Fatigue resistance of continuous glass fiber/polypropylene composites: Temperature dependence

The effect of testing temperature on the fatigue resistance of continuous glass fiber/polypropylene (CGF/PP) composites was studied. Fatigue resistance curves (or S-N curves) were obtained at −40°C, 23°C and 50°C. Both on an absolute stress basis and on a normalized stress basis (with respect to the yield stress at the temperature considered), the S-N curves showed that CGF/PP composites had excellent fatigue performance at 23°C and that their performance was actually improved at −40°C (below T_g of the PP matrix). The S-N curves at 50°C showed that, although the composite flexural strength was reduced because of PP matrix softening, their fatigue performance remained relatively high, as it is controlled by the CGF reinforcement. Comparison with a CGF/thermoset isophthalic polyester composite of identical fiber architecture and similar flexural strength at 23°C indicated that the properties of the thermoplastic PP matrix provided improved fatigue resistance, both on an absolute and a normalized basis, especially below the glass transition temperature. It was concluded that the fact that the fatigue performance of the CGF/polyester composite is only weakly temperature-dependent, while that of the CGF/PP composite is strongly temperature-dependent, does not necessarily mean that it shows superior performance. Polym. Compos. 25:622–629, 2004. © 2004 Society of Plastics Engineers.     

Fatigue properties of hemp and glass fiber composites

The fatigue properties of nonwoven randomly oriented short hemp fiber mat and chopped strand mat (CSM) glass fiber reinforced polyester composites have been studied, mainly in tension–tension mode. Despite having poorer absolute fatigue strength, the hemp fiber composites exhibited less fatigue sensitivity as compared with the CSM glass fiber composites in tension–tension fatigue. This could be correlated with the lower stiffness degradation observed during fatigue of the hemp fiber composites as compared with the glass fiber composites at the same normalized peak stress levels. Also, images recorded during fatigue loading showed that the hemp fiber composites were better at resisting crack formation and growth than the glass fiber composites. These results suggest that hemp fiber composites have the potential to replace glass fiber composites in applications where components are subjected to fatigue loads but the stress levels are of moderate value. POLYM. COMPOS., 35:1926–1934, 2014. © 2014 Society of Plastics Engineers     

Rheology of thermoplastic matrix short glass fiber composites

An experimental investigation of the processing of glass-fiber reinforced polypropylene is presented. Final fiber length distribution, chopped strand disgregation, matrix and composite rheological properties, die swelling, and surface morphology are analyzed. Strand disgregation is observed to increase with shear rate and fiber concentration and to decrease with the length of the die. Final fiber length distribution appears to be independent of die length but decreases with fiber concentration and shear rate. The viscosity and first normal coefficient functions show a linear dependence with shear rate and increases with fiber concentration. The extruded filament surface shows a minor roughness when the shear rate increases. The results of this experimental characterization give useful information for determining the influence of processing variables on the final properties of short fiber reinforced polypropylene and constitutes the first part of a more ambitious project that also includes the development of a modeling strategy of the processing behavior of short fiber composites.     

Effect of molecular weight and fiber diameter on the interfacial behavior in glass fiber/PP composites

In this study, the effects of fiber diameter, molecular weight of the matrix polymer, and interfiber spacing in glass fiber-reinforced polypropylene composites were investigated on the interfacial microstructure. The influences of the surface state of the fiber and the heat-treatment condition on the interfacial morphology and the spherulitic formation process in the matrix were also investigated. Consequently, it was found that both the fiber diameter and molecular weight of the polymer significantly influence the thickness of the transcrystalline layer. Also, as the interfiber spacing becomes smaller, the spherulites in the matrix polymer are not seen to be formed between the transcrystalline layers developed on the glass-fiber surface. In addition, the radius of the largest spherulites in the matrix polymer was found to be about the same as the thickness of transcrystalline region and to largely depend on the holding time at the crystallization temperature and cooling condition (or rate). © 1998 John Wiley & Sons, Inc. J Appl Polm Sci 67:1191–1197, 1998     

Effect of matrix ductility and interface treatment on mechanical properties of glass fiber mat composites

The influence of matrix properties on randomly oriented glass fiber epoxy composites has been studied. It is shown that an increased ductility (flexibility) of the matrix does not result in greater elongation to failure of the composite under tensile and flexural loads. The tensile (and flexural) strength and the modulus of elasticity are decreased as the ductility of the resin is increased. It is concluded that since the matrix material is subjected to a triaxial state of stress when the composite specimen is subjected to uniaxial loads, the effect of matrix modulus, Poisson's ratio, and yield strength are more important than the matrix ductility measured under uniaxial stress. The effect on mechanical properties of various surface treatments applied to the fibers is also investigated. Finally, scanning electron micrographs are presented showing matrix cracks, fiber debonding, and fiber pull-out.     

