Seth Putterman
University of California, Los Angeles
A physicist links magnetism, force and fatigue.
If a metal bar is repeatedly stretched and released it becomes fatigued and, eventually, ruptures. The latter can occur suddenly and unexpectedly: sometimes materials scientists can find no obvious thermodynamic hint that a steel rod is about to break. I am interested in fatigue because it parallels other phenomena that concentrate energy density, such as triboluminescence, whereby diffuse stress makes a crystal glow.
In both triboluminescence and fatigue, applied forces cause molecular rearrangements. But fatigue also involves nanometre-sized defects that accumulate during the useful life of a piece of metal and organize themselves into a soft spot. Recently, Sidney Guralnick and his colleagues at the Illinois Institute of Technology in Chicago measured how much work is needed to complete each 'stretch and release' cycle in rods of AISI 1018 steel, a common low-carbon steel that is used in vehicle parts such as gears (S. A. Guralnick et al. J. Phys. D Appl. Phys. 41, 115006; 2008). This allowed them to follow changes in the material's response to force as it fatigued.
A shift occurred at merely 12.3% of the time to rupture. What is happening inside the steel at this point is mysterious, but the number holds true even when the useful life of identically manufactured rods varies by a factor of 200.
Further clues will no doubt come from steel's piezomagnetism — the fact that its magnetism varies with the degree of stretch it experiences. This relationship is complex: even when the metal is so slightly strained that it goes back to its original shape on release, its magnetic field does not return to the pre-stretched state. One day investigations into this property may uncover the organizing principle of the nanometre-sized defects that underlie metal rupture.
Comments
This is an old game and if you do a real literature search over the past 70 years there will be a number of references, my guess one could easily find 100 references on this topic (lots of masters thesis). The correlations with EM response and superficial hardness because of the rapid saturation of properties early in the fatigue cycle due to mechanical annealing of the structure is well known. Also the alloy 1018 is not one commonly used for gearing unless you want to study rapid wear and ductile failure. Metallurgically being able to identify a percentage of life due to fatigue, is only useful economically in critical components where loss of life is involved if failure occurs to justify the measurement. However, the liabilities the measurer incurs are usually prohibitive which has limited applications in the past both for fatigue and creep measurements. The real interest in this problem is explaining the EM response from a complex ferromagnetic material like a 1018 steel as the substructure develops. There is no simple model analogous to conductivity that gives a workable constitutive equation for ferro magnetism. The ferromagnetic response is not easily incorporated into Maxwell's equations because of the collective nature of the quantum statistics required to describe ferro magnetism. The net result of this ignorance is that multi physics modeling software that is commonly applied to these types of problems has to be directly tested against experiment to judge the errors, typically in such applications errors easily exceed 100%.
Posted by: j. wallace | July 30, 2008 07:26 PM