Influence of Anatomical, Physical, 

and Mechanical Properties of 

Diffuse-Porous Hardwoods on 

Moisture Durability of Bonded 

Assemblies

Daniel J. Yelle

Ashley M. Stirgus

United States Department of Agriculture

Forest

Service

Forest Products

Laboratory

General Technical Report

FPL–GTR–244

September

2016
Abstract

Studying wood adhesive bond durability is challenging because 

wood is highly variable and heterogeneous at all length scales. In 

this study, three North American diffuse-porous hardwoods (hard 

maple, soft maple, and basswood) and their adhesively bonded assemblies were exposed to wet and dry cyclic tests. Then, their density differences were related to bond durability. Physical and mechanical properties (compression-shear) of the solid wood were 

measured along with mechanical properties of epoxy-bonded assemblies. Specific gravity tests gave the expected result (hard maple > soft maple > basswood), but volumetric swelling–shrinkage 

values for basswood were as high as those for hard maple. Tangential swelling–shrinkage values for basswood were approximately 

equal, suggesting that basswood can withstand swelling stresses 

with minute deformation. The compression-shear test showed that 

solid wood lost half its strength after vacuum-pressure soaking but 

recovered well after redrying. The epoxy-bonded assemblies also 

lost substantial strength after vacuum-pressure soaking, but soft 

maple and basswood assemblies completely recovered their 

strength after redrying. The goal of this study was to further the 

understanding of how density affects wood adhesive bond durability under a single wet–dry cycle in which swelling and shrinkage 

stresses are greatest. This study found that epoxy bond durability 

begins to deteriorate from the swelling–shrinkage stress in the 

range of densities between that of hard maple and soft maple. For 

species that have low density and high volumetric swelling–shrinkage (such as basswood), density is a controlling factor for moisture 

durability of bonded assemblies.

Keywords: bond durability, diffuse-porous, physical properties, 

compression-shear, epoxy

September 2016

Yelle, Daniel J.; Stirgus, Ashley M. 2016. Influence of anatomical, physical, and mechanical properties of diffuse-porous hardwoods on moisture 

durability of bonded assemblies. General Technical Report FPL-GTR-244. 

Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 8 p.

A limited number of free copies of this publication are available to the public from the Forest Products Laboratory, One Gifford Pinchot Drive, Madison, WI 53726-2398. This publication is also available online at 

www.fpl.fs.fed.us. Laboratory publications are sent to hundreds of libraries 

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The Forest Products Laboratory is maintained in cooperation with the University of Wisconsin.

The use of trade or firm names in this publication is for reader information 

and does not imply endorsement by the United States Department of Agriculture (USDA) of any product or service.

Contents

Introduction........................................................................ 1

Experimental ...................................................................... 2

Physical Properties of Solid Wood ................................. 2

Mechanical Properties from Compression-Shear

Parallel-to-Grain Test ..................................................... 3

Results and Discussion....................................................... 3

Physical Properties of Solid Wood ................................. 4

Mechanical Properties from Compression-Shear

Parallel-to-Grain Test ..................................................... 5

Conclusions........................................................................ 7

References.......................................................................... 7

Conversion Table

English unit Conversion factor SI unit

inch (in.) 25.4 millimeter (mm)

pound per square

inch (lb/in2) 6894.7 pascal (Pa)

temperature (°C) = [temperature (°F) − 32]/1.8

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Influence of Anatomical, Physical, and

Mechanical Properties of Diffuse-Porous 

Hardwoods on Moisture Durability of 

Bonded Assemblies

Daniel J. Yelle, Research Forest Products Technologist

Ashley M. Stirgus, Physical Science Technician Intern

Forest Products Laboratory, Madison, Wisconsin

Introduction

Durability of wood adhesive bonds exposed to moisture is 

an important topic in the wood products industry. When 

kiln-dried wood is used for adhesive bonding, the hygroscopic and anisotropic nature of wood can lead to swelling 

and shrinking stresses upon exposure to moisture content 

(MC) above or below the equilibrium moisture content 

(EMC) used during bonding. The stresses are produced by 

dimensional changes that occur from the swelling or shrinking of the wood cell wall in all three orthogonal directions 

