Unidirectional Materials Research Articles

Integration of cell formation and layout design in the unidirectional loop material handling environment

Integration of cell formation and layout design in the unidirectional loop material handling environment

Antiplane crack analysis of a functionally graded material by a BIEM

Elastostatic analysis of an antiplane crack in a functionally graded material (FGM) is performed by using a hypersingular boundary integral equation method (BIEM). An exponential law is applied to describe the spatial variation of the shear modulus of the FGM. A Galerkin method is applied for the numerical solution of the hypersingular traction BIE. Both unidirectional and bidirectional material gradations are investigated. Stress intensity factors for an infinite and linear elastic FGM containing a finite crack subjected to an antiplane crack-face loading are presented and discussed. The influences of the material gradients and the crack orientation on the stress intensity factors are analyzed.

Shear mode growth of short surface-breaking RCF cracks

Surface-breaking cracks occur in railheads due to unidirectional material flow caused by ratchetting from repeated contact loading. This type of crack is frequently observed on the gauge corner of high rails in curved track, where wheel sliding and high tangential forces occur. The present investigation examines the growth of a similar type of crack in a twin disc test specimen, using elastic–plastic finite element calculations and fracture mechanics theory. A two-dimensional finite element (FE) model was used to simulate the rolling contact fatigue twin disc test and crack behaviour for four short cracks: 0.05, 0.1, 0.2 and 0.4 mm. The results from the FE calculations were used in a Hobson-based, elastic–plastic fracture mechanics model. The Pineau criterion was employed to determine the governing crack growth mechanism, either shear or tensile. Net crack growth rate was defined as the difference between the rate of advance at the crack tip and crack mouth truncation by wear. Shear crack growth dominated for all four crack lengths and the net crack growth rate was greater than zero. The paths of crack advance were calculated for each crack; they show that the three shortest continue to grow parallel to the surface of the disc. The 0.4 mm crack, however, grows upward; thus, it may lead to spalling failure. The results are compared with others from similar twin disc tests and discussed in terms of their application to real train traffic situations where crack coalescence may occur.

A Multi-Commodity Network Design Approach For Automated Guided Vehicle Systems

This paper presents a multi-commodity network design approach to solve the problem of simultaneously locating I/O stations and determining the orientation of path segments in a flexible manufacturing environment. A unidirectional material handling system is assumed, as they are more common in industry than bi-directional systems mainly due to their simpler control. The problem is modeled using a mixed integer linear program with the objective of minimizing material handling costs. The model is used at the design stage of the system.

A review of the cutting of composite materials

AbstractThe increased use of composite materials has led to an increase in demand for facilities to machine them. There are significant differences between the machining of metals and alloys and that of composite materials, because composites are anisotropic, inhomogeneous and are mostly prepared in laminate form before undergoing the machining process. In most cases, traditional metal cutting tools and techniques are still being used. While the process of metal cutting has been well researched over the years, relatively little research has been carried out on the cutting of composite materials. This paper presents a brief review of research on the cutting of fibre reinforced polymer (FRP) composites and medium-density fibreboard (MDF). Most of the research published is concentrated on the chip formation process and cutting force prediction with unidirectional FRP materials. A review of some recent research on the prediction of cutting forces for MDF is also presented.

A review of the cutting of composite materials

The increased use of composite materials has led to an increase in demand for facilities to machine them. There are significant differences between the machining of metals and alloys and that of composite materials, because composites are anisotropic, inhomogeneous and are mostly prepared in laminate form before undergoing the machining process. In most cases, traditional metal cutting tools and techniques are still being used. While the process of metal cutting has been well researched over the years, relatively little research has been carried out on the cutting of composite materials. This paper presents a brief review of research on the cutting of fibre reinforced polymer (FRP) composites and medium-density fibreboard (MDF). Most of the research published is concentrated on the chip formation process and cutting force prediction with unidirectional FRP materials. A review of some recent research on the prediction of cutting forces for MDF is also presented.

Complete In-plane Elastic Characterisation under Tensile Tests of Angle-Ply Laminates Composed of Polymer-Matrix Layers

In this paper we present a new strategy to completely characterise the in-plane elastic properties of a large range of angle-ply laminates using only unidirectional tests. We consider laminates having the same number of identical plies in the α and – α directions. This new method uses some preceding results found by Verchery for orthotropic laminates, namely the conditions of existence of a specific direction ω, in which the shear-extension coupling is null. The characterisation of the laminate is then made using the results of three tensile tests: two in the orthotropy axes, and the third one in the ω direction, in order to have always a pure one-dimensional state of stress. We show that for the most common unidirectional fibre-reinforced materials, the angle ω is, in most cases, close to the α direction of the fibres. This result permits a complete experimental characterisation of the laminate, which then does not need any a priori knowledge of the elastic properties of the elementary layer. In addition, it provides a simple method to verify the predictions of the laminate behaviour obtained by the Classical Laminated Plate Theory (CLPT) when the elementary layer is completely known. The paper ends with numerical examples and with the results of some experimental tests.

