Compressive measurement engineering strain

Most deformations result in the material being strained, for instance, elongated or compressed (exceptions occur when the material is translated or rotated as a whole, without changing shape). In the uniaxial powder compression example, the deformation decreases the height of the powder from the initial value Hq to the current value H (Fig. 4A). Often used measures of uniaxial strain include the engineering strain... 

Although measures of engineering stress and engineering strain assume constant cross-sectional area of the rod being stressed, a material deformed elastically longitudinally (in compression or tension) has an accompanying lateral dimensional change. This is described by Poisson s ratio, i/. If a tensile stress produces an axial strain +e and... 

The present ISO standard for creep is ISO 80131 which specifies procedures for measurements in compression and shear. In earlier standards, creep and stress relaxation were covered in the same documents and creep in tension was included. One reason for the separation was that stress relaxation became more important for seal performance, whereas creep remained a more minority interest. Measurements in tension were dropped on the basis that engineering components are not generally stressed in this manner. However, it is worth noting that, if a general indication of creep performance is required, the strains in tension can be relatively large and only quite simple apparatus is necessary. Such a simple method is included in the ISO standard for tension set described in Section 3.2. The British equivalent, BS903 Part A152 is identical to ISO 8013. 

Fig. 5.17 Unconfined compression stress-strain curves and experimentally measured temperature increase ATa as a function of strain for PS (Dow 685), LDPE (Dow 640), and PP (LG H670). The initial test specimen was at 26°C and the crosshead speed of the compressing har with the load cell was 25.4 mm/min. The specimen dimensions were 101 mm diameter and 71 mm height. [Reprinted by permission from M. H. Kim, Ph.D Thesis, Department of Chemical Engineering, Stevens Institute of Technology, Hoboken, NJ (1999).]...

Wiliams and Ford developed plain strain compression test which was initially applied to metals [Williams and Ford, 1964]. It was based on the fact that strain is easier to measure in compression test. The same test method may be used for polymer blends to obtain total deformation curves up to high levels of strain that may be encountered in engineering applications. Williams had further explained the application of this technique to polymers [Williams, 1964]. 

Based on the load-strain and load-deflection measurements, PSZT exhibits non-linear stress-strain behavior. A plot of linear-elastically computed stress (or engineering stress) versus strain for poled-depoled specimens tested at room temperature, 75, 86, 105 and 120°C is shown in Fig. 3. Deviations from linear-elastic behavior initiate at a nominal stress level of approximately 20 30 MPa for specimens tested at room temperature and 10 20 MPa for specimens tested at an elevated temperature. Furthermore, the extent of non-linearity increases as the testing temperature increases. Conversion of the load-strain data to the true stress-strain behavior was achieved by implementing the approach first described by Nadai and adapted by Chen et al. ° The true compressive (o-c) and tensile stresses (at) were calculated as follows ... 

On a material science level, thermoplastics also have different behaviors than many other materials. For starters, most thermoplastics are anisotropic that is, they have different properties when measured in different directions. They also have different behavior in compression than they do in tension. And their mechanical behavior is nonlinear, in that their behavior does not follow the traditional linear stress-strain relationship seen in metals. This means the classic engineering equations for structural calculations are not always accurate. 

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