What is CTOD Test?
CTOD test is used to determine fracture toughness (fracture resistance) of a material with an artificial crack, and a test to determine crack tip opening displacement to generate unstable fracture (limit CTOD value). It is usually carried out by loading a CTOD test specimen on a 3-point bending system as shown in below figure. These tests are made using a full-plate thickness specimen containing a machined notch which is extended by fatigue cracking.
When a material having a defect is applied a load, a crack will propagate rapidly originating from the defect, resulting in fracture at a certain temperature range (unstable fracture).
CTOD Test standards
BS 7448 (Part 1-4): Fracture mechanics toughness tests
ISO 12135: Metallic materials – Unified method of test for the determination of quasistatic fracture toughness
ASTM E 1820: Standard Test Method for Measurement of Fracture Toughness
WES 1108: Standard Test Method for Crack Tip Opening Displacement (CTOD) Fracture Toughness Measurement
BS7910: Guide on Methods for Assessing the Acceptability of Flaws in Metallic Structures.
ISO 15653: Metallic Materials – Method of test for the determination of quasistatic fracture toughness of welds
Importance of CTOD Test
The CTOD test special form of fracture toughness test used when some plastic deformation can occur in the material prior to failure. This allows the tip of a crack to stretch and open, hence also called ‘tip opening displacement’.
Which Properties do we get from a CTOD test?
The information that is of interest is the load at failure, displacement of the opening (width) at the time the crack propagated. This information, along with load/displacement data collected throughout the test loading, is then used to determine the material’s toughness. There is considerably more analysis of the testing variables, which are simulating conditions that are required for accurate CTOD tests, but that is not the focus of this column.
The results from a CTOD test can help determine or predict when necessary repairs are needed to a structure. In many cases, a small crack in a structure doesn’t mean it will fail or that it requires immediate repair. Being able to schedule repairs when deemed necessary can save time and potentially tens of thousands of dollars or more in lost production. The CTOD test saves downtime and unnecessary repairs, and it reduces the overall costs to maintain a structure.