Statistical characteristics of out‐of‐plane permeability for plain‐woven structure

Since out‐of‐plane permeability of fiber preforms is a function of the number and arrangement of stacked layers, either many layers of preforms or numerous experiments are required to obtain an exact out‐of‐plane permeability experimentally. The reason is that there exist nesting and phase shifting when the preforms are laid up. From a statistical viewpoint, the effect of the number of preform layers on the out‐of‐plane permeability was analyzed by adopting an analytical model proposed in this study. Numerical simulation for a unit‐cell constructed based on geometry of the preform was carried out to validate the analytical model as well as experimental measurements of the permeability. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

Influence of textile parameters on the in‐plane permeability characteristics of non‐crimped fabric preforms

Material parameters such as the permeability of dry reinforcing textiles are key variables for modern composite production using liquid composite molding (LCM) technique. Nowadays numerical filling simulations are required for predicting the mold filling behavior. Inaccurate predictions can lead to a high risk of air inclusion and corresponding need for cost‐intense revision of the mold design. Permeability values of the textiles used in the process are basic requirements for a numerical filling simulation, since the permeability is directly linked to the filling behavior. Nevertheless, the permeability values of non‐crimped fabrics (NCF) which are used for aerospace and automotive structures are rare. In this study the influence of textile parameters of NCF on the in‐plane permeability has been investigated using a capacitive in‐plane permeability measurement technology. The results show the influence of the roving filament number as well as the used stitch length on the in‐plane permeability. It is confirmed that the textile grammage is not affecting the in‐plane permeability of NCF reinforcements. The results of this study are valuable for textile selection with specific permeability data as well as for numerical filling simulations. POLYM. COMPOS., 37:1854–1863, 2016. © 2015 Society of Plastics Engineers     

Influence of out‐of‐plane fiber orientation on reaction‐to‐fire properties of carbon fiber reinforced polymer matrix composites

This work investigates the influence of the out‐of‐plane orientation of carbon fibers on the reaction‐to‐fire characteristics of polymer matrix composites. A deep insight into combustion processes is gained, which is necessary to fully understand and assess advantages of composites with out‐of‐plane fiber angles. Epoxy‐based Hexply 8552/IM7 specimens with primarily low fiber angles between 0° and 15° are characterized by cone calorimetry. Heat release during fire is greatly affected by the out‐of‐plane fiber angle because of the thermal boundaries created by the fibers. The advancement of the pyrolysis front during fire was determined from peak heat release rates and validated by temperature measurements along the back surface of the panels, representing a novel method of determining position‐dependent pyrolysis migration velocity. These measurements show a transverse shift in pyrolysis front velocity for increasing out‐of‐plane fiber angles. Pyrolysis pathways between the fiber boundaries facilitate faster combustion through the composite thickness, especially for increasing angles from 0° to 15°. It was determined that under the chosen conditions, the pyrolysis front advances approximately 4 times faster when propagating parallel to the fibers than perpendicular.     

A simplified in‐plane permeability model for textile fabrics

In this paper, a simplified in‐plane permeability model for textile fabrics is developed. The model is based on a rectangular unit cell geometry, the one‐dimensional Stokes equation for flow in the channels or gaps between fiber tows, and the one‐dimensional Brinkman equation for flow in fiber tows. Three different textile fabrics are considered in the model: plain woven, 4‐harness, and bidirectional stitched fiberglass mats. The model incorporates the effect of porosity changes on permeability of fiber preforms under compression, which usually occurs in the molding process. To verify the validity of the model, the theoretical values are compared with a set of permeability measurements. Good agreement is found between the model prediction and the permeability measurements in the porosity ranges of ϕ ≤ 0.59 for plain woven fiber mats, ϕ ≤ 0.60 for 4‐harness fiber mats and ϕ ≤ 0.62 for bidirectional stitched fiber mats.     

