Linearly increasing stress test

Linearly increasing stress test Examples of Stress Corrosion Cracking see Models... 

A. Atrens, C.C. Brosnan, S. Ramamurthy, A. Oehlert, I.O. Smith, Linearly increasing stress test (LIST) for see research, Meas. Sci. Technol. 4 (1993) 1281-1292. 

N Winzer, A Atrens, W Dietzel, G Song, KU Kainer, Comparison of the linearly increasing stress test and the constant extension rate test in the evaluation of transgranular stress corrosion cracking of magnesium. Materials Science and Engineering A, 2008, 472, 97-106. 

The HDT is the temperature at which a standard deflection occurs for defined test samples subjected to a given bending load and a linear increase in temperature. The stresses usually selected are 0.46 MPa (HDT B) or 1.8 MPa (HDT A) and must be indicated with the results. In any case, the polymer cannot be used under this load at this temperature. 

The influence of fillers has been studied mostly at hl volume fractions (40-42). However, in addition, it is instructive to study low volume fractions in order to test conformity with theoretical predictions that certain mechanical properties should increase monotonlcally as the volume fraction of filler is Increased (43). For example, Einstein s treatment of fluids predicts a linear increase in viscosity with an increasing volume fraction of rigid spheres. For glassy materials related comparisons can be made by reference to properties which depend mainly on plastic deformation, such as yield stress or, more conveniently, indentation hardness. Measurements of Vickers hardness number were made after photopolymerization of the BIS-GMA recipe, detailed above, containing varying amounts of a sllanted silicate filler with particles of tens of microns. Contrary to expectation, a minimum value was obtained (44.45). for a volume fraction of 0.03-0.05 (Fig. 4). Subsequently, similar results (46) were obtained with all 5 other fillers tested (Table 1). 

Hopkins and Campbell also carried out similar tests with a pin-on-disc tester in which the stress in the contact zone was calculated as 117 MPa, reducing to 1.3 MPa. The results of these tests were completely different, showing a linear increase in wear life with increasing initial film thickness over the whole range studied, as shown in Figure 7.10. 

Figure 15 shows how the average fatigue lifetime of PS depends on frequency for two different stress amplitudes. The variation appears to be a linear one on this log-log plot, with the number of cycles to fracture increasing with increase of frequency, and at essentially the same rate for both stress amplitudes. For the rubber modified HIPS the fatigue endurance is plotted as a function of frequency in Fig. 16. Here too the lifetime increases with increase of test frequency and again the variation is a linear one on a log-lot plot. The slope of these curves is also essentially independent of stress...

During the test, the axial stress is increased in small increments and the axial deformation, which may be time-dependent, is recorded. The results may be plotted as an axial stress-strain relationship which is non-linear (the rate of strain increase declines with increasing stress), or as bulk density (or voidage, or void ratio) as a function of the compaction stress. 

The isothermal tensile fatigue tests were performed at 566°C (1050°F) in air. The specimen geometry was the same edge loaded tensile geometry used in the creep tests. The test cycle involved linearly increasing the tensile stress from the minimum value (14 MPa) to the maximum value over a one second period. The sample was held at the maximum stress for one second and then the stress was linearly reduced to the minimum value over an additional one second period. 

The transition point where the number of AE events drastically increases can be graphically determined under the assumption that the number of events linearly increases as a function of the axial stress. Fig. 11.41 exem-plarily shows the number of events versus the axial stress which was measured during a triaxial compression test of a rock salt specimen (diameter 60 mm, length 300 mm) at a confining pressure of 6 MPa (Alkan and Pusch, 2002). 

In a set of similar creep tests on the same material where the level of applied stress is varied at constant temperature, the effect of increasing stress normally is to decrease the creep modulus at corresponding test times. This is consistent with experience and with the theory of linear viscoelasticity. However, experimental data occasionally will show the opposite efl ect, or the creep curves at different stress levels will cross. This is probably due to experimental variation, and in such cases the experimental data may be regarded collectively as estimates of creep behavior over the range of the applied stesses involved. 

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 ... 

The mechanical properties of PP have been studied under pressures up to about 7 kbars by Hears et al. They observed a linear increase in peak yield stress with pressure in a series of tensile tests. 

Number-average molecular weights are Mn = 660 and 18,500 g/ mol, respectively (15,). Measurements were carried out on the unswollen networks, in elongation at 25°C. Data plotted as suggested by Mooney-Rivlin representation of reduced stress or modulus (Eq. 2). Short extensions of the linear portions of the isotherms locate the values of a at which upturn in [/ ] first becomes discernible. Linear portions of the isotherms were located by least-squares analysis. Each curve is labelled with mol percent of short chains in network structure. Vertical dotted lines indicate rupture points. Key O, results obtained using a series of increasing values of elongation 0, results obtained out of sequence to test for reversibility. 

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