DEPARTMENT OF MECHANICAL AND PRODUCTION ENGINEERING
COURSE CODE: MPEN 511E
COURSE TITLE: NON-DESTRUCTIVE TESTING METHODS
CAT I: MODEL ANSWERS
1. Explain three considerations for selection of a non-destructive testing (NDT)
inspection system.
(3 marks)
(i) Test principle or method
(ii) Types of defects
(iii)Cost of testing
(iv) Automation level
(v) Portability
(vi) Range of components and materials to be tested
2. Describe four defects introduced during the manufacturing processes such as
welding of components.
(4 marks)
(i) Porosity
(ii) Nonmetallic inclusions
(iii)Lack of fusion
(iv) Cracks incomplete root penetration concavity at the root of the weld
(v) Excessive penetration
(vi) Overlap
3. Explain three factors that affect sensitivity in magnetic particle testing and
inspection.
(3 marks)
(i) Orientation of the discontinuity with respect to induced magnetic field
(ii) Nature of particles carrying fluid
(iii)Strength of magnetic field
(iv) Shape of the component
(v) Size, shape and properties of the particles
4. Describe two application ranges and two limitations of liquid penetrant testing
and inspection method.
(4 marks)
Applications
(i) Quality control inspection of production of many manufacturing processes
(ii) Aircraft undercarriage inspection including wheels
(iii)Identification and sorting of materials using phase sensing equipment
(iv) Measuring the thickness of either conducting and non-conducting coatings
Limitations
(i) Only surface breaking defects can be detected.
(ii) Only materials with a relatively nonporous surface can be inspected.
(iii)Pre-cleaning is critical since contaminants can mask defects.
1
(iv) Metal smearing from machining, grinding, and grit or vapor blasting must be
removed.
(v) The inspector must have direct access to the surface being inspected.
(vi) Surface finish and roughness can affect inspection sensitivity.
(vii) Multiple process operations must be performed and controlled.
5. Describe the procedural steps of development stage of the liquid penetrant
method.
(6 marks)
(i) Surface preparations
(ii) Penetrant Application
(iii)Penetrant Dwell
(iv) Excess Penetrant Removal
(v) Developer Application
(vi) Indication Development
(vii) Inspection
6. Describe four suitable characteristics that penetrants in liquid penetrant testing
method must possess.
(4 marks)
(i) Spread easily over the surface of the material being inspected to provide complete and
even coverage.
(ii) Be drawn into surface breaking defects by capillary action.
(iii)Remain in the defect but remove easily from the surface of the part.
(iv) Remain fluid so it can be drawn back to the surface of the part through the drying and
developing steps.
(v) Be highly visible or fluoresce brightly to produce easy to see indications.
(vi) Not be harmful to the material being tested or the inspector.
7. Explain DC magnetization and AC magnetization in magnetic particle testing.
Describe advantages and limitations in each case.
(6 marks)
Direct current magnetization obtained from storage batteries is the most desirable
current to use, since it penetrates more deeply into test specimens than any other
current. The disadvantage of the direct current obtained from storage batteries is that
there is a specific limit to the magnitude and duration of current, which can be drawn
from the battery before recharging. Battery maintenance is costly and can become a
source of environmental waste.
Alternating current magnetization is also used for detection of surface cracks like
fatigue cracks. AC inspection units should be equipped with proper current controls.
An advantage of using AC is that the parts being inspected with this current can be
easily demagnetized by gradually reducing the current to zero. The limitation with
AC magnetization is its inability to penetrate deep into component being tested
2