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Variation of impact bending inthe wood of Pinus sylvestris L. inrelation to its position in the treep. de Palacios

L Garcia EstebanA. Guindeo

F. Garc ia FernandezA. Fernandez Canteli

N. Navarro

AbstractThe response of the wood of Pinus sylvestris L. to impact bending using the Charpy method was studied in 2940 defect-free,oriented test pieces from the central board of 10 trees felled during scheduled cutting, with the values obtained being related tothe position of the test piece in the tree, both from the pith to the outside and in ten ns of its height in the tree. For this purpose theabsorbed energy was obtained by using an instrumented drop-weight impact tester. The tests were carried out in stable hygro-

thermic conditions of 655 percent and 202 "C, and the wood was conditioned und er the same conditions before testing. Thecharacterisation was carried out on test pieces with a cross section of 20 mm by 20 mm and a length of 150 mm. The resultsobtained show that impact bending de crease s the closer the test piece is to the pith and the higher it is in the tree, with the decreas ebeing greater horizontally than vertically. In addition, it was shown that there is a significant relation between the numbe r of ringsand the impact response of the wood.

A,lthough impact response tests with the Charpy methodhave been used extensively on other construction materials.they have not been applied to wood to any great extent. Instructurally complex materials like wood, the most appropri-ate method for studying impact bending behavior is theCharpy method in its instrumented version, with electronicdevices that allow the situation of the load on the test pieceand its deformation to be known step by step, with samplingfrequencies of up to 1 megac ycle. (Sims 1988. Blackman andWilliams 1999, Tan 2000),

In conifers, impact brittleness is attributed to anatomicalfactors characteristic of earlywood; in fact, when wood is sub-jected to impact, it undergoes a local deformation at the pointof contac t, giving rise to microfissures w hich do not propag ateeasily due to the differences in the nature of its anatomicalelements. In this way the concentration of strains on the edgeof the crack which prod uces the fracture of the material cea seswhen a discontinuity is reached, with the level of stress havingto increase greatly for a new fracture to propagate (Gordon

1968). Similarly, when the fracture propagates perpendicularto the grain, in the radial direction, it tends to slow dow n w henit reaches the earlywood zone (Schniewind and Centeno1973).

The authors are. respectively. Univ. Lecturer (Doctor). SeniorUniv. Lecturer. Univ. Professor, and Associate Lecturer, Universi-dad Polilecnica de Madrid, Escuela Tecnica Superior de Ingenierosde Montes. Departaniento de Ingenieria Forestal. Catedra de Tec-nologia de la Madera. Ciudad Univers i taria , Madrid . Spain(paloma.depa!acios(a;upm.es , lu is ,garciai^Jupm,es , antonio .guindeoi^up m.es. francisco,gareia@upni.es); Univ. Professor. Uni-versidad de Oviedo. Escuela Politecnica Superior de Ingenieria. De-partamciito de Conslruccion e Ingenieria de Fabricacion. Campus deViesques. Gijon. Spain (afc(a)uniovi,cs); and Senior Univ, Lecturer,Universidad Politecnica dc Madrid. Escuela Universitaria de Arqui-teetura Tecniea, Departaniento de Construcciones Arquitectonicas ysu Control. Catedra de Construccion III. Ciudad Universitaria s/n,Madrid. Spain (nieves.navarroCuJupm.cs). This paper was receivedfor publication in .lune 2007. A rticle No, 10371,Forest P roducts Society 2008.Forest Prod. J.58(3):55-60.

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Figure 1. Obtaining test pieces.

One of the indicators most commonly used to evaluatewood toughness is specific weight (Kolimann 1951), althoughon other occasions ring width has been considered abetterindicator for toughness than specific weight, primarily in co-nifers. In fact, the presence of uv enile w ood modifies the spe-cific weight of latewood in the early years of tree growth, af-fecting response to impact bending.

Other factors such as slope of grain greatly reduce the en-ergy absorbed on impact (Bod ig and Jayne 1993).The combination of these factors affects the energy ab-sorbed during mechanical tests, with it being difficult to de-temiine the effect of a single one of the variables involved.This is why defect-free, oriented test pieces with straight grainwere used in this study, and the test pieces belonging to juve-

nile and mature wood were differentiated.Although the variation of wood properties in the trunk is arule rather than an exception, the variation models can differgreatly between genera and species (Burdon et al, 2004). Forthis reason, and b ecause the wood oi'Pinus sylvestris L. is oneof the most commonly used woods in construction, the varia-tion of dynamic response of defect-free, oriented wood wasstudied acco rding to its position in the tree, both from the pithto the outside and in terms of its height in the tree. K nowledgeof this variation enables the selection of more appropriatepieces of wood in consideration of their position in the tree,for use in high dynamic stresses, for example in end-notched

beams, holes in beams and other problems related to tensionperpendicular to the grain.

