Distortion of Norway spruce timber - Springer Link

Originalarbeiten á Originals

Holz als Roh- und Werkstoff 59 (2001) 94±103 Ó Springer-Verlag 2001

Distortion of Norway spruce timber Part 1. Variation of relevant wood properties M. Perstorper, M. Johansson, R. Kliger, G. Johansson

94

The overall aim of this study and series of papers is to increase the understanding of the mechanisms that govern moisture-related distortion of Norway spruce timber. In this ®rst paper the experimental study is described and the variation of wood properties presented. The study comprises 40 Norway spruce (Picea abies) trees from one fast-grown and one slow-grown stand in southern Sweden. From the trees 240 studs (45 ´ 70 ´ 2500 mm) were taken for measurement of distortion. Wood properties were measured on small specimens (13 ´ 13 ´ 200 mm) cut from the studs. Spiral grain angle was found to vary from approximately +3° (left-handed) close to pith to zero 150 mm from pith with a strong individual variation. The material from the fast-grown stand had a larger spiral grain angle compared with the slow-grown material. Spiral grain was poorly correlated to other parameters. Presence of knots had a substantial in¯uence on longitudinal shrinkage (al) measurements. Specimens with large knots (KAR > 33%) had almost 100% higher longitudinal shrinkage than specimens without knots. It should be pointed out, however, that measuring shrinkage in small specimens containing even small knots can create a problem with regards to the obtained results, especially results of al. It was found that presence of compression wood in several growth rings more than doubled the longitudinal shrinkage. For the radial and tangential direction the presence of compression wood decreased shrinkage with about 30%. The ratio between tangential and longitudinal shrinkage was 49 for normal wood whereas for compression wood the ratio was 13. These results con®rm the theory that the micro®bril angle governs shrinkage. Longitudinal shrinkage decreased slightly with increased distance from pith whereas radial and tangential shrinkage did not display any substantial radial variation. The fast-grown material had generally a higher longitudinal shrinkage and lower transverse shrinkage Mikael Perstorper, Marie Johansson, Robert Kliger (&), Germund Johansson Chalmers University of Technology, Department of Structural Engineering, Division of Steel and Timber Structures, 41296 GoÈteborg, Sweden The authors gratefully acknowledge the ®nancial support from, The Swedish Council for Building Research (BFR), proj. 950172±6, The Swedish Forestry and Agricultural Research Council (SJFR), proj. 20.0150/95 and the CF LundstroÈm Foundation.

than the material from the slow-grown stand. About 50% of the variation in longitudinal shrinkage was explained by radial position, density and ring width. Density and ring width did explain 60% of the variation in radial shrinkage but only 30% of the variation in tangential shrinkage.

Verwerfung von Fichtenschnittholz Teil 1. Variation der relevanten Holzeigenschaften Ziel dieser Arbeit ist ein tieferes VerstaÈndnis der Mechanismen, die das Verwerfen von Fichtenschnittholz verursachen. In diesem ersten Teil wird die Variation der Holzeigenschaften vorgestellt. Vierzig BaÈume (Picea abies) von einem schnell- und einem langsamwachsenden Standort wurden verwendet. Daraus wurden 240 KanthoÈlzer der Abmessung 45 ´ 70 ´ 2500 mm geschnitten und daran die Verwerfung bestimmt. Holzeigenschaften wurden an kleinen Proben (13 ´ 13 ´ 200 mm) gemessen, die aus diesen KanthoÈlzern hergestellt wurden. Der Faserwinkel variierte zwischen +3° in der NaÈhe der MarkroÈhre bis 0° bei 150 mm Abstand vom Mark mit groûer individueller Streubreite. Die Proben vom schnellwachsenden Standort hatten einen groÈûeren Faserwinkel als die vom langsamwachsenden Standort. Die Korrelation des Faserwinkels zu anderen Holzeigenschaften È sten hatte eiwar nur sehr schwach. Anwesenheit von A nen bedeutsamen Ein¯uû auf das longitudinale Schwinden (al); es lag bei hohen Astanteilen (KAR > 33%) fast 100% hoÈher als bei astreinen Proben. Es allerdings muû betont werden, daû bei der Messung an kleinen Proben È ste problematisch sind, speziell fuÈr al. schon kleine A Weiter zeigte sich, daû Anteile von Druckholz in einigen Jahrringen das longitudinale Schwinden mindestens verdoppeln. Radiales und tangentiales Schwinden wird durch Druckholzanteile um etwa 30% verringert. Das VerhaÈltnis zwischen tangentialem und longitudinalem Schwinden betrug 49 fuÈr normales Holz, bei Druckholz lag dieses VerhaÈltnis bei 13. Die Ergebnisse stuÈtzen die These, daû der Mikro®brillwinkel das Schwinden regelt. Longitudinales Schwinden nahm mit zunehmender Entfernung vom Mark langsam ab, waÈhrend radiales und È nderungen tangentiales Schwinden keine wesentlichen A aufwiesen. Schnellwachsendes Holz zeigte allgemein hoÈhere Schwindwerte als langsamwachsendes Material. Rund 50% des longitudinalen Schwindens werden erklaÈrt durch die Parameter radiale Position, Dichte und Jahrringbreite. Dichte und Jahrringbreite erklaÈren 60% des radialen Schwindens, aber nur 30% des tangentialen Schwindens.

