Wood is a hygroscopic material and its mechanical properties change with moisture content below the fiber saturation point. Fracture characteristic is one of most important mechanical properties of wood and often is taken as an important safety design factor for wood as an engineered structural material. The aim of this paper is to investigate the effect of moisture content changes on wood fracture properties, focusing on fracture behavior and patterns with different crack orientations of Betula platyphylla Suk. and Pinus bungeana in two different desorption processes. These two desorption processes were, respectively, from fiber saturation point to an equilibrium moisture content corresponding to 82.4%RH at 20ºC and from 82.4%RH to 35.2%RH at the same temperature of 20ºC

Lu, Zhenyou

Wang, Liyu

Zhao, Guangjie

Revistas Universidad del Bío-Bío

Maderas. Ciencia y tecnología 8(1): 25-30, 200625ISSN 0717-3644ISSN online 0718-221X1School of Technology, Beijing Forestry University, Qinghua East Road 35, Haidian District, Beijing, P.R.China, 100083. E-mail:liyuwang2000@yahoo.com.cn2College of Material Science and Technology, Beijing Forestry University, Qinghua East Road 35, Haidian District, Beijing, P.R.China, 100083. E-mail: zhaogjws@263.net.3School of Technology, Beijing Forestry University, Qinghua East Road 35, Haidian District, Beijing, P.R.China, 100083. E-mail: Luzy@bjfu.edu.cnCorresponding author: : liyuwang2000@yahoo.com.cnReceived:‘October 24, 2005. Accepted: March 26, 2006WOOD FRACTURE PATTERN DURING THE WATER DESORPTIONPROCESSLiyu Wang1, Guangjie Zhao2, Zhenyou Lu3ABSTRACTWood is a hygroscopic material and its mechanical properties change with moisture contentbelow the fiber saturation point. Fracture characteristic is one of most important mechanicalproperties of wood and often is taken as an important safety design factor for wood as an engineeredstructural material. The aim of this paper is to investigate the effect of moisture content changeson wood fracture properties, focusing on fracture behavior and patterns with different crackorientations of Betula platyphylla Suk. and Pinus bungeana in two different desorption processes.These two desorption processes were, respectively, from fiber saturation point to an equilibriummoisture content corresponding to 82.4%RH at 20ºC and from 82.4%RH to 35.2%RH at the sametemperature of 20ºC.Keywords:  water desorption,  wood fractureINTRODUCTIONWood fracture performance is influenced by temperature and moisture content with moisturecontent having significant impact. Smith and Chui (1994) sampled material in green conditionfrom flat sawn boards from three premature red pines grown in New Brunswick, Canada. Theyconcluded that reduction in moisture content at test from the fiber saturation point to 18% resultedin an increase in mode I energy for complete separation of fracture surfaces and crack growth inthe longitudinal direction. However, below 18% MC, any reduction in MC at test led to a reductionin fracture energy. Kretschmann and Green (1996) studied effects of moisture content and specificgravity for clear southern pine and concluded that mode II stress intensity factor increases as MCdecreases, reaching a maximum between 10 and 12% MC for all levels of density. They alsofound that mode I stress intensity factor increases as MC decreases, reaching a maximum between7 and 9% MC for all levels of density. In our previous paper (Wang et al. 2003), wood fracturepattern was investigated under different relative humidity environment in moisture adsorptionprocess. The objective of this work was to investigate the wood fracture behavior in moisturedesorption process on the effect of water transition on wood fracture properties, using a similartechniques as discussed in the previous paper (Wang et al. 2003).Maderas. Ciencia y tecnología 8(1): 25-30, 200626Universidad del Bío - BíoEXPERIMENT MATERIALS AND METHODSTest specimen preparationIn the experiment, Betula platyphylla Suk. and Pinus bungeana were used and the sampling methodfollowed the Chinese National Standard GB1927-91(1992):for tree sampling collection for physicaland mechanical tests of wood. Specimens included four different orientations TL, TR, LR, LT planes.Here, the first letter denotes direction normal to crack plane and the second one denotes direction ofcrack. An edge crack was cut in the center of the specimen using a fine saw blade and the crack tip wassharpened.  