An acoustic emission study on the fracture behavior of continuous glass fiber/polypropylene composites based on commingled yarn

The fracture behavior of continuous glass fiber reinforced polypropylene composites made of commingled yarn in the form of biaxial (±±45°) noncrimp warp‐knitted fabric, twill woven fabric, and swirl mat, respectively, was investigated by virtue of single edge notched tensile (SEN‐T) specimens. These composite laminates were manufactured by compression molding and cooled at two different rates (1°C/min and 10°C/min) during the last processing phase of the laminates. The failure mechanisms were studied by acoustic emission (AE) analysis. AE amplitude ranges corresponding to the individual failure modes have been identified. For biaxial noncrimp fabric reinforced materials, the failure mechanisms involved in the fracture procedure are governed by the interface related failure events. Higher cooling rate, which is accompanied by better fiber/matrix adhesion, results in not only the increase in the relative proportion of high‐amplitude failure events, but also the occurrence of a large quantity of fiber fracture events. For woven fabric and mat reinforced composites, fiber‐dominated failure mechanisms result in the higher fracture toughness when compared with biaxial noncrimp fabric composites. Under this circumstance, the change in cooling rate only results in the difference in the relative frequency of the individual failure modes. In addition, it is found out that the initiation fracture toughness of SEN‐T specimens can be easily assessed by marking the load value which corresponds to the first point of AE signals emitted stably in AE events‐displacement curves. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers     

Impact behavior of aramid fiber/glass fiber hybrid composites: The effect of stacking sequence

Aramid fiber/glass fiber hybrid composites were prepared to examine the effect of stacking sequence on the impact behavior of thin laminates. The effect of position of the aramid layer on the impact properties of hybrid composites was investigated using driven dart impact tester. The delamination area and fracture surface of hybrid composites were analyzed for correlation with impact energy. The addition of glass layer to aramid layer reduced the impact resistance of hybrid composite due to the restriction in the deformation of aramid layer. The position of aramid layer resulted in variations in the impact behavior of hybrid composites. When the aramid layer was at the impacted surface, the composite exhibited a higher impact energy. This was attributed to the fact that the flexible layer at the impacted surface in thin laminates can experience larger deformation. In three‐layer composites, the aramid fiber‐reinforced composite ( AAA ) exhibited the highest total impact energy due to high impact energy per delamination area (1EDA) in spite of low delamination area. Aramid fiber and glass fiber‐reinforced composites showed a different impact behavior according to the change of thickness. This was attributed to the difference in the energy absorption at interface between laminae.     

Wear response of glass fiber and ceramic tile-reinforced hybrid epoxy matrix composites

Iranian Polymer Journal - This study presents the tribological behavior of epoxy matrix composites containing two different fillers. The composites contain fillers with different particle sizes...     

Effect of different fiber orientations on compressive creep behavior of siC fiber-reinforced mullite matrix composites

The compressive creep behavior of monolithic mullite and a composite made of mullite reinforced by 40 vol% SiC fiber were investigated at temperatures from 1100 to 1200°C and under stresses from 5 to 55 MPa in air with a loading direction parallel and perpendicular to the fiber direction. For both situations the composite exhibits better creep resistance than monolithic mullite, although there is a creep anisotropy. The improvement in creep resistance when the fibers are parallel to the loading directions is due to the shedding of the applied stress on the SiC fibers, and the improvement in creep resistance when the fibers are perpendicular to the loading direction occurs because the fibers inhibit the lateral deformation of the mullite matrix along the fibers. The improvement mechanisms of the composites were confirmed further by their creep-recovery study, which indicated that the two types of composite specimens exhibit both an apparent creep-recovery behavior on load removal, due to the relaxation of the residual stress state between the mullite matrix and the SiC fibers after unloading. ©     

