Fatigue Mechanisms in High-Strength Silica-Glass Fibers

We use experimentally determined crack growth data for silica glass and a fracture mechanics model for delayed failure to predict the fatigue behavior for high-strength silica-glass fibers. The results of this model indicate that fracture mechanics methods can be used to adequately describe the fatigue behavior observed for high-strength silica-glass fibers at room temperature in humid conditions. The key feature to properly interpreting the fatigue of high-strength fibers is the use of a fracture-rate law in which the crack extension rate increases exponentially with applied stress. We show that a fracture mechanics approach to highstrength fiber fatigue can provide the basis for identifying additional fatigue mechanisms that may control failure in more aggressive fatigue environments.     

Toughness optimization of glass‐fiber reinforced PA 6/PA 66‐based composites: Effect of matrix composition and colorants

Mixing of polyamide 6 (PA 6) and polyamide 66 (PA 66) is integrated in the trend of development of new and improved materials by combination of different polymers and some reinforcing materials to polymer composites. The specific polymer composite PA 6/PA 66 reinforced with short glass‐fibers combines the good coloring of PA 6, and the small moisture absorption of PA 66. Technical applications of PA 6/PA 66 composites are mainly used in the automotive industry. Specific requirements of this industry lead to the necessity to optimize the material resistance against crack propagation of the PA 6/PA 66 composites, using mechanical and fracture mechanical methods. So, the present investigations focus on fracture mechanics toughness optimization of the PA 6/PA 66 composites, including unstable and stable crack growth. The aim of this toughness optimization is to find out the optimal mixing ratio of PA 6/PA 66. Applications of PA 6/PA 66 in the automotive industry and specific client wishes are the main reasons for black‐coloring of the PA materials. The influence of several black‐colorants (carbon black, nigrosine, spinel, iron oxide) on mechanical and fracture mechanical properties of the PA composites is also investigated using fracture mechanical methods. As experimental fracture mechanical method, preferentially, the instrumented Charpy impact test (ICIT) and the new cut method to determine the stable crack growth of glass‐fiber reinforced materials was used. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Fracture behavior of short fiber reinforced thermoplastics I. Crack propagation mode and fracture toughness

The fracture behavior of several short glass fiber reinforced thermoplastics has been studied. The fracture toughness of these materials may be related to local crack propagation mode, which is found to be highly rate dependent. At low test rates the crack growth in the reinforced polymers tend to follow a fiber avoidance mode, creating a greater area of new surfaces, which in conjunction with greater degree of interfacial debonding and fiber pullout friction leads to a higher fracture resistance. An increase in loading rate in general results in a more straight and flat crack path, as well as a lesser extent of fiber debonding and pullout. Therefore the fracture toughness is reduced although the frequency of fiber breakage is increased. The fracture behavior of these short fiber reinforced polymers appears to be dictated by the matrix properties when the loading rate is high.     

Influence of thickness and processing history on fatigue fracture of nylon 66. Part II: Crack tip morphology

Fatigue crack propagation rates in injection molded nylon 66 were previously shown to be strongly affected by prior processing history. To provide a physical basis for the observed acceleration in crack growth rates, microtomed sections were cut through the tips of stable fatigue cracks and examined by optical microscopy. A reduction in spherulite size occurs with reprocessing along with an accompanying decrease in the amount of deformation at the crack tip. For the initially processed nylon 66 this deformation consists of a vast array of independently initiated craze-like zones. Patchy type regions observed on the fatigue fracture surface are similar in size to the initially formed crazed zones. Crack advance occurs by the breakdown and coalescence of the crazed regions via matrix shearing. The extensive damage zone is believed to result in a reduction in stress intensity at the crack tip thereby reducing the crack propagation rates. For the reprocessed nylon 66, one observes fewer crazes and a sharper fatigue crack tip with a consequent acceleration in crack propagation rates and a smoother fracture surface.     

