The Analysis of Creep Behaviours of Wood and Wood Composite Materials (I)

木材及木質複合材料潛變之解析（一）

彭武財(W. T. Peng)

《林產工業》 19卷3期 (2000/09) Pp. 389-398

摘要

潛變是一種變形，係材料經過負荷至一持續時間，而以震動或靜止狀態下產生者。此種變形通常開始時急速，而後逐漸緩慢進行或達至平衡狀態，甚至終於有破損發生。有些材料之潛變僅隨HOOK'S之彈性法則，有些則隨NEWTON'S之粘性原理，而有時亦似隨BINGHAM之塑性理論或其他原理，以單純地或複雜地組合而進行者。木材及木質複合材料之潛變舉動通常適合由較複雜組合而成者來表示之。基本上，影響木材及木質複合材料之潛變舉動之因子可以由外在的與內在的兩方面加以說明之。前者如潛變持續之時間，負荷大小以及周圍之狀態如濕氣，溫度及含水率等。後者則更為複雜，諸如樹種，負荷方向，材料之種類、其密度、構成方式包括如組成單位基料之大小，組成方向，粘膠劑之種類及其粘著性，製造變數以及組合單位基料之破損情形，產品之後處理等。此等影響因子係由潛變實驗中所得來之數據作為解析其潛變舉動時之主要參考資料。其中尤其如溫度，濕度以及含水率最為突出。此乃由於此三因素對於木材及木質複合材料之物理及機械特性有高度之敏感性與嚴酷性所致。此尤其對於由木材粒片，剝切片以及木纖維所組合而膠固之材料更呈顯著。此乃由於此等材料在製作過程中由熱壓所遺留下來之剩餘壓縮應力在高溫高濕狀態下放散出來，引起其厚度之膨張與膠著點之拉斷等缺陷出現所致！此等影響因子其實不是容易地可以獨立地被分述出來，譬如由UF及PF膠合之粒片板之潛變舉動常見有彼此矛盾的現象；高含膠率之板類並不一定有較良好之抗拒潛變之效果；減小粒片之形狀亦不致有較差之潛變抗拒；板類由鉋屑及精細剝切片所製成者不一定常見有其顯著之差異等。但，由較大及較薄之剝切片所製成之木質複合板類基本上較粗短粒片所複合製成者有較低之潛變。又抗彎韌性通常與潛變性質相悸。一般言之，木材對潛變之抗拒最大，其次依順序為合板、長剝切片板、方薄片板、粒片板至纖維板為最小。在正常之情形下，木材在其50%之最大破壞應力水平下可以期待有其線型性。合板則最高為30%，方薄片板及粒片板則在20%左右。如由潛變速度來定位潛變限度時，木材則被認定略少於50%之最大破壞應力水平，粒片板則約15%，而纖維硬板則差不多1.5%而已。木材及木質複合材料所求得之潛變曲線通常較難轉換成適合於較簡單之數學方程式中。在統計允許之範圍內，與潛變特性有相關之諸因子亦可引進該適合之方程式中，以便得到更深入而詳細之表示。同樣地，木材及木質複合材料之潛變舉動亦可適合以粘彈性之機械模型來作表示。其彈性、粘性以及塑性或其他諸特性可以由適恰之方程式或機械模型中求得之。四因素Voigt模型加上一滑機可以適恰地代表纖維硬板之潛變舉動。又非一般化Voigt機械模型加上一滑機與否，可以勉強地代表木材及木質複合材料之潛變舉動。此等材料亦加以長．短持續潛變與其交替潛變測驗結果發現纖維硬板勉可適納Boltgmann之疊合原理。由單板被覆之粒片板較無單板複覆者，其遲緩函數曲線顯示在較短暫之遲緩時段內其值較低，且又少有變化。而其遲緩頻譜（Retardation spectrum）值之高低與其持續時間亦與上情形相似。然而纖維硬板之遲緩函數值則依其所負加之應力大小而呈不同高低之緩弛S字型狀，而其遲緩頻譜則全呈鈍圓狀鍥刀型狀。

