Why is the Japanese Sword Curved?
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WHY IS THE JAPANESE SWORD CURVED 7 WILLIAM N. WEINS*, Ph.D., P.E. and PETER BLEED**, Ph.D. * University of Nebraska, Department of Mechanical Engineering, Lincoln, Nebraska 68588-0656 ** University of Nebraska, Department of Anthropology, Lincoln, Nebraska 68588-0368 ABSTRACT The history and methodology of Japanese swordmaking are reviewed with particular reference to materials and processes. In order to gain a better understanding of the structure/property relationships in Japanese swords, a 16th century katana was analyzed metallographically and by attaching strain gages and cutting to relieve the residual stresses in the blade. The results indicate that the curve in the blade can be attributed to the volumetric expansion of the untempered martensite on the cutting edge, which also causes the edge to be in a state of residual compression. The residual compressive stresses are one of the key elements of the Japanese sword which give it superior toughness and cutting ability. INTRODUCTION Japanese swords have been described as the best edgedweapons ever made. In the popular media, these swords are ascribed fantastic cutting abilities and are presented as symbols for the heroic Samurai warriors of old Japan. The realities of Japanese swords are even more interesting than these popular perceptions. The swords of old Japan are marvels of pre-industrial technical complexity. Sword making in ancient Japan was an activity that transcended craft to be considered a fine art. Often the swordsmith himself was considered a religious practitioner engaged in a sacred activity. His work involved many precise steps and close interaction with a number of other specialists. The canons of sword design were highly refined and carefully maintained so that sword connoiseurship reached a high level. The technical excellence of Japanese swords, as documented by their ability to cut and withstand the rigors of battle, exceeds even the fables of popular literature. Swords were also at the center of a complex social and political system. Only the Samurai, members of the military ruling class of feudal Japan, could wear swords. For them, swords were a weapon, a status symbol, and an object of personal adornment. To meet these needs, a distinctive fencing style - kendo - was developed and sword fittings came to have great artistic and symbolic complexity. Given this technological and social elaboration, it is not surprising that fine Japanese swords are prized today as art objects and are sought by collectors. The technical characteristics of Japanese swords have been described in great detail by many Japanese sources as well as in a number of Western publications [1-71. There have also been a number of specific studies of the metallurgy of theso Mat. Res. Soc. Symp. Proc. Vol. 185. - 1991 Materials Research Society
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their swords [8-12], so that a great deal is known about the These studies have detailed technology and composition. technical characteristics of Japanese swords and revealed their fully technical sophistication. They have
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