Feedback from repairers, insurers and vehicle manufacturers say that the inclusion of stiffer, higher grade materials and more efficient impact load paths are leading to damage which is being underestimated.
For quite some time, BMW were pressing home the message that “carbon fibre doesn’t bend” to get the automotive industry to understand the different behaviours of a CFRP (carbon fibre reinforced plastics) structure and the huge amounts of energy it can absorb and transmit. Of course, we know carbon fibre can and does bend when formed to do so, but we need to start thinking in different ways. Engineering of car bodies has reached the critical point whereby it’s immensely challenging, both from an engineering perspective, to include more of the higher grade steels. The Volvo XC90, awarded top SUV for 2015 by Australia’s Best Cars, is probably the epitome of the limit for boron steel inclusion, with almost the complete occupant cell being shrouded in two layers. To further increase boron inclusion would require applications in the front and rear structures, which just isn’t possible as these need to reform progressively and that leaves Volvo to consider more aluminium, or even composites.
Mixed material car bodies are real and here now. The Mercedes S Class, with 52 material types in 36 thicknesses joined by 17 different joining processes was probably the flagship for this, but the new BMW 7 series “carbon core” body features 10 material types if you ignore the four different CFRP technologies and count these as one. If you include vehicles such as the forthcoming Toyota Prius 4, which features steel, aluminium and composites, then you can see how fast mixed material has trickled down from the S Class to mainstream and higher volume vehicles.
Now we are also well into the era of mixed-materials panels with the BMW 7 series and Jaguar XF both featuring hybrid B pillars, just two examples of this with many more emerging as OEMs sell these solutions “off the shelf” to vehicle manufacturers. So, a panel of two (or more) materials is something we have to get our heads around now. VW has even investigated applying hybrid panels to the MQB platform structure, which could, if implemented lead to literally millions of these panels being built into vehicles every year, which offsets the material cost.
Let’s be clear, the car makers don’t like engineering this complexity as it’s risky, difficult and expensive, but they have no choice. “We can do no more with steel,” was the statement from one senior development engineer from a vehicle manufacturer. So, it’s inevitable that as they have to face up to it, so does the repair community. Without simplifying it too much, perhaps rather than seeing how far the impact forces have progressed into a damaged car, we now need to consider where and how the impact forces have dispersed.
Press-hardened Hinge-Pillar and side rocker structures are becoming the norm as, whilst an expensive material choice, it enables a comparatively simpler structure to be engineered around this skeleton. These are not tasked with deforming, but instead to absorb and disperse the energy through rigid intersections with the food members and floor members, and perhaps into rigid castings that form the basis of the drivetrain mounting and chassis rails. This is equally and perhaps even more effective for small vehicles with straight impact absorbing chassis legs and sill structures transmitting much greater energies than ever before through stiff nodes to other profiles made of equally high grade materials. In this way, the small family car of today can absorb impact loads that would have been unimaginable 10 to 15 years ago.
So, we have a number of challenges;
*How far and where have the impact forces been transmitted to?
*Have the profiles involved in absorbing and transmitting the force deformed?
l For hybrid (mixed materials) panels, how have both materials been affected and have either exceeded their limits?
*Have the non-deforming materials (aluminum casting or CFRP or CFRP/steel components been overloaded and fractured?
Solutions and strategies for some of these are emerging. The Lamborghini strategy of ultrasonic diagnosis of CFRP structures, courtesy of their “flying doctor” service, has now been extended to Audi for the R8 coupé. For this to be effective, a programmed software package is required – specific panels, or even sections of the panels – so that the ultrasound can make sense of what it is seeing. This is a positive step towards being able to safely diagnose delamination or cracks, either on the opposite surface or even inside the composite material, and of course can detect cracks that are too small for the inspecting engineer to be able to visually detect. This can become a clear process providing an output for clear evidence to the insurance engineer of the structural state at the time of testing.
