Wood Sci. Technol. 23:215 227 (1989) W o o d S c i e n c e a n d T e c h n o l o g y

�9 Springer-Verlag 1989

Mechano-sorptive creep mechanism of wood in compression and bending *

P. Hoffmeyer, Lyngby, Denmark and R.W. Davidson, Syracuse N.Y., U S A

Summary. A model is introduced which links the mechano-sorptive behaviour of wood subjected to moderate and high compression or bending stresses parallel to grain to the formation of slip planes in the cell wall. Slip plane formation is dependent on the breaking of hydrogen bonds, which process is directly related to the amount of moisture change. The dramatic change of microfibril orientation in slip plane zones cause an increase of the longitudinaI shrinkage/ swelling and a decrease of the modulus of elasticity. These features of slip plane formation account for both the magnitude and the oscillation of the excessive mechano-sorptive creep associated with compression and bending parallel to grain. A summary is given of the character- istics of the mechano-sorptive effects, and the model is discussed in the light of these effects.

Introduction

Wood, which is al lowed to change its moisture content under load, exhibits dramat ic creep behavior, especially when subjected to bending or compression parallel to grain. This so-called mechano-sorpt ive effect has been known for almost 30 years (Arm- strong, Kingston 1960). Al though the topic has been the subject of numerous inves- tigations, the cause of mechano-sorpt ive behavior is still not fully understood.

This paper introduced the hypothesis that the microfailures, known as slip planes, are the main cause of mechano-sorpt ive behavior of wood subjected to modera te or high compression or bending stresses parallel to grain. Mechano-sorpt ive effects exist even for wood subjected to low stresses and for other modes than compression or bending parallel to grain. The proposed model however, is only encompassing the excessive effects typical of compression and bending parallel to grain.

Suppor t for the hypothesis is drawn mainly from already published results. How- ever, results from the authors ' supplementary experiments are also included. A quan- tification of the significance of slip planes relative to other possible mechano-sorpt ive mechanisms is part of a more extensive t reatment now in preparat ion.

The paper is one of the results of a project on the influence of changing moisture content on the mechanical behavior of wood, currently underway in a co-operation between College of Environmental Science and Forestry, State University of New York, and the Technical University of Denmark. Support for this project is provided by the Danish Technical Research Council and by the USDA Co-operative Research Program (proj. 85-FSTY-9- 0112). The support is gratefully acknowledged

216

The mechano-sorptive effects

P. Hoffmeyer and R.W. Davidson

An appreciation of the characteristics of the mechano-sorptive effects is necessary to assess the validity of the model presented below. An up-dated and extended version of the detailed review by Grossman (1976) is therefore presented. Particular emphasis is placed on the effects as seen in compression and bending. The typical features of mechano-sorption are:

a. Deformations increase during desorption (see e.g. Armstrong, Kingston 1962; Hearmon, Paton 1964).

b. The first sorption step causes an increase in deformation. All following sorption steps normally cause a decrease of deformation at low or moderate stresses (see e.g. Armstong, Kingston 1962; Hearmon, Paton 1964) and an increase of deformations at high stresses (Schniewind 1967; Mohager 1987).

c. The mechano-sorptive deformation is virtually time-independent and influenced only by the magnitude of moisture change below fiber saturation (see e.g. Arm- strong, Kingston 1962; Leicester 1971).

d. The increase in deformation of initially saturated wood is much greater than in initially dry wood (Armstrong, Kingston 1962).

e. Instantaneous elastic recovery following unloading is equal to (see e.g. Gibson 1965) or greater than (Mohager 1987) the initial elastic deformation. Thus, the anatomical features responsible for mechano-sorptive effects increase the elastic '~ back" of the wood.

f. Deformations are "frozen" for constant moisture content. After unloading and instantaneous elastic recovery, a large proportion of the total deformation is maintained. The deformations are ~ by further moisture cycling (Chris- tensen 1962; Eriksson, Nor~n 1965). Larger recovery during adsorption than during desorption (Armstong, Kingston 1962).

g. Constant moisture flow through wood, causing no local changes of moisture content, does not result in any mechano-sorptive effect (Armstrong 1972)

h. Mechano-sorption leads to failure in shorter time and/or at lower loads (Hearmon, Paton 1964; Schniewind 1967; Schniewind, Lyon 1973; Mohager 1987).

i. Modulus of elasticity is progressively reduced. Bethe (1969) demonstrated a linear relationship betwen total creep and MOE after termination of a series of 6 mois- ture cycles. A reduction of up to 40% of MOE was found.

j. The amplitude of the oscillation of the mechano-sorptive creep curve tends to increase linearly with total creep (authors' interpretation of data from Hearmon, Paton 1964; Gibson 1965; Mohager 1987). See Fig. 1.

k. Unloading causes the amplitude of the oscillation of the creep curve to drop. It then tends to stay constant regardless of total creep (authors' interpretation of data from Gibson 1965; Mohager 1987).

