Flow control using localized induction heating in a VARTM process

Voids formed during the mold filling stage of the vacuum assisted resin transfer molding (VARTM) process become defects in the fabricated parts. Active flow control is one way to eliminate these defects by guiding the flow along a desired path during the mold filling stage of the process. Building upon previous work of the authors [Johnson R, Pitchumani R. In: Proceedings of the thirty-fourth international SAMPE technical conference, MD, USA, vol. 34(1); 2002, p. 250–61; Johnson R, Pitchumani R. Enhancement of flow in VARTM using localized induction heating. Compos Sci Technol 2003;63(15):2202–15; Johnson R., Pitchumani R. In: Proceedings of the fourteenth international conference on composite materials, CA, USA; 2003, Paper# 0861; Johnson R, Pitchumani R. Simulation of active flow control based on localized preform heating in a VARTM process. Compos Part A-Appl Sci Manuf, in press doi:10.1016/j.compositesa.2005.09.007], this paper presents implementation of an active flow control using induction heating as a means of locally reducing viscosity to counteract the effects of nonhomogeneity in the permeability of preform layups in a prototype VARTM process. Feedback of flow front locations during the filling stage of the process is used together with a numerical process model to arrive at decisions on the trajectories of the induction coil and the coil voltage, so as to maintain a uniform flow progression without exceeding a prescribed maximum temperature limit. A flow front following control strategy is implemented in a lab-scale experimental setup and tested on several preform layups exhibiting spatial permeability variation, as well as in the case of preforms with mold inserts. Results of these studies demonstrate that active flow control is capable of reducing the fill time, improving the flow front uniformity throughout the duration of the mold fills, and eliminating dry spot formation.     

Simulation of the perpendicular recording process including image charge effects

This paper presents a complete model for the perpendicular recording process in single-pole-head keeper-layer configurations. It includes the influence of the image-charge distributions in the head and the keeper layer. Based on calculations of magnetization distributions in standstill situations, the model describes the relaxation process that takes place if the activated head is shifted along the recording layer, periodically switching its head field. The magnetization distributions thus derived are used in combination with a model for the readback process to calculate the readback flux and voltage pulses. For the sake of arithmetical convenience, the model was applied to a recording configuration with a thick single-pole head, but it can also be used for calculations with other head shapes, e.g. thin single-pole heads.     

Fatigue damage development in new fibre metal laminates made by the VARTM process

This paper investigates the tensile and fatigue properties of a newly developed fibre metal laminate (FML) manufactured using the vacuum assisted resin transfer moulding (VARTM) method. This manufacturing method allows the glass fibre reinforced epoxy and 2024‐T3 aluminium FML to be prepared at lower cost than conventionally manufactured FMLs. However, in order for the resin to infiltrate the FML, the metal sheets need to be perforated. These perforation holes act as crack initiators and reduce the FML's performance. Tension and fatigue test results of three different designs are reported and compared to mechanical property predictions. Additionally, single sheet Al alloy specimens were tested in order to analyse the influence of the drilling method.     

Optimization of the VARTM process for enhancement of the degree of devolatilization of polymerization by-products and solvents

Devolatilization of the polymerization by-products and the impregnation solvent during Vacuum Assisted Resin Transfer Molding (VARTM) of the polyimide polymers is analyzed using a combined continuum hydrodynamics/chemical reaction one-dimensional model. The model which consists of seven coupled partial differential equations is solved using a finite element collocation method based on the method of lines. The results obtained reveal that the main process parameters which give rise to lower gas-phase contents in the VARTM-processed polymer matrix composites are the vacuum pressure and the tool-plate heating rate. Lower tool-plate heating rates are found to be beneficial since hey promote devolatilization of the impregnation solvent at lower temperatures at which the degree of polymerization and, hence, resin viscosity are low.     

Deformation response and constitutive modeling of vinyl ester polymer including strain rate and temperature effects

The deformation behavior of vinyl ester polymer under monotonic tensile loading is characterized and modeled. The Standard Linear Solid model, which is a physical model, was used and modified to represent the stress–strain behavior of this polymer over a wide range of strain rates and temperatures. This model was also used to predict the stress-relaxation and short-term creep behavior of this material. The comparisons between the predictions and experimental data from tensile and relaxation tests demonstrate that this model can represent the deformation behavior of the material reasonably well.     

