Non-destructive Evaluation NDE

1. Non-destructive EvaluationNDE Dept. of Physics and Materials Science City University of Hong Kong References: H.E. Davis, G.E. Troxell, in chapter 16 of “The Testing of Engineering Materials”, 1982. J.S. Ceurter et al., “Advanced Materials Processes” (April 2002), p.29-31. T. Adams, “Advanced Materials Processes” (April 2002), p.32-34. Dr. Jonathan C.Y. Chung: NDE

2. Various Purposes • Locate defects (Why ?) • Determine dimension, physical, or mechanical characteristics • Determine Residue Stress (XRD) Dr. Jonathan C.Y. Chung: NDE

3. Advantage of Knowing the defects • Defects are usually stress raiser • Stress raiser can cause pre-mature failureOver design to overcome pre-mature failureBulky/heavy design • Catastrophic/sudden/unpredicted failureloss of lives and money • Quality control • Better design Dr. Jonathan C.Y. Chung: NDE

4. Better design (example) Consider a rectangular bar 10mm x 5 mm which will be used to support some load. The steel chosen had yield strength, tensile strength and fracture toughness being 600MPa, 900MPa and 40MPam. If the corresponding design safety factors are 1.2, 1.6 and 1.5 respectively. What is the allowable load? • Yielding failure (>25 kN) • Tensile fracture (>28.1 kN) • Fracture toughness (crack size dependant)2 mm: 16.8kN; 1mm: 23.6kN; 0.1mm: 75.2kN Dr. Jonathan C.Y. Chung: NDE

5. Yield strength (plastic deformation) area = 10 mm x 5 mm = 50 x 10-6 m2 max. load = (yield strength x area)  safety factor = (600MPa x 50 x 10-6 m2) 1.2 = 25 kN (plastic deformation at load > 25 kN) Dr. Jonathan C.Y. Chung: NDE

6. Tensile strength (Catastrophic failure) area = 10 mm x 5 mm = 50 x 10-6 m2 max. load = (tensile strength x area)  safety factor = (900MPa x 50 x 10-6 m2) 1.6 = 28.1 kN (tensile fracture at load > 28.1 kN) Dr. Jonathan C.Y. Chung: NDE

7. Fracture Toughness (require information of crack length) KIC =   (a) Assume geometric correction factor,  = 1 max = KIC /(a) Max load =  x A  (safety factor) = KIC /(a) x A  (safety factor) = 40MPam /(3.1416 x a) x 50 x 10-6 m2  (safety factor) When a = 2 mm, max load = (2000  0.07927)/1.5 = 16.8 kN When a = 1 mm, max load = (2000  0.05605)/1.5 = 23.6 kN When a = 0.1 mm, max load = (2000  0.01772)/1.5 = 75.2 kN Dr. Jonathan C.Y. Chung: NDE

8. NDE methods for location of defects Surface defects detection • Visual inspection • Liquid penetrant test • Magnetic particle method Internal defects detection • Magnetic particle method • Radiographic methods • Electromagnetic methods • Eddy current method • Barkhausen Noise Inspection Principle Material defects (grinding damage, re-tempering burn, Re-hardening burn, residue stresses • Acoustic methods Dr. Jonathan C.Y. Chung: NDE

9. Visual inspection • It should never be omitted. • Use low-power magnifying glass or microscopes (remember to take permanent photographic record) • Surface roughness: • Touch inspection using finger along the surface (2-3 cm/s.) • Light reflection method • No-parallex method • Penetrant test Dr. Jonathan C.Y. Chung: NDE

10. Penetrant test • Suitable for locating surface discontinuities, such as cracks, seams, laps, laminations in non-porous materials. • Applicable to in-process, final, and maintenance inspection. • ASTM E 165 General procedure: • Thoroughly clean the surface • Apply penetrant on the surface • Liquid penetrant enter small openings by capillary action • Remove liquid completely and apply developer (dry or wet) • The penetant bleed out onto the surface showing the location of the surface defect Dr. Jonathan C.Y. Chung: NDE