(tangential (T), radial (R), and longitudinal (L)) both externally and internally to the wood, thus creating strain-induced stresses (Peck 1957, Stamm 1935). Most adhesives do 

not absorb stresses as well as the bulk wood because they do 

not swell and shrink like wood. Thus, a large strain can develop between the adhesive and the wood, leading to internal stress on the bondline in addition to any applied loads. 

These stresses that occur in bonded wood products must be 

absorbed by the bondline, allowing the forces to be distributed and the energy to be dissipated. If stresses are concentrated, bond failure is likely to occur under wet conditions.

Not all adhesives create durable bonds with wood, especially when exposed to wet conditions. Certain adhesives, 

such as phenol-formaldehyde (PF) and resorcinol-formaldehyde (RF), have been shown to bond tremendously well to 

most wood species and provide highly durable bonds. Other 

adhesives, such as epoxies, do not bond as well to wood 

(AITC 1992) but do provide sufficient dry strength for certain applications (such as wood beam repair). When failures 

do occur under severe moisture tests, they typically occur 

within the epoxy interphase region closest to the wood surface (Frihart 2003). This region displays substantial interfacial stress upon exposure to wet conditions because the 

wood swells and shrinks, while the epoxy does not to any 

significant degree (Frihart 2006b). Therefore, studying adhesives such as epoxies that do not bond well to wood gives 

results that allow for a better understanding of why bonds 

fail. For example, if PF or RF is used to study bond durability, it would take several wet–dry cycles to achieve bond 

failure; this type of failure is normally within the bulk wood. 

Thus, one learns little from these studies because these adhesive bonds are just as durable as the bulk wood itself. 

When epoxies are used, bond failure will typically occur 

within one or two wet–dry cycles. Because epoxies are 

known to have poor tensile elongation, bond failure caused 

by tensile strain between wood and epoxies is inevitable. 

This allows us to analyze the bond failures and develop hypotheses on their failure mechanisms.

Further study is necessary for understanding bond durability 

of epoxy-bonded assemblies with different wood species. 

The degree of wood swelling under wet conditions is hypothesized to proportionally affect internal stress on the 

bondline. In a previous study, compression-shear strength 

(ASTM D 905 (1994a)) and percentage delamination 

(ASTM D 2559 (2004)) of epoxy-bonded assemblies for 

white oak, hard maple, aspen, Sitka spruce, and southern 

yellow pine were evaluated (Frihart 2006a). The less dense 

and lower strength aspen and Sitka spruce formed more durable bonds in both ASTM D 905 and D 2559 type tests 

compared with white oak, hard maple, and southern yellow 

pine. These results suggested that density of wood must play 

a substantial role in bond durability. However, missing from 

this previous study are measured physical properties of the 

wood and how the intrinsic swelling and strength of the 

wood affect bond durability. The following unanswered 

questions still remain: (1) What are the density and swelling–shrinkage properties of the solid wood species?

(2) What are the shear strengths of solid wood at 12% EMC, 

at full water saturation (FWS) and at 12% EMC after drying 

from FWS to 12% MC? (3) What effect does the intrinsic 

strength of these wood species have on the bond durability 

of epoxy-bonded assemblies? (FWS is defined here as the 

maximum amount of water that the wood specimens were 

able to forcibly absorb into the wood lumens and cell wall 

based on the pressure and time in this test. This is not the 

same as fiber saturation point as the specimens may not be 

at a point of maximum swelling.) The purpose of this study 

was to investigate the bond durability of three uniformly 

grained diffuse-porous hardwoods with respect to density
General Technical Report FPL-GTR-244

2

and swelling–shrinkage properties. We determined the physical and mechanical properties of solid wood and their 

bonded assemblies. The bonded assemblies were further 

evaluated using fluorescence microscopy. The results of this 

study may shed more light on the mechanisms of bond durability as a function of wood density and its close correlation 

to dimensional changes.

