AUTOMOTIVE DESIGN COMBATING
FATIGUE FAILURE
Patrik Ölund explains how clean steel offers new design possibilities for longer-lasting, high- performance components
Clean steel can increase component performance and reduce size and weight
A
utomotive component designers are on a constant search for materials that are lighter, stronger and capable of handling ever greater and more complex loads. Tey particularly value high mechanical strength. Yet although this is important, it is actually metal fatigue that accounts for the majority of premature failures in service. To combat this, Ovako has focused on developing clean steels in which the small inclusions that give rise to fatigue failure are closely controlled. Fatigue failure occurs when a component fails after a number of repeated loadings, even when the peak load is well below the material’s ultimate tensile stress (UTS). Te fatigue performance of a material is determined by carrying out laboratory tests, usually in the form of rotating bending fatigue (RBF) testing. Te number of cycles to failure at
varying levels of stress is recorded – the statistical nature of fatigue requires a number of tests to be carried out. Eventually, a “safe load” or “fatigue load
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limit” is established’ at which the sample will survive without failure beyond a certain number of cycles (typically 3–10 million).
INCLUSIONS INFLUENCE FATIGUE LIFE Extensive, long-term investigation of the fatigue performance of steels has identified clearly that defects such as non-metallic inclusions can initiate fatigue failures. Tis is because inclusions, typically larger sulphides in conventional steels and smaller oxides in cleaner steels, act as localised stress raisers. Tat means they cause the nominal load on a component to multiply beyond its safe fatigue limit. Tis results in cracks that propagate under cyclic loading, eventually resulting in failure. A clean steel that contains smaller sized defects will therefore provide a longer fatigue life than conventional steel. Fig. 1 illustrates the strong relationship
between defect size and fatigue strength. When larger defects are present a lower stress will cause fatigue failure.
Conventional steels often have significant amounts of defects from
60 to 80 µm or even larger, with a resulting fatigue strength of around 400 MPa. Yet in a clean steel, where inclusions are limited to less than 20
µm, the fatigue strength increases to over 600 MPa and even higher.
Te most important difference
between conventional steel and clean steel is the probability of detrimental defects occurring in the area of a component subjected to high load, where fatigue failure could initiate. Using a clean steel decreases this probability significantly. Tere is also much less scatter in the test results. So, using a clean steel reduces the need to use a high safety factor to take account of the potential variability in performance.
NEW OPPORTUNITIES Te major design opportunities offered by steel cleanness is important for
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