Effect of deformation on knitted glass preform in‐plane permeability

The excellent processing properties of knitted preforms for composite applications, as formability and permeability, are not sufficient to compensate the poor mechanical characteristics of the resulting material. Initial preform deformation is a way to improve these properties, but it modifies the permeability and changes the optimal infiltration conditions. This article presents an experimental setup and discusses the reliability of the permeability measurements. Experimental results show that the course‐wise permeability is significantly modified by the deformation of the fabric, whereas the wale‐wise permeability is quite insensitive to the deformation. In an equivalent isotropic system, the deformation essentially influences the anisotropy ratio. POLYM. COMPOS., 32:18–28, 2011. © 2010 Society of Plastics Engineers     

Influence of cure conditions on out‐of‐autoclave bismaleimide composite laminates

Bismaleimides (BMI) are thermosetting polymers that are widely used in the aerospace industry due to their good physical properties at elevated temperatures and humid environments. BMI‐based composites are used as a replacement for conventional epoxy resins at higher service temperatures. Out‐of‐Autoclave (OOA) processing of BMI composites is similar to that of epoxies but requires higher cure temperatures. Polymer properties such as degree of cure and crosslink density are dependent on the cure cycle used. These properties affect mechanical strength as well as glass transition temperature of the composite. In the current research, carbon fiber/BMI composite laminates were manufactured by OOA processing. The void content was measured using acid digestion techniques. The influence of cure cycle variations on glass transition temperature and mechanical strength was investigated. Properties of manufactured specimens were compared with that of conventional autoclave cured BMI composites. Laminates fabricated via OOA processing exhibited properties comparable to that of autoclave cured composites. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43984.     

Effects of postcure temperature variation on hygrothermal‐mechanical properties of an out‐of‐autoclave polymer composite

The effects of cure temperature variation on the properties of an out‐of‐autoclave polymer composite manufactured using Cycom 5320 8HS prepreg were investigated using different postcure temperatures of a two‐stage cure cycle. In addition, the effects of adverse environmental conditions on the cure temperature variation were studied by conditioning the samples in an environmental chamber until they reached moisture equilibrium. The state of cure was obtained using a differential scanning calorimeter and dynamic mechanical analyzer. The mechanical properties were obtained using short‐beam shear (SBS) and combined loading compression (CLC) test methods. The state of cure obtained showed increases in total heat of reaction, degree of cure, and glass transition temperature as the postcure temperature increased. The SBS and CLC strengths showed an increasing trend as postcure temperature increased. Good correlations were obtained between the material's cure temperatures, state of cure, and mechanical properties for room temperature dry and hot wet conditions. The study showed that the state of cure can be used to define, monitor, and verify the cure quality. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 3090–3097, 2013     

Evolution of mechanical properties during cure for out‐of‐autoclave carbon‐epoxy prepregs

Extent of cure and rheological properties were obtained for out‐of‐autoclave materials, Cycom 5320‐8HS and Cycom 5320‐PW, for the manufacturer recommended cure cycle using differential scanning calorimeter and encapsulated sample rheometer (ESR), respectively. Rheological properties from ESR were further used in designing the cure cycles to study the evolution of mechanical properties. Five panels were cured at different cure stages using the designed cure cycles and coupons were tested for short beam shear and combined loading compression properties at different cure stages. To correlate the mechanical properties with its respective glass transition temperature, dynamic mechanical analyzer was used to obtain the glass transition temperature for the coupons obtained from the respective panels. Statistical results showed significant difference in short beam shear and combined loading compression properties up to vitrification, however, no significant difference was observed on these mechanical properties after vitrification. The observed linear trend between degree of cure (DOC) and glass transition temperature (T_g) was validated using DiBenedetto relation. Linearly increasing trend between DOC and glass transition temperature (T_g) for different cure states suggests that both DOC and T_g can be used interchangeably to define the state of material. A good correlation was observed between material cure state and the mechanical properties. A mathematical model was also proposed to determine the short beam shear and combined loading compression properties based on material cure state. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41548.     

A gas flow method for determination of in‐plane permeability of fiber preforms

Resin flow plays a crucial role in many composite manufacturing processes. The most important parameters used in modeling and designing mold filling are the permeability of the fibrous preform, which is a kind of flow conductance and a property of the reinforcement, and the viscosity of the resin. The extent reaction, or degree of cure, is also important and causes change of chemical during mold filling. To determine the permeability of fiber preform searchers have been using liquid flow analysis. In this study, a new scheme for determining permeability using gas flow is proposed. In conventional liquid flow methods, radial propagation of the polymer into a porous medium is measured and used to determine permeability, whereas in the gas flow method, the several different preform geometries is measured and used. The effectiveness of the gas flow method was verified by comparing it with conventional methods.     