Experimental

In this study, we used three different kiln-dried wood species obtained from Wisconsin-based lumber companies, 

Tilia americana (basswood, low specific gravity (SG)), Acer 

rubrum (soft maple, medium SG), and Acer saccharum

(hard maple, high SG), all with straight grain, little or no 

heartwood, and minor defects. These diffuse-porous hardwoods were chosen for consistent vessel distribution 

throughout the growing season, straight grain, and fairly low 

extractive contents. All lumber was stored at 80 °F/65% relative humidity (RH) (12% MC) for approximately 3 months

before experiments began. To test that EMC was reached for 

each wood species, we measured mass changes of pre-cut 

blocks from the lumber on a weekly basis. After the weight 

change levelled out to approximately 0%, the wood was 

considered ready for experiments. All mass measurements 

were made on a Mettler–Toledo (Columbus, OH) balance 

(±0.001-g accuracy). All length, width, and thickness measurements were made using a Mitutoyo (Kanagawa, Japan) 

digital caliper (±0.0001-in. accuracy).

Physical Properties of Solid Wood

In total, 90 specimens for physical property measurements 

were prepared with dimensions of 0.75 × 0.75 × 0.75 in. For 

each species, 30 specimens were labelled with either B 

(basswood), SM (soft maple), or HM (hard maple). On the 

label, a dash with SG, V, or RT for SG, volumetric shrinkage, and radial–tangential shrinkage was added. For example, B-SG-1 is the first basswood specimen for SG measurements. For physical property measurements, the procedures 

subsequently outlined detail the modifications of ASTM D

2395 (2007) for SG, ASTM D 143 (1994b) (section 19) for 

volumetric swelling–shrinkage, and ASTM D 143 (section 

20) for radial and tangential swelling–shrinkage.

Growth Rings

The wood selected for physical property tests was initially 

separated by similarity in growth ring properties. For example, each of the 90 cubed specimens designated for SG, V, 

and RT was measured for number of growth rings per inch 

and growth ring width. This was performed to minimize differences associated with growth rings and potentially help in 

explaining any differences related to growth rate.

Specific Gravity

For SG measurements, ASTM D 2395 method A procedure 

was followed. With the 30 specimens equilibrated to 12% 

EMC, we measured length (L), width (R), and thickness (T) 

for the specimens and put L, R, or T on the spots the measurements were taken. Weight (I) of each specimen was 

taken at 12% EMC. After all specimens were measured in 

dimensions and mass, they were placed into an oven maintained at 100 °C for 48 h, removed, and placed into a desiccator (W.A. Hammond Drierite Co., Ltd., Xenia, OH) and 

weighed. With the following equation, SG was calculated at 

12% EMC and at FWS:

SG = K · W/[1 + (M/100)] · L · R · T

where

K = 0.061 (when weight is in grams and volume is in cubic 

inches),

W is either I for initial weight of specimen at 12% EMC or 

S for swollen weight at FWS,

M is sample moisture content at time of measurement (%),

L is length at 12% EMC or FWS,

R is radial dimension at 12% EMC or FWS, and

T is tangential dimension at 12% EMC or FWS.

