Influence of injection molding on the flexural strength and surface quality of long glass fiber-reinforced polyamide 6.6 composites

The objective of this paper was to process a polyamide 6.6 composite reinforced with long glass fibers (50 wt.%) using a design of experiments to determine the processing conditions that simultaneously maximize the flexural strength and the surface quality of the molded composite. The analyzed factors were the barrel temperature profile, the injection speed and the screw speed. To maximize the flexural strength response variable and the surface quality, all studied parameters should be maintained at the higher levels. The analysis of microstructure of composites molded demonstrates that this combination of parameters promotes a greater orientation of the fibers in the outer layers, as well as prevents the migration of glass fiber to surface of the composite. This kind of microstructure is favorable to a better surface quality, and a greater flexural strength.     

The characterization of low cost fiber reinforced thermoplastic composites produced by the DRIFT™ process

A new, low cost process for hot-melt impregnation of continuous reinforcing fibers with thermoplastic polymers is described. This technique can be used to fabricate various product forms including discontinuous, long-fiber products for compression molded parts, continuous fiber products for pultrusion, filament winding, and woven fabric applications. Mechanical data are presented for composites with various fiber and polymer combinations. Effects of fiber orientation and length on mechanical properties are discussed, and the effect of fiber–polymer bonding on impact strength and microstructure are shown. It is shown that the low cost and high performance achieved with this approach has the potential to expand applications of thermoplastic composite materials.     

Prediction of fiber orientation structure for injection molded short fiber composites

Structure of fiber orientation in injection molded short fiber composites is predicted by the numerical analysis. To analyze the packing stage as well as the filling stage, a compressible generalized Hele-Shaw model is adopted. A numerical scheme free from coordinate transformation is developed for three-dimensional shell-like geometry. Flow-induced fiber orientation can be predicted by solving evolution equations for the orientation tensor with a suitable closure approximation. Fibers are mainly oriented toward the flow direction near the top cavity wall due to high shear rates, while they are randomly oriented near the centerline of cavity where low shear rates prevail. Thus, the molded parts show the skin-core structure of orientation. Structure of fiber orientation continues to change during the packing stage due to additional velocity gradients – which are likely to align fibers more towards the flow direction. Electronic Publication     

Characterization of a peek composite segmental bone replacement implant

A composite material of polyetheretherketone and short, chopped E-glass fibers was used to produce a segmental bone replacement implant. Problems with current metallic implants include stress-shielding of the surrounding bone and subsequent loosening of the implant. A better match between the bulk material properties of the implant and the bone it replaces can decrease the occurrence of these problems. Composite materials were chosen because their properties can be tailored to match the requirements. Material selection was accomplished with the aid of modeling software, which predicted the composite properties based on its composition and fiber directional parameters. Prototype parts were completed through a series of in-house molding and machining processes. Sections complete with an embedded metallic porous surface were tested to measure the strength of the attachment of the surface. The molded parts were characterized both destructively and nondestructively. The results of tensile tests performed on molded parts were comparable to those using commercially supplied samples. The fiber orientation was measured to verify the random positioning of fibers throughout the part, as assumed in the initial material selection. Ultrasonic C-scanned images confirmed that the molded parts had a very low density of air pockets or voids.     

氧化铝短纤维增强铝基复合材料的蠕变破坏行为

研究了挤压铸造Al₂O₃短纤维增强铝基复合材料在350℃恒应力条件下的蠕变行为。蠕变试验过程中采用中断实验的方法对复合材料的显微组织进行观察,发现复合材料在蠕变过程中纤维发生断裂,弱界面发生破坏以及基体合金在应力作用下发生变形。根据复合材料在蠕变三个阶段中显微组织的变化情况,对其宏观蠕变行为进行了分析,认为位错在复合材料中滑移和攀移控制整个蠕变过程,并提出了短纤维增强金属基复合材料的蠕变断裂机理,合理地解释了复合材料的蠕变过程。      

Effect of fiber array on damping behaviors of fiber composites

This study aims to investigate the fiber array effect on modal damping behaviors of fiber composites. Three different fiber arrays, i.e., square edge packing (SEP), square diagonal packing (SDP), and hexagonal packing (HP), were considered to represent the microstructures of the unidirectional composites. Repeating unit cells (RUCs) suitable for describing the characteristics of the microstructure were adopted in the generalized method of cell (GMC) micromechanical analysis. The energy dissipation concept was then employed to calculate the specific damping capacities of composites in the material principal directions. The specific damping capacities obtained from micromechanical analysis were regarded as the equivalent damping properties homogenizing in the composites. In conjunction with the modal shapes of the composite structures determined from the finite element analysis, the specific damping capacity was extended to characterize the corresponding modal damping of the composite rods and plates. Both free–free and clamped-free boundary conditions were taken into account in the composite structures. Results indicated that the structures constructed from the composites with SDP fibers exhibit better damping behaviors than the other two cases.     

