EXPERIMENTAL CHARACTERIZATION OF PORTUGUESE STRINGS MECHANICAL PROPERTIES Elisa Costa, Pedro Serrão, A. M. R. Ribeiro, Virgínia Infante Department of Mechanical Engineering, Instituto Superior Técnico, Lisboa,

15th International Conference on Experimental Mechanics, Faculty of Engineering, University of , 22-27 July 2012

1- INTRODUCTION 4- TENSILE TESTS 6- DIMENSIONAL CONTROL

To perform a dimensional control, the 6 strings A,B and E from stainless steel and steel were cut Portuguese are instruments quite popular in Portugal, due to its Tensile tests where performed in stainless steel and steel strings. Two and embalm, so that a measure was perform in a scanning electron microscope. For each string, history and connection to the typical music, . It is an instrument with six different types of supports were used. On the first one, the string is supported three measures where done. In the table, the mean of this three values are shown. double courses, which can be tuned in pairs in two different ways: Lisboa around a mechanism and fixed with a bolt on the top and on the bottom. On

(Si, La, Mi, Si, La, Re) and (La, Sol, Re, La, Sol, Do). The length of the second one, it was decided to attach the string in the support as in the

the strings are different for each type: 44 cm and 47cm for Lisboa and Portuguese guitar stringing process with a loop at each end. The second

Coimbra type. fixating system was left since the strings break earlier and produce

Different string materials are used. In this poster we will present results on unreasonable results due to the twists applied. The strings also break nearby

the two types of plain strings available for Portuguese guitar and used by the zone of the strings close to the loop since some defects are induced.

the musicians. One is stainless steel spring wire , the other is high carbon Those are also typical rupture points that cause failure the while the a) b) c)

spring steel (music wire) . The strings gauges tested are for the Lisboa musicians are playing. Figure 7 – Steel strings. Images from the scanning electron microscope. a)Si; b)La;

Portuguese guitar tuning. Obtaining knowledge about the characteristics of For the graphical results presented the first type of fixation mechanism was c)Mi.

the usual strings used on Portuguese guitar will be a step on understanding used. The behavior shown in the graphics are not the typical curve that Figure 3 – Fixing systems the differences between the sounds both types produce. appear in tensile tests. The reason for this comes simply on the fact that is for tensile tests. not possible to stretch the strings equally in the beginning of each test. The mean of two tests of each string are presented in table as well as he a) b) c) Figure 1 – Portuguese guitar (Type Lisboa). comparison with the ASTM standard of the minimum and maximum allowed Figure 8 – Stainless steel strings. Images from the scanning electron microscope. values. a)Si; b)La; c)Mi.

2- OBJECTIVES Music Wire Diameter (Mm) Diference (%) Stainless Steel Diameter (Mm) Diference (%) B – Si 234.135 1.8 B – Si 250.000 4.2 The guitar sound and playability is influenced by the physical properties of the strings The main goal A – La 252.187 0.9 A – La 254.688 1.9 on this work is to achieve the maximum knowledge on each type of string selected to better characterize and understand which parameters might be important to take into account in the sound E - Mi 321.875 2.5 E - Mi 325.625 1.8 produced by them. Table 5- Values measure in the scanning electron microscope.

The difference presented in the table was obtained by comparing the difference between the measured

and the nominal diameter values with the nominal values. Both types of strings present differences

comparing to the real value of less then 5%. 3- CHEMICAL COMPOSITION Figure 4 – Tensile tests results – 0.25mm A string: Steel Figure 5 – Tensile test results -0.25mm A sting: Stainless music wire. steel AISI 302. Ultimate Ultimate String Diameter Stress Min Max String Diameter Stress Min Max Characterizing the material is important to distinguish this types of strings. The chemical [MPa] [MPa] 7- CONCLUSIONS composition is a way to identify which elements are present and to verify if they have the right B – Si 0.23 2789.5 2670 2970 B – Si 0.24 2582.5 2205 2415 amount of the elements to introduce them in a certain category of materials. A – La 0.25 2717 2650 2950 A – La 0.25 2303.3 2205 2415 E - Mi 0.33 2760.3 2570 2852 E - Mi 0.32 2468.2 2180 2385 The tests performed and the results obtained are important to characterize the static properties of the Table 2 – Tensile test results - steel music wire. Table 3 – Tensile test results - stainless steel. strings. This work covers a lack in the knowledge of Portuguese guitar strings from the type steel and Elemento Peso % % Atómica stainless steel. It is also a step to understand the differences that musicians notice between these C 6.69 24.23 The new strings tested meet the minimum ultimate tensile stress (UTS) of ASTM standard strings with similar properties. 0 1.23 3.34 requirements. The UTS is 10% to 15% lower for stainless steel as expected. Cr 17.38 14.54 Mn 1.25 0.99 Fe 65.78 51.22 Ni 7.68 5.69 REFERENCES Total 100.00 100.00 5- VICKERS TESTS Table 1 – Chemical composition of the Figure 2 – Chemical composition spectrum. string tested. Hardness and micro-hardness test was performed to a steel string. The results are presented in .Inácio O., Santiago F., Cabral, P. C. , “The Portuguese Guitar Acoustics: Part 1 Vibroacoustic table. Measurements”, Proceedings of the IV Iberoamerican Acoustics Congress,13th-17th September 2004, Guimarães, Portugal, 2004. The amount on the table of carbon comes by defect, which means that the quantity of elements that Test Force (Kg) Dureza (HV) .A.V. Olver, D. Wilson, P. Crofton, Investigation of service failures of steel music wire, Engineering are not identified are introduced in the quantity of carbon, so this amount doesn’t represent the Microhardness 0.1 664 Hardness 0.1 664 Failure Analysis, 14, 1224-1232, 2007 reality. Given the amount of Cr and Fe it is accurate to identify this type of string of a stainless steel .ASTM A228/A228M-07 Standard Specification for Steel Wire, Music Spring Quality ASTM Table 4 – Vickers tests results. Figure 6 – Embalm string. type. International Hardness is an important parameter to take into account since it affects the useful life and playability .ASTM A313 / A313M - 10e1 Standard Specification for Stainless Steel Spring Wire ASTM of the string. A harder string surface will offer greater resistance to mechanical wear, which means International that it will retard surface structural deterioration from interactions with fingernails, picks, and metal frets.

This research was performed within the project “Análise dinâmica das cordas da guitarra portuguesa e sua interação com o corpo do instrumento” (Dynamical analysis of Portuguese guitar strings and their interaction with the instrument body), PTDC/FIS/103306/2008, carried on by IDMEC/IST, ITN and ISMAE under a research grant by FCT. 15th International Conference on Experimental Mechanics, Faculty of Engineering, University of Porto, 22-27 July 2012