Fatigue behavior of filament‐wound glass fiber reinforced epoxy composite tubes under tension/torsion biaxial loading

A study of filament‐wound glass fiber/epoxy composite tubes under biaxial fatigue loading is presented. The focus is placed on fatigue lives of tubular specimens under tension/torsion biaxial loading at low cycle up to 100,000 cycles. Filament‐wound glass‐fiber/epoxy tubular specimens with three different lay‐up configurations, namely [±35°]_n, [±55°]_n, and [±70°]_n lay‐ups, are subjected to in‐phase proportional biaxial cyclic loading conditions. The effects of winding angle and biaxiality ratio on the multiaxial fatigue performance of composites are discussed. Specimens are also tested under two cyclic stress ratio: R = 0 and R = −1. The experimental results reveal that both tensile and compressive loading have an influence on the multiaxial fatigue strength, especially for [±35°]_n specimens. A damage model proposed in the literature is applied to predict multiaxial fatigue life of filament‐wound composites and the predictions are compared with the experimental results. It is shown that the model is unsuitable for describing the multiaxial fatigue life under different cyclic stress ratios. POLYM. COMPOS. 28:116–123, 2007. © 2007 Society of Plastics Engineers     

Effect of methylene chloride sorption on the mechanical properties of poly(aryl-ether-ether-ketone) (PEEK)

The effect of sorbed methylene chloride on the tensile strength and fatigue crack growth (FCG) resistance of PEEK were determined. PEEK sorbs up to 23 wt% methylene chloride; the transport process is essentially Case II, that is, the methylene chloride advances as a sharp front. Sorbed methylene chloride significantly reduces the tensile strength of neat PEEK and the strength reduction is linearly proportional to the amount of solvent sorbed. FCG rates in neat PEEK are increased by the methylene chloride sorption. At saturation, the FCG rates are two orders of magnitude higher than in dry PEEK. Methylene chloride plasticizes the resin, thereby reducing its glass transition temperature (T_g), tensile strength, and FCG resistance.     

Peak stress intensity dictates fatigue crack propagation in UHMWPE

The majority of total joint replacements employs ultra-high molecular weight polyethylene (UHMWPE) for one of the bearing components. These bearings may fail due to the stresses generated in the joint during use, and fatigue failure of the device may occur due to extended or repeated loading of the implant. One method of analysis for fatigue failure is the application of fracture mechanics to predict the growth of cracks in the component. Traditional analyses use the linear elastic stress intensity factor K to describe the stresses near a loaded crack. For many materials, such as metals, it is the range of stress intensity, ΔK, that determines the rate of crack propagation for fatigue analysis. This work shows that crack propagation in UHMWPE correlates to the maximum stress intensity, K_max, experienced during cyclic loading. This K_max dependence is expected due to the viscoelastic nature of the material and the absence of crazing or other cyclic load dependent crack tip phenomena. Such a dependence on a non-cyclic component of the stress allows cracks to propagate under load with little or no fluctuating stresses. Consequently, traditional fatigue analyses, which depend on the range of the stress to predict failure, are not always accurate for this material. For example, significant static stresses that develop near stress concentrations in the component locking mechanisms of orthopedic implants make such locations likely candidates for premature failure due the inherent underestimate of crack growth obtained from conventional fatigue analyses.     

Characteristic analysis of biaxially stretched PVDF thin films

By use of biaxial stretching process of poly(vinylidene fluoride) fabrication to gain high piezoelectricity is the focus in this article. The influence of different stretch ratio and temperature are investigated and compared to uniaxial stretching process. The Fourier transform infrared spectroscopy and X‐ray diffraction are used to observe the β‐phase fraction and the degree of crystallinity. The piezoelectric coefficients (d₃₃, d₃₁) and the sensitivity are also measured. All the above characteristic examinations show the same consequences that equi‐biaxial stretching process employed with same stretch ratio in both axial directions of poly(vinylidene fluoride) fabrication is the focus in this article. And heating temperature would have adequate piezoelectricity approximate to uniaxial stretching. In observation of surface morphology by using scanning electron microscopy, uniformity in biaxial direction is gained. However, biaxial stretching with higher stretch ratio of R = 4 × 4 would produce porosity and crack. Via atomic force microscopy, biaxial stretching with higher stretch ratio attains better surface chain orientation and smoothness. To sum up, the biaxial stretching approach has the advantage of low stretch ratio requirement to preserve good fabrication quality. Set up with stretch ratio in‐between R = 3 × 3 and R = 4 × 4 at 80 °C is suggested to use in biaxial stretching. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46677.     