Factors determining the performance of thermoplastic polymer/wood composites; the limiting role of fiber fracture

Thermoplastic polymer/lignocellulosic fiber composites were prepared with a considerable range of matrices and fibers in an internal mixer. Tensile properties were determined on bars cut from compression molded plates. Local deformation processes initiated around the fibers were followed by acoustic emission testing supported by electron and polarization optical microscopy. The analysis of results proved that micromechanical deformation processes initiated by the fibers determine the performance of the composites. Debonding usually leads to the decrease of composite strength, but decreasing strength is not always associated with poor adhesion and debonding. The direction of property change with increasing wood content depends on component properties and interfacial adhesion. Good interfacial adhesion often results in the fracture of the fibers. Depending on their size and aspect ratio, fibers may fracture parallel or perpendicular to their axis. At good adhesion, the maximum strength achieved for a particular polymer/wood pair depends on the inherent strength of the fibers, which is larger for perpendicular than parallel fracture. Inherent fiber strength effective in a composite depends also on particle size, larger particles fail at smaller stress, because of the larger number of possible flaws in them. A very close correlation exists between the initiation stress of the dominating local deformation process and composite strength proving that these processes lead to the failure of the composite and determine its performance.     

Effect of physicochemical structure of natural fiber on transverse thermal conductivity of unidirectional abaca/bamboo fiber composites

To study the effect of the microstructure of natural fiber on the transverse thermal conductivity of unidirectional composite, abaca and bamboo fibers were unidirectionally aligned to fabricate epoxy composites by a resin transfer molding (RTM) technique. The transverse thermal conductivity of these two types of composites was measured in a steady-state platform. X-ray diffractometer and scanning electron microscopy were applied to analyze the microstructure and morphology of both fibers and composites. The results indicated that the transverse thermal conductivity showed two types of tendencies with fiber content increasing: increasing for bamboo fiber composites, and decreasing for abaca fiber composites. The microstructure and theoretical analysis suggest that the lumen structure plays a great role rather than crystal structures and chemical compounds on the transverse thermal conductivity of unidirectional composites, which is useful for further development and design of natural fiber reinforced composites with better thermal insulation property for people’s daily life.     

Optimization of course locations in fiber-placed panels for general fiber angle distributions

Fiber-reinforced composites are usually designed using constant fiber orientation in each ply. In certain cases, however, a varying fiber angle might be favorable for structural performance. This possibility can be fully utilized using tow placement technology. Because of the fiber angle variation, tow-placed courses may overlap and ply thickness will build-up on the surface. This thickness buildup affects manufacturing time, structural response, and surface quality of the finished product.     

Microstructure variability and macroscopic composite properties of high performance fiber reinforced cementitious composites

High performance fiber reinforced cementitious composites have made major advances in recent years, to the point where they are being adopted in building and bridge constructions. The most significant advantage of HPFRCC over conventional concrete is their high tensile ductility. However, the tensile strain capacity has been observed to vary, most likely as a result of the variability of the microstructure derived from the processing of these materials.This paper describes the composite property variability, as well as the variability of the material microstructure. Scale linkage is discussed. In particular, the tensile stress–strain curves, and the crack pattern on uniaxially loaded specimens are presented. The treatment of random fibers in micromechanical models, and tailoring of matrix flaw size distribution for saturated multiple cracking are examined. It is suggested that robust composite properties can be achieved by deliberate control of microstructure variability. Some open issues concerning the randomness of microstructures and possibly related macroscopic behavior are also identified. Further gains in composite property control may be expected from improvements in characterization and modeling of the microstructure randomness.     

非均质秸秆纤维复合材料保险杠蒙皮刚度分析

目的 研究非均质秸秆纤维复合材料保险杠蒙皮的刚度性能.方法 采用试验与模拟分析的方法,通过共混挤出与化学发泡注塑工艺制备微发泡秸秆纤维/聚丙烯(SF/PP)复合材料试样,通过试验测试非均质结构试样的力学性能与微观结构,通过有限元分析手段建立非均质微发泡秸秆纤维/PP复合材料结构分析模型,并分析非均质材料保险杠蒙皮的刚度...     