The role of slip plane formation

Ultimate compression- or bending failure parallel to grain of wood is always preceded by formation of minute failures of the cell wall (see e.g. Bienfait 1926; Kisser, Steininger 1952; Dinwoodie 1968).

Mechano-sorptive creep in wood 217

au 2, / /

10 / . / y > , / t- < _ J tl.I

5 / 3 / / / / , r

,,/,J ~,~ ,7

0 I I i

1 2 RELATIVE AMPLITUDE

Fig. 1. Relationship between relative creep and relative amplitude of the mechano-sorptive oscillation for moisture content variations of the order 15-18%. 1: Mohager (1987); Fig. 3.20; E= 16.0 GPa; ~r=20 MPa. 2: Mohager (1987); Fig. 3.20; E= 14.5 GPa; a=20 MPa. 3: Mohager (1987); Fig. 3.20; E= 12.0 GPa; r MPa. 4: Gibson (1965); Fig. 1.5: Hearmon, Paton (1964); Fig. I

The re-orientation of S2-microfibrils in the zones of minute failures (Frey- Wyssling 1953) was first revealed by polarized microscopy (Robinson 1920) as distinct lines or planes extending through the S2-1ayer and oriented at a typical angle relative to the fiber axis. The term slip plane was coined by Bienfait (1926) to convey the failure mechanism whereby one part of the S2-wall slips relative to another.

The angle between the slip plane and the longitudinal direction of the cell wall is of the order of 60 degrees (see e.g. Dinwoodie 1974). Thus, the angle between the microfibrils in the slip zone of the S2-1ayer and the longitudinal direction is also approximately 60 degrees, provided that there is no change of density of the wood tissue. I f microfibrils are packed looser in the slip plane zone than in the surrounding undisturbed tissue, this angle decreases.

A SEM-micrograph of slip planes is shown in Fig. 2. Two typical features should be noted: Slip planes tend to form in pairs, and the orientation, as seen at the lumen face, is perpendicular to the orientation of the S2-1ayer.

It is hypothesized that the re-orientation of microfibrils in the slip plane zone changes the mechanical and physical properties of the wood. The modulus of elastic- ity decreases and the longitudinial shrinkage/swelling increases in proport ion to slip plane intensity.

The number of slip planes is known to be a function of stress level, moisture content and duration of load (see e.g. Wardrop, Dadswell 1947; Kisser, Steininger 1952; Dinwoodie 1968; Keith 1971; Kitahara et al. 1981). These parameters are all

218 P. Hoffmeyer and R.W. Davidson

Fig. 2. SEM-micrograph of slip planes in compression tested spruce (Picea abies). Arrows indi- cate the location of two of the numerous slip planes in the S2-wall

decisive for creep. The suggestion of a connection between creep and slip plane formation consequently is not new. However, it is proposed that slip planes are formed at a higher rate and at lower stresses in changing moisture conditions than at constant moisture content. Very likely, the deformations involved in slip plane cre- ation imply breaking of hydrogen bonds. This process is obviously facilitated by moisture changes, which must be associated with breaking and remaking of hydrogen bonds.

The key elements of slip plane formation with respect to mechano-sorptive effects in compression and bending are suggested to be the following:

- Slip planes form in proportion to the amount of moisture change. Longitudinal shrinkage/swelling increases in proport ion to number of slip planes and thus in proport ion to amount of moisture change. The elastic, viscoelastic and plastic properties of the wood changes in proport ion to the number of slip planes and thus in proport ion to amount of moisture change. Figure 3 shows a model of slip planes as seen in longitudinal sections of the

S2-wall in the dry and wet states respectively. It is assumed that water sorption results in a swelling perpendicular to the microfibrils only. As a result of the swelling of the S2-wall (a --, a'), the slip plane angle changes (0 ~ 0') and the length of the slip plane zone increases (L ~ L'). The variables of Figure 3 are interrelated as follows:

tan 0' = (a'/a) tan 0 (1)

