基于分组截尾数据Weibull分布的参数估计

Weibull分布是寿命试验中最常见的分布之一,因而基于分组截尾试验数据下的Weibull分布的参数估计是很有研究价值的。介绍了一种基于分组截尾数据而寿命服从Weibull分布的试验样本,并得到失效概率的Bayes估计。最后由最小二乘法得到其参数估计。     

Simple Shear Test of Interface between Sand and Structure under High Stress

By using a simple shear apparatus that is developed from RMT-150B,a series of simple shear tests of interface between saturated sand and structure under high normal stress were carried out to study constitutive law and shearing stiffness of interface to provide some theoretical evidences for design and calculation of projects concerned with mechanical problem of interface.In test,sand was consolidated in upper shearing box firstly,then,shear was started and testing data was recorded under high and constant normal stress.During the process,sand can deform relatively freely.It is shown that the relation curve between shear stress and shear displacement under high normal stress in simple shear tests is different from that in direct shear tests,and the complete shearing deformation process of interface under high stress can be described as a nonlinear elastic-perfect plastic form;Weibull distribution with three parameters can overcome the mathematical deficiencies of hyperbola in NEPP.The new model which is established by substituting Weibull distribution for the hyperbola of NEPP better agrees with the testing data.The regressive results of the initial shearing stiffness demonstrate that the linear function is slightly better than power function to express the relation between the initial shearing stiffness and normal stress.Based on the new interface model,the formula of tangential shearing stiffness is proposed,meantime,the change trend of tangential shearing stiffness of the new model is compared with that of NEPP.     

Comparing normal,lognormal and Weibull distributions for fitting diameter data from Akashmoni plantations in the north-eastern region of Bangladesh

Statistical probability density functions are widely used to model tree diameter distributions and to describe stand structure. The objective of this study was to compare the performance of normal, logarithmic-normal and three-parameter Weibull distributions for fitting diameter data from Akashmoni (Acacia auriculiformis A. Cunn. ex Benth.) plantations grown in the north-eastern region of Bangladesh. Data from 96 plots, established in 24 plantations in north-eastern Bangladesh and ranging in age from 1 to 6 years, were used for this study. The parameters of the Weibull distribution were calculated using maximum likelihood estimation (MLE) and moment estimation (ME) methods. The goodness of fit of normal, lognormal, Weibull MLE and Weibull ME were tested using one-sample Kolmogorov-Smirnov (KS) tests. The KS test results showed that both lognormal and Weibull distributions were equally effective for describing the diameter distributions of these Akashmoni plantations grown in the north-eastern region of Bangladesh.     

Characterising the diameter distribution of Sal plantations by comparing normal,lognormal and Weibull distributions at Tilagarh Eco-park,Bangladesh

For many years foresters have been using statistical probability density functions to describe and characterise stand structure. Predicting the current and future yields of a stand is essential for successful stand and timber management. Implicit prediction of current yield is accomplished by using diameter distribution methods. All diameter distribution yield systems predict the number of trees per unit area by diameter class. In this study, the normal, lognormal and the three-parameter Weibull probability density function were compared to characterise the diameter distributions of Sal (Shorea robusta) plantations grown at Tilagarh Eco-park, Bangladesh. Data from 70 plots, established in three plantations, were used for this study. The Weibull parameters were estimated by the maximum likelihood and moments estimator methods. A one-sample Kolmogorov–Smirnov test was used for the goodness of fit for all models. The Kolmogorov–Smirnov test results showed that both lognormal and Weibull distributions were suitable to characterise the diameter distributions of Sal plantations in the study area and may be applicable for other Sal forests in Bangladesh.     

