Aircraft Structural Components Design Philosophy- Damage Tolerant Design Concept

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Aircraft structural components is the main load carrying member. How efficiently structure will take load depends on its structural components design and material.Thus , primary consideration in aircraft design is placed on structural components.

Generally,  three different types of  design philosophies used in designing of structural components which are such as safe-life , fail-safe and damage tolerant design.

Please read below article on  safe-life and fail-safe design philosophy.

https://aircrafttechnicalinfo.blogspot.com/2021/08/aircraft-structural-components-design.html



Damage Tolerant Design Concept ; 


As we know fail safe structure so designed that after failure of one part there is still sufficient strength in the reminder part to prevent catastrophic failure until damage part found by routine inspection.The inspection criteria developed for fail safe design failed to specify inspection technique and inspection frequencies were not sufficient to detect discrepancies prior to catastrophic failure.

A major difficulties with safe life and fail safe design was their testing was done in laboratory environment with near perfect condition. Aircraft service variable such as aircraft utilization and climatic condition were not taken into consideration while components testing.To take into account the aircraft operating variable and rare perfect condition, the damage tolerant design evolved for structural components.  


The damage tolerant design philosophy accept the existence of minor flaws in components.The expected growth of minor flaws in the component and establish proper inspection method to identify flaws before they become critical to the aircraft airworthiness.  


Due component safety factor , damage tolerant designed component withstand the load even in existence of minor flaws in the component.To determine the safety factor of different component, design engineer who estimate the point at which flaws would be identified through inspection and based on this calculation safety factor of different component is determined by design engineer.

 

The feature which are considered for designing damage tolerant components are

1.Multiple load path construction

2.Use of crack stopper to control the rate of crack growth.

3.Providing adequate residual static strength.

4.The use of specially developed material less prone to damage.

5.Material should provide a controlled slow  rate of crack propagation after crack initiation combined with high residual strength.  





Fig 01 Damage Tolerant Design


Damage tolerant component safety is ensured through routine inspection , slow crack growth properties and crack arrest capabilities so that crack will not reach to its critical length before the next inspection.As the requirement of damage tolerant component is routine inspection and good design to carry load in presence of flaws in the component. Thus, damage tolerant design limited to component where we have enough component geometry for inspection and good design for multiple load path.Safe life design concept use for structural component where damage tolerant design is not feasible.Typical example of component damage tolerant design not feasible is landing gear ,engine mount and their attachment.


 Damage Tolerant Design Example ;






Fig 02 Wing Spar Damage Tolerant Design 


The above picture which shows the example of wing spar where crack observed on front spar.


Case 1

Crack detected on wing front spar and aircraft completed one year since new. Crack length is more then the threshold and flight cycle or hours is low so in this case immediate rectification action as per maintenance data has to be carried out.


Case 2

Crack detected on wing front spar, crack length is small and well within limit.As per the graph shown  in the picture, we can determine crack growth period with help of crack size and current flight cycle or hours.Crack growth period and inspection interval depends on crack length and flight cycles or hours.If crack growth period is small, the inspection interval will short to check crack condition and accordingly take rectification action.

 

 




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