Cell formations in the uni-directional loop material handling environment

The formation of machine cells and part families in a uni-directional loop material handling facility has been considered in this paper. The decision parameters include demand data and routing sequence of parts, processing times, unique and duplicated machine types, machine capacities or workload, and the uni-directional uni-cyclic material handling facilities. In this unique cell formation process, a divide and conquer algorithm is proposed to quickly estimate the intercell material movement cost that includes both forward flow and backward flow, instead of the typical measure that minimizes the number of intercell movements between machine cells. The detail of the proposed three-phase approach and the significance of material flow features are illustrated with two examples. Additional computational studies include 18 instances that are adapted from the literature, with the large data set up to 37 machines and 45 part types. The experimental results indicate that the approach is efficient and effective for performing cell formations in this particular material handling environment. The cell formations, which are developed by the criteria of minimizing the number of intercell movements and minimizing the actual intercell flow cost, are also compared.

Micromechanics models for non-linear behavior of piezo-electric fiber reinforced composite materials

Abstract Two non-linear micromechanics models, referred to as `XY piezo-electric fiber reinforced composite (PFRC) non-linear model' and `YX PFRC non-linear model', are proposed for predicting the non-linear behavior of unidirectional PFRC materials, which are subjected to a high monotonic electric field. These two models are developed following the similar procedures for the XY model and YX model, proposed previously for the linear properties of woven composite materials. Emphasis in this paper is placed on the lowest order of non-linearities, that is, dependent variables depend quadratically on independent variables. The required closed-form formulas for the effective electroelastic constants of PFRC materials are obtained using the non-linear constitutive equations for purely piezo-electric materials and the iso-field assumptions. The single- and double-loading conditions are introduced when deriving these closed-form formulas. For the sake of validating the present non-linear micromechanics models, a numerical study is carried out for four different types of materials including LiNbO3/piezo-polymer polyvinylidene fluoride (PVDF), PZT-5H/PVDF, PZN-4.5%PT/PVDF and PZT-7A/Araldite D PFRC materials. It is noted that the predicted variation trends of T3, D3 with S3 and E3 for the first three PFRC materials are consistent with those for the corresponding purely piezo-electric ceramics or crystals. For the effective properties along the fiber direction, the relevant closed-form formulas obtained using the present non-linear models are exactly the same as those derived based on the rule of mixtures. The effective linear constants C11, C12, C13, C22, C23, e31, e32, e33 and e33 predicted using the present models are consistent with those proposed by other researchers. For PZT-7A/Araldite D PFRC materials, the predicted results of S11+S12 and e33T/eo correlate well with the measured results available in the literature.

Scheduling for improving productivity of the automated manufacturing system

In this paper jobshop scheduling problem was considered on automated manufacturing systems with the closed loop and unidirectional material handling system. The objective of this research is to develop and evaluate heuristic scheduling procedures to improve productivity by minimizing makespan. Especially travel time of material handling system as well as processing time was considered in the proposed alogorithm. The heiristic algorithms are implemented for various cases. The results show that the proposal algorithms provide better solutions in productivity, frequency, job waiting time and the number of waiting jobs than the random scheduling algorithm.

Experimental and theoretical studies of fabric compaction behavior in resin transfer molding

Experimental and theoretical studies have been conducted on the compaction behavior of fabric preforms in the resin transfer molding process. Compaction experiments were carried out for three types of preforms, i.e. continuous strand mats, plain woven fabrics, and unidirectional knitted materials. The major factors contributing to the preform compaction behavior were identified. Then, in-situ contact pressure between adjacent fabric layers during compaction was measured using a TekScan system. The results of the pressure measurement motivated the improvement in the existing compaction model. Thus, the theoretical part of this paper focuses on modeling the compaction behavior of woven-fabric preforms with uniform, linear, and sinusoidal inter-layer contact pressure distributions. Analytical expressions for the relations among the fiber volume fraction, the applied compressive force, and the preform thickness reduction, have been established. Finally, the improved model is applied to predict the geometry of the resin channels of the preform under compaction. This knowledge can be readily used for evaluating the resin permeability as a function of applied pressure in the resin transfer molding process.

