United States

Department of

Agriculture

Forest Service

Forest

Products

Laboratory

Research

Paper

FPL-RP-512

Identifying

Bacterially Infected

Oak by Stress Wave

Nondestructive

Evaluation

Robert J. Ross

James C. Ward

Anton TenWolde
Abstract Acknowledgments

Bacterially infected wood, called wetwood, is often not

visually apparent in logs or green lumber. When kiln

dried, lumber containing wetwood is prone to develop

costly defects. The objective of this study was to

determine the effectiveness of a stress wave

nondestructive evaluation (NDE) technique to detect the

presence of wetwood, thereby allowing separation of

bacterially infected and noninfected lumber before kiln

drying. On average, this NDE technique correctly

identified 84 percent of bacterially infected red oak

lumber. However, the technique was less effective on

white oak, correctly identifying an average of 45 percent

of the infected lumber. The difference in results between red and white oak needs to be further investigated. Because this NDE technique shows promise,

especially for red oak, and because analysis is rapid,

implementation of this technique into a sawmill operation needs to be examined.

March 1992

Ross, Robert J.; Ward, James C.; TenWolde, Anton. 1992. Identifying

bacterially infected oak by stress wave nondestructive evaluation.

Res. Pap. FPL-RP-512. Madison, WI: U.S. Department of

Agriculture, Forest Service, Forest Products Laboratory. 6 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 53705-2398. Laboratory publications are sent to

more than 1,000 libraries in the United States and elsewhere.

This research was funded in part by the National

Hardwood Lumber Association and by the USDA

Office of Competitive Grants. We are grateful to

Theodore Mianowski, Charles J. Hillary, and Kirston

Buczak, Forest Products Laboratory, for assistance

with the experimental work. We also thank Webster

industries, Inc., Bangor, Wisconsin, for technical

assistance and for conducting the mild drying phase

of the study.

The authors also acknowledge the following individuals for their helpful comments: William Simpson,

Research Forest Products Technologist and

Stephen P. Loehnertz, Research General Engineer,

both of the USDA Forest Service, Forest Products

Laboratory, Madison, Wisconsin; Robert L. Youngs,

Professor of Forest Products, Virginia Polytechnic

Institute and State University; and James P.

Armstrong, Associate Professor of Wood Science,

West Virginia University.

The Forest Products Laboratory is maintained in cooperation with the

University of Wisconsin.
Identifying Bacterially Infected

Oak by Stress Wave

Nondestructive Evaluation

Robert J. Ross, Supervisory General Engineer

James C. Ward, Research Forest Products Technologist

Anton TenWolde, Supervisory Research Physicist

Forest Products Laboratory, Madison, Wisconsin

Introduction

Wetwood is an abnormal type of heartwood. Lumber

containing wetwood is difficult to kiln dry without developing costly defects (Ward and Pong 1980). Wetwood

develops from the infection of living trees by anaerobic

bacteria, decreasing the mechanical properties of wood

(Ward and Zeikus 1980). Consequently, oak lumber

containing wetwood (also called bacterial oak) is more

prone than normal (uninfected) oak lumber to develop

honeycomb, ring shake, and deep surface checks when

kiln dried under normal conditions (Ward and Groom

1983, Ward and Hart 1985).

It is difficult to recognize the presence of wetwood in

lumber on the green chain during mill operation; therefore, the drying defects that develop in oak wetwood are

unexpected (Ward and Groom 1983). These unexpected drying defects in oak can be costly to the

hardwood lumber industry. It is not easy to differentiate

between various causes for drying defects. However,

the Hardwood Research Council estimates that as a

result of wetwood-related drying defects in oak lumber,

losses can total as much as 500 x 106

 board feet

(nominal 1.2 x 106

 m3) per year and cost as much as

$25 x 106

 per year (John A. Pitcher, personal communication).

Wetwood also limits the potential for lumber drying at

accelerated schedules. Forest Products Laboratory

(FPL) drying tests showed that accelerating kiln drying

of red oak at elevated temperatures increases the

incidence of honeycomb in normal lumber. However,

losses from honeycomb in lumber containing wetwood

were three times greater than were similar losses for

normal lumber (Ward and Groom 1983).

A rapid, nondestructive evaluation (NDE) technique that

can identify wetwood in oak lumber before drying could

minimize these drying losses. If lumber containing

wetwood could be detected and separated from normal

wood, separate kiln drying schedules could be used to

minimize drying defects. Wetwood in oak can now be

detected with destructive, time-consuming laboratory

tests, but a rapid, economical nondestructive test

suitable for commercial operating conditions is needed

(Ward and Hart 1985, Ward and Simpson 1987).

