Parallel to the grain behaviour and NDT correlations for chestnut wood (Castanea sativa Mill.)

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1 Parallel to the grain behaviour and NDT correlations for chestnut wood (Castanea sativa Mill.) A. O. Feio University of Minho Department of Civil ngineering Portugal J. S. Machado LNC National Laboratory for Civil ngineering, Timber Structures Division Portugal P. B. Lourenço University of Minho Department of Civil ngineering Portugal ABSTRACT: L'obiettivo di questo lavoro è basato nella descrizione del comportamento meccanico del legno di castagno sotto compressione e traccione paralleli al grano. Dopo una revisione dei problemi addetti a caratterizzare il legname sotto questi tipi di caricamenti, una prova meccanica è stata installata. Gli esemplari del legname usati nel programma di prova sono stati divisi in due gruppi: legno nuovo di castagno (), che mai gli non è stato usato strutturalmente nondimeno viene dai ceppi che potrebbero essere usati come tali; e legno vecchio di castagno (), che già sono stati usati negli elementi strutturali dalle costruzioni antiche (data e origine sconosciuto). Le correlazioni fra le proprietà meccaniche e NDT (velocità ultrasonica di impulso e resistenza perforante) inoltre sono fornite, consideranti la densità del legno. 1. INTRODUCTION Timber is one of the most used materials in the roofs and floors of monumental constructions in Portugal. In particular, Chestnut (Castanea sativa Mill.) is usually present in noble constructions, given not only its mechanical and durability properties, but also its aesthetic characteristics. The design of timber structural applications is deeply related with its exceptional capacities in resisting compression solicitations along the grain, wood is also many times used in structures with tension loads. Given the significant higher resistance capacity of timber elements along the grain as compared with perpendicular to the grain (Feio et al., 04b) a rule of thank in timber design consists on avoiding tension perpendicular to the grain. The last decades witnessed developments in the testing techniques, methods and equipments that allow, diminishing the subjectivity and increasing the level of the structural analysis, diagnosis and inspection of historical constructions. NDT is greatly used due to the fact that their application does not affect the present structural integrity and safety of the structure. Presently these methods are used individually or in combination with other techniques. The effectiveness (in terms of results) could be increased if some laboratorial tests were used to study the variability of the mechanical characteristics of the wooden elements (Togni, 1995). These methods can be classified in two distinct groups: Global Test Methods (GTM) and Local Test Methods (LTM) (Bertolini et al., 1998). The first ones include the application of the ultrasonic and vibration methods. The LTM, with the utilization of the Resistograph (Rinn, 1994) and the Pilodyn (Gorlacher, 1987) as the most common NDT devices, play usually a role support in the evaluation of wooden elements and structures. The application concerns the evaluation of the incidence and severity of defects in the material state of conservation (Machado & Cruz, 1997), with the aim of comparing the residual section with the variations of density, usually associated with the loss of mass. These methods present several advantages such as their practical utilization, transport and efficiency. The main scope of the present work consists in the characterization of the mechanical behavior of chestnut wood (Castanea sativa Mill.) under compression and tension parallel to the grain. The mechanical characterization aims at obtaining the elastic properties (namely the modulus of elasticity and the Poisson coefficients) and the compressive and tensile strength parallel to the grain, in laboratory with a pre-defined set-up for mechanical testing. Correlation of these properties with NDT techniques will be also analyzed.

