Mode II wood fracture characterization using the ELS test

This paper describes experimental and numerical studies on the application of the end loaded split test to mode II wood fracture characterization. A new data reduction scheme, based on the specimen compliance and on the equivalent crack concept, is proposed. The method presents three main advantages relatively to the classical methodologies: it does not require crack measurement during propagation; it accounts for the root rotation at the clamping point and includes the effect of the fracture process zone at the crack tip. The new procedure was numerically validated using a two-dimensional finite element analysis including a cohesive damage model, which allows the simulation of crack initiation and growth. The results demonstrated the good performance of the model and the applicability of the end loaded split test for mode II wood fracture characterization.     

用单轴拉伸实验研究HIPS的银纹增韧现象

高抗冲聚苯乙烯 (HIPS)是通过颗粒填充进行改性以提高其力学性能的工程塑料 ,通过单轴拉伸实验对其银纹损伤及增韧现象做初步探讨 ,发现银纹增韧机理是橡胶增韧中主导因素 ,分析得到受拉HIPS的弹塑性损伤本构方程的一般形式 .     

Microstructural study of carbonized wood after cell wall sectioning

Wooden blocks of Japanese cedar (Cryptomeria japonica) were carbonized at 700 and 1,800 °C. The microstructure was analyzed by transmission electron microscopy (TEM) and μ-Raman spectroscopy of the inner planes of wood cell walls. The predominant structure was of a turbostratic nature and no heterogeneity was observed originating from the original cell walls. TEM observations of samples carbonized at 1,800 °C showed ordered regions in the surface layer of cell walls. This result was supported by polarized μ-Raman analysis. It may be caused by the deposition of carbon compounds volatilized from the cell walls during pulse current heating.     

Analysis of creep and modulus loss of the wood cell wall

This paper proposes a nonlinear constitutive model for the wood cell wall based on the nonequilibrium thermodynamics. The wood cell wall is modeled as a long fiber-reinforced composite with cellulose microfibril enclosed by hemicellulose and lignin. An internal variable is introduced into the Helmholtz free energy of the cell wall system, to describe the modulus loss of hemicellulose due to moisture absorption. The viscoelastic behavior of the wood cell wall changes with its moisture content, which leads to different creep evolutions even under the same loading level. To account for this phenomenon, another internal variable is introduced to depict the creep behavior of the wood cell wall, which is correlated with the irreversible energy dissipation processes such as stick–slip mechanism in the wood cell wall. Based on five elastic coefficients of transverse isotropy predicted by the present model, the creep behaviors of the wood cell wall with different microfibril angles are theoretically analyzed and show good agreements with experiment results.     

基于3D打印的木材细胞壁仿生设计

木材中起骨架作用的纤维素是以不同螺旋结构的微纤丝形式存在于细胞壁中。本文通过将3D打印技术与仿真模拟相结合,研究木材细胞壁的纤维螺旋增强结构。使用微晶纤维素(MCC)/聚乳酸(PLA)复合材料,在对MCC/PLA复合材料各项性能进行测试的基础上,借助3D打印技术构建木材细胞壁螺旋结构,通过改变纤维取向和纤维孔状结构编程合成结构的力学功能。有限元仿真则用于强调纤维在刚性单元之间的载荷传递机制中的关键作用。结果表明：通过编程纤维的取向和结构可以宏观调控结构的性能,其中纤维的交叉结构作为一种优化设计可以用于提高结构成型制品的力学性能。这些结构可以被组装成更大的系统,用于构建具有优化特定功能的模块化复合材料;在异质结构设计和新型复合材料制造领域中均具有潜在的应用价值。     

Mixed-mode I/II wood fracture characterization using the mixed-mode bending test

In this work fracture characterization of wood under mixed-mode I/II loading is addressed. The mixed-mode bending test is used owing to its aptitude for easier alteration of mode ratio. Experimental tests were performed covering a wide range of mode ratios in order to obtain a mixed-mode fracture criterion for the maritime pine (Pinus pinaster Ait.) in the RL crack propagation system. A data reduction scheme based on beam theory and crack equivalent concept was used to overcome some difficulties inherent to the test. The method does not require crack length monitoring during propagation and provide an entire resistance curve allowing easier identification of the fracture energy. A numerical analysis using cohesive elements was also performed to validate the method. The linear energetic fracture criterion was proved to be the most adequate to describe the failure envelop of this wood species.     