Low‐velocity impacts in continuous glass fiber/polypropylene composites

The low‐velocity impact behavior of a continuous glass fiber/polypropylene composite was investigated. Optical microscopy and ultrasonic scanning were used to determine the impact‐induced damage. At low impact energy, the predominant damage mechanism observed was matrix cracking, while at high energy the damage mechanisms observed were delamination, plastic deformation, which produced a residual specimen curvature, and a small amount of fiber breakage at the edge of the indentation on the impacted face of the specimens. The impact load vs. time signals were recorded during impact and showed that the load corresponding to the onset of delamination was independent of the impact energy in the range tested. The load at which the onset of delamination occurred corresponded to the values obtained by performing a linear regression of the delaminated area, obtained by ultrasonic scanning, as a function of the impact force. Tensile and flexural tests performed on impacted specimens showed that the tensile and flexural residual strengths and the flexural modulus decreased with increasing incident impact energy, while the post‐impact residual tensile modulus remained constant. The dynamic interlaminar fracture toughness was evaluated from the critical dynamic (impact) strain energy release rate of specimens with a delamination simulated by an embedded insert. The results are compared with the interlaminar fracture toughness values obtained during subcritical steady crack growth.     

Polyester composites reinforced with noncrimp stitched glass fabrics: Experimental characterization of composites and investigation on the interaction between glass fiber and polyester matrix

The primary purpose of the study was to investigate the anisotropic behavior of different noncrimp stitched fabric reinforced polyester composites. The effects of geometric variables on composite structural integrity and strength are illustrated. Hence, tensile, three‐point bending flexural and short beam shear tests were conducted up to failure on specimens strengthened with different layouts of fibrous plies in noncrimp stitched fabric. The remark, based on the observations while tensile testing, is that the stress–strain curves of polyester based composites were linear in the direction of fibers. However, in the matrix dominated orientations nonlinear relation between the stress and the strain was observed. Another aim of the present work was to investigate the interaction between glass fiber and polyester matrix. The experiments, in conjunction with scanning electron photomicrographs of fractured surfaces of composites, were interpreted in an attempt to explain the interaction between glass fiber and polyester and were interpreted in an attempt to explain the instability of polyester resin–glass fiber interfaces. It was concluded that the polymer was either deposited between adjacent fibers or as widely separated islands on the fiber surface. Infrared spectra of the cured polyester and its glass fiber composite were obtained by Fourier transform infrared spectroscopy. POLYM. COMPOS., 2008. © 2007 Society of Plastics Engineers     

Fatigue life of coir fiber reinforced PP composites: Effect of compatibilizer and coir fiber contents

The fatigue behavior of polypropylene/coir fiber composites was investigated. Composites were prepared according to an experimental statistical design, in which the independent variables coir fiber and compatibilizer content were varied. The compatibilizer used was maleic anhydride grafted polypropylene (PP‐g‐MA). Compatibilizer free composites were also prepared. Composites were prepared in a corotating twin‐screw extruder and the mechanical behavior of polypropylene/coir fiber composites were assessed through monotonic (tensile) and cyclic (fatigue) tests. Fatigue load controlled tests were conducted under tension–tension loads at a frequency of 6 Hz. The fracture mechanism was accompanied by surface fracture analyses using scanning electron microscopy (SEM). The results indicated the need for using compatibilizer in the composites; however, increase in compatibilizer content did not affect composite fatigue lifetime. Coir content was the variable with the strongest effect on composite properties; increasing this variable caused significant increase in fatigue life. POLYM. ENG. SCI., 53:2159–2165, 2013. © 2013 Society of Plastics Engineers     

Effect of fiber misorientation on the tensile strength of compression molded continuous fiber composites

Resin cross-flow during compression molding of unidirectional sheet molding compound composites, such as CSMC and XMC, may cause severe misorientation of the continuous fibers in the outer layers. The extent of fiber misorientation depends on the type of molding compound, the length of cross-flow, and the location of the charge in the mold. The tensile strength is reduced in the direction of cross-flow with decreasing mold surface coverage. However, since severe fiber misorientation is generally restricted to the outer layers, increasing the number of plies improves the tensile strength to the level observed with little or no misorientation.     

Effects of fiber aggregates on the transverse mechanical behavior of commingled PP–glass fiber composites

In previous articles, mechanical models were proposed to predict the reinforcement effect of polymers by particulates as well as unidirectional fibers over wide ranges of volume fraction of fillers and temperatures. On the basis of image analyses and the definition of representative morphological motifs, these models are able to predict the viscoelastic properties of quasi‐isotropic and unidirectional composites or to extract the behavior of a phase, such as the interphase in filled rubbers or the transcrystalline phase in semicrystalline polymers. In this work, based on a 2D image processing, this approach is extended to predict the viscoelastic properties of commingled PP–glass fiber composites. It is shown that fiber aggregates, composed of fibers and surrounding polymer, might be considered as the reinforcing phase. In addition, the different failure modes of these composites are separated as a function of the volume fraction of fillers or temperature. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 99: 3466–3476, 2006     