2 2. GOMTRY OF SPCIMNS 2.1 Compressive Tests Specimens Size and Orientation The average size of the tests specimens was originally mm. Ultrasonic tests were carried out in these specimens and, afterwards, all of them were cut in three samples of mm : two of specimens were tested in laboratory up to failure and the other specimen was used for the NDT tests (Resistograph and Pilodyn 6J). Forty seven wooden specimens of chestnut wood, divided in two distinct groups, were tested: new chestnut wood () and old chestnut wood (). All wood comes from the Northern region of Portugal and the specimens were produced in such a manner that uniform density distributions and similar density ranges for the individual specimens were achieved. The visual grading and inspection of the groups was carried out and in every specimen knots, slope of the grain and colouring of the timber were investigated, and the worst specimens were discarded. Specimen dimension of mm were used with the longitudinal axis being oriented in the parallel to the grain direction. The tops of the specimens were parallel to each other and perpendicular to the longitudinal axis. All the specimens were conditioned in a climatic chamber capable of keeping a temperature of ± 2ºC and a humidity of 65 ± 5%. Before testing the cross section of the specimens were slightly wetting in water, to guarantee that failure initiates and develop away from the ends of each specimen. This causes a slight local straightening, which is enough to avoid failure in the cross section. 2.2 Tensile Tests Specimens Size and Orientation As far as possible the test length was kept clear of usual defects. To prevent fracture at the grips care was taken avoiding large knots at the transition from test length to the clamped section. Following the standard Nbr7190 (1997), the specimens adopted must possess a geometry as the one 4 3 represented in Figure 1. The volume of each specimen is approximately 3.2 m. Their overall length was 330 mm while the gauge section was 2 mm long and about 7 mm in thickness. However, and aiming at a better test performance, the tension specimens were modified in the tops, (see Figure 1 ). This modification was based on the difference between the effective grip area (machine grips), significantly minor, and the original grip area (tension specimens). Some preliminary tests were conducted and in the majority of these testing-tests was verified that failure was associated to the specimens crush in the grip area. Forty five wooden specimens of chestnut wood, divided in two distinct groups, were tested: and. Once again, all wood comes from the Northern region of Portugal and specimens were conditioned in a climatic chamber. L L Dimensions in mm Figure 1 Specimens geometry: standard specimen and adopted specimen. 3. TST ST-UP FOR DSTRUCTIV TSTING 3.1 Compressive Tests Destructive Tests The tests were carried taking into account the method specified in the standard Nbr7190 (1997) and were conducted in a mechanical tests universal machine (Baldwin), with a loading cell capable of introduce a maximum load of 300 kn and a minimum load of 6 kn. Additionally a feed (Schenk system), acquisition and amplification data system (both from HBM Spider 8) was used, to obtain and to register all the data (see Figure 2).

3 The rate of loading was displacement controlled and typically causes attainment of the maximum stress in 2 five minutes or less ( v = 4 mm / s ). Compressive failure parallel to the grain is a progressive and stable process, and the failure mechanism has been found to be insensitive to the loading speed. However, this does not mean that other mechanical properties are unaffected by it (Renaud et al., 1996). One spherical bearing platen was used to improve the alignment of the specimens and promote uniform stress distribution on cross section surfaces. Figure 2 Test set-up for destructive tests: general view. The measurements of the horizontal strains in the specimens were done by two pairs of mechanical strain gauges (HBM_DD1 type, one in each one of the faces), placed on opposing faces of the specimens to eliminate the effect of bending, see Figure 2. The measurements of the vertical strains in the specimens were done by a pair of mechanical strain gauges, placed in the arms of the test machine. Previously, a series of calibration tests of the apparatus was carried out, for the purpose of verifying the agreement between the vertical displacements in the faces of the tests specimens, measured using mechanical strain gauges, and the vertical displacements test machine platens and arms, measured by means of LVDT's. The compressive strength parallel to the grain ( f ) is the conventional value determined by the residual specific deformation of 1%, according to the NBr7190 standard (1997). The modulus of elasticity is equal to the slope of the linear part on the stress-strain relationship (Figure 3), defined by the points ( σ %;ε% ) and ( σ 50 %;ε 50% ) corresponding respectively to % and 50% of the conventional stress, in compression perpendicular to the grain, and it is given by: c,0 σ = ε 50% 50% σ ε % % where σ % and σ 50% are the compressive stresses corresponding to % and 50% of the conventional stress ( f ), and ε % and ε 50% are the specific strains corresponding to the values of σ % and σ 50%. The relative moisture and the temperature during the tests were registered by an electronic device. During the tests, the average values of temperature and relative moisture were 21 ± 2ºC and 48 ± 6%, respectively. σc,0 (MPa) (1) f c,0 σ50% Preload Cycle 1% Offset σ% 0 ε% ε50% εc,0 (mm/mm) Figure 3 Stress-strain relationship: definition of the compressive strength parallel to the grain f ) following the NBr7190 standard (as based upon an offset of 1%). (

