A technical investigation into sword fractures
Swords - and why they break - were the subject of a detailed technical investigation by former engineering student Richard Tasker, summarised here. Richard's interest stems from dad Brian, of Kent’s White Star Sword Dancers, and his investigation formed part of his final year studies.
After about two years’ regular use, blades can suddenly fracture. The steel is brittle, so that little of the energy stored in the highly stressed blade is absorbed by fracture, leading to violent failure.
The project looked at the cause of failure and suggested possible modifications to increase blade life, make failure more predictable and be more economical. A sample blade material was very strong 1% carbon steel (tensile strength = 1633 MN/m2), one of a family of water-quenchable alloys named W1 tool steel.
Manufacture is believed to involve hot rolling, quenching then tempering at 320°C, to give a microstructure of tempered martensite, with high hardness and strength but low ductability. Poor toughness causes low energy absorption on final failure.
Blades most commonly fail at the steel lap joint near the swivel, where stress is concentrated. Characteristics include a large number of crack nucleation points and a widespread final fracture zone.
Another failure point is around a third of the way along the blade, where nominal stress is low but grinding against adjacent swords damages the blade edge. A sample showed a large “glassy” area with growth marks radiating some way from the edge before final failure.
Analysis showed that, although swords most often break at the swivel joint, nominal stress is at its lowest here, with stress concentration around 1.53. In the mid-section, a defect of 21 μm deep could be tolerated without initiating fatigue; once begun, it could grow as long as 5.84 mm before failure.
The blades' surface showed residual tensile stress, probably due to the quenching process. Surface treatment to induce residual surface compression could significantly improve blade life. The best way to achieve this would be to shot peen the blade before assembly. Alternatively, a hypo-eutectoid steel blade with a nitrided case would give a tougher core and strong, hard case with residual compressive stress, absorbing more energy on failure.
Replacing steel with composite construction could show benefits. E-Glass fibres set within polyester (both inexpensive materials) would produce a very strong blade but with lower stress on flexing and less fatigue. The danger from breakage would reduce as, on fracture, fibre absorbs most of the stored energy in the flexed blade.
But would it sound right? Ed.