The fatigue strength of welded structures and components are limited by the performance of their welded joints.
Welded joints are susceptible to develop cracks when subjected to fatigue loads. When cracks are discovered during non-destructive testing, factors such as applied and internal stresses and the material properties are relevant when considering likelihood of failure of the weld/component, herewith the acceptance level for the detected flaws can be determined. The line between the need of weld repair/enhancement and allowable stress levels can be made rationally based on fracture mechanics principles. Guidance on the application of the technique is presented in BS7910. The aim is to assess whether a given fabrication is adequate for its intended purpose, taking into account all the relevant factors. The applications of Engineering Critical Assessment (ECA) to assess the influence of some relevant factors are shortly explained below.
ECA-Fitness-for-purpose assessment of existing structures
Fracture Mechanics principles are used to assess the significance of flaws discovered during in-service inspections. This enables decisions on, repair or continued operation (without repair), to be made on a rational ‘fitness-for-purpose’ basis, thus avoiding unnecessary repairs while maintaining the safety of the structure’s integrity at all time
ECA-based weld acceptance criteria
Engineering Critical Assessment can be applied during fabrication. Critical flaw sizes, posing an unacceptable risk to the future integrity of a given welded component/structure, can be determined using fracture mechanics principles. This takes into account worst-case estimations of the material properties (including any potential degradation during future service-life) and the intended service loading. The accept or reject criteria used when fabrication flaws are discovered during manufacturing inspections, are normally less conservative (while still safe) than the good-workmanship-based criteria incorporated in common welding standards.
ECA-based material selection
Engineering Critical Assessment can be used during initial concept design to compare and contrast the behavior resulting from the use of alternative materials. The information can help to identify the most economical solution among the various materials being considered.
ECA-based maximum allowable stress determination
When flaws are discovered during in-service inspection, and found to be unacceptable from a fitness-for-purpose viewpoint, the most common outcome is either to withdraw the structure from service, or to order immediate repair.
Provided the service stresses are well understood and possible to verify, it is possible to opt for a third solution. This involves the continued operation (without repair) provided that the stresses are reduced to below the critical level determined by fracture mechanics approach. If the service stresses are not well known or are simply not possible to reduce the only remedy is the application of life extension techniques e.g. Ultrasonic Peening.
Fracture mechanics calculations can often be used as the most rational basis in order to get an opinion about the probable role that a given parameter might have played in a failure. These parameters are: stress level, pre-existing flaw, etc.
Fatigue assessments can be performed using the fracture mechanics approach or the alternative S-N curve approach. The latter is also referred to as the Design Rule approach. The fracture mechanics approach is particularly well suited for the investigation of the influence of a given parameter (e.g. changing the environment, material, nondestructive inspection capability, etc) on the probable fatigue life. The S-N curve approach is most suitable for design calculations. It is possible to use both approaches in tandem, e.g. in life extension studies to determine the enhancement in fatigue life to be expected as a result of applying a given fatigue life improvement method such as Ultrasonic Peening