AL = L' -- L = b(cos 20 - cos 20') (2)

L = acot 0 - bcos 20 (3)

Mechano-sorptive creep in wood 219

dry wet

Fig. 3. Model of slip plane as seen in a longitudinal section of the S2-wall in dry and wet condition

An illustration of the order of magnitude of the slip plane induced swelling can be obtained assuming the following values of the variables:

0 = 60 degrees a ' = 1.003 a for one percent change of moisture b = 0.2a

Here, the swelling of the S2-wall is asumed to be equal to the gross transverse swelling of wood. The value of b is estimated from electron micrographs (Keith 1970).

Using equations (1)-(3) gives a longitudinal swelling strain of the slip plane zone equal to 0.06% per percent moisture change. This then is also the order of magnitude of the longitudinal swelling of wood with closely packed slip planes. As a comparison, the longitudinal swelling of slip plane free wood is normally of the order 0.01% per percent moisture change.

M e c h a n o - s o r p t i v e m o d e l

An idealized diagram of the mechano-sorptive creep behavior for wood in compres- sion or bending at moderate and high stresses is presented in Fig. 4. The curve is suggested to consist of the following 3 components: - A creep deformation resulting from normal creep mechanisms, which are merely

accelerated by moisture changes. An oscillating deformation, the amplitude of which is a result only of load induced changes of shrinkage/swelling properties.

- A progressive elastic deformation in excess of the initial elastic deformation. This component results from the load induced change of microstructure.

The various phases of the mechano-sorptive creep curve are discussed below in the light of a slip plane based model:

Constant low moisture. Deformation increases. Slip planes develop slowly as a func- tion of stress level, moisture content and time.

1st sorption. Deformation increases. Slip planes develop rapidly due to the change of moisture as well as due to the fact that slip planes generally develop more rapidly at high moisture content levels. The rapid increase of deformation during this step can not normally be counteracted by the swelling of the few slip planes formed during the constant, low moisture phase.

220 P. Hoffmeyer and R.W. Davidson

z

/ /Zl / I I "~

o

/ o ~ / ~ I I I C~ V \ ~ . I 1o = ~ ,,~ I ~ ! '\ \ ..N-. , - - 13. 03 x ~ / , ; / %V-; o. a- - ,

si "~ o ~ , \ 2' " - - Q ta "13 , _ X ' ~ " P

TIME Fig. 4. Typical mechano-sorptive creep curve for wood subjected to bending or compression

All desorption phases. Deformation increases due to the longitudinal shrinkage of already existing slip planes in the compression zone. New slip planes develop in proportion to the change of moisture thereby causing additional deformation. Fewer slip planes develop during desorption than during sorption due to the relatively longer times spent at low moisture content levels during sorption.

2nd and following sorption phases. Deformation normally decreases, as the addition- al deformation, caused by formation of new slip planes, is less than the longitudinal shrinkage of already formed slip planes. At high stresses, however, where the forma- tion of new slip planes is more rapid, the balance is turned, and deformation increases.

Amplitude (So) of mechano-sorptive creep curve. This is a measure of the change of longitudinal shrinkage/swelling properties induced by the load, i.e. a measure of the number of slip planes present. As slip planes are at the same time the major contrib- utors to deformation, it follows that e0 is proportional to mechano-sorptive creep (see Fig. 1).

Elastic recovery (gi)- Upon unloading there is a recovery of most of the initial elastic deformation, sl, plus the additional elastic deformation, eisp, stored in the slip planes.

Delayed recovery (e,ec). Having dried under load, the slip planes are now locked in the compressed position. Unloading causes tension in the slip plane zones and residual compression in the adjacent, non-failed tissue. Subsequent sorption of moisture caus- es the slip planes to give in to the tension stresses and a decrease of deformation results. Following moisture cycles further relax the slip planes thus causing additional recovery.

Amplitude (s;) of oscillation after unloading. This is a measure of the changed longitudinal shrinkage/swelling of the unloaded, slip-plane containing wood. The difference between s 0 and s~ is suggested to relate in part to different microfibril angles of slip planes of stressed and non-stressed wood.

Mechano-sorptive creep in wood

Supplementary experiment

221

Al though evidence in suppor t of the effect of slip plane format ion is already available in the li terature, a supplementary experiment was carried out. It was aimed specifical- ly at i l lustrating the effect of pre-compression on modulus of elasticity and longitudi - nal shrinkage/swelling.

