Failure and controls of metals

Maurizio Vedani 2019-2020

FAILURE AND CONTROL OF METALS

Chi l’ha dura la vince! (by Fra) 1

Sommario

1. Principles of Linear Elastic Failure Mechanism ......................................................................................... 6

Notch effect (stress concentration factor) .................................................................................................... 6

Griffith theory ................................................................................................................................................ 8

Stress intensity factor .................................................................................................................................... 9

Plasticity correction ................................................................................................................................... 9

Evaluation of fracture toughness (K ) in plane strain conditions ........................................................... 10

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2. Fracture mechanism ................................................................................................................................ 12

Microstructural aspects of failure ............................................................................................................... 12

Ductile mechanisms ................................................................................................................................. 12

Brittle mechanisms .................................................................................................................................. 14

Toughening mechanisms in metals ............................................................................................................. 15

Toughening of a ductile material ............................................................................................................. 15

Toughening of a brittle material .............................................................................................................. 15

3. Fracture models ....................................................................................................................................... 17

Fracture mechanism maps .......................................................................................................................... 17

Fracture mechanism in brittle materials: the cleavage fracture ................................................................. 17

Statistical issues related to defects in brittle materials .......................................................................... 18

Fracture mechanism in materials undergoing brittle-to-ductile transition ................................................ 18

Rice & Thomson model............................................................................................................................ 19

Smith model............................................................................................................................................. 20

Fracture models for ductile materials: microvoid development................................................................. 20

Nucleation of voids .................................................................................................................................. 20

Void growth process ................................................................................................................................ 20

4. Features about grain boundaries and atom segregation ........................................................................ 21

Grain boundary cohesion ............................................................................................................................ 21

Theory for the migration of atoms at grain boundaries .............................................................................. 22

Temper embrittlement ................................................................................................................................ 23

5. Hydrogen embrittlement ......................................................................................................................... 24

Mechanisms of hydrogen embrittlement ................................................................................................... 25

Hydrogen-induced blistering ................................................................................................................... 26

Hydrogen-induced cracking ..................................................................................................................... 26

Hydrogen attack ...................................................................................................................................... 26

Cracking from hydride formation ............................................................................................................ 27

Delayed fracture ...................................................................................................................................... 27

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6. Liquid metal embrittlement .................................................................................................................... 28

LME in reducing cohesive strength (may be Brittle) ................................................................................... 29

LME in reducing fracture elongation (ductile metals) ................................................................................. 29

LME – phenomenological aspects ............................................................................................................... 30

Examples of LME.......................................................................................................................................... 30

7. Fatigue failure .......................................................................................................................................... 31

Safe-life approach .................................................................................................................................... 32

Focus on damage tolerant approach ........................................................................................................... 32

Fatigue crack nucleation and growth mechanisms ................................................................................. 33

Fatigue crack growth rates ...................................................................................................................... 34

Propagation of short cracks ..................................................................................................................... 35

Thermal fatigue ........................................................................................................................................... 35

TMF – practical examples: cylinder heads ............................................................................................... 36

TMF – practical examples: cracks in tools ............................................................................................... 36

8. Creep failure ............................................................................................................................................ 37

The creep curves.......................................................................................................................................... 37

Constitutive equation .................................................................................................................................. 38

Microstructural aspects of creep................................................................................................................. 38

Deformation mechanisms associated to dislocation motion .................................................................. 39

Deformation mechanisms associated to diffusive flow .......................................................................... 39

Creep failure ................................................................................................................................................ 39

4. Wedge cracking ................................................................................................................................... 40

5. Cavitation ............................................................................................................................................. 40

9. Residual stresses ...................................................................................................................................... 41

Classification ................................................................................................................................................ 41

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Type macroscopic residual stress ........................................................................................................ 41

:

Type microscopic residual stress ....................................................................................................... 42

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Type atomic residual stress .............................................................................................................. 42

Residual stresses induced by metalworking operations ............................................................................. 43

Effect of residual stress on static mechanical behaviour ............................................................................ 44

Effect of residual stress on Fatigue behaviour ............................................................................................ 44

10. Distorsions ........................................................................................................................................... 44

Irreversible distortions ................................................................................................................................ 44

Irreversible size distortions ..................................................................................................................... 45

Irreversible shape distortions .................................................................................................................. 45

A practical example ..................................................................................................................................... 46

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11. Residual stresses measurements ........................................................................................................ 47

Mechanical methods ................................................................................................................................... 47

The Almen test......................................................................................................................................... 47

Hole drilling strain gauge method ........................................................................................................... 48

X-ray diffraction methods ............................................................................................................................ 49

Assessment of residual stress field from XRD spectra ............................................................................ 50

Diffraction sources other than standard X-rays ...................................................................................... 52

Indirect methods ......................................................................................................................................... 52

12. Solidification defect ............................................................................................................................. 54

Solidification in ingots ................................................................................................................................. 54

Solidification in continuous casting ............................................................................................................. 55

Defects in foundry castings ......................................................................................................................... 55

Surface defects in castings ...................................................................................................................... 56

Bulk defects in castings ............................................................................................................................ 57

Welding defects ........................................................................................................................................... 59

13. Plastic deformation defect .................................................................................................................. 61

Hot working ................................................................................................................................................. 61

Failure mechanism .................................................................................................................................. 61

Hot- vs. cold deformation ............................................................................................................................ 62

Hot brittleness in ferrous alloys .................................................................................................................. 63

Secondary elements on grain size control................................................................................................... 65

Effect of fibrous structure and texture ........................................................................................................ 65

Cold-forming defects ................................................................................................................................... 67

14. Failure analysis .................................................................................................................................... 68

The general approach to failure analysis..................................................................................................... 68

Macroscopic analysis ............................................................................................................................... 68

Microscopic analysis ................................................................................................................................ 68

Extraction and handling of samples for failure analysis .............................................................................. 68

Non-Destructive testing .................................................................................................................................. 69

15 Visual inspection ..................................................................................................................................... 69

16 Liquid penetrant method ....................................................................................................................... 70

Liquid penetrant ...................................................................................................................................... 71

Developer ................................................................................................................................................ 71

Advantages and limitations ..................................................................................................................... 71

17 X-Ray inspection & tomography NDT ..................................................................................................... 72

X-ray generation and tubes ..................................................................................................................... 72

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Gamma rays - Radioactive decay ............................................................................................................. 73

X-ray NDT ................................................................................................................................................. 73

Features of radiographic images ............................................................................................................. 74

Advantages and limitations ..................................................................................................................... 74

Tomography ............................................................................................................................................. 75

18 Magnetic particle NDT ............................................................................................................................ 76

Testing procedure .................................................................................................................................... 76

Testing requirements: ............................................................................................................................. 76

Magnetization of the workpiece ............................................................................................................. 77

Examination methods and final treatment ............................................................................................. 78

Magnetic powders ................................................................................................................................... 78

Advantages and limitations ..................................................................................................................... 78

19 Eddy current NDT ................................................................................................................................... 79

Detection of defects and measurements techniques ............................................................................. 80

Advantages and limitations ..................................................................................................................... 80

20 Ultrasound NDT ...................................................................................................................................... 81

Ultrasounds waves .................................................................................................................................. 81

Production and use of US ........................................................................................................................ 82

Probes: Piezoelectric transducers ........................................................................................................... 82

US wavelength vs. flaw detection ........................................................................................................... 83

Applications of US for NDT ...................................................................................................................... 84

US testing methods ...................................................................................