Figure 2. D ART TESTER drop-weight machine.Materials and methods

The wood was obtained fiom 10 Pinus sylvestris L. trees inthe Valsain Forest inSegovia, Spain, from region of prov-enance 10 in Sierra dcG uada rrama (Catalan 1991). From eachtree the first 10 m of trunk w ere use d, with the first 2-m logbeing removed in order to avoid disperse data (Baonza et al.2001, Climent et al. 2003, Xu and Walker 2004). Some

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autbors attribute this abnormal behavior to the fact that the cellwall microfibrils have a greater angle in the lower part of tbetrunk ihan in the higher part (Donalson 1992, Evans et al, 2001,Evans and Kibblewhite 2002). In fact, physical-mechanical

Figure 3. Detail of the test.Tet t rWM 5853

G O O O -5500-5000-4500-4000-

J 3 5 0 0 -*g3000-i 2500-

2000-1500-1000-5 0 0 -

MalwiillD IMPACTD

(1

y1

r

characteristics are usually obtained from breast height, al-though some standards are more restrictive and requireheigbts of between 2 and 4 ni from the base (Giiler et al. 2004).A single radial board was obtained,, 5-cm thick throughoutits length, Tbe board was cut into 1.12-m sections, from whichstrips of wood w ere obtained from the pith to the outside of thetrunk, with a cross section of 35 by 35 mm. These were driedand conditioned in a chamber at a temperature of 202 C anda relative humidity of 655 percent. Final test pieces of 20 by20 by 150 mm were then obtained (Fig. 1). A 45'' notch wasmade in each test piece. 2-mm deep, with a radius of 0.25 mmat the base of tbe notch in accordance with the specificationsof the standard Charpy test {ASTM 2005).During the tests the bygrothermic conditions of tbe labora-tory were maintained stable at 655 percent and 202 C , be-cause in most species there is a considerable increase in thetoughness of the wood both when its moisture content (MC)increases(Gbelmeziu 1938, Bodigand .layne I9 93)a ndw henthe temperature increases, provided that the wood bas a bighMC. as wilh a low MC toughness decreases when the tem-perature increases (Zerbe 1956). Once the test bad been car-

ried out, the MC of the wood w as detennined in order to checkthat the results could be com pared. This was carried out in anoven at 1O32 C to anhydrous state (AENO R 2002).In order to assess the impact bending the instrumentedCbarpy method was used, by means of a DART TESTERdrop-weight tester of the CEAST brand (Fig. 2) and aDAS4000 data acquisition program, using a hammer with amass of 3200 g and a drop height of 1000 mm. A striking tipwith a radius of 1.5 mm w as chosen. This parameter proved tohave a major influence on tbe energy transmitted (Tanaka etal, 1995). The distance between supports was chosen as 100mm (Fig. 3) , Tbe total energy of the test piece fracture processwas obtained by the integration of the force curve throughout

tbe displacement of the test piece from the initial mom ent ofimpact until fracturing {Fig. 4).For the speed, Kalthoffs {1996)recommendations were taken intoconsideration. Tbe fit of the re-sponse was done by reducing theforce of impact until the grapb wasachieved. The Charpy testing m ethodwas chosen because the support sys-tem at tbe two ends of tbe test pieceremoves the effect caused by clamp -ing in the Izod method (McCow an etal. 2000). Deflexion during the test

was determined using Eq. [I]:

Tenwetute'C

0.0 0.2 Q4

Scald of Y A mV min Y max

(JO GOOOtl

a 6 Q8 1.0 1,2

PornU daiinilianX F p diinnn rp

H l.S

Force (N)3 5 4 9 0 0

1,8 ZO Z2 Z 4 Z6t imelai t l

Cur tot vJut [mi] d |irm)1 2 0 7 4 8 3

2.8 1 0

e i v(m/i ]

12

3 34

1 4 I S

E ! J ]1 1 1 5

3,8 4,0

Pojm17081

32 18Figure 4. Instrumented response diagram.

where:.V = deformation of the test pieceat the point of impactV,, ^ initial pendulum velocitym ^ pendulum massF = force indicated by tbestrain gauge/ = time interval from the initialmoment at w hich the loadis applied to the test piece

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lableLog

1. Energy1,2

values (J).Strip (x LiJ

1.2 3,4 5,6Strip

Figure 6. Variation of the energy for each log in terms of thstrip of wood, with LSD confidence intervais at 95 percent.affirmation eoincides with studies made by other researcherswhich dem onstrate that response to impact bending is greatethe greater the density of the wood is (Koch 1985), and greaterin mature w ood than in juvenile wood (Evans et al. 2000). Infact, strips (1,2) (3,4) and (5.6) show characteristics of uvenilewood, wh ile strips (7,8) begin to lose these characteristics.