Control of strength anisotropy of metal matrix fiber composites

Summary This paper examines theoretically the stress distribution around fiber breaks in a unidirectional reinforced metal matrix composite, subjected to axial loading when plastic yielding of the matrix is allowed to occur. The composites considered have a ductile interphase, bonding the matrix to the fiber. The likelihood of failure of a fiber adjacent to the existing broken fiber is considered. Detailed and systematic results are given for composites with a wide range of fiber volume fractions, Young's modulus of the fibers and the matrix, interphase properties and Weibull modulus for the strength of the fibers. The objective is the optimization of these material and geometric variables to ensure global load sharing among the fibers in the longitudinal direction, which will give the composite good longitudinal strength. Calculations are carried out for transverse loading of the composite to determine the effect of the ductile interphase on the yield strength. Characteristics of the ductile interphase are determined that will provide good longitudinal strength through global load sharing and a relatively high yield strength in the direction transverse to the fibers. This, in turn, will allow control of the strength anisotropy of uniaxially reinforced metal matrix composites.     

Electrical properties of hybrid carbon black/carbon fiber polypropylene composites

The electrical conductivity and morphology of injection molded polypropylene based composites containing two conductive fillers, carbon black (CB) and carbon fibers (CF) were studied. Injection moldings containing both, CB and CF, where the content of each filler was above its own percolation threshold, resulted in similar or lower values of overall composite volume resistivity compared with the resistivity of systems filled only with CB at the corresponding content. However, the resistivity of two-filler systems is always higher than the resistivity of systems filled only with CF at the corresponding content. The morphology and fiber length analysis of the injection molded composites are quite intriguing. Fiber orientation in the injection molded two-filler systems was found to be almost perpendicular to the melt flow direction, with no significant skin-core fiber orientation patterns, contrary to the typically observed fiber orientation in injection molded fiber filled composites. Moreover, the CF breakage in the presence of the CB was found more intense than when just CF is used, resulting in shorter fibers with narrower length distributions. This unexpected fiber behavior is responsible for the unexpected electrical behavior. However, the coexistence of CB and CF electrically conductive networks, supporting each other, was confirmed, in spite of the mechanical disturbances caused by the presence of fibrilar and particulate fillers.     

碳密封涂覆光纤在编织复合材料中的应用

碳密封涂覆光纤由于具有良好的强度性能和抗疲劳能力,已应用于海底光缆、军用制导光纤以及苛刻环境下的光纤传感系统中。本文作者研究了碳密封涂覆光纤在三维碳/环氧编织复合材料中的应用。将该光纤成功地编入编织复合材料试件并成型。对该光纤在编织前、后以及成型后的光学性能进行了测试。设计了使用碳密封涂覆光纤的偏振式光纤传感器,并用其对试件处于弯曲受力状态的情况进行了测试。     

Stress concentration factors in two-dimensional composites: effects of material and geometrical parameters

In the study of fracture processes in composite materials, the interactions between broken and intact fibers are of critical importance. Indeed, the redistribution of stress from a failed fiber to its unfailed adjacent neighbors, and the stress concentration induced in these, determine the extent to which a break in one fiber will cause more breaks in neighboring fibers. The overall failure pattern is a direct function of the stress concentration factors (SCFs). In this paper, we propose a new model for the SCFs in two-dimensional unidirectional composites containing broken fibers. A closed-form expression is derived for the SCF profiles as a function of material and geometrical parameters. The model differs significantly from earlier schemes, as the local effect of a fiber break on nearest neighbors is much milder than previously calculated, both as a function of the inter-fiber distance and of the number of adjacent broken fibers. Comparison with experimental results for silicon-carbide/epoxy composites demonstrates the validity of the proposed scheme. Since the overall fracture pattern in fiber composites is a direct function of the SCFs, the model may help shed light on fracture nucleation and growth in composites.     

