Discontinuous Growth Bands Research Articles

硬質ポリ塩化ビニルの疲労き裂進展と破面形態

The fatigue crack propagation behavior and the fracture surface appearance under several load conditions in polyvinyl chloride were investigated. The crack closure behavior was also examined. The fatigue crack propagation rate, da/dN, under constant amplitude loading increased with increasing stress ratio in the relationship of da/dN versus the stress intensity factor range, ΔK. This effect of stress ratio on da/dN was closely related to the crack closure behavior when the stress ratio was not so high. Discontinuous growth bands (D.G.B.) on the fracture surface seemed to be difficult to be formed when the stress ratio was very high. The width of D.G.B. was found to be dominated by the effective stress intensity factor range, ΔKeff. The crack extension during loading a single peak overload was very large and so it could be clearly identified on the fracture surface. This region of the crack extension on the fracture surface revealed granular markings containing many voids, which was similar to that found in D.G.B.. The amount of this crack extension was about equal to the size of the plastic zone at the crack tip formed by the overload under a plane strain condition.

Fatigue propagation behaviour of polystyrene/polyethylene blends

Fatigue crack propagation (FCP) of injection-moulded polystyrene (PS) and 95/5, 85/15 and 70/30 PS/high-density polyethylene (HDPE) blends at loading frequencies of 2 and 20 Hz was studied. The FCP results showed that increasing the HDPE content caused a progressive reduction of the fatigue crack growth rates, especially when a styrene/ethylene– butylene/styrene (SEBS) terpolymer was added as a compatibilizer. Increasing the loading frequency also led to a fatigue crack growth rate reduction. Moreover, the fatigue crack growth rates were lower at a given cyclic stress intensity factor range, ΔK, when the crack propagated normal, instead of parallel, to the melt-flow direction during injection moulding. Fractographic observations indicated that discontinuous growth bands (DGBs), associated with the fracture of crazes in the plastic zone, were present through most or all of the fracture surfaces of the PS/HDPE specimens. In the presence of sufficient HDPE, these DGBs were formed by the initiation, growth and coalescence of large dimples initiated at HDPE particles ahead of the microscopic crack front, similar to a multiple crazing effect. The loading frequency effect on the FCP behaviour of these blends is attributed to a time-dependent deformation process. It is concluded that the FCP behaviour of these blends is strongly affected by the loading direction with respect to the matrix and minor phase orientation, by the presence of a compatibilizer, by the composition of the blend and by the testing conditions. © Chapman & Hall.

A fracture mechanics approach to environmental stress cracking in poly(ethyleneterephthalate)

Environmental stress cracking of amorphous poly(ethylene terephthalate) in aqueous sodium hydroxide was studied by using a fracture mechanics approach. Compact tension specimens were machined from injection-molded plaques and used to determine creep crack growth rate as a function of applied stress intensity factor. The effects of caustic concentration and polymer molecular weight on creep crack growth behaviour were determined. Fractographic analysis of the fracture surfaces showed that crack growth proceeded by the formation of novel discontinuous growth bands. The structure and mechanism of formation of these bands are discussed.

Comparison of the fatigue propagation behaviour of polystyrene and 95/5 polystyrene/polyethylene blends

The fatigue crack propagation (FCP) behaviour of polystyrene (PS) and 95/5 PS/high density polyethylene (HDPE) was studied at cycling frequencies (f) of 0.2, 2 and 20 Hz. At the latter two frequencies, the fatigue crack growth rates (FCGRs) in the blends were lower than in PS, especially when a styrene/ethylene–butylene/styrene (SEBS) triblock copolymer was added as a compatibilizer. The fractographic features observed were analysed in detail. Discontinuous growth bands (DGBs), associated with the fracture of crazes, formed at low FCGRs in PS and at low and high FCGRs in the blends. Large dimple-like features formed at intermediate FCGRs and fatigue striations at high FCGRs were observed in PS. The observations indicated that the reduction in FCGR when HDPE was added to PS was associated with the presence of stretched HDPE particles which fractured behind the crack front, with increased particle–matrix adhesion favoured by the compatibilizer. An increase in cycling frequency decreased the FCGRs, with the fractographic observations indicating that this effect was associated with a decrease in the time-dependent deformation in the fracture process zone slightly ahead of the crack tip.

