Research On Creep Behavioral Regularity Of Wood Beam In Varying Humidity

Creep of wood beams in varying humidity of dynamic environment was calledmechano-sorptive creep (MSC). Because there was essential distinction between MSC andthe creep in uniform humidity, which was also called normal creep, and the deflection ofMSC was so much larger than that of normal creep, the law and the results of MSC havebeing drawn great interesting in the developed countries since the first discovery of MSC byArmstrong in1960. However, since there was no acknowledgeable scientific conclusion onthe law of MSC, the mathematic calculation of the MSC failed or has being blocked and theindustry could hardly take it into engineering application. The outstanding symbol of thissituation is that, up to now, any testing standards for creep, even in developed countries, arestill ruled with the law in uniform humidity. The suspense of MSC make people distrustingand worrying about the long term anti-bending capability of laminated veneer lumber (LVL)which was widely used as a flange of wood I-beam or the other structural members in indoorconstant varying humidity. Therefore, this research wanted to deepen the investigation ofthe law and the appearance of the MSC of solid wood of Populus (PSW in brief; as abreakthrough and control) and the LVL of Populus bonded with PF resin (P-PF-LVL) andthe influences by the content and distribution of moisture, load and the processing swellingand shrinkage, esp. those in the first adsorption, with the innovated systems for orthogonalexperiments and analyses.In order to satisfy various aspects of requirements for creep characteristic in varyinghumidity such as space system for constant temperature and varying humidity or uniformhumidity, testing, monitoring and analysis system of creep deflection, swelling andshrinkage of kinds of wood beams, supporting system for testing specimens, load and itsstart system, and the dynamic, real-time, precision, continuity, stability, security andcontrollability of dynamic support system, etc., this research firstly made a study on thetesting principle and its relationship with technical conditions of testing system and then acomplete set of creep testing system in varying humidity for wood beams was formed so asto make sure the smooth development of this research and the scientific, validity andreliability of testing process and results, through design, fabrication or outside processing ofmechanical system, entrusting specialized units to make design and fabrication of hardwareand software of testing, monitoring, data collecting, and analysis system according to the requirements presented by this research.Results from the contrast test on the law of creep in varying humidity and creep inuniform humidity confirmed the point view of predecessors in the opinion of creep directionpresented by this research: a) Creep in uniform humidity was just normal creep that itsdeflection was continually positive, while creep in varying humidity was a kind of creep thatthe direction of MSC was different in the adsorption and in the desorption(excluding the firstadsorption); b) difference of moisture gradient(DMG) may be the motivation of MSC; c)total creep deflection in varying humidity was larger than that in uniform humidity；what’smore, this research discovered and confirmed that:1) Although the overall behavior anddefection of creep in varying humidity was the same as that of creep in uniform humidity,the fundamental difference was that creep in uniform humidity was a kind of static creepthat its moisture condition, moisture content, and basic properties of material were allconstant while creep in varying humidity was a kind of dynamic creep that basic mechanicalproperties and section size of wood beam resulted from the change of moisture conditionand moisture content were varying and the law of creep in the adsorption and that in thedesorption were different, according to the static state and the dynamic state of moisturecondition;2) minus creep only happened in the early stage of the adsorption. The directionof creep in the middle and late stage of adsorption (period that DMG significantly reduced)were positive.Results from the L9(34) orthogonal creep test in varying humidity which used DMGand load(P) as the variables made an unprecedented discovery and validation that:1) P wasthe basic cause of creep in varying humidity, magnitude of positive deflection of P creepwas decided by the magnitude of P;2) DMG was the motivational factor of direction anddeflection of creep in varying humidity; According to the definition of orientation ofmoisture gradient(the direction of DMG was positive in the desorption while minus in theadsorption), the direction of DMG determined the direction of MSC. Positive DMGdetermined the creep in desorption was positive. Minus DMG on the one hand, determinedthe creep in the adsorption was minus. On the other hand, combined with P changed thedirection performed by creep in the adsorption. Magnitude of MSC was determined by themagnitude of DMG and its interaction with P;3) DMG made a boost effect on P(DMGp)which was similar with power relationship and resulted in a large incensement of positivedeflection of creep in the desorption(total creep deflection in the desorption was absolutelylarger than total creep in uniform humidity which was the maximum of varying humidity condition), if stimulated in the early stage of creep(peak period of creep), creep deflectionwas greater in the first adsorption, even there was positive deflection in the subsequentadsorption by the stimulation；Magnitude of creep in desorption, positive deflection of creepin the first adsorption and whether there