Fabrication of honeycomb-patterned cellulose material that mimics wood cell wall formation processes

Wood cell wall, composed of polysaccharides (cellulose and hemicelluloses) and an aromatic polymer (lignin), exhibits a honeycomb-like alignment. We have been making attempts to fabricate cellulose-based materials to reconstruct these wood components artificially, mimicking their formation processes. Those attempts are aiming not only at better understanding of the significance and the function of each wood component, but also at providing a novel, biomass-based polymer material with functionality. This article outlines a protocol to prepare honeycomb-patterned cellulose films with two different polymorphisms, carrying different pore sizes, as a basic framework of the artificial cell wall structure. It also illustrates the effect of the presence of hemicellulose and lignin on the physical property of the honeycomb-patterned cellulose films, when they were adsorbed onto the films.     

Estimation of width of fracture process zone in spruce wood by radial tensile test

Fracture behavior of spruce wood under radial tension was analyzed using nonlinear fracture mechanics as wood is classified as a quasi-brittle material. Stress-strain relationship with a strain-softening branch was obtained by digital image analysis and stress redistribution process, and the energy release rate of serial end-matched specimens was measured by performing a single-edge-notched tension test.The width of the fracture process zone (FPZ) was estimated by comparing two kinds of fracture energies. One was the dispersion energy per unit area to model strain localizations using a discontinuum model of damage theory, by integrating the stress-strain function with the strain-softening branch. The other was the energy release rate to determine crack growth. From this analysis, we determined that the width of the FPZ ranged from 0.3 mm to 0.5 mm in the radial direction. However, for a few specimens, the approximate stress-strain function could not be fitted into the stress-strain relationship obtained by the image analysis; it was observed that the fracture planes of these specimens tended to be more or less inclined.     

Damage of the cell wall during extrusion and injection molding of wood plastic composites

To study material damage in wood cells during any transformation process, one must consider the molecular architecture of natural cellulosic fibers, which may eventually impact the overall mechanical behavior of wood fibers. In particular wood species, anatomical features and mechanical properties of the cell wall may determine the potential for stress transfer in hybrid materials. In this study, we quantified wood cell damage in terms of the stiffness reduction of the S2 layer for the cell wall by measuring Young’s modulus with nanoindentations of the cell wall before and after processing. We then propose and validate a modified rule of mixtures based on a damage parameter affected by the latewood proportion and cell wall properties.     

The essential work of fracture through the wall of a uPVC pipe

The specific essential work of fracture, w _e, has been measured for a relatively thick walled uPVC pipe as a function of position through the wall of the pipe. w _e was highest at the surface of the pipe and decreased significantly at the centre of the pipe wall. The variation in w _e through the wall of the pipe correlated with the processing level of the uPVC material as measured by the critical temperature, T _c. The variability in the measured values of w _e was substantially higher in the centre of the pipe where the processing levels were lower. This was likely to be a result of the variability in the microstructure of the material where poor processing had introduced regions of poor fusion of primary PVC particles.     

Improved fracture toughness of ultrahigh strength steel through control of non-metallic inclusions

In recent years, ultrahigh-strength steels, which can be employed successfully at yield strengths of 1400 MPa or higher, have been used increasingly for critical structural applications in aircraft and aerospace vehicles. Most recently, there has been increased demand, however, for ultrahigh-strength steel with superior plane-strain fracture toughness, K _IC, and for the steels suitable for large-sized structural applications; isotropy regarding the property has especially been required. One potential solution to this problem is to control nonmetallic inclusions of the steels. This review concentrates on recent topics concerning improved K _IC of ultrahigh-strength steels, i.e. low-alloy and highly alloyed secondary hardening steels, through control of non-metallic inclusions. The major factors controlling the property are discussed for each of the techniques.     

