Use of Leicester's “rheological model for mechano-sorptive deflections of beams”

Summary The model for mechano-sorptive deformation proposed by Leicester is used to predict stress relaxation of wood drying under constraint from apparent creep of drying wood under constant load. The predictions are compared with measurements on four pairs of beams, one beam in each pair being tested under constant load, the other at constant deflection. Agreement was sufficiently close to demonstrate the value of the model.     

Duration-of-load and creep effects in strand-based wood composite: experimental research

The results from a duration-of-load and creep testing program on a thick strand-based composite product, using wood from forests attacked by the mountain pine beetle, are presented. The constant loading of experimental beams lasted for 1?year. The long-term deflection was monitored and recorded at a pre-set frequency. Time-to-failure data were also obtained for all the broken specimens. Ramp load test was carried out at different rates of loading to investigate its influence on short-term strength. Finally, the fatigue test was conducted with a triangular cyclic load history, and the strain history was obtained to elucidate the low-cycle fatigue behavior of the composite.     

A global rheological model of wood cantilever as applied to wood drying

In the process of wood drying inevitable stresses are induced. This often leads to checking and undesired deformations that may greatly affect the quality of the dried product. The purpose of this study was to propose a new rheological model representation capable to predict the evolution of stresses and deformations in wood cantilever as applied to wood drying. The rheological model considers wood shrinkage, instantaneous stress–strain relationships, time induced creep, and mechano-sorptive creep. The constitutive law is based on an elasto–viscoplastic model that takes into account the moisture content gradient in wood, the effect of external load, and a threshold viscoplastic (permanent) strain which is dependent on stress level and time. The model was implemented into a numerical program that computes stresses and strains of wood cantilever under constant load for various moisture content conditions. The results indicate that linear and nonlinear creep behavior of wood cantilever under various load levels can be simulated using only one Kelvin element model in combination with a threshold-type viscoplastic element. The proposed rheological model was first developed for the identification of model parameters from cantilever creep tests, but it can be easily used to simulate drying stresses of a piece of wood subjected to no external load. It can therefore predict the stress reversal phenomenon, residual stresses and maximum stress through thickness during a typical drying process.     

木材静曲蠕变成套测试系统的研发

为解决木材变湿蠕变测试普遍存在的环境参数(温湿度和风速)控制方式和控制精度不足、蠕变变形量测试灵敏度和精度不足、数据智能采集和处理以及系统安全防护不足等问题,在集成现有技术的基础上研发出一套木材蠕变测试系统。该系统包括环境气候箱、蠕变测试机架、木材蠕变变形量及干缩湿胀测试单元、荷载同时装卸单元、数据采集和处理单元、系统保障和防护单元6个部分。该系统可在温度为0~70℃、相对湿度为10%~98%、风速为0.3~5.6 m/s时对木材或木基材料的普通蠕变和机械吸附蠕变进行长时间稳定的测试、记录,蠕变变形量及木材厚度方向的干缩湿胀检测精度为±0.01 mm,数据采样间隔在1 s~24 h范围内可调。与现有木材蠕变测试系统相比,本系统在多参数自动同步获取、数据采样精度、运行稳定性、安全性和环境风速可调性等方面有独特优势。通过前期对木材112 d循环变湿蠕变量、干缩湿胀量及环境温湿度参数的观测和测试结果分析,证实该套检测系统可对木材的蠕变挠度、干缩湿胀量、环境温湿度等参数进行长期连续稳定的检测、记录和显示,所有测试指标均能达到设计预期。同时,由于该系统具有多参数协同检测和精度较高等特性,使得实测木材变湿蠕变比采用喷蒸变湿处理试件所获蠕变测试结果在局部地方存在明显差异,如吸湿过程中的木材静曲挠度并不都是反向减小,而是取决于吸湿速率和外荷载的竞争关系,从而为木材静曲变湿蠕变机理的揭示提供了有力证据。该系统的研制为精确可控变温变湿环境下木材蠕变机理的研究提供了新的平台。     

Long term bending creep of wood in cyclic relative humidity

Summary Long term creep and recovery test results of wood under a bending load of 10 MPa stress and subjected to relative humidity cycling are presented. In spite of the vast number of humidity cycles during these tests, the results do not give evidence of an existence of a mechano-sorptive creep limit. The results also indicate that the recovery is not complete on unloading when the humidity is cycled. A mechano-sorptive model that fits the test results is proposed. According to the model, the elastic bending deflection in a cyclic load of 10—3 MPa and subjected to natural outdoor relative humidity can be about doubled to account for the creep under ten years loading.     

