Crack initiation effects

Figure 5.313 shows the stress-crack initiating effect of non-additivated, chemically differently structured lubricant oils. Poly-a-olefins, silicone oils, and perfluoro-ether oils have a strong stress-crack initiating effect. Ester oils and polyglycols (soluble and insoluble in water) counteract stress-cracking [965]. 

Separate the metal from the environment with a physical barrier. Many corrosion inhibitors make use of this principal to protect metals. Proper use of an appropriate inhibitor may reduce or eliminate pitting. Pits are frequently initiation sites for corrosion-fatigue cracks. The effectiveness of inhibitors depends upon their application to clean metal surfaces. An example of this method is the use of zinc coatings on steel to stifle pit formation. 

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. 

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. 

In neutral solutions the application of cathodic polarisation prevents crack initiation and this could be taken to indicate that hydrogen embrittlement is not the operative mechanism, since the discharge and entry of hydrogen might be expected to fracture the specimen more readily. The beneficial effect of cathodic polarisation has been interpreted , however, to result from more rapid film repair in the alkaline catholyte generated by the cathode reaction. The film serves as a barrier to rapid hydrogen entry. Consistent with this is the observation that in an environment of low pH (e.g. 10 N HCl) where film formation would not be expected, cathodic polarisation has no effect upon crack propagation. 

The explanation of the effect of secondary inclusions on the delocalization of shear banding is based on the concept of modification of the local stress fields and achieving favorable distribution of stress concentrations in the matrix due to presence of inclusions. This leads to a reduction in the external load needed to initiate plastic deformation over a large volume of the polymer. As a result, plastically deformed matter is formed at the crack tip effectively reducing the crack driving force. Above approximately 20 vol% of the elastomer inclusions. 

Starting from a crack that is small compared with a wavelength, the effect on V z) when the lens is directly over the crack initially increases with crack depth. The chief information about the crack depth lies in the phase there is a weak resonance when the crack depth is comparable with the Rayleigh wavelength, and beyond that the contrast tends to become independent of the crack... 

Zhai, T.-G., Lin, S., and Xiao, J.-M. (1990). Influence of non-geometric effect of PSB on crack initiation in aluminium single crystal. Acta Metallurgia et Materialia 38, 1687-92. [282]... 

The typical effect of rate on the measured value of K,c and the associated type of crack growth can clearly be seen in Fig. 4 for an epoxy polymer based upon a DGEBA resin and cured with various amounts of TETA. The values of K ci and K ca are those for crack initiation and arrest (Fig. 2b) respectively and the difference between them characterises the amount of crack jumping which has taken place. When the difference... 

Impact tests may be performed with notched or unnotched specimens, but the results cannot be compared. The presence of a notch induces a great part of the crack initiation energy. The speed of the striker or of the crosshead may be varied from 1ms-1 to several kms-1 for ballistic tests strain rates may vary in the range of 10-1 up to 100 s-1. Very often the energy for failure is divided by the effective cross-sectional area or the deformed volume, in order to calculate, respectively, the surface or the volume resilience. Using the compliance formula and simple assumptions, GIc may be directly calculated (Williams, 1984). 

As well as producing these broadly beneficial effects, thermal microstresses can also degrade the strength of composites. The tensile components of stress can help in crack initiation. In a composite with a uniform distribution of particles, the tensile components act only over distances comparable with... 

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