Pirelli's investigation into its tyre blowouts at the Belgian Grand Prix has pinpointed debris as the main cause after it found 63 cuts in all the tyres it used over the race weekend.
Sebastian Vettel launched an angry tirade at Pirelli after experiencing a tyre failure on his car while running third, which the Italian company initially put down to wear from Ferrari's optimistic one-stop strategy. It occurred just two days after Nico Rosberg experienced a high-speed tyre failure during practice that saw his car pitched into a spin on the entry to the high-speed Pouhon corner.
After launching an extensive investigation into the failures, Pirelli found 63 cuts in the tyres it analysed after the race, compared to an average of 1.2 cuts per circuit in the previous 15 events (including race and tests). The FIA revealed that the remaining tread thickness on Vettel's rear tyres was approximately 30% at the time of the failure, making it more susceptible to damage from debris. On one of the other tyres Pirelli examined post-race, the cut had reached the belt of the tyre but had not resulted in a failure.
In reaction to the incidents, Pirelli has proposed a study in conjunction with the FIA into the way in which circuits are cleaned. It says the FIA reacted to fears of debris cuts on the tyres in Belgium by cleaning the track after qualifying, but still experienced cuts in the race.
The main conclusions presented to the FIA were:
Tyres used by different teams during the race were examined by cutting sections through the tyre, none showed any signs of internal delamination (hence fatigue).
Tyres used by different teams during the race were run on a test rig and were subjected to a representative race duty cycle, again none showed any signs of internal delamination (hence fatigue).
A highly unusual number of cuts were reported by Pirelli engineers during the Spa weekend, one of which reached the belt and caused tyre failure on Nico Rosberg's car.
Microscopic examination was conducted on sections taken through some of the cuts found in tyres used by different teams in the race, the results showed one other case where the cut had reached the belt but without causing failure.
The remaining thread thickness on Sebastian Vettel's rear tyres was approximately 30% at the time of the failure, making the tyre more susceptible to damage from even small pieces of debris.
Defending the structural integrity of its tyres, Pirelli said: "The tests carried out by Pirelli on the tyres used at Spa have confirmed the absence of any structural problems. Pirelli has undertaken in-depth analysis on the materials and production processes used, utilising two different methods of tests and checks.
"Since the start of 2015, 13,748 slick tyres have been used: including on especially severe tracks like Sepang, Barcelona and Silverstone. No problems have ever been discovered, underlining the fundamental solidity of the product."
As a result of the findings, the FIA says it is "willing to consider any safety recommendations made by the tyre supplier for the Italian GP and for the rest of the season."
Pirelli went on to give a more detailed explanation of how the cuts caused the failures on the cars of Vettel and Rosberg.
"If even a small piece of debris - made of carbon or any other particularly sharp material - penetrates and cuts the various structural parts of a tyre (which is obviously subject to high-speed use, and more susceptible if used for a prolonged period) without penetrating the actual structure, this can cause a failure that is different to that found in the event of a normal puncture, which is characterised by a loss of tyre pressure. And the former was the type of event seen on Sebastian Vettel's tyre at Spa.
"As for Nico Rosberg, in whose case the tyre usage was less, the tyre held up - as the footage clearly shows - and the failure was not instantaneous. For four corners previously, an element of the internal structure of the tyre was visible, coming out of the tread pattern. This highlighted the existence of the damage and the consequent start of the tyre's attrition.
"Throughout the Spa weekend (including practice, qualifying and the race) cuts caused by debris were found on the tyres of other drivers, which damaged the construction but did not cause any failures."