1 Introduction

next two papers, modelling of twist and modelling of bow and spring are presented.

1.1 Background Distortion of timber products is a major obstacle for an extended use of wood in the building industry. In fact, wood is loosing market shares to steel due to the lack of straightness that timber products too often exhibit (Johansson et al. 1990, 1994). The contractors choose steel since the production costs are low for the whole building system. One reason for this increasingly important problem is that the wood resource is changing towards fastgrown, small-diameter logs with more complex properties. The timber producing industry can resolve this problem in different ways. The development of composite materials is one way to homogenise and stabilize wood. However, the cost of these products are often far too high to compete with steel-based alternatives. The so-called Engineered Wood Products have taken (or defended) market shares from wood rather than from steel. Another way is to start with the basic problem and seek to understand the mechanisms that govern distortion. With an increased knowledge of this process the relevant wood properties can be identi®ed. The processing and development efforts can be directed towards the most important parameters that have direct impact on the product quality.

2 Experimental 2.1 Materials The material for this study came from two large-diameter stands of Norway spruce in southern Sweden; Toftaholm and AskenaÈs. Characteristics of the two stands are given in Table 1. They represent a fast-grown and a slow-grown material with an average growth ring width of 4.7 and 2.8 mm from pith to bark. Material from 40 trees were used in this project. The material was taken from the second log from the root-end, i.e. the upper butt-log, see Fig. 1. Each log was cut according to 3 ex log resulting in three members measuring 70 ´ 290 (in mm) after drying to 12% moisture content. The central yield was then ripped into six studs with the dimensions 45 ´ 70 ´ 2900 (in mm) after planing, see Fig. 1. The remaining full-size specimens were used in another study on large-dimension timber, see Perstorper et al. (1994). The studs represented three different radial positions: outer, intermediate and core. 2.2 Specimen preparation From the top-end of each stud, a 200 mm long, more or less, knot-free section was cut for the measurement of wood properties. The wood properties of this section were supposed to be representative for the whole stud. The section was ripped into three slices, each 13 mm thick, according to Fig. 1. The spiral grain angle h ± was then measured on the face nearest to the bark on all slices (A-BC) using the scribing method. Special attention was paid to scribe at a position on the slice that really represented the tangential face. Three scribe lines were made to verify the measurements. The actual sawing pattern deviated from the ideal one in many cases. The pith was therefore not always parallel to the edges of the slices. Such a deviation may lead to a large grain angle with respect to the edges of the slice. The spiral grain angle may however be low at the same time. The position and direction of the pith in relation to the slice edges were therefore recorded which

