共查询到20条相似文献，搜索用时 187 毫秒
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介绍了一个具有新内涵的最小耗能原理并讨论了它在流变固体力学中的应用.根据基于最小耗能原理的本构关系理论,导出了Bingham体及Maxwell体的本构关系;根据基于最小耗能原理的最小功耗原理,建立了准静态粘弹性力学率型变分原理的两种基本型式.所举的两个算例表明,根据上述新变分原理的两种基本形式,可求得问题的正确解答. 相似文献
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平面直角坐标转换可采用最小二乘法矩阵表达的法方程解算，但由于工程测量坐标数位通常较多，矩阵求解常超过E-500等小型计算设备的数据处理能力，为此，根据最小二乘原理，本文推导了用于平面直角坐标转换的另类平差方法，即公式计算法。利用公式计算坐标转换参数，无需专用矩阵程序，普通函数类计算器亦可方便求解，具有较强的野外实用价值。 相似文献
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宁辉 《中南林业调查规划》2000,19(1):1-4
叙述了非线性回归的原理，着重介绍了Ｍａｒｑｕａｒｄｔ迭代法和加权最小二乘法，并结合二元非线性材积模型给出了具体的参数估计方法和应注意的问题。 相似文献
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林分生长模型建立方法 总被引：1，自引：0，他引：1
在林分生长模型建立中,通过分析变量间的显著影响选择变量．研究模型的评价与检验指标等方法,为建立优选模型提供参考．文中对最小二乘拟合模型中的异方差和多重共线性问题进行了分析,提出了加权最小二乘法、逐步回归剔除法等处理方法． 相似文献
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我国南方集体林区公益林补偿原理与方法的研究 总被引：6，自引：0，他引：6
林业分类经营是当前我国林业工作改革的重点，而公益林补偿问题又是林业分类经营工作中的热点和难点。我们在福建省永安市全国林业分类经营规划试点过程中，试图对南方集体林区不同类型的公益林提出了具体补偿办法，并认为采用损失结合成本费用补偿的办法（即最小补偿原理）比较符合我国目前的国情。 相似文献
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在贮木场工程设计中，对场地平整和填挖方量的计算把最小二乘法的原理应用于方格网法，能使填挖实现基本平衡，工程量得到合理控制，节省平整场地的工程投资。 相似文献
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样地法是测度物种多样怀最常规的方法。本文通过科学地布设样地，借助于Ｐｉｅｌｏｕ教授的总体物种多样性信息度量法，得出了两组具重要意义的结论：（１）八大公山常绿落叶阔叶林和落叶阔叶林两种典型植眸物种多样性估计值分别为４．０２９８、３．８０４２；最大值为４．０４９２、３．８０７３。（２）欲较全面地反映八大公山两种被的上多样性全貌，其调查样方的最小表现面积应不小于：５０００ｍ＾２和４５００ｍ＾２。 相似文献
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将针叶材的超微结构破坏分为细胞壁间分离和细胞壁断裂两种形式。在制备其超细木粉过程中,涉及到针叶材细胞的破壁力计算时,提出以细胞壁中存在大量的原生细观缺陷作为已存在的裂纹,将单一细胞壁看作是带有裂纹体受拉应力的平板,利用弹塑性断裂力学中J积分与积分路径无关这一常数的性质,避开裂纹尖端的弹塑性区域进行理论计算,应用J积分与应力强度因子在平面应力下的关系,计算出针叶材细胞壁断裂韧性的大小,并将不同针叶树种的木材细胞断裂韧性数值与其相应的细胞壁抗拉强度实验结果相比较,得出以此理论计算木材细胞断裂韧性值较为合理,表明利用J积分计算木材细胞壁断裂韧性这一方法是可行的。 相似文献
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Jørgen Lauritzen Jensen 《Journal of Wood Science》2005,51(6):566-571
A quasi-non-linear fracture mechanics model based on beam on elastic foundation theory is applied for analysis of the double
cantilever beam (DCB) specimen for determination of fracture energy of wood. The properties of the elastic foundation are
chosen so that the perpendicular-to-grain tensile strength and fracture energy properties of the wood are correctly represented.