Several techniques have been investigated. For example, Harris and Taras (1986) reported success using

green density to isolate defect-prone oak lumber.

We investigated use of stress wave NDE techniques.

Currently, these techniques are successfully used to

evaluate the mechanical properties of a wide range of

wood-based materials, and in the wood products

industry, to grade veneer and evaluate wood structural

components.

Background

Stress wave NDE techniques use low-stress motions to

measure two fundamental material properties: energy

storage and dissipation. Energy storage is manifested

as the speed at which a wave travels in a material; the

rate at which a wave attenuates is an indication of

energy dissipation.

A commonly used technique that employs stress wave

NDE technology utilizes simple time-of-transmission

measurements to determine speed of sound. The

technique uses a mechanically or ultrasonically induced

impact to impart a wave into a specimen. Piezo-electric

sensors are placed at two points on the specimen and

used to sense passing of the wave. The time it takes

for the wave to travel between sensors is measured and

used to compute wave propagation speed.

Stress wave NDE methods are commonly used in

industrial applications for testing the properties of both

wood and nonwood products. With nonwood manufactured materials (i.e., metals, polymers, ceramics,

composites, and concrete), much of the NDE research

effort has been qualitative and directed toward detecting

intrinsic material flaws that may occur in the manufacturing process. Nondestructive evaluation research and

industrial applications with wood and wood-based

products are concerned more with predicting the
strength properties of wood than with detecting manufacturing flaws.

A review of stress wave NDE techniques and their

application to wood-based materials was presented by

Ross and Pellerin (1991). They noted several research

studies that examined the effects that various types of

biological degradation have on stress wave parameters.

Results from these studies revealed that speed-ofsound transmission was sensitive to strength losses

induced by fungal attack.

Stress wave NDE studies at FPL and Oregon State

University evaluated the use of both impact stress wave

and ultrasonic pulses to detect wetwood, sapwood, and

normal heartwood in green hemlock and true fir

samples and full-size dimension lumber (Ward and

others 1985). Measurements made along the grain

showed that impact stress wave and ultrasonic pulse

measurements were effective in distinguishing heartwood boards with less than 100-percent moisture

content from the heavier sapwood and wetwood boards

with greater than 100-percent moisture content. No

distinction could be made between wetwood and

sapwood boards if their weights, or densities, were

similar. Wetwood in aspen and white fir could be

distinguished from sapwood by use of pulsed electric

currents (Ward 1984) but this method was unsuccessful with red oak (Ward and Groom 1983).

Stress wave or other NDE tests that detect strength

differences between wetwood and normal wood might

be especially useful to detect wetwood containing

Clostridium. This type of bacteria degrades pectin and

the chemical bonds between wood cells (Schink and

others 1981). An FPL drying study using elm reported

that honeycomb and ring failure did not develop in

wetwood boards that were not infected by clostridial

bacteria (Boone and Ward 1977).

Mechanical tests with green wood of various species

showed that wetwood can be weaker than normal wood

when the results are corrected for differences in density

(Haygreen and Wang 1966, Knutson 1968, MacKay

1975, Seliskar 1950, Wilcox 1968).

Because oak wetwood develops honeycomb and ring

failure more readily than does normal wood during kiln

drying, NDE tests to detect wetwood in oak should be

designed to measure reductions in strength across the

grain rather than along the grain. Honeycomb susceptibility suggests that bacterially infected oak has been

weakened in strength perpendicular to the rays, and

ring failure suggests a weakening at the interface of the

earlywood with the latewood of the previous year.

Objectives and Scope

The objective of this study was to determine if measuring speed-of-sound transmission across the width of

boards would be an effective technique to detect the

presence of wetwood before kiln drying red and white

oak lumber.

Experimental Procedures

Materials

Thirty-two red and white oak trees were selected from

timber growing on different sites in Wisconsin, northeast

Iowa, and southeast Minnesota: 18 northern red oak

(Quercus rubra L.), 12 white oak (Quercus alba L.), and

2 black oak (Quercus velutina Lam.). During the selection process, trees and logs were screened for the

presence and absence of bacterial infections. Increment cores and wood shavings were taken from the

heartwood and examined for odor and color. Final tree

selection was based on this inspection. Bacterially

infected heartwood in red and black oaks usually had

sour rancid odors, and infected white oaks had sour

vinegar-type odors. Color differences between bacterially infected and normal (noninfected) oaks cannot

always be noted, but sometimes infected heartwood will

have a greenish cast that does not exist in normal

heartwood. We used these identification techniques,

because they served as a baseline in previous research. A minimum of two increment cores per tree

were also aseptically removed from standing trees

and later cultured for presence of bacteria in the

heartwood.

