The failure of a component can have serious consequences and cause safety concerns along with unnecessary costs associated with downtime. Product recalls are also a real possibility just like Subaru’s recall back in November 2018. Understanding why a component fails and knowing how to put clear processes in place helps to minimise the risk of this happening.
The main aims of our failure investigation are not just to determine the mechanism and root cause of failure but to suggest improvements that could be made to mitigate the risk of it occurring again. The most common failure mechanism for a spring is due to fatigue. Spring fatigue is due to repeated operation between two (or more) loads, causing a varying stress on the wire. Although these stresses will be below the ultimate tensile strength of the material, they can still cause failure as the repetitive action can cause a crack initiation which then quickly propagates under the influence of its repeated use.
However, identification of the failure mechanism is rarely enough information to mitigate the failure occurring again; the root cause of the failure needs to be identified. A major contributing factor to the life of the spring is the surface quality of the wire, particularly the presence of a defect that may serve as the initiation and catalyst for the failure. Surface defects, such as drawing, coiling marks or microstructural defects, can act as stress concentration points, raising the local stress and helping in the initiation of fatigue failures. Identification of these defects which lead to failure allow for future prevention. Corrosion and wear can also lead to damage to the surface of the wire creating an initiation point for cracks. Therefore, it is important not only to determine the failure mechanism but the root cause of failure to prevent re-occurrence.
The age old saying goes ‘prevention is better than the cure’ and here at the Institute of Spring Technology (IST) we advise on how to identify the failure risks, so the life of the spring can be improved. Shot peening is a tried and tested method of accelerating a stream of particles (shot) at the component’s surface, putting the spring under residual compressive stress, increasing its resistance to fatigue crack formation. Another way of preventing failure is to specify a higher strength material or an improved surface quality so the life of the spring can be optimised. Finally, for springs that are too small to shot peen, an optimised stress relief heat treatment can be applied to improve the life of springs for high duty applications. Using shaved or super clean raw material and electro polishing can be advantageous too.
Failure can also be caused by corrosion, stress corrosion cracking, hydrogen embrittlement, liquid metal embrittlement, overloading the spring, or too much relaxation of the spring and general wear and cracking of the spring. With decades of experience in analysing springs failures and an on-site laboratory, IST will be able to identify the exact root of the failure so robust solutions can be implemented.
Our test house offers a comprehensive range of specialist mechanical tests for all types of springs and non-spring components. These vital facilities enable manufacturers, engineers and end users from across the world to understand how a component failed and what improvements can be made to make them safer and last longer.