Volumetric Swelling and Shrinkage

For the volumetric swelling–shrinkage (V) measurements, 

ASTM D 143 (section 19) procedure was followed. With the 

30 specimens equilibrated at 12% EMC, L, R, and T for each 

of the specimens were measured and labelled with L, R, or 

T on the spots the measurements were taken. Weight (I) was 

taken for each specimen at 12% EMC. We placed the 30 

specimens (plus 1 dummy specimen per species) into a vacuum-pressure soak (VPS) tank, filled up the tank, and pulled 

a vacuum at 14 lb/in2(gauge pressure). The vacuum was 

held for 15 min, and water pressure was slowly added at

65 lb/in2(gauge pressure) and held for 1 h. This cycle was 

performed three times in total. The tank was then opened, 

and the dummy specimen for each species was split open to 

observe a full saturation gradient. The water-saturated specimens were then weighed (S), and the wet dimensions (L, R, 

and T) were taken in the same spot as at 12% EMC. Volumetric swelling (from 12% EMC to FWS), volumetric 

shrinkage (from FWS to 12% EMC), and SG (at FWS) were 

calculated. For example, swelling was the difference between swollen volume and volume at 12% EMC, divided by 

volume at 12% EMC. Likewise, shrinkage was the difference between swollen volume and volume at 12% EMC, divided by swollen volume.

Radial and Tangential Swelling and Shrinkage

For the radial and tangential swelling–shrinkage measure

ments, ASTM D 143 (section 20) was followed. With the 30 

specimens equilibrated at 12% EMC, we measured R and T

and labelled them with R or T on the spots the measurements were taken. Weight (I) was taken of each specimen at
Influence of Anatomical, Physical, and Mechanical Properties of Diffuse-Porous Hardwoods on Moisture Durability of Bonded Assemblies

3

12% EMC. We placed the 30 specimens into a VPS tank, 

filled up the tank, and pulled a vacuum at 14 lb/in2(gauge 

pressure). The vacuum was held for 15 min, and water pressure was slowly added at 65 lb/in2

(gauge pressure) and held 

for 1 h. This cycle was performed three times in total. After 

this cyclic soaking, we measured the weight (S) of the saturated specimens and the wet dimensions (R, T). The specimens were placed into a humidity room at 65% RH and 80 

°F to return to 12% EMC. The specimens were allowed to 

equilibrate for 1 week. Measurements were taken again of 

weight and dimensions for each specimen (R, T) at 12% 

EMC. The percentage swelling to FWS and percentage 

shrinkage back to 12% EMC were calculated similarly to 

that previously described for volumetric swelling and 

shrinkage.

Mechanical Properties from CompressionShear Parallel-to-Grain Test

Solid Wood

For compression-shear parallel to the grain of solid wood 

(ASTM D 143, section 14), we made 12 assemblies for each 

species, totalling 36 assemblies. From each assembly, we 

cut out four block-shear specimens with the dimensions as 

shown in Figure 1. From each assembly, one specimen was 

tested in compression-shear under ambient conditions

(80 °F/65% RH) and a second specimen was tested in compression-shear after a VPS cycle.

Figure 1. Dimensions of the ASTM D 143 (section 14) and 

reduced size D 905 specimen, where A = 1.25 in., B = 1.0 in., 

C = 0.25 in., and D = 0.25 in.

A third specimen was tested in compression-shear using 

ASTM D 905 fixture (no offset) after the VPS cycle and 

conditioning back to EMC of 12%. The total number of 

specimens tested was 3 species  12 assemblies  3 conditions = 108 compression-shear specimens. Additionally, we 

measured width and thickness of each specimen at 

12% EMC and after VPS to determine tangential and radial

swelling after VPS, and we weighed specimens at 

12% EMC and after VPS to determine percentage moisture 

gain.

Epoxy-Bonded Assemblies

For the compression-shear parallel-to-grain tests of epoxybonded assemblies (ASTM D 905, reduced specimen size), 

we made eight assemblies for each species, totalling 40 assemblies. FPL-1A epoxy resin was prepared and applied as 

previously described (Vick and Okkonen 1997). A fluorescent pigment (0.1% w/w, L-212, Beaver Luminescers, Newton, MA) was added to the epoxy resin mixture before application. From each assembly, four block-shear specimens 

