Crack tip opening displacement (CTOD) testing

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 1. 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 arrangement
Figure 1. CTOD Test arrangement

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

CTOD Sample preparation

To prepare a specimen for a CTOD test, a notch is machined in the center 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 Notch is shown in the below figure.

CTOD Test specimen
CTOD Test specimen

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’.

Unlike the inexpensive 10mm by 10mm square Charpy-V test piece with a sharp machined V notch, the CTOD specimen can be the full thickness of the material, will contain a genuine crack, and will be loaded at a rate more representative of actual service conditions. Usually, three tests are carried out at the relevant temperature to ensure consistency of CTOD results.

The test piece itself is ‘proportional’ – the length, depth, and thickness of each specimen are interrelated so that, irrespective of material thickness, each specimen has the same proportions.


Charpy V Notch vs. CTOD

The most common toughness test used is the CVN test. It is performed by taking several small machined samples from the test piece and breaking them with impact at the minimum operating temperature for the part. CVN testing is a relatively inexpensive and straightforward test that measures qualitative values.

Results from a CVN test verify that the operating temperature did not fall below the material’s ductile-brittle transition temperature. Why is this important? Because when a material is utilized below this temperature, it is prone to brittle failure. A brittle failure means there will be little or no warning before the material catastrophically fails. This is similar to a piece of glass or ice-breaking when force is applied. This type of failure mechanism must be avoided at all costs.

CTOD testing is performed on a sample that is representative of an actual part thickness (or scaled appropriately) and is subjected to in-service loading conditions and temperatures. The test part has an opening machined into it along with a fatigue crack of known dimensions. The piece is then placed under a gradual load, reflecting service conditions until the crack sufficiently opens or cleaves, which may be either partial or complete failure.

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. 

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