Sample trees were felled and bucked into 9-ft- (2.7-m-)

long logs and hauled to FPL. As shown in Figure 1,

6-in.- (140-mm-) long disks were cut from the ends of

each 9-ft log leaving an 8-ft (2.4-m) log. Radial sections

were cut from the log discs, wrapped in aluminum foil,

and frozen. Wood specimens from the frozen sections

were used later for more definitive microbial culture

tests than could be made with the initial screening tests

from the trees. The pattern of wetwood formation in the

tree is usually pyramidal, resulting from bacterial

infections that start in the roots and lower trunk and

progress upwards. Therefore, the presence of bacterial

infection in the disks allowed us to “map” the likely

location of bacterial infection in the log (Figs. 1, 2) and

make a preliminary estimate of the amount of bacterial

infection from the original location of the board in the

log. Figure 2 illustrates how various combinations of

normal and bacterial heartwood result when boards are

sawed from logs.

2
Butt end Lower disc

Figure l-Cutting diagram used to collect specimens from oak logs.

Figure 2—Example of bacterial infection pattern

in a tree and the lumber obtained from it.

Table 1—Oak boards used in NDE stress

wave tests

Species Grade

Number Volume

of (board

boards feet)a

Red oak

White oak

FAS

Selects

No. 1 C

No. 2 C

No. 3 C

All grades

FAS

Selects

No. 1 C

No. 2 C

No. 3 C

All grades

59

56

114

80

61

370

30

16

39

26

25

136

297

261

557

393

301

1,809

149

76

191

128

124

668

All 8-ft logs were sawed into 1-3/16-in.- (30-mm-) thick

lumber, and FPL staff graded it according to the rules of

the National Hardwood Lumber Association. Sample

boards consisted of the following five lumber grades:

Firsts and Seconds (FAS), Selects (Sel), Number One

Common (No. 1C), Number Two Common (No. 2C),

a

1 board foot = nominal 2.4 x 10-3m

3

.

and Number Three Common (No. 3C). The number

and volume of sample boards according to species and

grade are presented in Table 1. The red oak category

included black oak and northern red oak boards.

3
Table 2—Presence of bacterial infection by species and lumber gradea

Species Grade

Total boards in grade (%) Total volume in grade (%)

Normal Mixed Severe Normal Mixed Severe

Red oak FAS/Selects

No. 1 C

No. 2 C

No. 3 C

Ail grades

White oak FAS/Selects

No. 1 C

No. 2 C

No. 3 C

All grades

63

55

36

25

48

76

69

65

52

68

13

11

20

21

15

9

8

8

4

7

24 62

34 55

44 35

54 23

37 48

15 77

23 68

27 66

44 53

25 68

12

11

20

22

15

8

8

8

4

7

26

34

45

55

37

15

24

26

43

25

aNormal boards have less than 30 percent infected heartwood; mixed boards have between 30 and 70 percent

infected heartwood; severe boards have greater than 70 percent infected heartwood.

To confirm the preliminary estimates of the amount of

wetwood per board, each board was examined for the

presence and amount of normal heartwood and for

bacterially infected heartwood. Sample boards were

divided into three categories: normal, mixed bacterial,

and severe bacterial. Normal boards contained less

than 30 percent infected heartwood; mixed bacterial

boards contained between 30 and 70 percent infected

heartwood; and severe bacterial boards contained

70 percent or more infected heartwood. If there was uncertainty about the presence of infection, samples were

removed for inspection by microscope and culturing.

The red oak category contained more boards with

bacterially infected heartwood than did the white oak.

Table 2 shows that the amount of bacterial heartwood

increased with decreasing lumber grade. This occurs

because clear boards in the higher lumber grades are

generally sawn from the outer shell of the butt and

upper log, and knotty boards in the lower grades are

sawn from the interior where wetwood is more prevalent

(Fig. 2).

Speed-of-Sound Transmission

We measured speed-of-sound transmission across

the boards at 1-ft- (0.3-m-) intervals along the length

of each green board. Stress waves were induced in

the boards by impact, and the transit time measured

with a commercial stress wave timing device (Fig. 3).