枣庄市主要树种直径结构比较研究

分析了枣庄市市中区3种主要树种黑杨、刺槐和侧柏的直径结构特征,运用5种分布函数正态分布、Weibull分布、对数正态分布、Gamma分布以及logistic分布对这3种树的林分的直径分布进行了拟合,同时用卡方统计量对5种分布进行了检验,拟合和检验得出了一致的结果。     

长白山过伐云冷杉恢复林分主要树种径阶生长与枯损模拟

以长白山过伐林区金沟岭林场的云冷杉林4个局级固定样地连续12年的观测数据为研究对象,利用固定样地内主要针叶树种红松、冷杉和云杉,从1978年到1984年6年内的胸径与定期平均生长量对应值数据,建立林木径阶生长转移概率模型,预估林木径阶平均生长量,并利用1990年观测数据进行检验,结果表明:所建概率模型实际应用误差较小,精度较高;同时还分析了1978年至1990年12年间云冷杉林的枯损林木株数分布特征,通过模型模拟和检验,表明Weibull分布函数适用于异龄混交林的枯损株数分布模拟。     

新丰林场华北落叶松人工林的直径结构分析

森林直径结构对开展森林经营活动有重要作用。本研究以新丰林场的15块华北落叶松人工林为研究对象,采用样地调查的方法,利用Weibull分布对华北落叶松人工林林分直径结构进行研究,并对Weibull分布结果进行卡方(x2)检验。结果表明:66.7%符合Weibull分布密度函数,反应了调查地区华北落叶松人工林的直径分布不太合理,林分密度需进行适当的调整。     

华梵前山校区风速统计分布之探讨

研究针对华梵前山气象站2002年5月至2006年12月之小时风速资料进行统计分析,探讨华梵前山风速资料之分布是否合乎韦伯分布(Weibull distribution)及莱利分布(Rayleigh distribution).研究中韦伯分布使用动差法(Method of moment)及最大概似法(Method of maximum likelihood)求取参数;莱利分布使用动差法求取参数.进一步以K-S检定,判断各理论累积机率分布与观测值之累积机率是否相符;再以误差绝对值之总和判断何种分布较为适合.分析结果显示:韦伯分布与莱利分布均可适用于描述全部数据之机率分布.将资料分为夏半年(4-9月)与冬半年(10月至翌年3月)时,韦伯分布为最适用之机率分布函数;且夏半年之参数以最大概似法求得最佳,而冬半年则以动差法较佳.此结果可作为校区风蚀、防灾、生态、环境风场特性及评估风能潜势研究之参考.     

工厂化集中育秧模式下荆州市双季早稻低温冷害风险分析

利用荆州市6个国家气象站1981-2014年早稻生育期和日平均气温观测资料,统计传统露地育秧(TS)情况下早稻的生育期和积温,设定比传统方式提前10d播种于温室集中育秧(CS-10)和提前20d播种于温室集中育秧(CS-20)两种育秧模式,推算出相应的生育期,利用日平均气温观测资料和weibull分布模型,统计分析当地采用这两种育秧模式时早稻关键生育期冷积温(指早稻某生育期内冷害临界温度指标与日平均气温差值的累积值)变化及概率密度分布,比较分析3种育秧模式下早稻遭遇低温冷害的风险。结果表明,相较于TS模式,提前10d集中育秧时,移栽-分蘖始期、幼穗分化始期、抽穗开花始期、灌浆结实始期和收获始期分别提前14d、10d、8d、11d和7d;提前20d集中育秧时,各生育期则分别提前24d、15d、11d、17d、10d。其中,轻度冷害发生概率在发育前期(移栽-分蘖期、幼穗分化期)逐渐下降,发育后期(抽穗开花期、灌浆结实期)逐渐增加,而中度以上冷害的发生概率在发育前期增幅较大,其中提前10d集中育秧时,移栽-分蘖期和孕穗分化期的冷害概率分别为14.7%、49.7%;提前20d集中育秧时,冷害概率分别可达57.1%、71.7%,是传统育秧模式的数倍。因此,为降低冷害风险、保障早稻稳产,荆州市早稻集中育秧模式的适宜播种期应不早于3月25日,即比传统育秧模式提前7d左右。