The TL and LT specimen dimensions were 85 (length) x 15 (width) x 5 mm (thickness).The corresponding dimensions for TR samples were 95 x 20 x 10 mm, and those for LT samples (Pinusbungeana) were 120 x 20 x 10 mm.Fracture testingThe strong and weak degree of stress field near the tip of crack is determined by stress intensityfactor KI. Plane strain fracture toughness KIC is the critical stress intensity factor when wood crackstarts to propagate under static loading. Specimens were tested by universal mechanical testing machineat a rate of 0.5 mm/min and for each specimen, images were captured at various load levels up tofailure. The test method of fracture toughness followed the Chinese National Standard GB4161-84(1985) for plane-strain fracture toughness of metallic materials. This method is not completely suitablefor an orthotropic material such as wood, but it is acceptable when the orthotropic main axis directionis superposed with the crack surface direction, as well as the crack propagation direction. Therefore,the above method is suitable for TL and TR specimens because of their crack propagation along thewood initial crack. But for LR, LT specimens, as crack growth sloped from the initial crack direction,the fracture toughness value measured can only be considered nominally for reference.Water desorption experimentIn order to inspect wood fracture pattern under different relative humidity in desorption process,the samples were initially dried to the fibre saturation point and then exposed, respectively, to 82.4%RH and 35.2%RH at 20ºC. The relative humidity environment was created by the sulfuric acid waterysolution and the temperature was controlled by a constant temperature box. During the desorptionprocess, some specimens were taken out at different times to determine their moisture content andfracture nature.RESULT AND DISCUSSIONWood fracture pattern in the first desorption step (82.4%RH)Crack propagation pathIn the desorption process, specimens’ crack propagation path was the same as that in the moistureabsorption process (Wang et al., 2003). Regardless of specimen types, all of their crack propagationoccurred at the crack tips. The crack propagation direction of TL and TR specimens was consistentwith the original crack direction, and the fracture occurred as soon as their cracks were initiated.However, the crack of LT specimen started along initial crack direction, but, after very shortperiod of time, it changed the direction and propagated along the fiber direction. The crackpropagation of LR specimen was complex, starting along initial crack direction in very shortperiod of time and then propagating in folding linear direction as shown in Figure 1.As observed from both the adsorption in previous work (Wang et al., 2003) and from thiswork, the crack propagation path has the same pattern and is not by moisture content change.Maderas. Ciencia y tecnología 8(1): 25-30, 200627Wood Fracture...: Wang et al.FIGURE 1. Crack propagation pattern in desorption process for the four types of specimensFIGURE 2. Load and displacement curve for specimens during the first step of desorption to 82,4% RH.Critical load determinationFrom the experiments for TL and TR specimens, it was observed that the fracture occurred as soonas the crack started to expand. Or in other words crack did not have obvious subcritical expansion. Inthese cases, the maximum load when the specimen ftacured is termed as the critical load, and the measuredloading and displacement curves in the first step of desorption are illustrated in Figure 2.From Figure 2, it is seen that for TL and TR specimens, the curve of load (P) versus displacement(V) follows an approximate linear relation in the early stage of the loading, and the critical load nearlyMaderas. Ciencia y tecnología 8(1): 25-30, 200628Universidad del Bío - Bíosuperposed the maximum load. Once crack started to grow, the load rapidly dropped and the test specimenswould fracture.But for LT, LR specimens, crack would propagate slowly with different degrees and no obvioussymptoms were observed when crack began to propagate before specimens fracture. From the point, themaximum fracture load was no longer taken as the critical load. During specimen testing, «burst out»sound was frequently heard clearly during rupture. Thus the 1st «burst out» load was taken as thecritical load PQ as indicated by a start in Figure 2. Seen from figure 2, even if reaching the critical loadPQ, at which crack started to lose stability and propagate, the load continues to increase with crackgrowth. The specimen still has high load-bearing capacity after the 1st rupture. After reaching the largestload Pmax, the load dropped with crack growing and displacement increasing. By this point the specimenhad gradually lost its load-bearing capacity.FIGURE 3  Fracture toughness of specimens during the first desorption stage at 20ºC (82.4%RH)Fracture toughnessFigure 3 shows the results of fracture toughness at