11. Enhancing the penetrant test • Strike the part to force the liquid out of the defect • Fluorescent-penetantdepth of surface defects may be correlated with the richness of color and speed of bleed out • Filtered-particle inspection:-This method depends on the unequal absorption into a porous surface of a liquid containing fine particles in suspension.-Preferential absorption causes the fine particles in the solution to be filtered out and concentrated directly over the crack, producing a visual indication. • Cracks on Non-conducting materials:-A cloud of fine electrically charged particle is blown over the surface, causing a buildup of powder at the defect. Dr. Jonathan C.Y. Chung: NDE

12. Magnetic Particle Test • Use to locate the defects at or near the surface of ferromagnetic objects. • The magnetic particles tends to pile up and bridge over discontinuities. • A surface crack is indicated by a line of the fine particle following the outline of the crack. • A subsurface defect by a fuzzy collection of the fine particles on the surface near the defect. • Fatigue crack in an airplane gear. • Orientation of cracks • Some cracks are more difficult to detect. • DC current is often employed, since it permit deeper defects detection. Dr. Jonathan C.Y. Chung: NDE

13. Permanent magnets with soft iron keepers Dr. Jonathan C.Y. Chung: NDE

14. Fixture for yoke induction of longitudinal magnetic field Dr. Jonathan C.Y. Chung: NDE

15. Leakage Flux Dr. Jonathan C.Y. Chung: NDE

16. Fatigue cracks in airplane gear detected by the magnetic-particle method Dr. Jonathan C.Y. Chung: NDE

17. Orientation of magnetic fields Dr. Jonathan C.Y. Chung: NDE

18. Some cracks are more difficult to detect Dr. Jonathan C.Y. Chung: NDE

19. Threshold indications of near-surface cavities Dr. Jonathan C.Y. Chung: NDE

20. Radiographic methods • X-rays method (Exograph) • Gamma rays (Gammagraph) • Neutron • Infra-red (FT-IR) imaging Dr. Jonathan C.Y. Chung: NDE

21. X-ray method (ASTM E 94) • High energy photon (short wavelength, high frequency) can penetrate materials better • Formation of the radiograph • X-ray source • Arrangement for radio graphing a welded joint • Xeroradiography: static electricity, fine powders, specially coated Al plate, image available in seconds • On-line Soft X-ray scanning: low energy X-ray • Influence of size of source and sharpness of image • Interpretation of the radiograph: (e.g. Radiograph of a 20 mm weld) • Quality of image • Safety(Biology effect) Dr. Jonathan C.Y. Chung: NDE

22. Formation of a radiograph Dr. Jonathan C.Y. Chung: NDE

23. X-ray source • X-ray method (seconds/minutes) is faster than gamma-ray method (hours) • The quality of the image depends on the stability of the high voltage electron tube and the penetration power of the x-ray. • Industrial units (40-400kV) • High resolution system (30-150kV) • High energy system (>400kV) Dr. Jonathan C.Y. Chung: NDE

24. Radio graphing a welding joint Dr. Jonathan C.Y. Chung: NDE

25. Interpretation of Radiographs • Contrast due to difference in thickness, density, composition. • Gas cavities and blowholes are indicated by well defined circular dark areas. • Shrinkage porosity appears as fibrous irregular dark region having an indistinct outline. • Cracks appear as darkened areas of variable width. • Sand inclusions are represented by gray or black spots of an uneven or granular texture with indistinct boundaries. • Inclusions in steel castings appear as dark areas of definite outline. In light alloys the inclusion may be more dense than the base metal and thus cause light areas. Dr. Jonathan C.Y. Chung: NDE

26. Influence of size of source on sharpness of image Dr. Jonathan C.Y. Chung: NDE

27. Radiograph of a 20 mm weld Dr. Jonathan C.Y. Chung: NDE

28. Quality of image • The absorption increase rapidly with the thickness exponentially • The longer the wavelength, the greater the absorption. • Penetrameter: a calibration device helps in determining the smallest detectable defect Dr. Jonathan C.Y. Chung: NDE

29. Radiation Monitoring and Safety • Observe the rules, regulation and monitoring measures set by the local and international nuclear and radiation monitoring bodies. • Be EXTREMELY careful, don’t perform this in a rush. • Once the operation manual have been set, the engineers and technicians must follow it STRICTLY. • Don’t make arbitrary compromise. • Get advices from the licensed radiographers. • Select appropriate personal monitoring devices. Dr. Jonathan C.Y. Chung: NDE

