TENSILE DRYING STRESS VALUE IN DEPENDENCE OF THE SHRINKAGE RESTRAINT METHOD

Jerzy Majka
Department of Hydrothermal Wood Processing and Modification of Wood, The August Cieszkowski Agricultural University of Poznan, Poland

ABSTRACT

The subject of study was determination of maximum values of tensile drying stress in samples of European beech (Fagus sylvatica L.) wood in radial direction. In drying stress studies the measurement of maximum stress is axial force, present at cross section of the sample due to complete restraint of shrinkage. In the presented research this assumption was executed in two ways: in the effect of cycle loading of the sample (used in numerous experiments so far) and with the use of extensometer. The research results prove the relationship between the drying stress values and the shrinkage restraint method.

Key words: shrinkage, restraint, drying stress, drying, European beech.

INTRODUCTION

One of the most significant purpose of the study of drying stress in wood has been determination of their maximum values. In numerous research so far, the maximum (critical) values of tensile drying stress have been described as a result of total restraint of wood shrinkage [1, 3, 4, 9]. Total restraint of sample shrinkage was obtained due to successive increase of tensile loading, ensuring preservation of initial sample size in the direction of loading. The stress value in a sample was calculated as the algebric sum of cycle loads by the area of the cross-section perpendicular to the direction of the force action. The obtained value of tensile drying stress (defined as maximum) may be too low, as cycle loading of wood samples is the reason of increase of speed of stress relaxation, in comparison with typical rheological process, without loading [7]. Justification of the above statement may be the results of research conducted by Widłak [10], which show that the values of tensile drying stress generated in the conditions of successive loading are decisively lower, as compared with the values determined in conditions where force in sample could not totally restraint its shrinkage. In view of this fact, comparative studies were carried out, with the purpose of determining the effect of drying shrinkage restraint on the values of generated drying stress.

MATERIALS AND METHODS

The research was made with the use of European beech (Fagus sylvatica L.) wood specimen in radial direction (Fig.1). This species of wood is most frequently used in the research of drying stresses due to the special susceptibility for checks in the drying process of timber.

The measurement of maximum (ctitical) stress is axial force, present at cross section of the sample due to complete restraint of shrinkage. Total restraint of wood shrinkage has been obtained in two ways:

1. Method A – used in numerous experiments so far, where insignificant momentary strain is allowed, and then the sample is elongated to reach its initial size (the stress value in a sample was calculated as the algebric sum of cycle loads by the area of the cross-section perpendicular to the direction of the force action);

2. Method B – in which cycle loads of the sample was replaced by extensometer, which allows to preserve the initial sample size (the stress value in a sample was calculated as the force, measured by a extensometer by the area of the cross-section perpendicular to the direction of the force action).

During experiments, the decrease in the moisture content and deformations of unrestrained specimen witout loading of a twin specimens were measuring simultaneously in a laboratory drier chamber with forced air rate ca. 1.5 m/s.

Unrestrained shrinkage of the sample (moisture strain) was calculated in the following way:

(1)

where: L_i – the initial length of a sample, mm; – length of a sample, when moisture content reaches the value MC_i, mm.

The experiments had been continued until a marked stress stagnation or decrease occurred.

RESULTS AND ANALYSIS OF EXPERIMENTS

Based on the research results, it was found out that drying stress value is dependent on the shrinkage restraint method. The results of comparision experiments shows Table 1 and Figures 2-4.

* Defined as:

To determine correlation between the use of shrinkage restraint method and the value of generated stress, the Fisher test was used, in which the hypothesis had to be verified, that the use of comparable methods of drying shrinkage restraint is stress of, on average, the same value.

To present quantitative relation, assessment of linear parametric function was used (so called basic contrast), that can be presented in the following way [1]:

(2)

where: φ – contrast value, µ_i – mean, c_i – coefficient.

In the analysed comparison, contrast may be presented as:

(3)

and expresses comparison of the effects of the use of method A and method B on the assessment of generated tensile drying stress. On the basis of calculations it was found that an average value of tensile drying stress obtained in method A is at least 48.5% and not more than 55.4% lower than stress value obtained in method B.

CONCLUSIONS

Based on the research results, it was found out that the value of tensile stress determined in the cycle load experiment (method A) cannot be described as maximum, as it was adopted in numerous research so far, as the stress is considerably lower than probable real values, determined in the conditions of total shrinkage restraint with the use of extensometer (method B). Higher values of stress determined with the use of method B at the same time confirm the earlier observations of Widłak [10]. Moreover, the results of presented comparison of results obtained in experiments allow to state that cycle load method (method A) may not adequately reflect the mechanism of stress generation in dried wood. The facts presented above have to be taken into consideration when the major purpose of research is to determine the possible real values (critical) of drying stress.

ACKNOWLEDGEMENTS

This study is a part of researches that was supported by the State Committee for Scientific Research, as research project 3P06L028 23.

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