1.2 Related studies During the last decades many research groups have been working with moisture-related distortion of timber (Mishiro and Booker 1988; Fridley and Tang 1993; Perstorper et al. 1995; Ormarsson 1995; Danborg 1994; Forsberg 1997; Sandberg 1997). Mechanical models have been developed by Stevens and Johnston (1960) and Balodis (1972) for the twisting distortion. Recently, Ormarsson (1995) developed an extensive ®nite element model as tool for an increased understanding of the distortion of timber. In most of the above mentioned studies, the wood properties necessary for a successful modelling have not been measured in great detail. The aim of this study was to record crucial wood properties, such as shrinkage (longitudinal, tangential and radial), occurrence of compression wood and spiral grain angle as well as the standard properties; density and ring width. By using these properties, models for distortion can also be tested quantita- Table 1. Description of the fast-grown stand at Toftaholm and the slow-grown stand at AskenaÈs tively. 1.3 Aim The overall aim of this study and series of papers is to increase the understanding of the mechanisms that govern moisture-related distortion of Norway spruce timber by extensive measurement of relevant wood properties. This study is presented in a series of papers. In this paper an overall description of the project is given. However, the main focus is on the variation of the measured wood properties and their interdependence. In the

Tabelle 1. Beschreibung des schnellwachsenden Standorts bei Toftaholm und des langsam wachsenden bei AskenaÈs

Diameter at breast height [mm] Age [years] Site class Volume per hectar [m3sk/ha] Stems per hectar Mean growth ring width [mm] Dito, 0±100 mm from pith [mm]

Fast-grown (Toftaholm)

Slow-grown (AskenaÈs)

360 65 G34 370 500 4.7 6.2

400 105 G36 790 420 2.8 3.6

95

96

Fig. 1. Specimen preparation Bild 1. Herstellung der Proben

made it possible to correct the measured grain angle accordingly. A left-handed spirality was de®ned as positive (S-helix), see Fig. 1. The slices were then cut into ®ve sticks per stud measuring 13 ´ 13 ´ 200 (in mm). The stick from the B-slice was cut at a position were the edges would coinside well with the radial and tangential directions. A small rivet with a rounded head was mounted on the end of each stick to create a distinct measurement point for the longitudinal shrinkage. A small part of the stick from the B-slice was turned in order to increase precision and to facilitate measurement of the radial and tangential shrinkage. The distance from the pith to the centroid of each stud was determined (r). In some cases the actual sawing pattern deviated from the ideal one to a large extent. Therefore, distance from pith r is a better indicator of the radial position than the stud groups (Core-Intermediate-Outer). For the sticks the radial distance r was also calculated.

2.3 Shrinkage measurement A special device based on digital displacement transducers was developed for the shrinkage measurements. Great efforts were made to ensure that the position of the stick was the same for each measurement. A high degree of repeatability was therefore reached. The maximum deviations for repeated recordings were approximately 0.003 mm and 0.010 mm for the longitudinal and transverse measurements, respectively. The device is further described by Bengtsson (1997). The sticks were weighed at both moisture stages as well as after oven-drying. The moisture content u was determined according to the following equation: mu m0 uˆ …1† m0 where

mu ˆ mass before drying m0 ˆ mass after drying to 103 °C. The shrinkage strain e and shrinkage coef®cient a were determined as:

L1 L2 eˆ L1 e aˆ u1 u2 where L1 ˆ size at moisture stage 1 L2 ˆ size at moisture stage 2 u1 ˆ moisture content at stage 1 u2 ˆ moisture content at stage 2

…2† …3†

2.4 Distortion measurement The studs were placed in a climate-controlled room and suspended vertically to avoid any in¯uence of gravitation on the distortion development. A device for distortion measurements based on digital transducers was developed, see Fig. 2. Firstly the stud was placed with the ¯at face downwards on the device resting on three pins with rounded heads, two at the bottom end and one at the top end. Two transducers at the top end (#1 and #2) recorded the twisting distortion and one transducer at mid-span recorded the bow (#3), including the gravitation effects. The stud was then turned on its side in order to record the spring deformation with the mid-span transducer. Each stud was put onto the device in the same way at each measurement. 2.5

By mistake, 50 (20%) of the ``B-sticks'' were not cut out Moisture stages with the growth rings parallel to the edges of the stick. All material was placed in the climatic chamber and These specimens had therefore to be omitted in the anameasured at four moisture stages, see Table 2. Distortion lysis regarding transverse shrinkage. of the studs was measured at the end of each moisture

Fig. 2. Test set-up for distortion measurements Bild 2. Bestimmungsverfahren der Verwerfungen

