3. Low Cycle Fatigue (LCF) - ULisboa · PDF fileKeywords: Experimental tests, Railway,...
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Mechanical behavior assessment of railway material J.Nunes Mechanical Engineering Department, Instituto Superior Técnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal Abstract The main goal of this paper is to present the evaluation of the mechanical properties of railway material, using tensile stress tests, low and high cycle fatigue tests and to describe the resulting fracture surfaces. These tests were conducted under ASTM norms E8–04 and E 606 – 92, for tension testing of metallic materials and strain-controlled fatigue, respectively. The identification of the nucleation site was done in a binocular microscope and portray of the fracture surfaces was achieved with a OPTICA microscope. Keywords: Experimental tests, Railway, Fatigue, Fractographic Analysis, Crack Propagation. 1. Introduction The virtual possibility of a railway component to fail is always cause for concern. The accumulation of damage due to fatigue can reduce greatly the expected service life of the rail track, with passing years this problem has only aggravated. [1,2] The objective of this work is to present the findings produced in an array of mechanical tests, and assess the damage railway components are subjected to. 2. Tensile stress test The machine used to perform the tensile test was the uniaxial electro-mechanic machine Instron 3369. The tests were conducted under the ASTM Norm E8-04 [1] and the specimens were of the shaped displayed in Figure 2-1. Figure 2-1: Standard Round Tension Test Specimen [1] Three samples in total were tested. One was machined out of a high-speed train wheel, and the other two from a rail. 3. Low Cycle Fatigue (LCF) Two samples were tested in conditions of low cycle fatigue, under ASTM Norm E606-92 [2]. Each specimen performed 15 cycles or less, during each of the selected strain amplitudes, displayed in Table 3-1. Sample Strain [%] CB4 0.2 0.3 0.4 0.5 0.6 CC2 0.4 0.5 0.6 0.8 1 Table 3-1: Strain amplitudes used in LCF The samples used were of the shape described in Figure 3-1.
Transcript of 3. Low Cycle Fatigue (LCF) - ULisboa · PDF fileKeywords: Experimental tests, Railway,...
Mechanical behavior assessment of railway material
J.Nunes
Mechanical Engineering Department, Instituto Superior Técnico,
Av. Rovisco Pais, 1049-001 Lisboa, Portugal
Abstract
The main goal of this paper is to present the evaluation of the mechanical properties of railway material, using tensile stress tests, low and high cycle fatigue tests and to describe the resulting fracture surfaces. These tests were conducted under ASTM norms E8–04 and E 606 – 92, for tension testing of metallic materials and strain-controlled fatigue, respectively. The identification of the nucleation site was done in a binocular microscope and portray of the fracture surfaces was achieved with a OPTICA microscope. Keywords: Experimental tests, Railway, Fatigue, Fractographic Analysis, Crack Propagation.
1. Introduction
The virtual possibility of a railway component to fail is always cause for concern. The accumulation of damage due to fatigue can reduce greatly the expected service life of the rail track, with passing years this problem has only aggravated. [1,2] The objective of this work is to present the findings produced in an array of mechanical tests, and assess the damage railway components are subjected to.
2. Tensile stress test
The machine used to perform the tensile test was the uniaxial electro-mechanic machine Instron 3369. The tests were conducted under the ASTM Norm E8-04 [1] and the specimens were of the shaped displayed in Figure 2-1.
Figure 2-1: Standard Round Tension Test
Specimen [1]
Three samples in total were tested. One was machined out of a high-speed train wheel, and the other two from a rail.
3. Low Cycle Fatigue (LCF)
Two samples were tested in conditions of low cycle fatigue, under ASTM Norm E606-92 [2]. Each specimen performed 15 cycles or less, during each of the selected strain amplitudes, displayed in Table 3-1. Sample Strain [%]
CB4 0.2 0.3 0.4 0.5 0.6
CC2 0.4 0.5 0.6 0.8 1
Table 3-1: Strain amplitudes used in LCF
The samples used were of the shape described in Figure 3-1.
Figure 3-1: Cylindrical specimen used in LCF, [2]
4. High Cycle Fatigue (HCF)
Several test samples were subjected to HCF testing, in various case scenarios. These scenarios are described in the following subsections.
4.1. Case I
Case I is a proportional loading between
axial and torsion forces of √ with synched phases.
a)
b)
Figure 4-1: Case I loading scenario
4.2. Case II
Case II is a proportional loading between
axial and torsion forces of √ and 90º out-of-phase loading.
a)
b)
Figure 4-2: Case II loading scenario
4.3. Case III
Case III is proportional loading of between
axial and torsion of √ , although in this case the axial force is always negative, resulting in a crushing fatigue test. This test is meant to simulate the real-life scenario of a passing wheel over a rail track.
a)
b)
Figure 4-3: Case III Loading Scenario
5. Results
In the ensuing subsections the results for the various test performed are presented.
