Flexural fatigue of marine laminates reinforced with woven roving of E-glass and of Kevlar® 49 aramid

A comparison of the flexural fatigue characteristics of E-glass and Kevlar® 49 aramid in polyester and vinyl ester resin hand lay-ups typical of boat hull laminates is presented. Data on unidirectional E-glass and aramid composites from epoxy preimpregnated tapes are reported for comparison. The data indicate that while the initial flexural strength of E-glass woven roving laminates is greater than that of aramid laminates, the runout stress of aramid laminates at 10⁶ fatigue cycles is similar or superior to glass. S-N curves for aramid laminates are flatter indicating better flexural fatigue resistance.     

Morphology and tensile properties of unreinforced and short carbon fibre reinforced Nylon 6/multiwalled carbon nanotube-composites

The morphology and the tensile properties of unreinforced and short carbon fibre (SCF) reinforced Nylon 6/multiwalled carbon nanotube (MWCNT)-composites are investigated. The morphology analysis shows that MWCNT and SCF are randomly oriented in the composites. Furthermore, the SCF fail due to fibre pull-out, while the MWCNT fail due to fracture. Young's modulus and tensile strength of SCF reinforced Nylon 6 and Nylon 6/MWCNT-composites increase with increasing total filler volume content. Replacing SCF by MWCNT further enhances Young's modulus and the tensile strength. An additive modelling approach leads to better results at low MWCNT-volume contents, while at higher MWCNT loadings a multiplicative modelling approach results in a better approximation of the experimental data. Thus the SCF reinforced Nylon 6/MWCNT-composites behave at low MWCNT-volume contents like a polymer composite containing two different types of fillers, while at higher MWCNT loadings a behaviour of a short fibre reinforced nanocomposite is observed.     

Fatigue mechanisms of thermoplastics

Failure in fatigue by the mechanism of crack initiation and propagation originationg at flaws and stress concentrations has been fairly well established. It explains the fatigue phenomena in highly elastic materials, namely metals and certain thermosetting plastics such as reinforced polyester and epoxy resins. We have found, however, that a different mechanism dominates the fatigue behavior of thermoplastics. Specifically, the mechanism involves the generationa of heat within the material due to viscous damping or hysteresis. This paper presents fatigue, damping data and temperature measurements during cyclic stress to support this conclusion on three themoplastic resins of widely different mechanical properties, polyetrafluouoethylene (PTFE), Nylon 6, and polymethyl-methacrylate (PMMA). In addition to the usual S-N fatigue curves, we wil show how surface temperature changes with fatigue life and how this change is affected by stress, Frequency, crystallinity, specimen geometry and other parameters. Alos, the loss compliance of the materials will be presented as afunction of temperature to show the relationship of fatigue to damping properties.     

混杂纤维复合材料的平面剪切性能

研究了基体韧性和铺层方式对玻璃纤维、碳纤维及其混杂纤维复合材料平面剪切性能的影响。结果表明，玻璃、碳及其混杂纤维复合材料的平面剪切应力-应变曲线均具有非线性特征；在脆性基体中选用混杂结构，其复合材料的剪切性能具有正的混杂效应。     

Fatigue damage studies in high strength sheet-molding compound fiberglass composites

The materials considered in this investigation were oriented-hybrid-glass-fiber-reinforced vinyl ester resin sheet-molding compound (SMC) composites. The composition of one material was 60 percent continuous glass and 5 percent random fibers, while the other was 45 percent continuous glass and 20 percent random fibers. The S-N curves for the two materials were determined in the continuous fiber direction. The fatigue stress levels ranged from approximately 20 to 80 percent of each material's respective ultimate tensile strength. The residual tensile strengths of the fiber-reinforced composites were not seriously reduced until near final fracture. The moduli of the composites in the fiber direction are also not reduced in any detectable manner. Damping measurements have shown that the damping increases during fatigue and that it is a sensitive measure of matrix and interface damage, generally debonding and cracking. It is shown by damping measurements that damage accumulates differently for two materials studied. The material with the higher percent of continuous glass appeared to be more cycle-dependent, while the other was more stress-dependent.     

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.     