Mechanical modeling of the transcrystalline interphase behavior in commingled PBT/glass fiber composites

In a previous work, a mechanical model was proposed to predict the reinforcement of amorphous polymers by particulates as well as by unidirectional fibers over wide ranges of volume fractions of fillers and temperatures (or frequencies). This model is based on both the definition of a representative morphological pattern (RMP), accounting for the presence of fiber clusters, and a quantitative morphology analysis, based on the percolation concept. In this work, such an approach is extended to describe the viscoelastic properties of a semicrystalline polymer, poly(butylene terephthalate), commingled with 30 and 50 vol % of unidirectional glass fibers. It is found that aggregates constituted by both fiber clusters and a transcrystalline region (TCR) can act as the continuous phase. Based on the use of a mechanical model in a reverse mode, the actual viscoelastic behavior of this TCR is extracted and compared to that displayed by the unfilled polymer. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 2513–2524, 2000     

Carbon-matrix composites with continuous glass fiber and carbon black for maximum strain sensing

Electrically conductive glass-fiber-reinforced polymer composites have been prepared by adding carbon black, and carbonization processes have been applied to the resulting matrices. The carbonized composites were found to show characteristic changes in resistance during cyclic tensile tests, in which the resistance increased in the loaded state was retained even after unloading. Pyrolysis temperature dependence of the residual phenomena was investigated in order to understand the effects of the carbonized matrix and the carbon black network. The residual behavior became more pronounced with increasing pyrolysis temperature until 500 °C, while that diminished over 600 °C. The thermal decomposition of the matrix was almost completed up to 500 °C, and the shrunk matrix coexisting with glass fibers had a residual tensile stress along the fiber direction. The matrix carbonized at higher than 600 °C showed an increase in conductivity, which disrupted the strain-sensitive percolation network and hence the resistance response. These results showed that irreversible change in the carbon black network under the internal tensile stress provided the residual phenomena.     

增材制造连续碳纤维增强金属基复合材料性能

采用增材制造工艺方法进行具有高比强度、密度小等优良性能连续碳纤维增强金属基复合材料的直接制备。研究了连续碳纤维表面改性、路径搭接率、打印喷头温度、基板温度、打印速度等过程处理方法及工艺参数对所制备金属基复合材料抗拉强度的影响。研究结果表明,对连续碳纤维原材料实施表面改性处理,可以实现制备过程中熔融金属基体与连续碳纤维之间的良好浸润复合,以提高复合材料的抗拉强度;增大路径搭接率,可以有效提高增材制造复合材料内部纤维的体积占比,从而增大其抗拉强度;升高打印喷头温度、基板温度、打印速度,可以减小熔融金属表面张力,提高其流动性,并有利于沉积层间实现良好重熔,从而有效避免在已沉积层表面裂纹处和路径搭接区凹坑处形成气孔缺陷,进一步提升复合材料的抗拉强度。     

Long fiber reinforced polyamide and polycarbonate composites. II: Fatigue behavior and morphological property

Fatigue behavior and morphology of long glass fiber reinforced semicrystalline polyamide (nylon 6,6) and amorphous polycarbonate (PC) composites were investigated. The fiber length distribution in the molded samples was calculated by image analyzer. The tension-tension fatigue loading tests at various levels of stress amplitudes were studied. The two-parameter Weibull distribution function were applied to obtain the statistical probability distribution of experimental data. A good correlation existed between the experimental data and the Weibull distribution curves. Straight line S? N curves of long glass fiber reinforced semicrystalline polyamide and amorphous polycarbonate composites at various probabilities were established. The stiffness of the composite under tension-tension fatigue loading was measured. The thermal stress history was also investigated by thermo-imaging techniques during fatigue life testing. Further, failure morphology was examined by scanning electron microscopy (SEM). The results showed that the fracture behavior of the ductile damage in polyamide is different from the brittle damage in polycarbonate.     

Modification of the dynamic damping behavior of epoxy/glass fiber composites via fiber coating with functional latices

A series of functionalized latices were synthesized using: n-butyl acrylate (BuA), butyl methacrylate (BuMA), and glycidyl methacrylate (GMA). The particle microstructure was controlled by a sequential addition of monomers. The glass fibers were commercially roll-coated using a mixture of these latices and amino-silane. The specimens were filament wound, compression molded, and cured. The effects of the morphology and reactivity of the latices were analyzed in dynamic as well as flexural modes. The uniformity of the coating as well as the nature of the latex are responsible for the mechanical behavior of the composites.