Nine matched specimens (10 m m x 10 mm x 35 ram) of spruce (Pica abies) were compressed to strains in the range from 0% to 5% using a strain rate of l % / h . The specimens were water saturated and mainta ined at 80 ~ during compression. The specimens were then unloaded and taken through cycles of moisture variat ion (dry to water saturat ion) until the longitudinal shrinkage/swelling had reached a constant level. They were subsequently compression tested and the modulus of elasticity regis- tered.

I t proved difficult to avoid macrofai lures in the spruce for strains in excess of 3 percent and virtually impossible to go beyond 5 percent. An addi t ional 7 specimens of beech (Fagus sylvatica) were therefore taken to 9 percent strain under the same condit ions as the spruce. No macrofai lures were observed at this strain level for beech. However, polar ized light microscopy revealed closely packed slip planes in all cells. Two specimens were taken to 9 percent strain and subsequently allowed to dry while mainta ining the load. The resultant total strain amounted to 18 percent. The speci- mens were unloaded and later water soaked and dried while measuring the length changes.

Results are presented in Table 1. The relat ionship between total deformat ion and resultant free swelling from oven-dry condit ion to water saturat ion is shown in Fig. 5. The compression and subsequent "freezing" of deformat ion is shown in Fig. 6.

Table l. Modulus of elasticity and free longitudinal swelling as a function of pre-compression

Degree of Moisture Modulus of Ultimate Total pre-compression content elasticity stress swelling % % MPa MPa %

Beech (Fagus sylvatica): 9.0 9.0 9.0 9.0 9.0 9.0 9.0

Spruce (Picea abies): 0.0 0.2 0.5 1.0 1.5 2.0 3.0 4.0 5.0

0.0 3,250 77 1.19 4.2 1,800 71 -

11.0 1,700 48 1.25 15.3 1,340 47 1.39 19.4 1,090 37 1.39 24.5 1,340 36 1.06 85.0 1,140 41 1.56

117 4,150 15 0.04 102 3,850 17 0.06 124 3,300 17 0.07 113 2,170 15 0.18 101 1,770 15 0.27 131 2,660 18 0.30 130 1,770 17 0.43 114 1,570 13 0.59 122 1,240 16 0.75

222 P. Hoffmeyer and R.W. Davidson

9 , % 8

z �9 7 �84

< 6,

5' �9 kl. UJ g, tm -J 3 < I-- 2 0 F-

1

0

01 i 12 0'.0 .2 o'.~ o.~ o18 11o i. ~ i i ~ L O N G I T U D I N A L SWELLING

Fig. 5, Relationship between level ofpre-compression and free longitudinal swelling from oven- dry condition to water saturation. Legend: x = spruce (Picea abies). *= average of 6 specimens precompressed to 9% (Fagus sylvatica)

80% , , " I / Orig ina l length of moisture I

sa tura ted specimen J I , , , I ec, on l

while d r y i n g under load

Elastic sp r i ng -back ~ _ _ upon un loading (1.3%)

Creep recovet 'y a f te r 2 days (0.7%)

Moisture sa tu ra t ion . 1 R e l e a s e of in terna l stresses

O v e n - d r y i n g . Sl ip plane i induced shr inkage (2.5%)

I I I

/ I

90% 100%

Fig. 6. Change of length of beech (Fagus sylvatica) compressed in moisture saturated condition and subsequently allowed to dry while under load

Discussion

The hypothesis that load induced changes of longitudinal movements is the cause of the oscillation o f mechano-sorptive creep was suggested by Ba~ant (1985) and Hunt and Shelton (1987). Recently Hunt and Shelton (1988) discussed possible structure related explanations for this behaviour. They did not, however, indentify slip plane formation as a potential explanation.

A quantification of the slip plane caused shrinkage/swelling necessary to account for the oscillation, can be obtained e.g. from results of Mohager (1987).

Mechano-sorptive creep in wood 223

According to Fig. 1, the amplitude t o of the shrinkage/swelling induced oscillation for bending may grow up to 2.2 times the initial deformation ei for moisture oscil- lations of approximately 18%. For specimen No. 2, which had a relatively high load, and was a medium stiff specimen, the initial deformation was approximately 0.14%. The moisture content oscillation was approximately 18%. The amplitude caused by a change of the longitudinal shrinkage/swelling of the compression zone only, consequently corresponds to a load induced longitudinal shrinkage/swelling of less than 2.2 �9 2 �9 0.14//8 ~ 0.03 % per percent change of moisture. The corresponding amplitude of the unloaded specimen may be somewhat less, which means that slip plane induced shrinkage/swelling properties of the order less than 0.03%/% for an unloaded specimen explains the maximum creep oscillation.