4 In the preload cycle no information (strain or load) was recorded and the loading procedure was maintained. The requirement of this preload cycle was to permit and homogenize a perfect accommodation of the material avoiding slacks. 3.2 Tensile Tests Destructive Tests The destructive tests in tension parallel to the grain were conducted in a mechanical tests universal machine (INSTRON Model 4483) with a loading cell capable of introducing a maximum load of 0 kn. Additionally a feed, acquisition and amplification data system which was idealized for these particularly tests and that allowed, after the preparation and mounting of the full bridge transducers used, to obtain and to register all the data. The measurements of the vertical and horizontal strains in the specimens were done by two pairs of bonded strain gauges, placed on opposed faces of the specimens to eliminate the effect of bending. This measured strain is considered to represent strain at a point acceptable from a macro-mechanics perspective, considering that wood is an inhomogeneous material at a microscopic scale. These strain gauges are based on the resistance variation of a conductor when deformed (see Figure 3). Lead Wires a B Figure 4 Strain gauges. TML s Tokyo Sokki Kenkyujo Co., Ltd B-1 adhesive, which is highly resistant to moisture, was used to bond all strain gauges to the specimens. The recommended curing temperature for the adhesive is +36º. All bonded strain gauges were wired to a full bridge transducers with one dummy gauge as the temperature compensation strain gauge and compensating the bending resultants strains (load eccentricities). The compensating strain gauges were of the same lot as the active strain gauges and were mounted on similar specimens. The pair of vertical strain gauges (parallel to the grain; TML L-60-11) and the pair of horizontal strain gauges (perpendicular to the grain; TML L--11) were mounted (each distinct pair in each face) in the central section of the specimens, see Figure 5a. All the specimens were previously prepared: first grease and rust were removed from the bonding surface, then they were lightly polished with an abrasive paper of #1~180. After these two operations the specimens were wiped with acetone and the strain gauges installation position was marked, see Figure 5b. F F L--11 L--11 L F F Figure 5 Bonded strain gauges: scheme in the two opposite faces; and in a specimen. The tests were conducted in accordance to the Nbr7190 (1997) standard and the rate of loading was 3 mm/min. This normative change was necessary, because the tests were performed under displacement control

5 and not under force control. Simultaneously, photographs were taken at different stages. In the preload cycle no information (strain or load) was recorded and the loading procedure was maintained. The relative moisture and the temperature during the tests were registered by an electronic device. During the tests, the average values of temperature and relative moisture were 21 ± 2ºC and 46 ± 8%, respectively. 4. TST ST-UP FOR NON-DSTRUCTIV TSTING 4.1 Ultrasonic Tests Compressive Tests The methodology followed was based on the transmission method (or shadow method), which is based on two transducers located in two opposite faces, as transmitter and receiver. During the tests the ultrasonic equipment Pundit/Plus was used with cylinder-shaped transducers of 150 khz composed by piezoelectric ceramic crystals involved in a steel box. The tests were carried with two types of signal transmission: (1) Direct Method, parallel to the grain ( d = 30 cm), and (2) Indirect Method ( d = cm ), where d is the distance between transducers, see Figure 6. The elastic properties of wood were estimated by the measurement of stress wave propagation time in these directions and average values were considered in all measurements. At least two readings per specimen were generally made but a third one was added if the two first readings differed significantly. In all tests, coupling between the transducers and specimens was ensured by a conventional hair gel and a constant pressure was applied by means of a thick (2 mm) soft rubber spring, allowing adequate transmission of the elastic wave between the transducers and the specimen under testing, transmitting the coupling force without loading the transducer. Figure 6 Test set-up: Direct Method, parallel to the grain, and Indirect Method. 4.2 Ultrasonic Tests Tensile Tests The ultrasonic emission was made using the same equipment than for compression tests. Three different kinds of emission tests were realized: (1) Indirect Method ( d = cm ); (2) Indirect Method ( d = 45 cm ) and (3) Direct Method, parallel to the grain (see Figure 7). The propagation times of the ultrasonic wave were recorded and the material assumed to be continuous and homogeneous. In all methods average values were considered. In all tests, coupling between the two cylinder-shaped transducers and specimens was assured by a conventional hair gel and a constant pressure was applied by means of a rubber spring, allowing adequate transmission of the elastic wave between the transducers and the specimen under testing.