Fatigue behavior and modeling of short fiber reinforced polymer composites including anisotropy and temperature effects

Effects of anisotropy and temperature on cyclic deformation and fatigue behavior of two short glass fiber reinforced polymer composites were investigated. Fatigue tests were conducted under fully-reversed (R = −1) and positive stress ratios (R = 0.1 and 0.3) with specimens of different thicknesses, different fiber orientations, and at temperatures of −40 °C, 23 °C, and 125 °C. In samples with 90° fiber orientation angle, considerable effect of thickness on fatigue strength was observed. Effect of mold flow direction was significant at all temperatures and stress ratios and the Tsai–Hill criterion was used to predict off-axis fatigue strengths. Temperature also greatly influenced fatigue strength and a shift factor of Arrhenius type was developed to correlate fatigue data at various temperatures, independent of the mold flow direction and stress ratio. Micromechanisms of fatigue failure at different temperatures were also investigated. Good correlations between fatigue strength and tensile strength were obtained and a method for obtaining strain–life curves from load-controlled fatigue test data is presented. A fatigue life estimation model is also presented which correlates data for different temperatures, fiber orientations, and stress ratios.     

嵌入加强筋的缝合泡沫夹芯结构复合材料VARTM工艺树脂充填模拟及验证

在缝合泡沫夹芯结构复合材料的泡沫中嵌入轻质的加强筋板,可以在不增加缝合密度并且在只增加较少质量的前提下,增强复合材料制品整体的强度和刚性。文中对真空辅助树脂传递模塑成型(VARTM)工艺树脂在嵌入加强筋的缝合泡沫夹芯结构复合材料预成型体中充填过程进行模拟和验证研究。采用一种矩形流道模型代替沿加强筋与泡沫空隙间的树脂流动,并对其等效渗透率及孔隙率进行计算;通过PAM-RTM软件模拟了嵌入加强筋板的缝合泡沫夹芯结构VARTM工艺的树脂充填过程,并建立了流动可视化实验装置与模拟对比,结果表明模拟与实验相当吻合。而模拟与实验的结果均表明加强筋的引入可以在局部加强树脂沿厚度方向的流动,但是会延缓树脂对整个预成型体的充填。     

Experimental Study of Thickness Gradient Formation in the VARTM Process

Although the vacuum assisted resin transfer molding (VARTM) process has been the focus of much attention in the past several years, it is only recently that research tools have been developed to investigate fundamental process mechanisms associated with it. One area of concern for composites manufacturers is the ability to maintain dimensional tolerances throughout the process cycle. It has been well known that a thickness gradient forms during VARTM, leading to corresponding gradients in fiber volume fraction and the associated mechanical properties. In this work, the formation of these thickness gradients is investigated using an array of linear variable differential transformers (LVDTs) to monitor surface displacements during infusion. SMARTweave sensors are used to correlate resin flow with the surface displacements for infusions, using either a line resin source or point resin source. Software is used to display the surface displacements as a three-dimensional plot so that the entire part surface may be visualized during infusion.     

Layerwise modeling of progressive damage in fiber-reinforced composite laminates

This paper investigates the effects of discrete layer transverse shear strain and discrete layer transverse normal strain on the predicted progressive damage response and global failure of fiber-reinforced composite laminates. These effects are isolated using a hierarchical, displacement-based 2-D finite element model that includes the first-order shear deformation model (FSD), type-I layerwise models (LW1) and type-II layerwise models (LW2) as special cases. Both the LW1 layerwise model and the more familiar FSD model use a reduced constitutive matrix that is based on the assumption of zero transverse normal stress; however, the LW1 model includes discrete layer transverse shear effects via in-plane displacement components that are C ⁰ continuous with respect to the thickness coordinate. The LW2 layerwise model utilizes a full 3-D constitutive matrix and includes both discrete layer transverse shear effects and discrete layer transverse normal effects by expanding all three displacement components as C ⁰ continuous functions of the thickness coordinate. The hierarchical finite element model incorporates a 3-D continuum damage mechanics (CDM) model that predicts local orthotropic damage evolution and local stiffness reduction at the geometric scale represented by the homogenized composite material ply. In modeling laminates that exhibit either widespread or localized transverse shear deformation, the results obtained in this study clearly show that the inclusion of discrete layer kinematics significantly increases the rate of local damage accumulation and significantly reduces the predicted global failure load compared to solutions obtained from first-order shear deformable models. The source of this effect can be traced to the improved resolution of local interlaminar shear stress concentrations, which results in faster local damage evolution and earlier cascading of localized failures into widespread global failure.     