Effects of low-energy impact and thermal cycling loadings on fatigue behavior of the quasi-isotropic carbon/epoxy composites

Effects of low-energy impact loading and thermal cycling on fatigue behavior of carbon fiber reinforced epoxy (carbon/epoxy) laminates are examined. A low-energy of 0.62 Joules was adopted to impact carbon/epoxy laminates prior to thermal cycling exposure and fatigue test. The temperature ranged between 60 and −60 °C for thermal cycling and the stress ratio of 0.1 with a frequency of 3 Hz for fatigue loading were used. Impact performances were tested on the virgin specimens and the thermal-cycling exposure specimens. Residual tensile strength and fatigue tests were performed on the laminate composites after being subjected to thermal cycling. The relationship between tensile strength reduction and fatigue performance after thermal cycling was investigated. Stiffness degradation during fatigue testing was monitored; the differences in stiffness for these three composites (virgin specimens, low-energy impacted specimens, low-energy impacted and thermal-cycling exposure specimens) were compared and the coupling effects of low-energy impact and thermal fatigue were studied. Furthermore, the S-N curves were also plotted and the variation was compared on the aforementioned three composites. SEM was used to examine the difference in fracture morphologies on the composites with and without suffering low-energy impact and thermal fatigue.     

Pultruded fiber reinforced thermoplastic poly(methyl methacrylate) composites. Part II: Mechanical and thermal properties

A novel process has been developed to manufacture poly(methyl methacrylate) (PMMA) pultruded parts. The mechanical and dynamic mechanical properties, environmental effects, postformability of pultruded composites and properties of various fiber (glass, carbon and Kevlar 49 aramid fiber) reinforced PMMA composites have been studied. Results show that the mechanical and thermal properties (i.e. tensile strength, flexural strength and modulus, impact strength and HDT) increase with fiber content. Kevlar fiber/PMMA composites possess the highest impact strength and HDT, while carbon fiber/PMMA composites show the highest tensile strength, tensile and flexural modulus, and glass fiber/PMMA composites show the highest flexural strength. Experimental tensile strengths of all composites except carbon fiber/PMMA composites follow the rule of mixtures. The deviation of carbon fiber/PMMA composite is due to the fiber breakage during processing. Pultruded glass fiber reinforced PMMA composites exhibit good weather resistance. They can be postformed by thermoforming, and mechanical properties can be improved by postforming. The dynamic shear storage modulus (G′) of pultruded glass fiber reinforced PMMA composites increased with decreasing pulling rate, and G′ was higher than that of pultruded Nylon 6 and polyester composites.     

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.     

Fatigue of glass fiber reinforced injection molded plastics. II: Tensile versus flexural loading

A previous paper reported fatigue data for several glass fiber reinforced thermoplastic materials by measuring their S-N, stress versus number of cycles to fail, behavior. It was demonstrated that both fiber orientation and loading mode, tensile versus flexural, strongly influence the resultant S-N data. The orientation effects are considered real, reflecting the local fiber orientation distributions in the samples, which are determined by geometry, processing and material variables. However, the higher fatigue lives observed in flexural loading are considered an artifact. Specifically for this class of materials, it is concluded that the use of linear elastic beam bending theory equations to calculate bending stresses are inappropriate. It is shown that a plasticity-based correction factor makes the flexural S-N data equivalent to the tensile results. In fact this correction applies equally well to monotonic loading, not just fatigue. Although it is concluded that tensile loading is preferred, some cases where flexural loading may be advantageous are indicated. Polym. Compos. 25:569–576, 2004. © 2004 Society of Plastics Engineers.     

Nylon fiber reinforcement for polyurethane composites

The potential of a nylon 6 fiber was evaluated for composite applications involving rigid polyurethane systems. A study showed that the use of COMPET N, a nylon 6 based fiber surface treated to enhance wetout and adhesion, in an ISP polyurethane system results in composites with higher impact values and lower weight than those reinforced with glass fiber. A 2 percent loading of COMPET N matt increased impact values by more than 300 percent over an unreinforced control while a 3 percent loading of glass matt resulted in increases of less than 100 percent. A drop impact test showed COMPET N fabric reinforced composites to absorb twice as much energy as an unreinforced and glass reinforced composite without major damage. These advantages have also been observed in COMPET N/Glass hybrids, in which layer orientation can be used to produce composites with specific properties. Cut fiber reinforced composites were found to exhibit similar properties as fabric reinforced ones resulting in the commercialization of a canoe end cap containing cut nylon fiber. Additional property benefits such as improved impact fatigue and decreased stress cracking have also been observed in nylon 6 reinforced composites resulting in the consideration of these materials for a wide range of applications.     

