non-destructive evaluation nde

Non-destructive EvaluationNDE

Dept. of Physics and Materials ScienceCity University of Hong Kong

References:

1. H.E. Davis, G.E. Troxell, in chapter 16 of “The Testing of Engineering Materials”, 1982.

2. J.S. Ceurter et al., “Advanced Materials Processes” (April 2002), p.29-31.3. T. Adams, “Advanced Materials Processes” (April 2002), p.32-34.

112/04/22 Dr. Jonathan C.Y. Chung: NDE1

Various Purposes

• Locate defects (Why ?)• Determine dimension, physical, or mechanical

characteristics• Determine Residue Stress (XRD)

112/04/22 Dr. Jonathan C.Y. Chung: NDE 2

Advantage of Knowing the defects

• Defects are usually stress raiser• Stress raiser can cause pre-mature failure

Over design to overcome pre-mature failureBulky/heavy design

• Catastrophic/sudden/unpredicted failureloss of lives and money

• Quality control• Better design


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 40MPam. If the corresponding design safety factors are 1.2, 1.6 and 1.5 respectively. What is the allowable load?

(a)Yielding failure (>25 kN)(b)Tensile fracture (>28.1 kN)(c)Fracture toughness (crack size dependant)2 mm: 16.8kN; 1mm: 23.6kN; 0.1mm: 75.2kN


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)


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)


Fracture Toughness (require information of crack length)

KIC = (a)Assume geometric correction factor, = 1max = KIC /(a)

Max load= x A (safety factor)= KIC /(a) x A (safety factor)= 40MPam /(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 kNWhen a = 1 mm, max load = (2000 0.05605)/1.5 = 23.6 kNWhen a = 0.1 mm, max load = (2000 0.01772)/1.5 = 75.2 kN


NDE methods for location of defects

Surface defects detectionVisual inspectionLiquid penetrant testMagnetic particle met

hod


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

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 methodNo-parallex method

Penetrant test


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 165General procedure:General procedure: Thoroughly clean the surfaceThoroughly clean the surface Apply penetrant on the surfaceApply penetrant on the surface Liquid penetrant enter small openings by capillary Liquid penetrant enter small openings by capillary

actionaction Remove liquid completely and apply developer (dry or Remove liquid completely and apply developer (dry or

wet)wet) The penetant bleed out onto the surface showing the The penetant bleed out onto the surface showing the

location of the surface defectlocation of the surface defect


Enhancing the penetrant test Strike the part to force the liquid out of the defect Fluorescent-penetant

depth 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.


Magnetic Particle Test Use to locate the defects at or near the surface of ferromagnetic o

bjects. The magnetic particles tends to pile up and bridge over discontinuit

ies. A surface crack is indicated by a line of the fine particle following th

e 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 detect

ion.


Permanent magnets with soft iron keepers


Fixture for yoke induction of longitudinal magnetic field


Leakage Flux


Fatigue cracks in airplane gear

detected by the magnetic-particle

method


Orientation of magnetic fields


Some cracks are more difficult to detect


Threshold indications of near-surface cavities


Radiographic methods

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


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)


Formation of a radiograph


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)


Radio graphing a welding joint


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.


Influence of size of

source on sharpness of image


Radiograph of a 20 mm weld


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


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.


Biological Effects• Relaxation lengths of various shielding materia

ls.• Estimated radiation does to U.S. population• Acute doses of penetration radiation.


Relaxation lengths of various shielding materials


Estimated radiation does to U.S. population


Acute doses of penetration radiation.


Neutron Radiography


a. Brass bullet with gunpowder

b. Steel airbag inflator with packets of fast-burn pyrotechnic

c. 38 mm long turbine blade

d. Turbine blade with flaw

a

dc

b

FT-IR imaging


Inclusion in polypropylene film IR spectra showing impurities (1) ester and (2) amide.

Red: ester Red: amide

Perkin-Elmer FT-IR imaging system


FT-IR imaging


An image a fly’s wing

Fingerprint image


PCB sample


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.


Encircling CoilsIf 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.


Effect of similar inner and outer defects on flux pattern and measurement


Barkhausen Noise Inspection


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).


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


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.


Detection of defects by ultrasonic waves


Oscilloscope screen of ultrasonic tester


2-D image (C-scan)

single depth


3-D image Multiple depth (only the layer with problem is shown)

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


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.


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.


Electronic device for measuring surface roughness


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.

Ultrasonic tester for

measuring thickness from one side only.


non-destructive evaluation nde

Documents

allowable load

kic amax load

m2 safety factorwhen

advanced materials processes

yielding failure

kntensile fracture

nonporous materials

ndefracture toughness