Fatigue testing illustration

In the case of a pressure vessel subjected to cyclic loading (as here) cracks can grow by fatigue and a vessel initially passed as safe may subsequently become unsafe due to this crack growth. The probable extent of crack growth can be determined by making fatigue tests on pre-cracked pieces of steel of the same type as that used in the pressure vessel, and the safe vessel lifetime can be estimated by the method illustrated in Case Studv 3. 

Significant scatter is often evident in time to failure data obtained from stress rupture tests conducted on either neat materials or on bonded joints. This scatter may obscure trends and frustrate the user. Results are typically plotted as load level versus the time to failure, a form that is analogous to S-N plots used in fatigue tests (see Durability Fatigue). In keeping with the principles of polymer physics, the time to failure axis should be plotted on a log scale, as illustrated in Fig. 1. Many creep-rupture models for homogeneous materials are based on forms like... 

In the above illustrations, the number of cycles does not exceed 10. In conventional fatigue tests, seldom more than 10 cycles are applied to check fatigue lives. However, in some industries, the required design lifetime of many components often exceeds 10 cycles. Note that materials with well-defined endurance limits do not necessarily show infinite fatigue lives when tested at cycles above 10. Time constraints usually prevent the performance of such extended tests. 

FIGURE 5.15 Schematic illustration of cyclic fatigue testing. 

Figure 1.8 Illustration of typical molded flat sheet fatigue-testing specimens, (a) Flat sheet fatigue specimen with rectangular cross-section (b) flat sheet fatigue specimen with circular cross-section.

Figure 1.14 Illustration of an eccentric machine for flexural oscillation fatigue tests.

Figure 1.17 Illustration of cantilevered fatigue-testing specimens per ASTM D671.

The diagram in O Fig- 46.18 illustrates this case. Spot weld bonded H-samples (see O Fig. 46.15) start the fatigue test at a stress level of around 60 kN and end after six million cycles at around 30 kN, which is the starting level of the Metal Active Gas (MAG)-welded sample. 

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... 

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