The addition of a polyamide thermal break in an aluminium window system or an aluminium façade is now commonplace among specialist window manufacturers due to their proven ability to improve the thermal performance of the building to which they are later fitted. Polyamide isolation strips need to be carefully installed within the internal section of an aluminium profile in order to achieve the best insulation results possible and leading profile manufacturers achieve this by a process of near simultaneous knurling and crimping. Without highly-engineered thermal break rolling machines being utilised in the production process of an aluminium profile, it is possible for the shear strength of aluminium windows to be compromised, however. Even when machines for insulating aluminium profiles are used, it is preferable to test the shear strength of the finished product. Not only will this allow engineers to fully understand the design limits of the framing materials they are working with - in terms of the weight of glazing they can hold - but it will also offer a greater certainty over the longevity the windows themselves, given the respective shear strengths of the various products that might be on offer.
"Glass ﬁber-reinforced polyamide composite is an increasingly important engineering material due to its high level of mechanical performance and temperature resistance." - Aziz Hassan (et al) in the Journal of Reinforced Plastics and Composites.
Tensile Strength Aluminium Window Methodologies
The shear strength aluminium windows can achieve is often much greater than those outside of the fenestration industry imagine is possible. Moreover, there is some doubt among some building designers over the possibility of manufacturing aluminium profiles with the rigidity required for larger spans of glazing if polyamide isolation strips are also included. Perhaps this is because with older technology, polyamide thermal breaks were sometimes simply hammered into their aluminium housing, rather than being more gently rolled into a knurled groove, thus causing the profile to be weakened. There again, some thermoplastics used for creating thermal breaks are well-known to trap water which can undermine the tensile strength of the frames into which they are fitted. Despite the use of modern water-resistant plastics and thermal break rolling machines today, such perceptions continue to exist. Thankfully, two principle methods for testing the shear strength aluminium windows have achieved are in common usage. The first is DIN EN 14024, a set of rules of procedure laid down by the DIN Standards Committee. The second is NF252, a particular certification of French standard system that handles the quality of both industrial and consumer products.
DIN EN 14024
Along with inter-laminar shear strength tests, under EN 14130, in-plane shear modulus by plate twisting, under EN 15310, and double V-notched shear testing approaches, under ASTM D5379, among others, DIN EN 14024 is the most commonly-used standard for testing aluminium windows with thermal insulation. In some scientific studies, a range of factors – from the orientation and reinforcement efficiency of polyamide inserts to their fibre-related properties – are also considered. However, DIN EN 14024 is now an industry standard in much of Europe and, under chapter 5.4, deals with issue of shear strength. Under this section of the standard, compressive stress is measured in such a way that any shift in a profile can be plotted against the force applied to it. The formula for doing so is f = 5/384* (1- 5/8 ß2 + 16/25 ß4) * (qo * l4) / (E * Iid). In later chapters of the standard, issues such as measuring the deformation of aluminium profiles using the after ageing method are specified.
Under the French NF system, the control, the quality and the safety of products are all considered to be of a high enough standard prior to any product receiving this prestigious marque. Despite the need to fulfil three areas of the standard, in terms of shear strength, the level of rigidity is often greater than is found in other testing areas, certainly when compared in terms of Newtons per millimetre. Under NF252, a list of complying products is freely available to download.