different elapsed time during the first stage ofdesorption (82.4%RH). Different specimen types had different fracture toughness values. But for identicalspecimen types under different time of equilibrium, their fracture toughness values were different.Specimen fracture toughness increased gradually with elapsed time during desorption, and the fracturetoughness of TL and TR specimens was quite close and its value was much lower than those of LR andLT specimens.Wood fracture pattern in the second moisture desorption stage (82.4% to 35.2%RH)Crack propagation pathIn the second desorption process, specimens’ crack propagation path was basically the same as thatin the first desorption process. Thus the results in Figure 1 can represent those for specimens in thesecond desorption process.Critical load determinationThe maximum load is higher in the second stage of desorption as the moisture content is lower. Ascan be seen from the experiment, the moisture desorption process would not strongly influence thecritical load. Thus a similar critical load determination method to the first moisture desorption processmay be used by the second moisture desorption process. With desorption time growth in the moisturedesorption process, TL, TR specimens would retard a while to fracture after getting up crack. Becausethe time was very short, only several seconds, therefore the biggest load was thought as the critical load.Maderas. Ciencia y tecnología 8(1): 25-30, 200629Wood Fracture...: Wang et al.Regarding LT, LR specimens, critical load was still the load at some moment when the first clearly «rupture» sound was heard.Fracture toughnessSeen from figure 5, in the second desorption process (82.4% to 35.2%RH ), specimen fracturetoughness decreased with elapsed desorption time. This was opposite to that observed in the first stageof desorption. Further examination for relationship between fracture toughness and moisture content isshown in Figure 6, below the fiber saturation point, for moisture content from 0 to 17%, wood fracturetoughness increased as MC increased, and reached the maximum value at the MC 17%. The fracturetoughness subsequently decreased with increasing of MC from 17% to 28%. Similar results have beenfound for fracture load for resistance to cleavage could reach the maximum value at the moisture content18% (Mamada and Aoki 1992). Futher work is needed to understand the reasons for such behavior.FIGURE 5  Fracture toughness of specimens during the second desorption stage at 20ºC (J82.4%RH 35.2%RH)FIGURE 4.  Load and displacement curve for specimens during the second desorption stage al 20ºC(82.4%RH 35.2%RH)Maderas. Ciencia y tecnología 8(1): 25-30, 200630Universidad del Bío - BíoFIGURE 6 Relationship between moisture content and fracture toughnessCONCLUSIONIn this work, it was found that for a given type of specimen, the crack propagation path in desorptionfollows a similar pattern to adsorption as observed in previous studies (Wang, et al., 2003).Specimen crack orientation was found to influence crack propagation path. The direction of crackpropagation for TL and TR specimens was basically along the initial crack direction, namely TL specimenspropagate along the fiber direction, TR specimens along the radial direction or along wood ray direction.Moisture content in the wood influenced wood fracture toughness at a given temperature. For woodmoisture content change from fiber saturation point state to an equilibrium moisture content at 82%RHand 20ºC, fracture toughness for all of the specimens gradually increased with elapsed time. However,with further desorption from 82%RH to 35%RH, wood fracture toughness decreased with time.REFERENCESChinese National Standard.1985. Standard test method for plane—strain  fracture toughness ofmetallic materials. GB4161-84.Chinese National Standard.1992. Method of sample tree collection for physical and mechanicaltests of wood.GB 1927-91.Kretschmann, D.E.; Green, D.W. 1996. Moisture content specific gravity for relationships forclear southern pine. In: Proceedings of the International Wood Engineering Conference.New Orleans,LA. pp. 536–542.Smith, I.; Chui, Y.H. 1994. Factors affecting mode I fracture energy of plantation-grown red pine.Wood Sci. and Technol. 28:147—157.Mamada, S.; Aoki, S. 1992. A test for wood dynamic cleavage. Bull.Niigata Univ. For. (25): 141—146.Wang, L. Y.; Lu, Z.Y.; Zhao, G.J. 2003. Wood fracture Pattern during the Water Adsorption Process.Holzforschung 57 (6): 639–643

Wood fracture pattern during the water desorption process

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Wood fracture pattern during the water desorption process

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