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Predicted Fracture Behavior of Shaft Steels with Improved Corrosion Resistance
Department of Marine Engineering and Ship Power Systems, Faculty of Maritime Studies Rijeka, University of Rijeka, Rijeka 51000, Croatia
Department of Engineering Mechanics, Faculty of Engineering, University of Rijeka, Rijeka 51000, Croatia
Author to whom correspondence should be addressed.
Hugo F. Lopez
Received: 14 January 2016 / Accepted: 14 February 2016 / Published: 19 February 2016
One of the crucial steps in the shaft design process is the optimal selection of the material. Two types of shaft steels with improved corrosion resistances, 1.4305 and 1.7225, were investigated experimentally and numerically in this paper in order to determine some of the material characteristics important for material selection in the engineering design process. Ultimate tensile strength and yield strength have been experimentally obtained, proving that steel 1.4305 has higher values of both. In addition, J-integral is numerically determined as a measure of crack driving force for finite element models of standardized fracture specimens (single-edge notched bend and disc compact tension). Obtained J values are plotted versus specimen crack growth size (Δa) for different specimen geometries (a/W). Higher resulting values of J-integral for steel 1.4305 as opposed to 1.7225 can be noted. Results can be useful as a fracture parameter in fracture toughness assessment, although this procedure differs from experimental analysis.
steel 1.4305; steel 1.7225; fracture
Material selection is a crucial step in the process of engineering design. Optimal selection of the material can significantly reduce the possibility of failures, along with understanding the nature and stress intensity that occurs in a designed structure. Engineering practices usually distinguishes one or few causes of failure: excessive force and/or temperature-induced elastic deformation, yielding, fatigue, corrosion, creep, etc. Selection of improper materials may have a negative effect on operational lifetime cycle and result in flaw appearance, which can cause structural failure.
A successful material-selection process implies reconciling requirements, such as appropriate strength of a material, sufficient level of rigidity, heat resistance, etc. For structures susceptible to crack growth, it is necessary to ensure that the material has been selected on the basis of fracture mechanics parameters.
Considering shaft design, the fracture mechanics approach must be used in order to account for high stresses and harsh operating conditions. Implementation of the fracture mechanics approach has the benefit of reducing potential failures, such as the fatigue induced fracture presented in a study of marine main engine crankshaft failure [1
]. The agitator steel shaft failed due to an inadequate design, which was incapable of withstanding torsional-bending fatigue during operation [2
]. The gearbox shaft failure occurred due to high stress concentrations at the corners of the wobbler of the shaft, causing fatigue crack initiation [3
]. Improved design and machining practice suggested that this would help to prolong service life of the component. A forklift collapsed due to failure of axle shaft, caused by material inclusions and poor heat treatment [4
Most of the mentioned failures occurred on steels typically used in the manufacturing of shafts intended for use in harsh environments, where a higher corrosion resistance is necessary. To be able to properly choose a suitable material for such an environment, characterization of a material is essential.
Fracture mechanics parameters that define material resistance to crack propagation are usually determined through experimental investigations of the material under consideration. Fracture behavior is usually estimated using some of the well-established fracture parameters, such as stress intensity factor (K
-integral, or crack tip opening displacement (CTOD). J
-integral is appropriate for quantifying material resistance to crack extension when dealing with ductile fracture of metallic materials, which includes nucleation, growth, and coalescence of voids [5
]. For a growing crack, J
-integral values can be determined for a range of crack extensions (Δa
) and can be presented in the form of the J
-resistance curve. This curve is usually obtained experimentally following standardized procedures, but it can be successfully complemented or even substituted by numerical methods, e.g., the finite element (FE) method. Some of the recent articles on this topic include discussion on the accuracy of J
-integral obtained by experiments, two-dimensional (2D) FE analysis, three-dimensional (3D) FE analysis, or the Electric Power Research Institute method [6
-integral and CTOD are related through plastic constraint factors evaluated using 3D FE analyses of a clamped, single-edge tension specimen [7
]. Methodology to evaluate 3D J
-integral for finite strain elastic-plastic solid using FE analysis is proposed [8
]. Stress intensity factors and T-stress of 3D interface cracks and notches are computed using the scaled boundary FE method [9
This paper presents a comparison of numerically predicted J-values taken from the measure of crack driving force for two types of steel commonly used in shaft manufacturing, steels 1.4305 and 1.7225. Obtained material data may help designers to find the best solution in appropriate material selection.
This work has been financially supported by the Croatian Science Foundation under the project 6876, and by the University of Rijeka under the projects 13.09.1.1.01 and 13.07.2.2.04. Funds for covering the costs to publish in open access publications have been provided.
Goran Vukelic and Josip Brnic conceived the idea of the research; Josip Brnic performed the experiments; Goran Vukelic analyzed the data; Goran Vukelic performed numerical analysis; Goran Vukelic and Josip Brnic wrote the paper.
Conflicts of Interest
The authors declare no conflicts of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.
The following abbreviations are used in this manuscript:
Disc Compact Tension
Single Edge Notched Bend
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(a) Test specimen (dimension in mm); (b) testing machine.
Uniaxial engineering stress-strain (σ
) diagrams for the considered materials [16
Optical micrograph of steel 1.4305; as-received material, soft annealed and cold drawn, cross-section of the specimen, aqua regia, 1000×.
Optical micrograph of steel 1.7225; as-received material, soft annealed, cross-section of the specimen, 4% nital, 1000×.
FE model of: (a) SENB specimen; (b) DCT specimen.
Comparison of numerically-predicted and experimentally-obtained J values for SENB specimens of 1.6310 steel.
Numerically-predicted J values for steel 1.4305 using FE models of: (a) SENB specimen; (b) DCT specimen.
Numerically-predicted J values for steel 1.7225 using FE models of: (a) SENB specimen; (b) DCT specimen.
Chemical composition of considered materials (wt%).
Yield strength (σYS) and tensile strength (σTS) of the considered materials [16,17].
|Material||σYS (Mpa)||σTS (Mpa)||CVN (J)||KIc (MPa·m1/2)|
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