In‐plane compression of constrained preforms

Preforms constructed from a plain‐weave, glass fabric were compressed in‐plane within a fixture that mimicked the constraints of a closed mold. Typically, a gap was left between the bottom of the preform and the floor of the fixture; upon Initial compression, the preform slid within the fixture, which allowed the friction between the preform and the fixture wall to be measured. The preform began to compress as it contacted the floor of the fixture. The deformation was proportional to the applied stress until a critical stress was reached. Above this stress, the preform sustained damage in the form if localized buckling and a corresponding decrease in mechanical integrity. The in‐plane compressive behavior varied with system parameters, such as preform geometry, fabric orientation, and clamping stress and was shown to be strongly dependent on friction of the preform against the fixture wall. A model was developed to describe the contribution of preform friction with the fixture wall to the in‐plane compressive behavior of constrained preforms.     

Investigation of fiber orientation states in injection‐compression molded short‐fiber‐reinforced thermoplastics

Injection‐compression molding (ICM) has received increased attention because of its advantages over conventional injection molding (CIM). This article aims to investigate the effects of five dominating ICM processing parameters on fiber orientation in short‐fiber‐reinforced polypropylene (SFR‐PP) parts. A five‐layer structure of fiber orientation is found across the thickness under most conditions in ICM parts. This is quite different from the fiber orientation patterns in CIM parts. The fibers orient orderly along the flow direction in the shell region, whereas most fibers arrange randomly in the skin and the core regions. Additionally, the fiber orientation changes in the width direction, with most fibers arranging orderly along the flow direction at positions near the mold cavity wall. The results also show that the compression force, compression distance, and compression speed play important roles in determining the fiber states. Thicker shell regions, in which most fibers orient remarkably along the flow direction, can be obtained under larger compression force or compression speed. Moreover, the delay time has an obvious effect on the fiber orientation at positions far from the gate. However, the effect of compression time is found to be negligible. POLYM. COMPOS., 31:1899–1908, 2010. © 2010 Society of Plastics Engineers.     

Determination of in‐plane permeability of fiber preforms by the gas flow method using pressure measurements

A method is described for measuring the in‐plane permeability of orthotropic fibrous preforms using gas flow. The method is based on an optimization process between computed and measured pressures at various locations in the mold during steady state gas flow through the enclosed preform. The computed pressure is obtained by the control volume finite element method (CVFEM). This method was demonstrated by using a specially designed mold with multiple ports for gas injection and pressure measurement and it was shown that it can be implemented easily and yields consistent and reliable results.     

Friction and wear behavior of glass fabric/phenolic resin composites with surface‐modified glass fabric

Untreated, air‐plasma‐bombarded, and β‐aminoethyltrimethoxylsilane‐silanized glass fabric (GF) was used to prepare GF/phenolic composites by dip coating in a phenolic adhesive resin and successive curing. The tribo‐performances of these GF/phenolic composites sliding against AISI‐1045 steel were evaluated with a pin‐on‐disc wear tester. The chemical composition of the untreated and surface‐treated GF was analyzed with Fourier transform infrared spectroscopy and X‐ray photoelectron spectroscopy. The interfacial regions between the phenolic resin and GF and the worn surfaces of the composites were analyzed with scanning electron microscopy. The results show that the GF/phenolic composite with β‐aminoethyltrimethoxylsilane‐silanized GF had the highest load‐carrying capacity and best tribo‐performance, and it was followed by the composite with plasma‐treated GF. The improved tribo‐performance of the GF/phenolic composite made of surface‐treated GF was attributed to the strengthened interfacial bonding between the treated GF and the phenolic adhesive resin. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Damage development in a noncrimp‐glass fabric reinforced epoxy composite material

The relationship between textile architecture and the damage sequence under tensile loading has been investigated experimentally for a composite material reinforced with a noncrimp glass‐fiber textile of configuration [0°, +45°, 90°, −45°] stacking sequence based on epoxy resin matrix cured with high‐temperature hardener. The system chosen for this work consists of a bifunctional epoxy, diglycidyl ether of bisphenol A, cured with a tetrafunctional amine, diaminodiphenyl sulfone (DDS). This system ensures to obtain a rigid material with excellent mechanical properties in order to observe, analyze, and identify the process and progress of the generated damage and the failure mechanism which leads to the materials fracture. The properties have been studied for each ply direction at 0°, +45°, 90°, and −45° in order to make a comparative assessment of the influence of the polyester (PES) yarns in zig‐zag and unidirectional geometry, that hold together the four plies in the textile, in the composite damage generation. The laminates were uniaxially tensile loaded until final fracture occurred. It was found that PES threads have an effect on cracking progression depending on the textile orientation. POLYM. COMPOS., 2009. © 2009 Society of Plastics Engineers     

