Damage Tolerance Evaluation – Understanding the importance of answering “Why do we do what we do and how to do it?”
Why do structures need to tolerate damage (whatever the source may be) and how did we get where we are today in our understanding and implementation of Damage Tolerance Evaluation and what lies ahead ?
This blog offers some insight into explanations for the above questions.
There is no doubt that the Flight Safety is at its best. The Year 2017 has been noted as the safest year as far as the civil aviation is concerned. (LINK)
All aspects of Airborne components (structures, systems, engines etc) and supporting non-Airborne components (runway conditions, communications etc) have their own contribution to it. Focusing particularly on the structural integrity evaluation and its contribution to overall safety, the safety outcome can be attributed to Regulations, Maintenance Practices and Design Philosophy/Activities among others.
And these are all governed by the advances in technology and improvement in understanding of behaviour of materials, components and structures. The advances are generally followed by the identification of problem areas that the then-current regulations/maintenance/design criterion did not envisage or comprehend.
On the military front some failures prompted USAF to launch ASIP (Aircraft Structural Integrity Program) investigating all aspects of fatigue and coming up with practical solutions (evaluation & product improvements) and some other failures highlighted the need for the concept of manufacturing flaw.
On the civil aircraft front ; some failures highlighted the importance of considering the stress concentration in fatigue evaluation, some failures highlighted the impact of considering realistic fatigue spectrum in testing be it coupon level or full scale. Some other failures highlighted the concept of Widespread Fatigue Damage.
Overall, it has been accepted that the damage in the structures can not be avoided/eliminated but need to be tolerated. Damage Tolerance Evaluation depend on understanding of the behaviour of materials throughout the operation of the structure.
There are ample examples in history to highlight that our understanding of behaviour of materials needs constant upgrading. The fundamental understanding (theory), engineering representation (mathematical formulation) are pursued by academies, industries; the studies of which, it is fair to say, are driven by the product developments. Unfortunately in many cases it is the failure of the products that indicate the need for a new concept.
While the safety is ensured as discussed above for the current technologies, the demand of higher reliability of production/operations and the need for design optimisation (design to cost for ex) keep pushing the boundaries of our understanding on the existing technologies and imposes new technologies in the product developments.
It is imperative to expose the the engineers involved in the detailed calculations that support the overall structural integrity to the overall picture of Damage Tolerance Evaluation. They should fully comprehend “Why do we what we do and how to do it”? That will enable the engineers to comprehend/explain/plan (rather than reacting to a failure that occurs in-service after product development) what needs to be done when a new material/technology is introduced and is faced with the task of ensuring structural integrity using that material/component/structure.
AeroSIFT training on Practical Approach of Fatigue and Damage Tolerance Evaluation is prepared keeping this principle in mind.