Jean-Philippe Laures joined TWI as a Senior Research Engineer in the Engineering Department in 1989, where he is responsible for software development. He graduated as Diplôme d'ingénieur in Lyon, France in 1987.
In the late 1980s Jean-Philippe completed his masters and doctorate degrees at Tufts University, Medford, in the United States. As a trainee engineer he worked with Single Buoy Moorings in Monaco on the evaluation of mesh generation finite element packages, and FEM modelling and analysis of offshore structures.
New computer software can help engineers to assess the acceptability of weld discontinuities in fabricated structures, as Jean-Philippe Laures explains.
The presence of material discontinuities means that few structural components are totally free from risk of failure. Traditional standards and codes of practice allow only very small flaws to be present in a structure. These tolerable flaw sizes are usually based on relatively arbitrary criteria. In metal structures flaws such as cracks, pores, lack of fusion and undercut are often found in even the best quality welds.
The problem becomes even more acute when repairing a defect by excavating material and then re-welding. It cannot be certain that such a repair will be successful, since the process may have introduced new defects, shortening the life of the structure.
Considerable resources are wasted every year in unnecessary repairs or over-conservative design.
Defect assessments
Advances in fracture mechanics have allowed engineers to assess discontinuities more accurately, thus offering the potential for reducing the number of repairs and over-designed structures. The British Standards Institution issued a Published Document (PD6493) in 1980 giving guidance on assessing the acceptability of discontinuities in welded structures.
More recently, BSI committee WEE37, including TWI and CEGB staff, has been compiling a single document to give guidance on the latest assessment techniques. The new document, PD6493:1990, will be a considerably extended version of its predecessor to accommodate three levels of assessment of discontinuities in the fracture clauses, while the fatigue clauses remain virtually unchanged. PD6493:1990 will therefore reflect the 'older' PD6493:1980 assessment, as well as the CEGB R6 Revision 2 and Revision 3 documents.
Fracture
Levels 1, 2, and 3 of the fracture assessments are arranged in increasing order of accuracy of the results and data required, and therefore in decreasing order of conservatism and increasing cost of the assessment procedure.
Level 1 is a screening level, using a semi-empirical LEFM (linear elastic fracture mechanics) and EPFM (elastic-plastic fracture mechanics) method (used in the older PD6493:1980). It sets up a simple and quick assessment for qualifying the less dangerous cases.
Level 2 uses a modified strip yield model based on the EPFM Dugdale analysis (assuming that the plasticity is all contained in the plane ahead of the crack tip: CEGB R6 Revision 2). It is the appropriate level for most current assessments, giving a less conservative result, but it is still accessible by simple techniques.
Level 3 performs a tearing analysis utilising estimation of the J integral and its differential dJ/da from the material's stress-strain and R curves, making it possible to calculate the criterion of crack extension J), and the criterion of instability of crack extension (dJ/da)/(CEGB R6 Revision 3). It is intended to be the least conservative (most accurate) assessment, but usually needs much more input data and expertise, and often needs a computer.
Fatigue
The fatigue assessments are defined by two approaches. The so-called simplified approach is based on the method described in BS 5400 and relates the life of a cracked component to its geometry, its structural design and the applied stress spectrum via experimental S-N curves. The other fatigue assessment approach uses fracture mechanics methods (the Paris Law) which relate the incremental increase in crack length to an arbitrary incremental stress cycle.
By integrating over the stress spectrum the structure will experience over its lifetime, the engineer can therefore determine the crack growth law for the given application. Again, this more detailed assessment often needs more input data and use of a computer. By combining fatigue and fracture one can thus assess the acceptability of discontinuities in a structure, taking account of the static load (the maximum load to be seen by the component) and the dynamic load (representing the loading seen by the component over its whole lifetime).
A text animator for PD6493
It became clear early in the revision of PD6493 that fracture and fatigue assessments could often be performed only by experts. Furthermore, some assessments (such as Level 3 of fracture and the integration of the Paris Law in the fatigue clause) could not easily be undertaken without the help of a computer. As an illustration of the complexity of the task, the fracture and fatigue assessments contain approximately 100 paragraphs and 50 clauses, excluding introductory material. Each clause refers to applicable equations depending on the case being studied, involving over 90 different parameters and over 150 different equations.
It should normally take an expert no more than a few hours to carry out Level 1 or Level 2 fracture assessments. On the other hand, Level 3 fracture and the fatigue assessments may take some days of analysis. Inexperienced users may also take some hours to determine which clauses and which equations need to be applied in a particular case.
The need is clear for 'intelligent' software to decide which clauses to apply, to ask only for essential data, and to perform the appropriate assessment. Indeed, such software would allow non-expert and expert alike to perform even the more complex assessment in about half an hour.
TWI staff developed the idea of designing software to guide the user through the document. The software would encode TWI interpretation of the BSI document and, whenever necessary, perform numerical calculations; it would, in a sense, 'animate' the document.
A Group Sponsored Project (GSP5571) was therefore set up to develop a Text animator for PD6493:1990, after TWI had carried out a prototyping and a feasibility study. TWI is now running a user group to finalise the validation and quality assurance of the software.
The scope of the software embraces all fracture assessment levels and the fatigue assessment clauses covering the Paris Law integration. It is expected that the final product will be complete in March 1991. Until then, industrial members of TWI will be welcome to join the user group.
The software
The text animator, PC6493, is a complex decision tree designed to reflect TWI's interpretation of the document. No previous knowledge of the document is required of users; it is assumed only that they are qualified engineers and will therefore understand the basic language used.
The software reproduces a consultation with an expert, creating a dialogue between the user and the computer. The software selects the appropriate clauses depending on the answers given by the user. It also gives guidance on appropriate default values, asks only for the required data, and finally performs the calculation scheme and displays results as messages ( eg 'The defect is acceptable'), or as graphs relating the various parameters ( ie Failure Assessment Diagram: FAD). For the more experienced user, the software will also display intermediate values in the calculation.
In conclusion
The objective of producing the text animator was to bring the expertise of TWI staff to the engineer's desk. It is estimated that such a program allows an engineer to resolve about 80% of flaw assessments, many of which would be considered 'trivial' by the expert. This allows the engineer to produce better designs by increasing the use of such techniques, and to avoid unnecessary repairs while being able to identify and report cases needing outside expertise.
From TWI's point of view, the text animator not only allows experts to perform their tasks more quickly and efficiently, but also frees their time to concentrate on the assessments which need individual specialist analysis.
PC6493 is the first of a series of tools; TWI is currently involved in developing software to help communicate knowledge, expertise, and data across the materials engineering field. The aim is to build a network of databases, expert systems, and computerised codes of practice. Indeed, assessment of flaws is complex and a single software item could not cover the whole subject. Several complementary pieces of software are at prototype stage or under development as follows:
- Fracture toughness database;
- Fatigue crack growth database;
- Defect interaction calculation;
- Charpy-fracture toughness correlation;
- Weld distortion prediction;
- Expert system on pipeline technology.
We shall publish more on these in due course.