97

Table 2. Moisture stages Tabelle 2. Feuchtestufen

Moisture stage

Temperature

Relative humidity

Duration

Moisture content at the end of conditioning period

1 2 3 4

20 20 20 20

85% 30% 85% 30%

6 5 4 3

15.6% 7.2% 14.4% 7.8%

°C °C °C °C

months months months months

98

Fig. 3. Radial variation in ring width based on measurements at the top end of each stud Bild 3. Radiale Streuung der Jahrringbreite. Messung am EndstuÈck der Kantholzproben

stage when equilibrium was reached. The sticks were measured and weighed at the end of the ®rst two moisture stages. The length of stick A1, A5, B, C1 and C5 as well as the radial and tangential size of stick B were recorded at the ®rst two moisture stages, see Fig. 1. The sticks were also weighed at the same time. The sticks were ®nally dried at 103 °C and weighed to determine the moisture contents. The density was determined on the basis of the weight and volume of the sticks.

3 Variation in wood properties

2.8 mm for the slow-grown stand and 4.7 mm for the fastgrown stand.

3.2 Density The density of the studs corresponded very well with the density of the sticks. In Fig. 4, the radial variation in density for the sticks at moisture stage 2 (»7% moisture content) is shown. As expected, a radial variation corresponding to the ring width variation was found. The average density was 427 kg/m3 for the slow-grown material and 373 kg/m3 for the fast-grown material.

3.3 Spiral grain angle 3.1 The spiral grain angle varies generally from a spirality of Ring width approximately +3° (left-handed) close to the pith to zero The average ring width for each stud was determined from or in some cases to a negative spirality (right-handed) photocopies of the top end of the studs. As expected both closer to bark, see Fig. 5. There is also an indication of a stands display a rather strong systematic radial variation slight decrease in grain angle in the absolute vicinity of the in ring width, see Fig. 3. The average ring width was pith. These observations coincide very well with results

Fig. 4. Radial variation of density at moisture stage 2 (»7% moisture content) for sticks, 13 ´ 13 ´ 200 mm Bild 4. Radiale Streuung der Dichte bei Feuchtestufe 2 (ca. 7% rel. Feuchte) fuÈr StaÈbe der Abmessung 13 ´ 13 ´ 200 mm

Fig. 5. Radial variation of spiral grain angle Bild 5. Radiale Streuung des Faserwinkels

from other studies of Norway spruce (Danborg 1994; Jensen et al. 1994; Dahlblom et al. 1997, etc). The material from the fast-grown stand has generally a higher grain angle than the slow-grown material. This difference is partly explained by the fact that the grain angle variation is coupled to the growth ring number. Many studies indicate that the age of the cambium-initial governs the spiral grain angle to a large extent. At the same distance from the pith, the slow-grown material has a higher growth ring number than the fast-grown material. Thus, the slow-grown material should have a lower grain angle at the same radial position. The fast-grown material has an average spiral grain angle of 2.0° and the slow-grown 1.5°. This overall difference between the stands is statistically signi®cant (ttest, p ˆ 0.0002). The radial variation differs somewhat between the stands. The spiral grain decreases with a steeper gradient for the fast-grown stand. Spiral grain is rather poorly correlated to radial position when looking at all trees at the same time. However, when studying individual trees, a strong correlation is obtained between spiral grain and distance from the pith in many cases. The level and variation pattern is very different from tree to tree, see Fig. 6, which explains the poor overall correlation.

3.4 Influence of compression wood and knots on shrinkage parameters It was not always possible to cut out a perfectly ``clear'' section from the studs for the wood properties measure-

ment. Some sections and sticks contained both knots and compression wood. In order to take these anomalies into account all sticks were visually examined. The presence of compression wood is dif®cult to characterize. However, the sticks were grouped in the following categories by visual examination: CW-0: No compression wood CW-1: Widened latewood band in one or several growth rings CW-2: Dominating latewood bands in one or several growth rings For the knots the Knot Area Ratio, KAR, was used as a parameter. The knot area ratio is the percentage of a cross section that a projection of the knot(s) cover. The following groups were used: KAR-0: KAR ˆ 0 KAR-1: 0 < KAR £ 33% KAR-2: KAR > 33% The presence of compression wood and knots has a substantial impact on shrinkage, see Figs. 7, 8, 9 and Table 3. The presence of knots increases the longitudinal shrinkage due to the ®bre deviations. Specimens with large knots (KAR > 33%) had almost 100% higher longitudinal shrinkage than specimens without knots. Transverse shrinkage is not affected since the measurement position was chosen at a knot-free part of the sticks. For normal wood, the mean values of the ratio between transverse and longitudinal shrinkage are 22.6 and 48.7 for