Fatigue fracture of reaction injection molded (RIM) nylon composites

This study was undertaken to determine how milled glass fibers affect the fatigue resistance of reaction injection molded (RIM) nylon 6. Specifically the effects of glass content, fiber length, orientation, and surface treatment were investigated. The fatigue crack growth rates for unfilled and glass-filled samples were observed to follow the well-known Paris equation in terms of dependence on cyclic stress intensity factor. For the unfilled nylon a line shaped zone was observed in advance of the crack tip. Fractography results suggest that the zone was the projection of the actual crack tip profile through the thickness of the sample rather than a distinct plastic or deformation zone. The fatigue fracture surface exhibited a patchy type structure with features 50–150 μm in size, suggesting a void coalescence type of mechanism as has been reported for injection molded nylons. A diffuse damage zone, several millimeters in size, was observed at the crack tip for the glass-filled RIM nylon 6. The zone was observed to pulsate with the applied oscillating load. The growth of the damage zone volume with increasing crack length (and thus increasing stress intensity factor range) followed the Paris law, as did the crack growth rate data. The damage mechanism is attributed to void formation and microcracking at the fiber–matrix interface. The results of this study show that, for milled glass-reinforced RIM nylon 6, the crack growth rates were much more rapid than observed for injection-molded nylon 6 containing chopped glass fibers. This difference is attributed to the greatly reduced glass fiber lengths for the milled glasses.     

Fatigue crack propagation in,graphite fiber reinforced nylon 66

The rate of fatigue crack propagation in graphite fiber reinforced nylon 66 was measured. A model of the form å = β [K_max^1-γ ΔK^γ]^r was used to correlate the rate of crack propagation å with the maximum stress intensity K_max and the amplitude of the stress intensity ΔK experienced by the notched specimen during the fatigue test. The quantities β, γ and r were constant at fixed temperature and frequency of the test. It was also found that there exists both an upper and a lower threshold of stress intensity for the slow ropagation of damage during fatigue. The mechanism of crack propagation in the short graphite fiber reinforced nylon was found to be similar to the growth and fracture of crazes in thermoplastics. The propagation of damage at the crack tip is controlled by matrix deformation, cavitation, fiber breakage and fiber pullout. Damage can propagate in the absence of crack growth until a critical point is reached at which time the material fractures catastrophically.     

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.     

Failure of Glass with Subthreshold Flaws

Conventional postthreshold crack analysis cannot be used to predict the strength and fatigue behavior of glass with subthreshold flaws. Therefore, a fracture mechanics model for failure of glass with subthreshold indentation flaws was developed. This model accounts for both the near- and farfield residual stresses associated with the indentation impression. It is shown that these stresses play a major role in the initiation and subsequent propagation of cracks that eventually cause failure. The model predicts "pop-in" of a well-developed crack and failure under continuous and discontinuous crack growth in both inert and fatigue conditions. The results of experiments with bare fused silica fibers with indentation subthreshold flaws in inert and fatigue (water) environments were in good agreement with the predictions by the model.     

Essential work of fracture (EWF) analysis for short glass fiber reinforced and rubber toughened nylon‐6

The effect of fiber content on the fracture toughness of short glass fiber reinforced and rubber toughened nylon‐6 has been investigated using the essential work of fracture (EWF) analysis under both quasi‐static and impact rates of loading. Under quasi‐static loading rate, matrix plastic deformation played a major role. Addition of 10 wt% of short glass fibers into a rubber toughened nylon‐6 matrix improved the fracture toughness substantially. This is due to the synergistic effect that comes from matrix yielding and fiber related energy absorption such as fiber debonding, fiber pull‐out and fiber fracture. With further increasing the glass fiber content, up to 20 and 30 wt%, even though plastic deformation could still take place on the fracture surfaces, the depth of the fracture process zones was much smaller when compared with the system with 10 wt% of glass fibers. The reduction in fracture process zone caused the reduction in fracture toughness. Under impact loading rate, the unreinforced blend still fractured in a ductile manner with gross yielding in the inner fracture process zone and the outer plastic zone. The unrein‐forced blend therefore possesseed higher fracture toughness. For the fiber reinforced blends, the matrix fractured in brittle manner and so fracture toughness of the reinforced blends decreased dramatically. The impact fracture toughness increased slightly after incorporation of a higher weight percentage of glass fibers.     

Application of the J-integral concept for the description of toughness properties of fiber reinforced polyethylene composites

For the determination of toughness properties of HDPE/glass fiber and HDPE/cotton fiber composites, an instrumented Charpy impact test has been used. The interpretation of impact load-deflection curves has been carried out with several concepts of fracture mechanics. Here the limits of linear elastic fracture mechanic (LEFM) have been shown. The change of toughness properties with increasing fiber volume can be described for short fiber reinforced composites with the help of the J-integral concept in a suitable mode. An application of the conventional Charpy impact test will result in an overestimation of material behavior because of the energy of crack propagation. With the help of a micromechanical model to describe failure processes, taking account of energy dissipative processes, it is possible to calculate fracture mechanical material parameters. Because of the peculiarities of deformation and fracture behavior, the application of elastic-plastic fracture mechanic (EPFM)-concepts for fiber reinforced polymers is required.     

