The fatigue of synthetic polymeric fibers

The fatigue properties of a number of different types of fibers have been investigated and failure under cyclic loading conditions compared to that caused by simple tensile loading. Polyamide, polyester, and polyacrylonitrile fibers have been studied and all have been found to fail by fatigue mechanisms. The loading conditions have been monitored by a fiber fatigue apparatus developed for this purpose and the fracture morphologies inspected by scanning electron microscopy. In all of the cases which are considered in detail, fatigue failure of the fibers has been found to occur when cycling from zero load to a maximum load of about 60% of the tensile strength. Fatigue failure is accompanied by a distinctive fracture morphology, clearly different from the tensile fracture morphology and involving crack propagation along the fiber at a slight angle to its axis, although the mechanism which causes this in the acrylic fiber is probably different from that for the polyamide and polyester fibers.     

Bending and torsional fatigue of nylon 66 monofilaments

Using newly developed test equipment, the fatigue behavior of nylon 66 monofilaments was studied under two loading conditions, pure bending or simple torsion. For each mode, results are expressed in terms of the measured decay in stiffness with numbers of cycles over a range of maximum applied strain levels. Fatigue lifetimes are presented in S–N format where the log number of cycles of fatigue for a 40% decay in stiffness (N) is plotted as a function of applied strain (S). The failure mechanism for these fibers in each fatigue mode reflects the morphology of semicrystalline-oriented synthetic fibers. In torsion, many longitudinal cracks form around the perimeter of a fiber as the result of cleavage of the relatively weak interfibrillar bonds in nylon 66. In bending, cracks form within kink band boundaries and grow at an oblique angle to the fiber axis. © 1992 John Wiley & Sons, Inc.     

Modeling deformation of a melt-infiltrated SiC/SiC composite under fatigue loading

Results from fatigue experiments done on a SiC/SiC composite are presented. A micromechanics-based model is used to study the observed behavior under cyclic loading. The model includes consideration of progressive damage, creep and oxidation of the fiber and matrix. Comparison of model predictions with test data showed that the deformation during fatigue in this material is explained primarily by damage in the form of matrix microcracking and interface debonding, in combination with creep under the cyclic load. Stiffness of the material was observed to not change significantly during fatigue indicating that the contribution of fiber fracture to deformation is limited. Fiber fracture however was found to determine final failure of the composite. Failure under cyclic fatigue loading was found to be affected by load transfer from the matrix to the fiber due to damage and creep, and by progressive degradation of the load-carrying fibers due to the combined effect of oxidation and load cycling.     

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.     

Prestressed polymeric matrix composites: Longevity aspects

Elastically prestressed polymeric matrix composites (EPPMCs) are produced by stretching fibers (e.g., glass) within the composite during matrix curing. The resulting prestress can enhance mechanical performance, without increasing section dimensions or weight. Viscoelastically prestressed polymeric matrix composites (VPPMCs) can provide similar benefits, these being produced by subjecting polymeric fibers (e.g., nylon 6,6) to a creep load, which is released prior to molding. Although VPPMCs offer simplified processing and flexibility in product geometry, long‐term viscoelastic activity within the prestressing fibers is sensitive to time‐temperature limitations. In this study, nylon 6,6 fiber‐polyester resin samples were subjected to accelerated ageing. Using time‐temperature superposition, the samples were maintained at 70°C for 2,298 h, representing a 20‐fold ageing increase over previous work. Subsequent Charpy impact testing (at 20°C) demonstrated that the VPPMC samples absorbed ∼40% more energy than corresponding control (unstressed) counterparts; i.e., no deterioration in impact performance was observed, over a duration equivalent to ∼25 years at 50°C. In contrast, the longevity of EPPMCs remains unknown, but it is suggested that progressive localized matrix creep at the fiber‐matrix interface regions may cause a deterioration in elastically generated prestress with time and/or elevated ambient temperatures. POLYM. COMPOS., 37:2092–2097, 2016. © 2015 Society of Plastics Engineers     

Monotonic and Cyclic Fatigue Behavior of High-Performance Ceramic Fibers

Monotonic and cyclic fatigue behavior of single fibers or fiber fabrics are of significant interest, since fiber assemblies or fiber-reinforced composite materials in structural applications are often subjected to cyclic loading. Studying the cyclic fatigue behavior of fibers is particularly difficult because of their small diameter (∼10 μm) and high aspect ratio. In this paper, we report results of monotonic tension and tension–tension fatigue behavior of two sol–gel-derived ceramic fibers: Al₂O₃–SiO₂–B₂O₃ (Nextel 312) and Al₂O₃ (Nextel 610). Nextel 312 exhibited a great deal of variability in tensile strength, reflected by a Weibull modulus of 4.6, versus Nextel 610, which had a Weibull modulus of 10.5. Our experiments showed clearly that cyclic loading was more damaging than static loading and, thus, resulted in a lower cyclic fatigue life compared with static loading. The fracture behavior under fatigue loading was distinctly different from that under monotonic loading. It is believed that processing-induced flaws acted as crack initiation sites, and that the cyclic loading induced subcritical cracking, followed by coalescence of cracks immediately prior to failure.     

