Metal Fatigue: Facts and Lessons from Fatigue Tests

Transcription

1 Metal Fatigue: Facts and Lessons from Fatigue Tests September 26th October 31st 2016 Aalto University, Espoo, Finland given by Emeritus Prof. Yukitaka Murakami Kyushu University, Japan 1

2 Members of teaching assistants Mari AMAN Aalto Univ. Kentaro WADA Kyushu University 和田健太郎 Yuzo TANAKA KMTL (Kobe Material Testing Laboratory, Co. Ltd.) 田中裕三 2

3 Why Facts and Lessons from Fatigue Tests? 3

4 Contents 1. Orientation 2. Introduction 3. Fatigue test: Experience, Observation, Analysis and Discussion 4. Varieties of Fatigue Fracture Origins Don t be lost in the forest of fatigue. 5. What is fatigue damage? 6. Stress concentration 7. Stress intensity factor 8. Crack propagation Law and reality 9. Crack closure Nonpropagating crack and fatigue threshold 10. Long crack and short crack 11. Notch effect and Size effect 12. Scatter of fatigue data and fatigue design 13. More extensive topics which are not treated in the lecture 14. Presentations by the teams and discussion - Summary toward your future work 4

5 Summary and Conclusion Metal Fatigue is Mechanics of Crack Don t be lost in the forest of fatigue! 5

6 Let s experience fatigue test and observe what happens! 6

7 Grouping: Names of each team Team A: d = 50mm Adeyinka Abass, Vu Le, Ivan Yashchuk, Kentaro Wada Team B: d = 100mm Kristoffer Brink, Lehtimaki Eero, Andrew Roiko, Ville Heikkila Team C: d = 300mm Jasmin Jelovica, Oskari Ryti, Gowtham Radhakrishnan, Juho Jalava Team D: d = 500mm (0.5mm) Risto Juujärvi, Teemu Lahtinen, Tommi Seppänen Team E: d = 1000mm (1mm) Suvi Papula, Roman Mouginot, Joonas Kähkönen, Antti Forsström Other participants: Prof. Michel Stoschka, Visting Prof. Martin Leitner, Post doctoral researcher Pasquale Gallo 7

8 The first fatigue test by Team D Team D: d = 500mm (0.5mm) Risto Juujärvi, Teemu Lahtinen, Tommi Seppänen Applied stress: 350MPa Test frequency : f = ~33, 2000rpm N f =? 8

9 2. Introduction Facts: More important than studying models and simulations 1. Chemical composition and Mechanical properties Sample material: A typical low carbon structural steel 2. Fatigue test Tension-Compression, Rotating bending, Plate bending, Reversed torsion, Mixed mode fatigue test Fatigue limit: Definition and the relationship between mechanical properties. HV and Tensile strength 2. Crack initiation: Examples. Difficult to define. Variety of crack initiation sites 3. Crack propagation : Examples 4. Objective: Study fatigue process and Extension of the acquired knowledge to 9

10 Importance of valid experimental data for the approval of models (Y. Murakami M. Endo, Int. Fatigue, 1994, Vol.16, p.176) Care must be exercised when the value of fatigue limit of a plain (smooth) specimen s w0 is employed as one of the basic data of a model, because there is a possibility that s w0 itself may include the influence of small defects. The data for tension-compression fatigue of high strength steels must also be carefully checked before use, because misalignment of specimens in fatigue test and bending of specimen shape due to heat treatment always tend to lower the fatigue strength. Unusually low fatigue strength of data 105 must therefore be questioned. Company engineers do not usually have the fatigue data requested in the models proposed by many researchers. Therefore, models that do not need fatigue tests will be welcome by them, or suggestions for obtaining data equivalent to fatigue data should be presented. They are naturally very careful to use models that do not have a declared prediction error. Be careful to models! 10

11 3. Fatigue Test and Replica Method Observation by Teams Material: A structural carbon steel. Tensile strength s ult = 436MPa?, HV = 132? Grouping: Team A: Specimen contains an initial artificial defect of a drilled hole with 50mm diameter. Applied stress s a = X MPa. Team B: Specimen contains an initial artificial defect of a drilled hole with 100mm diameter. Applied stress s a = X MPa. Team C: Specimen contains an initial artificial defect of a drilled hole with 300mm diameter. Applied stress s a = X Mpa Team D: Specimen contains an initial artificial defect of a drilled hole with 500mm diameter. Applied stress s a = X Mpa Team E: Specimen contains an initial artificial defect of a drilled hole with 1000mm diameter. Applied stress s a = X MPa. Problem 1: Exchange the data and predict the fatigue life for other applied stress levels. Problem 2: Predict the fatigue limit for the specimen of each team. 11

12 Grouping: Names of each team Team A: d = 50mm Adeyinka Abass, Vu Le, Ivan Yashchuk, Kentaro Wada Team B: d = 100mm Kristoffer Brink, Lehtimaki Eero, Andrew Roiko, Ville Heikkila Team C: d = 300mm Jasmin Jelovica, Oskari Ryti, Gowtham Radhakrishnan, Juho Jalava Team D: d = 500mm (0.5mm) Risto Juujärvi, Teemu Lahtinen, Tommi Seppänen, Team E: d = 1000mm (1mm) Suvi Papula, Roman Mouginot, Joonas Kähkönen, Antti Forsström Other participants: Prof. Michel Stoschka, Visting Prof. Martin Leitner, Post doctoral researcher Pasquale Gallo 12

