FEATURE SPRINGS & SHOCK ABSORBERS
Within the springs industry, valve springs are considered some of the top performers. Typically manufactured from material with the tightest tolerances, highest strength and
with minimal inclusions, their design is optimised in many ways. But despite being exposed to some of the most advanced processes available to improve life, this does not make them immune to failure. In this article I will go through one of the rare instances where a precision engine valve spring failed prematurely. The question is why? Dr. Conor McCaughey, Metallurgist at the Institute of Spring Technology, comments
UNDER INVESTIGATION A
valve spring for use in a high performance combustion engine
was supplied to the IST after it failed. Manufactured from super clean silicon chrome vanadium wire, this had also been exposed to a variety of advanced post processing techniques including dual shot peening to improve the fatigue life and relaxation. This spring design had been in service for years without any premature failures and with no new changes to the design or manufacture. So why did it fail?
THE EXAMINATION The failure occurred at a single point in the centre of the spring. Initial examination of the fracture face indicated that it failed from the inside coil position, the region of highest stress. However, there was no evidence of damage, defects or corrosion pits on the surface of the wire. Examination of the rest of the spring highlighted some areas of wear and damage, but nothing beyond what would be expected of a spring in service. Higher magnification images taken of the fracture face (Figure 1, below) indicated that the failure did not initiate from the surface of the wire but slightly below it. It seemed probable that the spring failed via fatigue from a sub-surface point close to the inside coil position, but it was unclear what the root cause of failure was. So, samples were taken and the
microstructure observed, defects measured, and the hardness recorded.
spring are located on the surface of the wire at the inside coil position. If this inclusion had been present on the surface it would have either been detected by the wire manufacturer or removed during the peening process. Being subsurface, it remained undetected and was in a region where crack initiation was likely. The spring was shot peened to remove
Figure 2 Under a microscope, it clearly shows a small (roughly 30µm across) indent at the centre of the propagation region close to the surface of the wire
Figure 1: Higher magnification images taken of the fracture face indicated that the failure did not initiate from the surface of the wire but slightly below it
However, none of the results obtained shed light on why the spring had failed. The breakthrough came when the
fracture faces were imaged using a scanning electron microscope. Figure 2, above, shows the initiation region from one of the fracture faces. It clearly shows a small – roughly 30µm across – indent at the centre of the propagation region close to the surface of the wire. This was exactly where the optical images indicated the initiation would be. A similar indent was seen on the other fracture face. This led to the conclusion that the spring failed via fatigue, initiated at an inclusion. However, the spring had been
manufactured from super clean material so how could an inclusion have caused failure? The answer is, as good as wire manufacturing is, and as clean as the material is, it is impossible to remove all inclusions. It is even stated on most, if not all, super clean material that an occasional inclusion will be present in the material in excess of 30µm. This might be well known in the spring and wire industries, but it doesn’t seem to be as widely known by end users. However, what was particularly
unfortunate is not only that the inclusion was present, but its location. It is known that the highest stresses in a compression
10 JULY/AUGUST 2020 | DESIGN SOLUTIONS
surface defects and impart beneficial residual stresses at the surface. The beneficial residual stresses at the surface, however, hide a dirty little secret: they are balanced out by detrimental residual stresses subsurface. Whilst the beneficial residual stresses reduce the peak stress, the detrimental ones act in the same direction as the applied stresses, leading to localised increased stress. As fatigue life is dependent on the stress applied this will lower the life of the spring if there is a defect present, around which a fatigue crack can initiate, e.g. an inclusion.
‘INFINITE LIFE’: USE CAUTION This should not dissuade anyone from shot peening springs. In the vast majority of springs, peening will increase the fatigue life. It is only in a tiny proportion of cases where an inclusion is present in just the wrong area that this will become a catastrophic problem. As the surface quality of the wire gets
better and better, you can end up in a situation where the spring will never fail from a surface defect but initiations caused by subsurface inclusions or microstructural deflects. These can sometimes be smaller and way beneath the region discussed here affected by shot peening, at a relatively low stress, and cause failure after a huge number of cycles (hundreds of millions or even billions). Although this might be considered by most to be equivalent to infinite life it is not. Manufacturers and customers should therefore use extreme caution when quoting ‘infinite life’.
Institute of Spring Technology
www.ist.org.uk
/ DESIGNSOLUTIONS
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