When investing in custom shade sails, it is vital to understand how and why shadecloth behaves as it does when tensioned in two directions at the same time, which is what occurs when a shade sail is installed. At Polyfab, we ensure that the engineering and design of the elastic property of our custom shade sails are of the highest quality.

Fabric stress and strain response:

For elastic material, there is a linear relationship between stress and strain, with the slope of the line termed Elastic Modulus. A material is said to be ‘stiff’ if the line is steep, meaning the Elastic Modulus is high.

If a fabric is stretched in one direction only, it will extend in the direction of the load, and contract in the perpendicular direction. The applied load is termed stress, and the fabric deformation is termed strain. For an anisotropic material like shadecloth, if the extension in the less ‘stiff’ direction is high, then the contraction will still occur in the other, even if an equal force is applied in that direction. 

If the fabric is stretched in both directions simultaneously (termed biaxial loading), there will be a positive strain in the direction of the applied loads, and a negative strain on the opposite direction.

Biaxial testing:

The Biaxial Test results display the net effect of a force that causes elongation in the direction it is applied offset by the contraction induced by an equivalent force applied in the perpendicular direction. Under Biaxal loading, the stiffness in the weft direction is heightened, relative to the uniaxial case, due to the contraction triggered by a high elongation in the warp. However, in the warp direction the stiffness remains essentially the same as that in the uniaxial case. This is because there is so little elongation in the weft direction, that the contraction caused by it in the opposite warp direction is negligible.

All Polyfab shade cloth behave in this way, thus, under biaxial loading are very much stiffer in the weft direction than the warp. The results also display that the elongation response to be applied loading is also non-linear and shows pronounced hysteresis during, and permanent plastic deformation after each cycle; however, the response starts to approximate linearity and to display greater stiffness after the first cycle. These results can be used to derive the Elastic Parameters for the shade cloth material.

It is important to state that biaxial test results are not coherent with the theories of linear elasticity in Hooke’s law, which is the basic premise behind the relationship between stress and strain in finite element CAD models. Mechanical conditioning would partly remove the worst eccentricities of the fabric’s behaviour but is impracticable to execute this on site.

Polyfab’s shade cloth material is very ‘forgiving’, and much improvisation in tensioning occurs on site during installation as further compensation for the practical complexities of establishing the finished fabric profile.

Why does this matter?

Although the above is technical, it is important in understanding why Polyfab is the leader in this space. Our shade cloths are designed to be durable by reducing the strain across the cloth. This ensures both safety and longevity of our products.

Polyfab are recognised as one of the premium providers of safe, robust fabrics as we go the extra mile to help customers make informed decisions. Our open and transparent approach will help give your customers the peace of mind they need to invest in commercial shade sails with confidence.  For further information on custom shade sails, contact Polyfab via our contact form.