The influence of temperature and absorbed water on the fatigue crack propagation in nylon-6,6

The effect of absorbed water on the fatigue crack propagation (FCP) of nylon-6,6 was investigated over a range of test temperatures and is correlated with dynamic mechanical properties. Both the storage modulus, a measure of specimen stiffness, and the loss compliance, a measure of energy dissipation and hysteretic heating, influence FCP response. At a given temperature, fatigue resistance is greatest for a given water content corresponding to an optimum combination of storage modulus, E′, and loss compliance, D″. The use of an empirical shift parameter to normalize the temperature dependence of the FCP behaviour of nylon with various water contents is discussed.     

Fatigue of hybrid epoxy composites: Epoxies containing rubber and hollow glass spheres

A diglycidyl ether of bisphenol A (DGEBA) epoxy resin was modified by incorporation of varying concentrations of hollow glass spheres (HGS) and/or reactive liquid rubber (CTBN). The fatigue crack propagation (FCP) behavior and mechanisms of such materials were studied in detail. A synergistic phenomenon reported for static fracture toughness was also observed in the FCP resistance of such composites. Optical microscopy studies revealed that the interactions between the stress fields of the crack-tip process zone and HGS cause plastic-zone branching, which in turn gave rise to synergistic toughening. It is also shown that process zone - second phase particle interactions cause a transition in FCP behavior of rubber-modified epoxy polymers, similar to that observed in metal alloys. Consequently, the process zone toughening mechanisms are active only above a certain stress intensity range. Conversely, FCP resistance of both modified and unmodified epoxies were the same below the transition.     

The effect of molecular weight on fatigue crack propagation in nylon 66 and polyacetal

The influence of molecular weight on fatigue and fracture behavior in nylon 66 (N66) and polyacetal (PA) is examined. Fatigue crack propagation (FCP) resistance and apparent fracture toughness (K_cf) in these two semicrystalline polymers increase with increasing molecular weight in a manner consistent with that reported for another semicrystalline polymer (HDPE) as well as for several amorphous polymers. The improved FCP resistance with increasing molecular weight is attributed to the development of a molecular entanglement network that more effectively resists cyclic-load-induced breakdown. A type of discontinous crack growth is identified in PA at 100Hz and in N66 (2.6% H₂O) at 50 Hz and compared with that observed in amorphous polymers.     

Fatigue crack propagation in mica-filled polyolefins

Edge notched samples of polypropylene (PP) and high-density polyethylene (HDPE) containing different mica concentrations were tested in mode I tensile loading. Crack growth was approximated by a non-linear regression of exponential form using statistical software (SAS). Characterization of fatigue crack propagation (FCP) was made using the Paris-Erdogan law. The crack front in PP was preceded by a wide plastic zone in which craze developed, leading to a discontinuous crack growth. Using spline functions, a margin between maximum and minimum FCP rates, recorded during the crack progression, is presented along with the average FCP rates. It is shown that mica-reinforced PP samples exhibit higher FCP rates than unfilled PP. In HDPE, mica reduces FCP rates resulting in a higher resistance to fatigue crack propagation. Effect of test frequency is presented for unfilled polymers and 10 percent mica concentration by weight in both matrices. An increase in the test frequency has no significant effect on FCP rates for both raw and mica-reinforced PP. Unfilled and mica-filled HDPE show noticeable decrease in FCP rates with increasing frequency.     

Fatigue crack propagation behavior of cellulose esters

The effect of plasticizer concentration on fatigue crack propagation (FCP) rate in cellulose acetate-propionate (CAP) was determined. Compact tension specimens were machined from 6.2 mm-thick injection molded plaques and tested on an MTS servohydraulic testing machine using a sinusoidal waveform with a frequency of 1 Hz. Two FCP mechanisms were identified: a crazing mechanism, which dominated at low values of stress intensity factor range, ΔK, and a shear yielding mechanism, which dominated at high values of ΔK. The value of ΔK at the onset of the transition from the crazing mechanism to the shear yielding mechanism was a function of plasticizer concentration, and therefore yield strength of the CAP. The transition in crack propagation mechanism created a V-shaped feature on the fracture surface, which could be used to weight the contributions from the two crack propagation mechanisms to the overall FCP rate.     