were cut with the dimensions shown in Figure 1. From each 

assembly, one specimen was tested in compression-shear 

under ambient conditions (80 °F/65% RH) and a second 

specimen was tested in compression-shear after a VPS cycle. A third specimen was tested in compression-shear after 

the VPS cycle and conditioning back to EMC of 12%. The 

assembly material left over was used for light and fluorescence microscopy sections for determining adhesive penetration into lumens. The total number of specimens tested 

was 3 species  12 assemblies  3 conditions = 108 compression-shear specimens. Additionally, we measured width 

and thickness of each specimen at 12% EMC and after VPS 

to determine tangential and radial swelling after VPS and 

we weighed specimens at 12% EMC and after VPS to determine percentage moisture gain.

Results and Discussion

A major contributing factor in wood that controls how much 

dimensional change will occur is its density (or SG). There 

are many other factors that come into play as well, such as 

extractives, juvenile wood, reaction wood, and slope of 

grain just to name a few. However, density has undoubtedly 

the largest effect on dimensional change. Density may seem 

like a simple factor to control when using substrates that are 

homogeneous and have a predictable, repeatable structure at 

all length scales. However, wood is a challenging material 

in that it has a highly heterogeneous structure at all length 

scales. Therefore, controlling the density factor is not so 

simple. One way to narrow down the variability of wood 

species to study with respect to density is to use species 

known for uniformity in pore distribution. Such species include the diffuse-porous hardwoods, which compared with 

ring-porous hardwoods, typically have more predictability

when it comes to anatomical structure in that the vessel pore 

diameters are uniformly distributed throughout the growth
General Technical Report FPL-GTR-244

4

ring. As density increases in diffuse-porous species, the fiber cell content will increase but the vessel distribution will 

remain quite uniform. Figure 2 shows light micrographs of 

the three North American diffuse-porous hardwoods used in 

this study, which vary in density. Each of these images 

shows an earlywood-to-latewood transition (that is, one 

growth ring) as indicated by the horizontal band of dense

fiber cells.

Figure 2. Transmission light microscopy images of solid 

wood cross sections (30-μm thickness) of (a) basswood, (b) 

soft maple, and (c) hard maple taken at 10× and (d) basswood, (e) soft maple, and (f) hard maple taken at 16×

Physical Properties of Solid Wood

A summary of all measured physical properties is shown in 

Table 1. The following section attempts to describe differences between the wood species tested.

Growth Rings

Major differences are seen between hard maple and soft maple. Hard maple has almost twice as many rings per inch 

than soft maple, thus giving ring widths in hard maple approximately half those of soft maple. Basswood showed 

rings per inch and ring widths close to those of hard maple. 

One might expect that a slower annual growth rate may lead 

to greater density. This is not always true and is more species dependent. Therefore, growth rate does not appear to

influence wood density in the case of the diffuse-porous 

hardwood specimens examined in this study.

Table 1. Measured physical properties of wood species

using ASTM standard methodsa

Property Hard maple Soft maple Basswood

Growth rings per inchb 10.2 (2.0) 5.6 (0.5) 8.3 (0.3)

Growth ring width (in.)b 0.09 (0.02) 0.16 (0.03) 0.11 (0.02)

SG at 12% EMC c 0.67 (0.01) 0.48 (0.01) 0.38 (0.01)

SG at FWSc 0.61 (0.00) 0.45 (0.01) 0.35 (0.00)

Volumetric swelling (%)c 14.1 (0.8) 9.9 (1.0) 13.6 (1.3)

Radial swelling (%)c 4.4 (0.8) 2.9 (0.8) 2.4 (0.5)

Tangential swelling (%)c 9.1 (0.9) 6.5 (0.5) 8.4 (0.7)

Volumetric shrinkage (%)c 12.3 (0.6) 9.0 (0.8) 12.0 (1.0)

Radial shrinkage (%)c 1.9 (0.7) 0.7 (0.7) 1.8 (0.4)

Tangential shrinkage (%) 7.6 (0.9) 5.2 (1.1) 8.2 (0.7)

aStandard deviation is shown in parentheses; SG, specific gravity; EMC, 

equilibrium moisture content; FWS, full water saturation.

bn = 30.

cn = 10.

