The Crack Tip Opening Displacement
or CTOD Test measures the resistance of a material to the propagation of
a crack. CTOD is used on materials that can show some plastic deformation before
failure occurs causing the tip to stretch open.
Accurate measurement of this displacement is one of the essentials of the test.
In the CTOD test the specimens are proportional. If the thickness is
represented by 'A', then the depth will either be 'A', for a square cross
section, or, '2A' for a rectangular cross section with the standard length being
To prepare a specimen for a CTOD
test, a notch is machined in the centre of the
specimen and then an actual fatigue crack is carefully induced at the base of
the notch. The crack must be long enough to pass through any area displaying
plastic deformity caused by the machining process.
The actual test is performed
by placing the specimen in 3 point bending and accurately measuring the amount
the crack opens. For this purpose a strain gage is employed, mounted to a clip
between two precisely placed knife edges at the mouth of the machined notch.
The crack tip opening is
plotted against the load applied. There are three basic types of fracture
behavior with this test: brittle fracture, pop-in, and ductile. The first curve
shows a brittle fracture with little or no plastic deformation. The curve shows
a pop-in where the crack initiates in a brittle manner but is soon arrested by
tougher more ductile material. This behavior can occur many times giving the
curve a saw tooth appearance. And the third curve depicts a completely plastic
or ductile behavior.
Locating the notch correctly in the material being tested
is important. A fatigue crack positioned incorrectly will not sample the
required area thus invalidating the test.
Polishing, etching and
metallurgical examination are often used to provide the required accuracy
in notch placement. These techniques may also be employed after the test to
provide additional confirmation of the validity of the test.
A low stress range is
common when performing the fatigue cracking. Employing high stresses to speed up
the process can cause large area of plastically deformed material to form ahead
of the fatigue crack invalidating the test.
Examination of the fatigue
crack surface is necessary to determine the success or failure of the
test. The length of the crack itself is accurately measured. If the length of
the crack is not within the specified limits the test is invalid. If the crack
is not in a single plane, or at an angle to the machined notch, or, if the crack
is not in the proper region, the test is invalid.