Fatigue testing curve

At contact fatigue tests of different steel and cast iron types was used the acoustic emission technique. Processed records from the AE analyser show importance of acoustic response of tested surface continuous sensing. In graphs are obvious characteristic types of summation curves, or may be from significant changes of AE signal course identified even phases of the wear process. 

When constant stress (5) amplitudes are encountered, the process is known as high cycle fatigue, because failure generally occurs only when N exceeds 10 cycles. Data from high cycle fatigue tests are reported in the form of an 5 vs Ai curve, as shown in Figure 4b (7). 

Rotating Beam Fatigue Test for Steel Cords. The purpose of this test method is to evaluate steel cord for pure bending fatigue (121). The test sample consists of a 3-mm diameter mbber embedded with steel cord. Different bending stress levels are appHed and the time to failure is recorded. The test stops at 1.44 million cycles. The fatigue limit is calculated from S—N (stress—number of cycles) curve. 

ASTM Special Technical Publication No. 91 discusses in detail the important ramifications to be considered in the various statistical aspects of fatigue testing. Most often, the fatigue curves as well as the tabulated values... 

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. 

To measure n (defined in Eq. (12.30)) directly from velocity versus K curves is often difficult and time consuming. Fortunately, a simpler and faster technique, referred to as a dynamic fatigue test (not to be confused... 

Under normal service conditions, stresses due to cyclic loading are small and deformations can therefore be considered to be elastic. However, cyclic loading may cause fatigue of the adhesive. A conventional fatigue testing technique involves determination of so-called S-N curves, where S is the stress amplitude and N is the number of cycles to failure. At this point, it would be useful to define some important parameters that are used in the phenomenological description of the fatigue phenomenon. Based on Fig. 33.5, the stress amplitude can be defined as in Eq. (3) we use t since samples were loaded in shear. 

For example, Melin et al. [20] showed that fatigue tests of impact-damaged laminates in tension—compression fatigue at R values of — 1 and —5 have almost identical stress-life (S-N) curves when the lives are plotted in terms of maximum compression stress. Tests at an J value of -1 have a mean stress of zero and equal excursions into tension and compression, whereas tests atR= -5 will have stress cycles with tension... 

Figure 16.41 Electrochemical impedance spectroscopy (ECIS) Nyquist plot of Re-hn impedance curves at various stages during low-cycle flexure fatigue testing of woven GFRP (specimen failure occurred at 7000 cycles) [99].

In Fig. 5.11(c) the results of the fatigue tests, all containing compressive load cycles, are summarized, where the maximum compressive stress in each load cycle (o i ) is plotted against the number of cycles, N, for two different laminates. It is evident that pure compressive cycling results in an extremely flat versus N curve, as already shown in Fig. 5.11. Introducing... 

Plot of stress, S, vs. number of cycles, N, required to cause failure of similar specimens in fatigue test. Data for each curve on the S-N diagram are obtained by determining fatigue life of a number of specimens subjected to various amounts of fluctuating stress. The stress axis may represent stress amplitude, maximum stress, or minimum stress. A log scale is usually used, especially for the N-axis. 

---