Fatigue specimens

Endurance limit To develop S-N curves the fatigue specimen is loaded until, for example, the maximum stress in the sample is 275 MPa (40 ksi) (Fig. 2-43). At this stress level it may fail in only 10 cycles. These data are recorded and the stress level is then reduced to 206 MPa (30 ksi). Tliis specimen may not break until after 1,000 stress cycles at this rather low stress level. 

Fig. 4. Photograph of PS fatigue specimen cycled for 80% of its expected fatigue life at a stress amplitude of 17.2 MPa and at 21 Hz...

Fracture surfaces of the cyclically deformed fatigue specimens were observed at low magnification in order to identify the zones of fatigue initiation and final failure, at higher magnifications in order to identify the regions of microscopic crack formation and growth and microscopic plastic mechanisms. 

Figure 5 shows the dimensions of the fatigue specimen. Nominal thickness is 0.125 in. Although laminate thicknesses may vary from 0.090 to O.lSOin., the grips will easily accommodate these variations. 

Fractographic examinations were performed on fracture surfaces of both tensile and the unidirectional fatigue specimens tested at various temperatures. An extensive discussion and numerous fractographs of both the base metal and welded... 

Contrary to macroscopic samples where the fracture surface of fatigue specimens is usually normal to the tensile stress direction, thin sheets are similar to micro-wires. They undergo a very large amount of plastic deformation and neck down to zero cross-section, ending in irregular knife edge rupture, very much like a high-temperature creep rupture. 

Figure 1.15 Dimensions of a constant force flexural fatigue specimen. Reprinted with permission from ASTM D671-93. 1993 ASTM International...

In another illustration (Fig. 7.5), an S-N type curve is illustrated for Si3N4. Here, as in Fig. 7.3, static and cyclic fatigue are compared. Typical fractographs for cyclic and static fatigue specimens are shown in Fig. 7.6a, b, respectively. In... 

A high-magnification SEM fractograph of the portion between the two semicircular marks in the cyclic fatigue specimen is shown in Fig. 7.6c, after... 

Fig. 7.6 SEM fractographs of fatigue specimen a cyclic fatigue, b static fatigue, and c cyclic fatigue at high magnification [12], With kind permission of John Wiley and sons...

A significant difference between the appearance of fracture surfaces of fatigue specimens tested at 75 and -300 F at the same stress is that the specimens tested at -300 F show more "rubbing" action. It appears that the fracture growth is not as rapid as that in the case of the specimens tested at 75 F. The extent of fatigue damage on the specimen prior to fracture indicates the combined effects of a slower crack propagation rate and the increased tensile properties. 

FIG. 5—Photomicrograph showing fretting caused surface cracks in a metallographic section through a TI-6AI-4V fretting fatigue specimen. 

Since the scaiming electron microscope (SEM) became available, investigators have sometimes exposed a tensile or fatigue specimen that is pre-corroded and then tested to failure by tension or by fatigue. Either method inherently causes failure at the site of the deepest surface flaw. SEM examination of the fractured faces can then accurately measure the maximum depth of pitting. 

The extremely low ductility values at ambient temperature and the increased ductility with increasing temperatures strongly influence the observed fracture mode. Tfensile and fatigue specimens indicate that the predominant fracture modes are cleavage at low temperatures due to dislocation pile-up and intergranular fracture above the brittle-ductile transition temperature (Ref 13-17). 

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