ASSESSMENT OF CRACK INITIATION IN PRESS FITS OF RAILWAY …esistc24.mecc.polimi.it/Berlin/ESIS_TC24_Berlin_2010_varfolomeev.pdf · Fatigue Strength and Fatigue Life of Railway Axles

© Fraunhofer-Institut für Werkstoffmechanik IWM

I. Varfolomeev, M. LukeFraunhofer Institute for Mechanics of Materials IWM

Freiburg, Germany

ESIS TC24 Workshop

Fatigue Strength and Fatigue Life of Railway Axles

October 11-12, 2010, BAM Berlin

ASSESSMENT OF CRACK INITIATION IN PRESS FITS OF RAILWAY AXLES


2

Outline

Introduction, motivation

Material EA4T

Fatigue tests

Fretting fatigue tests

Assessment of crack initiation in press fit

Conclusions


3

Motivation

Press fits are of special concern in axle design and inspection practice (crack initiation and potential propagation)

Design rules for axles require a reduction of the maximum net stress amplitude for press fits, as compared to axle free surface

Stress reduction factor is mainly determined empirically based on full scale tests and depends on the axle design, e.g. solid vs. hollow axle, material, transition geometry (D/d)

Such an approach does not directly employ the knowledge of material S-N curves, so that comprehensive investigations have to be performed for individual axle design and material combinations


4

Acceptable stresses – after EN 13104

Nominal stress 240 MPa

Nominal stress 132…150 MPa


5

Current practice

Definition of the allowable net stress based on full scale tests

Representative number of tests is required for statistical data analysis

Time, costs

Transferability to other materials and geometries?

Reference: Traupe et al., Safe and Economic Design of Running Gears, IMAB TU Clausthal, 2004


6

Goal and scope of the study

Correlation between standard fatigue S-N curves and fretting fatigue data

Plane fatigue and fretting fatigue tests

Assessment of crack initiation in press fit

Fracture mechanics analysis

Experimental setup and methodology similar to e.g. Lykins, Mall et al. (2000-2004)


7

Material tensile properties (EA4T)

0

100

200

300

400

500

600

700

800

0,00 0,05 0,10 0,15 0,20 0,25 0,30

εtechnisch

σ tec

hnis

ch [

MPa

]

PA1-Z1PA1-Z3PA1-Z4PA1-Z6PA1-Z8PA1-Z9PA1-Z11PA1-Z13PA1-Z14

außen

Mitte

Kern

Strain [mm/mm]

Stre

ss [M

Pa]

outer surface


8

Fatigue tests

Rotary bending tests of cylindrical specimens, D = 10 mm

R = -1


9

Fatigue data (S-N curves)

LCF and HCF test data, R = -1

Estimated endurance limit σD < 375 MPa

150

250

350

450

550

1E+4 1E+5 1E+6 1E+7

N, cycles

σ a, M

Pa

107106105104

median curve

run-outs


10

Standard material characterization

Present results and reference data

Traupe et al. (2004)

Cherny et al. (2008)

150

250

350

450

550

1E+4 1E+5 1E+6 1E+7

N, cycles

σ a, M

Pa

107106105104

median curve

σD: Ø9 mm

σD: Ø170

size effect


11

Data scatter and size effect

150

200

250

300

350

400

450

500

550

1E+4 1E+5 1E+6 1E+7 1E+8N, Zyklen

108106105104 107

σ a, M

Pa standard specimens, e.g. D = 10 mm


12

Data scatter and size effect

150

200

250

300

350

400

450

500

550

1E+4 1E+5 1E+6 1E+7 1E+8N, cycles

108106105104 107

σ a, M

Pa standard specimens, e.g. D = 10 mm

large specimens (components)

small specimens

Endurance limit vs. specimen size:Ref. Cherny, Workshop on Damage Tolerance of Railway Axles13th–14th of October 2008, Milano


13

Fretting fatigue tests

Test setup developed and adopted for resonant testing machine of type TESTRONIC (Russenberger Prüfmaschinen AG)Contact force ≤ 4 kNNet stress ratio R = 0.1


14

Specimen for fretting fatigue tests

Rectangular cross section 6×3 mm² or 10×3 mm²

Contact pads


15

Fretting fatigue tests: summary


16

Fretting fatigue tests: summary

5 specimens → 9 tests → 5 cracked surfacescrack length > 0,5 mm → 2 tests crack length of some 40 μm → 3 testsno cracks → 4 tests


17

Fretting fatigue tests: FE analysis

EP aSWT εσmax=

Element length ≥ 10 μm

Fatigue damage parameter according to Smith-Watson-Topper:


18

Plane fatigue vs. fretting fatigue results

Comparison in terms of the net stress

150

250

350

450

550

1E+4 1E+5 1E+6 1E+7

N, cycles

σ a, M

Pa

plane fatigue

fretting fatigue, crack initiation

fretting fatigue, no crack initiation

107106105104


19

Plane fatigue vs. fretting fatigue results

Comparison in terms of the SWT parameter

150

250

350

450

550

1E+4 1E+5 1E+6 1E+7N, LW

σ a o

r P S

WT,

MPa

plane fatigue, d = 10 mm



median curve, fretting fatigue tests

107106105104

Test Nr. 10

Test No. 3

Size effect: about 30-fold difference in the size of the high-stressed area in plane vs. fretting fatigue specimens increase of fatigue endurance

Shift of the median curve ×5

×2 scatter band of fretting fatigue data


20

Full scale tests for press fits

Traupe et al., Safe and Economic Design of Running Gears, IMAB TU Clausthal, 2004

Reference: Traupe et al. (2004)

Overall 16 tests on full scale axles, D/d = 1.08

Cracks in press fits in 5 cases: net stress 190 to 200 MPa, 2.5×106

to 6.4×106 load cycles


21

Evaluation of full scale tests

Finite-element modelling

Both elastic and elastic-plastic analyses

Variation of the coefficient of friction


22

Evaluation of full scale tests


23

Summary of fatigue tests

150

250

350

450

550

1E+4 1E+5 1E+6 1E+7N, LW

σ a o

r P S

WT,

MPa





full-scale press fit test, SWT

107106105104

Endurance limit according to DIN EN 13103/13104


24

Summary of fatigue tests

150

250

350

450

550

1E+4 1E+5 1E+6 1E+7N, LW

σ a o

r P S

WT,

MPa





full-scale press fit test, SWT

full-scale press fit test, net stress

107106105104

Endurance limit according to DIN EN 13103/13104

net stress approach


25

Conclusions

Results demonstrate a correlation between plane fatigue and fretting fatigue data (account for size effect!)

Lifetime of press fits can be predicted making use of the knowledge of material S-N curves and stress state in the component

Successful prediction for 5 out of 9 small specimens + full scale specimen


26

Thank you for your attention

Documents

ASSESSMENT OF CRACK INITIATION IN PRESS FITS OF RAILWAY …esistc24.mecc.polimi.it/Berlin/ESIS_TC24_Berlin_2010_varfolomeev.pdf · Fatigue Strength and Fatigue Life of Railway Axles