Airworthy Material ..!
There is nothing like airworthy material either by definition or by name. However, the aircraft design have to meet the safety and airworthiness requirements, therefore the aerospace designer chooses materials with due care to meet the material property behaviour, process of manufacturing and qualification.Design Structural Weight
Structural weight is an extremely sensitive issue in aircraft design. Increase in structural weight in air frame, aero engines or guided weapons have snowballing effect. An extra kg of weight may cause an extra kg of fuel burn for each flight and thus reduce range or payload, for an aero engine, over weight by one kg may cause a loss of as high as 30 hp engine power and 1 kg extra weight for an guided weapon may require additional 6- 7 kg of propellant to be burnt. Aerospace designers therefore choose material with higher specific strength (UTS/Density) and specific stiffness (Young's modulus/density).
Scatter in Material Property
Scatter in material property is unavoidable and the designers have to use the minimum of the test values while estimating the specific strength or specific stiffness. The effect of this scatter on short term property (static strength or UTS) is usually low. For example UTS of duralumin may be 40-44 kg/mm^2, the designer is required to use 40 kg/mm^2 for his analysis.
The scatter in the dynamic properties like LCF, and HCF (low cycle and high cycle fatigue) are usually high. This is reflected in the upper and lower bound of the S-N curve shown in the figure. The scatter or the width of the band is usually due to reasons which are both internal and external to the test specimen or the components. The external factors include dimensional accuracy, surface finish and presence of any stress concentration. The internal factors which give rise to the high scatter bandwidth are the presence of unwanted impurity element that provide nucleation points for initiation of crack and help in crack propagation.
Aerospace Grade Material
Aerospace metallurgists have devised methods for controlling these impurities to ppm (parts per million) level. Residual gases (O, H, N, Ar, He), non metallic S, P and metalloids impurities of Pb, Bi, Sb, As, Ag, Cu, Ti, Te are controlled to extremely low levels ( 0.3 ppm) where their deleterious effects are tolerable. These activities are normally not done for general engineering material processing. The trace elements in the general engineering materials are up to 500 ppm (0.05%). SAE Aerospace Material Specification (AMS) indicate the level of controlling the trace elements and impurities.
Bandwidth of S-N Curve
Bandwidth of S-N curve is significantly lower for Aerospace Materials compared to the General Engineering materials due to the improved metallurgical processing. Thus for the same deign fatigue life, an aerospace material can be stressed to much higher level compared to an general engineering material as shown in the figure. This gives the aerospace grade material an extra advantage to reduce the structural weights of the stressed components.
Categorisation of Parts
As per the airworthiness standard Def Stan 00 970, a part can be categorised into different grades taking cognizance of the strength and stiffness requirement of the part as shown below:
Grade A
A part shall be graded as 'A' if the deformation or failure of the part would result in one or more of the following:
- Structural Collapse
- Loss of Control
- Failure of Motive power
- Inability to operate or unintentional operation of any system or equipment essential to the safety or operational function of the aeroplane.
- Incapacitating injury to any occupant
- Unacceptable unserviceability or maintainability.
Grade B
All other parts not covered under category 'A'.
Selection of Material Based on Component Grade
For grade 'A' Components
Material conforming to aerospace grades
For grade 'B' Components
They may be manufactured by aerospace grade material or less controlled material like general engineering grades or proprietary material at the discretion of the designer.
