Robert J. Ross, Kent A. McDonald, Lawrence A. Soltis
USDA Forest Service, Forest Products Laboratory, Madison, WI
Patrick Otton
U.S. Navy, Charleston Navy Yard, Charlestown, ME
ABSTRACT
The USS Constitution is the oldest floating commissioned ship in
the U.S. Navy. Recently, the USDA Forest Service, Forest Products Laboratory,
was involved in developing an inspection methodology for the use of Navy
personnel responsible for maintaining the ship. Several nondestructive
evaluation (NDE) techniques were used to assess the condition of fasteners
in the ship and the general condition of the wood comprising it. Radiography
and ultrasonic techniques were used to assess the condition of the copper
pins used as fasteners. Stress wave NDE techniques were used to locate
areas of degradation in the wood. This paper describes the stress wave
techniques employed and results obtained from their use.
Keywords: NDE, deterioration, USS Constitution, historic
structures
1. INTRODUCTION
The USS Constitution is the oldest floating commissioned ship in
the world and still a part of the U.S. Navy. Launched on October 21, 1797,
the ship is currently in drydock, preparing for the 200th anniversary of
her launching.
Personnel from the U.S. Navy responsible for maintaining the ship have
investigated a variety of nondestructive testing techniques to assess the
condition of the wood in the ship. Radiography and ultrasonic techniques
were used to assess the condition of copper pins used as fasteners. Stress
wave nondestructive evaluation (NDE) techniques were used to locate areas
of degradation in the wood. The objective of this paper is to describe
the stress wave techniques utilized and present results obtained from their
use.
2. BASELINE INFORMATION
Stress wave NDE techniques have been researched and are utilized frequently
to inspect large timber structures.1 As an introduction to the
stress wave technique, a schematic of the stress wave concept for detecting
deteriorated areas within a rectangular wood member is shown in Figure
1.
Figure 1 - Test setup used to locate deteriorated member.
A stress wave is induced by striking the member with an impact device
that is instrumented with an accelerometer that in turn emits a start signal
to a timer. A second accelerometer, which is coupled to the member, then
responds to the leading edge of the propagating stress wave and sends a
step signal to the timer. The elapsed time for the stress wave to propagate
between the accelerometers is displayed on the timer. The underlying premise
for use of this technique is that the speed, hence the transmission time,
at which a stress wave travels through a wood member is indicative of the
member's condition.
For example, it has been shown that a stress wave travels at speeds
that are significantly slower in deteriorated wood when compared with sound
wood. Consequently, this technique has proven to be an effective method
for locating large, degraded areas in timbers. Table 1 is a summary of
published applications of these techniques for locating deteriorated regions
in timbers. Note that previous efforts were aimed at locating degraded
areas in softwood timbers. No information was found on use of these techniques
for assessing degradation in hardwood timbers. In addition, limited baseline
information exists on speed of stress wave transmission values for hardwood
species (Table 2).
Table 1-Summary of research on use of stress wave technique for decay
detection in timber structures
Reference
Type of structure
Type of wood product
Test procedure
Analysis
(2)
Bridge
Douglas Fir, glulam, creosote pressure treated
Speed of stress wave perpendicular to grain, across laminations at
1-ft transmission
Stika spruce clear 2x4's
Southern Pine clear 2x4's
10
9
19,400 (52)
16,800 (60)
Laboratory study
(12)
Douglas Fir clear 2x8's
10
16,200 (62)
Laboratory study
(13)
Southern Pine clear 2x2's
Southern Pine knotty 2x6's
10
10
17,000 (59)
16,800 (60)
Laboratory study
(14)
Douglas Fir
12
3,000-5,260
(333-190)
Laboratory study
(3)
Douglas Fir
11
3,854-5,494
(259-182)
Inspection of college football stadium
(4)
Douglas Fir
-
7,500 (133)
Inspection of gymnasium
(15)
Southern Pine
9
16,390-19,231 (61-52)
Field study of decay
(16)
Northern Red White Oak
Green
4,464 < 2,500
(224 > 400)
Laboratory study of bacterially infected lumber
Smulski reported on speed of stress wave transmission values parallel to
the grain for sugar maple, yellow birch, white ash, and red oak.5
Armstrong and others determined speed of stress wave transmission values,
both parallel and perpendicular to the grain for birch, yellow poplar,
black cherry, and red oak.6 McDonald measured speed of stress
wave transmission for three hardwood species (beech, hickory, and red oak)
in longitudinal, radial, and tangential directions.17
Figure 2 - Test setup used to established speed of stress wave transmission
parallel to grain in live oak specimens.
Many timbers in the USS Constitution are from live oak, a hardwood
species. Consequently, baseline information on speed of stress wave transmission
in live oak was needed before we could proceed with using the stress wave
technique on the members in the USS Constitution. Thus, we conducted
a series of laboratory experiments to determine baseline speed of stress
wave transmission values for live oak specimens prior to inspecting the
ship.
To determine speed of stress wave transmission values parallel to the
grain, we tested 80 0.5- by 0.375- by 12.0-in. (0.01- by 0.01- by 0.30-m)
live oak specimens, using the experimental setup shown in Figure 2. All
specimens were conditioned to approximately 12 percent equilibrium moisture
content. An average value of 105 microsec/ft (9,524 ft/sec) was found.
Values for these specimens ranged from 71 to 151 microsec/ft (6,622 to
14,085 ft/sec).
To determine speed of stress wave transmission values perpendicular
to the grain, we tested 20 5- by 12- by 12-in (0.1- by 0.3- by 0.3-m) live
oak specimens, using the setup illustrated in Figure
3. An average value of 278 microsec/ft (3,597 ft/sec) was found. Values
for these specimens ranged from 210 to 476 microsec/ft (2,101 to 4,762
ft/sec).
3. INSPECTION OF MEMBERS
All deck beams (four decks of approximately 32 beams each), various knees,
stern post, stem keelson, and keel were examined using the setup illustrated
in Figure 1. Baseline stress wave transmission times for sound live oak
were calculated for the various thickness members. Significantly lengthy
transmission times indicated the presence of deterioration.
Figure 4, Figure
5, Figure 6, Figure
7a, and Figure 7b illustrate the
results we obtained. Note that the numbers on the drawings represent the
stress wave transmission time for that point in the member. Significantly
longer transmission times were indicative of deteriorated wood. Inspection
of these members after they were removed from the ship revealed that the
severity of degradation corresponded to increases in transmission times.
This finding is in agreement with previously reported results.
4. CONCLUDING REMARKS
Stress wave nondestructive evaluation techniques were used to successfully
locate deteriorated wood in the USS Constitution. The technique
that was utilized involved inducing a stress wave in the member in question
and monitoring the time it takes to flow through the member, perpendicular
to the grain. Significantly longer transmission times were observed when
deteriorated wood was present.
5. REFERENCES
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