Fig. 6. Examples of radial variation of spiral grain angle for two individual trees. Radial position is here represented as the Z-coordinate, see Fig. 1 Bild 6. Beispiele fuÈr die radiale Streuung des Faserwinkels fuÈr zwei BaÈume. Die Radiale Position entspricht hier der Z-Koordinate in Bild 1

99

Fig. 7. In¯uence of presence of compression wood (CW) and knots (KAR) on longitudinal shrinkage coef®cient al (n ˆ 1188). There were no specimens with the combination CW-2 /KAR-2. (Box plot showing 10th, 25th, 50th, 75th and 90th percentile) Bild 7. Ein¯uû von Druckholz (CW) und Astanteilen (KAR) auf den longitudinalen Schwindungskoef®zientenal (n ˆ 1188). Proben der Kombination CW2/KAR-2 sind nicht vorhanden. Die 10±25-, 50-, 75- und 90% Perzentilen sind dargestellt

100

Fig. 8. In¯uence of compression wood on transverse shrinkage (n ˆ 190). (Box plot showing 10th, 25th, 50th, 75th and 90th percentile) Bild 8. Ein¯uû des Druckholzanteils auf die Querschwindung (n ˆ 190). Die 10±25-, 50-, 75- und 90% Perzentilen sind dargestellt

Fig. 9. In¯uence of compression wood on shrinkage coef®cient ratio. Specimens with knots are excluded (n ˆ 148). (Box plot showing 10th, 25th, 50th, 75th and 90th percentile) Bild 9. Ein¯uû des Druckholzanteils auf das VerhaÈltnis der Schwindungskoef®zienten. Die 10±25-, 50-, 75- und 90% Perzentilen sind dargestellt

the radial and tangential direction respectively. For specimens with a lot of compression wood (CW-2) the ratios drop to 5.9 (radial) and 13.3 (tangential), see Table 4 and Fig. 9. Appearently, the high micro®bril angle normally associated with compression wood makes it less anisotropic with respect to shrinkage. Presence of compression wood increases longitudinal shrinkage and decreases transverse shrinkage.

3.5 Influence of radial position and stand type on longitudinal shrinkage In the result presentation, which will follow regarding longitudinal shrinkage, only the clear sticks (CW-0, KAR-0) are included.

The mean value of the longitudinal shrinkage coef®cient was 0.0078, see Table 5, which is in accordance with several other studies of Norway spruce. The material from the slow grown stand displays a signi®cantly lower longitudinal shrinkage compared with the fast grown material (0.00715/0.00852, p < 0.0001). This difference is most likely associated with the greater relative amount of latewood for the slow-grown material. Kyrkjeeide (1991) among others has shown that the micro®bril angle of the S2-layer is signi®cantly greater for earlywood than for latewood. A large micro®bril angle is normally associated with a large longitudinal shrinkage coef®cient. The difference between the stands is therefore not surprising.

3.6 Influence of radial position and stand type on radial and tangential shrinkage In the result presentation, which will follow regarding transverse shrinkage, only sticks without compression wood (CW-0) are included. The overall mean value of the radial shrinkage coef®cient is 0.151 which coincides very well with other studies. Persson (1997) reports a value of approximately 0.17 for Norway spruce. The slow grown material has a distinctly higher radial shrinkage (0.176) compared with the fastgrown material (0.129), see Fig. 11, which most likely can be explained by the greater relative amount of latewood for slow-grown material. The tangential shrinkage for the material in this study is similar to what is reported in the literature. The overall mean value is 0.334 and the difference between the stands is small but statistically signi®cant (p ˆ 0.0001). According to Fig. 12 the tangential shrinkage appears to be lower close to pith. The main reason for this difference is most probably the in¯uence of ring curvature on the measurements since the measurement length is 13 mm. However, it can be noted that Cown and McConchi (1983) report a small radial increase in tangential shrinkage for Radiata pine.

