AMC1 27.613 Material strength properties and design values

COMPOSITE SANDWICH PANEL

(a)     Qualification of the manufacturing process

The conditions outlined in the guidance standard AC 21-26, ‘Quality Control for the Manufacture of Composite Materials’ are considered to be relevant to composite sandwich PSE structure.

The qualification is intended to demonstrate that the combination of material, tooling, equipment, procedures, and other controls, making up the process, will produce representative parts having consistent material properties that conform to design requirements.

As part of the process qualification, destructive and non-destructive inspection (NDI) should be conducted to determine conformity to specified design requirements and check the suitability of the resulting product by assessing features such as:

—                   uniformity of the adhesive fillets between honeycomb core cell wall and skin; in particular, the process should ensure that on both faces of the honeycomb core a regularly shaped fillet (meniscus) be established;

—                   absence of ‘telegraphing’ effects and waviness on the skins of the sandwich panel;

—                   distortion of the core cells — this defect could be particularly critical for highly curved panels unless suitable precautions are taken during fabrication (e.g. core thermal performing);

—                   presence in the adhesive of unacceptable levels of porosity or humidity;

—                   disbonds between core and cells; and

—                   weak bonds.

(b)     Material strength and determination of design allowables

The strength properties of the sandwich panels should be established in order to ensure that the probability of structural failure due to material and process variability is minimised.

Because of the peculiarity of the sandwich panel construction, the material properties should be established on a specimen that is fully representative of the panel construction in terms of skin, core material and curing cycle.

Design features such as transition zones from solid laminate to core/skin should be also tested with a representative specimen for determination of strength properties.

It is expected that at least the following static allowables be established according to the statistics required in CS 27.613:

—                   Adhesive shear strength;

—                   Shear core strength (ribbon and transverse direction);

—                   Core compression strength;

—                   Flatwise strength;

—                   Flexural strength;

—                   Compressive strength; and

—                   Bearing strength (for a specimen representative of all the panel areas where fasteners are installed and subject to significant bearing stresses).

In determining the above properties, the effect due to humidity uptake, highest and lowest temperature expected in service, manufacturing defects up to limit of acceptability and allowable in-service damage defined in maintenance documents, if any, should be considered. For PSEs, impact damages should also be assessed in accordance with CS 27.573.

The validity of the engineering formula used to establish analytical design allowables should be always verified by dedicated experimental activity in order to assess the effects of the manufacturing process (e.g. curing pressure which is normally limited to the crush core strength) and environmental conditions on the allowables predicted by these formulas.

(c)      Damage tolerance and residual strength

(1)     Threat survey and damage modes

Further to good processing, and when meeting the damage tolerance and fatigue evaluation of composite rotorcraft structures requirements of CS 27.573, the applicant should clearly demonstrate that a robust structure has been produced by showing that:

—                  a thorough damage threat survey has been completed which identifies and defines all threats, including impacts, heat, moisture, etc. and the potential for interaction of these threats is addressed;

—                  all damage modes have been identified for the configuration when subject to all likely threats, paying particular attention to all likely damage modes which might not be readily detected.

For impact threats, this requires testing throughout the threat impact energy ranges up to a readily detectable damage using a range of appropriate impactor geometries, including blunt impactors up to 4 inches diameter⁽¹⁾, and a range of impactor stiffnesses, e.g. for hail threat damage (if appropriate), such that all competing damage modes can be identified. Representative boundary conditions should be used in the substantiating test campaigns; and

—                  all potentially undetectable damage modes (not only disbonds and weak bonds) have been simulated in testing (up to appropriate dimensions such that detection becomes possible, and the dimension of such damage has been quantified such that ultimate load (UL) can be maintained up to this level). The possibility of interaction between threats, e.g. impact and heat, should be considered in the simulation and substantiation process.

Note: Witness structures can be used in service, provided that a consistent and conservative correlation can be demonstrated to exist between the witness indications on the witness structure and the damage (all likely modes and extents) considered in the critical structure.

The recommendations for threat assessment and blunt impact evaluation are also addressed in AC 27.573.

⁽¹⁾ An alternative impactor diameter may be proposed by the applicant, based on the results of the damage threat survey.

(2)     Residual strength after extensive damage or degradation

The part should be sized to sustain the required residual strength, in accordance with CS 27.573(d)(4)(ii)(B), with extensive damage or degradation of the most critical skin to core bond between available arrestment features. Such damage or degradation should be readily detectable to assure damage tolerance for bond failures which experience has shown not to be extremely improbable.

It is also expected that relevant fatigue testing at specimen level, representative of a design point (e.g. fastened joint) and typical panel configuration, be performed in order to assess the effects of:

—                  material/manufacturing process variability;

—                  environmental condition;

—                  allowables manufacturing defects; and

—                  impact damages.

(d)     Instructions for continued airworthiness (ICA)

The ICA include clear instructions to inspect⁽²⁾ (and repair), both internally and externally:

—                   all load paths, e.g. up to load transfer fittings, joints, and other significant changes in stiffness and section, for damage following an overload event, e.g. impact, heavy landing, excessive gust, etc.;

—                   all structure regularly exposed to extreme temperatures, e.g. local to engine outlets for aircraft used extensively in hot climates, etc. Although inspections intervals should have been justified according to the level of detectability and residual strength capability during certification substantiation based upon a damage threat survey, experience has indicated that the potential for interaction between heat and damage can be problematic.

⁽²⁾ paying particular attention to:

—                   repaired structures; and

—                   any existing, and potentially related, ICA, e.g. existing ADs, etc.

[Amdt 27/10]