Properties of films and fibers obtained from Lewis acid-base complexed nylon 6,6

A nylon 6,6 complex with GaCl₃ in nitromethane (4-5 wt% nylon 6,6) was prepared at 50-70 °C over 24 h for the purpose of disrupting the interchain hydrogen bonding between nylon 6,6 chains, resulting in amorphous nylon 6,6, and increasing the draw ratio for improving the performance of nylon 6,6 fibers. After drawing, complexed films and fibers were soaked in water to remove GaCl₃ and regenerate pure nylon 6,6 films and fibers. FTIR, SEM, DSC, TGA, and mechanical properties were used for characterization of the regenerated nylon 6,6 films and fibers. The amorphous complexed nylon 6,6 can be stretched to high draw ratios at low strain rates, due to the absence of hydrogen bonding and crystallinity in these complexed samples. Draw ratios of 7-13 can be achieved for complexed fibers, under low strain rate stretching. This study indicates that nylon 6,6 fibers made from the GaCl₃ complexed state, using a high molecular weight polymer, can reach initial moduli up to 13 GPa, compared to initial moduli of 6 GPa for commercial nylon 6,6 fibers. Lewis acid-base complexation of polyamides provides a way to temporarily suppress hydrogen bonding, potentially increasing orientation while drawing, and following regeneration of hydrogen bonding in the drawn state, to impart higher performance to their fibers.     

Apparent Enhanced Fatigue Resistance under Cyclic Tensile Loading for a HIPed Silicon Nitride

Cyclic tensile loading tests of a commercial HIPed silicon nitride at elevated temperatures have indicated apparent "enhanced" fatigue resistance compared to static tensile loading tests under similar test conditions. At 1150°C, stress rupture results plotted as maximum stress versus time to failure did not show significant differences in failure behavior between static, dynamic, or cyclic loading conditions, with all failures originating from preexisting defects (slow crack growth failures). At 1260°C, the stress rupture results showed pronounced differences between static, dynamic, and cyclic loading conditions. Failures at low static stresses (<175 MPa) originated from environmentally assisted (oxidation) and generalized creep damage, while failures at similar times but much greater (up to 2 x) cyclic stresses originated from preexisting defects (slow crack growth failures). At 1370°C, stress rupture results did not show as pronounced differences between static, dynamic, and cyclic loading conditions, with most failures originating from environmentally assisted (oxidation) and generalized creep damage.     

Creep enhanced adsorbtion of water or aqueous zinc chloride solution increases the creep rate of nylon 6,6

The creep behavior of nylon 6,6 at 21°C was significantly altered when the local “dry” environment was changed to water mist or an aqueous zinc chloride mist. Nylon 6,6 was found to exhibit logarithmic creep because the relation between the log of the strain rate and the creep strain was linear with a negative slope. The effect of changing the creep environment from dry to wet, with the addition of moisture from an ultrasonic humidifier was to decrease the negative slope by 50–70% within 5–10 min. This effect could be interpreted as a decrease in modulus, which allowed for easier creep deformation. Based on the stress‐free diffusivity of water in nylon and the dimensions of the test sample the time to saturate the sample was estimated to be about 100 h. Therefore, there appeared to be synergism between the creep deformation and the environment that dramatically enhanced the rate of saturation and slowed the decrease in the creep rate. The tentative explanation provided is that the aqueous solutions, by binding to the hydrogen bonds in nylon, are dragged into the sample during creep deformation, and the dragged‐in aqueous solution then plasticizes nylon. This is analogous to the conclusion in another recent study that showed that deformation, during a hardness test, in the presence of aqueous zinc chloride, transported the solution species deeper into the sample than could be reasonably explained by ordinary diffusion processes. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 494–497, 2001     

Structural changes in polyester fibers during fatigue

Structural changes occurring during the fatigue failure of polyester fibers have been identified, and a comparison has been made with untested fibers and fibers which were subjected to cyclic loading conditions which did not produce fatigue. Fatigue failure was seen to result in a distinctive fracture morphology. Infrared spectrometry and X-ray diffraction revealed a lowering of crystallinity under fatigue conditions but not under other loading conditions. Transmission electron microscopy and electron diffraction revealed the creation of amorphous zones which are supposed as coalescing to form an amorphous band seen along and ahead of the fatigue crack. The zone just ahead of the fatigue crack tip is shown to contain voids. Crack propagation involves, therefore, the joining up of these voids and development along the amorphous band.     