6 (c) Figure 7 Ultrasonic tests: Indirect Method ( d = cm); Indirect Method ( d = 45 cm) and comparison between the two specimens; (c) Direct Method, parallel to the grain. 4.3 Resistograph Tests The use of the Resistograph allowed to obtain the density profile of the used tests specimens, see Figure 8a. Drilling was made parallel to plan RT (plain TL and LR), which, in real cases, represents the accessible face of the timber elements. Figure 8 NDT devices: Resistograph and Pilodyn 6J. For all the specimens, from the graphs obtained, a resistographic measure (RM) was determined. The selected resistographic measure represents the ratio between the integral of the area of the diagram and the height of the tests specimens (see q. 2). Using this quantity, the Resistograph results can be easily compared with the values of density and the strength values. h Area RM = 0 h (2) 4.4 Pilodyn 6J Tests The Pilodyn 6J is a device that allows, through the release of a spring that transforms the elastic potential energy into impact energy, to measure the penetration of a metallic needle with 2.5 mm of diameter. This impact is responsible for the penetration of the needle in the surface of the specimens, allowing to register the depth penetrated by the needle in plan TL or in plan RL of the specimens, see Figure 8b. The Pilodyn 6J was used with the aim of correlating the density of each specimen with the depth reached with the needle of the device (surface hardness or resistance to superficial penetration). 4.5 Density Determination Compressive Tests Density was measured according to NP-616 (1973) standard. Given the conditioning conditions of the specimens, the average density is determined for a moisture content of 12%, given by: ρ 12% m = V 12% 12% (3)

7 5. XPRIMNTAL RSULTS FOR DSTRUCTIV TSTING 5.1 Compressive Tests Destructive Tests Results The results of the destructive tests are presented in Table 1 summarizing the test statistics. As stated above compressive failure parallel to the grain is a progressive and stable process, usually deemed to be a result of shear stresses. Table 1 Compression parallel to the grain results (average values): and c,0 Poisson f c,0 (GPa) ν LR ν LT ν (N/mm²) TR Average No. 47 CV c,0 Poisson f c,0 (GPa) ν LR ν LT ν (N/mm²) TR Average No. 47 CV In general the group present higher values, for the elastic and strength properties, than group, i.e., the mechanical characteristics of the old wood are, usually, slightly higher than the new wood (11-14%). It is not clear but it is likely that old specimens have been obtained from larger trees and old grown trees. Time, acting alone, does not change the mechanical and physical properties of wood. The group present higher CV values: this fact could be related with the origin of some trees that can come from distinct places (they are from the same region but not from the same stand). The group is more homogeneous which reveals a careful work in the selection of the trees. Poisson s ratios presented reveal good agreement with the literature values. Poisson s ratios present, in the three considered planes, higher CV values. This could be justified due the inherent inhomogeneity of wood. Under compression parallel to the grain, structural changes initiate as the stress level and instability increases with formation of macroscopic kinks. It was observed that each specimen develops one or, at maximum, two principal gross shear band(s) at relatively large strain, i.e. at a strain beyond the maximum stress ( f %. c,0, max ), observable with the naked eye, see Figure 9. This/these shear band(s) formed at a strain of Figure 9 illustrates the stress and radial/longitudinal strain diagram, for a specimen in compression parallel to the grain. As one can see initially the response was linear elastic and the limit of proportionality occurred at a longitudinal strain of about 0.01 and at a longitudinal strain of about 0.018, a gross shear band formed f c,0,max Stress (MPa) Kink Formation Gross Shear Band(s) Formation Radial Strain (mm/mm) µlr = Longitudinal Strain (mm/mm) Figure 9 Stress-Strain diagram, for a specimen in compression parallel to the grain. The group (see Table 2) present higher values, for the characteristic compression parallel to the grain characteristics, than group, i.e., the compression strength and modulus of elasticity values of the old wood are, usually, slightly higher than the new wood (varies in the range 12%-33%). These values were obtained according to the N384 standard (1995).

