Charpy V-notch testing enables engineers to make judgments about risks of brittle fracture
occurring in steels but a CTOD test measures a material property- fracture toughness.
Fracture toughness data enables engineers to carry out fracture mechanics analyses such as:
- calculating the size of a crack that would initiate a brittle fracture under certain stress conditions at a particular temperature
- the stress that would cause a certain sized crack to give a brittle fracture at particular temperature
This data is essential for making an appropriate decision when a crack is discovered during inspection of equipment that is in-service.
A CTOD specimen is prepared as a rectangular (or square) shaped bar cut transverse to the axis of the butt weld. A V-notch is machined at the center of the bar, which will be coincident with the test position - weld metal or HAZ.
A shallow saw cut is then put into the bottom of the notch and the specimen is then put into a machine that induces a cyclic bending load until a shallow fatigue crack initiates from the saw cut. The specimens are relatively large – typically having a cross section B x 2B and length ~10B (B = full thickness of the weld). The test piece details are shown in the schematic below.
Fig.1. Proportional rectangular cross section CTOD specimen
CTOD specimens are usually tested at a temperature below ambient and the temperature of the specimen is controlled by immersion in a bath of liquid that has been cooled to the required test temperature. A load is applied to the specimen to bend cause bending and induce a concentrated stress at the tip of the crack and a clip gauge, attached to the specimen across the mouth of the machined notch, gives a reading of the increase in width of the mouth of the crack as the load is gradually increased. For each ‘test condition’ (position of notch and
test temperature) it is usual practice to carry out three tests.
The schematics below illustrate the main features of the CTOD test.
Fig.2. Typical test arrangement. The specimen can be easily immersed in a cooling bath
Fig.3. Position of CTOD specimen immediately prior to crack propagation
Fracture toughness is expressed as the distance that the crack tip opens without initiation of a brittle crack. The clip gauge enables a chart to be generated showing the increase in width of the crack mouth against applied load from which a CTOD value is calculated.
An application standard or Client may specify a minimum CTOD value that indicates ductile tearing. Alternatively, the test may be ‘for information’ so that a value can be used for an engineering critical assessment. A very tough steel weldment will allow the mouth of the crack to open widely by ductile tearing at the tip of the crack whereas a very brittle weldment will tend to fracture when the applied load is quite low and without any extension at the tip of the crack. CTOD values are expressed in millimetres - typical values might be << ~ 0.1mm = brittle behaviour; > ~1mm = very tough behaviour.