Fatigue behavior of neat and long glass fiber (LGF) reinforced blends of nylon 66 and isotactic PP

This paper focuses on the study of the fatigue behavior of neat and long glass fiber (LGF) reinforced nylon 66/PP-blends. The fatigue was characterized using Parislaw plots in the stable crack growth acceleration range. The fatigue crack propagation (FCP) is presented as a function of the crack growth per cycle (da/dN), the amplitude of the stress intensity factor ΔK, and of the strain energy release rate ΔG. It was also of interest to compare the order of performance found in fatigue to that in the static fracture test. The fracture surfaces were characterized with SEM to determine the failure mechanisms. Further, thermographic camera recordings were used to study the size of a “heated” area (ΔT = 2°C) that developed around the crack tip during the cyclic loading of LGF-PP with different amounts of maleic anhydride grafted PP (PP-g-MAH). For the neat materials, a different order of performance was detected under static and cyclic loading. This was explained by the different failure mechanisms observed after static and cyclic fracture that were related to different stress states of the specimens during the fracture process. On the other hand, the LGF-blends showed a similar order of performance during the static and the fatigue test. This was explained by the observation that similar fiber related failure mechanisms occurred in the composite, both after failure caused by the static and cyclic loading, respectively. For the LGF-PPs with varying PP-g-MAH content, the order of performance in fatigue did not correspond to the size of the “heated area” around the crack tip. This was caused by a change in the composite failure mechanisms, which contributed differently to the size of the “heated area” and to the fatigue performance.     

单向聚酯帘线/橡胶复合材料的疲劳损伤机理

利用较Ｘ射线技术和扫描电子显微镜技术,研究了在周期载荷下单向聚酯帘线／橡胶复合材料的疲劳机理。结果表明,在高应务下,单向聚酯帘线／橡胶复合材料的疲劳损伤以聚酯帘线断裂为主,在中应力下,单向聚酯帘线／橡胶复合材料的疲劳损伤是逐步进行的;在你力下,单向聚酯帘线／橡胶复合材料的疲劳损伤仅有局部的界面损伤。     

Weatheirng study of glass-fiber reinforced polyester sheets by scanning electron microscopy

Evidence is presented of the main steps in the physical breakdown of glass-fiber reinforced polyester (GRP) composites on outdoor weathering. The chronological sequence is fiber ridging, rupture of the resin layer covering ridging fibers or fibers running close to the surface, spalling of the resin at the site of failure and subsequent erosion, fiber prominence and formation of a network of microcracks. Breakdown is believed to be caused by a type of stress fatigue imposed on the composite by cyclic variation of humidity and temperature in conjunction with solar radiation, and by the action of water and oxygen. The under side of the exposed GRP sheeting shows only incipient breakdown, indicating that solar radiation is an important factor. Countermeasures suggested to reduce breakdown include techniques to keep fibers away from the surface, use of resins with better thermal and moisture characteristics, and use of resin formulations with the best light stability.     

Fatigue performance of an injection‐molded short E‐glass fiber‐reinforced polyamide 6,6. I. Effects of orientation,holes, and weld line

This article presents the experimental results of stress‐controlled fatigue tests of an injection‐molded 33 wt% short E‐glass fiber‐reinforced polyamide 6,6. The effects of specimen orientation with respect to the flow direction, hole stress concentration, and weld line on the fatigue life have been considered. In addition, the effect of cyclic frequency has been examined. In addition to the modulus and tensile strength, the fatigue strength of the material was significantly higher in the flow direction than normal to the flow direction, indicating inherent anisotropy of the material caused by flow‐induced orientation of fibers. The presence of weld line reduced the modulus, tensile strength, failure strain, and fatigue strength. The fatigue strength of specimens with a hole was lower than that of un‐notched specimens, but was insensitive to the hole diameter. At cyclic frequencies ≤ 2 Hz, failure was due to fatigue, and fatigue life increased with frequency. However, at cyclic frequencies > 2 Hz, the failure mode was a mixture of fatigue and thermal failures, and fatigue life decreased with increasing frequency. POLYM. COMPOS., 27:230–237, 2006. © 2006 Society of Plastics Engineers.     