關鍵字

潛變舉動 ； 實驗方程式 ； Voigt模型 ； 降伏點 ； 非一般化Voigt模型 ； Bingham塑性 ； Boltgmann之疊合原理 ； 遲緩函數 ； 遲緩頻譜

並列摘要

Creep is a type of deformation or strain which resulted from an imposition of force or stress and a given duration time statically, cyclically and/or dynamically on the materials. This deformation or strain usually increases drastically first, and then, become gradually or reaching to an equilibrium, or even get failure of materials by the given time. Creep behaviours of some materials simplely following Hook's elastic law or Newton's viscous principle, and some are resembling of the Bingham's theory or other principles, by the combining of them simplely and complexely. Creep behaviours of wood and its relative composite materials especially fit into those complexely combined models manipulated by these theoris or principles for aequring more adequate expressions. The factors influencing creep behaviours of wood and wood composite materials basically can be specified from the external and the internal phases. For the case of former phase, the creep duration time, imposed creep stress and the surrounding environments, such as humidity, moisture content and temperature etc. The latter phase usually are more varied, such as wood species, load direction, type of materials, their density, structural formation including geometry and orientation of the formed elements, the type of adhesives and their adhesions, the manufacture variables and the degrees of brokendown of structural elements as well as the after treatments etc. These factors are important parameters for considering the analysis of creep behaviours which obtained from the empirical data. Especially, the temperature, humidity and the moisture content are most pronounced factors since these parameters are highly sensitively and severely affect their physical and mechanical properties. The materials which formed and consolidated with wood particles, flakes or even the fibers bonded with the varied binders show particularly significant. The reason is .their residual compressive stresses during the hot pressing has to be released as soon as they are exposed in higher temperature and humidity condition, and consequently, the increasing of thickness springbock and fracturing the bonds thereafter, inevitablely. These factors however, are difficult to be clearly described, e.g. the creep performance of particleboards bonded with UF and PF resins showed contradictorily; boards with high resin content not always disply an improved creep resistance; decreasing of particle size also not always shaus the increasing of creep; boards composited with the shavings and the engineered flakes are not usually tend to be significantly different etc. Wood composite panels made from larger and thinner flakes basically display less creep than that of those made from cubical particles, and following by the plywood, strandboard, waferboard, particleboard and the fiberboard in order. Creep behaviours of solid wood responses in normal condition can be expected to be linearity under the applied stress of less than 50% of short-term static ultimate strength. Plywood shows maximum around 30%, and the waferboard, particleboard response an about 20%. On the other hand, from the rate of creep to determine the creep limit stress for solid wood is found to be a slightly less than 50%, particleboard is found to be around 15% and hardboard is only around 1.5% of respective ultimate stresses.The creep behaviours of wood and wood composite materials also can be expressed by the viscoelastic models. Their elasticity, viscosity and the plasticity or other variables can be determined either from the fit equations or from these assumed mechanic models. Four element Voigt model with a slip mechanism can be fit properly to the creep behaviour of hardboard. The degeneralized Vogit model with or without slip mechanism also can barely be fix to the particleboard and solid wood. The suitability of superposition principle for these materials was also examined and found that the handfoard could farely complied it fased on the results of their long-and short-duration creep and creep recovery tests. Particleboand with veneer overlaied showed greatly lower with less changed retardation function ψ in a comparatively shorter retardation time τ, and the level, time period of the retardation spectrum L(τ) also showed similanly as above deacribed in contrast with those oftained from the foard with no veneer overlaied. The retardation function ψ for the hasdfoand seemed to be the loosely S-shaped curves which were varied with the leuels of stress applied. Their retardation spectrum L(τ) however, showed somewhat rarndly wedge-shaped curves for all of those different stresses applied.

並列關鍵字

Creep behavior ； Empirical equation ； Voigt model ； Yield point ； Degeneralized Voigt model ； Bingham plasticity ； Boltzmann’s Superposition principle ； Retardation function ； Retardation spectrum