Volume 7, Issue 3 (May 1979)
A Simple and Sensitive Method of Monitoring Crack and Load in Compact Fracture Mechanics Specimens Using Strain Gages
The calibrated strain on the back face (the face opposite that from which the slot is machined) of compact tension (CT) and T-type wedge-opening-loading (WOL) specimens provides a method for measuring crack length when the load is known or for measuring load when the crack length is known. The method is simple, reliable, sensitive, and inexpensive. A good correlation was achieved between strain measurements on a CT specimen and values computed from a two-dimensional finite element analysis. The method has good potential for developing into a more sensitive crack length measurement technique than has previously been achieved.
Calibration tables and graphs are reproduced that describe the relationship between crack length, back-face strain, and load for CT and T-type WOL specimens of any size and thickness and for any linear elastic material.
The method has several advantages over the closely related crack opening displacement (COD) technique for some test situations and these are described. In particular, the back-face strain increases linearly with crack length for constant stress intensity conditions except for very deep cracks in CT specimens. The overall characteristics render the technique ideal for incorporation into computerized/automated crack growth testing.
For constant back-face strain, the stress intensity was shown to decrease with increase in crack length for both CT and T-type WOL specimens. This decrease is more pronounced than for corresponding constant COD testing and this provides a good technique for obtaining threshold fatigue or stress corrosion conditions.