Growth of fatigue cracks in polycarbonate

The rapid increase in the rate of application of thermoplastics in engineering design problems and the interest in the structural use of these materials have resulted in the requirement of comprehensive information about the behaviour of thermoplastics when subjected to cyclic loading conditions. In addition to the “total fatigue life” data already available for many materials, attempts have been made to analyse the crack initiation and steady crack growth processes and determine the effects of parameters such as mean load, frequency and crack geometry on the rate of crack propagation. The results of an investigation of these aspects of fatigue crack growth in a brittle thermoplastic, polymethylmethacrylate (PMMA), have already been reported. In this paper, the results of a test program devised to study the behaviour, at room temperature and in air, of a polycarbonate, (PC), under similar loading conditions, are presented. Fracture Mechanics concepts have been used to analyse the results. It was found that a relationship of the form ?_N = β λⁿ already shown to predict the cyclic fatigue crack propagation rate in PMMA, is also applicable to polycarbonate. However, when the effects of frequency and loading rate were studied, it was found that after the magnitude of parameter K?( = ΔK/half the periodic time) exceeded 4000 lbf in. ^?3/2 s^?1, the influence of the mean level of stress intensity factor, K_m, became negligible in comparison to the effect of ΔK.     

Fatigue crack growth in polymers. I. Effect of frequency and temperature

The effects of frequency, from 0.1–100 Hz, and temperature, ?60°C to +21°C, on fatigue crack propagation in poly (methyl methacrylate) and polycarbonate were investigated. A cyclic crack propagation law proposed by Arad-Radon-Culver, namely where λ is (K_max²-K_min²) and K_max and K_min are the respective values of maximum and minimum stress intensity factor, was applied to describe a relationship between crack growth and cyclic life. Cyclic tests performed in tension between zero load and K_max showed a linear relationship between the crack lengths and the number of cycles for all temperatures and frequencies tested. It was found that, in general, the cyclic crack growth decreased with decreasing temperature and increasing frequency. However, important exceptions to this rule have been noted.     

Fatigue crack growth in polymers. II. The interaction of mean stress,frequency, and temperature

Fatigue crack propagation was studied in large centernotched plate specimens of two polymers, poly(methyl methacrylate) and polycarbonate, under tensile cycling conditions. Tests were performed at frequencies of 0.1, 5.0, and 20 Hz, and at two temperatures, ?60 and +21°C. The interaction of mean stress intensity, frequency and temperature was investigated. It was found that in tests performed at constant frequency or temperature, the fatigue crack propagation rates were dependent on both the range of the applied stress intensity factor and its mean value, K_m. All propagation rates increased with increasing K_m. Also, the threshold stress intensities decreased with increasing K_m, suggesting very low levels of ΔK for non-propagating cracks, certainly below the 0.4 K_c, the minimum level investigated here. In PMMA, decreasing crack growth rates with increasing frequency were established for a wide range of K_m. However, in PC crack growth rates increased substantially with increasing frequency. Finally, “upper and lower transition points” were noted on crack growth curves of both materials.     

Fatigue crack growth behavior and mechanism of closed‐cell PVC foam

The applicability of linear‐elastic fracture mechanics parameters (ΔK and K_max), elastic–plastic fracture mechanics parameter (ΔJ), and time‐dependent fracture mechanics parameter (C*) to characterize fatigue crack growth (FCG) rate of closed‐cell polyvinyl chloride foam was investigated in the present work. The effect of stress ratios (R = 0.1 and 0.4) on FCGs was observed when the ΔK, K_max and ΔJ were used as fracture mechanics parameters. As a fracture mechanics parameter that combines ΔK and K_max, the K* successfully characterized FCGs (da/dN) at R = 0.1 and 0.4. While, a time‐dependent fracture mechanics parameter (C*) successfully correlated da/dt of creep crack growth (CCG) test, but it failed to correlate da/dt of FCG tests. The FCGs at both R = 0.1 and 0.4 were cyclic dependent, while the CCG was time dependent. For cyclic‐dependent crack growth, the interaction between polymer‐chain scission and small scale crack‐tip blunting was the main mechanism, whereas the interaction between polymer‐chain pullout and large scale crack‐tip blunting dominated fracture process for time‐dependent crack growth. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers     

