Fatigue fracture of long fiber reinforced nylon 66

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

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.     

A preliminary study of the fracture morphology of acrylic fibers

A preliminary study has been made of the tensile and fatigue fracture morphology of acrylic fibers. Tensile breaks show a characteristic granular fracture surface which suggests the separate failure of fibrillar units in the fiber structure. In some instances, there are separate crack and final failure regions; in others, the fracture is in transverse steps linked by axial splits. The main characteristic in tensile fatigue is axial splitting of the fibers. The loading conditions for fatigue failure are less severe than in steady loading there is no zero minimum load criterion for fatigue failure as found in some other fibers.     

Toughness Optimization of SBR Elastomers – Use of Fracture Mechanics Methods for Characterization

Elastomer materials are used in a wide application range and subjected to different loading from which failure of the material results. Because this failure is caused by initiation and propagation of cracks, the application of fracture mechanics methods for the assessment of the material is obvious. A short summary of the methods of technical fracture mechanics including possibilities of determination of crack resistance curves is given. Vulcanizates on the basis of SBR 1500 with various sulfur and carbon black contents were investigated. For describing the crack initiation and crack propagation behavior, several fracture mechanics examination methods were applied. Tear‐analyzer results were used to assess the crack propagation behavior under fatigue‐like loading conditions. Furthermore, for the characterization of the crack resistance of the materials under impact‐like loading conditions, instrumented tensile‐impact tests were performed. To obtain information about the initiation and propagation of a stable crack, quasi‐static fracture mechanics tests were applied. The results of the several tests are discussed in dependence on sulfur and carbon black contents. We found a non‐monotonous behavior of the toughness as a function of carbon black loading. An explanation is given in connection with a percolation‐like transition in filler morphology on larger length scales.

Schematic crack propagation curve for characterizing the fatigue behavior of the vulcanizates recorded in a TFA test.     

Micromechanisms of interlaminar fracture and fatigue

The interlaminar fracture and fatigue properties of AS/3501-;6 graphite/epoxy are discussed from a mechanistic point of view. Particular emphasis is placed on the interaction between the loading mode and the local geometry of the interlaminar zone and on how this affects the stresses close to the crack tip and the resulting failure path. Delamination growth under Mode I loading is shown to depend on the likelihood of fiber bridging occurring and on how effective these bridged fibers are at diverting strain energy away from the crack tip. The Mode II behavior is controlled by both the work required to shear the fibers from the matrix and the ease with which tensile failure of the matrix between the fibers can occur.     

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.     

Fatigue fracture behavior of chlor-alkali membranes

The fatigue fracture behavior of chlor-alkali monolayer membranes is investigated under a condition similar to the membrane electrolysis cell service environment, i.e., at 86°C in brine solution. The linear elastic fracture mechanics (LEFM) approach, i.e., da/dN vs. AK, is implemented in this study. It is found that the fatigue crack propagation behavior of the chlor-alkali membranes can be successfully characterized via the LEFM technique. The fatigue fracture surfaces of the membranes are also investigated using stereo-optical microscopy and are found to show typical fatigue striation patterns. Detailed experimental procedures for preparing and testing of the membranes are presented.     

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.     

Improving the crack resistance of bulk molding compounds and sheet molding compounds

Fiber-reinforced plastics exhibit two types of mechanical failure: gross fracture and microcracking. Gross fracture involves both matrix and fiber failures. Principal resistance to crack propagation derives from partial decoupling of fibers and then stressing, remove finite volumes of them to fracture. Classical concepts of fracture mechanics can be applied to such composites, though modifications of methodology to treat anisotropy and other special effects are required. Microcracking occurs principally in the matrix phase and usually accompanies cyclic fatigue, drop impact, bending, or rapid cooling from molding temperatures. It lowers composite stiffness, environmental resistance and may reduce strength. Matrix resins require high fracture toughness to minimize or eliminate microcracking. This paper discusses cracking in bulk molding compounds and sheet molding compounds, complex materials containing high percentages of glass fibers and calcium carbonate filler. Microcracking can be greatly reduced by tire addition of small amounts of a rubber to the polyester matrix. Various tests such as impact, bending, acoustic emission and crack propagation demonstrate the improved toughness properties which result. No sacrifice of original strength characteristics occurs, and markedly improved resistance to damage has been noted with rubber modified epoxy and polyester matrix resins.     

The superposition model and environmental crack propagation in low-density polyethylene

A study has been made of dynamic and static fatigue failure phenomena in low-density polyethylene (ρ = 0.912) of MF17 in a methanol environment using SEN specimens. The type of failure, namely, crack propagation, was the same in both conditions and the results were analyzed using fracture mechanics concepts. A previous model for correlating environmental dynamic and static fatigue crack growth in metals was applied to this polymeric material, and good agreement was found between experimental and predicted behavior when the strain-rate sensitivity of the threshold value of the stress intensity factor was taken into account.     