Present results (Fig. 5) show a maximum longitudinal swelling of 0.75% and 1.3% for spruce and beech respectively. This corresponds to approximately 0.03% and 0.05% per percent change of moisture and consequently explains the oscillation. The swelling predicted by the model (Fig. 3.) for wood with closely packed slip planes is 0.06% per percent change of moisture.

An apparent obstacle to the general application of the suggested model for low stresses is the belief that slip planes are formed only above stress levels of approxi- mately 50% for bending and 60% for compression (Kisser, Steininger 1952; Din- woodie /968; Keith 1970, /971, /972; Kitahara et al. 1981). Dinwoodie (/968) did report to have found slip planes even below 25% of ultimate stress. However, such few slip planes may in fact have been artifacts produced during specimen preparation.

In contrast, many of the investigations on mechano-sorptive behavior show effects at lower stress levels. It must be realized however, that all investigations on slip planes have been carried out at constant moisture. And in fact, all the mechano-sorptive tests have used this very condition as a control, for which no effect could be demonstrated.

The explanation may be that not only do slip planes form at a higher rate at varying moisture, but they also form at lower stress levels.

An early indication of this being the case is given by Armstrong and Kingston (1962), who " . . . found compression failures to be present in beams and compression specimens drying under load. In the beams, the compression failures occurred to- wards the compression side. No failures were evident in beams maintained at constant moisture content while under load". The stress levels applied in these tests ranged from 24 to 38% and 18 to 40% of the green wood short-term strength in compression and bending respectively.

Further support can be found from investigations on the behavior of ammonia- treatment wood. Although the effect of ammonia on the properties of wood is more dramatic, the basic mechanism of altering the properties is the same as that of water (Davidson, Baumgardt 1970; Bariska, Schuerch t977).

Bach (1974) reported of an impressive 97-98% reduction of modulus of elasticity in compression parallel to grain for ammonia-treated wood. From 10 GPa for dry wood it dropped to 0.2-0.3 GPa. In comparison, modulus of elasticity in tension dropped only from 10 GPa to 4 GPa. Such wood subjected to bending, will conse- quently exhibit almost 20 times larger strain in the compression side than in the tension side. Similar, but much less pronounced behaviour, was reported for wood subjected to moisture change (Kingston, Armstrong /951; Armstrong, Kingston, a962).

224 P. Hoffmeyer and R.W. Davidson

Earlier unpublished results by one of the authors show that ammonia-bent wood abounds in slip planes towards the compresson face. It may therefore be concluded, that the vast deformation of the compression side is due to slip plane formation, and furthermore that the bond breaking capability of ammonia is the cause.

Bach demonstrated non-linearity of the short term stress-strain relationship even below a stress of 6 MPa. This indicates that slip planes form at such low stresses when ammonia-treated.

At very low stresses, where no slip planes are likely to form, any mechano-sorp- tire effects should consequently be of the same order for tension and compression. This is in accordance with the findings of Hunt (1986), who demonstrated no signif- icant difference for the two modes for stresses of 7.5 MPa. The comparatively mod- erate mechano-sorptive effect at such low stress levels is believed to be the results of the increased or decreased general alignment of structural elements when subjected to tension or compression respectively.

Ammonia-treated wood subjected to bending, allowed to dry in the deformed state, will "freeze" its new form. Upon rewetting, the compression side will swell and the wood regains much of its original shape (Davidson, Baumgardt 1970). This behavior is also analogous to the mechano-sorptive behavior of wood.

Figure 6 shows how water saturated beech subjected to 18% compression, and allowed to dry under load, maintains most of the deformation upon unloading. As such deformations are known to be the result of slip plane formation, it may be concluded, that the "frozen" form is caused by the relative immobility of slip planes at constant moisture content. A single wetting of the specimen results in an almost complete recovery. However, the free longitudinal shrinkage/swelling is now greatly enlarged, obeying the relationship shown in Fig. 5.