Tensile Creep and Creep-Recovery Behavior of a SiC-Fiber─Si3N4-Matrix Composite

The tensile creep and creep-recovery behavior of a unidirectional SiC-fiber/Si₃N₄-matrix composite was investigated at 1200°C in air. A primary objective of the study was to determine how various sustained and cyclic creep loading histories would influence the creep rate, accumulated creep strain, and the amount of strain recovered upon specimen unloading. The key results obtained from the investigation can be summarized as follows: (1) A threshold stress of 60 MPa was identified, below which the creep rate of the composite was exceedingly low (∼10⁻¹² s⁻¹). (2) Periodic fiber fracture was identified as a fundamental damage mode for sustained tensile creep at stresses of 200 and 250 MPa. (3) Because of transient stress redistribution between the fibers and matrix, the creep life and failure mode at 250 MPa. were strongly influenced by the rate at which the initial creep stress was applied. (4) Very dramatic creep-strain recovery occurred during cyclic creep; for cyclic loading between stress limits of 200 and 2 MPa, 80% of the prior creep strain was recovered during 50-h-creep/ 50-h-unloading cycles and over 90% during 300-s-creep/ 300-s-unloading cycles. (5) Cyclic loading significantly lowered the duration of primary creep and overall creep-strain accumulation. The implications of the results for microstructural and component design are discussed.     

Asbestos replacement aspects: the effect of asbestos pretreatment on the tensile properties of epoxy-Kevlar-asbestos hybrid composites

This paper presents data on the tensile performance of hybrid composite materials consisting of pretreated asbestos (chrysotile variety), Kevlar fibers, and epoxy resins. Asbestos was precoated with a well-adhered film of nylon 6,6, a polymer especially compatible with the epoxy phase. The proposed pretreatment procedure involves serial application to the asbestos of two immiscible solutions of hexamethylenediamine and adipoyl chloride, or impregnation in nylon 6,6 solution. Combination of the treated asbestos grades with high modulus aramid fibers was followed to investigate the reduction of the asbestos content in only-asbestos-based engineering plastics. The results from the tensile tests revealed composition regions of a possible positive hybrid effect, i.e. a better performance was effected than that encountered in the only-Kevlar or only-asbestos-filled composites at the same fiber content.     

GPC技术在合成纤维相对分子质量及其分布测试中的应用

综述了凝胶渗透色谱(GPC)技术在腈纶、涤纶、聚酰亚胺纤维、氯纶、氨纶、尼龙纤维和聚乙烯醇纤维的相对分子质量及其分布测试中的应用的研究进展.重点对各合成纤维的GPC测试条件进行了总结,揭示了色谱条件的优化机理,并对GPC方法开发方向及在合成纤维工业中的进一步应用提出了建议.指出在以后的合成纤维生产和研究过程中,可以挖掘...     

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.     

Tensile Creep Behavior of a Vitreous-Bonded Aluminum Oxide under Static and Cyclic Loading

Creep deformation and rupture behavior of a vitreousbonded aluminum oxide was investigated under uniaxial static and cyclic tensile loadings at 1000°, 1100°, and 1175°C. The material was more creep resistant, i.e., having lower creep strain rates, under cyclic loading compared to that under static loading. For the same maximum applied stress, the ratio of steady-state creep rate under static loading to that under cyclic loading at 1100°C was approximately 100. However, the value of this ratio decreased to about 10 when the testing temperature was raised to 1175°C or lowered to 1000°C. Under static loading the material had more propensity to develop creep damage in the form of micro- and macrocracks, leading to early failure, whereas under cyclic loading the creep damage was more uniformly distributed in the form of cavities confined to the multigrain junctions. Viscous bridging by the grain boundary second phase may be the primary contributor to the lower creep deformation rate and improved lifetime under cyclic loading.     

Polymerization compounding composites of nylon‐6,6/short glass fiber

Nylon‐6,6 was grafted onto the surface of short glass fibers through the sequential reaction of adipoyl chloride and hexamethylenediamine onto the fiber surface. Grafted and unsized short glass fibers (USGF) were used to prepare composites with nylon‐6,6 via melt blending. The glass fibers were found to act as nucleating agents for the nylon‐6,6 matrix. Grafted glass fiber composites have higher crystallization temperatures than USGF composites, indicating that grafted nylon‐6,6 molecules further increase crystallization rate of composites. Grafted glass fiber composites were also found to have higher tensile strength, tensile modulus, dynamic storage modulus, and melt viscosity than USGF composites. Property enhancement is attributed to improved wetting and interactions between the nylon‐6,6 matrix and the modified surface of glass fibers, which is supported by scanning electron microscopy (SEM) analysis. The glass transition (tan δ) temperatures extracted from dynamic mechanical analysis (DMA) are found to be unchanged for USGF, while in the case of grafted glass fiber, tan δ increases with increasing glass fiber contents. Moreover, the peak values (i.e., intensity) of tan δ are slightly lower for grafted glass fiber composites than for USGF composites, further indicating improved interactions between the grafted glass fibers and nylon‐6,6 matrix. The Halpin‐Tsai and modified Kelly‐Tyson models were used to predict the tensile modulus and tensile strength, respectively.