Specific Gravity

The SG of the wood used in this study was measured at both 

12% EMC and FWS after the volumetric swelling test. The 

density differences were clear: hard maple > soft maple > 

basswood. As expected, when the wood was in a swollen 

state, density decreased for all wood species. In this case, 

hard maple had the most dramatic SG change from 

12% EMC to FWS with a 9% decrease. Under the same MC 

change, soft maple had a 6% decrease and basswood had an 

8% decrease.

Volumetric Swelling and Shrinkage

In this study, the change in volume going from 12% EMC to 

FWS is considered volumetric swelling. The change in volume going from FWS back to 12% EMC is called volu

metric shrinkage. Not surprisingly, hard maple showed the 

greatest volumetric swelling and shrinkage of all three species tested. However, basswood also displayed high volumetric swelling and shrinkage, close to that of the higher 

density hard maple, despite its much lower density. One factor that may be influencing this is that basswood is known 

for having considerably more tension wood than most hardwood species. Tension wood, made up of a gelatinous (G) 

layer formed toward the lumen, has nearly pure cellulose 

and can form in place of the S2 or S3 layers (Côte and Day 

1965, Daniel et al. 2006). This G-layer makes tension wood 

much more susceptible to moisture absorption leading to 

more dramatic dimensional changes (Panshin and de Zeeuw 

1980, Skaar 1972).

Radial and Tangential Swelling and Shrinkage

The two orthogonal dimensions that change the most with 

moisture are radial and tangential, with tangential swelling–

shrinkage being typically twice that of radial (Panshin and 

de Zeeuw 1980). Therefore, these dimensions were measured for swelling and shrinkage. As for radial swelling and
Influence of Anatomical, Physical, and Mechanical Properties of Diffuse-Porous Hardwoods on Moisture Durability of Bonded Assemblies

5

shrinkage, hard maple showed the highest values. The radial

swelling was similar for soft maple and basswood, but radial

shrinkage was slightly higher for basswood. For tangential

shrinkage and swelling, hard maple and basswood showed 

high values, similar to what was shown for volumetric 

swelling and shrinkage. Interestingly, basswood tangential

swelling and shrinkage values were almost equivalent. Of 

course, these values come from a single wet–dry cycle, but 

because after swelling and shrinkage, basswood returned to 

the initial sample dimensions at 12% EMC, basswood displayed more of an elastic response compared with hard maple or soft maple. This result suggests that basswood is more 

effective at restoring initial swelling stresses than is hard 

maple or soft maple, possibly because of its lower density. 

Another possibility for these results is that soft maple and 

hard maple, being higher density species than basswood, 

were not fully equilibrated to 12% MC after 1 week of storage at 80 °F/65% RH. If this were the case, the maple specimens may not have fully shrunk to their equilibrium 

dimension.

Mechanical Properties from CompressionShear Parallel-to-Grain Test

Okkonen and River (1988) studied several factors that affect 

the apparent strength of solid wood and adhesive-bonded 

joints under compression-shear. They found that, using 

ASTM D 905 and D 143 test methods, the presence or absence of an offset in the shear plane was the most consistent 

and powerful factor. Also, specimen double-notching consistently gave strengths equal to or greater than the singlenotched shape. The underlying conclusion was that the

D 905 test (no-offset) and the D 143 test (offset) are not 

comparable. Thus, if comparison between solid wood and 

adhesive-bonded specimens is necessary (as is the case for 

this study), all specimens (solid and adhesive-bonded) 

should be tested in the D 905 double-notched configuration 

with no-offset, have equal slopes of grain, have T bonding 

surfaces, and have similar rings per inch and ring width for 

compression-shear tests. Figure 1 shows the specimen dimensions used for this study. Both ASTM tests performed in 

this study gave results that represent testing to failure.