8 Table 2 Compression parallel to the grain: characteristic values c,0,05 (GPa) f c,0, 05 (N/mm²) c,0, 05 (GPa) f c,0, 05 (N/mm²) Tensile Tests Destructive Tests Results After the destructive tests the patterns observed on Figure were registered: (c) (d) Figure Failure patterns observed on the destructive tests: shear failure; shear and tension failure; (c) pure tension failure; and (d) splinter. Tensile strength parallel to the grain ( f t, 0 ) is determined from the maximum load applied to a specimen, following the NBr7190 (1997) standard. This value corresponds to the ideal situation where the stress-strain diagram does not present a significant slope in the first linear part, but if any slope in the first linear part is observed the tensile strength is calculated in that point. ach load-extension diagram was reduced to a true stress-true strain plot; from these, yield strengths were determined using a strain displacement that was equivalent to a 0.3% offset. The characteristic values (mean or 5-percentiles) are normally given in the design codes. In the present work a 5-percentile value ( f - characteristic strength value) was determined by ranking all the test values for a group in ascending order. The 5-percentile value is the test value for which 5% of the values are lower, as recommended by N 384 (1995). A number of linear regression equations were attempted between pairs of elastic constants and also between elastic constants and density. No significant relationships were found for the Poisson s ratios ν LT as function of density, t, 0 or f t, 0. Table 3 and Table 4 present the average results obtained with the bonded gauges and the global results, respectively. Analyzing the results obtained the f t, 0 and t, 0 values present high CV s (23% for and % for ). The results can be explained taking into account the global analysis of the results without give any particular attention to the observed failure pattern groups, which obviously had a great influence in the strength of the specimens. However, this could be a good solution because in practical situations one can not guarantee which kind of failure pattern will occur. It was assumed that the reason for the different failure behaviour and the different tensile strengths were caused by slope grain deviations. One problem arises when only the mechanical gauge was used: when this gauge jumps (due the failure of the specimen), the test procedure was aborted for safety reasons. On this way, tensile strength values are

9 lower with the mechanical gauge than the bonded gauges. Because of this the characteristic values presented in Table 3 are related with the first campaign, where bonded gauges were used. Table 3 Tension parallel to the grain: characteristic values t,0,05 (GPa) f (N/mm²) (GPa) f (N/mm²) Table 4 Results obtained with the mechanical strain gauge (destructive tests global results) t,0,05 (GPa) f (N/mm²) (GPa) f (N/mm²) Average No CV CORRLATIONS BTWN NON-DSTRUCTIV AND DSTRUCTIV TSTS 6.1 Compressive Tests Ultrasonic Tests The results of the Indirect Method have more significance in terms of practical applications since timber elements in service generally do not have their cross section surfaces available. Table 5 provides the values measured for all specimens. On the basis of the below data, one may assume that the acoustic wave transmission preferably follows the direction of greater stiffness and overall strength showing a dependence of the wave propagation on the elastic properties of material. Again, the results of the group were slightly higher than with the group. Table 5 Indirect method: and Dynamic modulus of elasticity (GPa) Average No CV Correlations with lasticity Modulus and Strength Figure 11 and Figure 12 illustrate the results between, f and din using the Indirect Method. It is possible to conclude that using a simple linear regression one can predict the and f once used an ultrasonic analysis. This good capacity of prediction is not only the result of a well coordinate and efficient application of the ultrasonic method but it is also result of the inclusion of a visual information of the specimens c,0 (GPa) c,0 = 0,004. din r 2 = 0.70 c,0 (GPa) c,0 = 0,005. din r 2 = din - IM (N/mm²) din - IM (N/mm²) Figure 11 Relation between and din using the Indirect Method: ; and.

10 f c,0 (N/mm²) p f c,0 = din r 2 = 0.67 fc,0 (N/mm²) f c,0 = din r 2 = din - IM (N/mm²) din - IM (N/mm²) Figure 12 Relation between f and din using the Indirect Method: ; and. 6.2 Compressive Tests Resistograph Method Correlation with Density Figure 13a and Figure 13b show the correlations between resistographic measure and density for the group and group respectively, and as a main conclusion generally the resistographic measure values are higher for the group than for the group: this could be explained based on the average density of the group be higher than the density of the group. Resistographic Measure (cm) RM = ρ r 2 = 0.71 Resistographic Measure (cm) RM = ρ r 2 = Density (kg/m³) Density (kg/m³) Figure 13 Correlation between resistographic measure and density: ; and Correlation with Modulus of lasticity and Strength Figure 14 and Figure 15 present the potential of this method in the prediction of the modulus of elasticity and longitudinal compressive strength of each specimen for the group and group, respectively. No relevant differences can be pointed unless the higher resistographic measure values of the group as a consequence of the higher average density of the group in comparison with the group. Resistographic Measure (cm) RM = c, r 2 = c,o (GPa) Resistographic Measure (cm) RM = c, r 2 = c,o (GPa) Figure 14 Relation between resistographic measure and elasticity modulus: ; and.

