‘Patching’ is emerging as a growing trend within car body structural engineering

It’s nothing new, but it offers useful solutions for engineering in relation to better crash performance without the weight increases and complication of additional reinforcements of high grade materials, whilst adding challenges for the joining process.

Patching is the process of reinforcing a panel with another material in specific areas. By reinforcing just a specific area, rather than a complete panel, only the main panel needs joining. Take the B–post for example; typically this is made up of the B–post outer, the reinforcement, and the inner. Joining these three into the roof and, into the sill, can be complicated enough. And, with the likes of Honda choosing new processes to join a stack of four panels including ultra-high strength steel, incorporating another panel to support the B–post structure would add more complication. So, adding a ‘patch’ behind a specific section of the inner, or the reinforcement, can make a stronger panel with predictable deformation in an impact.


How, and where, these patches are applied varies between models. The current Mercedes C Class (W205) has a small patch in the footwell, linking and reinforcing the bulkhead, A-post and floor, to better enable the car to achieve the required performance in the Insurance Institute for Highway Safety (IIHS) small overlap crash test. This crash test is a huge challenge for many vehicle manufacturers, although Mercedes has mastered it with a strategy of deflection, and this simple addition.

Subaru applies a patch into the upper A–post of the new WRX. This involves the new process of spot welding the patch to the reinforcement panel, before they are pressed into shape. Volvo too has implemented patches in the new XC90, creating ‘soft zones’ behind the rigid boron steels to enable controlled deformation of the structure in an impact.


In many ways this is the trend driver. With the requirement to lose vehicle weight to meet emissions targets, the percentage of advanced materials is increasing, with the Audi TT having just 5.5% mild steel content, and the C Class just 12%. With higher content of stronger, less ductile materials, ‘engineering in’ controlled deformation to absorb impact forces is getting harder, so these softer materials can enable these relatively brittle materials to deform, rather than fracture.

With this, repairers need to start looking out for non-expanding pads within the sill and B–posts. These are not foam that hasn’t expanded, but specific materials to prevent the panel fracturing and failing. In no circumstances should they be replaced with foams.

We know that the development of composite patches is ongoing. Rather than seeing a carbon fibre panel, we may soon see a steel or aluminium panel reinforced by a composite section, bonded in key areas. This could be carbon fibre reinforced, glass fibre reinforced, or even supported by a woven fibre hollow core. However, all of these will present a new set of challenges for us in repair.


The issue for the repair industry is the patching process, be that composite or with conventional materials, which may well lead to reductions in the scope for sectioning a panel. In many cases only a complete panel replacement may be possible and, if intrusive requiring adjoining panels to be removed to access the joint, it could result in more total losses. Another challenge for repairability, and one of many we face.

The repair technology centre at Thatcham is constantly looking for solutions to these emerging and established trends, so that insurers, and repairers, can repair cars. Starting next month, I’ll begin showing more specific examples of some of the work the team here does, to benefit the industry with efficient and safe repair data.

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