Temperature and frequency effects on fatigue crack growth in acrylonitrile–butadiene–styrene (ABS)

AbstractFatigue crack growth (FCG) in a commercial‐grade acrylonitrile–butadiene–styrene (ABS) over the temperature and frequency ranges of 10–70°C and 0.01–10 Hz was studied. A model for the effects of temperature and frequency on the FCG rate was refined. The refined model is shown to accurately predict FCG rates in ABS. Three different types of fatigue fracture surfaces have been found. The first type is characterized by discontinuous growth bands; the second, by a rather smooth surface; and the last, by a rough surface relative to the second. The transition between the first and second types was found to be dependent on temperature and frequency as well, whereas the transition between the second and last types was found to be only dependent on temperature. These findings are discussed in relation to crazing. The apparent activation energy (ΔHth) was evaluated for both the first and second types to be 19.22 kJ/mol. © 1995 John Wiley & Sons, Inc.

Fractographic analysis of fatigue crack propagation in an amorphous copolyester

The fatigue crack propagation (FCP) behaviour of amorphous glassy polymers has been the topic of numerous investigations [1-8]. These studies have shown that fatigue crack propagation typically occurs by the formation of striations and/or discontinuous growth (DG) bands. A striation is a fracture surface marking which corresponds to the position of the crack tip as a result of an individual cycle or load excursion. A DG band, on the other hand, is a fracture surface marking which corresponds to an increment of crack growth caused by multiple cycles. Striations and DG bands may be found on the same fatigue fracture surface; DG bands typically occur at locations associated with slow crack growth rates, whereas striations occur at locations associated with rapid crack growth [1]. Fatigue fracture surfaces are commonly examined by scanning electron microscopy (SEM), despite the fact that this technique suffers from some limitations. In conventional scanning electron microscopy, speci- mens must be coated with either carbon or a conductive metal in order to eliminate surface charging and to reduce radiation damage. Although the new generation of field emission-based scanning electron microscopes obviate the need for suface coating, accurate measurements of topographical features in three dimensions cannot be obtained. A relatively new type of microscopy, scanning force microscopy (SFM), on the other hand, is capable of providing three-dimensional images of surfaces in air or under a fluid, without the need for a vacuum or special surface treatment [9]. A primary objective of this study was to determine the applicability of SFM for fractographic analysis of polymeric materials. Fatigue crack propagation tests were performed on an amorphous, glassy thermoplastic copolyester of dimethyl terephthalate with ethylene glycol and 1,4- cyclohexanedimethanol (EASTAR~'ETG, Eastman Chemical Company). Compact tension specimens were machined from 6.3-ram thick injection-moulded plaques of the copolyester. FCP tests were performed at room temperature on a MTS closed-loop servo- hydraulic testing machine using a sinusoidal wave- form with a frequency of 10Hz. The tests were conducted in load-control using a minimum-to- maximum load ratio of 0.1. Crack lengths were measured by computer using an elastic compliance technique. FCP rates,