Effect of pores on crack propagation behavior for porous Si3N4 ceramics

The mechanism of interaction between crack-tips and pores in the porous ceramics was investigated in this paper. By using the J-integral numerical method, the effect of the pores on the stress-intensity factors of the crack tips is analyzed quantitatively. The presence of the pores may enhance or weaken the stress-intensity factors. The simulation results show that the pores in front of the crack will enhance the stress-intensity factors of the crack tip and promote the crack propagation and the pores at the side of the crack will weaken the stress-intensity factors of the crack tip and prevent the crack propagation. At the same time, the effects of different pores on the stress distribution around the crack-tip, with different pore-crack tip distance or positions, are presented.     

Role of crack tip shielding mechanisms in fatigue of hybrid epoxy composites containing rubber and solid glass spheres

The fatigue crack propagation (FCP) resistance of epoxy-based composites containing various concentrations of solid glass spheres (SGS) and/or reactive liquid rubber (CTBN) was examined. The FCP results show that the simultaneous use of rubber and solid glass spheres (hybrid composites) results in synergistic improvement in FCP resistance of composites through the entire crack growth regime. The nature of synergistic interactions was elucidated by careful examination of the fatigue fracture surfaces and the subfatigue fracture surfaces of fatigue samples. It was shown that when rubber particles cavitate in the vicinity of the glass spheres, regardless of the nature of the interface, glass particle debonding from the matrix is suppressed due to a change in the crack tip localized stress state. This, in turn, results in improved pinning/bridging efficiency of the glass spheres. Furthermore, it was shown that crack tip plastic zone-rubber particle interactions induce a transition in FCP behavior of rubber-modified epoxies. Consequently, crack tip shielding mechanisms become active when the size of the plastic zone at the crack tip becomes large compared to the size of the rubber particles. © 1995 John Wiley & Sons, Inc.     

Crack-tip Degradation Processes Observed during in situ Cyclic Fatigue of Partially Stabilized Zirconia

It is proposed that reduced transformation zone widths in Mg-PSZ in cyclically versus critically propagated cracks are due to reductions in the crack-tip toughness, consistent with an intrinsic cyclic fatigue mechanism. Cyclic fatigue crack growth in Mg-PSZ was observed in situ in a SEM. Following cyclic fatigue, the samples were critically broken and the fracture surfaces observed. Extensive crack bridging by the precipitate phase was observed near the crack tip, and it is proposed that this crack bridging significantly affects the material's intrinsic toughness. Frictional degradation of the precipitate bridges occurs during cyclic loading and hence reduces the critical crack-tip stress intensity factor for crack propagation. Reductions in the critical crack-tip stress intensity factor also lead to reductions in the transformation zone widths during cyclic loading and hence the level of crack-tip shielding caused by phase transformation. This appears to be the mechanism of cyclic fatigue. A degree of uncracked ligament bridging was also observed and is linked with the frequency of random large precipitates. However, analysis shows that its effect upon crack growth rates under cyclic load is limited.     

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.     

New method for extracting fracture toughness of ceramic materials by instrumented indentation test with Berkovich indenter

Applying finite element analysis, a method is proposed for evaluating fracture toughness of ceramic materials by instrumented indentation with Berkovich indenter. The crack-tip K_I (Stress intensity factor) of Berkovich-produced crack is numerically calculated by using virtual crack closure technique, in particular, three kinds of crack pattern, i.e., radial crack, transition crack and half-penny crack are identified and their crack fronts meet the equi-K_I requirement. The validity of the proposed method is verified by instrumented indentation tests on standard SRM2100 (Si₃N₄) and CRM156 (Fused Silica) samples. Comparison with six representative conventional indentation methods indicates that the proposed method has advantages including wide application range, high accuracy and applicability to different crack patterns. Additionally, it’s revealed that the conventional indentation fracture toughness formulae derived from Lawn-Evans-Marshall formula tend to exhibit larger test error when applied to materials of relatively high indentation work ratio W_e/W_t.     

The effect of fiber-matrix adhesion on the fatigue fracture of glass-fiber-reinforced poly(vinyl chloride)

Fatigue crack propagation (FCP) of injection-molded glass-fiber-reinforced poly(vinyl chloride) was examined as a function of fiber-matrix adhesion (coupling) and fiber content at different load levels. Considering the entire FCP history, from crack initiation to critical propagation, it is shown that fatigue lifetime and fracture toughness of coupled composites increase with fiber weight fraction. Uncoupled material exhibits the highest fracture toughness at 10 wt% fiber, yet its fatigue life is considerably shorter. Damage analysis indicates that fiber debonding, pullout, and particularly fiber fracture seem to contribute to the higher fatigue lifetime noted in coupled composites. The Crack Layer Theory is employed to describe the observed FCP behavior. The effective enthalpy of damage parameterizes the resistance of the composite to FCP in terms of the observed mechanisms.     

