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Jørgen Lauritzen Jensen 《Journal of Wood Science》2005,51(6):577-582
A quasi-non-linear fracture mechanics model based on beam on elastic foundation theory is applied for analysis of the splitting
failure of dowel joints loaded perpendicular to grain. Simply supported beams symmetrically loaded by two dowels are considered,
and the effects of edge distance, dowel spacing, and distance between dowels and supports are accounted for. The foundation
modulus used in the beam on elastic foundation model is chosen so that the perpendicular-to-grain tensile strength and fracture
energy properties of the wood are correctly represented. This ensures that a conventional stress analysis and failure criterion
lead to the same solution as the compliance method of fracture mechanics. A semiempirical efficiency factor is proposed to
account for the influence of the total beam depth, which does not enter the beam on elastic foundation model, but the effect
of which is evident from tests. It is shown that the so-called Van der Put/Leijten model, which recently has been adopted
in Eurocode 5, appears as a special case of the model presented. Tests on simply supported beams with a single dowel joint
at midspan are compared with the theoretical predictions. Various edge distances, beam depths, and spans were tested. 相似文献
2.
白杨刨片横纹压缩流变性能 总被引：4，自引：3，他引：4
用参数分离法对手工刨切的厚为0.8、0.5、0.35、0.25、0.15mm的白杨刨片进行了横纹压缩流变试验。结果表明,用上述几种厚度的刨片所分别叠成的厚约10mm的试件,其柔量大于厚度为10mm木块试件的柔量;刨片越薄,柔量越大。弹性变形及瞬间塑性变形的增长,意味着实木经过刨切,因反复受力而使木材高聚物分子及细胞壁组织都受到影响,并有一定程度的破坏。实木经过刨切成为薄片后再层叠起来所组成的试样,在横纹压缩流变性能方面所体现的变化,一定程度上与木块受压所发生的变化,在趋势上是一致的。然而,由于前者试样系由薄刨片层叠而成,因而在影响其流变性能的诸多因素中,尚有刨片厚度及刨片层叠方向二者明显的影响作用。 相似文献
3.
引用“辐热积”概念,建立叶面积指数模型和基于光合的干物质积累模型。将叶面积指数模型和高斯积分法相结合,依据丹参的冠层结构特点,建立三层模型。选用光合直角双曲线模型,并考虑温度对最大光合速率的影响,量化丹参的呼吸消耗;去除生长后期气温下降、光照减弱等因素造成丹参地上部分枯萎、干物质减少,使后期模拟更为精确,从而建立基于生理生态过程的丹参光合生产和干物质转化积累动态模拟模型。用均根方差、相对误差及拟合指数对独立的试验2数据进行检验,叶面积指数和干物质模型模拟值和实测值间决定系数R≥0.992,模拟值和实测值间符合度达到极显著水平,回归相对误差均在10%左右,拟合指数均接近于1。 相似文献
4.
Jørgen Lauritzen Jensen 《Journal of Wood Science》2005,51(5):480-485
A linear elastic fracture mechanics model for calculation of the splitting strength of dowel-type fastener joints loaded perpendicular
to grain (Van der Put/Leijten model) has previously been presented, and now forms the basis for design in Eurocode 5. The
original Van der Put/Leijten model was derived using a number of simplifying assumptions, e.g., that the normal forces in
the cracked parts of the beam can be ignored, leading to a solution that does not involve the effect of an initial crack.
In the present article an extended version of the Van der Put/Leijten model is derived without any simplifying assumptions,
and it is shown that the original Van der Put/Leijten model appears as a special case, namely by assuming that only contributions
from shear deformations are significant. The model presented here involves the effect of an initial crack and may be characterized
as a generalized linear elastic fracture mechanics model. Results of tests showing the influence of initial cracks of various
lengths are presented and compared with the predictions. 相似文献
5.
董玉库 《东北林业大学学报》1991,19(1):22-29
胶合板、细木工板、刨花板等用作家具及木结构构件时的变形,可按正交各向异性平板理论进行理论分析,且用微分方程来描述。本文通过广义简支边法和直接求解法,对在均布载荷下,两对边简支,另两对边自由的正交各向异性平板弯曲微分方程的解进行了理论推导及分析。得出如下结论:①两种推导方法所得结果可变换成完全相同的表达形式;②不同条件下级数解的收敛速度均很快;③纠正了日本学者平岛义彦关于此问题研究的部分失误;④理论计算值与部分实测值之间尚存在一定偏差,有待于进一步研究。 相似文献
6.
文章定义了一类条件参变量正交系统，它是普通正交理论的一种推广。Ｚ正交理论是以常微分方程定性理论中的中心─焦点问题为背景提出的，期望随着这一理论的进一步完善，能够推动解决中心─焦点系统的焦点量上界问题。 相似文献
7.
Three softwood samples and one hardwood sample were tested under a tension load applied along the radial direction using small
clear specimens and the local tension strain was measured using the digital image correlation method. We successfully obtained
a stress-strain curve with a strain-softening branch by calculating the stress using the strain distributions in the vicinity
where the specimen ruptured. The continuous digital imaging of the specimen proved to be very effective for measuring the
strain in quasi-brittle materials such as wood under tension. The nonlinearity of the stress-strain curve was quantified using
two parameters representing the deviation from linear elasticity, and the formula of the stress-strain curve was deduced from
the interrelation between these parameters. This formula is expressed quite simply by using the modulus of elasticity along
the radial direction and another constant that is unique to the material.
Part of this article was presented at the 56th Annual Meeting of the Japan Wood Research Society, Akita, Japan, August 2006 相似文献
8.
Empirical relationships between remotely sensed vegetation indices and canopy density information, such as leaf area index
or ground cover (GC), are commonly used to derive spatial information in many precision farming operations. In this study,
we modified an existing methodology that does not depend on empirical relationships and extended it to derive crop GC from
high resolution aerial imagery. Using this procedure, GC is calculated for every pixel in the aerial imagery by dividing the
perpendicular vegetation index (PVI) of each pixel by the PVI of full canopy. The study was conducted during the summer growing
seasons of 2007 and 2008, and involves airborne and ground truth data from 13 agricultural fields in the Southern High Plains
of the USA. The results show that the method described in this study can be used to estimate crop GC from high-resolution
aerial images with an overall accuracy within 3% of their true values. 相似文献
9.
为提升马铃薯片品质,采用含不同质量分数氯化钙、葡萄糖酸锌、抗坏血酸的溶液流体处理垂直磁场下的马铃薯片,结果表明,马铃薯片置于磁感应强度1 500 Gs环境下利于溶质的渗透,流速为0.05 m/s处理的马铃薯片品质改善显著,若电解质流体流速过高则会干扰溶质的渗透。空白样即未施加磁场的情况下,产品品质均无任何改善。该技术可实现对马铃薯片的品质提升,包括微量元素的营养强化和保脆处理,且弱电解质即抗坏血酸在流场中也受垂直磁场的影响并富集于马铃薯片组织表面从而使其具有抑制褐变的效果。 相似文献
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