13 Specimen for bending fatigue 120 φd h 4-Φ4.5 R10 Cross section of drilled hole Drilled hole φ (Dimension unit: mm) Thickness: 2mm t2 10 Nominal size of drilled hole Diameter d (mm) Depth h (mm)

14 Specimen size Depth h 120 φ d Cross section of drilled hole Drilled hole W =10mm, t = 2mm Nominal diameter and depth of drilled hole Specimen ID h (mm) W t Team A B C D E Diameter of Specimen Width Thickness drilled hole ID W (mm) t (mm) d (mm)

15 Predict the fatigue limit s FL of the specimen of your team before fatigue test and after every lecture! Prediction with accuracy ±40MPa is very good. Prediction with accuracy ±20MPa is excellent. We denote today s prediction by s FL1. You may modify your prediction every day. We denote it by s FLi. Decide the stress amplitude for your next fatigue test. 15

16 Time necessary for fatigue test and lecture schedule Speed of the testing machine: For standard use, f =~20Hz. At highest <30Hz. f = 20Hz Cycles /h = 7.2x10 4 Cycles/day = 1.7x10 6 f = 30Hz Cycles /h = 1.08x10 5 Cycles/day = 2.6x10 6 Necessary time for N = 10 7 is 92.6hrs = 3.86days= ~4days Lecture days (10days): September 26, 28 October 3, 5, 10, 17, 19, 24, 26, 28 Examination: 31 October 16

17 Material: A typical low carbon steel, JIS SS400 Yield stress σ y [MPa] Chemical composition(mass%) C Si Mn P S Fe Bal. Mechanical properties Ultimate tensile strength σ Ult [MPa] Elongation δ [%] Vickers hardness at core HV Microstructure 100 μm 25 μm

18 Vickers hardness variation from surface to core The surface of specimens was polished by 2mm after plane grinding. HV measured by load P = 100g Locations of measurement Mean values of 5points Surface polish ~2μm : 215HV (210, 216, 220, 212, 215) ~5μm : 213HV (208, 210, 220, 216, 212) ~20μm: 169HV (167, 167, 173, 172, 166) Hardness at core : 133HV (126, 152, 128, 137, 123) (HV = 133 measured by load P = 10kg) 18

19 Fatigue Data: S-N Data s s 1 Stress s 2 Fatigue limit N f1 N f2 S-N Curve N f Number of Cycles to Failure 19

20 100 μm φ0.5-0: N=0 20

21 Vickers hardness test

22 The principle of the test Indenter Indentation F α=136 Test force square-based pyramidal diamond indenter d 1 d 2 Vickers hardness tester Specimen Indentation Make a indentation to the test surface by the indenter. Vickers hardness number (HV) is based on the indentation test force divided by the surface area of the indentation.

23 Calculating formula d HV = Test force : F (kgf) Surface Area of the indentation (mm 2 ) = F (N) d 2 2sin- α 2 = F (N) d 2

24 Examples HV and Diagonal length Diagonal length of the Indentation d (mm) Test Force F (N) Vickers hardness HV0.1 HV0.3 HV10 HV A rough indication Material Al alloy S45C: normalizing (Carbon steels for machine structual use) SCM425: heat-treated (Low-alloyed steels for machine structual use) SK85: quench and temper (Carbon tool steels) HV 40~ ~ ~350 > HV30 417HV10 406HV HV0.1 Even in the same hardness, indentation size is different in the test force Ex. 417HV10 417: hardness number HV: Vickers hardness scale 10: Used test force in kgf

25 More precise information on HV Locations of measurement and Average of 5 points HV below the surface (Polished by ~5mm):Load 100g 213HV (208, 210, 220, 216, 212) HV below the surface (Polished by ~20mm):Load 100g 169HV (167, 167, 173, 172, 166) HV at the core part of the specimen: Load 10kg 133HV (126, 152, 128, 137, 123) 25

26 10 mm Depth from the surface 5 mm HV = 213 Depth from the surface 20 mm HV = 169 Depth from the surface 1 mm HV = mm

27 2 mm Depth from the surface 5 mm HV = 213 Depth from the surface 20 mm HV = 169 Depth from the surface 1 mm HV = 133

28 Note related to Standards Position of indentation Standard JIS Z 2244 ISO Distance between indentations >3d for steel and copper alloys and >6d for light metals Distance from the center of the indentation to the edge of the specimen >2.5d for steel and copper alloys and > 3d for light metals ASTM E384 >2.5d >2.5d >2d Specimen thickness >1.5d

29 Replica Method Replica Method is a nondestructive sampling method that records and preserves the topography of material surface. Practical procedure Equipment Apply and Dried Transfer printing Observation Measurement Replica film mirror-reversed Specimen (wet surface with acetone) carbide CracK CracK carbide 29

30 Equipment Replica film (Cellulose acetate film, 35μm) Acetone (CH 3 COCH 3 ) Microscope and Ocular micrometer Cut to the appropriate size for specimen. Double-sided tape Glass slide Tweezers Black marker pen 30

31 Apply, Dry and Transfer printing Specimen Replica film Acetone Specimen Replica film Drop acetone on to the specimen surface. Cover with the replica film immediately. Wait 3 minutes to dry the film. Take off the film. 31

32 Observation Reason for coating with black Replica film lens lens 32 Back face light light Transfer face Replica film Transfer face Back face Double-sided tape Glass slide Coating with black ink on the back face of the replica. With transfer face up, paste to the double-sided tape while stretching it. Cut of unnecessary parts Observation

33 Examples of Replica Method Metallographic structure Creep void (SEM) SEM observation is possible after coating the replica surface with metal such as Au. This is a conventional technique for microscopic observation of microstructure. 33