Mechanics of Crack Tip Opening Displacement


Crack tip opening displacement is kind of a destructive testing that is applied to measure the fracture toughness of the material. In shortened form the method is known as CTOD. In this technique a notch is made on the test material and subjected to bending at specific temperature (usually at low temperature) until the crack initiates. Bending is applied in such a way that the crack tip opens widely and the extent of opening without brittle fracture is measured as toughness.  The technique proves quite useful in repair works when the crack is appeared during service. Following is the demonstration on mechanics of crack tip opening displacement.
Specimen
Specimens are taken across the axis of weld metal or heat affected zone. The dimensions of test specimen are demonstrated in codes and standards. A rectangular piece is cut in 10T x 1T x 2T (L x W x H) where T is total thickness of weld. From the middle of rectangular specimen a V-cut is indented inside. It is to be noted that V-cut is applied inside the notch, not at the surface of specimen. See the figure.

Technique
Test technique is as simple as bending a long bar. The specimen with V-cut is inserted in bending equipment and subjected to cyclic bending until a crack initiates. A bending former is pressed at the bottom so that notch is in tension. Stress concentration at the tip of crack, causes the crack to open widely. A gauge is connected to the opening which measures the length of gap created by the widening of crack tip. The experiment provides the statistics about the increase in opening against the load and the required CTOD value in millimeter is calculated.

Test Conditions
  • Temperature – The experiment is performed normally at low temperature. For this the specimen is put inside the cooled liquid or gas to achieve the required temperature.
  • Position – location of notch on weld metal or HAZ.


What information it gives?
CTOD value assists to determine the toughness and brittleness of weld metal. Tougher the test weld, wider will be the tip opening. In this case considerable tip opening is observed when applied load is increased. Bending load pulls the crack edges apart. On the other hand slight loading may cause fracture in brittle test piece. Here brittle crack is developed without any widening.

It also assists in determination of maximum crack size that can cause fracture. This information is quite helpful when a crack is inspected in weld metal during service. It gives an idea about the extent to which the crack can withstand particular conditions of temperature and load. Moreover determination of maximum load/stress that can cause failure for specific crack size at particular temperature hence, making it easier to make judgments accordingly to prevent failure. 

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