Testing and assessment

TWI is well known for its work on fracture toughness, crack growth resistance and crack arrest testing of nuclear steels and other materials, particularly when the properties of welds and heat affected zones are required. Analysis and interpretation of test data and metallography of fracture surfaces is undertaken in-house. Fracture toughness data is analysed according to the relevant standards, whilst new methods such as the Master Curve allow small scale specimen data to be statistically analysed and extrapolated to higher thickness and strain rates.

One of TWI's strengths is its versatility in devising:

• new testing methods,
• testing at extreme conditions
• testing very large and small specimens.

The comprehensively equipped laboratory contains a wide range of rigs, actuators and testing machines, with customised instrumentation and dataloggers. In addition, TWI has two drop weight testing facilities for traditional Pellini testing and impact resistance studies.

As well as ferritic and austenitic steels, materials tested have also included zirconium alloys, inconel, monel, and graphite blocks. There is a wealth of test data and experience available. Materials engineers can relate the macroscopic properties of welds and heat affected zones to the microstructures produced by the welding process.

Recent project work for the nuclear industry has included:

• the development of fracture toughness test methods for mis-matched and dissimilar metal welds
• crack arrest tests using instrumented Charpy specimens and wide plates
• standard and sub sized fracture toughness testing of weld and heat affected zone material is carried out regularly on reactor materials, while the validation of transport flask materials requires tests at high strain rates.

Tube Stress TM

In addition to materials testing, TWI also has specialist skills in the measurement of residual stress. An array of different techniques are available depending on the application. TWI expertise in the field is internationally recognised, for example, TWI has been the leader of the European Framework Project VORSAC (Variation of Residual Stress in Aged Components).

The block removal, layering and splitting technique was developed at TWI and has been used to determine the through thickness distribution of residual stress associated with letter box weld repairs. A special device (Tube Stress ^TM ) to measure remotely the residual stress in welded tubes was developed for the French nuclear industry. Recently, TWI has been assessing residual stresses in large PWR primary circuit piping components.

Contact us at: power@twi.co.uk