Fatigue fracture of long fiber reinforced nylon 66

The fatigue behavior of long fiber reinforced nylon 66 has been investigated by measuring fatigue crack propagation rates of injection molded samples. Plaques varying in thickness from 3 to 10 mm were employed for nylong 66 containing either glass, carbon or aramid fibers. Both conventional chopped, short fiber reinforcements and pultruded long fiber filled nylon 66 were examined. Long fiber reinforced nylon 66 exhibits improved fatigue resistance as shown by decreases in fatigue crack propagation rates compared to short fiber filled composites. Using a fracture mechanics analysis, it is shown that the improvements are due primarily to the higher moduli of the long fiber reinforced nylon 66, with only a slight increase in the calculated strain energy release rate associated with fatigue crack growth. For short or long glass fibers, and for short carbon fibers, the effects of fiber orientation on fatigue crack growth rates can be predicted from the fracture mechanics model. More significant effects of fiber length on fatigue fracture energies are noted for long aramid and long carbon reinforced nylon 66. It is also shown that thicker plaques can exhibit poorer fatigue fracture behavior owing to their inferior core sections.     

复合材料疲劳S-N曲线的建模

本文建立了复合材料疲劳寿命曲线的“有效缺陷模型”,阐明了模型中两个参数的物理意义。通过实验验证,说明该模型可以较好地预测复合材料的疲劳寿命。利用该模型,仅需较少的试验即可绘制出新材料的Ｓ－ Ｎ 曲线。     

Pultruded fiber-reinforced poly(methyl methacrylate) composites. II. Static and dynamic mechanical properties,environmental effect,and postformability

This paper presents a novel process developed to manufacture poly(methyl methacrylate) (PMMA) pultruded composite. The mechanical, thermal, and dynamic mechanical properties, environmental effect, postformability of various fiber (glass, carbon, and Kevlar 49 aramid fiber) reinforced pultruded PMMA composites have been studied. Results show mechanical properties (i.e., tensile strength, specific tensile strength, tensile modulus, and specific flexural strength) and thermal properties (HDT) increase with fiber content. Kevlar fiber/PMMA composites possess the highest specific tensile strength and HDT, carbon fiber/PMMA composites show the highest tensile strength and tensile modulus, and glass fiber/PMMA composites show the highest specific flexural strength. Pultruded glass-fiber-reinforced PMMA composites exhibit good weather resistance. These composite materials can be postformed by thermoforming under pressure, and mechanical properties of postformed products can be improved. The dynamic shear storage and loss modulus (G′, G″) of pultruded glass-fiber-reinforced PMMA composites increased with decreasing pulling rate, and their shear storage moduli are higher than those of pultruded Nylon 6 and polyester composites.     

Flexural fatigue behavior of injection-molded composites based on poly(phenylene ether ketone)

Flexural fatigue tests were conducted on injection-molded short fiber composites, carbon fiber/poly(phenylene ether ketone) (PEK-C) and glass fiber/PEK-C (with addition of polyphenylene sulfide for improving adhesion between matrix and fibers), using four-point bending at stress ratio of 0.1. The fatigue behavior of these materials was presented. By comparing the S-N curves and analyzing the fracture surfaces of the two materials, the similarity and difference of the failure mechanisms in the two materials were discussed. It is shown that the flexural fatigue failure of the studied materials is governed by their respective tensile properties. The matrix yielding is main failure mechanism at high stress, while at lower stress the fatigue properties appear fiber and interface dominated. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65: 1857–1864, 1997     

Effect of frequency on the fatigue behavior of [±45]4s laminate of carbon fiber reinforced polyetheretherketone (PEEK) composites under tension-tension loading

Fatigue behavior of carbon fiber reinforced poly etheretherketone (PEEK) laminated composite was investigated. The [±45]_4s AS-4/PEEK laminated composites under static tensile measurement at various test temperatures were conducted. Three tension-tension fatigue loading frequencies, 1 Hz, 5 Hz and 10 Hz, were selected to study the effect of frequency on the fatigue behavior of [±45]_4s AS-4/PEEK laminate. The survival probability of experimental fatigue life data under different stress amplitude tests were estimated and analyzed by the median rank order-statistic cumulative-distribution function and the Weibull distribution function. The S-N curves at different fatigue loading frequencies exhibited a trend of two-segment straight line curves. The increase in surface temperature of specimens was found and the thermal stress history was also investigated by thermo-image techniques during fatigue life testing. The fatigue failure mechanism was investigated by X-ray radiography.     

