Question C1 A 3-mm thick metal plate is known to contain 4 discontinuities: Discontinuity 1 - 1-mm deep surface breaking

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Question C1 A 3 Mm Thick Metal Plate Is Known To Contain 4 Discontinuities Discontinuity 1 1 Mm Deep Surface Breaking 1 (126.71 KiB) Viewed 12 times

Question C1 A 3-mm thick metal plate is known to contain 4 discontinuities: Discontinuity 1 - 1-mm deep surface breaking linear crack, • Discontinuity 2 - Sub-surface linear crack parallel to and 0.25 mm below surface, • Discontinuity 3 –Sub-surface void 2.5 mm below surface, • Discontinuity 4 - Sub-surface void 0.5 mm from surface. Figure 3 shows a cross section of the plate. The discontinuities are labelled 1, 2, 3, and 4. A surface probe with a skin depth of 1 mm is connected to an eddy current flaw detector and is balanced at position B. The flaw detector impedance display is then calibrated to trace a horizontal lift-off signal drawing from B to A. The X-Y gains are adjusted to 1:1 ratio with 80% FSH representing as 1-mm. 1 B >C -D →E →F Figure 3
(a) If the probe is balanced to Air, explain with an impedance plane diagram, how permeability and conductivity of steel and aluminium may produce different sets of indications. (3 marks) (b) Copy Figure 3 to your answer sheet. Illustrate an accurate trace line that will show on the impedance display when the surface probe moves from position B to C. (3 marks) (c) Describe in detail, how discontinuity 1 disturbs the induced current and why it can be detected by the primary coil in a surface probe. (4 marks) ON 이 Page 7 of 11 Question C1 (continued) (d) No indication is detected when the probe moved over discontinuity 2. Explain what the reasons may be and suggest a remedy action to improve sensitivity. (3 marks) (e) Explain why high frequency surface probes cannot detect discontinuity 3. (3 marks) (f) Determine if a 600-Hz probe may detect discontinuity 3, given the metal plate is an aluminium with magnetic permeability of 1.26 x 106 and electrical conductivity of 35.4% IACS. (2 marks) (g) State the concerns when interpreting indications at position F and suggest alternate solutions to improve accuracy. (2 marks)