Initiation, crack

Crack initiation and stage one growth that deepens the intrusions within the plane of high shear stress. 

Stage two crack propagation of well-defined cracks on the planes of high tensile stress in the direction normal to the maximum tensile stress. 

CF cracks are always initiated at the surface, unless there are near-surface defects that act as stress concentration sites and facilitate subsurface crack initiation. Crack initiation takes place independently of fatigue limit in air as it can be decreased or eliminated through the increase of dissolution rates at anodic sites. Localized corrosion such as pitting favors fatigue crack initiation through stress concentration and a local acidic environment. The two main mechanisms of CF are anodic slip dissolution and HE (80). 

The materials and corrosive environments have been classified (81) into the following three groups on the basis of surface corrosion conditions  

Bulk surface films, such as three-dimensional oxides. 

Parallel markings, shaped like a quarter ellipse, occur on some fracture surfaces (Fig. 9.3a). A surface crack has initiated when a blunt object pressed on the product surface (Fig. 9.3b). As this crack spreads sideways, the object penetrates the product and twists the two sides in opposite directions. This double torsion loading causes the crack to advance more rapidly on the lower surface in tension. The characteristic markings are due to momentary hesitations of the crack front. 

Holes or sharp corners are familiar causes of stress concentrations. Near such a feature, the most tensile principal stress reaches a maximum value. 

Local maximum stress Stress in absence of feature 

Stress concentration factors contours of principal tensile stress (MPa) for (a) a central hole in a plate under tension—contours of relative stress 10 MPa tensile stress at the ends (b) I mm radius Internal corner in a product subjected to bending. 

Elastic stress concentrations cannot explain most product failures, since yielding nearly always occurs before crack initiation. However, they indicate locations where yielding is likely to occur first. Therefore, the failure stress in Charpy impact tests (Section 9.5.1) should not be calculated using the notch q value. Craze formation is another form of (localised) yielding, which also modifies the stress distribution in the product. Section 9.4.4 shows that craze breakdown may occur at a critical opening displacement, rather than at a critical stress. Hence, elastic-plastic analyses must be used for most polymer product failures. 

The movement of mobile dislocations plays a significant role in the fatigue failure preparation. If a specimen is subjected to alternating stresses the generation of dislocations is known to occur. There is a threshold stress tg at which this process begins [81] 

A force is known to attract dislocations to the surface of a specimen. Energy required to produce a certain amount of slip inside a solid is about twice that required to produce the same amount of slip at the surface. Motion of the dislocations caused by cycling loading occurs long before the yield strength is reached. 

However, the process of the crack nucleus extension is in fact reversible up to some point in time.  

1) Preparatory processes of fatigue and their reversibility can be discovered by the work function technique because of formation of surface steps. The electronic work function can be defined as a smallest amount of energy required to remove an electron from the surface to a point outside the metal with zero kinetic energy. The generation of surface steps early in the fatigue process results in a distinct decrease in work function [83]. 

The surface area of the critical crack embryo is determined from the equality 2yS = Fi + F2- (173) 

In most cases, a fatigue fracture is starting at a highly loaded position of the component. The high load may be caused by an overload, generating some initial damage in the component that is hard to detect, but may cause ultimate failure. 

Local stress concentrations are often caused by notches (see chapter 4). Notches may be part of the design (e. g., at bearings or at undercuts), may be caused during manufacture (e.g., tool marks caused by metal cutting), or may be due to imperfections in the material (e.g., casting pores, brittle precipitates). 

In section 5.2.3, we saw that microscopic defects or cracks are usually irrelevant under static loads because they are smaller than the critical crack length from equation (5.27). If loads are cyclic, much smaller defects, like casting pores or inclusions, can initiate fatigue cracks. The fatigue strength of a material is thus much more sensitive to the manufacturing process and material defects than the static strength. 

Even if the component is initially defect-free, it is not guaranteed that no cracks will form. Cracks are initiated by a roughening of the surface of the component under cyclic loads (see figure 10.6) caused by plastic deformation. This deformation is due to dislocation movement at stresses below the yield strength Rpo.2, which is insignificant under static loads. However, dur- 

According to section 3.2, the yield strength iJpo.2 is defined as the stress corresponding to a plastic strain of 0.2%. For this, some amount of dislocation movement is necessary. 

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BE-5249 Optimization of methodologies to predict crack initiation and early growth In comoonents under comolex creeo-fatiaue loadina tC FAT) Dr. S. Holdsworth GEC ALSTHOM Ltd... 