Figure 6 shows a deerease of the energy curves for each logas the height increases, which is also confirmed by other authors (Niklas 1997), attaining a clearly defined asymptotictendency, similar to that studied in Pinus Radiata for MOE byBurdon et al. (2004). An increase in the value obtained thefurther the strip is from the pith is also observed, with thistendency decreasing after strips (5.6). It ean also be observedthat except for one isolated point (strips 3,4; log 2) the data aa whole cannot be considered significantly different, althoughit would be if logs at two extremes, such as 2 and 8, werecompared.

The regression fit to explain the variation of the energy inthe radial and longitudinal directions forstrips( 1.2), (3,4) and(5,6) gave rise to a potential function, and for strips (7,8) itgave rise to an exponential function (Fig. 7) whose coefficients are shown in Table 3 with the 95 percent confidenceintervals.Multiple linear regression analysis enables Eq. [3] to be defined as a model for obtaining the energy va lue:Energy -ax trunk + hx force + ex strip + dx ring + e

[3where the coefficients, as well as the 95 pereent eonfideneeintervals and the /j-value of each parameter, are shown inTable 4. In accordance with the coefficient R^ the modewould explain 85.8 percent of the variability of the energywith all parameters, in addition, being significant for a level oeonfidenee of over 99 pereent.

Although it was determined that the dynamic response othe wood is related to its density and the presence of juvenile

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strip 1.2fitted strip 1.2strip 3.4fitted strip 3.4strip 5.6fitted strip 5.6strip 7.8fitted strip 7.8

15Figure 7. Fitted curves of the variation of energy in height.Table 3. Fitting parameters of the energy variation curves (Fig. 5).Strip Function Coefficient a Coefficient h

1,23.45.67.8

9.805( 8.083,1 1.53) -0.516 4 (-0.64()S.-(1.3923)11.3(10.16.12.43) H).3O84 (-0.3751 .-0.2417)10.27(-280.6.301.1) -0.1105 (-3.824.3.603)ll . 5i (9.54L 13.48) -0.03684 (-0.0688 .-0.0048)

Table 4. Fitting parameters of the model of energy vari-ability (Ff = 0.858).ParameterLogForceStnpRingConstant

Constantsahcde

Value (confidence interval 95%)0.0612(0.0275.0.0945)0.0050(0.0049.0.0051)

-0.1719 (-0.2505,-0-0933)0.0771 (0.0657 0.0885)

-6.9564 (-7.3087,-6.6041)

p-value0.00040.0000O.fM)OO0 . 0 0 0 00 . 0 0 0 0

or mature wood, further studies will enable the influence ofwood anatomy on the dynamic properties to be correlated andquantified through the use of certain anatomical elements(length and tangential diameter of the tracheids. cell wallthickness, juven ile wood, fibril a ngles, ete).

Conclusions There is a statistically signifi-cant relation in the wood ofPinus sylvestris between thenumber of rings per eentime-ter and the energy absorbed byd y n a m i c r e s p o n s e of thewood, with the variation be-ing greater the lower the nu m-ber of rings is. The response of the wood onimpact increases the furtherthe test piece is from the pith,and it deereases the higher thetest piece is in the tree. Pieces of wood subjected tohigh dynamie stresses mustcome from the lower logs (ex-eluding the basal log) andmust contain as little juvenilewood as possible.

Literature citedAtnerican See. for Testing and Materials(ASTM). 2005. Standard tes t methodsfor determining Ihe Izod pendulum im-p a c t r e s i s t a n c e of p l a s t i c s . A S T MD-256-05. ASTM. West C onshohocken,Pennsylvania. 20 pp.Asociaeion Espafiola de Nor ma l i zae i on(AENOR). 2002. Moisture content of apiece of sawn timber. Part I. Determina-t ion by oven drymet hod - UNE- EN13183-1:02 AENOR. 8 pp.Baonza . M . V . . A. Gutier rez , and J . M.Grau. 2001. Variations in density, hu-midity, heartwood and bark according to

Ihe height in the trunk in pino laricio. In :Proe. ofthe Third Spanish Forest Con-gress Vol- V. pp. 632 638 (in Spanish ).English abstract available.B l ackman . B . R . K . and J . G . Wi l l i ams .1999- Impact and high rate tcsling ofcomposi tes : An overview. In: Proe . ofthe NATO Advanced Sludy Insi . on Me-chan i c s of Compos i t e Ma t e r i a l s andS t r u c t u r e s . N A T O Sci. S e r i e s 3 6 1215- 224 .Bodig, J . and B.A. Jayne. 1993. Mechanicsof wood and wood composites. Chapter6, Dynamic Behavior. Kreiger Publish-ing Co.. Malahar, Florida.