Fatigue crack growth and fracture in glass sphere-filled nylon 6

The common degrading effect of glass beads on the static fracture energy and the fatigue crack propagation response in nylon 6 materials is examined by conducting fracture mechanics tests and by considering the progress of cracks through the composites. The scanning electron micrographs indicate that the cracks travel through regions of polymer matrix and also along the interfaces between polymer and glass beads. It is demonstrated that, although fracture of the polymer regions requires considerable energy, cracking of the interfaces usually absorbs very little. Thus, the crack propagation is preferably concentrated on these microstructural regions, which is the cause of the decrease in fracture energy and increase in fatigue crack growth rate with increasing amount of glass spheres in the composite. Partial properties of the matrix and the interface are introduced in order to describe the fracture behavior and to improve the understanding of the gross fracture processes. The combination of these partial properties with the volume fraction of filler and certain geometrical factors by a modified rule of mixture leads to critical values for the failure of the composites, which are in reasonable accord with the measured fatigue and fracture data.     

Influence of thickness and processing history on fatigue fracture of nylon 66. Part I: Crack propagation measurements

The influence of sample thickness on fatigue crack propagation rates in injection molded nylon 66 was determined by preparing 12.7 mm thick plaques along with more conventional 3.0 mm thick samples. Initial results suggested a large effect of thickness as the crack propagation rates were accelerated in the thicker samples and the stress dependence was also increased. Since the calculated thickness for a plane stress to plane strain transition in nylon 66 is 9.0 mm, it was thought that these results were related to the stress state at the crack tip. However, a more thorough study of the thicker plaques has now demonstrated that neither the magnitude nor the stress dependence of the fatigue crack growth rates is necessarily changed under plane strain conditions as similar results can be obtained for thick and thin plaques. It is suggested that the earlier results were confounded by a previously unrecognized processing history effect which does accelerate fatigue fracture. The latter effect is shown by thermal analysis and optical microscopy to be related to a rearrangement of the polymer network during melt processing.     

Dynamic behavior of short aramid fiber‐filled elastomer composites

Short fiber reinforcement is a suitable way to improve the tribological properties of elastomers. However, rubbers products are often exposed to highly dynamic mechanical loadings. Hereby it is crucial to study the change in dynamic behavior due to the addition of short fibers. Therefore, these properties were investigated in terms of dynamic mechanical thermal analysis, heat build‐up (HBU), and fatigue crack growth resistance under cyclic loadings for two different rubber compounds. A peroxide cured ethylene–propylene–diene rubber (EPDM) and a sulfur cured natural rubber (NR) compound were chosen and reinforced with two types of short aramid fibers. It was found that the short fibers could contribute to the improvement in the crack growth resistance, HBU, and the dynamic mechanical behavior of the composites depending on the testing conditions. POLYM. ENG. SCI., 54:2958–2964, 2014. © 2014 Society of Plastics Engineers     

相容剂对长玻纤增强尼龙66树脂力学性能和流变行为的影响

采用自行研制的熔体浸渍包覆长玻纤装置,制备了长玻纤增强尼龙66(LGF-PA66)复合材料.研究了相容剂乙烯-辛烯共聚物接枝马来酸酐(POE-g-MAH)、三元乙丙橡胶接枝马来酸酐(EPDM-g-MAH)对LGF-PA66力学性能和流学行为的影响.结果发现:当相容剂质量分数为2.5%时,复合材料的拉伸强度最大,缺口冲击强度在相容剂质量分数为0%～10.0%范围内近似线性的增加,不同相容剂对力学性能的影响相似.运用了拉伸强度模型和缺口冲击强度模型对实验结果进行了解释.相容剂用量的增加导致了平衡扭矩线性的提高,但对实际加工并没有带来太大的影响.     

A theory for the flexural strength of steel fiber reinforced concrete

A theory is presented to predict the flexural tensile strength of concrete reinforced with short, discontinuous steel fibers randomly oriented and uniformly dispersed in a cement-based matrix. The theory is based on a dual criterion of crack control and composite mechanics. The first crack in the fibrous composite occurs due to bond slip. The fracture process consists of progressive debonding of fibers during which slow crack propagation occurs. Final failure occurs due to unstable crack propagation when fibers pull out and the interfacial shear stress reaches the ultimate bond strength. The theory is supported by test data on fiber reinforced concrete, mortar and paste.     

