Stress raisers

For exampie, east irons have internai diseontinuities as severe as the stress raiser... 

The presence of stress raisers, including sharp comers and imperfect welds, produces locally high stress levels. These should be avoided where possible or taken into account when designing the materials for use in environments in which they are susceptible to stress corrosion cracking or corrosion fatigue. 

The test method may specify either a notched sample or an unnotched sample. If a notched specimen is specified, the dimensions of the notch are also given. Figure 15.31 is an example of these dimensions for a metal sample. In the Charpy test the notched sample is struck from behind the notch. In the Izod test the notch is facing the hammer. The notch, called a stress raiser, concentrates the stresses applied by the impact load. It makes the material brittle and increases the elastic limit in the notch area. Without the notch, many materials will bend without fracture, and the capacity to absorb energy will not be accurately determined. 

The areas where a test piece is attached to clamps and cut edges are preferential sites for cracking. It is generally good practice to coat clamped areas with an ozone resistant paint (which does not affect the rubber in any way) but cut edges are best left. For most purposes a Hypalon-based paint is satisfactory. Clamps, even when made of material such as aluminium, should be soaked in ozone prior to use. Any pattern or flaws on the test piece surface will also tend to act as stress raisers and show preferential cracking. 

Experimental Evidence. Morphology. Figure 3 (33) shows in phase contrast microscopy the development of crack or craze patterns around rubber particles in a toughened polystyrene. The lack of dependence of crack inclination on direction of stress is especially marked in this micrograph, and can be explained only by reference to dynamic branching rather than to crack or craze nucleation by stress raisers. Schmitt and Keskkula refer to the lines as craze cracks and cracks. ... 

Pitting can also occur under atmospheric conditions. The corrosion starts at the break and continues to undercut the coating, forming a rather heavy tubercle of hard rust or scale with the pit underneath the original metal. The corrosion products help to isolate the aggressive medium inside the pit. These are common in marine environments as well as various industries where strong corrosive conditions exist.26 Also, pits with their mouth open (uncovered) exist and are responsible for loss of thickness and can act as stress raisers. 

A statistical analysis based on General Linear Model (GLM) was developed to analyse the influence of loading and off-axis angle on damage of composite laminates. The void content in the composite specimens acted as stress raiser resulting in cracks initiation, propagation and failure as tensile loads progressively applied. Analysis of Variance (ANOVA) was performed for void contents to check the statistical differences caused by the experimental errors. 

Notches act as stress raisers and redistribute the applied stress so as to favor brittle fracture over plastic flow. Some polymers are much more notch sensitive than others, but the brittleness temperature depends in general on the test specimen width and notch radius. Polymers with low Poisson ratios tend to be notch sensitive. Comparisons of impact strengths of unnotched and notched specimens are often used as indicators ofthe relative danger of service failures with complicated articles made from notch sensitive materials. 

Fatigue characteristics of the alloy were evaluated by testing standard eye specimens 100x40x5.6 mm in size with a stress raiser ac = 2,8. The tests were carried out at two levels of loading (1) Omax = 600 MPa, Pmax = 65.92 kN and (2) cmax = 400 MPa, Pmax = 43.95 kN. The load application frequency was f = 2-3 Hz, R = 0. 

Typical destructions of standard eye specimens after fatigue tests are shown on fig. 3. During the process of cyclic tests of the eye specimens the fatigue cracks initiated on the surface of holes of 20 mm in diameter (stress raisers) and propagated outward to the external part of the specimens. 

On the other hand, it can also be concluded that the defect-free weld of alloy T110 is a less effective stress raiser than the radius transition in a specimen tested. 

However, real solids contain many flaws, which become stress raisers and initiate cracks at loads much below those required for the separation of an ideal body, even if only tensile forces are applied. The stressed system is unstable and cracks expand rapidly, accelerating to high propagation velocities only the bonds at the crack tips are breaking at any instant. 

The test may be carried out on plain rectangular bars, but most often a carefully defined notch is molded or machined into the face to be struck (Figure 12.8). The impact tests are often regarded as a means of assessing the resistance of a material to shock where notches or stress raisers generally are present. The ratio of impact strength of unnotched to that of notched specimen is sometimes regarded as a measure of the notch sensitivity of a material. 

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