Burdon. R.D. . R.P. Kihblewhite. J .C.E. Walker. R.A. Megraw , R. Evans.and D.J. Cown. 2004. .luvenile versus mature wood: Anew eoneept,orthogonal tocorcwood versus outcrw-ood. wilh special reference toPinus radiala and P. laeda- Forest Sci. 5()(4):399-415.Catala n. G. 199 1. Region s of origin of Pinus sylvestris L. and Pinus nigraAm. subsp. salzmannii (D unal) Franco in Spain (in Span ish). INI A andICONA. Madrid. Spain.Climent. J.. M.R. Chamhe!. L. Gil. and J.A. Pardos. 2003 . Vertical heart-wood variation panems and prediction of heartwood volume inPinuscanariensis Sm. Forest Ecol. Manag. 174(l-3):203-211.Donalson. L.A. 1992. Within- and between-trce variation in miciofibrilangle in Pinus radiata. New Zealand J- ofForestry Sci. 22:77-86-F.vans. J.W.. J.F. Scnft. and D.W. Green. 2000. Juvenile wood effeet inred alder: Analysis of physical and mechanical data to delineate juve-nile and mature wood zones. Forest Prod. J. 50(7-8):75-87.Evans. R. andR.P. Kibblewhite. 2002. Controlling wood stiffness inplantation softwoods. In : Proe. of 13th international symposium on

Cool lie i cut R-'

0.3015 (-294-2. 294.8)0.95620.96420-76990.6405

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non-dcstnjclive testing of u'ood, Univ, of California, Berkeley, ForestProducts Soc. Madison, Wisconsin,

R, Booker, and R .P, Kibblewhite. 2001 . Variation of mi-crofibril angle, density and stiffness in fifty radiata pine trees, ln \ Proc.of 55th annual general Appita conference, Hobart, Tasmania, Austra-lia.Ghelme^iu. N, 1938. Res. on the impact resistance of timber. Uolz alsRoh- und W erkstoff U 15):585-6OI.Gordon. J.H. 1968, The New Sci, of Strong Materials. Penguin Books,269 pp,Giilcr, C . I . Bektas, and A,M . Basturk. 2004. Some mechanical proper-ties of eastern beech wood. J. of the Inst, of Wood Sci, 16(4):223-229.Kalthoff J,F, 1996. Fund ame ntals for the assessm ent of data measu redby impact test using notched and precracked Charpy specimens. Inter-nal Rept, Experimentcllc Mcchanik, Ruhr-Universitat Bochum.Koch. P, 1985, Utilization of Hardwoods Growing on Southern PineSites. USDA Forest Scrv, . Washington. D,C,Kollmann. F. 1951, Tech, of Wood and W ood Based Materials. Vol. 2.Springer Verlag. Berlin. Germany.McCowan, C,N,. D,P, Vigliotti. and T,A. Siewert, 2000, Evaluation ofABS Plastic Impact Verification Specimens, Pendulum Impact Test-ing: A Century of Progress, STP 1380, ASTM. pp. 210-217.

Niklas, K.J. 1997. Mechanical properties of black locust (Robinia pseudoacacia L,) wood. Size and age-dependant variations in sap- andheartwood. Annals Bot-London 79:265-272,Schniewind. A.P. and J,C. Centeno. 1973, Fracture toughness and d uration of load fiictor. Wood Fiber Sci, 5{2):I52-I59,Sims, G.D. 1988. Understanding Charpy impact testing of compositelaminates. ln \ Proc, of the 6th Inlcr. Conf on Composite Materials 33 ,494-3 ,507.Tan. L, 2000, Determining dynamic fracture parameters in compositematerials through instrumented Charpy impact testing. PhD thesisUniversidad de Oviedo, Departaniento de Construccion c Ingenieria dFabricacion, Madrid, Spain,Tanaka, M ,, Y, Ohno . H. Horigome, H. Tani. K, Shiota, and A. Misawa1995, Effects of the striking edge radius and asymmetrical strikes onCharpy impact test results. In : Pendulum impact machines: Procedureand specifications for verification, STP 1248, ASTM. Philadelphia, pp153-167.Xu. P. and .I,C.F. Walker, 2004. Stiffness gradients in radiata pine treesWood Sci, and Tech. 38:1-9,Zerbe, J.I. 1956. Sources of variability encountered in dclcrmining theimpact strength of wood and the effect of incremental loading on absorbed energy and elastic properties, thesis. State Univ. of New YorkCollege of Forestry.

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