Notch Effects

Defined as the ratio between the peak stress Umax and the stress that would act in the same point of the reduced section (1 — a) without considering the local stress amplification, with a being the notch length and I the width of the section containing the notch. The local stress intensification give rise to the suspicion that the fatigue limit oy or 5/ must be consequently reduced by the same factor fe, to a value aj or Sj 

This section is of particular interest as a consequence of the fact that almost all ceramics have flaws, many in form of microcracks or even cracks on a larger scale. These are outcomes of the fabrication techniques however, reducing such pores or cracks to a viable, minimal level is very costly. Many such cracks end at surfaces, where they act as stress raisers. Notches may also be considered as cracks, though the difference between notches and cracks is only a matter of size, not kind. Thus, the study of notched specimens has practical significance, because, in engineering components, fatigue starts mostly at the surface, at those places where stress- 

In Fig. 7.62, the adjusted notch-fatigue life fits within the scatter band of the S-N curve. In this case, the fatigue-stress level in the notched specimens was adjusted by using the damaged area, which extends about 0.4 mm into the specimen, and using the undamaged area beyond it. (The stress level is the load divided by the area and, to this end, an adjustment had to be made). 

Equation (7.16) is often represented by a different symbol, Kf, the notch factor, relating the strength of smooth specimens to that of notched specimens, but having the same meaning as  

Numerous experimental observations indicate that notches are among the key factors that determine the fatigue strength of structures and play an important role in the estimation of fatigue life and the strength of some structures exposed to 

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A crack count of 30-80 cracks/mm is desirable to maintain good corrosion resistance. Crack counts of less than 30 cracks/mm should be avoided, since they can penetrate into the nickel layer as a result of mechanical stress, whilst large cracks may also have a notch effect. Measurements made on chromium deposits from baths which produce microcracked coatings indicate that the stress decreases with time from the appearance of the first cracks . It is more difficult to produce the required microcracked pattern on matt or semi-bright nickel than on fully bright deposits. The crack network does not form very well in low-current-density areas, so that the auxiliary anodes may be necessary. 

Internal plastic thread The strength of plastic threads is limited, and when molded in a product involving either an unscrewing device or a rounded shape of thread similar to bottle-cap threads, they can be stripped from the core. Screw threads, when needed, should be of the coarse type and have the outside of the thread rounded so as not to present a sharp V to the plastic that can produce a notch effect. 

The reason for this is that explosions exert extraordinarily great forces, and that the formation of cracks is favored by any weakness in the structure, since the tension peaks attain very high values in the vicinity of acute angles (notch effect, see Fig. 48). Such holes, in particu-... 

The tough-brittle transition temperature is hard to define it is, of course, strongly dependent on the conditions, such as the time scale of the experiment, notch effects etc. The brittleness temperature is, in general, being determined by a series of standard impact tests, carried out at different temperatures when 50% of the samples are broken in a brittle way, then the brittleness temperature has been reached. 

The load-displacement curves for C(T) tests of the neat EpoxyH were almost linear until the final unstable fracture. The fracture toughness value in 77K-LNj was 210 J/m and that in RT-air was 120 J/m. Thus the toughness increased by 1.8 times by changing the test environment from RT-air to 77K-LN. Brown and co-workers have found that amorphous polymers crazed in 77K-LNj, but not in a helium or vacuum at about 78K [20-22]. They have also reported that the stress-strain behavior of all polymers, amorphous and crystalline, is affected by at low temperatures [22]. Kneifel has reported that the fracture toughness of epoxy in 77K-LNj is higher than that in RT-air and 5K, and that the reason for this is the reduced notch effect by plastic deformation [23]. Then, the increase of the fracture toughness of the neat EpoxyH in this study is probably caused by the similar effect. 

As well, a notch may do the same with regard to the diffusion from the points of view of the geometry and the stress effects on the transport phenomenon, if compared with the stress-unassisted diffusion in a smooth cylinder. In particular, the range of the disturbing effect of a notch on stress in assisted transport phenomena in solids can be estimated from fig. 4, where vanishing of the notch effect corresponds to fairly radial flow trajectories, or concentration contour bands parallel to the cylinder surface, the same as it occurs in smooth bars. 

Eliminate notches in design or minimize notch effect as much as possible carefully inspect welds and base material eliminate or minimize vibrations, stress levels. 

Etch-stop at dielectric interfaces is another common occurrence in surface micromachining, when the etch terminates on the sacrificial or isolation oxide beneath the active silicon layer. Notching effects appear in high-aspect ratio (narrow)... 

Notch Effect - The effect of the presence of specimen notch or its geometry on the outcome of a test such as an impact strength test of plastics. Notching results in local stresses and accelerates failure in both static and cycling testing (mechanical, ozone cracking, etc ). 

Fig. 37. The notching effect. When the polysilicon film clears, the oxide charges up to the point of deflecting oncoming ions which then etch the sidewall. Electrons can t reach the bottom of the trench to neutralize the positive charge.

Notch effect. There may be fibre and matrix damage through the thickness, causing a local loss of stiffness. This has the same effect as an insertion or hole that raises a stress concentration, but probably less than 3 as in the case of a circular hole. 

Notch effect Local buckling Global buckling Global/local Buckling... 

Various transitions between titickn a in h produfct A. notch effect, a severe Itoss riser B. a small radius atihe cottier, Ov Itirge natius at the cortiesr, and D. with a large, unaotehed radiiis... 

In Fig. 4.6A, the product shows a notch effect, a severe stress riser causing a serious weakness in the corner. A small radius (Fig. 4.6B) is better, and a large radius (Fig. 4.6C).or a slope (Fig. 4.6D) is best for strength in the comer area, provided the slope blends into the straight portion with a large radius and without a notch. 

In Fig. 4.18B, there is a radius R, on the core this radius should be large enough to permit easy flow of the plastic. However, the result of such a design is a thickening at the comer which can result in more shrinkage at this point, creatiag sink marks as indicated by heavy lines. Also, unless the product really requires a comer shape such as that shown in Fig. 4.18B, the material there is wasted. In addition, shmp inside comers in any piece made from steel (or any other material, including plastics) are bad for the life of the product (dr mold part) due tin the notch effect created by such an inside comer. 

Fracture Strength. The fracture strength, ac, of composites shows a range of results, but ac cannot exceed that of the matrix. The notch effects and residual stress concentration could lower it further. Whether or not the filler particles behave as precursors of cracks depends on the particle diameter, d. This can be qualitatively understood by the fact that the stored deformation energy, C/, is a function of whereas the fracture surface energy, y, is proportional to d. Applying the Griffith criterion of fracture initiation,... 

The mechanical properties of aluminum-sprayed steel remain unimpaired other than a slight loss in fatigue strength owing to the notch effects caused by preliminary grit blasting. The fatigue... 

In case of tensile, bending, torsion, or alternating loads, roughness may be of particular importance for the component s lifetime namely, if roughness values exceed a certain limit, so that stress concentration as a result of the notching effect occurs. Near a notch stress increases disproportionately intense and may reach unexpectedly high values (Thum et al. 1960). Size and type of the notch influence the... 

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