Characterization of a rigid silicone resin

Silicone resins have been used as binders for ceramic frit coatings and can withstand temperatures of 650°C to 1260°C. Conceptually, silicone resins can potentially be used as matrices for high temperature fiber‐reinforced composites. The mechanical and thermal properties of a commercially available silicone resin, Dow Corning® 6‐2230, were characterized. Neat 6‐2230 resin was found to have inferior room temperature mechanical properties such as flexural, tensile and fracture properties when compared to epoxy. The room temperature flexural properties and short beam shear strength of the silicone/glass composites were also found to be lower than those of epoxy/glass composite with similar glass content. However, the silicone resin had better elevated temperature properties. At an elevated temperature of 316°C, the retentions of flexural modulus and strength were 80% and 40% respectively of room temperature values; these were superior to those of phenolic/glass. Unlike the carbon‐based resins, the drop in flexural properties of the silicon/glass laminates with temperature leveled off with increase in temperature beyond 250°C. The resin weight loss at 316°C in 100 cm³/min of flowing air was small compared to other carbon‐based resins such as PMR‐15 and LaRC TPI. Only Avimid‐N appeared comparable to Dow Corning® 6‐2230.     

Some effects of matrix and interface properties on the fatigue of short fiber-reinforced thermoplastics

The fatigue behavior of injection-molded tensile bars of short-fiber-reinforced theromplastics is described and related to the fatigue behavior of the matrices and the strength of the fiber/matrix interface. A brittle matrix system based on polyphenylene sulfide is shown to behave in a similar manner to long-fiber composites. Glass-fiber reinforcement in this matrix gives fatigue sensitivity that correlaes with that of unimpregnated glass fiber strands, while carbon-fiber rein-forcement gives better fatigue resistance. A well-bonded, due-tile matrix system based on nylon 6,6 gives matrix-controlled fatigue sensitivity. Fatigue data for glass- and carbon-fiber-reinfoced nylon 6,6 superimpose on the matrix fatigue data when normalized by the ultimate tensile strength. Another ductile matrix, polyetherther ketone, is very fatigue-resistant, but its composite progressively loses its reinforcing effect in fatigue, apparently due to interface failure. A transitional matrix, polysulfone, shifts from ductile to fatigue-crack-dominated failure as the cyclic stress is reduced. Its composites show an analogous failure mode shift, and the high cycle-fatigue response is correlated with fatigue-crack-growth data.     

A comparison between the static and fatigue properties of glass‐fiber and carbon‐fiber reinforced polyetherimide composites after prolonged aging

The static and fatigue behavior of two fiber‐reinforced composites was characterized after samples were subjected to both natural and accelerated aging. The composites consisted of a thermoplastic matrix (PEI: polyetherimide) reinforced with glass or carbon fabric. Natural aging involved exposing samples to the elements over a period of two years, while accelerated aging was conducted in a saline solution during 200 days. Subsequently, static (tensile and interlaminar shear) and fatigue tests were carried out with the aim of determining the mechanical properties of the materials after exposure. A negligible decrease in the value of these properties was observed, while different behavior was detected depending on the type of aging of the material.     

二维与三维机织复合材料的力学性能实验研究

通过对超厚三维正交机织复合材料及二维机织层合板分别进行拉伸和压缩实验，研究比较两复合材料刚度和强度特性的差异；研究发现无论是三维机织材料的拉、压，还是二维层合板的拉、压的应力一应变曲线都可近似为直线关系，而且具有脆性破坏的特点；三维复合材料的拉、压强度要高于二维层合板，是由于不同的增强相结构及纤维含量造成；不同的破坏模式对材料强度影响很大。