An experimental study of the water-assisted injection molding of glass fiber filled poly-butylene-terephthalate (PBT) composites

The purpose of this report was to experimentally study the water-assisted injection molding process of poly-butylene-terephthalate (PBT) composites. Experiments were carried out on an 80-ton injection-molding machine equipped with a lab scale water injection system, which included a water pump, a pressure accumulator, a water injection pin, a water tank equipped with a temperature regulator, and a control circuit. The materials included virgin PBT and a 15% glass fiber filled PBT composite, and a plate cavity with a rib across center was used. Various processing variables were examined in terms of their influence on the length of water penetration in molded parts, and mechanical property tests were performed on these parts. X-ray diffraction (XRD) was also used to identify the material and structural parameters. Finally, a comparison was made between water-assisted and gas-assisted injection molded parts. It was found that the melt fill pressure, melt temperature, and short shot size were the dominant parameters affecting water penetration behavior. Material at the mold-side exhibited a higher degree of crystallinity than that at the water-side. Parts molded by gas also showed a higher degree of crystallinity than those molded by water. Furthermore, the glass fibers near the surface of molded parts were found to be oriented mostly in the flow direction, but oriented substantially more perpendicular to the flow direction with increasing distance from the skin surface.     

Explicit cross-link relations between effective elastic modulus and thermal conductivity for fiber composites

Explicit cross-link relations between effective elastic modulus and thermal conductivity for composites with different fiber orientation are derived with help of Mori-Tanaka micromechanical method. Numerical cross-link relations are also established by digital-image-based finite element method, and they compare favorably with the analytical cross-link relations especially for the composite with aligned fibers and planar randomly oriented fibers. Both analytical and numerical cross-link relations agree well with the experimental results available in the literature. For the composite with space randomly oriented fibers, the numerically obtained cross-link relations are insensitive to the fiber’s shape, and the analytical cross-link relations are weakly dependent on fiber’s shape. In sum, the sensitivity of cross-link relations to the fiber’s shape depends on the extent of anisotropic behavior of fiber composites. Such cross-link relations can be potentially applied for predicting the difficult-to-measure elastic modulus from the measured thermal conductivities.     

Experimental determination and modeling of thermal conductivity tensor of carbon/epoxy composite

The modeling of thermal behavior of composite parts during their forming requires an accurate knowledge of their thermo-physical properties. Because of the heterogeneous nature of composites, the thermal conductivity tensor appears to be the most tricky to determine experimentally but also to model. A wide range of experimental methods can be found in the literature in order to measure either in-plane or transverse conductivity of composite parts, but very few succeed in performing it on dry preform or uncured laminates. In this study, the effective thermal conductivity tensor of carbon/epoxy laminates is investigated experimentally in the three states of a typical LCM-process: dry-reinforcement, raw and cured composite. Samples are made of twill-weave carbon fabric impregnated with epoxy resin. The transverse thermal conductivity is determined using a classical estimation algorithm, whereas a special testing apparatus is designed to estimate in-plane conductivity for different temperatures and different states of the composite. Experimental results are then compared to modified Charles & Wilson and Maxwell models. The fiber crimping of a ply is also taken into account in modeling. The comparison shows clearly that these models can be used to predict the effective thermal conductivities of woven-reinforced composites provided that the material properties are well known.     

Fabrication and characterization of fully biodegradable natural fiber-reinforced poly(lactic acid) composites

Polymer composites were fabricated with poly(lactic acid) (PLA) and cellulosic natural fibers combining the wet-laid fiber sheet forming method with the film stacking composite-making process. The natural fibers studied included hardwood high yield pulp, softwood high yield pulp, and bleached kraft softwood pulp fibers. Composite mechanical and thermal properties were characterized. The incorporation of pulp fibers significantly increased the composite storage moduli and elasticity, promoted the cold crystallization and recrystallization of PLA, and dramatically improved composite tensile moduli and strengths. The highest composite tensile strength achieved was 121 MPa, nearly one fold higher than that of the neat PLA. The overall fiber efficiency factors for composite tensile strengths derived from the micromechanics models were found to be much higher than that of conventional random short fiber-reinforced composites, suggesting the fiber–fiber bond also positively contributed to the composites’ strengths.     

纤维束分布对复合材料有效性能的影响

针对纤维束增强相在基体材料中的分布方式不同,建立了由固体基体和纤维束增强相两相介质组成的复合材料细观力学模型。假设该模型的细观结构呈周期性均匀分布,纤维束内的纤维接触是光滑的。采用二尺度展开法计算了复合材料的有效性能,得出了不同微结构分布的复合材料的刚度系数、横向弹性模量、泊松比和剪切模量随纤维束体分比的变化曲线,并将数值结果与实验数据进行了比较。研究表明,数值结果与实验数据有较好的吻合,增强相的分布直接影响到复合材料的力学性能。