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.

2
Various Purposes

• Locate defects (Why ?)
• Determine dimension, physical, or mechanical
  characteristics
• Determine Residue Stress (XRD)

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

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 (gt25 kN)
• Tensile fracture (gt28.1 kN)
• Fracture toughness (crack size dependant)2 mm
  16.8kN 1mm 23.6kN 0.1mm 75.2kN

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 gt 25 kN)

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 gt 28.1 kN)

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

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

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

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

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.

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.

13
Permanent magnets with soft iron keepers
14
Fixture for yoke induction of longitudinal
magnetic field
15
Leakage Flux
16
Fatigue cracks in airplane gear detected by the
magnetic-particle method
17
Orientation of magnetic fields
18
Some cracks are more difficult to detect
19
Threshold indications of near-surface cavities
20
Radiographic methods

• X-rays method (Exograph)
• Gamma rays (Gammagraph)
• Neutron
• Infra-red (FT-IR) imaging

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)

22
Formation of a radiograph
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 (gt400kV)

24
Radio graphing a welding joint
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.

26
Influence of size of source on sharpness of image
27
Radiograph of a 20 mm weld
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

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, dont perform this in a
  rush.
• Once the operation manual have been set, the
  engineers and technicians must follow it
  STRICTLY.
• Dont make arbitrary compromise.
• Get advices from the licensed radiographers.
• Select appropriate personal monitoring devices.

30
Biological Effects

• Relaxation lengths of various shielding
  materials.
• Estimated radiation does to U.S. population
• Acute doses of penetration radiation.

31
Relaxation lengths of various shielding materials
32
Estimated radiation does to U.S. population
33
Acute doses of penetration radiation.
34
Neutron Radiography
a
b

1. Brass bullet with gunpowder
2. Steel airbag inflator with packets of fast-burn
   pyrotechnic
3. 38 mm long turbine blade
4. Turbine blade with flaw

c
d
35
FT-IR imaging
Inclusion in polypropylene film
IR spectra showing impurities (1) ester and (2)
amide.
Red amide
Red ester
36
Perkin-Elmer FT-IR imaging system
37
FT-IR imaging
An image a flys wing
38
Fingerprint image
39
PCB sample
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.

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.

42
Effect of similar inner and outer defects on flux
pattern and measurement
43
Barkhausen Noise Inspection
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).

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

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 lt25mm 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.

47
Detection of defects by ultrasonic waves
48
Oscilloscope screen of ultrasonic tester
49
2-D image (C-scan)single depth
3-D image Multiple depth (only the layer with
problem is shown)
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

51
Enamel and paint coating thickness

• The reluctance of the magnetic circuit of the
  sensitive gauge head when placed on a coated
  steel surface varies with the thickness of
  enamel/paint.
• The gauge head is calibrated to read thickness
  directly in thousands of an inch.

52
Nickel coating thickness

• One type of instrument employs a portable spring
  balance for test.
• Thickness of nickel coating on nonmagnetic base
  metals is determined by force required to detach
  the magnet from the coating.
• The greater the thickness of the nickel coating,
  the larger the force required.

53
Electronic device for measuring surface roughness
54
Concrete test hammer
A NDT impact test for determining the hardness,
and the probable compressive strength of concrete
in a structure is by causing a spring-loaded
hammer inside the tube automatically to strike
the concrete.
55
Ultrasonic tester for measuring thickness from
one side only.