Structural Characterization of Ancient Japanese Swords from MAAS Using Neutron Strain Scanning Measurements

The current paper presents a forensic study that aims to characterize non-invasively the laminated structure of a set of Samurai’s swords, part of the East Asian Collection of the Museum of Applied Arts and Sciences (MAAS) in Sydney, via strain scanning measurements. Neutron residual stress analyses were undertaken on samples of well-known origin, time period and authorship to create a reference database on the main lamination methods developed by Japanese swordsmiths. The benchmark data were cross-matched with results obtained from a mumei (no-signature) blade in order to attribute its manufacturing tradition based on qualitative and quantitative data rather than stylistic criteria. Maps of two stress components and d0-values in the transverse crosssection of each sword were determined as a result of the neutron diffraction experiment with submillimeter spatial resolution. Since these two material characteristics are induced by the manufacturing process, analysis and comparison of the results allows drawing conclusions about variability or similarity of the actual production techniques of the Japanese swords. Introduction Japanese swords are famed among all others all over the world as the most effective in terms of hardness, resilience and, last but not least, aesthetics [1]. Their forging technique was almost unique; steel lumps, obtained from the furnace, were strongly pre-treated to obtain a homogenous and purified multilayered sheet. Distinctive carbon steels, characterized by different hardness, were shaped and specifically used for different parts of the blade components to optimize their mechanical feature (Fig. 1) [1]. Since the Koto age, 10-17 century AD, five different traditions developed distinctive construction techniques that evolved during the following historical periods [2]. However, the actual techniques that were used by the early sword-smiths were never documented and the necessary information was orally transmitted from the master to his most skilled pupils. In spite of the large amount of studies published on the subject, different manufacturing techniques are still not fully understood. Until recently, only expendable samples were investigated by standard analytical techniques, which mainly require sample cutting or are based on surface analysis. Nowadays, neutron diffraction [4] and neutron imaging methods [5] have been demonstrated to be the most suitable tools to qualitatively and quantitatively characterize composition and micro-structural properties of metal artifacts in a non-destructive way, mandatory for well conserved museum collections. Residual Stresses 2016: ICRS-10 Materials Research Forum LLC Materials Research Proceedings 2 (2016) 443-448 doi: http://dx.doi.org/10.21741/9781945291173-75 444 In the current study we attempted systematic research of the Japanese blades in order to investigate whether the laminated structure of a set of Samurai’s swords can be determined in non-destructive manner by means of neutron tomography, neutron diffraction stress analysis or both. The research is based on part of the East Asian Collection of the Museum of Applied Arts and Sciences (MAAS) in Sydney. The Japanese sword collection contains both samples of well-known origin, time period and authorship (group 1) as well as a group of mumei (no-signature) blades (group 2). Group 1 allowed us to build up a reference database on the main lamination methods adopted by Japanese swordsmiths and therefore to be used as benchmark data that can be cross-matched with the results obtained from group 2 in the attempt of attributing the corresponding manufacturing tradition to the unknown blades. Although a number of different blades were analyzed, in the current work we report only the neutron diffraction stress analysis on blades which were classified as katana, while the neutron tomographic data and results on wakizashi will be published separately somewhere else. Samples The attributes of the four katana from of the East Asian Collection of MAAS that were used in the forensic investigation are reported in Table. 1. Origin, time period and authorship are known only for three of them by transliteration of the signature engraved on their hilt. According to stylistic study, only the time period can be assigned for blade H6856 while the manufacturing tradition it belongs to is still uncertain. The physical description of the swords is given in Table 2, where the blades’ total length and thickness and width, measured in the mid length (where neutron diffraction measurement were made) are reported. Experimental: neutron diffraction stress analysis The neutron diffraction stress analysis in objects like Japanese swords is difficult because of two reasons. First, a technical aspect, a high spatial resolution is required, ~0.5 mm, since individual multiple layers can be of sub-millimeter size while a typical thickness of a blade is 5-7 mm. At the same