5.1. Tensile Stress Test
The test for the rail samples are displayed in Erro! A origem da referência não foi encontrada., and for the wheel in Figure 5-2.
Figure 5-1: Tensile test results for rail sample
Figure 5-2: Tensile test result for wheel sample
5.2. LCF
In Figure 5-3 and Figure 5-4 the LCF results are displayed for all the strains rehearsed.
Figure 5-3: LCF results for sample CB4
Figure 5-4: LCF Results for sample CC2
Using the maximum values of each cycle for each sample we can compare the material in a tension-strain graph.
Figure 5-5: Comparison of stress-strain results from tensile stress tests and LCF
5.3. HCF
In Figure 5-6 the results for HCF test are displayed in a diagram. On Table 5-1 the HCF used in the elaboration of the diagram are displayed.
Figure 5-6: HCF results in S-N diagram
Type of loading
Von Mises equivalente
stress [ ]
No. of cycles
Case I
444.81 41652
420.34 91250
409.22 115869
378.09 1000000
Case II
499.93 45213
483.17 88654
461.88 345221
456.49 834527
454.49 1000000
Case III
847.73 36486
815.39 48324
722.84 126583
657.13 1000000
Case III (With notch)
718.27 49476
663.15 74562
648.25 121643
583.59 211024
Table 5-1: Table of HCF results
5.4. Fractography
In the following subsections the angle of the cracks that arose from the HCF tests, and surfaces and the progression of the
crack of some samples will be displayed. On the final subsection four tables are provided for the reader to have a better understanding of all the angles of all the samples.
5.4.1 Case I
Specimen 2
Figure 5-7: Location and angle of initiation crack
a) b)
Figure 5-8: a) Location of crack relative to surface; b) Progression of crack over
surface
5.4.2 Case II
Figure 5-9: Location and angle of initiation crack
a) b)
Figure 5-10: a) Location of crack relative to surface; b) Progression of crack over
surface
5.4.3 Case III
Main Crack Secondary Crack
Cra
ck
No.
1
a) b)
Cra
ck N
o.2
c) d)
Figure 5-11:Location and angles of a) Main crack No.1;b) Secondary crack
No.1;c) Main crack No.2;d) Secondary crack No.2
a) b)
Figure 5-12: a) Location of crack relative to surface; b) Progression of crack over
surface
5.4.4 Case III with notch
Main Crack Secondary Crack
Cra
ck N
o.1
a) b)
Cra
ck N
o.2
c) d)
Figure 5-13 Location and angles of a) Main crack No.1;b) Secondary crack
No.1;c) Main crack No.2;d) Secondary crack No.2
a) b)
Figure 5-14: a) Location of crack relative to surface; b) Progression of crack over
surface
5.4.5 Table of results
5.4.5.1 Case I
Sample Crack 1 Crack 2
2 22.5º
3 26.5º 27.8º
Figure 5-15: Crack angles for case I
5.4.5.2 Case II
Sample Crack 1 Crack 2
2 0º
4 0º 0º
5 0º
Figure 5-16: Crack angles for case II
5.4.5.3 Case III
Sample Crack 1 Crack 2
Main Crack Secondary Crack Main Crack Secondary Crack
1 21º 57.2º 22.5º 39.6º
2 23.7º 45º - -
3 41.5º 78.6º 34.7º 57.9º
Figure 5-17: Crack angles for case III
5.4.5.4 Case III with notch
Sample Crack 1 Crack 2
Main Crack Secondary Crack Main Crack Secondary Crack
1 44.7º 36.5º 44.6º 26.0º
2 34.5º 45.7º - -
3 11.8º 33.9º 31.0º 12.1º
4 34.2º 45º - -
Figure 5-18: Crack angles for case III with notch
6. Conclusions
From the tensile stress tests we can conclude that the wear of the rail induces changes in the mechanical properties of the material, and that the mechanical properties of wheel and rail are different, maybe due to relative wear of the components
From the LCF tests we can observe a softening of the material for low strains and tolerance for hardening for higher strains.
On the HCF tests the case I loading is the most severe , case III is the one that endures higher stresses for a given fatigue life, and case III with notch influences significantly life and tolerance to damage when compared to case III without notch.
On the fractography evaluation we see that case I promotes cracks that develop with 25º, and case II with 0º. Case III with or without notch has the propensity to develop multiple cracks with two directions each.
References
[1] ASTM, E 8-04 Standard Test Methods for Tension Testing of Metallic Materials.
[2] ASTM, E606-92 Standard Practice for Strain-Controlled Fatigue Testing.
[3] S. L. Grassie, “Rolling contact fatigue on the British railway system: treatment”.
[4] M. M. Z. M. F. R. K. Dang Van, “A comprehensive approach for modeling fatigue and fracture of rails”.
