# Fatigue Failure of a Pressing Machine
## 2 Analysis of the Failure  
After about 500,000 work cycles, the fracture occurred in the upper guide bar (see Figure 1). The fatigue crack initiation was in the internal thread of the guide bar in the cross-section between the bolt end and the bottom of the threaded rod guide opening. The fracture surface appearance displayed in Figure 2 demonstrates the high rate of fatigue crack propagation across the overall cross-sectional area. The residual fracture of the whole cross-section is only about one-third of total cross-sectional area. While assessing the causes of premature fracture of the guide bar, it was necessary to analyze the material at the fracture site and the working cycles of the device in terms of stresses acting in the cross-section where the fracture occurred. A sliver of base material directly cut out from the guide bar in the fracture site was selected for metallographic analysis of its microstructure and hardness measurement. By mechanical splitting, subsequent grinding, and fine polishing, the microscopic structure was enhanced after etching (Figure 3). A number of facts can be ascertained from the microstructural analysis of the steel. * The material is non-eutectic steel with a ferritic-pearilic structure which has not been heat-treated. * Carbon content is 0.45 \(\div\) 0.48%. * The possible alloying element is chromium (in the unprocessed state, it practically does not have any impact on the strength and deformation properties of the steel). * The microstructure of the material is highly homogeneous without structural debris, massive deposits, cracks, burnt oxides, etc. * In view of the small number of deposits, it can be assumed that the steel has a low sulphur and phosphorus content and was obtained by a two-stage metallurgical treatment. * The structure of the steel does not contain any structural regions that would serve as local initiators of the origin and growth of the fatigue crack. Figure 2: Fracture surface. The measured hardness values (Figure 4) with minimal scattering are confirmed by the conclusions of the metallographic observation of the structural homogeneity of the steel under consideration and its sufficient plastic properties (ductility). The hardness determined for steel strength is about 495-530 MPa. The fatigue limit of such material can be calculated by methods [8; 9] to an approximate value of 220 MPa. In order to assess the cause of the fracture, it is necessary to further know the operating loading in the critical cross-section of the guide bar in the form of a stress time course. Within the available options, the following procedure was selected:  
* direct measurement of stresses on the external surface of the critical cross-section of guide bar during operation and
* determine the notch effect of the internal thread analytically and on the model using FEM (finite element method). Along the critical cross-section of the guide bar, strain gauge sensors were installed at 120\({}^{\circ}\) spacing, so that it was possible to differentiate the effect of bending moments and normal force [10; 11] (Figure 5). This procedure allowed the point of maximum cross  
Figure 4: Hardness measurements at the site of the guide bar crack in both directions. Figure 3: Metallographic facet of the steel (magnified 250 times, etched in nital). sectional stress to be determined using equations for the calculation of the stress from normal force and the maximum stress from the bending moment:  
\[\sigma_{N}=\frac{1}{3}E(\varepsilon_{i}+\varepsilon_{ii}+\varepsilon_{iii})\]  
\[\sigma_{B}=\frac{2}{3}E\sqrt{\varepsilon_{i}^{2}+\varepsilon_{ii}^{2}+ \varepsilon_{iii}^{2}-(\varepsilon_{i}\cdot\varepsilon_{ii}+\varepsilon_{ii} \cdot\varepsilon_{iii}+\varepsilon_{i}\cdot\varepsilon_{iii})} \tag{1}\]  
where \(E\) is [PERSON]'s modulus and \(\varepsilon_{i}\) are values of the relative deformations measured by individual sensors.