was positive deflection in the subsequent adsorptionor not were all determined by the magnitude P, DMG and DMGp;4) P made blockage onminus deflection induced by DMG. Its magnitude was determined by the magnitude of Pand DMG; This blockage determined there was positive or negative creep deflection in theadsorption and the magnitude of minus creep deflection in the adsorption;5) A newgeneralization of MSC’s law was presented, MSC behavior and difference between thepositive creep direction and minus creep direction was basically caused by the independenteffect and direction of DMG and P, their dynamic competition with each other, boost orblockage effect and the happening time of all these effect in the creep process; Direction ofDMG determined there was positive creep in the desorption while there was minus creep inthe adsorption; Direction of creep in the adsorption(especially the first adsorption) wasdetermined by the magnitude of DMG and P, DMGp, P’s blockage on minus deflectioninduced by DMG, and the happening time of these effect. When DMGp was the leadingfactor, there would be positive creep deflection in the adsorption. When P’s blockage wasthe leading factor, there would be minus creep deflection in the adsorption.Analysis and results from test on P-PF-LVL in above-mentioned condition showed that:1) the overall law of LVL’s creep in varying humidity and creep in uniform humidity wasthe same as that of solid wood;2) because the internal structure of LVL and solid wood weredifferent and in this test static bending modulus of elasticity (MOE) of P-PF-LVL(10370Mpa) was less than that of PSW (14730Mpa). That was to say, the bending resistanceability of P-PF-LVL was lower than that of PSW, the positive creep effect induced by P andDMGp on P-PF-LVL was larger than that on PSW, which resulted in the differentappearance time of minus creep in the adsorption between P-PF-LVL and PSW. In addition,the positive creep deflection in the first adsorption was much larger, even there was positivecreep in the second adsorption.3) The emergence of positive creep in the second adsorptionfurther validated the significant findings of this study, i.e., direction and magnitude wasdetermined by the dynamic competition, boost or blockage effect between DMG and load.At the same time, analysis and results from above-mentioned tests showed that:1)dynamic change of internal moisture condition and moisture content of wood beams in thecreep process in varying humidity resulted in different degree of swelling or shrinkage; Just as creep deflection in varying humidity, swelling and shrinkage of wood beam mainlyoccurred in the early stage of adsorption or desorption;2) The swelling and shrinkage couldnot alter the basic law of mechano-sorptive creep of the wood beams but affected themeasure value of deflection from either adsorption or desorption;3) the accumulatedthickness swelling and shrinkage within the2cycles respectively occupied-0.63%of thetotal bending creep deflection(TBCD) of the Populus and1.27%of that of the Populus LVL.Creep deflection in varying humidity was much larger than that in uniform humidity.Creep test in2.5varying humidity cycles from RH42to80under25%failure load on100*10*5mm(length*width*depth) PSW showed that, TBCD of PSW after2cycles in RH42%~80%~42%followed with an adsorption up to80%was0.468mm, which was1.80times as those (0.260mm) in the same period in the uniform RH80%. The TBCD of P-PF-LVL after2cycles in RH42%~80~42%followed with an adsorption up to80%was1.403mm, which was1.46times as those (0.961mm) in the same period in the uniformRH80%.Because creep resistance ability of P-PF-LVL in uniform humidity in this test was lowerthan that of PSW(creep deflection of P-PF-LVL in uniform humidity(0.961mm) was3.70times than those of PSW(0.260mm)) or because static bending MOE of P-PF-LVL was lessthan that of PSW, although creep resistance ability of P-PF-LVL in varying humidity wasbetter than that of PSW(the ratio of P-PF-LVL’s creep deflection in varying humidityto thatin uniform humidity(1.46) was notably lower than that of PSW(1.80), the TBCD of P-PF-LVL(1.403mm) after2.5varying humidity from RH42%to RH80%under25%failure loadwas3.00times than those(0.468) of PSW. Therefore, the basis for improving creepresistance ability of LVL was to improve MOE of LVL, lower the absorption ability of LVLand made strict controls on manufacturing process of LVL(especially for Populus LVL).Based on the above results of research, this paper puts forward suggestions:1) theproducers, designers, and users of wood beams together with relevant standards and normsshould take the effect of creep in varying humidity on the deflection of wood beams inservice into account;2) our nation should make a supplement of relevant standards andnorms, such as creep in uniform humidity, creep in varying humidity, normal moisturecontent, et al to describe and analyze the creep behavior of wood beams clearly.The scientificity of this research was to reveal the law of MSC’s behavior. Thepracticability of this research was to provide a basis and methods for the quantitative analysisof MSC and engineering application, analysis and design of service ability of IB and fabrication and quality control of IB. The prime innovation of this research was to present anew complete interpretation about law of MSC’s behavior especially in the first adsorptionboth at home and abroad, then the innovation of think methods, test equipment andpresentation or clarification on some nouns concept, such as creep in varying humidity, MSC,OMG, NMC, etc. Deficiencies of this research was that shortage of research strength andtesting equipment resulted in the lack of testing numbers and testing sizes.