Quasistatic loading

This paper discusses the impact of wind action on natural-draft cooling towers. The structure of the wind load may be divided into a static, a quasistatic, and a resonant part. The effect of surface roughness of the shell and of wind profile on the static load is discussed. The quasistatic load may be described by the variance of the pressure fluctuations and their circumferential and meridional correlations. The high-frequency end of the pressure spectra and of the coherence functions are used for the analysis of the resonant response. It is shown that the resonant response is small even for very high towers, however, it increases linearly with wind velocity. Equivalent static loads may be defined using appropriate gust-response factors. These loads produce an approximation of the behavior of the structure and in general are accurate. 11 refs, cited. 

The aim of this review is to concentrate mainly on these fundamental aspects of the fracture behavior of glassy thermoplastics. In the first Section, following an outline of the relevant fracture mechanics theory, the optical interference method is described and the nature of the results obtainable from it is discussed. The next Section then considers the behavior of cracks and crazes in specimens subjected to quasistatic loading, whilst the final Section examines the role of crazing associated with fatigue crack growth. 

Fig. 33. Maximum craze width 2v as a function of crack speed d reference curve (at the top) is for continuously moving cracks under quasistatic load (comp. Fig. 11a)...

For craze zones at the tips of static and of moving cracks under quasistatic loading conditions it has been shown in Sect. 3 that the normal stress acting on the craze zone and the modulus E can be derived from the measured craze dimensions using the Dugdale model. 

Uniaxial and triaxial compression tests of silicon-carbide ceramics under quasistatic loading conditions were performed by Brannon et al. [33]. Their SiC-N specimens were prepared in the form of a right circular cylinder, as is indicated schematically in Fig. 1.43a next to the experimental set-up. 

The presence of cells marks the distinctive difference between bulk composite and composite foams. The level of porosity measures the amount of empty space within the matrix and varies with foam density. For the case of syntactic foams, different microstructures or levels of porosity can be created through varying the type and amount of microspheres. Details of the microspheres used by Wouterson et al. (2007b) to prepare the epoxy syntactic foams are listed in Table 2.1. In the fracture toughness assessment under quasistatic loading, SEND specimens were loaded in a three-point bend (3PB) geometry. Due to the difference in density between the various types of microspheres, densities of foams with equivalent amounts of microspheres vary. The difference in density makes the comparison of the properties of foams nonrelevant. In order to compare the performance of foams, the specific mechanical/fracture properties are used. 

The magnitude and nature of the load are considered in formulating the design. The load may be essentially quasistatic, cycHc, or impact. Many stmctural failures, for example, have been caused by supposedly innocuous stmctural details welded in place without any consideration given to their effect on fatigue properties. The service temperatures are also important, since they affect the fracture resistance of a material. 

The cohesive surface considered in the foregoing is based on observations made under quasistatic conditions. In particular, the incubation time for craze initiation is neglected and a critical stress state for craze nucle-ation is used (Eq. 11). For dynamic loading, a time-dependent craze initiation criterion is to be included in the kinetics, since the characteristic timescale associated with the loading can be comparable to that involved in the craze nucleation process. If the time for craze initiation is accounted for, another timescale is involved in the competition between crazing and shear yielding that determines whether or not crazing takes place. Therefore, a switch... 

Fig. 36. Relationship between craze width 2v and length s during crack propagation under fatigue and quasistatic (circular area) loading...

Experimental results show typical values of nj/no in PMMA at break in the range of 2-3 and in PC of 1.4-1.5 leading to craze refractive indices p of 1.15-1.09 and 1.19-1.12, respectively. A similar restriction on the range of values of Hj/ng is found in the experimental results for quasistatic and cyclically loaded cracks in the other sections of this review. Hence, with reference to Fig. 2.6, the variation in craze refractive index is not nearly as great as might initially have been expected. 

The elastic modulus by this dynamic method is slightly higher than by load extension, a quasistatic method. Tests involving elastic deformations, where either stress or strain is held constant, are called static tests. In quasistatic tests, stress or strain is changed slowly with time, and in dynamic tests, stress and/or strain are varied rapidly with time. 

Figure 9 is for spherical probe and shows that even at very low crosshead velocity the viscoelastic effects considerably increase the adherence force compared to the elastic (or quasistatic) adherence forces at fixed load (point C) or fixed grips (point D). 

We have already referred to various kinds of data on mechanical behavior of polymers. We are now going to consider methods of acquisition of such information. The most fi equently used are the so-called quasistatic methods which involve relatively slow loading. Tension, compression, and flexure belong here. The quasistatic methods have to be distinguished from so-called transient tests which include stress relaxation and creep. There are also impact tests and dynamic mechanical procedures which will be defined later. 

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