CTOD Testing per BS 7448, ISO 12135 and ASTM E1820

What CTOD Measures and When a Specification Asks for It

CTOD is how far the faces of a sharp crack open at its tip before the crack starts to grow. It measures toughness in the presence of a flaw, which a Charpy energy cannot. Specifications call for it wherever a weld must carry a flaw it is allowed to keep: thick sections, low temperatures and high stresses.

• Pipelines: API 1104 Annex A, where CTOD on weld metal and heat-affected zone sets the flaw acceptance criteria of an Engineering Critical Assessment.
• Offshore structures: ONGC Specification 2009F Rev. 8, where CTOD with a fracture mechanics assessment can support welding without post-weld heat treatment, with the Company's written approval.
• Operator and consultant specifications: ONGC, EIL, DNV and API specifications call for CTOD on welded coupons.
• Pipe mills: seamless and welded line pipe qualified against a project specification, often with a third-party witness.
• Defence: high strength steel weld coupons for naval programmes.

Specimens, Notch Location and the Fatigue Pre-crack

A CTOD value is only as good as the place its crack tip sits. The specimen is machined so the notch lands in the zone the specification names, weld metal or heat-affected zone. A fatigue pre-crack is then grown from the notch at room temperature, before the specimen is brought to test temperature and loaded.

1. Specimen. Single edge notched bend (SENB) specimens, usually B x 2B, or compact tension C(T) specimens, machined in TCR's own machine shop. Thickness 8 mm to 32 mm across the full temperature range, 8 mm to 50 mm at room temperature, and SENB over 50 mm to 100 mm, depending on the strength of the material.
2. Notch position. Weld centreline, fusion line or a set distance into the heat-affected zone, as the specification requires. For a weld metal test, at least 75 per cent of the crack front must lie in the weld metal.
3. Fatigue pre-crack. Grown in air at room temperature under controlled cyclic load, so the crack is sharp and the specimen is not overloaded.
4. Crack front. After the test the specimen is broken open and the pre-crack front is measured through the thickness: at nine points under ISO 15653 and ISO 12135, or at 25, 50 and 75 per cent of thickness with crack-to-notch angles where the order asks for it.
5. Notch location confirmed. Post-test metallography sections the specimen to show where the crack tip actually sat. On a heat-affected zone notch, that section is what makes the result valid.

Sizing the Test Plate

Each B x 2B SENB specimen for a 65 mm weld comes from a blank 140 mm wide, the full plate thickness and 700 mm long, cut across the weld with the seam at its centre. One set of three needs three such blanks, and machining takes 2 to 4 mm off the thickness.

• One plate, one set: a 700 mm x 550 mm plate with the seam across it gives exactly three 140 mm blanks once 30 mm is discarded from each edge, and nothing for a retest.
• Send for retests at the start: supply enough plate for a retest set with the first consignment, so a retest does not wait for new welding.
• B x B specimens: about 75 mm wide x full thickness x 350 mm per blank, but ISO 12135 recommends B x B only for material 75 mm and thicker; on a 65 mm weld it is an exception the client must accept.

CTOD After H2S Hydrogen Pre-charging

In wet hydrogen sulphide service, hydrogen diffuses into the weld and lowers its toughness. TCR's procedure, developed for Larsen and Toubro, charges the pre-cracked specimen in H2S before the CTOD test, so the result is the toughness the weld keeps in service rather than in clean air.

1. Charge. At least 96 hours fully immersed in NACE TM0177 Solution A saturated with H2S.
2. Transfer. Loading starts within 20 minutes of removal from the solution. A specimen that misses the window goes back into solution for at least 24 hours.
3. Test. CTOD per ISO 15653, ISO 12135 or BS 8571, then the fracture face is examined for hydrogen damage and reported with the CTOD value.
4. Compare. Where the specification asks for both, CTOD in air and after charging on the same weld show how much toughness hydrogen takes away.

The H2S bench is the one that runs HIC and SSC qualification on the corrosion and sour service page; hydrogen embrittlement that does not come from H2S, such as cylinder steel and plated parts, is on the hydrogen embrittlement page.

What to Send Before Testing

Most CTOD delays start before the sample arrives: a test plate welded at the wrong heat input, or too little parent metal to machine the specimen with its notch on the weld centreline. Agree the standard, the notch positions, the temperature and the specimen count before the plate is welded.

• Test plates welded to the procedure being qualified, with enough parent metal either side for a B x 2B specimen. For electrode qualification, plates of 250 mm x 25 mm x 250 mm with the weld at the centre.
• The specification and its clause: standard and edition, notch positions, test temperature, acceptance value and any witness requirement.
• Specimen count: at least three valid specimens per notch position, and six where CTOD in air and after H2S charging are both required.
• For ONGC work: a test plate whose carbon equivalent is within 0.02 of the maximum production value, welded at the highest production heat input.
• International shipments: the shipping invoice states that the material has no commercial value and is sent for testing only.