The observation that the short term modulus of elasticity is reduced after mechano-sorptive creep exposure (Bethe 1969) is in accordance with the slip plane based model: Wood, in which slip planes are introduced prior to static compression testing, shows a decrease of modulus of elasticity in proportion to the amount of pre-compression (Table 1). Spruce subjected to 5% pre-compression is seen to have suffered a 70% reduction of modulus of elasticity. Similarly, beech subjected to a 9% pre-compression suffers a reduction of 80-90%.

The mechano-sorptive effect is believed to be virtually time-independent (Arm- strong, Kingston 1962; Leicester 1971). On the contrary, slip plane formation is believed to be time-dependent (Wardrop, Dadswell 1947; Kingston, Armstrong 1951; Kisser, Steininger 1952; Dinwoodie 1968; Keith 1971; Kitahari et al. 1981). Very likely, the effect of moisture changes normally just obscures the effect of time. For longer times, however, the mechano-sorptive effect should therefore be enlarged.

Experimental support for this view is available. Ranta-Maunus (1973, 1975) showed plywood subjected to bending to exhibit normal mechano-sorptive behavior. However, if the plywood was first subjected to creep at constant moisture content for 6 months, the first sorption phase resulted in decreasing deformation. This behavior is presumably the result of the forming of a large number of slip planes already during the first, constant moisture phase.

Mohager (1987) subjected wood, which had been removed from old buildings, to bending under varying moisture. His results show a similar decrease of deformation during the first sorption phase. In addition, the amplitude t o was large already

Mechano-sorptive creep in wood 225

during the first moisture cycle. This behavior is to be expected from wood having developed slip planes during its prior function as a load bearing member of a con- struction.

Constitutive equations to account for the mechano-sorptive behavior of wood have been proposed by a number of authors (Leicester 1971; Ranta Maunus 1971, 1973; Rybarczyk, Ganowics 1974; Ba2ant 1983; Hunt 1986; Mukadai, Yata t986, 1987; Mgtrtensson, Thelandersson 1987). Such equations usually superimposes upon the normal creep elements two additional elements. One element takes into account the stress free longitudinal movement. The other element relates creep rate to a stress dependent function of the rate of moisture change. The usual relationship between rate of longitudinal movement and rate of moisture change can be used to substitute one for the other in the latter element, thus producing an element of "stress induced shrinkage/swelling" (Ba~ant 1983; M~rtensson, Thelandersson 1987). The two mechano-sorptive elements may be expressed as

~msg = 8s -~- a " S L - l ss I i4)

in which dm~g is the total mechano-sorptive creep strain rate, ~ is the rate of stress free shrinkage/swelling, a is a constant or function describing mechano-sorptive behavior, and SL is the applied stress level.

In relation to the proposed morphological model, this equation fails to take into account the moisture change induced creep oscillation. An improved equation would therefore be 5msg = ~(1 + b- SL[+ SI) + a . eL . I~sl (5)

in which b is a constant or a function describing the mechano-sorptive change of shrinkage/swelling behavior.

Still, not all the features of the slip plane based model are accounted for by the equation. The fact that the elastic properties are progressively and lastingly changed should also be reflected in the constitutive equation. The elastic strain must be made a function of moisture content, stress level and the total amount of change of moisture content while under load.

Similarly, the recoverable creep must be made a function of amount of change of moisture content after unloading. A further elaboration on the pinned slider model (Hunt 1982) or the racket model (Ba~ant 1983) may prove useful for this purpose.

Conclusion

The proposed mechanism satisfactorily explains all effects characteristic of mechano- sorptive behavior for wood subjected to moderate and high compression or bending stresses parallel to grain. The model accounts for the excessive mechano-sorptive effects characteristic for these two modes as opposed to the tension mode.

A prerequisite of the model is the formation of cell wall failures known as slip planes. The model therefore is meaningful only for stresses high enough to cause such slip planes. At constant moisture content slip planes form at stresses of the order of 50% of ultimate stress. At varying moisture content slip planes form at lower stress levels. However, the threshold level of stress for varying moisture content is not known. Further research is therefore needed to assess the general validity of the proposed model.

226 P. Hoffmeyer and R.W. Davidson

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Mechano-sorptive creep in wood 227

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(Received August 24, 1988)

P. Hoffmeyer Associate professor Building Materials Laboratory Tech. University of Denmark Building 118 DK-2800 Lyngby Denmark

R. W Davidson Professor Dept. Wood Products Engineering Coll. Environm. Sci. and Forestry State University of New York Syracuse, N.Y. 13210 USA