Oxidation, corrosion, fatigue … The debates of the Evry court on Friday took on the appearance of a very technical but essential course to establish responsibilities for the fatal derailment of a train in 2013 at Brétigny-sur-Orge station ( Essone).
“We would like to understand the difference between oxidized, tired and corroded when we talk about bolts,” said the president of the chamber, Cécile Louis-Loyant, to a legal expert. “Is it visible to the naked eye? Does this state affect the strength of the material?”
On July 12, 2013, the Paris-Limoges accident caused 7 deaths and more than 400 victims – physically and/or psychologically injured.
In question: the reversal of a splint, a large staple joining two rails, which pivoted around one of the four bolts. How ? When did the other three bolts break? This temporality, a major issue in the trial, is the subject of sometimes bitter exchanges between the SNCF defense and the lawyers for the civil parties.
The investigating magistrates, who called on experts for seven years of investigation, support the hypothesis of a slow process of degradation of the track apparatus, in particular of the bolts showing corrosion and fatigue.
On this basis, they asked to judge the SNCF (criminal heiress of SNCF infra, in charge of maintenance), the SNCF Réseau track manager (formerly Réseau ferré de France) and a former railway executive, for “involuntary homicides” and ” unintentional injury”.
The SNCF vigorously disputes this reasoning and has commissioned other specialists, who support the hypothesis of a sudden disassembly of the coupler, due to a defect in the steel. With bolts that would have yielded very shortly before the tragedy.
– “Alarm signal” –
After several days of battles of expertise, the president asked to redefine three states observed on certain bolts in question: oxidation, corrosion and fatigue.
The oxidation of a bolt is “visible to the eye”, identifiable in particular by its “orange color”, explains Laurent Régnier, specialist at the Cetim laboratory.
“It can be cleaned with a rag, by hand”, details the expert, but the part does not find a “new condition” because, if there has been oxidation, it is that the material “begins to react” . And this oxidation does not affect the strength properties of a bolt.
Oxidation can appear within “three, four days”, as “after a few hours”, finally notes Mr. Régnier, “it all depends on the environment”.
“And corrosion, is it also visible to the naked eye?”, continues the president. Yes, in particular by “the deposits of brown color”. It can be eliminated with “an appropriate chemical solution or a particular mechanical action such as abrasive paper”, answers Laurent Régnier.
Thursday, the expert had also compared this “corrosion to a disease”. “You have a material that oxidizes at first, which takes on an orange color in two, three days. If you do nothing, it will increase (…) you will eat the material. We will measure loss of thickness on the material”.
“What is a tired room?” Then asks the president.
“Fatigue is a mode of ruin” which corresponds to “moderate stress”, explains Laurent Régnier. A bolt does not “break 100% in fatigue”: “a shock” causes “the final sudden break” of the bolt.
This fatigue, “not visible to the naked eye”, was noted through microscopic examinations, which revealed “small stop lines”. It’s “like a tree that you cut down: you have the possibility of telling its age according to the lines”.
“The railway worker can see if a part is oxidized if it is orange; he can see if it is corroded if it is brown; but he cannot see if it is tired”, summarizes the president, who believes that “the only way avoiding fatigue is prevention”.
A bolt that breaks on a splint, which has four, is “an alarm signal”, considers another expert required by justice, Pierre Henquenet, whose conclusions are disputed by the SNCF.
Following the debates on Monday, they will focus on maintenance.