11 Resistographic Measure (cm) RM = 2.6. f c, r 2 = fc,0 (N/mm²) Resistographic Measure (cm) RM = f c, r 2 = fc,0 (N/mm²) Figure 15 Correlation between resistographic measure and f : ; and. 6.3 Compressive Tests Pilodyn 6J Method Correlation with Density Pilodyn 6J was used with the aim of correlating each specimen s density with the pin reached depth. The depth to which the pin penetrates shows to be inversely proportional to the density of the wood as expected. The pilodyn pin penetrates 7 to mm into wood, and the results show good correlations between density and Pilodyn 6J penetration. 2 The analysis of the results show a good correlation ( r varies in the range , for and respectively) between Pilodyn 6J estimates and wood density estimates, to the test conditions, see Figure Depth (mm) Depth = ρ r 2 = Density (kg/m³) Depth (mm) Depth = -0,039. ρ r 2 = Density (kg/m³) Figure 16 Correlation between depth and density: group; group. 6.4 Tensile Tests Ultrasonic Tests The results of the ultrasonic tests, taking into account the three different methods are now compared. The highest modulus of elasticity values were obtained to the Indirect Method ( d = cm ). This fact is directly related with the uniform section presented by the measured region: there were no narrow or contract zones in the specimen, such as happens in the other two methods, see Table 6. On the other two methods the average values are very similar. Table 6 Results obtained with the mechanical strain gauge (destructive tests) Direct Method Indirect Method (d = cm) Indirect Method (d = 45cm) din _ DM (GPa) din _ IM (GPa) din _ IM (GPa) Average No CV Figure 17 illustrates the correlations between the destructive tests and the ultrasonic tests. A significant (ranging between for the group and between for the group) allows to conclude about a good relations between the ultrasonic and the destructive tests, not only with this method but also with the two other methods. 2 r

12 25 (Indirect Method_d = cm) 25 (Indirect Method_d = cm) t,o(gpa) 15 t,0 (GPa) 15 5 t,0 = 0.57.din_IM r 2 = t,0 = 0.58.din_IM r 2 = din_im (GPa) din_im (GPa) Figure 17 Correlation between din _ IM and t, 0, for the Indirect Method: ; and. 7. CONCLUSIONS Comparing the characteristic tensile strength and modulus of elasticity values with the characteristic compressive results one can conclude that the characteristic strength values are slightly higher in tension parallel to the grain than in compression parallel to the grain ( 32%). The elastic moduli of elasticity results are also slightly higher in tension parallel to the grain than in compression parallel to the grain ( 25%). Table 7 presents the comparison results. Table 7 Comparison between compression and tension parallel to the grain characteristic values t,0,05 c,0,05 f t,0,05 f c,0,05 t,0,05 c,0,05 f t,0,05 f c,0, In this paper, both new and old sound chestnut wood are considered in the testing program. As a first conclusion, the mechanical characteristics of the old wood are, usually, slightly higher than the new wood. A reason for this it not clear but it is possible that the old specimens have been obtained from larger trees. The coefficients of Poisson found corroborated values found in the literature and single-parameter linear regressions showed good agreement between non-destructive parameters (Pilodyn 6J and Resistograph), and elastic values or density. 8. ACKNOWLDGMNT The first author gratefully acknowledges Foundation for Science and Technology (FCT), for PhD grant SFRH/BD/6411/01. The authors acknowledge also the support of Augusto de Oliveira Ferreira Lda. (specimen preparation and supply), and personnel of the Timber Structures Division and the Structural Testing Laboratory of LNC. 9. RFRNCS Bertolini, C.; Brunetti, M.; Cavallaro, P.; Macchioni N.; 1998 A non destructive diagonostic method on ancient timber structures: some practical application examples. Proceedings of 5th World Conference on Timber ngineering, Montreux. Presses Polytechniques Universitaires Romandes, Vol.I, pp uropean Committee for Standardization; 1995 N 384 Structural timber Determination of characteristic values of mechanical properties and density. Office for Official Publications of the uropean Communities. Luxembourg. Feio, A.; Machado, J.; Lourenço, P.; 04a Caracterização da resistência da madeira de Castanho à tracção paralela ao fio. CIMAD 04, 1º Congresso Ibérico. Guimarães, Portugal, UM, pp Feio, A.; Machado, J.; Lourenço, P.; 04b Caracterização de propriedades da madeira de Castanho na direcção perpendicular ao fio recurso a técnicas destrutivas e não destrutivas de ensaio. CIMAD 04, 1º Congresso Ibérico. Guimarães, Portugal, UM, pp

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