VOL. 10 July ISSUE YEAR 2009
in Vol. 10 - July Issue - Year 2009
Reflection on Metal Fatigue
In eleventh century Europe every soldier used to arm himself with a sword. One of the important criteria for a sword was that it should not break. This meant that the sword would have to be heavy to withstand the heavy impact. However the problem was heavier the sword the more energy it would require to maneuver and would mostly slow down an individual. The solution was found by the blacksmiths of Toledo who came out with swords which were lighter and would also not break. They had developed blades that were thin and lightweight and had beautiful proportions. They also could take and hold a keen edge. But most important, they had a characteristic unmatched by those from other sword makers: Toledo swords were so tough that they could be bent almost double, over and over, without breaking. That made them virtually indestructible in battle.
No other sword makers knew how to do this. The process was a closely guarded secret. Toledo swords became famous in medieval times and have remained so ever since. (In 1663 the poet Samuel Butler wrote in Hudibras of "The trenchant blade, Toledo trusty.").
Over the ages the secret of their manufacture was not revealed until the 1970s, when X-ray diffraction detected an induced compressive stress layer on the surface. A group of engineers familiar with the history of weapons unraveled the secret to the Toledo blacksmiths’ technique to the world. They examined a Toledo sword using modern techniques. X-ray diffraction & detected an induced compressive stress layer at the surface. Acid etching removed extremely thin layers, beneath which electron microscope surveys showed subsurface compression patterns that would have been made with a ball-peen hammer. The Toledo blades had not been peened with shot since steel shots were not available those days, but they had been peened, and that was the secret to their strength and durability.
Now this very secret of Toledo swords is being extensively used in the automotive and aviation industry to combat Metal failure because of Fatigue. The concept of metal fatigue was not completely unknown in the 1800s by the scientific and engineering communities, it was a concept that was neither thoroughly observed nor investigated until the twentieth century. Metal fatigue is the term used to describe the weakened condition of metal parts used in machinery, aircraft, vehicles, etc. after extensive continuous use.
The need for information on metal fatigue became increasingly important as aviation technology advanced. The metal parts of early aircraft sustained heavy damages--such as snapped-off wings and shattered propellers--now attributable to metal fatigue.
According to Shawn M. Kelly the concept of fatigue is very simple, when a motion is repeated; the object that is doing the work becomes weak. For example, when you run, your leg and other muscles of your body become weak, not always to the point where you can’t move them anymore, but there is a noticeable decrease in quality output. This same principle is seen in materials. Fatigue occurs when a material is subject to alternating stresses, over a long period of time. Examples of where Fatigue may occur are: springs, turbine blades, airplane wings, bridges and bones.
In materials science, fatigue is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. The maximum stress values are less than the ultimate tensile stress limit, and may be below the yield stress limit of the material.
Factors That Affect Fatigue-Life
• Cyclic stress state. Depending on the complexity of the geometry and the loading, one or more properties of the stress state need to be considered, such as stress amplitude, mean stress, biaxial, in-phase or out-of-phase shear stress, and load sequence.
• Geometry. Notches and variation in cross section throughout a part lead to stress concentrations where fatigue cracks initiate.
• Surface quality. Surface roughness causes microscopic stress concentrations that lower the fatigue strength.
• Material Type. Fatigue life, as well as the behavior during cyclic loading, varies widely for different materials: E.g. composites and polymers differ markedly from metals.
• Residual stresses. Welding, cutting, casting, and other manufacturing processes involving heat or deformation can produce high levels of tensile residual stress, which decreases the fatigue strength
• Size and distribution of internal defects. Casting defects such as gas porosity, non-metallic inclusions and shrinkage voids can significantly reduce fatigue strength.
• Direction of loading. For non-isotropic materials, fatigue strength depends on the direction of the principal stress.
• Grain size. For most metals, smaller grains yield longer fatigue lives, however, the presence of surface defects or scratches will have a greater influence than in a coarse grained alloy.
• Environment. Environmental conditions can cause erosion, corrosion, or gas-phase embrittlement, which all affect fatigue life. Corrosion fatigue is a problem encountered in many aggressive environments.
• Temperature. Higher temperatures generally decrease fatigue strength.
Infamous Fatigue Failures
Versailles Train Crash
On May 8, 1842 one of the trains carrying revelers on their return from Versailles to Paris, having witnessed the celebrations of the birthday of Louis Philippe, derailed and caught fire. Though the resulting conflagration mutilated the dead beyond recognition or enumeration, it is estimated that 53 perished and around 40 were seriously injured.
The derailment had been the result of a broken locomotive axle. Rankine‘s investigation of broken axles in Britain highlighted the importance of stress concentration, and the mechanism of crack growth with repeated loading.
De Havilland Comet
Metal fatigue became apparent to aircraft engineers in 1954 after three de Havilland Comet passenger jets had broken up in mid-air and crashed within a single year. Investigators from the Royal Aircraft Establishment at Farnborough in England told a public enquiry that the sharp corners around the plane’s window openings (actually the forward ADF antenna window in the roof) acted as initiation sites for cracks. The skin of the aircraft was also too thin, and cracks from manufacturing stresses were present at the corners. All aircraft windows were immediately redesigned with rounded corners.
• The Liberty Ships during World War II
• The 1980 capsize of the oil platform Alexander Kielland
• United Airlines Flight 232, Japan Airlines Flight 123, China Airlines Flight 611, Los Angeles Airways Flight 417, and El Al Flight 1862
• The 1957 plane crash of the "Mt. Pinatubo", presidential plane of Philippine President Ramon Magsaysay who died in the crash along with 24 others.
• The 1919 Boston Molasses Disaster has been attributed to a fatigue failure
• The 1998 Eschede train disaster (crash of an Intercity Express train).
• The 2005 crash of Chalk‘s Ocean Airways Flight 101