Failure mode of passenger and light truck tires
|Date Published||September 3, 2007|
Radial tire disablement can happen because of numerous causes. One failure mode is the separation between the steel belts. This separation usually starts at the edge and undergoes propagation circumferentially and radially. The portion of the belts still attached together eventually is not enough to counteract the centripetal force. The tread and second belt are then detached from the first belt. This failure mode can happen with or without rapid air loss.
When the detachment occurs, numerous internal rubber compounds and reinforcing materials are exposed and visible. It is common to find regular patterns at interfaces between compounds. These patterns suggest some marks left either by the liners used during the manufacturing process to handle the uncured compounds or some die imprints. These imprints might be interpreted that the compounds are unstuck and could lead to false conclusion on the cause of the disablement.
A paper published at the 2004 International Tire Exhibition and Conference by G. Bolden gives examples of marks found in brands of tires2.
This experimental study describes tests which were carried out on a light truck tire and a passenger tire built with a nylon cap ply.
The process marks were composed of liner marks impressed manually on the compounds during the tire assembly before the curing process in order to give a crisp appearance. The surface with radial striations generated by die marks were used as is. After exposure to various conditions, the tires were stripped at predefined interfaces.
Process marks occurring during the building of a light truck and a passenger tire stay after cure and are revealed when layers are stripped and interfaces are exposed.
Various conditionings show that the longer the time spent inflated at 65°C the crisper the process marks become without ever reaching the appearance of an unstuck surface (for example deliberately induced by silicone).
The observation by electronic scanning microscopy reveals a very different aspect of the surface between an unstuck surface by silicone and a peeled surface after conditioning.
No significant difference was found between conditioning of the tires inflated with air and with nitrogen. The effect of oxygen can be ruled out and the evolution of the properties at the interface is therefore induced by the time spent at 65°C. Even interfaces between two layers of the same compounds (between the second and third belt and between the first and second belt) reveal the original process marks.
Because of the asymmetry of the adjacent components, the presence of these process marks is sensitive to the direction of peeling and to the shape and thickness of the adjacent compounds.
Location of the liner marks
Liner marks and striations between compounds of light truck tire with three belts.
Two batches of tires, Michelin LT 225/75 R16 LRE (115/112R) LTX M/S Tubeless, were built with imprints of liners on different compounds. This line of tire has a belt package comprised of three belts. The outer (third belt) and middle belt (second belt) have the same angle and the inner (first belt) has opposite angle. A gum strip is laid on the edges of the three belts and between the first and second belt.
The first batch of tires had imprints of liner (Figs. 3 and 4) on both sides of the gum strip covering the edge of the three belts (Fig. 5) and on both sides of the third belt (Fig. 6). The second batch of tires had imprints of liner on both sides of the first belt, second belt and gum strip at the edges of the two steel belts.
Fig. 1 illustrates the position of the liner marks during the building of the tires.
The tread was extruded with a process which left radial striations on the surface which is placed in contact with the belts when the tire is built (Fig. 7). This surface was in contact with the third belt and the edge strips covering the edge of the three belts. These striations were present both in and between the areas where the liner marks were imprinted.
Liner marks and striations between compounds of a passenger tire with two steel belts and a nylon cap ply.
The Fig. 2 illustrates the placement of the liner imprints. They were placed such that they would appear at the interfaces between the first and second belt and between the second belt and the nylon cap ply. They were also placed on the gum strip.
Obtaining the liner marks
The liner marks were artificially produced by pressing the liner on both sides of the compounds between the plates of a hydraulic press without distortion of the belt (~100 Kg/centimeters2).
Conditioning of the tires
It is well known that the time spent at varying temperatures causes the properties of most of the compounds in a tire to evolve.
This is proven by the numerous studies carried out in the past years and published in the literature. This paper does not intend to find conditions which would duplicate the field but only to find the parameters which would act on the studied phenomena. The parameters were chosen as follows:
Inflation Pressure: 80 Psi for LT 225/75 R16 and 44 Psi for passenger P225/60 R16
Inflation Gas: air and nitrogen
(See Table I for details)
To investigate the effect of oxygen on the presence of process marks, the tires were inflated respectively with air and nitrogen.
Stripping of the Michelin LT 225/75 R16 LRE (115/112R) LTX M/S Tubeless
The tires were stripped between the tread and the third belt in the center of the tread and on each shoulder in the same direction. Only the interfaces which are in contact with the tread are relevant. Fig. 8 illustrates the principle of the peeling process.
Center of the tread
The center of the tread shows distinctly the radial striations. It also exhibits a pattern linked to the presence of the grooves and blocks of the tread. It is not directly associated with one or the other but nevertheless shows some periodicity.
The surface area with striations increases with the time of conditioning. (Figs. 9-14)
The shoulders exhibit a very strong asymmetry when peeled circumferentially in the same direction. When the shoulder is peeled in the same direction as the cut edges of the wires a tear occurs along the extremities of the belt edge. If the direction of peeling is opposite to the cut edges of the wires the peeling occurs at the interface. This becomes more pronounced when the time of the conditioning increases (Figs. 15 and 16).
