Case Study: Failure Analysis for the Rail Industry

Our failure analysis and testing services have been helping companies for over 70 years to understand why a spring failed and what steps can be taken to improve its life.
Springs are often critical components and are found in a huge number of products across many sectors including medical, oil and gas, aerospace, automotive and rail. Until a spring has failed the true impact is never fully understood in how vital a role these components play. Understanding springs is our business and as an impartial association and not-for-profit organisation we are at the heart of the spring manufacturing industry dedicated to supporting businesses worldwide.

We’ve built a close relationship with the rail sector over the years, mainly driven by the high volume of springs needed. Guided by this need, rail industry professionals are always looking to find ways to improve a springs performance, find out why a failure occurred and what preventative measures can be taken.

The train industry, like most transport sectors, has safety critical springs where failure could lead to property damage, injury and, in the most serious cases, death. Thanks to redundancies and safety procedures the failure of one spring is unlikely to cause any damage or injury. However, it can have a huge financial impact with rolling stock being taken off track for maintenance. Therefore, spring failure not only needs to be understood by determining the mechanism of failure and the root cause of that failure, but recommendations need to be made to reduce the risk of failure occurring again.

We have performed several failure investigations for the rail sector, mainly focusing on nested springs within the suspension system in the bogie. An example of one such failure will be summarised here to highlight the techniques used in our investigations.

Figure 1 shows a typical failure we received. The compression spring was the outer spring of a nested configuration with a failure located roughly one coil from the end coil, located at the bottom of the suspension set up.

Figure 1 shows that the protective coating had been removed from several areas, especially the coil to coil position close to the failure, and there was evidence of rust throughout the spring. Figure 2 shows one of the fracture faces. This shows that rust was present not only where the coating had been removed but also on the fracture face, making it difficult to identify the root cause of failure. Propagation marks from a fatigue failure are visible through the corrosion on the surface, emanating from the coil to coil position.

Indicating the failure occurred from damage seen in this position. It would be easy to say the spring failed due to fatigue, initiating from damage at the coil to coil position, but that is not the whole story. SEM, EDS and microstructural analysis are also implemented to determine if there was anything else that could have led to the failure. Further examination showed that the damage seen close to the initiation was most likely post failure damage as it was only present on one of the fracture faces. SEM imaging (Figure 3) highlighted a small surface defect at the apex of the initiation Indicating that the failure might not have been due to damage or corrosion but a material defect. Micrographs of the material showed that there were larger than expected surface defects and large carbides had formed close to the surface.

Due to the number of issues we found with the spring it was not clear exactly what had initiated the failure: corrosion pit, damage, microstructural abnormalities or surface defect. However, in our experience, the failure most likely initiated at the surface defect shown in Figure 3, was accelerated by corrosion and the damage to the bar was caused post-failure. Based on this analysis, recommendations were suggested to not only reduce the possibility of future failures of this nature but also to improve the spring to limit reoccurrence of any of the issues seen.

Carbide formation and poor surface quality indicate that either the raw material was not up to standard or the processing parameters used during hot coiling were not optimised. We recommended spot checking the material as it entered the facility and were able to recommend processing variables to reduce the probability of hot coiling causing defects in the material. A non-destructive check of the final product was also suggested to reduce the chances of poor-quality springs reaching the market.

Although the damage at the fracture face was determined to have been cause post failure there was wear and damage seen throughout the coil to coil position. If the spring had not failed due to a surface defect this wear would have probably caused premature failure. The damage seen in this region was not as significant as the matching region at the other end of the spring. This indicated that the orientation of the spring within the seats also might not be optimised and should be examined further to reduce damage to the surface and the coating on the spring.

There are several facilities worldwide that can perform failure analysis on mechanical components, but very few have over 7 decades of experience in spring design, manufacture and failure that we have. We truly understand every process of a spring, from design concept through to wire choice, standards, production methods, final use and how it performs in situ. This knowledge means we can identify the source of a failure and suggest improvements, giving the customer the performance from a spring while reducing the likelihood of future failures.

We continue to work closely with the rail industry and support their needs as the demand on the rail network grows. Please get in touch today for confidential and independent failure investigation services.