Solving tyre design flaw
By Dieter Disselbeck
In the area of road safety, since decades, the subject ‘tyre defects,’ and ‘tyre blow-out’ has been in spotlight in the media, for example as in Formula 1 accidents. Various breakdown statistics show that tyre damage stands besides electronic faults at the top of the list. In most cases these damages are caused by flaws in the design of tyres
Chairman Christopher Hart of the National Transportation Safety Board (NTSB) told
ABC News on October 27th, 2015: “When you see 33,000 accidents a year, in relation to defect tyres, we know we have a serious tyre problem and a good piece of that relates to recalls.” Tyre failures impose worldwide enormous human, social and economic cost on society.
This is not only a calamity, but also an engineering disaster.
The public definition of a “serious tyre problem” by NTSB calls for a solution and to remember the Code of Ethics of Engineers referenced in the ASME Constitution: “Engineers shall hold paramount the safety, health and welfare of the public in the performance of their professional duties.”
Safety is down to us all and demands all our commitment. All the more we are required to grasp arising possibilities to improve tyre quality. New materials, new findings and test results show promise for solutions.
The tyre industry has done a lot for the steady, on-going development of their products, but there is still a central flaw – namely weak and unreliable material adhesion. This design flaw is identified by the tyre manufacturers themselves through the tyre recalls.
For example Consumer Affairs reported on 10/17/2016: “Continental Tyre the Americas is recalling 14,567 Crosscontact LX20 tyres, size P275/55R20 111S, manufactured May 3, 2015, to May 9,2015. The tyres, sold as replacement tyres as well as original equipment on certain General Motors full size trucks and SUVs, may have insufficient adhesion within the belt package, resulting in tread wear, vibration, noise, or bulging areas on the tyre. The insufficient adhesion can cause partial or full tread/belt loss, increasing the risk of a crash.”
Since the introduction of radial tyres beginning the 1970’s approx. 45 million tyres have been recalled in the US, mostly due to poor adhesion causing delamination and subsequently catastrophic failures with the danger of fatal accidents. Compared to 5,176,852 tyres recalled in the years 2004 to October 27, 2015 there is no change of the situation over the decades.
Furthermore “the recall system is completely broken”, because less than 20% of the recalled tyres were physically sent back to the manufacturers (NTSB). Millions of officially dangerous products remain on the roads – in the US as well as other countries of the world, where US recalled tyres are also sold.
The tyre industry is always emphasising the importance of safety, but the fact is the industry did not acquire and does not have the ability up to now of making strong and durable connections between the plies, that will deliver long term product reliability and manufacturing robustness.
It is therefore still a most pressing goal, to use the know-how and experience of other industries like aircraft or chemical industry, to perfect the existing design of radial tyres mainly in view of suppressing delamination / separations, as the most common mode of failure.
Delamination is a serious subject of concern in engineering application of composite materials, because of the resulting degradation of structural integrity. The phenomenon arises because fibers lying in the plane of a ply do not provide reinforcement through the thickness and so the composite strength relies on the relatively weak matrix or adhesive layer to carry loads in z-direction. To mitigate the risk of failure, composite structures are reinforced.
The main toughening methodes are: Knitting, 3D-weaving, tufting and z-pinning or
conventional mechanical fasteners.
The result of application: High mechanical stability and the dispersion of the dynamic forces via the adhesive joints and the mechanical fasteners. The combination of adhesive bonding and mechanical fastening is an effective way for a fail-safe design and allows for example to manufacture primary aircraft composite structures.
In the area of road safety there is, since decades, again and again, the subject of “tyre defects, tyre blow-out” a spotlight in the media. It has been happening frequently in Formula 1 races. Various breakdown statistics (TÜV, ADAC truck service, Truck 24) show that tyre damage stands besides electronic faults at the top of the list. According to the figures of federal accidents statistics, tyres are also the number 1 technical cause of accidents with personal injury or death.
