Mechano-sorptive creep under compressive loading – A micromechanical model

The creep of paper is accelerated by moisture cycling, an effect known as mechano-sorptive creep. It has also been observed that the mechano-sorptive effects are larger in compression than in tension. In this paper a simplified network model for mechano-sorptive creep is presented. It is assumed that the anisotropic hygroexpansion of the fibres leads to large stresses at the fibre–fibre bonds when the moisture content changes. The resulting stress state will accelerate creep if the fibre material obeys constitutive laws that are non-linear in stress. Geometrical fibre effects are included in the model in order to capture experimental observations of the differences between paper loaded in tension and compression. Theoretical predictions based on the developed model are compared to experimental results for paper both under tensile and compressive loading at varying moisture content. The important features in the experiments are captured by the model, i.e. the creep is accelerated by the moisture cycling and the mechano-sorptive effects are larger in compression than in tension.     

An anisotropic fibre-network model for mechano-sorptive creep in paper

In this paper a simplified network model for mechano-sorptive creep is presented, which is a further development of an earlier paper [Strömbro, J., Gudmundson, P., 2008. Mechano-sorptive creep under compressive loading – a micromechanical model. International Journal of Solids and Structures 45 (9), 2420–2450.]. It is assumed that the anisotropic hygroexpansion of the fibres leads to large stresses at the fibre bonds when the moisture content changes. The resulting stress state will accelerate creep if the fibre material obeys a constitutive law that is non-linear. Fibre kinks are included in order to capture experimental observations of larger mechano-sorptive effects in compression than in tension. Moisture dependent material parameters and anisotropy in the fibre distribution have been introduced. Theoretical predictions based on the model are compared to experimental results for an anisotropic paper both under tensile and compressive loading at varying moisture content and it is found that the important features in the experiments are captured by the model. Different kinds of drying conditions have also been examined.     

Creep of composites with a porous fibre/matrix interface under variable loading

A micromechanical model of island-type fibre/matrix interface is described and experimental data are represented and analysed. Changes in the interface strength during a cycle loadings and, correspondingly, changes in the creep resistance of a composite due the island-like scheme of the fibre/matrix interface are described.     

金属蠕变与蠕变破坏理论综述

本文对金属在均匀单向与复杂应力状态下的蠕变与蠕变破坏行为及结构物蠕变下持久性发展了系统的试验与理论研究。 在试验研究中本文考虑了各种金属在恒应力与变应力下的行为,应力集中,加载方式对蠕变破坏的影响;同时还提出了材料的结构损伤的新的量测方法。     

On creep deformation in longitudinal bending of beams

Methods for calculating the creep strain in the longitudinal bending of beams were considered by many authors (e.g., see [1]). The computations permit determining the deflection variation as a function of time and the critical time in which the beam compressed by the forces directed along its axis loses its stability. In the present paper, we consider the creep deformation of beams of various cross-section and show that the critical time can be increased by changing their structure.     

Creep deformation of beams under compression and bending stresses

Methods for calculating the creep strain of beams or plates in compression were considered by several authors (e.g., see [1–3]).The goal of calculations is to determine the function describing the deflection increase and the time in which the deflection attains the maximum admissible value.In this paper, we consider the creep strain with regard to the common action of compression and bending stresses.     

黏弹性薄板蠕变屈曲的载荷-时间特性研究

通过对黏弹性薄板压屈的稳定性分析,着重讨论了蠕变屈曲载荷－时间的特性,理论分析表明：黏弹性薄板蠕变屈曲与材料的力学性能密相关,屈曲载荷不像弹性薄板为一定值,而是与时间相关的;在一定载荷下,经过一段时间后出现延迟屈曲,相关的实验研究也证实这一重要结论;这种延迟失稳问题在工程中有其重要的意义。     

The examination and interpretation of the properties of fibres using rheological measurements on non-uniform fibres

Summary Stress-strain, creep and torque-twist measurements on a large number of wool fibres of varying type are used to illustrate how fibre non-uniformities affect the observed behaviour and how the fibre material behaviour is extracted from the measurements. In this regard the concept of effective area of a cross-section is introduced in order to simplify the analysis. It is shown that fibre non-uniformities may hide the effects of certain structural features and if reasonably uniform fibres are not available it may not always be possible to derive the true material behaviour. Variation of properties between fibres can be useful in deriving significant structural information about the fibre material.With 7 figures and 2 tables     

A nonlinear creep model based on the concept of slip

Relations between the creep strain and the stress are obtained for a complex stress state on basis of the hypothesis that the tangent strain components depend, in a certain slip system, only on the history of variation of the corresponding shear stress component. It is assumed that the slip system orientation is chaotically distributed over all directions and the total strain of the polycrystal is considered as the sum of contributions of all slips. This permits describing the creep strains in an arbitrary stress state by using a common operator defining the dislocation slip in a certain slip system. The model can be used for arbitrary creep operators. Computational results are analyzed and compared with experimental data obtained by other authors.     

