Stress concentration level

Let us consider in conclusion the physical significance of stable crack fractal dimension in PASF samples. As it has been shown in Ref [12], the stress concentration coefficient Kg of triangular crack is given by the Eq. (5.10). In Fig. 8.6 the relation between parameters and for PASF sample (solvent - chloroform) is adduced, fi om which linear reduction follows at growth. Thus, the dimension for stable crack has the simple physical significance - it is the value, reciprocal to stress concentration level at crack tip. 

The difficulty in accurately estimating the degree of local concentration remains one of the principal reasons susceptibility to SCC in a specific environment or circumstance is difficult to predict. Measurement of nominal stresses or levels of corrodent in the bulk environment can be quite misleading as predictors of SCC susceptibility. 

In our study, the effect of moisture over the nonneutral pH range of 3-11, direct sunlight, ozone at a concentration level of 6000 ppm, and the effects of loading stresses, were investigated for the three commercial sealants. A characteristic variation of crosslink density for the typical silicone sealants is shown in Fig. 29. This figure depicts the results for the coupons exposed to moisture and sunlight. Initially upon exposure, the crosslink density of the sealants exhibit an increase due to the availability of residual uncurred crosslink sites... 

Basically, in the vicinity of a sharp comer all fringes converge toward the apex. Having a high density of lines at this point indicates the presence of high stress level. At a rounded corner there will be considerably less concentration. Besides the molding problems, sharp corners often cause premature failure because of the stress concentration. To avoid these problems, inside comer radii should be equal to one-half the nominal wall... 

Standard fatigue tests normally require extended periods of time to complete regardless of the type of stress/strain used. Accelerated fatigue tests change some conditions of the test in the hope of reaching the same end point in less time. Potential means of accelerating the tests include changing factors such as the temperature, frequency, chemical environment, levels of stress or strain, or the addition of stress concentrators. 

When designing sewer networks, particularly gravity sewers, reaeration is the major process that should be focused on to reduce sulfide formation and the formation of organic odorous substances (cf. Section 4.4). A number of hydraulic and systems characteristics can be managed to increase the reaeration rate and avoid or reduce sulfide-related problems. The hydraulic mean depth, the hydraulic radius, the wastewater flow velocity and the slope of the sewer pipe are, in this respect, important factors that are dealt with in Section 4.4. It should be stressed that it is not necessarily the objective to avoid sulfide formation (in the sewer biofilm), but the sulfide that occurs in the bulk water phase should be at a low concentration level. Therefore, the DO concentration in the bulk water phase should not be lower than about 0.2-0.5 g02 m-3, sufficiently high to oxidize sulfide before a considerable amount is emitted to the sewer atmosphere. 

A related issue has to do with the initial wafer-level uniformity (wafer thickness, wafer warp and bow, thicknesses of thin films across the wafer surface, uniformity of stress in such thin films across the wafer) and the subsequent impact on wafer-level polish performance. Some examination has been made of the impact of wafer warp and bow on the polish performance [68], where it was found that the initial warpage can have significant impact (with the implication that reclaimed wafers may not be appropriate monitors of wafer-level polish performance). Other work has considered inherent variation due to Von Mises stress concentrations at the edge of the wafer (conceptually, a downward pressure on the wafer causes lateral stress buildup near the edge of the wafer) [64]. 

Typically, a stressed sample of about 10 to 20% degradation is used to demonstrate the resolution among degradation products. A 10 to 20% degraded sample is used because it has a sufficiently high concentration level of critical related substance. Therefore, these related substances can be detected easily. In addition, 10 to 20% degradation is not too excessive, and the related substance profile should be close to that of a typical stability sample. 

It is not surprising that, given the importance of tearing and the different levels of result obtained from different geometries, a considerable number of tear tests have been devised which, in part, reflect the different stress concentrations found in various products. The arbitrary nature of the geometries means that, in general, the measured tear strength is not an intrinsic property of the material and it is difficult to directly correlate the results of laboratory tests with the performance of products in service. 

Many variables used and phenomena described by fracture mechanics concepts depend on the history of loading (its rate, form and/or duration) and on the (physical and chemical) environment. Especially time-sensitive are the level of stored and dissipated energy, also in the region away from the crack tip (far held), the stress distribution in a cracked visco-elastic body, the development of a sub-critical defect into a stress-concentrating crack and the assessment of the effective size of it, especially in the presence of microyield. The role of time in the execution and analysis of impact and fatigue experiments as well as in dynamic fracture is rather evident. To take care of the specihcities of time-dependent, non-linearly deforming materials and of the evident effects of sample plasticity different criteria for crack instability and/or toughness characterization have been developed and appropriate corrections introduced into Eq. 3, which will be discussed in most contributions of this special Double Volume (Vol. 187 and 188). 

Some practical cases are determination of residual stress in steel springs, the effect of mechanical loading on stress relaxation of machined and shot-peened nickel-base alloys,65 determination of residual stress level in turbine engine disks as they accumulate engine cycles,65 66 effect of manufacturing processes on residual stress, measurement of stress gradients in mechanical, electronic and structural components, effect of heat treatment on residual stress in steel coil springs, effect of variable heat treatment temperature on residual stress in iron alloys, measurement of stress in multiphase materials and composites and stress measurements at locations of stress concentrations. 

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