Solid Wood

In comparing the compression-shear stress of the solid wood 

specimens under dry, wet, and wet–dry, there are some similarities and differences. Figure 3 shows a bar graph of the 

results. With all species tested, the wood displayed a 50% 

decrease in shear strength going from dry to wet condition. 

However, when these species went from wet, then back to 

dry condition, the basswood was the only species to completely regain its original dry shear strength. Hard maple and 

soft maple did recover most of their initial strength but not 

completely. As was previously mentioned, this result could 

be because basswood has good elasticity and recovers well 

Figure 3. Compression-shear stress parallel to the grain of 

solid wood species (n = 12) (EMC, equilibrium moisture 

content).

from initial swelling stresses. However, it also could mean 

that the maple specimens did not completely reach 12% MC 

before testing, thus giving slightly lower strength values 

compared with initial values. Another plausible cause is that 

the hard maple and soft maple showed some loss in strength 

after swelling and shrinking because of cleavage of bonds 

between cells or within cell walls during the wet–dry cycle.

Epoxy-Bonded Assemblies

After the three species were bonded with epoxy, the compression-shear stress of the assemblies was tested in an identical fashion (Fig. 4). A major difference in these results 

compared with solid wood was that the epoxy-bonded hard 

maple lost more than 70% of its initial dry shear strength 

when exposed to wet conditions. In contrast, the epoxybonded soft maple and basswood lost 50% of its initial dry 

shear strength, thus behaving similarly to the solid wood. 

When the epoxy-bonded assemblies went from wet and then

Figure 4. Compression-shear stress parallel to the grain of 

epoxy-bonded assemblies (n = 12) (EMC, equilibrium moisture content).
General Technical Report FPL-GTR-244

6

back to dry condition, the hard maple lost even more 

strength compared with the wet condition. This result is different from that observed in Frihart (2006a) in which conditions were 70 °F/50% RH instead of the 80 °F/65% RH in 

this study, which may have affected the strength recovery 

mechanism. That is, a drier ambient condition (10% MC instead of 12% MC) may have allowed the epoxy-bonded 

hard maple to recover more of its initial strength. In contrast 

to the hard maple, shear strength of the soft maple and basswood assemblies actually increased 28% and 9%, respectively (beyond their initial dry strength) after the wet–dry 

cycle. This result is interesting because the lower density 

species appear to be completely unaffected by the wet–dry 

cycle. Although this is only one wet/dry cycle, the result 

suggests that there is probably some density (or range of 

densities) between that of soft maple and hard maple at 

which below this density, a wet–dry cycle will not affect the 

epoxy–wood bond and above this density, catastrophic failure of the epoxy–wood bond may occur. Tarkow and Turner 

(1958) demonstrated that an increasing density of wood, 

when exposed to a range of low to high RH, will exponentially increase the swelling pressure of the wood. Therefore, 

it is probable that the low-density basswood used in this 

study did not produce as much swelling pressure, and stress, 

as did the high density hard maple.

Bondline thickness may also play a role in epoxy-bonded 

assembly durability. To visualize how density of the wood 

affected bondline thickness, cross sections of the bonded assemblies were observed using fluorescence microscopy 

(Fig. 5). When bonding any wood adhesive assembly, a balance is required for proper adhesive flow and surface 

spread. The pressure should not be too high as to push out 

the entire adhesive from the bondline (i.e., starving the 

joint), but not too low so that little adhesive exudes from the 

bondline (i.e., poor spread). If this balance is met correctly, 

some clear differences between species density can be observed. The basswood assembly showed the thickest epoxy 

bondline, followed by soft maple, and then hard maple with 

the thinnest epoxy bondline (Fig. 5). These images also 

show that the basswood anatomy allowed the epoxy to penetrate the farthest into the wood. Thus, a more dense wood 

makes adhesive penetrability a challenge as is the case with 

hard maple.