As it can be seen in Fig. 1, the liner marks were localized radially on a width of about 2 inches. Their presence was directly linked to the path of the peeling and to the dissymmetry of tear that was explained above. No liner marks were observed when the rubber was torn but they appeared when the peeling follows the interface.
Tread and shoulder stripping
Center tread: The stripping was carried out between the tread and the third belt. The liner marks appeared almost across the full width of the tread. There was no observable difference between the tires inflated with air or with nitrogen. Three examples of conditioning time are shown in the Figs. 17-19.
Shoulder: The presence of liner marks and the striations are linked to the direction of peeling. Liner marks were observed on the surface of the gum strip placed over the edge of the three belts, when the direction of the peeling reveals the interface. (Figs. 20-21)
Stripping between steel belts
Belt2/Belt3: The two belts have the same angle and do not show any specific characteristic interface. The liner marks appear with no defined pattern.
Belt1/Belt2: The two belts have opposite angle and the pattern of peeling shows a very distinct circumferential symmetry of tear. This pattern has been described in past papers1. The center is a transition zone which width varies depending on the direction of peeling. The liner marks are therefore more clearly visible in one direction than the other (Figs. 22-23).
The Michelin P225/60 R16 97H Energy MXV4 plus Green X tires were stripped between the second belt and the nylon cap ply and between the first and second belts. Unlike the LTX it was not possible to strip between the tread and the nylon cap ply and on the shoulders most likely because of the difference in the thickness and the properties of the compound.
Some striations were revealed on the shoulder between the tread and the nylon cap ply. Only for the tires conditioned at 56 and 336 days. The striations were revealed only by the prolongation at the interface Tread/nylon cap ply during the stripping between the second belt and the nylon cap ply. This interface for the other conditioning times was not revealed. (Figs. 24-25)
The liner marks were not observed on the new tire. They started to appear at 56 days conditioning. It might have appeared early but the tires were not conditioned for 28 days. The interfaces between the nylon cap ply and second belt and the interface between first and second belt were the most visible and are revealed almost systematically in each condition. (See Figs. 26-29). The liner marks on the gum strips were not clearly visible.
Synthesis of observations
Tables II and III regroup the observations for all the conditions. They were tagged by ``yes'' or ``no'' if striations and liner marks were observed. The columns with N/A relate to interfaces where nothing was expected to be observed. Some observations could not be done because of the break of the compound during the stripping.
The visibility of the process marks is linked to the ability of the tear to be directed at the interface. The tire is made of materials of widely different rigidity and shapes along with the viscoelastic properties of the compounds, do not allow the tear to be guided precisely at the interface. Consequently the process marks appear irregularly and are sensitive to the direction of peeling.
Microscopic observations of the interfaces
In one of the tires the surface of the liner marks and the striations were coated with silicone grease which left the area unstuck. A sample of this surface was then compared with an electron scanning microscope to one of the peeled tire previously conditioned at 56 days. (Figs. 30-31)
The interfaces of both striations and liner marks reveal a very different appearance. As expected the interfaces of the striations and liner marks exhibit clear micro tear. When silicone is introduced the interfaces with striations and liner marks are smooth. (Figs. 32-35).
These observations suggest that the peeling induces micro tear very close to the interface and follows the general irregular contours of the surface left by the striations and the liner marks.
1. Daws, John W., ``Fractography of tire tread separation'' Paper 67 ACS Rubber Division, April 2003.
2. Bolden, Gary, ``Component interfacial tearing appearances'' ITEC 2004, Paper 51 Standard Testing Laboratory.
TECHNICAL NOTEBOOK Edited by Harold Herzlich
* * *
One failure mode of passenger and light truck tire disablement is the separation between the steel belts followed by the tread/exterior belt detachment. This failure mode can happen with or without rapid air loss.
When the detachment occurs, numerous internal rubber compounds and reinforcing materials are exposed and visible. It is common to find regular patterns at interfaces between compounds. These patterns suggest some marks left either by the liners used during the manufacturing process to handle the uncured compounds, or some die imprints. These imprints might be interpreted that the compounds are unstuck and could lead to false conclusion on the cause of the disablement.
It is shown that the process marks manually imprinted before curing are revealed during the stripping of the tire after cure and after conditioning. Conditionings show that oxygen has no noticeable effect on the appearance of the process marks and that the longer the inflated tires spent at 65°C the crisper they appear. As they are revealed when the tear follows the interface, their appearance is sensitive to the direction of peeling. The observation by electronic scanning microscopy of the surface shows a thin layer of micro tears which follows the contour of the process marks.
* * *
Jean-Claude Brico is a consultant at Forensic Tire Expertise. He worked for Michelin for 38 years in several positions, including research and development of rubber compounds, quality assurance of manufacturing and tire verification. During the last 13 years, he directed the Forensic activities of field tires in North America.
He has a master's degree in chemistry and is a member of Society of Automotive Engineers, ACS Rubber Division and the Tire Society. He can be reached by email at FTE@charter.net.