Building Block Testing

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 multiple failure modes that composite structures may experience. The building block approach is used to identify knock down or enhancement factors for environmental conditions when testing full-scale structures in a hostile environment is not feasible.

MERL has the required equipment and technical know-how to conduct tests at all levels up to the sub-element level for aerospace structures. MERL can also supply consultancy advice on the structural tests often carried out in the client’s own laboratories.

MERL staff have also worked in developing new types of building block tests specific to the application. The emphasis on this approach is to impart realistic service loads and environments on to the sub-structure or test coupon using innovative fixturing and equipment such as MERL’s multi-axial test machine.

For composite materials, 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 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 behaviour and failure mode of increasingly more complex structural assemblies which are structure and application dependent.