Compressive test and numerical simulation of center‐notched composite laminates with different crack configurations

Experimental and computational studies of the composite laminates with thin center notches under axial compressive loading are carried out. A series of compressive testing of the composites with different crack lengths and angles between the loading vector and 0° fiber direction were conducted. The damage mechanisms as well as load–displacement curves are obtained from the test to analyze the effects of crack dimensions on stress distribution and ultimate load. It was shown that the compressive strength of composites drastically reduces when the crack angle goes from 0° to 90°. By studying the fracture surfaces of the tested specimens, all initial cracks within the laminates are found to extend without a straight crack path until fibers fracture simultaneously. Cases that involve crack propagation are modeled for different crack dimensions with a 3D progressive damage finite element analysis using the Abaqus. Numerical simulations qualitatively reproduce the general observations made in the laboratory experiments. POLYM. COMPOS., 38:2631–2641, 2017. © 2015 Society of Plastics Engineers     

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.     

Effect of cooling rate and crack propagation direction on the mode 1 interlaminar fracture toughness of biaxial noncrimp warp‐knitted fabric composites made of glass/PP commingled yarn

The mode 1 interlaminar fracture toughness of biaxial (±45°) noncrimp warp‐knitted fabric composites made of glass/PP commingled yarn was investigated. The crack propagation along the warp and weft directions, respectively, was considered for the composites cooled at two different rates during laminate molding. The interlaminar fracture toughness was characterized by determining the critical strain energy release rate (G_IC) of initiation and propagation measured from the double cantilever beam tests. In the case of a slow cooling rate (1°C/min), most specimens possess pure interlaminar crack propagation and direction‐independence characteristics. Nevertheless, the high‐cooled (10°C/min) specimens fractured in both directions suffer extensive intraply damage (crack branching, debonding, and bridging of 45°‐oriented interfacial yarns) and knit thread breakage, leading to G_IC of propagation two times higher than that of the slow‐cooled specimens, and the clear difference in the G_IC values of initiation between the two directions may be due to the contribution of the knit thread breakage to the fracture energy. POLYM. COMPOS., 2008 © 2007 Society of Plastics Engineers     

High temperature failure envelopes for thermosetting composite pipes in water

Abstract

This paper presents the results of an investigation of the biaxial stress–strain behaviour of filament wound glass fibre reinforced composite pipes exposed to high temperature water. Two matrix systems were investigated: cycloaliphatic amine cured epoxy resin; and siloxane modified phenolic alloy. Water absorption tests on pipe using the two systems at 95°C showed equilibrium moisture contents of 0.5 and 4.5%, respectively, saturation being achieved within seven days at this temperature in both cases. The axial moduli of the pipes were determined at temperatures up to 160°C, using a bending test. Reductions were observed in the T _g of both systems in the water saturated condition. Biaxial loading tests were carried out on the two pipe systems at temperatures from 20 to 160°. The results are presented in the form of failure envelopes and stress–strain relationships under load. At the highest temperatures (above its T _g ), significant weakening of the epoxy system was observed, especially in matrix dominated loading conditions. In contrast, the failure envelopes for the phenolic system showed remarkably little temperature influence.     

Effect of stress state and polymer morphology on environmental stress cracking in polycarbonate

Environmental stress crazing or cracking (ESC), a long studied phenomenon, is the brittle failure of glassy thermoplastics, which are normally ductile, under the synergistic action of stress and certain surface active agents. This work involves a study of a polycarbonate—oleic acid system under two novel in‐depth conditions: multiaxial stress states and changes in the polymer morphology. Initial uniaxial creep tests showed that the formation of cracks rather than crazes is observed. Multiaxial testing is done using blister tests where the polycarbonate film is stressed using a pressurizing medium to form a blister that is in a biaxial state of stress. Changes in the polymer morphology are induced by orientation of the polymer film. On samples exposed to stress and surface active agents, the stress component that is perpendicular to the direction in which the crazes/cracks form appears to influence the crack patterns. The polymer orientation has a significant influence. The orientation not only induces crack formation in a direction parallel to the orientation (regardless of the direction of the major principal stress), but it also reduces the stress required for crack formation for all stress states. Any attempt at modeling the phenomenon of environmental stress cracking therefore needs to take these effects into account. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 550–564, 2003     

脆性材料的亚临界裂纹扩展和双向应力的影响

分析了陶瓷,玻璃等脆性材料的断裂机理和表征.指出了脆性材料的失效过程的三种不同的裂纹发展模式.实验研究了玻璃在静载下的亚临界裂纹扩展特征及其受双向应力的影响.从而为脆性材料的可靠性和寿命评价提供理论基础和手段.     