time many points are required to be measured since the exact number and location of layers are not quite known. Therefore a compromise between resolution, number of measurement points and experimental time per point needs to be found. Second, a theoretical aspect, since the experimentally measured peak shift combines two effects, variation of (macrostress-free) d0 and elastic macrostress, and both of them are expected to be present due to manufacturing process, identifying and separation of these two contributions is essential for the aim of the experiment. However, there is no chance for proper resolution of d0 since only non-destructive analysis is allowed for the museum items. Thus, Fig. 1. The most common lamination structures of the Japanese swords. Fig. 2. A generic mesh in the transverse crosssection of the sword for neutron diffraction mapping and a real example of the longitudinal stress map for sword H4839 (min: -446 MPa, max: 291MPa). Residual Stresses 2016: ICRS-10 Materials Research Forum LLC Materials Research Proceedings 2 (2016) 443-448 doi: http://dx.doi.org/10.21741/9781945291173-75 445 the approach described in the following was used as the best possibility to resolve d0 problem. The first problem was solved by use of ANSTO’s neutron residual stress diffractometer KOWARI [6], which can provide necessary sub-millimeter (0.5 mm) spatial resolution and feasibility of that was proved in the past experiments [7]. Although a full 2D mapping would be very desirable, due to time limitations only a number of one-dimensional scans were performed. However, the measurement points are selected in such a manner and such numbers, >50, that would allow distinguishing unambiguously between possible structures as shown in Fig. 2 and at the same time some interpolation can be applied to produce 2D maps. At each mesh point spaced by 0.5 mm through thickness and by 1.0 mm along the central line, strain measurement was carried out in three orthogonal directions (longitudinal, transverse, normal) as in a traditional stress scanning experiment. A nominal gauge volume as small as 0.5×0.5×0.5 mm3 was used for measurements of the longitudinal strain component (symbols in Fig. 2 are scaled to this gauge volume size), while it was enlarged to 0.5×0.5×20 mm3 for measurement of the normal and transverse, since this was allowed by blade geometry, with extension of the gauge volume along the longitudinal direction. Strain measurement has been done using wavelength of 1.67 Å that provides Fe(211) reflection at 2θ = 90° and typical accuracy of ±30 μstrain that was achieved for the normal and transverse components after an acquisition time of 4 minutes. For the longitudinal strain component, however, a typical accuracy was ±50 μstrain with a measurement time of 20 minutes. Overall, approximately 1.5 days of beamtime was used for each blade. In order to resolve the second problem, the following strategy was adopted. In contrast to the traditional stress experiment, where d0 must be provided and three stress components are calculated, in the current experiment, the zero-through-thickness-stress condition was applied enabling calculation of two stress components and one d0-value out of the d-spacings for three measured directions. This plane-stress condition should be fulfilled with good accuracy (within our Table 2: Swords characterisation. Thickness and width are taken at the point of maximum curvature of the blade. The length is measured as a straight line from the point to the notch where the back of the katana meets the mounting of the hilt. ID Dimensions, mm Full view H1360 Thickness = 5.7 Width = 24.5 Length = 69 H5378 Thickness = 7.4 Width = 28.5 Length = 76.6 H4839 Thickness = 5.4 Width = 25.2 Length = 72.5 H6856 Thickness = 5.7 Width = 26.8 Length = 63.5 Residual Stresses 2016: ICRS-10 Materials Research Forum LLC Materials Research Proceedings 2 (2016) 443-448 doi: http://dx.doi.org/10.21741/9781945291173-75 446 experimental errors) because of sample planar geometry, but the major concern would be the anisotropic behaviour of d0. Until slicing of a blade is allowed, this is the only practical assumption. However, some checks can be applied to verify the assumption. If the abovementioned assumption is accurate enough and stress and d0 are resolved correctly then the transverse stress component should satisfy with boundary conditions on top and bottom edges of the blades and the longitudinal integral stress balance condition can be (at least approximately) evaluated. Results and Discussion The neutron diffraction experiment resulted in maps of two stress components and d0-values in the transverse cross-section of each sword which were determined with sub-millimeter spatial resolution. These two material characteristics are the imprint of the production process and their study can help to identify the actual manufacturing techniques based on qualitative and quantitative data rather than stylistic criteria. Although 2D maps give a good idea about spatial distribution, in