This situation shows that a successful tyre operation, especially with trucks and SUVs, is difficult and that there is a need for product improvement. The former Hoechst AG, manufacturer of fibers/tyre cord, polymers, resins etc., was also engaged in developing solutions for their customers. The tyre problems as stability, weight, performance were similar to those, when developing aircraft composite structures or automotive sandwich-structures as cross-industry project. The key for solution was an interlocking the joining system.
When working on optimising tyre stability this know-how led to a proposed solution, in that the critical adhesive bonding of the actual design is secured by interlocking mechanical fastening. Based on this 3D-armouring network structure prototypes of the innovative “Net-tyre” were developed.
The main features of the innovation are: The net-tyre has a 3D-network structure as reinforcement within the tread. The structure follows the contour of the tread. The network consists of a new textile fabric with hexagonal open cells, permeated by rubber. The functional part of the rubber is interlocked with the structural part of 3D reinforcement and prevents risks of tread separation and tyre blow-out.
The network encases the tyre and provides cohesion and stability of the tyre body. In the vulcanisation process the fabric will be pressed by the tyre mould into the rubber, thereby the meshes of the fabric will open up, the rubber will penetrate the network creating an interlocking joining system. The network increases the energy absorption capacity, puncture resistance, damaging force, damaging deformation. Additionally the network structure offers the additional safety protection against tread loss, especially with retreaded tyres.
Due to the chemical bonding in actual tyre building, the choice of matching materials is restricted. Utilising mechanical fastening and anchoring the tread profiles, the new design allows greater flexibility in material selection and combination. This paves the way for optimising the performance properties of tyres, for example weight.
The elastic 3D-structure ensures functional safety for the cohesion of the tyre components and is barrier to cracks, crack propagation and other damage (e.g. breaks).
The feasibility of the network design has been successfully demonstrated by manufacturing and testing prototypes carried out by major tyre manufacturers. The prototypes were manufactured in-line on existing production facilities. Tests proved significantly better structural durability and operational reliability.
An elastic fiber network is an important component of dynamic biological structures that functionally require elasticity and resilience. Very distinctive fibernets are to be found, for example in the lung, aorta and the connective skin tissue. The system can absorb the acting forces, which arise by expansion and compression. It works like a shock-absorber. A characteristic feature is an elongation of 100 % to 150 %.
Such a fiber system is out of elastin, a very extensible fiber and collagen, a fiber with an enormous tensile strength, but little elongation. The prototyping of the Net-tyre was also carried out with an extensible fabric based on a combination of a high tenacity and an elastomeric fiber, developed by Hoechst AG.
The fabric with an areal weight of 270 g/m2 had an elongation of 133.9 % in longitudinal and 138.8 % in transversal direction. It is fascinating to see, how the Net-tyre prototype meets and emulates nature in design, in technological datas and in function. In nature’s organisms and in the tyre innovation a reinforcing elastic network, encasing the structure, effectively constrains the overstraining of the critical adhesive bonds and limits the deformation behaviour. It controls the dynamic forces and keeps the structure together.
Given the nature of the situation it would be of gross negligence not to note the teaching of nature for necessary modifications of tyre design. To clear the path for changes we have to reconsider firmly held opinions and conflicts of interest. It is opinion in tyre industry that only very low extensible reinforcement should be built into the layers. This prejudice of experts is wrong and do not lead to reaching the goal regarding the existing problems, nature‘s solution and test results of Net-tyre prototype. Why not check an elastic reinforcing network and a hybrid-fiber system that prevents overstraining?
For decades tyre industry has relied on adhesion bonding. In other industries it is, however, a widely known fact, that bonded joints often fail instantaneously and unexpected. Because of dynamic stress and abrupt load changes, tyres are subject to extreme loads, and adhesive bonding is not the optimum joining system for tyre manufacturing. Standards to ensure safety are not developed. Why not check a mechanical fastening system to make the adhesion joints safe?
* This article is based on the author’s presentation on the subject at the Tire Technology Conference in Hanover in February 2017. Disselbeck was Head of New Technologies and was formerly with Hoechst AG