A non-linear viscoelastic model with structure-dependent relaxation times: III. comparison with l.d. polyethylene creep and recoil data

The equations of a model previously proposed by the authors to describe the non-linear viscoelastic behaviour of polymer melts are elaborated here to predict creep and recoil responses.The comparison with existing data for both shear and elongational flow shows good agreement and supports the assumption of a material which is destructured by a prolonged application of high deformation rates.     

Analytical study of the load transfer in fibre-reinforced 2D composite materials

The phenomenon of the load diffusion from a fibre to a surrounding matrix is analysed for the 2D case. We use an approximate analytical approach based on the asymptotic reduction of the governing biharmonic problem into two harmonic problems. The comparison of the obtained solutions with known results of other authors shows an acceptable accuracy of the proposed asymptotic simplifications. All solutions are obtained in closed analytical form.The case of perfect bonding between fibre and matrix for a single fibre and for a periodic system of fibres is firstly considered. Then we study the influence of the interface stiffness. In the case when only a single fibre is loaded, the influence of all other fibres is predicted by means of a three-phase model. The proposed approach gives a possibility to solve the problems for a broken fibre and for a broken matrix, as well as for partly debonded fibres. The important problem of infinite matrix cracks is also solved in the present paper.The obtained results can be used for the calculation of pull-out and push-out tests, as well as for the investigation of the fracture of composite materials.     

Evidence and modelling of physical aging in green wood

Physical aging typical of semi-crystalline polymers was evidenced in green (never dried) wood from three tropical hardwood species. The assumption of uniform aging rate was verified by the construction of master curves from series of tensile creep tests in the fibre direction, performed at increasing time elapsed after a quench following heating above the glassy transition. The rheological response during periods of creep small enough to neglect the progress of aging was described by a model made of a spring in series with a parabolic dash-pot where only the characteristic time depends on the aging time. The model was able to describe results obtained by a previous author on softwood loaded transversally to the fibres. The possible role of a transient adsorption process consecutive to the quench is discussed.

Modeling long-term creep rupture by debonding in unidirectional fibre-reinforced composites

Delayed fracture due to debonding can be observed in many unidirectional fibre-reinforced composites when the fibre/matrix interface experiences creep. The aim of this work is to describe such a phenomenon within the recently proposed modeling framework of transverse isotropy that allows for a neat decomposition of the mechanical behavior into fibre-directional, transverse, and pure shear parts. Specifically, debonding is here chosen to be governed by the tension transverse to the fibres. One can then speak of a mode-I debonding if use is made of the terminology adopted in fracture mechanics. On another hand, the time-dependent response is attributed to the matrix constituent. As the role of this latter is to deform and support stresses primarily in shear, a viscoelastic behavior is introduced that affects solely the pure shear part of the behavior. We show that both characteristics can be easily embedded into the aforementioned formulation. Among others, the occurrence of tertiary creep is made possible to predict. It is otherwise found that the predicted debonding path always propagates along the direction of the fibres in agreement with many experimental observations found in the literature. On the numerical side, the algorithmic treatment of debonding is independent of the one for viscoelasticity. This renders the implementation within the context of the finite element method very easy.     

蠕变条件下刚性夹杂填充高聚物复合材料的延迟时间-蠕变荷载幂率关系研究

本文研发了不锈钢短纤维(SSFs)填充聚丙烯(PP)的新型高聚物复合材料,开展了在不同的蠕变荷载下材料的蠕变行为研究。实验证实,在相同的持载时间条件下,蠕变变形与蠕变载荷之间呈非线性关系,这个现象用传统的线性粘弹性理论难以解释。本文采用延迟时间与蠕变荷载相关的观点,对蠕变变形-蠕变载荷的非线性现象进行分析,发现了在对数坐标下延迟时间与蠕变载荷之间存在线性关系,从而得到了延迟时间与蠕变载荷之间的幂律关系。进一步的分析指出,蠕变载荷提升导致延迟时间增加的机理主要由材料体系的损伤所诱发。     

Constitutive equations for densification of matrix-coated fibre composites during hot isostatic pressing

