Fatigue phenomena when designing industrial steel structures – Part 1

Strength may be defined in terms of tensile yield stress, F_y, which is the point where plastic behaviour begins at unrestricted plastic flow. Strength or resistance may also be characterized in terms of the ultimate tensile stress, F_U, which is attained after yielding and significant plastic behaviour. An increase in strength is associated with plastic behaviour (due to strain hardening), until the ultimate tensile stress is reached (Fig. 1).

The most significant properties of steel that are exhibited by stress-strain curves are the elastic modulus (linear slope of the initial portion of the curve up to yield stress) and the existence of yielding and plastic behaviour, with some unrestricted flow and strain hardening, until the ultimate stress is attained. This behaviour occurs in a structural element that is under normal loading conditions. In many industrial buildings, other failure criteria need to be considered in the design. These failure criteria may be different based on the material properties and loading conditions. Fatigue, creep, the impact of strain rate and design temperature are some of the other failure modes that may need to be considered in the structural design of some industrial buildings.

Fatigue is a process in which damage accumulates due to the repetitive application of loads that may fall below the yield point. Fatigue is the initiation and propagation of microscopic cracks into macro cracks through repeated application of stresses. All structural steel materials contain metallurgical or fabrication-related discontinuities, and most also include severe stress concentrators.

The fatigue begins as an internal or surface flaw where the stresses are concentrated and consist initially of shear flow along slip planes. Over a number of cycles, this slip generates intrusions and extrusions that begin to resemble a crack. A true crack, running inward from an intrusion region, may propagate initially along one of the original slip planes but eventually turns to propagate transversely to the principal normal stress until observing a sudden fracture of the remaining cross-section (Fig. 2 to 4). The phenomenon may be problematic because a single application of the load would not produce any sign of defect, and a conventional stress analysis may lead to an assumption of safety that does not exist.

The fatigue life of a member is the amount of stress that can be sustained before showing any signs of failure. Some of the parameters that influence fatigue life are as follows:

The fatigue of materials subjected to cyclic loading is dependent upon not only the maximum stress level encountered, but also the range of the stresses applied. Generally, the greater the maximum stress and range, the greater the damage encountered. The accepted method is the maximum stress and R-value, which is a minimum to maximum stress level (Fig. 6 to 9). Usually, the five conditions that the R-value can take range from +1 to -1:

1. Stresses are fully reversed: R = -1
2. Stresses are partially reversed: R is between -1 and zero
3. Stress is cycled between a maximum stress and no load; the stress ratio R becomes zero
4. Stress is cycled between two tensile stresses; the stress ratio R becomes a positive number less than 1
5. An R stress ratio of 1 indicates no variation in stress, and the test becomes a sustained-load creep test rather than a fatigue test.

The geometric effect is often dominant. It depends on the stress gradient in the crack growth direction. In thick plates, the non-linear stress peak appears in a relatively deep surface layer, which gives rise to faster crack growth and a shorter fatigue life compared to similar details in a thinner plate. Also, the probability of introducing large flaws in welds is greater, as the extent of welding increases.

The microstructure of metals can influence the fatigue life of a member. The complex microstructures in structural metals and engineering alloys result in a number of potential sites where competing fatigue crack initiation mechanisms may occur.

It has been demonstrated that service temperatures as low as -50°C do not have a negative impact on the fatigue behaviour of steel. The fatigue strength at low temperatures grows slightly due to an increase in material tensile strength. However, in the case of governing the brittle fracture, low temperatures decrease the fatigue life of the element. Fatigue resistance also decreases at temperatures higher than 300°C due to creep phenomena. The reduction factor for temperatures above 100°C, without considering the creep phenomena, can be written as follows:

R_T=1.0376 -0.2239.10^-3 T -1.372.10^-6 T^{2 [2]}

where “T” is the ambient temperature.

As you can see, there are many components that can influence fatigue life. We have only begun to list them here, but keep your eyes peeled for Part 2 of this blog article where we continue to provide other possible causes of structural fatigue along with some strategies. Our engineers are here to help you, so feel free to contact us for more information or for support with any issues you may be experiencing with your structural components.