Fatigue Strength of Machines and Systems

1. Introduction

Fatigue properties have been a topic of investigation since the nineteenth century. Many researchers from that time, up until today, have tested and developed theories on this topic, but fatigue failures still occur. It is estimated that over 80% of failures of machine parts are caused by fatigue cracks [1]. To prevent fatigue cracks, different models are used, e.g., [2,3,4]. To use any of them, the fatigue properties of the material being used must be known. These properties are specified by various constants, and many models have been developed to estimate the material’s constants, from tensile tests to hardness measures. However, these methods can lead to a high rate of error. Especially, this could happen with new materials, using new heat treatments, or plastic deformation. That is why it is important to correlate the fatigue properties of a metal with the elements of a machine or system. Because of this, many structural components are experimentally tested, for example, bicycle frames [5], railway axles [6], or bridge crane shafts [7].

In most cases, the failure parts are investigated to find the cause of the problem [8,9]. The fracture surface can provide information on the propagation of the crack. Firstly, type of loading and magnitude are investigated [10,11]. Secondly, if there was impact on the surface of the element or any other unusual event [6,12]. Thirdly, if the component was manufactured according to the available documentation [7]. Fourthly, if there are material defects [13,14].

2. An Overview of Published Articles

New parts must fulfill many requirements. One of them is fatigue reliability. The failure of such parts of the machine can lead to life-threatening risks, large economic losses, or environmental damage. In these cases, it is essential to perform experimental tests; therefore, it is important to elaborate a test plan. Such case studies can be found in the paper by Monclova-Quintana et al. [15], where a bilinear P-S-N curve with Weibull distribution was also described. The relationship of a number of specimens with statistical test power can be found in another Special Issue paper by Strzelecki and Sempruch, which allows us to determine the required test number of specimens according to the reliability of failure. In another paper by Huňady et al., an upgrade of the universal testing machine is reported. Such an approach is ecologically friendly, as it uses old analog equipment with new digital control components.

Kharchenko et al. analyzed dynamic loads on drill rigs. Such cases of loading can lead to the accumulation of fatigue damage. The calculation of these loading cases is difficult for engineers because of unsteady operating modes, like the initiation of drilling. A mathematical model was proposed and verified in the work.

Ivanytskyi et al. presented a novel optical digital method for fatigue testing. The authors presented a testing stand for measuring pipes’ internal pressure and axial load. A theoretical–experimental method was proposed. Another approach to designing the pipeline structure can be found in a paper by Wei et al., in which they analyzed the residual strength of coiled tubing pipes affected by repeating banding.

Lie et al. utilized a new computer science method. Machine learning was used for fracture hit detection and the monitoring of oil/gas wells. The presented method can be used for design completion, spacing and infill, production management, and wellbore protection.

A fatigue study of a ball screw was presented by Lv et al. It was analyzed under extreme loads and ultra-low cycling conditions. This case of exploitation often occurs in the assembly part of the installation, or during the repair of the machine. However, it is rarely taken into account in engineering calculations of fatigue life.

Other interesting fatigue tests were presented by Nowicki et al., where three photopolymer 3D-printed materials (NextDent Denture 3D, NextDent C&B MFH Bleach, and Graphy TC-80DP), were used for the temporary restoration of dental implants. The tests imitated real mechanical and thermal loading in their clinical application. The authors gave recommendations for the use of each of the materials for the specific situations of individual patients.

3. Conclusions

Based on the research papers presented here, the following conclusion can be drawn: it is essential to provide fatigue tests in the context of the large dispersion of test results. Additionally, it was determined that fatigue crack occurs due to many factors, which were mentioned in the Introduction. Unfortunately, each factor is not adequately described in normative documents or other guidelines.