The variable parameter rheological model of wood

How to establish the rheological model to simulate creep behavior of wood and wood-based composites under change-load has not been solved in research of wood rheology. This paper presents a new model—variable parameter rheological model. The bending creep behavior of small clear poplar specimens under different constant load levels were examined. The load levels within 50% of rupture load of the specimens, and the experimental creep behavior were simulated by the variable parameter Maxwell model. The results show that using only one model of variable parameters may simulate the creep behavior of wood under different constant load levels very well. Applying the generalized Boltzmanns superposition principle, the variable parameter rheological model can be used to simulate the creep behavior of wood under change-loads conveniently and accurately.     

Overview of the use of ultrasonic technologies in research on wood properties

Two ultrasonic technologies, acoustic emission and acousto-ultrasonics, are reviewed in terms of characteristics and applications to wood and wood-based materials. The background is given on the two technologies, including the effect of wood on wave propagation and sensitivity to defects and other properties. The factors that affect wave injection and processing are reviewed for each technology. Four applications are given as examples: particleboard springback, creep/creep-rupture, adhesive curing, and deterioration detection. Recent developments in the technologies are discussed. Received 30 September 1999     

Fibre morphological effects on mechano-sorptive creep

The increased creep rate of paper under load during moisture cycling conditions as compared to that at high constant humidity is a problem in the use of packaging materials. In order to investigate the influence of morphological factors of the fibres on the occurrence and magnitude of this phenomenon, i.e. the occurrence of mechano-sorptive creep, studies on wood fibres isolated from different parts of spruce wood were performed. Thus, creep properties were studied on earlywood and latewood fibres from both juvenile wood and mature wood. In general, latewood fibres showed a higher degree of mechano-sorptive creep than earlywood fibres, and mature wood showed a higher degree of mechano-sorptive creep than juvenile fibres. The difference in mechano-sorptive creep rate between different fibres was shown to be correlated to the differences in fibril angle. The smaller the fibril angle the higher was the mechano-sorptive creep ratio. It was suggested that at fibril angles approaching 45° wood fibres do not exhibit mechano-sorptive creep.     

Cantilever experimental setup for rheological parameter identification in relation to wood drying

Wood exhibits a pronounced time dependent deformation behavior which is usually split into ‘viscoelastic’ creep at constant moisture content (MC) and ‘mechano-sorptive’ creep in varying MC conditions. Experimental determination of model rheological parameters on a material level remains a serious challenge, and diversity of experimental methods makes published results difficult to compare. In this study, a cantilever experimental setup is proposed for creep tests because of its close analogy with the mechanical behavior of wood during drying. Creep measurements were conducted at different load levels (LL) under controlled temperature and humidity conditions. Radial specimens of white spruce wood [Picea glauca (Moench.) Voss.] with dimensions of 110 mm in length (R), 25 mm in width (T), and 7 mm in thickness (L) were used. The influence of LL and MC on creep behavior of wood was exhibited. In constant MC conditions, no significant difference was observed between creep of tensile and compressive faces of wood cantilever. For load not greater than 50% of the ultimate load, the material exhibited a linear viscoelastic creep behavior at the three equilibrium moisture contents considered in the study. The mechano-sorptive creep after the first sorption phase was several times greater than creep at constant moisture conditions. Experimental data were fitted with numerical simulation of the global rheological model developed by authors for rheological parameter identification.     

Creep in chipboard

Summary Although previous papers in this series have shown that a 4-element rheological model can provide a very good representation of the creep deflection of chipboard in bending, a new set of long-term data shows that it has certain limitations as a model for predicting deflection up to 3^1/2 years. It was deduced in a earlier paper (Pierce et al. 1979) that the linear viscous flow component was likely to predict higher than actual deflections over a long period. This paper shows that view to be correct, and puts forward a modified 4-element model in which the viscous component is non-linear with respect to time. The resulting 5-parameter model is shown to be superior to the 4-parameter model for long-term predictions of creep deflection particularly at the lower stress levels, although it appears that the viscoelastic and viscous components of deflection are not as realistic as in the 4-parameter representation.     