Fatigue fracture of short-glass-fiber-reinforced poly(vinyl chloride): A crack layer approach

The fatigue behavior and fracture toughness of injection molded short-glass-fiber-reinforced poly(vinyl chloride) (sgfr-PVC) were investigated using the Crack Layer approach and fractography, Fatigue crack propagation (FCP) experiments in single-edge-notched (SEN) specimens were conducted concurrently with microscopic observations. Fracture was observed to propagate as a main crack surrounded by a layer of damage. The magnitude of damage was controlled by the content of glass fiber, which in turn controlled crack reduced acceleration and fracture toughness. FCP behavior was successfully described by the Crack Layer theory, which accounts for the damage associated with crack propagation. In absence of significant interfacial bonding, mechanical fiber/matrix interlocking provided the main resistance to crack propagation. Fiber-induced matrix deformation and fiber pull-out appeared to be the dominant energy absorbing mechanisms.     

Time-dependent craze zone growth at a crack tip in polymer solids

By considering the polymer bulk as a linear viscoelastic body and the craze zone at crack tip as a nonlinear damage zone, the control equation for craze zone growth has been derived. It is shown that for a time-independent craze-zone stress, the craze zone would grow only if the crack-tip stress intensity factor is changed. If the crack-tip stress intensity factor remains constant during loading, the growth rate of the craze zone length will be interrelated to the crack-tip stress, the craze zone length and the rate of change of the craze-zone stress. If both the craze-zone stress and the crack-tip stress intensity factor are time-independent, the craze zone length will be constant during the crack growth, which is the case of self-similar crack growth. Moreover, a new stress distribution model in craze zone is presented based on the constructed damage evolution law, and the lengthening and thickening of the craze zone at the crack tip are also formulated. The numerical calculations from the proposed model agree well with the published experimental data.     

Mechanical behavior of injection-molded polystyrene/polyethylene blends: Fracture toughness vs. fatigue crack propagation

Blends of polystyrene and polyethylene (PS/PE), including belnds in which a styrene/ethylene-butylene/styrene (SEBS) terpolymer was employed as a compatibilizer, were studied. Their rheology showed that the effect of the addition of SEBS to PS/PE blends was strongly affected by the blend composition and the shear rates involved in the blending and post-forming processes. The addition of PE to PS led to a reduction of fracture toughness compared with that of PS. This effect was attributed to the fine minor phase morphology of the blends obtained after extrusion blending and injection molding. The fatigue crack propagation (FCP) results showed that the fatigue crack growth rates were significantly reduced at low and moderate range of stress intensity factor (ΔK) by the presence of PE. Performance was enhanced when SEBS was present. The results also showed that both the fracture toughness and the FCP behavior of the blends were strongly dependent on the loading direction, the minor phase morphology, the composition of the blend, and, to a lesser degree, the presence of a compatibilizer. This study demonstrates that the fracture toughness and the FCP performance of such polymer blends can vary inversely.     

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.     

A simple viscoelastic model for fatigue crack propagation in polymers as a function of molecular weight

A simple theory is presented to explain the strong influence of molecular weight (M) on rates of fatigue crack propagation (FCP) in amorphous polymers. It is proposed that the equation describing FCP rates may be expressed as the product of two functions, one involving the stress intensity factor (ΔK), and the other characterizing the relaxation process occurring in the plastic zone. To provide a physical network in the plastic zone that can sustain fatigue loading, it is proposed that one needs a sufficient fraction of molecular fibrils per unit area ($\text{W1}$) whose lengths are greater than M_c, the critical value of M required for entanglement. This effect can be summarized as a generalized rate process (confined at the plastic zone) expressed by A exp (Bσ) where σ is a stress and A and B are constants (B including the volume of activation). It is deduced that M influences the activation volume through the values of $\text{W1}$ and W, the weight fraction of molecules whose M>M_c. Using the equation developed it was possible to correlate FCP data of PVC and PMMA as a function of M with a high degree of confidence. Also, the value of activation volumes obtained compared favourably with those in the literature for static tests. The complementary value of $\text{W1}$ for these polymers was also seen to approximate closely to the void fraction in a craze. Extension to other cases such as semi-crystalline materials also seems possible.     

