The strength of slate

In contrast to most rocks, slate is a very anisotropic rock, i.e. the mechanical properties highly depend on the orientation to the cleavage plane. Due to its most common usage as roofing material, the bending strength also known as flexural strength or modulus of rupture is considered as the most important parameter.

In case of flooring the hardness or abrasion resistance should be determined. Especially for mining shear strength, deformation modulus and cohesion are important to calculate the stability and therefore the safety of a mine.

The bending strength, also known as flexural strength or modulus of rupture is considered to be the most important strength parameter of roofing slates and is the only strength parameter which is required in the EN 12326 and ASTM C 120.
The bending strength is generally important for projecting elements in constructions or elements like architraves. On the roof and on the façade the slate is exposed to the wind which can deflect the slate shingles.
According to the experiences, the average bending strength of a slate lies between 50-80 MPa. Compared to other rock types, roofing slates have the highest bending strength.

The Young's modulus and Poisson's ratio are the two elastic constants which describe the elastic behaviour of a rock. The Young's modulus E is also known as modulus of elasticity and one can distinguish between the dynamic and static Young's modulus.
The dynamic Young's modulus is generally but not necessarily slightly higher than the static Young's modulus.
The Deformation modulus comprises elastic and plastic deformation. If slate is mined in a room-pillar system the larger the rooms become due to mining the higher will be the load on the pillars, leading to a plastic deformation.
The Poisson's ratio ν is the second elastic constant: The compression of the sample parallel to the applied stress is accompanied by an extension perpendicular to the stress. This is known as Poisson expansion and can be expressed as fraction or in percent. For isotropic linear-elastic materials the Young's Modulus E has the following relation to the shear modulus G and bulk modulus K:

E = 2 G x (1 + ν)

E = 3 K x (1 - 2ν)

The shear modulus is the ratio shear stress - shear strain while the bulk modulus describes the compressibility of a material.

The compressive strength is important, for example, for load bearing elements like columns or pillars in mines. The parameter describes the complete loss of cohesion of the rock along the plane of failure. To obtain compressive strength values simple uniaxial test can be carried out.

Values for the tensile strength of slate are rarely found in published literature. The determination of the tensile strength is mostly carried out with the Brazilian test. Here one measures values and calculates with an inappropriate formula because it is based on the Hooke's law which is valid for isotropic materials. But this is a common test and the values are "sufficiently correct" to work with.