This work addresses the development of physically based constitutive equations for the consolidation of fibre-matrix-void systems typically arising in the manufacture of matrix-coated fibre metal matrix composite materials. The analyses consider square array packing of the coated fibres under symmetrical in-plane compressive load and take into account the power-law creep of the matrix. Two models have been developed. The first is based on an energy approach in which assumed velocity fields in the deforming matrix are considered and are expressed in terms of an unknown parameter. In this way, the dependence of the deformation rate on volume fraction of voids and fibres is derived through the use of Hill's minimum principle for velocities. The second model makes use of micro-mechanical finite element modelling in which fibre, matrix and void are modelled explicitly. The micro-mechanical finite element model is developed for validation and comparison. Theoretical predictions are examined. The constitutive equation for consolidation derived from Hill's minimum principle shows good agreement with results obtained from micro-mechanical finite element modelling.     

岩石含水蠕变损伤模型的开发与应用

摘要：为研究岩石含水率对蠕变的影响,将含水开关与蠕变损伤阀值引入蠕变模型,实现了含水劣化与蠕变损伤在本构关系上的耦合,建立了岩石含水蠕变损伤模型。模型与某软岩的三轴压缩蠕变试验结果取得了较好的拟合效果,并由麦夸特法+通用全局优化（LM-UGO）算法反演出了模型参数随岩石含水率的变化规律。在C++语言和FLAC3D中Fish语言的环境下,通过推导模型本构方程的三维差分形式,并利用FLAC3D所提供的接口实现了模型的二次开发。通过对开发的岩石含水蠕变损伤模型进行算例验证和分析,验证了模型的可靠性,也得出了软岩巷道蠕变变形随围岩含水率增大而增加的结论,进一步反映出蠕变模拟计算中考虑岩石含水率的必要性。     

On the elastic properties of model suspensions as investigated by creep recovery measurements in shear

 The elastic properties of model suspensions with spherical monodisperse hydrophilic glass spheres that were dispersed in a Newtonian liquid were determined in creep and creep recovery measurements in shear with a magnetic bearing torsional creep rheometer. The creep and creep recovery measurements were performed depending on the applied level of shear stresses ranging from 0.19 Pa to 200 Pa. Since the recoverable creep compliances of the chosen suspending medium (i.e. a low molecular weight polyisobutylene) were far below the lower limit of the resolution of the creep rheometer it can be considered to behave as purely viscous. By applying a large shear stress in the creep tests the investigated suspensions with a volume fraction of Φ _t =0.35 behave as Newtonian liquids, too. For these suspensions no significant recoverable creep compliances could be detected, as well. In contrast to the Newtonian state of suspensions at high shear stresses, where a shear induced ordering of the particles can be expected, a non-Newtonian behaviour arises by applying a very low shear stress in the creep test. In this state large recoverable creep compliances were detected for the suspensions. The magnitude of the recoverable creep compliances of the suspensions exceeded the largest creep compliances of polymer melts that are reported in the literature by more than two decades. From the results obtained by creep recovery measurements with a magnetic bearing torsional creep rheometer it can clearly be concluded that the particle structure present in the chosen model suspension gives rise to a pronounced elasticity. Received: 21 November 2000 Accepted: 12 July 2001     

轮轨滚动接触力学的发展

轮轨滚动接触力学主要研究轮轨滚动接触过程中的作用行为。由于其研究的复杂性,目前在该领域的研究已基本形成既独立又关联的六个分支,它们分别是轮轨滚动接触蠕滑率/力理论、轮轨粘着、接触表面波浪形磨损、轮轨滚动疲劳、脱轨和轮轨噪音。本文综述了这几个方面研究的发展历史和现状。由于轮轨滚动接触作用的研究又是以轮轨滚动接触蠕滑率/力理论为基础的,故本文着重评述目前常用于车辆/轨道动力学和轮轨关系研究中几个经典滚动接触理论模型的优缺点。根据实际工程中轮轨作用存在的严重问题,并提出轮轨滚动接触理论及其试验在今后的研究方向和所要考虑的有关重要因素。      

表面裂纹蠕变分析的蠕变线弹簧模型方法

鉴于用通常的数值方法分析三维蠕变裂纹问题的困难,提出了一个三维表面裂纹蠕变断裂力学参量分析的蠕变线弹簧模型方法,并在非稳态蠕变条件下的位移、裂纹尖端J积分和C积分的工程估算公式及弹塑性线弹簧模型的基础上,建立了蠕变线弹簧模型方法的有关基本方程．具体分析计算了受均匀拉伸表面裂纹平板的J积分和C积分,并与三维有限元解进行了比较,其结果吻合良好．研究结果为进一步研究三维表面裂纹的蠕变扩展及寿命预报提供了基础．