Micromagnetic recording model of pole-tip saturation effects

Auxiliary write-head fields, either from simple analytic approximations or from finite-element computations, are used to record transitions on a Landau-Lifschitz-Gilbert model of thin-film recording media. A micromagnetic model of a magnetoresistive element is used to read back the transitions. Focus is on the impact of pole-tip saturation on the quality of the recorded track and subsequent playback. It is shown that with increasing write current, the quality of the recorded transitions degrades much more rapidly than would be expected from the analytical expressions for the write-head field. Specifically, head saturation causes the transition parameter to increase. At high write currents, poor write field gradients at the track edges result in a strong increase in transition parameter with read width percentage of the recorded track     

Mathematical modeling of the process of sublimation drying

A complex physicomathematical analysis of vacuum sublimation drying has been given. Approaches to calculation and designing of sublimation dryers have been proposed. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 79, No. 1, pp. 48–55, January–February, 2006.     

Mathematical modeling of the process compaction of wood

Within the framework of the mechanics of heterophase systems a mathematical model for the process of pressing wood has been offered; this model takes account of the influence of its complex rheological properties and surface phenomena in thin interlayers of water on the change in a porous structure. With numerical methods, a study has been made of the influence of the sample’s humidity and temperature fields on the strength and quality of the material obtained. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 78, No. 5, pp. 62–69, September–October, 2005.     

Finite element modeling of the filament winding process

A finite element model of the wet filament winding process was developed. In particular, a general purpose software for finite element analysis was used to calculate the fiber volume fraction under different process conditions. Several unique user defined subroutines were developed to modify the commercial code for this specific application, and the numerical result was compared with experimental data for validation. In order to predict the radial distribution of the fiber volume fraction within a wet wound cylinder, three unique user defined subroutines were incorporated into the commercial finite element code: a fiber consolidation/compaction model, a thermochemical model of the resin and a resin mixing model. The fiber consolidation model describes the influence of the external radial compaction pressure of a new layer as it is wound onto the surface of existing layers. The thermochemical model includes both the cure kinetics and viscosity of the resin. This model analyzes the composite properties and tracks the viscosity of the resin, which is a function of the degree of cure of the resin. The resin mixing model describes the mixing of “old” and “new” resin as plies are compacted. Validations were made by comparing image analysis data of fiber volume fraction in each ply for filament wound cylinders with the FEM results. The good agreement of these comparisons demonstrated that the FEM approach has can predict fiber volume fraction over a range of winding conditions. This approach, then, is an invaluable tool for predicting the effects of winding parameters on cylinder structural quality.     

Elastoplastic modeling of progressive interfacial debonding for particle-reinforced metal-matrix composites

Summary The purpose of this study is to model the elastoplastic behavior of particle-reinforced metal-matrix composites with particle-matrix interfacial debonding. The partially debonding process at the interface is represented by the debonding angles. The equivalent orthotropic elastic moduli are constructed for the debonded yet isotropic particles to characterize the reduction of the load-transfer ability in the debonded directions. To simulate the debonding evolution and the transition between various debonding modes, the volume fractions of various particles are expressed in terms of the Weibull's statistical functions. Micromechanical homogenization procedures are utilized to estimate the effective moduli and the overall yield function of the resultant multi-phase composites. The associative plastic flow rule and isotropic hardening law are postulated based on the continuum plasticity theory. The effects of partially interfacial debonding on the overall yield surfaces and stress-strain relations of the composites are investigated and illustrated via numerical examples as well.     

Analytical modeling of Raman lidar return,including multiple scattering

An analytical approach to modeling Raman lidar return with multiple scattering in presented. This approach is based on a small-angle quasi-single-scattering approximation developed earlier for elastic lidar sounding. An approximation of isotropic backscattering for the Raman-scattering case is proposed and tested. The computed results are presented and compared with known data. The approximation was found to be quite simple and provided a high accuracy of Raman lidar return calculations.     