To better understand what the mechanical property data 

might mean with regards to durability, the differences between the compression-shear stress results in solid wood and 

epoxy-bonded assemblies can be shown as fractions. For example, the epoxy-bonded shear stress divided by the solid 

wood shear stress will give an idea of how the intrinsic 

strength of wood compares with strength of its bonded assembly, giving a bonded/solid ratio. Table 2 shows a summary of the compression-shear results along with the 

bonded/solid ratios. From these ratios, it becomes clearer

Figure 5. Fluorescence microscopy images of wood-epoxy 

bondline cross sections at 16× (30-μm thickness): basswood (top); soft maple (middle); hard maple (bottom).

that basswood is the most durable species when it comes to

epoxy bonding because its stress ratios when wet, dry, and 

wet–dry were closest to each other. Soft maple also displayed ratios fairly close to each other. Hard maple displayed the most dramatically different ratios and thus would 

be considered the least durable epoxy-bonded species used 

in this study.
Influence of Anatomical, Physical, and Mechanical Properties of Diffuse-Porous Hardwoods on Moisture Durability of Bonded Assemblies

7

Table 2. Summary of compression-shear stress for solid wood and bonded assemblies

Hard maple Soft maple Basswood

Dry Wet Wet–dry Dry Wet Wet–dry Dry Wet Wet–dry

Solid (lb/in2) 2,885 1,458 2,638 1,814 876 1,631 1,524 764 1,559

Bonded (lb/in2) 1,532 412 300 1,402 748 1,795 1,271 631 1,382

Bonded/solid 0.53 0.28 0.11 0.77 0.85 1.10 0.83 0.83 0.89

Conclusions

Understanding the factors involved in wood adhesive bond 

durability has been a challenge considering the tremendous 

variability of wood from the macroscale down to even the 

nanoscale. This study focused on relating density differences to bond durability when the sample was exposed to 

sequential wet and dry cycling conditions. The species were 

common North American diffuse-porous hardwoods (hard 

maple, soft maple, and basswood) with clear density differences but with fairly similar and uniform anatomical features. The physical properties of the solid wood were measured, followed by mechanical property measurements (compression-shear) on both solid wood and epoxy-bonded 

assemblies.

The two major findings pertaining to the physical properties 

of the solid wood were the following. First, although SG 

gave the expected result (hard maple > soft maple > basswood), the volumetric swelling–shrinkage values for basswood were as high as those for hard maple, potentially because of tension wood typically found in basswood. Second,

the tangential swelling and shrinkage values for basswood 

were almost equivalent, suggesting that this species has 

good elasticity and can withstand swelling stress with minimal deformation.

The two major findings pertaining to the mechanical properties were the following. First, solid wood showed a 50% decrease in compression-shear strength after VPS, but recovered well after redrying to 12% EMC. Basswood showed a 

complete recovery of shear strength after redrying. Second,

the epoxy-bonded assemblies also showed dramatic strength 

loss after VPS, but only the soft maple and basswood assemblies completely recovered this strength after redrying to 

12% EMC.

This study demonstrated that density does indeed play a significant role in how wood adhesive bonds perform under 

wet–dry cycle durability tests. There seems to be a density 

or range of densities between that of hard maple and soft 

maple where epoxy bond durability begins to deteriorate 

from the swelling–shrinkage stress. This study also demonstrated that with species such as basswood with low density 

and high volumetric swelling–shrinkage, density is more of 

a controlling factor for bond durability. Future studies 

should explore more low-density species with a wide range 

of volumetric swelling–shrinkage values to better understand the reasons behind their good bond durability. Is it just 

because of their low density or are there other factors at 

play, such as pore size distribution and uniformity, type and 

frequency of cells, type and frequency of pits, reaction 

wood, juvenile wood, magnitude of the stresses at the 

bondline, etc.?

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