Brittle crack

It is possible to confuse SCC with other brittle cracking phenomena. Confirmation of SCC typically requires a metallographic examination. On thin-walled components, the surface from which the cracking originates may not be apparent. In these cases, a formal metallographic examination may be required to assure positive identification of the surface from which the cracks originate. 

Similarly it seems that retained austenite may be beneficial in certain circumstances , probably because the austenite acts as a barrier to the diffusion of hydrogen, although in high concentrations (such as those obtained in duplex stainless steels) the austenite can also act as a crack stopper (i.e. a ductile region in the microstructure which blunts and stops the brittle crack). 

Plastics are susceptible to brittle crack-growth fractures as a result of cyclic stresses in much the same way as metals. In addition, because of their high damping and low thermal conductivity, plastics are prone to thermal softening if the cyclic stress or cyclic rate is high. Examples of the TPs with the best fatigue resistance include PP and ethylene-propylene copolymers. 

This rather useful empirical expression is applicable to many electrodeposited materials [e.g., molybdenum, zinc, steel (10)]. The expression has been able, for instance, to provide an acceptable explanation for the phenomenon of the brittle cracking in chromium electrodeposits. It has been quite helpful in the general study and understanding of the functional connection between hardness and grain size values in many electrodeposits. 

The fracture strengths of polymers are generally lower than those of metals and ceramics. The mode of failure for thermosets is generally referred to as the materials being brittle. Cracks, related to bond breakage, occur at points of excess stress. These create weak spots and may lead to fracture if the applied stress, appropriate to create bond breakage, continues. 

Mono- and polycrystalline natural and synthetic materials are not subject to plastic strain and have no independent slip system. Stress concentration occurs in them at.crack tips and at flaws in the material, affecting the maximum strength which originates from the chemical or physical cohesion forces present. Non-plastic materials (crystals, rocks, ceramics, glass) show brittle cracks—forming at very low plastic strain—usually originating from surface flaws. 

Brittle erosion is the loss of material from a solid surface due to fatigue cracking and brittle cracking caused by the normal collisional force Fn. Materials with very limited capacity for elastic and plastic deformation, such as ceramics and glass, respond to particle impacts by fracturing. The yield stress for brittle failure Fb for normal impacts is about... 

A probable mechanism of erosion for plastics is illustrated in Fig. 6.2 [Briscoe and Evans, 1987]. Initially, a series of plastically deformed grooves can be formed by the abrasion of particle flows. The subsequent directional or random impacts of particles may push the deformed grooves from side to side. The fatigue limits of the plastics would eventually make the ridges between the grooves detach to form ribbonlike debris. Brittle cracks also occur when the wear tracks interact. 

There is, however, a complication besides ductile fracture also brittle failure may occur, which is not brought about by deformation and flow of the material, but by the initiation and propagation of brittle cracks. Since both processes also need time to develop, the time to brittle failure can also be represented by a curve, similar to... 

With such a complex failure behaviour, it is difficult to make a reliable estimation of the allowable stress, i.e. the stress at which, in the required duration of use (mostly 50 years for pipes for transportation of water), no failure will occur. Evidently, extrapolation of failure tests, even if they extend over a year, is insufficient. The transition from ductile to brittle crack failure, and the accompanying change in slope of the curve, may take place after several years One, therefore, resorts to tests at elevated temperatures, on the basis of the idea that a temperature increase will accelerate both failure mechanisms, and also the transition from ductile to brittle failure. 

The environment also plays a role in some environments brittle crack failure is strongly promoted. For example, detergents such as synthetic soaps can decrease the time to brittle failure of PE by a factor between 10 and 50 (see Figure 7.21). This phenomenon is known as stress corrosion or environmental stress cracking (ESC) (see further 8.5). 

Cyclic load frequency is the most important factor that influences corrosion fatigue for most material environment and stress intensity conditions. The dominance of frequency is related directly to the time dependence of the mass transport and chemical reaction steps involved for brittle cracking. 

Film-induced cleavage models. It has been suggested that dealloying and/or vacancy injection could induce brittle fracture. The model assumes that a brittle crack initiates in a surface film or layer and this crosses the film/matrix without loss of speed. The brittle crack will continue in the ductile matrix until it eventually blunts and arrests. Verification of this model needs better understanding of the surface films and brittle fracture. (Jones)5... 

The brittle cracking and subsequent debonding of films deposited on flexible substrates subjected to uniaxial strain is described theoretically and illustrated with Ni films evaporated on ion-etched polyethylene terephthalate (PET). It is shown that, if the materials deform elastically, the shear strength of the interface, x, may be evaluated from the length,... 

Figure 1. Schematic diagram of test specimen Ni film of thickness, t, on PET substrate of width w strained in direction of arrows. Dotted lines represent brittle cracking. The x-coordinate is measured in the direction of strain from the edge of a film segment.

ESC is mostly a surface-initiated failure of multiaxially stressed polymers in contact with surface-active substances. These surface-active substances do not cause chemical degradation of the polymer, but rather accelerate the process of macroscopic brittle-crack failure. Crazing and cracking may occur when a polymer under multiaxial stresses is in contact with a medium. A combination of external and/or internal stresses in a component may be involved. 

Some difficulties also arise for the interpretation of scratch tests carried out at progressively increasing normal load or indentation depth. Figure 3 indicates, for example, that a transition from ductile deformation to brittle cracking can occur when increasing the normal load whilst the contact strain is nominally fixed by the conical indenter angle. This is indeed observed in many polymer systems and the notion of a critical load at the ductile-brittle transition is largely used to characterize the scratch response. This depth... 

Fig. 7 Development of fatigue cracks in an epoxy/glass contact under gross slip condition (1Hz, displacement amplitude 60 xm) (from [97]). White arrows indicate the occurrence of crack initiation and propagation at the edge of the contact under the action of tensile stresses. The lateral contact stiffness, K, is essentially a measurement of the elastic response of the epoxy substrate within the contact zone. Brittle crack propagation is associated to a drop in stiffness due to the additional accommodation of the imposed displacement provided by crack opening mechanisms...

Types 1 and 4 tensile failures, which are of less interest to conservationists at this time, are caused by brittle crack propagation and by long axial splits, respectively. Such fractures can occur in ceramic and elastomeric fibers (both Type 1) and highly oriented aramids (Type 4). 

Figures 32 and 33 shows the variation of the ratio of distance x from the notch root to the position of a brittle crack nucleus to the radius of the notch root q with temperature for PAs and PEEK, respectively. The position of crack nuclei was measured...

All the analysis so far described has assumed that the material is linearly elastic. Almost all polymers, however, show some evidence of time dependence and are viscoelastic. Since we are generally concerned with small strain behaviour for brittle cracks, it is reasonable to suppose that the materials are linearly viscoelastic so that, for example, when computing a strain from a stress, we caimot write ... 

Several cautions are, however, in order. Polymers are notorious for their time dependent behavior. Slow but persistent relaxation processes can result in glass transition type behavior (under stress) at temperatures well below the commonly quoted dilatometric or DTA glass transition temperature. Under such a condition the polymer is ductile, not brittle. Thus, the question of a brittle-ductile transition arises, a subject which this writer has discussed on occasion. It is then necessary to compare the propensity of a sample to fail by brittle crack propagation versus its tendency to fail (in service) by excessive creep. The use of linear elastic fracture mechanics addresses the first failure mode and not the second. If the brittle-ductile transition is kinetic in origin then at some stress a time always exists at which large strains will develop, provided that brittle failure does not intervene. 

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