IIW White Paper

4 Needs and challenges in welding and joining technologies

most critical flaw type during fabrication most of such tests are targeted to define welding and respective heat treatment procedures for crack avoidance. For this, numbers of test procedures have been developed to assess the hot and cold cracking behaviour of welds. Significant progress has been achieved by the IIW to compare the various procedures in Round Robins and to provide respective specifications, guidelines and standards. Hence, numbers of heat treatment procedures for cold crack free welding of structural steels have been specified and standardised. Design aspects have been taken less into account during establishment of the respective test procedures. As a major design aspect, an evaluation of shrinkage restraint of welds is gaining attention among the IIW community now, because this parameter is significantly affecting the local and global stresses and strains in a joint. Through the pioneering IIWwork, procedures developed in the seventies have provided quantitative evaluation of the shrinkage restraint. These procedures have been improved in recent years and with the advancements in numerical modelling, more insight has been gained on how the shrinkage restraint affects the stresses and strains as well as respective cracking of joints. In recent years, significant improvements have been made in the testing procedures of the welds for determination of the tensile properties (local and global). The use of micro-scale specimens, for example, micro-flat tensile specimens (0.5 mm thick and 2.0 mm wide) have been developed to establish the very small weld volumes and HAZ gradients. The use of the micro-indentation technique has also been developed to test extremely small zones of weld joints. Fracture toughness using Charpy-V or fracture mechanics (CTOD or J-integral) test methods have been well- established for structural weldments. There exists ongoing discussions on the consequence and essence of higher constraint specimens (deeply notched three point SENB specimen) versus weld (single edge notched tension, SENT) specimens tested under tension (lower constraint) particularly for design and qualification of the girth welds in pipes. Testing under cyclic loading for S-N behaviour is well established to be used in fabrication and design stages as well as for design of welded structures. Furthermore, IIWhas developed unique Fatigue Design Guidelines including improvement techniques over many years and nowadays used by industry for design of welded components. With respect to fatigue crack propagation (FCP) test procedures, the community still uses testing standards developed for base materials. Here, there exists need for further development of FCP testing procedures taking into account the special features of the welds including narrow EB and laser beam welds, dissimilar joints etc. Fatigue testing, for instance, has been improved towards amore consistent evaluation of welds in comparison to barely testing of base materials. Also, corrosion testing of welds is meanwhile carried out in more realistic environments. Significant progress has been made in testing of component-like welded specimens and/or complex welded components under different loading conditions (uni-axial or multi-axial) as well as under the environmental conditions (e.g corrosive or sub-zero temperatures). These tests (in limited number) are usually accompanied with numerical simulations to predict or verify predictions of service performance. Obviously these tests contain the effects of welding residual stresses, as exist under the testing conditions of interest. Nevertheless, suitable test procedures have to be developed to simulate more complex, but respectively realistic conditions to gain more insight into the service behaviour of welded components in combination with modelling efforts. As further perspectives, sensorics and micro-electronics will increasingly be applied to both, materials related testing and, in particular, to the testing of welded/joined components. Also on-line monitoring and in-situ investigations (including structural health monitoring, SHM) during fabrication and service of welded structures will gain importance.

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Through Optimum Use and Innovation of Welding and Joining Technologies

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