It is shown that this particular choice of foundation stiffness makes a conventional maximum stress failure criterion lead
to the same solution as the fracture mechanics compliance method. Results of linear elastic fracture mechanics are obtained
as a special case by assuming an infinitely large value of the perpendicular-to-grain tensile strength. The quasi-non-linear
fracture mechanics model is compared with other models and with results of tests conducted to reveal the influence of the
geometrical properties of the DCB specimen. In addition, the appropriateness of choice of the foundation stiffness is investigated. 相似文献
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竹材横向断裂的物理模型与能量吸收机制:纤维束的断裂与抽拔 总被引：1，自引：0，他引：1
采用细观力学方法,研究竹材在横弯断裂过程中竹纤维束断裂与抽拔这2种损伤模式的能量吸收机制,并实际计算基本组织开裂、界面分层、竹纤维束断裂、竹纤维束抽拔4种损伤模式对竹材横弯断裂的增韧贡献。结果表明:1)不同组织结构在损伤演化过程中会因不同的能耗而具有不同的增韧贡献,在这4种导致竹材优良强韧性能的主要结构因素中,单位面积上纤维束拔出功对断裂功的贡献最大,其次是纤维束断裂;2)通过对试件的断裂总耗能试验值与按照4种损伤模型计算的耗能理论值进行比较,二者结果很接近,表明本文对竹材不同组织结构在横弯失效中的力学功能所建立的物理模型基本正确。 相似文献
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Jørgen Lauritzen Jensen 《Journal of Wood Science》2005,51(6):559-565
A quasi-non-linear fracture mechanics model based on beam on elastic foundation theory is applied for analysis of dowel joints
with a single dowel loaded perpendicular to grain. The properties of the elastic foundation are chosen so that the perpendicular-to-grain
tensile strength and fracture energy properties of the wood are correctly represented. It is shown that this particular choice
of foundation stiffness makes a conventional maximum stress failure criterion lead to the same solution as the compliance
method of fracture mechanics. Results of linear elastic fracture mechanics are obtained as a special case by assuming an infinitely
large value of the foundation stiffness. Results of tests on so-called plate joints are compared with theoretical predictions,
showing good agreement for variations in initial crack length as well as edge distance. 相似文献
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In this paper, an experimental analysis of the fracture parameters via the invariant J-integral for a cracked specimen made of wood is presented. The experimental test is realized using a sample made of Douglas fir loaded in opening mode. The sample geometry is a mix between the single edge notch and wedge splitting specimens, and the crack advances in the radial-longitudinal system, parallel to the wood rings. By using the optical mark tracking method, the displacement field evolution close to the crack tip is recorded during the test. The stress and strain fields are calculated using a finite element model generated from the experimental displacement fields. Further, the energy release rate is evaluated for different circular paths or crowns defined around the crack tip and for different loading values. 相似文献
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Axially loaded glued-in hardwood dowels 总被引：4，自引：0，他引：4
J. L. Jensen A. Koizumi T. Sasaki Y. Tamura Y. Iijima 《Wood Science and Technology》2001,35(1-2):73-83
The failure load of axially loaded hardwood dowels glued-in parallel to the grain direction of the jointed timber parts is
considered. Two simple theoretical solutions using linear elastic fracture mechanics/ideal plasticity and linear elastic stress
analysis, taking into account the finite shear stiffness of the bond line are, presented and compared with experimental results.
Theory shows that bond line shear strength is the governing strength property for ductile joints and fracture energy is the
governing strength property for brittle joints. Bond line shear strength and fracture energy are determined by means of curve-fitting.
Received 24 April 1997 相似文献
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《Wood material science & engineering》2013,8(4):176-185
Abstract A new testing method measuring the specific fracture energy of wood-based panels in Mode I is proposed. Three types of wood-based panels, i.e. oriented strand board (OSB), particleboard (PB) and medium density fibreboard (MDF) are investigated, using fracture energy and the industrial European standard method of internal bond strength according to EN 319. Double cantilever beam specimens are notched in the middle layer to introduce an initial crack. To apply tensile load perpendicular to the surface of the panels to open the crack in Mode I specimens were adhesively bonded to steel braces. Besides the calculation of the total fracture energy an advanced analysis of the load–displacement curve was also performed. Results of the fracture energy method were compared to internal bond strength (IB). Specimen shape is optimized for industrial purposes using double cantilever beams, while the determination of the fracture energy is performed by simple integration of the load–displacement curve. While IB showed a large scattering of data, the fracture energy test yielded statistically significant differences between the board types. 相似文献
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A quasi-nonlinear fracture mechanics model is presented for the analysis of row shear failure in timber connections with multiple
fasteners in a row. A prerequisite for use of the model is a known distribution on the fasteners of the total applied load.
It is shown that the ideal plastic and linear elastic fracture mechanics solutions appear as special cases of the quasi-nonlinear
model. The model offers strength predictions that include the effect of among others the number of fasteners in a row, fastener
spacing, row spacing, end-distance, edge-distance, fastener diameter, and material properties, such as shear strength, fracture
energy, and modulus of elasticity. Simple explicit expressions are obtained from the analysis for any known fastener load
distribution. For bolted connections, where bolt-hole gaps cause high loads on certain bolts at random, the model may be used
in simulation procedures or in probabilistic models. 相似文献