AMC 25.613 Material strength properties and material design values

1.       Purpose. This AMC sets forth an acceptable means, but not the only means, of demonstrating compliance with the provisions of CS-25 related to material strength properties and material design values.

2.       Related Certification Specifications.

CS 25.571 “Damage-tolerance and fatigue evaluation of structure”

CS 25.603 “Materials”

CS 25.613 "Material strength properties and material design values”

3.       General. CS 25.613 contains the requirements for material strength properties and material design values. Material properties used for fatigue and damage tolerance analysis are addressed by CS 25.571 and AMC 25.571(a).

When developing the material strength properties and material design values, the applicant should also consider potential anisotropies and establish all properties and design values relevant to the application of the material.

4.       Material Strength Properties and Material Design Values.

4.1.    Definitions.

Material strength properties. Material properties that define the strength related characteristics of any given material. Typical examples of material strength properties are: ultimate and yield values for compression, tension, bearing, shear, etc.

Material design values. Material strength properties that have been established based on the requirements of CS 25.613(b) or other means as defined in this AMC. These values are generally statistically determined based on enough data that when used for design, the probability of structural failure due to material variability will be minimised. Typical values for moduli can be used.

Aeroplane operating envelope. The operating limitations defined for the product under Subpart G of CS-25.

4.2.    Statistically Based Design Values. Design values required by CS 25.613(b) must be based on sufficient testing to assure a high degree of confidence in the values. In all cases, a statistical analysis of the test data must be performed.

The ‘A’ and ‘B’ properties published in the SAE ‘Metallic Materials Properties Development and Standardization (MMPDS) Handbook’ or ESDU 00932 are acceptable, as are the statistical methods specified in the applicable chapters/sections of these handbooks. Other methods of developing material design values may be acceptable to EASA.

The test specimens used for material property certification testing should be made from material produced using production processes. Test specimen design, test methods, and testing should:

(i)      conform to universally accepted standards such as those of the American Society for Testing Materials (ASTM), European Aerospace Series Standards (EN), International Standard Organisation (ISO), or other national standards acceptable to the Agency, or:

(ii)     conform to those detailed in the applicable chapters/sections of the SAE MMPDS Handbook, Composite Materials Handbook 17 (CMH-17), ESDU 00932 or other accepted equivalent material data handbooks, or:

(iii)     be accomplished in accordance with an approved test plan which includes definition of test specimens and test methods. This provision would be used, for example, when the material design values are to be based on tests that include effects of specific geometry and design features as well as material.

EASA may approve the use of other material test data after review of test specimen design, test methods, and test procedures that were used to generate the data.

The use of some materials and processes may allow the applicant to design parts for which the material strength and other properties are produced during production or repair. Consequently, the use of simple material test coupons (as typically produced, independent of the part) at the base of a typical test pyramid (e.g. as defined in AMC 20‑29 for ‘composite structures’) may not be representative of the material strength and other properties of the final part. When a higher test pyramid is required, then the applicant may need to reduce (for practical reasons) the number of specimens below what is normally expected for generating statistically significant values, e.g. as those associated with A and B basis data (as defined in the MMPDS Handbook). Therefore, other mitigating measures are likely necessary (e.g. coupon testing of prolongations, testing of coupons from sections of production parts, other sampling strategies, more intensive non-destructive inspection (NDIs), etc.). Until industry establishes standards for such situations, the applicant should agree with EASA whether and how to use test articles of a higher test pyramid, as well as associated small datasets, to generate material and design data. In that agreement, EASA may give credit to the applicant for applicable established practices.

4.3.    Consideration of Environmental Conditions. The material strength properties of a number of materials, such as non-metallic composites and adhesives, can be significantly affected by temperature as well as moisture absorption. For these materials, the effects of temperature and moisture should be accounted for in the determination and use of material design values. This determination should include the extremes of conditions encountered within the aeroplane operating envelope. For example, the maximum temperature of a control surface may include effects of direct and reflected solar radiation, convection and radiation from a black runway surface and the maximum ambient temperature. Environmental conditions other than those mentioned may also have significant effects on material design values for some materials and should be considered.

4.4.    Use of Higher Design Values Based on Premium Selection. Design values greater than those determined under CS 25.613(b) may be used if a premium selection process is employed in accordance with CS 25.613(e). In that process, individual specimens are tested to determine the actual strength properties of each part to be installed on the aircraft to assure that the strength will not be less than that used for design.

The applicant should have data available to understand if a material is anisotropic and should account for this condition during testing.

If premium selection is to be used, the test procedures and acceptance criteria must be specified on the design drawing.

4.5.    Other Material Design Values. Previously used material design values, with consideration of the source, service experience and application, may be approved by the Agency on a case by case basis (e.g. "S" values of "The Metallic Materials Properties Development and Standardization (MMPDS) handbook" or ESDU 00932).

4.6.    Material Specifications and Processes. Materials should be produced using production specifications and processes accepted by the Agency.]

[Amdt 25/1]

[Amdt 25/27]