What is the building block approach for structural evaluation

It is generally recognised that numerical analysis is supplemental to design of composite structures. However, it should always be used in conjunction with a rigorous test programme. The approach taken for compiling a test programme from materials selection to structural validation is termed the "building-block approach". The advantage of this approach is it allows experimental identification of the multiplicity of failure modes that composite structures may experience. It is also used to identify knock down or enhancement factors for environmental conditions when testing full-scale structures in a hostile environment is not feasible. Other building block tests allow guidance on the truncation of fatigue spectra and the variability inherent with fatigue testing. The building block approach is summarised in Mil-Hdbk-17 as:

1. Generate material preliminary design allowables.
2. Based on the design/analysis of the structure, select critical areas for subsequent test verification.
3. Determine the most strength-critical failure mode for each design feature.
4. Select the test environment that will produce the strength-critical failure mode. Special attention should be given to matrix-sensitive failure modes (such as compression, out-of-plane shear, and bondlines) and potential "hot-spots" caused by out-of-plane loads or stiffness tailored designs.
5. Design and test a series of test specimens, each one of which simulates a single selected failure mode and loading condition; compare to analytical predictions, and adjust analysis models or design allowables as necessary.
6. Design and conduct increasingly more complicated tests that evaluate more complicated loading situations with the possibility of failure from several potential failure modes. Compare to analytical predictions and adjust analysis models as necessary. Design (including compensation factors) and conduct, as required, full-scale component static and fatigue testing for final validation of internal loads and structural integrity. Compare to analysis.

The five levels of tests are constituent, lamina, laminate, structural element, and structural subcomponent tests.

Constituent Testing:
This evaluates the individual properties of fibres, fibre forms, matrix materials, and fibre-matrix pre-forms. Key properties include fibre and matrix density, and fibre tensile strength and tensile modulus.

Lamina Testing:
This evaluates the properties of the fibre and matrix together in the composite material form. Key properties include fibre areal weight, matrix content, void content, cured ply thickness, lamina tensile strengths and moduli, lamina compressive strengths and moduli, and lamina shear strengths and moduli.

Laminate Testing:
Laminate testing characterises the response of the composite material in a given laminate design such as quasi isotropic. Key properties include tensile strengths and moduli, compressive strengths and moduli, shear strengths and moduli, interlaminar fracture toughness, and fatigue resistance.

Structural Element Testing:
This evaluates the ability of the material to tolerate common laminate discontinuities. Key properties for the aerospace industry, include open and filled hole tensile strengths, open and filled hole compressive strengths, compression after impact strength, and joint bearing and bearing bypass strengths.

Structural Subcomponent or Full-scale Testing:
This testing evaluates the behavior and failure mode of increasingly more complex structural assemblies which are structure and application dependent.