Fatigue crack growth in polymers subjected to fully compressive cyclic loads

It is experimentally demonstrated in this work that the application of cyclic compression loads to polymeric materials, specifically high-density polyethylene and polystyrene, results in the nucleation and propagation of stable fatigue cracks. The cracks grow at a progressively slower rate along the plane of the notch in a direction perpendicular to the far-field cyclic compression axis. The overall characteristics of this compression fatigue fracture are macroscopically similar to those seen in metals, ceramics, as well as discontinuously reinforced inorganic composites. It is reasoned that the origin of this Mode I compression fatigue effect is the generation of a zone of residual tensile stress locally in the vicinity of the notch-tip upon unloading from the maximum far-field compressive stress. The residual tensile field is generated by permanent damage arising from crazing and/or shear deformation ahead of the notch-tip. Evidence for the inducement of residual tensile stresses on the crack plane is provided with the aid of micrographs of near-tip region where crazes are observed along the plane of the crack, i.e. normal to the compression loading axis. Compression fatigue crack growth in polystyrene is also highly discontinuous in the sense that the crack remains dormant during thousands of fatigue cycles following which there is a burst of crack extension, possibly in association with fracture within the craze. This intermittent growth process in cyclic compression is analogous to the formation of discontinuous growth bands during the tension fatigue of many crazeable polymers. The exhaustion of the near-tip residual tensile field and the increase in the level of crack closure with increasing crack length cause the fatigue crack to arrest. The universal features of this phenomenon are discussed in the context of ductile and brittle, non-crystalline and crystalline, as well as monolithic and composite materials.

冷間圧延されたポリカーボネートの疲労き裂進展

Fatigue crack propagation behavior in cold-rolled polycarbonate was studied under constant-amplitude loading and single peak overloading conditions. The results obtained in this experiment are summarized as follows:(1) Cold-rolling improves the resistance of fatigue crack propagation at low ΔK levels in polycarbonate, and restrains the formation of discontinuous growth bands which are easily formed at low ΔK levels in unrolled polycarbonate.(2) The acceleration factor of crack propagation due to a single peak overload is markedly lower in cold-rolled specimens than that in unrolled ones, and the fracture surface of acceleration region shows many fine tearing lines in cold-rolled ones in contrast to the granular markings in unrolled ones. This fact suggests that the cold-rolling restrains the craze formation at the crack tip, which causes lower acceleration.(3) Fatigue crack propagation after a single peak overload shows the so-called delayed retardation in cold-rolled specimens, which contrasts with an undelayed retardation in unrolled ones.(4) The maximum retardation rate after a single peak overload is lower in cold-rolled specimens than that in unrolled ones. This is considered to be due to the smaller scale of the overload-induced crack tip blunting or orientation hardening near the crack tip in cold-rolled ones.

SEM observations of broken fibrils at fatigue fracture in PVC

The structure of crazes in amorphous glassy polymers has been intensively studied and it is weU established that crazes have a lower density than the surrounding bulk material and that they consist of fibrils interspersed by voids, the fibrils being oriented primarily along the direction of the acting principal tensile stress (e.g. [1-4]) . This knowledge, especially of the fibril structure, has been mainly obtained by transmission electron microscopy of unbroken crazes in thin polymer films. After fracture of a glassy polymer the fracture surface should also reflect the remnants of the craze, especially of the fibril structure. However, an inspection of the extensive literature with regard to the appearance of fracture surfaces shows that most of the markings on the surfaces are produced by the fracture process itself and that there are only spare hints to the fibril structure. It is the aim of this letter to give direct evidence of fibrils on fatigue loaded fracture surfaces of PVC and to indicate a correlation between fibril structure and craze size. The choice of this load condition and material was due to the fact that the crack propagation of fatigue loaded PVC has been reported [5-7] to exhibit the interesting feature of so-called discontinuous growth bands: these fatigue markings arise at a retarded crack propagation following several hundred loading cycles during which the crack tip remains stationary; the band width is assumed to be equal to the craze zone length. Fig. 1 shows an example of such markings, which are oriented normally to the direction of crack growth. (The arrows indicate the direction of crack propagation.) In a previous examination of the micromorphology of and between the bands in PVC many microvoids have been found which decrease in size from 2 to 0.1 grn in the direction of crack growth [6, 7], as may also be observed in Fig. 1. From this it has been suggested that the cracking process occurs by a void coalescence mechanism