Microstructure and fracture mechanical response of wood

Investigations on the fracture properties of wood in relation to its microstructure are reported. The inhomogeneous and hierarchical structure of wood is addressed. Wood species, the influence of orientation, the role of structural features, like rays are considered and discussed. Likewise the mode of loading, which determines the mode of fracturing, and the influence of humidity have been studied by using new fracture mechanical techniques and ways of evaluation. The specific fracture energy has been determined under crack opening conditions. In-situ loading in an environmental scanning electron microscope (ESEM), which allows observation in moistured condition, has been performed in order to investigate the mechanisms of fracturing of wood on a sub-microscopic scale. In the nanometer range, especially the influence of the microfibril angle on deformation and fracture behaviour has been studied.     

Gradual transition zone between cell wall layers and its influence on wood elastic modulus

Aiming at bringing the modeled cell wall modulus to a more accurate level, for the first time, the gradual transition zone between the S1 and S2 layers was considered as an independent interlayer in the mechanic model (the interlayer model). In this study, the fibril angle and polymer contents in the interlayer were assumed to change gradually, continuously, and simultaneously as functions of the distance to the interface between the interlayer and S2. It was found that the location and thickness of the interlayer significantly influenced the results of the interlayer model. For the cell wall modulus, the excellent longitudinal predictions of replacing the outer S2 with the interlayer were coupled with the poor ones in the transverse direction. For the replacement of the inner S1, the situation reversed. Considering these coupling effects, the best configuration of location and thickness for the interlayer was obtained through a simple evaluation process. The results show the possibility to diminish the discrepancy between the measured and modeled longitudinal modulus by introducing the interlayer, while in the transverse direction, the discrepancy is reduced to a distinct extent but remains present. It was also highlighted that the transition zone located in the inner part of S1 has a critical impact on the performance of the interlayer model. Moreover, different functions for describing the transition were compared.     

Strength and fracture of lithium hydride crystals under uniaxial compression

We determine the conditions of fracture of lithium hydride crystals under uniaxial compression and establish the character of deformation and fracture of tested specimens and the ultimate compressive loads that can be withstood by the specimens. We also discovered and statistically analyzed a specific size effect on the fracture processes in crystals that is connected with the fact that the dimension of the crystal in the direction of applied compressive stresses is the principal factor that determines its compression strength. Russian Federal Nuclear Center, All-Russia Scientific Research Institute for Experimental Physics, Sarov, Russia. Translated from Problemy Prochnosti, No. 5, pp. 64–69, September–October, 1999.     

基体拉伸法测试二氧化钛薄膜界面结合强度

利用热氧化法在工业纯钛表面形成氧化层,利用改进后的基体拉伸法研究了500和800℃加热1h所形成的氧化层和基体的结合强度,结果发现裂纹形貌与界面结合强度有关.500℃所形成的氧化层较薄,与基体的结合强度高,＞970MPa,裂纹方向与载荷轴向呈±45°;而800℃形成的氧化层较厚,与基体的结合强度差,＜495MPa,裂纹基本与载荷轴向垂直.     

About the dynamic uniaxial tensile strength of concrete-like materials

Experimental methods for determining the tensile strength of concrete-like materials over a wide range of strain-rates from 10⁻⁴ to 10² s⁻¹ are examined in this paper. Experimental data based on these techniques show that the tensile strength increases apparently with strain-rate when the strain-rate is above a critical value of around 10⁰-10¹ s⁻¹. However, it is still not clear that whether the tensile strength enhancement of concrete-like materials with strain-rate is genuine (i.e. it can be attributed to only the strain-rate effect) or it involves “structural” effects such as inertia and stress triaxility effects. To clarify this argumentation, numerical analyses of direct dynamic tensile tests, dynamic splitting tests and spalling tests are performed by employing a hydrostatic-stress-dependent macroscopic model (K&C concrete model) without considering strain-rate effect. It is found that the predicted results from these three types of dynamic tensile tests do not show any strain-rate dependency, which indicates that the strain-rate enhancement of the tensile strength observed in dynamic tensile tests is a genuine material effect. A micro-mechanism model is developed to demonstrate that microcrack inertia is one of the mechanisms responsible for the increase of dynamic tensile strength with strain-rate observed in the dynamic tensile tests on concrete-like materials.