Discuss the advantages and disadvantages of each of the following methods for measuring (i) crack length and (ii) crack closure loads:
(a) Crack mouth opening displacement gage.
(b) Strain gages mounted in the wake of the crack, above and below the plane of the crack.
(c) Strain gages mounted immediately ahead of the crack tip.
(d) Back-face strain gage.
(e) DC electrical potential drop.
(f) AC electrical potential drop.
(g) Ultrasonics.
(h) Acoustic emission.
(i) Travelling microscope providing a direct optical image of the crack on the specimen surface.
Also discuss the experimental procedures you would use for the calibration of crack length and crack closure, where appropriate, for each of the above methods.