Fatigue hysteresis loop

FIG. 24 Thermomechanical fatigue at 25-80 °C thermal cycle and isothermal fatigue hysteresis loops at 25°C and 80°C are compared. Mechanical strain range was 1%. There was no hold in the 50-sec ramp time cycle. (From Ref 7.)... 

Furthermore, the history of a hysteresis loop plays an important role in the determination of lifetime and reliability of ferroelectric capacitors, especially for applications in ferroelectric memories. Three main effects are characterized in particular as changes in the hysteresis loop under various conditions, which are described later in this chapter as fatigue, retention, and imprint with the corresponding ways to measure these effects. 

Nano scale hysteresis loops of fatigued FeCaps... 

There are a number of indicators of fatigue damage that have attracted interest in the literature. During the life of a component subjected to fatigue, the material can exhibit changes in modulus, permanent offset strain, shape of the hysteresis loops, and temperature rise of the specimen surface. Direct evidence of matrix crack density can be obtained by surface replication, while a more detailed analysis of microstructural damage requires scanning electron microscopy (SEM). 

Fig. 6.11 Changes in the hysteresis behavior during the fatigue of unidirectional SiCf/CAS-II. The number of cycles (in thousands) is shown above each curve. Note that the average modulus, area of the hysteresis loops, and the permanent strain offset all change during fatigue. Failure took place at 3.21 x 106 cycles. After Holmes and Cho.12...

For non-transparent specimens, as shown by Bucknall and Stevens useful information relative to the deformation mode can be obtained by recording hysteresis loops as a function of cycles. Figure 6 shows hysteresis loops obtained at 0.2 Hz at various N values for PS tested at a stress amplitude of 24.1 MPa and Fig. 7 for HIPS tested at 17.2 MPa. For PS, with Nf = 1,451 cycles, there is no detectable change in loop area at this stress amplitude up to the final cycle. This illustrates the highly localized nature of the fatigue-induced damage zone in PS and indicates that, for this polymer, hysteresis loop observations are not an effective method for detecting craze... 

For a HIPS sample tested at a stress amplitude of 17.2 MPa and a frequency of 0.2 Hz, hysteresis loops taken at various cycles (Fig. 7) indicated that craze initiation was first observed for this sample after about 20 cycles, while 283 cycles were required to fracture. For similar fatigue tests carried out at the lower frequency of 0.02 Hz, the cycles to fracture were decreased (from 283 to 64) and loop asymmetry and craze formation began sooner, at about 1-2 cycles. The changes produced in hysteresis loops with cycling are shown in Fig. 19. With decrease of test frequency reduces, the entire S-N curve shifts to the left as shown by Fig. 18, and, because of the increased time for each cycle, fatigue induced craze initiation occurs earlier in the specimen lifetime. 

Fatigue-induced deformation mechanisms can be studied by measuring the volume changes. Another sensitive method is to monitor the hysteresis loops under tension-compression loading. 

Figure 12.5. Hysteresis loops developed during fatigue tests of ABS and HIPS [BucknaU, 1988].

Figure 7. The effect of cycles during dwell fatigue experiments on the area of hysteresis loops of the stress-strain curves...

Towo AN, Ansell MP (2008) Fatigue of sisal fibre reinforced composites constant-life diagrams and hysteresis loop capture. Compos Sci Technol 68 915-1924... 

Fig. 4. Plots of hysteresis loops for polyacetal fatigued in the low cycle (thermally dominated) and high cycle (mechanically dominated) regions. The numbers located by each hysteresis loop indicate the cycle number (13). To convert MPa to psi, multiply by 145.

An important issue is the influence of an electrochemical environment on the cyclic deformation behavior of metals [74,33-35]. As illustrated by the data in Fig. 1 for a carbon-manganese steel in high-temperature water, environment does not typically affect the relationship between stresses and strains derived from the maximum tensile (or compressive) points of steady-state (saturation) hysteresis loops [36]. Such loops should relate to elastic and plastic deformation prior to substantial CF microcracking. CF data of the sort shown in Fig, 1 are produced by either stress or total strain controlled uniaxial fatigue experiments, identical to the methods... 

Long-term fatigue behavior was also studied. The hysteresis loop after each cycle was observed to behave stochastically. As the crack opening displacement... 

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