A nonlinear compaction model for fibrous preforms

Based on the mechanics of porous media and physical insight gained from experimental observation, a model for predicting the nonlinear compaction of fibrous preforms in the resin transfer molding process is developed. A key physical constant — namely, preform bulk compressibility — is proposed to establish the relationship between the applied pressure and the preform bulk volume. The preform bulk compressibility is a function of fiber volume fraction and five parameters — the initial fiber volume fraction, the final (maximum attainable) fiber volume fraction, the initial pore volume compressibility, the fiber compressibility, and an empirical index. Results of compaction experiments on plain-woven fabric preforms and unidirectional non-woven materials support the validity of the model. Excellent agreement between theory and experiments has been obtained. The present model provides for fibrous preforms a nonlinear constitutive law whose coefficients can be physically interpreted.

Operational Decisions in AGV-Served Flowshop Loops: Fleet Sizing and Decomposition

This paper considers design and operational issues that arise in repetitive manufacturing systems served by automated guided vehicles (AGVs) in loop layouts with unidirectional material flow. Such systems are in widespread industrial use, and play an important role in modern manufacturing environments. The objective considered is the minimization of AGV fleet size, given the minimum steady state cycle time required to produce a minimal job set (or equivalently, given the maximum throughput rate). We also study whether the decomposition of a large AGV-served flowshop loop into several smaller loops improves productivity. The original loop and the decomposed design are compared with respect to the minimum cycle time needed for the repetitive manufacture of a minimal job set. When there are three or more machines in the loop, finding the optimal cycle time is an intractable problem. We therefore use the genetic algorithm developed in the companion paper [17], to identify whether the original or the decomposed design is more efficient. Our study suggests that many systems perform more productively as a result of decomposition. Finally, we discuss a joint sequencing issue that arises in decomposed systems with limited buffers between the loops, and analyze the tractability of all the relevant joint sequencing problems.

Operational Decisions in AGV-Served Flowshop Loops: Scheduling

This paper considers operational issues that arise in repetitive manufacturing systems served by automated guided vehicles (AGVs) in loops with unidirectional material flow. The objective considered is the minimization of the steady state cycle time required to produce a minimal job set (or equivalently, throughput rate maximization). Our models allow for delays caused by AGV conflicts. We define and analyze three nondominated and widely used AGV dispatching policies. For each policy, we describe algorithms and intractability results for combined job scheduling and material handling problems. We describe a genetic algorithm that estimates the cycle time within 5% on average for instances with up to 10 machines and four AGVs. Some related fleet sizing and loop decomposition issues are discussed in the companion paper [19].

Configuring layout in unidirectional loop manufacturing systems

This paper addresses the layout configuration problem of workstations in a unidirectional loop manufacturing system. In this popular unicyclic material handling facility, the material transporters depart from the input/output station, traverse each workstation exactly once, and then return to the input/output station. We show the important properties of this unidirectional material handling loop network. Based on these characterizations, both heuristic and branch-and-bound algorithms are proposed to solve such NP-complete layout problems. Computational experiments with problem sizes up to 100 workstations are reported. Both the problem size and the material flow density have been found to affect the solution quality and the computational efficiency. The proposed methods appear efficient and effective for solving these layout problems.

Wear behaviour of HVOF thermally sprayed WC–14Co–3Cr coatings

The present study describes the tribological properties of thermal sprayed WC–14Co–3Cr (wt-%) coatings produced by using the high velocity oxyfuel (HVOF) deposition process. The coatings have been characterised with respect to their chemical composition and microstructural features (SEM-EDX) and roughness (profilometry). Experiments using a tribometer (pin on disk configuration) under non-lubricating conditions have been performed with the aim of determining the wear resistance of these coatings. To ensure reproducibility of the wear tests, the coated samples have been ground to an average superficial roughness Ra of 0·230 μm. Steel balls of AISI 1086, 6 mm in diameter, were tested against coated and uncoated samples. The morphology of the wear tracks on both pins and discs was characterised using SEM and three-dimensional profilometry. An average friction coefficient of 0·596 and average wear rate of 1·21 × 10-4 mm3 Nm-1 werefound for the tribological pair AISI 1086 pin/ WC–14Co–3Cr coated disc. For this pair a mixed wear mechanism of both abrasive and adhesive wear was observed for the pin with unidirectional material transfer from the pin toward the coating.