Creep of four tropical hardwoods from Cameroon

Summary The knowledge of the behaviour of wood over time is important in the design of timber structures. This study, which is part of a program to better understand the behaviour of tropical woods, aims at showing the effect of creep in wood. Observations were made on the development of the deflection in a uniformly stressed beam under controlled temperature and humidity conditions. The results obtained show that for stresses not greater than 35% of the stress at failure, the behaviour of the species tested is linear. In these cases, the total relative creep is not more than 35%. Similar results have been obtained for four cameroonian species: Azobe, Tali, Sapelli and Movingui.This research was carried out in the Structural Analysis Laboratory of the Department of Civil and Urban Engineering, National Advanced Polytechnic School of Engineering. Yaounde. Director: Dr. A. Foudjet     

Creep in chipboard

Summary Previous linear and curvilinear regression models for predicting the creep deflection of timber and timber products have failed to provide an adequately good fit. However, this paper shows that the 4-element (and to a lesser degree the 3-element) rheological model provides an extremely good fit to chipboard creep data.A set of experiments has been carried out on the creep behaviour of five commercially-available types of chipboard under 3-point sustained loading at constant temperature and humidity. This range of board types encompassed three types of glue — UF, MF/UF and Pf- and was loaded at two stress levels-30% and 60% of the short term ultimate stress. The lifetime of these specimens ranged from 25 days to over 31/2 years until either failure occurred or the load was removed.Creep curves based on 3- and 4-element rheological models have been fitted to the data from each specimen using an iterative least squares computer program which we developed. The validity of the two models is discussed, together with studies on the comparative behaviour of different board types and the use of the models as predictive tools.     

结构胶合板蠕变测试方法的研究

木质材料的蠕变特性是衡量其能否适用于工程用途的重要指标。本文介绍了一种较为新颖的蠕变测试方法——应变测试法，即将结构用胶合板试件置于恒温恒湿恒定栽荷务件下，通过智能应变仪记录受栽后的位移，以实现长期的蠕变测定。经2个月的检测，结果表明，杨木结构用胶合板的蠕变曲线基本符合木材的三阶段蠕变理论，其结果真实地反映了材料在恒定外力下的变形情况，为进一步探求结构用胶合板的应用研究打下基础。     

Effect of load pulse shape on predicted damage accumulation in wood

Summary Load-duration (creep-rupture) effects in wood are often modeled using cumulative damage theory. In application, such as for duration of load reliability analyses, incremental damage owing to stochastic load pulse models is summed over an assumed reference period (e.g., 50 years), with failure defined in terms of the damage state variable. The effect of load pulse shape on cumulative damage is examined herein through the use of one such proposed damage accumulation model. An exponential damage rate model (EDRM) is used to calculate the damage owing to a rectangular load pulse (RECT), a triangular load pulse (TRI), and two trapezoidal load pulses (TRAP1 and TRAP2). The differences in the calculated damage predicted by the EDRM and resulting from the four load pulse shapes are presented and the effect of critical load parameters (e. g., load intensity and duration) are illustrated. Also, the effect of neglecting ramp loading and unloading is discussed; that is, the error in assuming RECT versus either TRAP1 or TRAP2 load pulses is assessed.     

Monitoring critical defects of creep rupture in oriented strandboard using acoustic emission: incorporation of EN300 standard

This creep rupture study in commercial oriented strandboard (OSB) used a 4-point flexural test to evaluate the dynamic property changes of a 300×1,000-mm specimen using an acoustic emission (AE) system. Compared to deflection, AE events were more sensitive to damage accumulation than deflection to final failure. Specimens were artificially notched on either the tension- or compression-side and were subjected to 80% stress level at ambient conditions. Defects on the compression side of the bending specimen were found to be more critical than on the tension side in creep-rupture. The in-plane fractures followed patterns of the valleys of low-density spots as defect trenches, demonstrating adverse effects of high variation in horizontal density. An impetus and rationale to incorporate regulatory quality inspection standards and product certification of structural OSB based on the control limits of ±10% panel density as stipulated in EN300 standard is discussed.Experiment conducted at the former University of California Forest Products Laboratory (Nondestructive Evaluation Center), Richmond, CA 94804, USA.     

Effect of microfibril angle on the longitudinal tensile creep behavior of wood

In this report, we undertook studies of the viscoelastic properties of wood from the viewpoint of the fine structure and properties of the constituent materials in the wood cell wall. To measure the mechanical properties of the wood as the behavior of the cell wall, it is required to perform the longitudinal tensile test using a homogeneous specimen. In this study, microtomed specimens of sugi (Cryptomeria japonica D.Don) earlywood were used for the creep test, which were conducted at the fiber saturation point. The substantial creep compliance of the cell wall was simulated using a simplified viscoelastic model consisting of a Voigt element and an independent spring in series. Based on the experimental results, the values of the parameters were optimized. The results were as follows: (1) the longitudinal tensile creep deformation tends to increase with the elapsed time, similar to the bending creep behavior; (2) the magnitude of the longitudinal creep function increases with MFA; and (3) each parameter in the simplified viscoelastic model is markedly affected by the MFA. Based on these results, the mechanism of the longitudinal tensile creep deformation of wood is discussed.     

