastic properties of the stitched multi-axial warp knit fabric composite materials. The fiber volume ratios determined by geometric parameters set by the representative volume and elastic behaviors of the in-situ constituent materials are used for the predictions. The crucial step in the analysis is to correlate the averaged stress states in the constituents by adopting bridging matrix. The predicted results are compared with the experimental results. It is found that the predicted results are in reasonably good agreement with the experimental results.

Abstract: The use of composite materials in many engineering structures has increased recently. The structures are jointed together by single or multiple bolted mechanical fastening to transfer the loads from one member to another. Applications of such joints require adequate knowledge of stress distribution in the vicinity of the boundary of pin hole. In this study, stress analysis is conducted with finite element method to study the stress distributions in the vicinities of both single-pin and multi-pin loaded holes of multi-axial warp knit fabric composite laminates. The perfect fit condition and contact with friction between the pin and hole without lateral constrain are assumed in the analysis. Effects of friction, stacking sequence, ratio of edge distance over hole diameter, ratio of width over hole diameter, number of pins, number of rows, number of column and hole patterns are evaluated. The results show they have a significant influence on joint performance.

Abstract: An elastopalstic analysis of the micromechanical approach is performed to investigate the stress transfer mechanism in a short fiber reinforced composites. The model is based on the New Shear Lag Theory (NSLT) which was developed by considering the stress concentration effects that exist in the matrix region near fiber ends. The unit cell model is selected as the Representative Volume Element (RVE) for the investigation of longitudinal elastoplastic behavior in discontinuous composites. Thus far, it is focused on the detailed description to predict fiber stresses in case of the behavior of elastoplastic matrix as well as elastic matrix. Slip mechanisms between fiber and matrix which normally take place at the interface are considered for the accurate prediction of fiber stresses. Consequently, onset of Slip points is determined analytically and it showed a moving direction to the fiber center region from the fiber tip as the applied load increases. It is found that the proposed model gives the more reasonable prediction compared with the results of the conventional model (SLT).

Abstract: The anisotropy of the tensile strength of plain-weave fabric is numerically evaluated by the finite element simulations. The plain-weave fabrics show complicated deformation behavior that is quite different from that of the continuum. The mechanics of woven fabric is not sophisticated yet enough to evaluate the strength and fracture mechanism in arbitrary stress conditions. The opacity of the tensile strength significantly diminishes the material reliability for the advanced use of fabrics. This study addresses the ideal tensile strength in arbitrary directions by using the pseudo-continuum model, which we have proposed to predict the deformation behavior and fiber stresses of the plain-weave fabrics. In this study, the numerical simulations of uniaxial extension in various directions are carried out by one finite element subjected to ideally uniform deformation, and we predict the breaking loads and elongations corresponding to the ultimate strength of the fiber.

Abstract: This article analyses the cutting mechanism of bolt-end according to coal side supporting. The authors put forward a new structure of FRP bolt-end by studying the structure of FRP bolt-end. The new structure is named as metal bushing and indentation. It is testified that the bolt with the structure can effectively support coal side in practice.