Of course, like so many other skills, this needs to be learned and ultrasound can take some getting used to. It remains to be seen whether all applications will require specific software to accurately diagnose the damage, but if it does, then this means having to refer every applicable vehicle to a facility with the correct equipment and software. If there is no cross-brand or “generic” applications, then this also means significant investment by the body shops, and even the franchised main dealers may baulk at yet more mandatory equipment.
BMW has developed a simpler solution for the 7 series. This involves a template panel that fits the CFRP lined Press-Hardened steel rocker reinforcement panel that has a uniform profile across its length, enabling a finite limit to acceptable deformation.
BMW is also one of those looking at visible inspection locations on the vehicle structure where fibre-reinforced plastics, including CFRP, can be visually inspected for damage. This in itself may lead to changes in how we think of vehicle damage assessment. A complete and accurate first-time assessment may no longer be possible as partial trim strip to expose these key points and inspection may be required in many cases, but re-assessment in itself could also be driven by the complex requirements of ADAS diagnosis and reinstatement.
Assessing the “conventional” parts of the structure may become a more demanding task as well. The alignment tolerances of a modern car body are so much finer than before as the body is so much more rigid, so measuring distortion to diagnose and reinstate body alignment will probably also require 3D mapping electronic technology for fast and precise measurement. Those that hold “manufacturer approved” or franchise status may have to invest in chassis jigs and brand-specific measuring, but this is again not a viable cross-industry solution. What’s more, given the further transmission of impact forces, where we measure will in all likelihood change. Thousands of vehicles with aluminium castings are reaching the Australian roads every year, and this number is increasing. These are not likely to deform and so will transmit the impact further than perhaps anticipated.
So, what is the solution? To progress successfully as an industry, a number of things need to happen:
*Identify the main elements (materials, nodes, construction and joining processes) that cause structural behaviour that is difficult to diagnose in these new structures. Perhaps a particular combination is more prone to be difficult for diagnosis than another, and perhaps another is more prone to failure/fracturing of composites or hybrid panels.
*Evaluate and validate the various diagnostic procedures and tools available to do this and ensure these are practical to use in the automotive repair environment. We need to clearly establish which diagnostic procedures and tools available to do this and ensure these are practical to use in the automotive repair environment. We also need to clearly establish which diagnostic processes can be relied upon to detect delamination of the opposite surface or internal defects of composites including carbon fibre. We should also consider that, at some point, we may wish to validate the joining in repair of composites. Much of the current composite expertise comes from the aviation industry and their volumes are lower and individual aircraft and repair costs are obviously massive, so much of that thinking needs to be adapted for us. But neither should we forget steel nor aluminium as we need to ensure we can efficiently diagnose damage to all structures – not just those that are composite inclusive.
*Creation of standards, training, repair data and competencies that are transparent and that protect the consumer, the insurer, the corporate vehicle owner and the smash repairer, so that all we know beyond doubt the structural integrity of each vehicle. This needs to take into account differing skill levels and to be applicable across a vast range of materials and construction strategies.
This, I believe, can be achieved, but we need to understand that we are on the crest of a wave of new body engineering strategies and materials, so what we see today with mixed-material bodies will not necessarily reflect what we see in 10 or even five years’ time; the pace of material engineering really is that fast. So, in some ways it’s like trying to hit a moving target, but it does develop our ability to cope with the core technologies until the pace of advancement settles.
Thatcham Research is probably best placed of all to lead on this. We have the close links with the relevant car makers and OEM’s who are at the lead of this fast-paced technical development, and we are geographically close to many leading academics who are enabling some of these advances. But perhaps most important is our many, many years’ experience of damage assessment and repair, our daily engagement with insurers and repairers and our research and validation testing capability. The fact that so many parties have approached us for collaboration on this already suggests there is much concern across the automotive industry as we’re all moving into unchartered water with car body structures – and it’s only through collaboration that we will manage the changes.