'Stress corrosion' the cause of Series 800 cracks?

‘Stress corrosion’ could be the cause

Industry insiders told Modern Railways that while investigations into the cause of the cracks on Hitachi trains continue, initial thoughts are that they are a form of stress corrosion cracking – a combination of stress and a chemical reaction. Substantial samples were cut from No 800026 and more severely damaged No 802205 and taken for metallurgical analysis by The Welding Institute and in Japan, while No 800013 was used as a testbed for a full repair programme. Nos 800013/026 were two of the four trains involved in the earlier issue where cracks were discovered around the yaw damper bracket bolsters during April.

The design authority in Japan responsible for the Class 80x product provided the train operators and Office of Rail and Road with a better understanding of the role of the lifting pockets and what risks may or may not follow should one become completely detached. This situation, whilst possible, is thought to be extremely unlikely given the equipment is secured at several points. Initial conclusions by engineers were that the small number of yaw damper-related cracks found in late April could be related to the weight of the diesel engines on those vehicles, but that the more numerous jacking point cracks are ‘definitely not related to vehicle weights’. It is noted that vehicles fitted with diesel engines have a different yaw damper setup to those without.

Although the Hitachi fleets were withdrawn en masse, there was an emerging view within the industry that that the discovery of further cracks in any significant numbers is unlikely. One senior insider told Modern Railways: ‘I don't think we will find any more cracks because we've been through the whole fleet now with a very thorough examination. The view of the engineers is that the cracks we were finding had been there a while because they weren’t showing any fresh metal. It appears that these cracks don’t develop “overnight” and whilst there might be one or two that possibly creep over the line, we're not expecting to find that many more units have developed cracks suddenly.’ Another source added that the cracks had not become visible on painted vehicles as the new paint was sufficiently flexible to remain intact, hiding the cracks beneath.


Repairs will be complex

There is widespread acceptance that an extensive repair programme will now be needed, covering all Hitachi’s Class 80x and Class 385 fleets and taking many months. Modern Railways understands that although the cracks aren’t in a weld, they will be repaired with a weld.

An insider explained ‘Although we think that Hitachi can demonstrate that the vehicles can run, it is clear that they will, in the medium to longer term, have to be repaired. There are a number of options being looked at for the long-term solution. One is whether a weld would solve the problem to everyone’s satisfaction. Hitachi might have to look at a slightly different assembly and look at whether they bolt it on and not weld it. The other option is whether to just cut that piece of the vehicle out and weld something new in.’ Engineers are also considering whether the fact that there is a connection via the bracket between the anti-roll bar, yaw damper and the lifting bracket might mean that a replacement should be designed in a different way.

When a plan is agreed, all electrical equipment must be disconnected to avoid damage during welding. Past experience with other fleets suggests it is more efficient to modify all vehicles in a set at the same time rather than take individual vehicles out of service. Early industry estimates are that it could take until the end of 2022 to complete repairs to all affected vehicles.

The repair process is complicated further by the fact it will require dedicated facilities with appropriately trained engineers. As one insider commented: ‘There are a limited number of people who have the right skills for welding railway vehicles and there are a limited number of places where you can take the vehicles, support them in the right place and do the welding itself’. The aluminium alloy used on the trains means the work must be overseen by The Welding Institute to ensure it is completed correctly. The work by Hitachi is being independently validated by consultancy Ricardo to ensure full compliance. At the time of writing, no definitive cause had been identified by Hitachi, nor any fix proved.