Section 6 Crack tip opening displacement tests

6.1.1 Unless agreed otherwise, tests are to be made on specimens of the full section thickness and which conform to a nationally agreed standard.

6.1.2 Normally the specimens are to be rectangular with the main dimensions as indicated in Figure 2.6.1 Outline dimensions of the preferred specimen and are to be tested in three point bending.

6.1.3 A subsidiary specimen as in Figure 2.6.2 Outline dimensions of the subsidiary specimen may be used by agreement.

6.1.4 In each case the notch is to be positioned at the centre of the loading span; its root radius is not to exceed 0,10 mm. The notch is to be extended by the generation of a fatigue crack to give an effective crack length of the dimension a. For this purpose, the fatigue stress ratio, R ₁, is to be within the range 0 to 0,1 and the fatigue intensity is not to exceed 0,63σ_γ B ^½ where σ_γ is the 0,2 per cent proof stress at the test temperature.

Figure 2.6.1 Outline dimensions of the preferred specimen

Figure 2.6.2 Outline dimensions of the subsidiary specimen

6.2.1 Whenever possible, tests are to be made using machines operating under displacement control. The type of control is to be recorded.

6.2.2 The test equipment is to be calibrated annually.

6.2.3 The crack opening displacement gauge is to have an accuracy of at least one per cent. It is to be calibrated at least once every day of testing and at intervals of no more than 10 tests. It should be demonstrated that the calibration is satisfactory for the test conditions.

6.3.1 Tests are to be made in a recognised test house in accordance with a nationally accepted standard.

6.3.2 Unless otherwise agreed, all tests on unwelded wrought material are to be made on specimens taken transverse to the principal working direction and are to be through thickness notched.

6.3.3 Where tests are made on weld material, the fatigue crack should be arranged to sample the maximum amount of unrefined weld metal.

6.3.4 Where tests are made on the Heat Affected Zone (H.A.Z.) of a weld, a K or single bevel weld preparation is recommended. The region of lowest fracture toughness in the Heat Affected Zone should be identified for the particular steel and weld procedure by means of preliminary tests. The fatigue crack is to be accurately positioned to sample as high a proportion of this critical region as possible and after testing has been completed, the specimen is to be sectioned to check that this has been achieved. Sufficient tests should be made to ensure that the critical region has been sampled in at least three specimens.

6.3.5 At least three valid tests are to be made for each material condition. Invalid tests are to be disregarded and the tests repeated.

6.3.6 Local pre-compression of the test specimen ahead of the notch is acceptable in order to provide an acceptably even fatigue crack front.

6.3.7 The temperature of the test piece is to be measured to within ±2°C over the range minus 196°C to +200°C and to within ±5°C outside this range. The temperature should be measured at a point on the specimen not farther than 2 mm away from the crack tip.

6.4.1 The test is to be regarded as invalid if:

1. the fatigue crack front is not in a single plane;

2. any part of the fatigue crack surface lies in a plane whose angle with the plane of the notch exceeds 10°;

3. the length of any part of the fatigue crack is less than 0,025W or 1,25 mm, whichever is the greater;

4. the difference between the maximum and minimum lengths of the fatigue crack exceeds 0,1W;

5. the difference between any two of the lengths of the fatigue crack at 0,25B, 0,5B and 0,75B exceeds 0,05W.

6.4.2 In addition, for tests on welds and Heat Affected Zones (H.A.Z.), the following criteria are to be complied with:

1. Weld metal. The fatigue crack front shall not extend outside the weld metal deposit and 80 per cent should be within 2 mm of the fusion line.

2. Grain coarsened H.A.Z.. The fatigue crack should be within 0,5 mm of the fusion line and should sample all of the grain coarsened H.A.Z. present. However, if fusion line irregularities prevent this, a sample including as much grain coarsened H.A.Z. as possible may be accepted.

3. Subcritical/intercritical H.A.Z. boundary. The fatigue crack is to sample the boundary between the subcritical and intercritical regions of the H.A.Z. However, if fusion line irregularities prevent this, a sample including as much relevant microstructure as possible may be accepted.

6.5.1 The test report is to include:

1. details of the material, its condition and size;

2. the thickness and width of the test specimen;

3. the fatigue pre-cracking conditions;

4. the test temperature and environment;

5. the test machine control system and rate of change of displacement or load;

6. crack length measurements;

7. force/displacement records, preferably in the form of an autographic record;

8. the critical crack opening displacement;

9. a photograph of the fracture;

10. any observation on the fracture surface.