Wohler fatigue limit

It should be remembered [39], that when evaluating data about fatigue limits in the pulsating or alternating mode which have been determined with inert fluid environments, that the corrosion fatigue may not only effect remarkably the reduction of the admissible stresses but also depends on the number of cycles (a continuous small slope of the Wohler-curve). 

Figure 11.44 Wohler curve in inert and in corrosive environment (schematic). The figure shows (a) a metal with a fatigue limit (b) a metal without a fatigue limit and (c) a metal undergoing corrosion fatigue.

The cylinder is subjected to a repetitive stress by the hydraulic pressure. Approx. 40 strokes are performed per hour so that after an assumed life of the press of 30 years, and two 8-hour shifts per working day, and 300 working days per year, the number of load alternations = 30 X 300 X 2 X 8 X 40 = 5,760,000 is attained. Therefore, the fatigue limit has to be calculated as per Wohler s diagram. 

This time the variation is about 15 %. The question arises now as to the knowledge of the fatigue limit of the plate containing 20 mm hole with respect to that oy of plate without hole. The Wohler s curve of the material is that already assessed in sample problem 4 of Sect. 6.3. It can be seen how the transition point N, between low cycle fatigue and high cycle fatigue, i.e., between plastic and elastic behavior, is at about — 0.0015 so that we can assume that in our case the behavior is mainly elastic and neglect the plastic component and draw the... 

The time to failure measured on non-precracked specimens represents the sum of the initiation and propagation times of a crack. Unfortunately, crack initiation, which often is a slow step, is a random phenomenon. As a result, the reproducibility of corrosion fatigue tests earried out with non-precracked specimens is usually mediocre. Furthermore, to determine a Wohler eurve, a large number of specimens is needed. Statistical methods are usually applied to determine the endurance limit in corrosion fatigue tests. 

As mentioned above, the fatigue behavior with damage development should be assessed by the Wohler curve and the fatigue endurance limit. The latter value is the most important parameter for design and construction purposes. For the description of the Wohler curves, different methods can be selected according to the plotting (semi- or double logarithmic scale). 

Figure 9.10. Wohler curves related to fatigue tests in compression of different carbides. The lower limit of loading is 250 Mpa...

---