Effect of Stress Concentrations

During service the impact behaviour of a plastic article will be influenced by the combined effects of the applied stress system and the geometry of the article. Although the applied stress system may appear simple (for example, uniaxial) it may become triaxial in local areas due to a geometrical discontinuity. Fig. 2.78 

In the laboratory the impact behaviour of a material could be examined by testing plain samples, but since brittle failures are of particular interest it is more useful to ensure that the stress system is triaxial. This may be achieved most conveniently by means of a notch in the sample. The choice of notch depth and tip radius will affect the impact strengths observed. A sharp notch is usually taken as 0.25 mm radius and a blunt notch as 2 mm radius. 

Graphs such as Fig. 2.79 also give a convenient representation of the notch sensitivity of materials. For example it may be seen that sharp notches are clearly detrimental to all the materials tested and should be avoided in any good design. However, it is also apparent that the benefit derived from using generously rounded comers is much less for ABS than it is for materials such as nylon or PVC. 

Impact strength also increases as the notch depth is decreased. The variation of impact strength with notch depth and radius may be rationalised for some materials by use of the linear elastic stress concentration expression. 

It has been shown that for acrylic, glass-filled nylon and methyl pentene there is reasonable correlation between the reciprocal of the stress concentration factor, K, and impact strength. However, for PVC good correlation could only be achieved if the finite dimensions of the sample were taken into account in the calculation of stress concentration factor. 

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Typical S-N (stress versus number of cycles) curves for various metals and composite materials are shown in Figure 6-4 [6-3]. The boron-epoxy composite material curve is much flatter than the aluminum curve as well as being flatter than the curves for any of the metals shown. The susceptibility of composite materials to effects of stress concentrations such as those caused by notches, holes, etc., is much less than for metals. Thus, the initial advantage of higher strength of boron-epoxy... 

Figure 13.6. Effects of stress concentration on breakability. Scanning electron micrographs (top frames) showing breaks of anchored silicon beams etched with (a) plasma, (b) KOH and isopropyl alcohol, and (c) KOH and optical micrographs (bottom frames) of printing results that demonstrate the relative ease of breakability, or the ability for a stamp to separate elements from the anchoring structures in each system. (Reprinted with permission from Ref. 54. Copyright 2007 American Institute of Physics.)...

This test has an inherent problem associated with the stress concentration and the non-linear plastic deformation induced by the loading nose of small diameter. This is schematically illustrated in Fig 3.17, where the effects of stress concentration in a thin specimen are compared with those in a thick specimen. Both specimens have the same span-to-depth ratio (SDR). The stress state is much more complex than the pure shear stress state predicted by the simple beam theory (Berg et al., 1972 ... 

Fig. 3.17. Effect of stress concentrations on short beam shear specimens (a) thin specimen (b) thick...

Finally we show that the present model can nicely predict the marked deleterious effects of stress concentrations such as dust particles on fibril breakdown. Suppose we imagine that the stress concentrating particles affect a certain volmne fraction cp of the polymer film. Under these conditions one can readily show using Eq. (A14) of Appendix II that the appropriate Pj is just the volume average of P f, the fibril failure probability of the unaffected film, and P, the fibril failure probability in the region affected by the stress concentration around the dust particle, i.e.,... 

The testing of laminates for strength, stiffness and toughness is standardized using the same philosophy that holds for metals and polymers, i.e., the use of long and narrow specimens. This is a very reasonable approach for isotropic materials, as it minimizes the effects of stress concentrations. 

With impregnation by an appropriate monomer, the main effect after polymerization is the sealing of the continuous capillary pore system, which reduces the effect of stress concentrations from pores and micro-cracks, thereby increasing the strength. A reduction of porosity by 10% doubles the strength. 

Note that it is very important that every stud is well rounded at its base to reduce the effects of stress concentration and to prevent breakage of the stud. i See Section 5.4, Chamfers and Radii.) The hole in Fig. 5.7B must be made by two core pins meeting at the narrow section. The sattie rulA apply as on p. 94, cofe pins meeting core pins. 

Ratio of the maximum stress in the region of a notch, or another stress raiser, to the nominal corresponding stress. SCF is a theoretical indication of the effect of stress concentration on mechanical behavior. Since it does not take into account the stress relief due to plastic deformation, its value is usually larger than the empirical fatigue notch factor or strength reducing ratio. External or internal cracks in a plastic caused by imposed stresses. 

Fracture mechanics studies the effect of stress concentrations that occur when a load is applied to a body containing a void, independent of the geometry or material of the body. By this definition it would seem logical that the fracture toughness of a material, as determined by specific fracture mechanics techniques, would be an appropriate design criterion. 

Figve 8.11 The effects of stress concentrations on the peak position of the C=C Raman band for a 3 mm thick cross-polymerized diacetylene-urethane plate deformed in tension. The data were obtained from spectra obtained at the different positions indicated (after [69])... 

Ti-8V-5Fe-1 Al Effect of stress-concentration factor on room-temperature notch stress rupture... 

Figure 15-10. The combined effect of stress concentration and molded-in stress due to the sharp...

EFFECTS OF STRESS CONCENTRATORS ON DAMAGE EVOLUTION IN SIOSIC... 

To understand the effect of stress concentration of SiC/SiC composites, several coupons with different notch geometries were tested in tension. The geometries tested include double-radius (DR), double-slit (DS) and standard dogbone (DB), and are own schematically in Figure 1, where the dimensions shown in parenthesis indicate width from notch tip to notch tip (net width). Damage initiation and progression was monitored by modal acoustic emission and a finite element analysis was also performed to compare experimraital data with that of a semi-analytic model. 

Effects of Stress Concentrators on Damage Evolution in SiC/SiC Composites... 

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