Table 3. Longitudinal shrinkage coef®cient al for different groups of specimen with respect to presence of compression wood and knots Tabelle 3. Longitudinaler Schwindungskoee®zient al fuÈr verschiedene Probengruppen mit Druckholz- und Astanteilen CWgroup

KARgroup

Count

Longitudinal shrinkage coef®cient, al [%] Mean

Std. Dev.

Median

CW-0 CW-1 CW-2 CW-0 CW-1 CW-2 CW-0 CW-1 CW-2

KAR-0 KAR-0 KAR-0 KAR-1 KAR-1 KAR-1 KAR-2 KAR-2 KAR-2

671 102 36 273 38 11 50 7 ±

0.781 0.959 1.705 1.001 1.279 1.662 1.412 1.649 ±

0.202 0.336 0.632 0.274 0.459 0.607 0.463 0.651 ±

0.765 0.968 1.617 0.966 1.234 1.580 1.366 1.586 ±

All

All

1188

0.931

0.368

0.854

The longitudinal shrinkage decreases slightly from pith to bark according to Fig. 10. This tendency has been found in several other studies of Norway spruce, e.g. SaaranpaÈaÈ (1994), Bengtsson (1997), Persson (1997), Dahlblom et al. (1997), and is attributed to the decrease in micro®bril angle of the S2-layer within the ``juvenile'' wood zone close to pith. Another in¯uence might be the proportion of latewood that is closely connected with ring width. The radial decrease in ring width, see Fig. 3, might therefore also play a role.

3.7 Summary of material data In Table 5, material data are summarised. It should be noted that the number of measurements are sometimes less than the nominal number of specimens simply due to measurement error and/or handling mistakes.

Table 4. In¯uence of compression wood on radial and tangential shrinkage. Specimens containing knots are not included. Mean values and standard deviation Tabelle 4. Ein¯uû des Druckholzes auf radiales und tangentiales Schwinden. Proben mit Astanteilen sind nicht enthalten. Mittelwerte und Standardabweichung CW-group

Count

Long. Shrink al [%]

Radial shrink. ar [%]

Tangent. shrink. at [%]

Shrinkage Ratio ar/a1 [±]

Shrinkage ratio at/al [±]

CW-0 CW-1 CW-2

117 23 8

0.773 (0.20) 0.968 (0.31) 2.054 (0.59)

15.1 (3.9) 13.5 (3.8) 10.5 (3.3)

33.4 (5.4) 30.0 (5.3) 24.2 (3.0)

22.6 (10.4) 18.2 (16.1) 5.9 (3.1)

48.7 (16.6) 39.5 (29.6) 13.3 (4.9)

All

148

0.876 (0.39)

14.7 (4.0)

32.5 (5.7)

21.0 (11.9)

45.4 (20.4)

Table 5. Summary of material data. Mean values and standard deviation. Specimens with knots or compression wood are omitted Tabelle 5. Zusammenfassung der Materialeigenschaften. Mittelwerte und Standardabweichung. Proben mit Astanteilen oder Druckholz sind ausgeschlossen Property

Count (all)

All

Fast-grown (Toftaholm)

Slow-grown (AskenaÈs)

Ring width, RW [mm] Density at stage 2 (u » 7%), q7,7 [kg/m3] Spiral grain angle, SGA[°] Moisture content at stage 1, u1[%] Moisture content at stage 2, u2 [%] Longitudinal shrinkage coef®cient, al [%] Radial shrinkage coef®cient, ar [%] Tangential shrinkage coef®cient, at [%] Shrinkage ratio at/ar [)] Shrinkage ratio ar/al [)] Shrinkage ratio at/al [)]

240 1198 637 1192 1192 671 153 153 153 117 117

4.45 (1.89) 400 (50.7) 1.7 (1.66) 15.6 (0.67) 7.16 (0.32) 0.781 (0.20) 15.1 (3.9) 33.4 (5.4) 2.30 (0.46) 22.6 (10.4) 48.7 (16.6)

5.72 (1.64) 373 (37.0) 1.96 (1.75) 15.7 (0.87) 7.26 (0.31) 0.852 (0.22) 12.9 (2.6) 35.1 (5.4) 2.51 (4.3) 17.8 (8.1) 43.3 (16.0)