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.     

Fatigue crack growth in polyethylene

Fatigue crack propagation (FCP) rates are studied in 6 mm thick specimens of high density polyethylene (HDPE) containing razor notches, Centrally-notched plates and single-edg notched bars are subjected to sinusoidal tension-compressio or tension-zero cycling at 0.5 or 2.0 Hz under load control a room temperature; crack growth is monitored using a travelling microscope. After many thousands of cycles with no observable damage at the tip of the razor notch, a craze like zone begins to form. This zone grows slowly until it reaches the length characteristic of a mature crack at the same ΔK. Crack growth proper then begins. The number of cycles to initiate crack growth falls linearly with increasing ΔK at the razor notch Subsequent crack growth is determined both by the current value of ΔK and by loading history. When ΔK is increasing, FCP rates follow a standard Paris law curve. However, reduced, FCP rates are observed following an overload.     

Thermal Shock Resistance of Laminated ZrB2–SiC Ceramic Evaluated by Indentation Technique

In this work, the thermal shock behavior of laminated ZrB₂–SiC ceramic has been evaluated using indentation‐quench method based on propagation of Vickers cracks and compared with the monolithic ZrB₂–SiC ceramic. The results showed that the laminated ZrB₂–SiC ceramic exhibited better resistance to crack propagation and thermal shock under water quenching condition, and the critical temperature difference (ΔT_c) of laminated ZrB₂–SiC ceramic (ΔT_c ≈ 590°C) was much higher than that of monolithic ceramic (ΔT_c ≈ 290°C). The significant improvement in thermal shock resistance was attributed to residual stresses enhancing the resistance to crack growth during thermal shock loading.     

Rolling contact crack growth in a PMMA disc

A two‐disc machine was used to study crack propagation rate in polymethylmethacrylate (PMMA) subjected to cyclic loading created by a rolling contact. An initial crack of known length was created in PMMA disc, and its growth under known load conditions continuously monitored. It appears that after initial rapid crack growth there is a period of noticeable reduction in the crack growth rate. A rate equation based on the Paris approach is derived and its coefficients found to be m_p = 0.705 and C_p = 2.93 × 10⁻⁴. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 2311–2317, 2000     

Dynamic stress intensity factors during viscoelastic crack propagation at various strain rates

Dynamic stress intensity factors K_D were measured by the caustic method and crack propagation velocity ? by the velocity gauge techniques for PMMA [poly(methyl methacrylate)] during dynamic crack propagation at various strain rates \documentclass{article}\pagestyle{empty}\begin{document}$ \rm \dot \varepsilon $\end{document} . No definite applied strain rate effects on the dynamic stress intensity factor were observed for applied strain rates ranging from 8.33 × 10^?4 to 30/sec; however, the test results do show crack propagation velocity dependency in K_D ～ ? relations. The high local strain rate region may be realized at the running crack tip even under the quasi-static loading case of \documentclass{article}\pagestyle{empty}\begin{document}$ \rm \dot \varepsilon $\end{document} = 8.33 × 10^?4/sec, since all the crack propagation velocities obtained were greater than 50 m/sec even up to 450 m/sec.     

Discrete element simulation of SiC ceramic containing a single pre-existing flaw under uniaxial compression

A model of a SiC ceramic containing a single pre-existing flaw was established based on the discrete element method. The effects of the flaw inclination angles, which ranged from 0° to 75°, on the mechanical properties of the specimen under uniaxial compression were studied. The evolution of the force-chain field, displacement field and stress field around the pre-existing flaw in the process from the load to failure was also analysed. The results showed that the flaw inclination angle affected the mechanical properties of the specimen as well as the initiation and propagation of the first crack. Based on the investigation of the force chain field, it was found that the distribution curve of the normal force carried by the parallel bond in the specimen with the corresponding angles under compression is similar to the “peanut” rose diagram, while the shear force distribution curve is similar to the "butterfly wings" rose diagram. In addition, in the analysis of the displacement field and the stress field, the displacement field around the flaw can be divided into four types in the process from specimen loading to its failure. Meanwhile, it was found that initiation of the first crack was affected by tensile stress. With the propagation of the first crack, the tensile stress concentration region at the flaw tip moved and dissipated correspondingly.