The unit of measurement is microseconds. The effective zone of each stress wave measurement for all

samples was 1-3/16-in. (30-mm) thickness and 6- to 9-

in. (150- to 230-mm) width. For analysis, all measurements were expressed in microseconds across a 7.5-in.

(190-mm) span of board width. Measurement accuracy

was ±1 μs.

4

Results and Discussion

Measurement of stress wave transit time across the

width of green boards ranged from 140 to more than

300 μs per 7.5-in. (190-mm) span. We found that

transit times for bacterially infected heartwood of red

and black oak lumber tended to be greater than 250 μs,

while normal heartwood usually had times less than

250 μs. This corresponds with a stress wave speed of

2,500 ft/s (760 m/s). Differences in stress wave travel

times between infected and normal wood were significantly greater than what would be expected from

differences in ring orientation (McDonald 1978). However, this pattern was not as clear for white oak where

stress wave transit times in infected wood were often

less than 250 μs and similar to transit times for normal

heartwood.

Figure 3—Technique used for measurement of

speed-of-sound transmission.
Table 3—Accuracy of speed-of-sound measurements compared to previously used techniques when

identifying normal and bacterially infected green lumber

Bacterial boardsb

Normal boardsa Mixed and severe Severe

Total Correctly Total Correctly Total Correctly

boards identified boards identified boards identified

Species Grade (number) (%) (number) (%) (number) (%)

Red oak FAS/Selects 72 100 43 72 28 82

No. 1 C 62 98 52 83 39 92

No. 2 C 27 93 51 90 35 100

No. 3 C 14 93 46 89 33 97

Ail grades 175 98 192 84 135 93

White oak FAS/Selects 35 100 11 9 7 14

No. 1 C 27 96 12 42 9 56

No. 2 C 17 88 9 67 7 71

No. 3 C 13 100 12 67 73

All grades 92 44

11

97 45 34 56

aNormal boards have less than 30 percent infected heartwood.

bMixed boards have between 30 and 70 percent infected heartwood; severe boards have greater than

70 percent infected heartwood.

We found that maximum transit time values for each

green board usually provided a clearer distinction

between boards in different wetwood categories than

did the board average of the readings. Table 3 shows

the relative success of wetwood identification using

(1) maximum stress wave transit time values for each

green board and (2) NDE classification categories

greater and less than 250 μs per 7.5-in. (190-mm)

width. For red oak, more than 93 percent of the normal

boards and more than 82 percent of the severe bacterially infected boards were correctly identified. For white

oak, more than 88 percent of the normal boards were

correctly identified, but not more than 56 percent of the

No. 1C severe bacterially infected boards. Successful

identification of bacterially infected white oak in the

No. 2C and No. 3C grades had an accuracy not greater

than 71 percent. Mixed bacterially infected boards of

both white and red oak were less easy to identify than

were the normal boards and the severe bacterially

infected boards.

Additional research is needed to explain the differences

between stress wave transit times for bacterially infected heartwood of red and white oak. One explanation might be related to the differences in microbial

populations between red and white oaks. Bacterial

cultures from infected heartwood of red oak usually

contained populations of anaerobic bacteria, such as

Clostridium spp., Fusobacterium sp., Lactobacillus

acidophilus, and Peptostreptococcus anaerobius.

Cultures from white oak often contained unidentified

microaerophilic actinomycetes. Actinomycetes are

filamentous branching organisms that superficially

resemble fungi but are related to true bacteria.

In the infected white oaks, the actinomycetes appeared

to attack mainly extractives such as the tannins, while in

the red oaks, the anaerobic bacteria appeared to attack

the compound middle lamellae and the extractives.

Bacterial disintegration of the middle lamella would

cause a weakening of the bonds between wood cells

and presumably have an effect on stress wave transit

times. Anaerobic bacteria were also cultured from

some infected white oaks, but usually from the inner

core of the logs where the low-grade lumber was sawn.

Conclusions

Nondestructive evaluation (NDE) stress wave measurements identified bacterially infected heartwood in green,

red oak lumber with an average accuracy of 84 percent.

Of the severe bacterially infected red oak boards,

93 percent were identified. The average accuracy of

NDE stress waves for identifying bacterial heartwood in

green, white oak was much less than the accuracy for

red oak (45 percent). Differences between the bacterial

populations in red and white oak may be responsible for

the dissimilarity of NDE stress wave measurements.

Further evaluation of stress wave analysis with white

oak and other species is needed. The practical effectiveness and how to integrate this technique into a

sawmill operation also needs to be examined.

5
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