Modeling of fatigue failure for SiC/SiC ceramic matrix composites at elevated temperatures and multi-scale experimental validation

The fatigue failure of ceramic matrix composites at elevated temperatures was predicted using the micromechanics method. Multiple micro-damage models were developed to describe the evolution processes of matrix cracking, interface wear, and fiber fracture during fatigue loading. On this basis, the fatigue life was calculated. To validate the fatigue failure model, multi-scale experiments were conducted. In the macroscale, the S-N curve was obtained by the fatigue test. In the microscale, multiple in-situ measuring methods were developed through which the matrix crack density, the interfacial shear stress, and the percentage of fracture fibers were obtained. Both the macroscale and microscale experimental results were in good agreement with the predicted results. Therefore, the fatigue failure model developed in the present work is accurate.     

Predicted Static Fatigue Behavior of Specially Coated Optical Glass Fibers

Long-term static fatigue behavior of optical glass fibers is controlled by their strength, fatigue resistance, and zerostress aging behavior. The effectiveness of four special coatings in preventing the long-term static fatigue deterioration of optical glass fibers was evaluated by determining the dynamic fatigue behavior and the effects of zero-stress aging on strength of the four specially coated optical glass fibers in water from 25° to 85°C. The results clearly show that the strength, fatigue resistance, and aging behavior varied significantly between these specially coated fibers. By analysis of these experimental results in terms of fracture mechanics principles, the predicted static fatigue behaviors of the four fibers were compared. Ideally the optimum fiber is one that exhibits a high strength, low strength variability, high fatigue resistance, and high aging resistance. Each of these specially coated fibers had a deficiency in at least one of these properties.     

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.     

Analysis of Effect of Crack Interaction on Nature of Strength Loss in Thermally Shocked Brittle Ceramics

A fracture mechanics analysis is presented to study the effect of collinear crack interaction on the crack stability and propagation behavior in a brittle material subjected to thermal stress failure. It is shown that collinear crack interaction may result in crack coalescence and complete failure.     

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.     

Combined Mode I-Mode III Fracture of Fatigue-Precracked Alumina

The mixed-mode fracture behavior of (cyclic) fatigue pre-cracked ceramic specimens was studied in combined tension-torsion loading. Circumferentially notched cylindrical rods of polycrystalline alumina were precracked in uniaxiai cyclic compression to introduce a concentric mode I fatigue crack. Subsequently, the rods were quasi-statically fractured in pure tension, pure torsion, and various combinations of tensile and torsional stresses to obtain the mode I-mode III fracture envelope. The introduction of torsional loads promotes severe abrasion between the crack faces. The critical stress intensity factor for fracture initiation increases by a factor of °2.3 as the loading mode is changed from pure tension to pure torsion. Fracture surface tortuosity and abrasion "shield" the crack-tip from the far-field tensile and torsional loads to cause an apparent toughening effect. The mechanisms of mixed-mode fracture in alumina are examined and consequences of the breakdown of the similitude concept implicit in the nominal use of fracture mechanics are discussed.     

Sensitivity of fracture strength in porous glass

We investigated the effect of porosity and crack size distributions on the fracture behavior of porous glass through a combined finite element and fracture mechanics method. Simulations showed that the effect of crack size distributions on the change in fracture strength with porosity decreases as the pore size to crack size ratio increases. For a pore size to crack size ratio of >∼4, the average failure initiating crack size decreases with increasing porosity. Two regions were defined to describe the relationship between porosity and fracture strength: Region I for porosity less than 20 vol.% and Region II for porosity greater than 20 vol.%. Simulation results were directly compared to the porous glass experiments from the literature.     

Material deformation and fatigue behavior characterization for elastomeric component life predictions

Failure analysis and prediction of fatigue life of elastomers are important issues due to wide usage of elastomeric components in many applications. Material deformation and fatigue characterization including both crack nucleation and crack growth are typically required for such analysis and predictions. This article discusses relevant material deformation properties obtained from experiments conducted under stress states of simple tension and planar tension. Cyclic transient behavior including Mullin's effect is also discussed. Then, fatigue crack initiation approach and experimental procedure and obtained data are discussed including modeling of the R ratio effect. Finally, based on the fracture mechanics approach, fatigue crack growth test procedure and properties are presented, including a model for R‐ratio effect. It is also shown that using the crack growth approach as a total fatigue life predictor gives reasonable results, as compared with the crack nucleation approach. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers     

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.     

Some observations on the fracture of poly(ethylene terephthalate) film

Recent work on the applications of linear elastic fracture mechanics to the problem of ductile tearing and fracture of poly(ethylene terephthalate) film is reviewed. It is shown that, even for a brittle plastic, there can be no direct correlation between the amount of slow crack growth and stress intensity factor. The Dugdale model, combined with a constant crack opening displacement criterion, is then used to correlate both initiation and final fracture results for poly(ethylene terephthalate) film extended in the extrusion direction. Also, the model is able to justify the use of linear elastic fracture mechanics for a limited range of crack initiation results.