Compressive fatigue in wood

Summary  An investigation of fatigue failure in wood subjected to load cycles in compression parallel to grain is presented. Small clear specimens of spruce are taken to failure in square wave formed fatigue loading at a stress excitation level corresponding to 80% of the short term strength. Four frequencies ranging from 0.01 Hz to 10 Hz are used. The number of cycles to failure is found to be a poor measure of the fatigue performance of wood. Creep, maximum strain, stiffness and work are monitored throughout the fatigue tests. Accumulated creep is suggested identified with damage and a correlation is observed between stiffness reduction and accumulated creep. A failure model based on the total work during the fatigue life is rejected, and a modified work model based on elastic, viscous and non-recovered viscoelastic work is experimentally supported, and an explanation at a microstructural level is attempted. The outline of a model explaining the interaction of the effect of load duration and the effect of the loading sequences is presented. Received 8 December 1997     

Mechano-sorptive creep in pulp fibres and paper

The factors affecting mechano-sorptive creep of wood and paper have been investigated for a long time. It has also been argued that single wood fibres do not exhibit mechano-sorptive creep and that the reasons for the accelerated creep under moisture cycling conditions instead are related to the bonds between the fibres. In order to examine the relevance of this argument, measurements on single pulp fibres of different composition were performed in tension, and the mechano-sorptive creep was compared to that of papers made from the same source of pulp fibres. All fibres tested were found to exhibit an increased creep rate during moisture cycling as compared to constant humidity conditions. Thus, pulp fibres show mechano-sorptive creep and in this sense behave similar to solid wood or paper products made thereof. A linear relation between the creep strain rate during cyclic humidity and the creep strain rate at a constant humidity was also noted for both fibres and paper. This relation was not affected by changes in hemicelluloses content or composition, neither for fibres nor for papers made of these fibres. However, in all cases, papers showed a much higher mechano-sorptive creep than the corresponding fibres they were made of.     

Transient moisture effects on wood creep

To gain insight into the physical nature of the coupling between mechanical stress and humidity variations, the behaviour of thin wood strips was studied using specially developed apparatus for creep/recovery and relaxation/blotting-out tests in a controlled humidity environment. The load time and the rate of viscoelastic creep were found to have little influence on mechano-sorptive creep. Moreover, creep trajectory curves for specimens with continuous and interrupted humidity cycles indicated divergence from simple creep-limit behaviour. The effect of transient moisture was also modelled numerically at the molecular level using an idealized cellulose-based composite. Preliminary results suggest that: (i) during free shrinkage, the cellulose chains in elementary fibrils may bend perpendicular to the planes of the hydrogen bonded sheets which form the crystalline lattice; (ii) transient hydrogen bonding between the crystalline cellulose and amorphous polymer owing to the introduction or removal of water may accelerate shear slip between the two phases in the presence of an external load. Received 6 July 2000 The financial support of the Swiss Federal Office for Education and Science is gratefully acknowledged.     

Creep and stress relaxation behavior for natural cellulose crystal of wood cell wall under uniaxial tensile stress in the fiber direction

We investigated the temporal changes in creep and stress relaxation behavior in both microscopic crystalline cellulose and macroscopic strain of wood specimen using Japanese cypress (Chamaecyparis obtusa Endl.) to understand the viscoelastic properties of wood cell walls. Specimens 600 µm in thickness were observed by the X-ray diffraction and submitted to tensile load. The crystal lattice strain of (004) plane and macroscopic strain of specimen were continuously detected during creep and stress relaxation tests. It was found that the creep compliance based on macroscopic strain showed a gradual increase after instantaneous deformation due to loading and then the parts of creep deformation remained as permanent strain after unloading. On the other hand, crystal lattice strain showed a different behavior for macroscopic strain; it kept a constant value after instantaneous deformation due to loading and then increased gradually after a certain period of time. These differences between macroscopic and microscopic levels were never found in the stress relaxation tests in this study. Relaxation modulus at the macroscopic level only showed a decreasing trend throughout the relaxation process. However crystal lattice strain kept a constant value during the macroscopic relaxation process. In addition, the microfibril angle (MFA) of wood cell wall has a role of mechanical behavior at microscopic level; crystal lattice strains were smaller with increasing MFA at both creep and relaxation processes. Creep compliance and stress relaxation modulus at the macroscopic level decreased and increased with increasing MFA, respectively. Our results on the viscoelastic behavior at microscopic level evidenced its dependency on MFA.