Dynamic behavior of delaminated plates considering progressive failure process

The paper is to study on dynamic response behavior of the delaminated composite plates considering progressive failure process. A formula of element stiffness and mass matrices for the composite laminates is deduced by using the first-order shear deformation theory combined with the selecting numerical integration scheme. A damping model is constituted by a generalized orthogonal damping model on basis of Adams' strain energy method. A virtual interface linear spring element is also employing for avoiding the overlap and penetration phenomenon between the upper and lower sublaminates at the delamination region. The failure analysis method for the delaminated plates under dynamic loading is established by a modified Newmark direct integral method in conjunction with Tsai's failure criterion and corresponding stiffness degradation scheme. By some numerical examples, the effects of frequency of dynamic load, delamination length and location, and reduction of structure stiffness during the progressive failure process upon dynamic behavior of the delaminated composite plates are discussed. The method and conclusions would be useful for composite structures designers.     

Statistical modeling of the film-coating process in a vacuum

We describe the application of the method of statistical modeling through the use of an electronic digital computer to solve the problem of the distribution of film thickness for various geometric and physical parameters in the elements of a deposition system.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 17, No. 1, pp. 140–144, July, 1969.     

Computer modeling of the write process in perpendicular recording

A self-consistent computer simulation model utilizing a ring head and single layer recording medium has been developed to study perpendicular recording. In the model it is assumed that the medium switches uniformly following the volume averaged field. The optimum recording field obtained from the model calculation at various head-medium spacings permits an estimation of head saturation limitations. The roll-off curve, which is the linear superposition of the alternating voltage pulses, is in good agreement with experimental data. The write spacing loss is spacing dependent and is 120d/λ (dB) at contact for a typical recording case. For contact recording the imaging effect is important in the write process. Optimum recording at contact is shown to produce a high overshoot and sharp leading edge in the transition while leaving the magnetization level below the coercivity; this yields an apparent "negative" transition length in terms of the arctangent transition model.     

Dynamic effects in progressive failure of structures

This paper considers the behaviour of loaded structures when a member (or members) breaks prematurely. This behaviour consists of a transient vibration of the remaining damaged structure as it moves towards a new state of equilibrium. Static calculations may predict that the structure, with the failed members omitted, is capable of safely bearing the external loading. This static approach to the load-bearing capacity of the damaged structure is implicit in many design codes for redundant, damage-tolerant, structures. But it is shown here that transient overloads induced by the sudden fracture of a member may cause progressive fracture of other elements before a new equilibrium state is reached. Sometimes the cascade of fractures arrests; sometimes catastrophic failure of the whole structure occurs. Experiments with a redundant radially-tied (spoked-wheel) structure confirm the prediction that there are structures which are statically safe, but which are dynamically unsafe. In spite of the simplistic nature of these experiments they shed light on several aspects of dynamic failure which will be important for real engineering structures.     

Analytical modeling of the extrusion process using the electrostatics concept

Analytical and numerical modeling of metal forming processes has been studied by many researchers during the past decades. So far, several methods have been introduced to model material behavior during forming processes. In this research, the analogy between the classical plasticity theorem and electrostatic field line formation has been investigated for the first time. It is shown that the velocity potential function of movement in the plasticity and the charge-free electrostatic field are both in the form of Laplace’s relation. The aim of the current research is to model the metal forming processes using the equations governing the electrostatic fields. To demonstrate the effectiveness of the proposed method, the extrusion process is used as the benchmark. A three-dimensional CAD model of the deformation region is constructed and two different voltages are applied to the entry and exit sections based on the geometrical and extrusion parameters. Then, the Laplace’s relation is solved numerically and the relative extrusion pressure, particle flow path in the deforming region, and maximum plastic strain are calculated.To verify the proposed method, different sections are modeled and the results are compared with those in the literature. Finally, some experimental tests are conducted for the extrusion process of the round billets to complex sections. Significant agreement has been observed between the experimental results and the analytical ones. It is shown that the extrusion process could be modeled easily and accurately using the mathematical formalism of the electrostatics.