30. Biological Effects • Relaxation lengths of various shielding materials. • Estimated radiation does to U.S. population • Acute doses of penetration radiation. Dr. Jonathan C.Y. Chung: NDE

31. Relaxation lengths of various shielding materials Dr. Jonathan C.Y. Chung: NDE

32. Estimated radiation does to U.S. population Dr. Jonathan C.Y. Chung: NDE

33. Acute doses of penetration radiation. Dr. Jonathan C.Y. Chung: NDE

34. a b Neutron Radiography • Brass bullet with gunpowder • Steel airbag inflator with packets of fast-burn pyrotechnic • 38 mm long turbine blade • Turbine blade with flaw c d Dr. Jonathan C.Y. Chung: NDE

35. FT-IR imaging Inclusion in polypropylene film IR spectra showing impurities (1) ester and (2) amide. Red: amide Red: ester Dr. Jonathan C.Y. Chung: NDE

36. Perkin-Elmer FT-IR imaging system Dr. Jonathan C.Y. Chung: NDE

37. FT-IR imaging An image a fly’s wing Dr. Jonathan C.Y. Chung: NDE

38. Fingerprint image Dr. Jonathan C.Y. Chung: NDE

39. PCB sample Dr. Jonathan C.Y. Chung: NDE

40. Electromagnetic methods • Magnetic measurement is sensitive to chemical composition, structure, internal strains, temperature and dimensions. • Limitations: • Magnetic properties cannot be simply related to the mechanical properties • Sensitive to internal strains and temperature. This is more significant when high frequencies or low magnetizing forces are employed. Dr. Jonathan C.Y. Chung: NDE

41. Encircling Coils If the test coil moved over a crack or defect in a metal plate, at a constant clearance speed, a momentary change will occur in coil reactance and coil current. Dr. Jonathan C.Y. Chung: NDE

42. Effect of similar inner and outer defects on flux pattern and measurement Dr. Jonathan C.Y. Chung: NDE

43. Barkhausen Noise Inspection Dr. Jonathan C.Y. Chung: NDE

44. Barkhausen Noise (Principle) • Magnetizing field causes the materials undergo a magnetization change in ferromagnetic material • This change is a result of the microscopic motions of magnetic domain walls within the metal. • Domain wall movement emit electrical pulse that can be detected by a coil of conducting wire. • These discrete pulses are measured in a bulk manner, resulting in a compilation of thousands of electrical pulses referred to as Barkhausen noise. • The amplitude of this signal magneto-elastic parameter (MP). Dr. Jonathan C.Y. Chung: NDE

45. Acoustic Methods (Sonic methods) Ultrasonic methods • Detection of defects by ultrasonic waves • Oscilloscope screen of ultrasonic tester • Ultrasonic Virtual Images: • 2-D image (C-scan) • 3-D image Dr. Jonathan C.Y. Chung: NDE

46. Ultrasonic NDT methods (ASTM E 127, E478, Eb500) • Frequency used: 100k-20MHz (audible: 20-20kHz) • Produced by piezoelectric crystals, such as quartz, in electric fields. An a/s voltage produces mechanical oscillations • The divergence angle depends on the ratio of the wavelength to the diameter of the source (e.g. In steel a sound at 5MHz has a wavelength of only 1.25mm, a crystal <25mm will have a small divergence angle • Usually one crystal probe both sends and receives sound • The probe is moved progressively along the surface • Cracks parallel to the waves reflect very little to the beam; hence, 2 tests normal to each other are required. Dr. Jonathan C.Y. Chung: NDE

47. Detection of defects by ultrasonic waves Dr. Jonathan C.Y. Chung: NDE

48. Oscilloscope screen of ultrasonic tester Dr. Jonathan C.Y. Chung: NDE

49. 2-D image (C-scan)single depth 3-D image Multiple depth (only the layer with problem is shown) Dr. Jonathan C.Y. Chung: NDE

50. To determine dimension, physical or mechanical characteristics • Thickness of paint and enamel • Nickel coating • Hardness tests • Moisture content by electrical means • Proof tests • Surface roughness tests • Concrete test hammer • Sonic method for measuring thickness Dr. Jonathan C.Y. Chung: NDE