3.18 (1.13) 427 (48.3) 1.47 (1.54) 15.5 (0.32) 7.07 (0.30) 0.715 (0.16) 17.6 (3.8) 31.8 (4.8) 2.05 (4.1) 27.6 (10.2) 54.6 (15.2)

101

Fig. 10. Radial variation of longitudinal shrinkage coef®cient al for the two stands Bild 10. Radiale Streuung des longitudinalen Schwindungskoef®zienten al fuÈr zwei standorte

102

Fig. 11. Radial variation in radial shrinkage coef®cient ar for the two stands Bild 11. Radiale Streuung des radialen Schwindungskoef®zienten al fuÈr zwei Standorte

Fig. 12. Radial variation in tangential shrinkage coef®cient at for the two stands Bild 12. Radiale Streuung des tangentialen Schwindungskoef®zienten al fuÈr zwei Standorte

Table 6. Correlation matrix based on linear regression (R). Specimens with compression wood or knots are omitted (n = 117). The density is based on weigth and volume at moisture stage 2 (u » 7%) Tabelle 6. Korrelationsmatrix der linearen Regression (R). Proben mit Druckholz oder Astanteilen sind ausgeschlossen (n = 117). Die Dichte ist bestimmt aus dem Gewicht und Volumen bei Feuchtestufe 2 (ca. 7% rel. Feuchte)

RW Density SGA al ar at at/ar ar/al at/al

Density

SGA

al

ar

at

at/ar

ar/al

at/al

r

)0.69 1

0.27 )0.18 1

0.65 )0.62 0.32 1

)0.70 0.76 )0.11 )0.60 1

)0.54 0.51 )0.02 )0.46 0.67 1

0.51 )0.63 )0.13 0.48 )0.81 )0.16 1

)0.69 0.79 )0.21 )0.84 0.86 0.56 )0.71 1

)0.66 0.71 )0.21 )0.89 0.71 0.70 )0.43 0.92 1

)0.34 0.32 )0.29 )0.54 0.16 0.27 )0.05 0.39 0.50

4 Correlation between growth characteristics and wood properties As expected, density is correlated to ring width and radial position, see Table 6. Using a 2-order polynom for the regression line, the coef®cient of determination is R2 ˆ 0.52. Spiral grain angle is poorly correlated to other parameters. Longitudinal shrinkage is associated to density, ring width and distance from pith. A multiple regression analysis using these parameters gives a coef®cient of determination of R2 ˆ 0.56. Radial shrinkage is highly correlated to density and ring width. Approximately 60% of the variation is explained by density and ring width together. This is not surprising since radial shrinkage is associated to the proportion of latewood in the growth rings. However, tangential shrinkage is less correlated to density and ring width; only 34% if the variation is explained by the parameters together. 5 Conclusions Spiral grain angle was found to vary from approximately +3° (left-hand) close to pith to zero 150 mm from pith with a large individual variation. The material from the fast-grown stand had a higher spiral grain angle than the slow-grown one. Spiral grain was poorly correlated to other parameters. Presence of knots in sticks 13 ´ 13 ´ 200 mm had a substantial in¯uence on longitudinal shrinkage measurements. Specimens with large knots (KAR > 33%) had almost 100% higher longitudinal shrinkage than specimens without knots. Compression wood had a profound effect on all shrinkage parameters. It was found that presence of compression wood in several growth rings increased longitudinal shrinkage with more than 100%. For the radial and tangential direction the presence of compression wood decreased shrinkage about 30%. The large micro®bril angle normally associated with compression wood should, according to theory, increase longitudinal shrinkage and decrease transverse shrinkage. Thus, these experimental results con®rm the theory that the micro®bril angle governs shrinkage. It can be noted that compression wood is less anisotropic compared with normal wood. For example, the ratio between tangential and longitudinal shrinkage is 49 for normal wood, whereas for compression wood the ratio is as low as 13. Longitudinal shrinkage decreased slightly with distance from pith whereas radial and tangential shrinkage did not display any substantial radial variation. The material from the fast-grown stand had generally a higher longitudinal shrinkage and lower transverse shrinkage than the slowgrown material. About 50% of the variation in longitudinal shrinkage was explained by radial position, density and ring width. Density and ring width did explain 60% of the variation in radial shrinkage but only 30% of the variation in tangential shrinkage.

References

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