Typically, ozone cracking initiates at sites of high stress (flaws) on the mbber surface. Thus, in general, mbber articles should be designed to rninirnize potential sites of high elongation such as raised lettering. Similarly, the use of clean molds helps to reduce the incidence of surface flaws. 

The chemical species that can lead to EIC in each alloy system are fairly well known although the exact mechanisms of crack initiation and propagation are not thoroughly understood. The species that promote EIC in one alloy system do not necessarily promote EIC in others. A discussion of EIC and the chemical species that promote it can be found in the Hterature (34). 

Separated Anode/Cathode Realizing, as noted in the preceding, that locahzed corrosion is usually active to the surrounding metal surface, a stress specimen with a limited area exposed to the test solution (the anode) is elec trically connec ted to an unstressed specimen (the cathode). A potentiostat, used as a zero-resistance ammeter, is placed between the specimens for monitoring the galvanic current. It is possible to approximately correlate the galvanic current 7g and potential to crack initiation and propagation, and, eventually, catastrophic fail-... 

Highly ductile materials tend to be more resistant to thermal fatigue and also seem more resistant to crack initiation and propagation. 

The level of stress may be, and generally is, much less than the yield strength of the metal. However, in general, higher stresses increase crack growth rate and the number of cracks initiated. 

The orientation of the cracks indicates that cyclic bending stresses or cyclic axial stresses generated by thermal expansion and contraction provided the responsible stresses. The large number of crack initiation sites and the tightness of the cracks indicate high-level stresses. 

Microstructural examinations revealed intergranular, sparsely branched cracks originating on the external surface. Some cracks initiated as transgranular fissures. 

Such degradation of the surface causes little effect on either flexural strength or flexural modulus of elasticity but the influence on the impact properties is more profound. In such instances the minute cracks form centres for crack initiation and samples struck on the face of samples opposite to the exposed surface show brittle behaviour. For example, a moulded disc which will withstand an impact of 12 ftlbf without fracture before weathering will still withstand this impact if struck on the exposed side but may resist impacts of only 0.75 ftlbf when struck on the unexposed face. 

Moulded plastics will also have crack initiation sites created by moulding defects such as weld lines, gates, etc and by filler particles such as pigments, stabilisers, etc. And, of course, stress concentrations caused by sharp geometrical discontinuities will be a major source of fatigue cracks. Fig. 2.72 shows a typical fatigue fracture in which the crack has propagated from a surface flaw. 

Fig. 2.79 shows the typical variation of impact strength with notch tip radius for several thermoplastics. The first important fact to be noted from this graph is that the use of a sharp notch will rank the plastics materials in a different order to that obtained using a blunt notch. This may be explained by considering the total impact strength as consisting of both crack initiation and crack propagation... 

Other factors which can affect impact behaviour are fabrication defects such as internal voids, inclusions and additives such as pigments, all of which can cause stress concentrations within the material. In addition, internal welds caused by the fusion of partially cooled melt fronts usually turn out to be areas of weakness. The environment may also affect impact behaviour. Plastics exposed to sunlight and weathering for prolonged periods tend to become embrittled due to degradation. Alternatively if the plastic is in the vicinity of a fluid which attacks it, then the crack initiation energy may be reduced. Some plastics are affected by very simple fluids e.g. domestic heating oils act as plasticisers for polyethylene. The effect which water can have on the impact behaviour of nylon is also spectacular as illustrated in Fig. 2.80. 

The surface finish of the specimen may also affect impact behaviour. Machined surfaces usually have tool marks which act as stress concentrations whereas moulded surfaces have a characteristic skin which can offer some protection against crack initiation. If the moulded surface is scratched, then this protection no longer exists. In addition, mouldings occasionally have an embossed surface for decorative effect and tests have shown that this can cause a considerable reduction in impact strength compared to a plain surface. 

Figure 8 shows the SEM images with a low level of strain (50%). It is clear that even with a low-strain level defects are initiated in the sulfur cured system with the formation of large cracks at the boundary layer between the two phases. However, in the peroxide cured system the mechanism of crack initiation is very different. In the latter case the NR-LDPE interface is not the site for crack initiation. In this case, stress due to externally applied strains is distributed throughout the matrix by formation of fine crazes. Furthermore, such crazes are developed in the continuous rubber matrix in a direction... 

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