Influence of fiber orientation and fiber content on properties of sisal‐jute‐glass fiber‐reinforced polyester composites

The incorporation of natural fibers with polymer matrix composites (PMCs) has increasing applications in many fields of engineering due to the growing concerns regarding the environmental impact and energy crisis. The objective of this work is to examine the effect of fiber orientation and fiber content on properties of sisal‐jute‐glass fiber‐reinforced polyester composites. In this experimental study, sisal‐jute‐glass fiber‐reinforced polyester composites are prepared with fiber orientations of 0° and 90° and fiber volume of sisal‐jute‐glass fibers are in the ratio of 40:0:60, 0:40:60, and 20:20:60 respectively, and the experiments were conducted. The results indicated that the hybrid composites had shown better performance and the fiber orientation and fiber content play major role in strength and water absorption properties. The morphological properties, internal structure, cracks, and fiber pull out of the fractured specimen during testing are also investigated by using scanning electron microscopy (SEM) analysis. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 42968.     

Experimental study and finite element simulation of a glass fiber fabric shaping process

A finite element simulation is proposed for the shaping of glass fiber fabric. The overall mechanical behavior of the fabric is obtained by combining the tensile behavior of a single thread and the current position of threads in the fabric. The constitutive model for a single thread in tension is based on a statistical approach and is identified using tensile tests. Shear and tensile tests have been carried out on fabric specimens to demonstrate that the behavior of the fabric mainly results from the contribution of each thread, the sliding between fiber threads having a small effect on the energy for the deformation mechanism of these fabrics. A numerical model for the shaping process is obtained based on a finite element approach using three- and four-node membrane shell elements. The formulation accounts for the large displacements and large strains involved in the process as well as the mechanical behavior. A drawing simulation is presented in the case of an hemispherical punch. The comparison with experimental results obtained in this case shows good agreement.     

An Investigation on triaxial compression of flexible fiber packings

This article examines the mechanical response of flexible fiber packings subject to triaxial compression. Short fibers yield in a manner similar to typical granular materials in which the deviatoric stress remains nearly constant with increasing strain after reaching a peak value. Interestingly, long fibers exhibit a hardening behavior, where the stress increases rapidly with increasing strain at large strains and the packing density continuously increases. Phase diagrams for classifying the bulk mechanical response as yielding, hardening, or a transition regime are generated as a function of the fiber aspect ratio and fiber–fiber friction coefficient. Large fiber aspect ratio and large fiber–fiber friction coefficient promote hardening behavior. The positions of boundaries between different regimes depend on the confining pressure and fiber flexibility. The hardening packings can support much larger loads than the yielding packings, but larger internal axial forces within fibers and larger fiber–fiber contact forces occur.     

The influence of surface modifications of glass on glass fiber/polyester interphase properties

Unsized glass fibers and planar glass substrates were subjected to low temperature plasma or wet-chemical process to modify the fiber or substrate surface and thus influence the interphase properties of the glass/polyester system. Plasma-polymerized thin films (interlayers) of organosilicon monomers (hexamethyldisiloxane and vinyltriethoxysilane) were deposited in an RF helical coupling plasma system on the glass surface. Commercial silane coupling agent (vinyltriethoxysilane) was coated onto an unmodified glass surface from an aqueous solution. Bonding at the glass/interlayer interface was analyzed by employing a micro-scratch tester together with an optical polarizing microscope for the planar samples. The results revealed that the adhesion bonding could be controlled by plasma process parameters. Scanning electron and atomic force microscopies enabled characterization of the film surface morphology. Chemical composition and chemical structure of prepared interlayers were characterized using X-ray photoelectron and infrared spectroscopies. Microcomposites (macrocomposites) were tested to evaluate the interfacial shear strength (short-beam strength) of the glass fiber/polyester interphase using the microbond test (short-beam shear). Our study indicated that the most efficient interphase could be prepared by plasma polymerization or wet-chemical process using the vinyltriethoxysilane monomer. The short-beam strength was 110% higher than that for untreated fibers in both cases.