高纤维含量的短切碳纤维增强尼龙流变行为研究

以40%高纤维含量的短切碳纤维增强尼龙(PA66/SCF(40%))复合材料为研究对象,采用高压毛细管流变仪对其挤出料粒进行稳态流变试验,并采用扫描电子显微镜(SEM)观察其注塑试样拉伸断面表观形貌,深入研究了高纤维含量下短切碳纤维增强尼龙的流变行为。结果表明,随着表观剪切速率增加,材料挤出过程中总压力降不断增加;随着温度增加,总压力降逐渐减小;PA66/SCF(40%)复合材料为假塑性流体,存在剪切变稀行为,在较高剪切速率下,纤维沿流动方向发生取向;材料挤出胀大比与弹性回复有关,挤出胀大比随剪切速率增加而增加,随温度增加而减小。     

增韧增强改性尼龙的研究及应用

综述聚烯烃弹性体、橡胶、苯乙烯系共聚物等增韧尼龙和玻璃纤维、碳纤维和芳纶纤维等增强尼龙及增韧增强相结合改性尼龙的研究进展。介绍了增韧增强尼龙的应用情况。     

Effects of fibers on the crystallization of poly(phenylene sulfide)

The isothermal crystallization of unreinforced poly(phenylene sulfide) (PPS) and PPS filled with glass, carbon, and aramid fibers was studied by differential scanning calorimetry. The Avrami exponent and rate constant are reported, but the crystallization half-times were used to compare the effects of different fibers on the rate of PPS crystallization. The aramid and carbon fibers decreased the crystallization half-time with the aramid fiber having the most pronounced effect. The glass fibers affected the crystallization half-time only at the higher crystallization temperatures. The aramid filled PPS exhibited anomalous degree of crystallinity behavior in that the degree of crystallinity passed through a minimum as a function of temperature. The other systems all exhibited increasing degree of crystallinity with increasing crystallization temperature. Finally, the Avrami plot for the aramid filled PPS is not linear, and the data are fitted better with two linear regions indicating that two types of crystallization processes may be present.     

Fatigue-resistant nylon alloys

Crystalline nylon 66 was modified by blending with both an amorphous nylon and a rubbermodified amorphous nylon. The ternary blends exhibit a 50–100-fold decrease in fatigue crack propagation rates, even at rubber concentrations of only 1 or 2%. These same blends do not necessarily exhibit improved impact strength and the examination of a variety of alloys and blends demonstrates that fatigue and impact fracture mechanisms are distinctly different. The fracture surface morphologies indicate that the basic fatigue fracture mechanism of craze coalescence for nylon 66 is not changed by alloying. However, the presence of the rubbery phase leads to cavitation and ductile drawing that retard the craze breakdown and coalescence processes without evidence of crack tip blunting. Surprisingly, the addition of rubber-modified nylon 66 to a nylon 66 matrix does not impart as great an improvement in fatigue resistance as does the miscible amorphous nylon. Also, alloys having improved impact strength are observed to exhibit inferior fatigue resistance. These results demonstrate that the excellent fatigue resistance of crystalline polymers can be improved even further by judicious selection of alloying ingredients optimized specifically for fatigue fracture. © 1994 John Wiley & Sons, Inc.     

Adhesive joints between carbon fiber and aluminum foam reinforced by surface‐treated aramid fibers

Short aramid fibers have been successfully used to reinforce the interface adhesive property between carbon fiber/epoxy composites and aluminum foam, and to form aramid‐fiber “composite adhesive joints.” In this study, to further improve the reinforcing effect of the aramid‐fiber‐reinforced adhesive joints, aramid fibers were ultrasonic treated to conduct different surface conditions. Critical energy release rate of the carbon fiber/aluminum foam sandwich beams with as‐received and treated interfacial aramid fibers were measured to study the influence of the surface treatment on aramid fibers. It was found that reinforcements in critical energy release rate were achieved for all samples with treated aramid fiber as measured under double cantilever beam condition. The interfacial characteristics of the short aramid fibers with different surface condition were investigated and discussed based on scanning electron microscopy observations. It is suggested that advanced bonding between aramid fibers and epoxy resin was conducted after surface treatment, and more energy was therefore absorbed through fiber bridging during crack opening and extension process. POLYM. COMPOS., 36:192–197, 2015. © 2014 Society of Plastics Engineers