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NDE of historic structures-USS Constitution

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.
Test setup used to locate deteriorated member.
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  Sound wood: 390 microsec/ft
Moderate decay: 557 microsec/ft
Severe decay: 741 microsec/ft
(3) Football stadium Douglas Fir, solid-sawn, creosote pressure treated Speed of stress wave transmission per- pendicular to grain, inspected in vicinity of connections Sound wood: 260 microsec/ft 
Incipient decay in center of members: 389 microsec/ft
1.5-in- (0.04-m-) Thick shell of solid wood: 649-778 microsec/ft
Decayed member: >1300 microsec/ft
(4) School gymnasium Douglas Fir, glulam arches Speed of stress wave transmission perpendicular to grain, inspected in vicinity of end supports Sound wood: 133 microsec/ft
Decayed wood: 267 microsec/ft
Table 2-Summary of research on speed of stress wave transmission values for various species of sound wood
 
Reference  Species  Moisture content (% ovendry)  Speed of stress wave transmission parallel to grain ft/s (microsec/ft) Speed of stress wave transmission perpendicular to grain ft/s (microsec/ft)  Comments
(5) Sugar Maple
Yellow Birch
White Ash
Red Oak
12 
11 
12 
11
12,790-16,820 (78-59) 
14,270-18,020 (70-55) 
13,000-16,680 (77-60) 
12,440-16,280 (80--61)
Laboratory study
(6) Birch 

Yellow Poplar 

Black Cherry 

Red Oak 

4-6 

4-6 

4-6 

4-6

17,090 (58)
(15,384-18,800) (65-53) 
17,910 (56)
(16,930-18,890) (59-53) 
16,900 (59)
(15,980-17,820) (63-56) 
16,530 (60)
(14,430-18,630) (69-54) 
4720 
(4589-4851)
4920 
(4592-5248) 
5020
(4758-5282) 
5410 
(5082-5738) 
(212)
(218-206)
(203)
(218-190)
(199) 
(210-189)
(185) 
(197-174)
Laboratory study
(7) Several  11 16,000-19,700 (62-51) Laboratory study
(8) Red Oak Veneer  12 10,800-14,400 (92-69)  Laboratory study
(9,10) Several species  12,000-17,100 (83-58)  Laboratory study
(11)  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

Test setup used to established speed of stress wave transmission parallel to grain in live oak specimens.
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

  1. R.J. Ross and R.F. Pellerin, Nondestructive testing for assessing wood members in structures-a review, FPL-GTR-70, U.S. Department of Agriculture, Forest Service, Forest Products Laboratory, Madison, WI, 1994.
  2. N.J. Volney, "Timber bridge inspection case studies in use of stress wave velocity equipment," In: Proceedings, Eight International Nondestructive Testing of Wood Symposium; 1991 September 23-25; Vancouver, WA., pp. 235-246, Washington State University, Pullman, WA, 1992.
  3. R.J. Ross, Quality assessment of the wooden beams and columns of Bay C of the east end of Washington State University's football stadium, Unpublished research, Washington State University, Pullman, WA, 1982.
  4. R.J. Hoyle and R.F. Pellerin, "Stress wave inspection of a wood structure," In: Proceedings, Fourth symposium on nondestructive testing of wood, pp. 33-45,Washington State University, Pullman, WA, 1978.
  5. S.J. Smulski, "Relationship of stress wave and static bending determined properties of four northeastern hardwoods," Wood and Fiber Science, 23:(1): 44-57, 1991.
  6. J.P. Armstrong, D.W. Patterson, and J.E. Sneckenberger, "Comparison of three equations for predicting stress wave velocity as a function of grain angle," Wood and Fiber Science, 23(1): 32-43, 1991.
  7. R.H. Elvery and D.N. Nwokoye,. "Strength assessment of timber for glued laminated beams," Paper II, In: Nondestructive testing of concrete and timber, organized by the Institution of Civil Engineering and the British Commission for Nondestructive Testing, 1969, June 11-12, pp. 105-110. Institute of Civil Engineering, London, 1970.
  8. J. Jung, Stress wave grading techniques on veneer sheets, FPL-GTR-27, U.S. Department of Agriculture, Forest Service, Forest Products Laboratory, Madison, WI, 1979.
  9. M.C. Ihlseng, "The modulus of elasticity in some American woods, as determined by vibration," Van Nostrand's English Magazine 19: 8-9, 1878.
  10. M.C. Ihlseng, "On a mode of measuring the velocity of sound in wood," American Journal of Science, 3d Series, 17(98): 125-132, 1879.
  11. C.C.. Gerhards, "Effect of earlywood and latewood on stress wave measurements parallel to the grain," Wood Science 11(2): 69-72, 1978.
  12. C.C. Gerhards, Effect of cross grain on stress waves in lumber, FPL-RP-368, U.S. Department of Agriculture, Forest Service, Forest Products Laboratory, Madison, WI, 1981.
  13. C.C. Gerhards, Effect of knots on stress waves in lumber, FPL-RP-384, U.S. Department of Agriculture, Forest Service, Forest Products Laboratory, Madison, WI, 1982.
  14. P.S. Rutherford, "Nondestructive stress wave measurement of incipient decay in Douglas Fir," M.S. thesis, Washington State University, Pullman, WA, 1987.
  15. R.F. Pellerin, R.C. DeGroot, and G.R. Esenther, "Nondestructive stress wave measurements of decay and termite attack in experimental wood units," In: Proceedings, Fifth nondestructive testing of wood symposium; 1985 September 9-11; Pullman, WA., pp. 319-352, Washington State University, Pullman, WA., 1985.
  16. R.J. Ross, J.C. Ward, and A. TenWolde, "Stress wave nondestructive evaluation of wetwood," Forest Products Journal, 44(7/8):79-83, 1994.
  17. K.A. McDonald, Lumber defect detection by ultrasonics, FPL-RP-311, U.S. Department of Agriculture, Forest Service, Forest Products Laboratory, Madison, WI, 1978.
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