Stamping of continuous fiber thermoplastic composites

Despite demonstrated success in low volume aerospace and defense applications, structural composites remain at the periphery of high volume industries such as construction, automotive, and consumer goods because of long cycle times. Stamping provides a means of making composite sheet products at rates ten to a hundred times faster than any existing continuous fiber processes. However, to make composites stamping a viable process, one must understand how the combination of fabric architecture, tool design, and process conditions interact to produce a part free of wrinkling and tearing. In this paper, the effect of temperature, stamping rate, and boundary constraints on the material deformation is presented. The focus of this study is a co-mingled glass/polypropylene fabric, in the form of a layer of unidirectional yarns held together by stitches. The results show that temperature variations have the greatest effect on deformation. In addition, a finite element model of parallel strips with linear constraints was shown to successfully simulate the sliding deformation or draw-in of the stitched unidirectional material.

Estimation of limit strains in disk-type flywheels made of a compliant elastomeric matrix composite undergoing radial creep

Fiber reinforced elastomeric matrix composites (EMCs) offer several potential advantages for construction of rotors for flywheel energy storage systems. One potential advantage, for safety considerations, is the existence of maximum stresses near the outside radius of thick circumferentially wound EMC disks, which could lead to a desirable self-arresting failure mode at ultimate speeds. Certain unidirectionally reinforced EMCs, however, have been noted to creep readily under the influence of stress transverse to the fibers. In this paper, stress redistribution in a spinning thick disk made of a circumferentially filament wound EMC material on a small rigid hub has been analyzed with the assumption of total radial stress relaxation due to radial creep. It is shown that, following complete relaxation, the circumferential strains and stresses are maximized at the outside radius of the disk. Importantly, the radial tensile strains are three times greater than the circumferential strains at any given radius. Therefore, a unidirectional EMC material system that can safely endure transverse tensile creep strains of at least three times the elastic longitudinal strain capacity of the same material is likely to maintain the theoretically safe failure mode despite complete radial stress relaxation.

Effect of Fiber Waviness on the High-Strain-Rate Behavior of Composites

The effect of fiber waviness on the high-strain-rate behavior of carbon/epoxy composite materials was investigated. Falling weight impact and Split Hopkinson Pressure Bar systems were developed for basic characterization of unidirectional aligned composites in compression at strain rates up to 1800 s-1. Seventy-two and forty-eight-ply unidirectional carbon/epoxy (IM6G/3501-6) laminates loaded in the longitudinal and transverse directions were characterized. Compression tests on 45° off-axis laminates of the same unidirectional material were also conducted to obtain the in-plane shear stress-strain behavior. Strain rates over a wide range, from quasi-static up to 1800 s-1, were recorded. An incremental analysis was then used to predict the high-strain-rate behavior of composites with fiber waviness using the results obtained from the strain rate characterization. It is shown that, under longitudinal compressive loading, the nonlinear stress-strain behavior shows significant stiffening as the strain rate increases due to the shear component involved.

Strain Rate Effects on the Transverse Compressive and Shear Behavior of Unidirectional Composites

Methods for dynamic characterization of composite materials were extended and applied to the study of strain rate effects under transverse compression as well as shear. Falling weight impact and Split Hopkinson Pressure Bar systems were developed for dynamic characterization of composite materials in compression and shear at strain rates up to 1800 sO. Strain rates below 10 s5l were generated using a servohydraulic testing machine. Strain rates between 10 sol and 300 sol were generated using the drop tower apparatus. Strain rates above 500 s-l were generated using the Split Hopkinson Pressure Bar. Seventy-two and forty-eight ply unidirectional carbon/epoxy laminates (IM6G/3501-6) loaded in the transverse direction were characterized. Off-axis (15°, 300, 450 and 600) compression tests of the same unidirectional material were also conducted to obtain the in-plane shear stress-strain behavior. Strain rates over a wide range, from 10-4 s-1 (quasi-static) up to 1800 s-1, were recorded. The 90-degree properties, which are governed by the matrix,show an increase in modulus and strength over the static values but no significant change in ultimate strain. The stress-strain curve stiffens as the strain rate increases. This stiffening behavior is very significant in the nonlinear region for strain rates between 10-4 s-I and 1 s51. For strain rates above 1 s-1, the stress-strain behavior continues this stiffening trend until it is almost linear at a strain rate of 1800 s-1. The shear stress-strain behavior, which is also matrix-dominated, shows high nonlinearity with a plateau region at a stress level that increases significantly as the strain rate increases.