Impact fracture morphology of nylon 6 toughened with a maleated polyethylene–octene elastomer

This study aimed at using scanning electron microscopy to study the Izod impact fracture surface morphology of super‐tough nylon 6 blends prepared by blending nylon 6 with a maleic anhydride‐grafted polyethylene‐octene elastomer (POE) in the presence of a multifunctional epoxy resin (CE‐96) as compatibilizer. The fracture surface morphology and the impact strength of the nylon 6 blends were well correlated. The fracture surface morphology could be divided into a slow‐crack‐growth region and a fast‐crack‐growth region. Under low magnification, the fractured surface morphologies of the low‐impact‐strength nylon 6 blends appeared to be featureless. The area of the slow‐crack‐growth region was small. There were numerous featherlike geometric figures in the fast crack growth region. The fractured surface morphologies of the high‐impact‐strength nylon 6 blends exhibited a much larger area in the slow‐crack‐growth region and parabola markings in the fast‐growth region. Under high magnification, some rubber particles of the low‐impact‐strength nylon 6 blends showed limited cavitation in the slow‐crack‐growth region and featherlike markings in the fast‐crack‐growth region. Rubber particles of high‐impact‐strength nylon 6 blends experienced intensive cavitation in the slow‐crack‐growth region and both cavitation and matrix shear yielding in the fast‐crack‐growth region, allowing the blends to dissipate a significant amount of impact energy. A nylon 6 blend containing 30 wt % POEgMA exhibited shear yielding and a great amount of plastic flow of the matrix throughout the entire slow‐crack‐growth region, thus showing the highest impact strength. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 1285–1295, 2000     

Fatigue crack propagation and damage evolution in PBT-GF and SAN-GF

Our earlier investigations of fatigue behavior in PBT-GF and SAN-GF with different fiber lengths have shown that fatigue crack propagation (FCP) can be described in terms of elastic-plastic fracture mechanics. In this work it is shown that the influence of structural material parameters on the resistance to FCP correlates with the extent of energy dissipation at the crack tip. With increasing fiber length, the zone of energy dissipation is increased. By means of microscopic investigations, the prevailing damage in the zone of energy dissipation is identified as micro cracks in the matrix.     

Additive effect of micro-damage zones from nano-scale crack tip and nano-grains for fracture toughness measurements of 3Y-TZP zirconia ceramics

A nano-scale crack tip around 500 nm wide introduced by femtosecond laser still affects the accuracy of fracture toughness K_IC measurements of 3Y-TZP zirconia ceramics with average grain size G from 200 to 500 nm. A simple formula was proposed to estimate the additive effect of crack-tip damage zones from an infinitely sharp crack to a nano-scale blunt notch. The error in fracture toughness measurements is less than 8 % if the nano-scale crack-tip width < 0.5·G. The intrinsic K_IC can be deduced from the simple formula if the nano-scale crack tip > 0.5·G. This study shows the same K_IC was deduced from two different sets of 3Y-TZP measurements with nano- and micro-scale notches of 500 nm and 18 µm wide. Furthermore, the simple formula specifies the relation between the fracture toughness K_IC and intrinsic strength f_t via grain size G, which means K_IC can also be estimated from f_t and G without testing pre-cracked specimens. K_IC values of 3Y-TZP from specimens with and without pre-cracks were compared.     

Crack tip damage analysis in fatigue fracture of epoxy resin

The application of the crack layer theory to fatigue crack propagation (FCP) in epoxy is discussed. A crack tip damage evolution coefficient μ is introduced to assess the extent of damage as a fraction of the damage associated with critical crack propagation. The results can be expressed in the form where dl/dN is the rate of FCP, G₁ is the energy release rate whose critical value is G_1c, and β is a phenomenological constant. Although no damage was detected from microscopic analyses, μ increases fivefold during stable crack propagation. Fractal analysis of fracture surface profiles provides a quantitative measure of the roughness associated with crack advance. The fractural measure d is found to evolve in a similar fashion as μ, suggesting the applicability of d to quantify crack tip damage evolution.