1. Applying the damage tolerance methodology on a railway component
Morgado T. L.1, 2; Leite de Sousa, M.G.3; Martins R. F. 1,3; Pereira M.F.C.4, Maurício A.4
1 ICEMS/IST-UL, Lisbon University, Av. Rovisco Pais, Lisbon, Portugal
2 Engineering Unit, ESTA/ Polithecnic Institute of Tomar, 2400 Abrantes, Portugal
3 UNIDEMI - Department of Mechanical and Industrial Engineering, FCT/ UNL, Caparica, Portugal
4 Centro de Petrologia e Geoquímica, IST/UL, Av.Rovisco Pais, Lisbon, Portugal
Introduction
There isn´t usual to apply the damage tolerance approach on the railway industry.
Casting process offer unique cost advantages over other manufacturing methods for many components, especially those having complex three-dimensional geometry. However, neither the presence of casting defects cannot be excluded nor their influence in the fatigue life of the component.
The damage tolerance concept assumes the existence of initial flaws in the component and its basic philosophy is based on the acceptance that damage will propagate and an adequate system of inspection should be implemented in order to monitor the damage.
In this work, fatigue life predictions of pre-existing cast defects present in cast steel of Portuguese railway couplings, used for coal transportation, are presented using specific crack growth relationships. This will avoid early retirement from service of these components and avoid extra costs.
4. Tomography
The equipment used was a micro-tomograph Skyscan1172 (Fig.3)
The analytical set-up consists on the combination of X-ray shadow microscopic system and a computer with tomographic reconstruction (NRecon), visualization and analysis software (Dataviewer, CTAn e CTVox).
The specimen analyzed had a rectangular shape with 4mm width and 1 mm thickness (Fig .4).
Samples were scanned at a voltage of 85 kV with current intensity at 116A.
The pixel size used was 4.80 μm
Fig. 3 – Micro-tomograph Skyscan 1172 available on IST in the Mineralogy and Petrology Laboratory. a) b)
Fig. 4 – a) Amplified sample for tomographic analysis. b) Sample placed on the tomography machine.
5. Porosities and Life Prediction
2. The Object of Study
Figure 5 shows an example of tomographic observation; relevant data about the size and shape of porosities was obtained.
As shown in fig. 1, the object of study is a cast steel coupling, obtained by sand cast with the specifications ASTM A , designed ‘ATLAS’. This railway component is used to connect the wagons between them and the locomotive.
In the calculations performed the following values were used:
Young’s Module 207 GPa
Poisson’s Coefficient 0.3
Yield Strength 427,7 MPa
Ultimate Tensile Strength 659,6 MPa
Fig. 1 – Coupler reconstruction in SolidWorks. a) b)
Fig. 5 – a) Tomography reconstruction in CTVox with different planes of view; b ) Sample reconstruction on CTvox with pores in red.
3. FEM
Finite element analysis (FEM) was carried out using tetrahedral solid elements.
It was found that the critical region lies in the connection between the head and the body of the component and that the maximum Von Mises stress does not exceed 316 MPa (Fig. 2).
Propagation curves da/dN vs ΔK for a probability of survival of 50% were used to calculate the fatigue life of the component, assuming superficial and embedded defects with dimensions similar to those measured using tomography.
BS7910 equations were used to calculate the stress intensity factor and a MatLab routine was written to calculate the fatigue life. a) b) a) b)
Fig. 2 – a) Coupling mesh and the values for the Von Mises stresses obtained by ANSYS; b) Coupling design improvement.
Fig. 6 - Fatigue Life Prediction assuming: a) superficial defects; b) Embedded Defects assuming.
6. Conclusions
Tomography results shows the geometry complexity of intrinsic defects of the material.
It is extremely important for the mechanical fatigue design, to consider the intrinsic
defects of the material.
The fatigue life prediction could allow the definition of inspection periods and extend
the fatigue life of the component.
Redesign study, showed that minor modifications will allow to obtain a theoretical infinite fatigue live.