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RECIPIENT'S CATALOG NUMBER TN-1703 DN044010 4. TITLE (and Subtitle) TYPE OF REPORT & PERIOD COVERED EVALUATION OF NONDESTRUCTIVE UNDERWATER i Not final; Jun 1981 — Aug 1983 TIMBER INSPECTION TECHNIQUES Co en 7. AUTHOR(s) 8. CONTRACT OR GRANT NUMBER(s) C. A. Keeney and S. E. Pollio 9. PERFORMING ORGANIZATION NAME AND ADDRESS 10 RR sa a ie peel TASK NAVAL CIVIL ENGINEERING LABORATORY Port Hueneme, California 93043 YF60.534.091.01.202B 11. CONTROLLING OFFICE NAME AND AODORESS | 12, REPORT DATE August 1984 Naval Facilities Engineering Command aan or DASEE Alexandria, Virginia 22332 14. MONITORING AGENCY NAME & ADDRESS(if different from Controlling Office) 15. SECURITY CLASS. (of this report) Unclassified 1Sa. DECLASSIFICATION’ DOWNGRADING SCHEDULE a 6. DISTRIBUTION STATEMENT (of this Report) Approved for public release; distribution unlimited. 17. DISTRIBUTION STATEMENT (of the abstract entered in Block 20, if different from Report) 18. SUPPLEMENTARY NOTES KEY WORDS (Continue on reverse side if necessary and identify by block number) o Underwater inspection, nondestructive testing, waterfront facilities, ultrasonic testing, impact testing, computerized axial tomography, timber inspection 20. ABSTRACT (Continue on reverse side if necessary and identify by block number) This report presents the assessment of potential techniques for underwater nonde- structive testing of timber piles. Three techniques are discussed: X-ray tomography, indirect ultrasonic testing, and impact testing. A computerized axial tomography (CAT) system has never been used underwater to date. However, studies concluded that the underwater application of tomography is technically feasible. A brief introduction to the proposed prototype underwater CAT system is presented. In addition, results of laboratory DD , ates 1473, ~— EDITION OF 1 NOV 65 IS OBSOLETE (continued) 0 0301 OO40elb O Unclassified SECURITY CLASSIFICATION OF THIS PAGE (When Data Entered) Unclassified SECURITY CLASSIFICATION OF THIS PAGE(When Data Entered) 20. Continued and field tests to evaluate the capability of an indirect ultrasonic system and impact system to accurately measure the percent cross-sectional wood loss are presented in detail. Library Card Naval Civil Engineering Laboratory EVALUATION OF NONDESTRUCTIVE UNDERWATER TIMBER INSPECTION TECHNIQUES, by C. A. Keeney and S. E. Pollio TN-1703 58 pp illus August 1984 Unclassified 1, Underwater inspection 2. Nondestructive testing I. YF60.534.091.01.202B | | | | | | | | | | | This report presents the assessment of potential techniques for underwater nondestructive | | testing of timber piles. Three techniques are discussed: X-ray tomography, indirect ultrasonic | l testing, and impact testing. A computerized axial tomography (CAT) system has never been | used underwater to date. However, studies concluded that the underwater application of | tomography is technically feasible. A brief introduction to the proposed prototype underwater | | CAT system is presented. In addition, results of laboratory and field tests to evaluate the capa- | | bility of an indirect ultrasonic system and impact system to accurately measure the percent | cross-sectional wood loss are presented in detail. l | | | | | | Unclassified SECURITY CLASSIFICATION OF THIS PAGE(When Data Entered) CONTENTS JENA OUG ILO, 69S oO) o Oo o46)-6, Gt 66 6,9 6 x WAGMEAROWIMD) 6 0.6 6 6 oO 66 8d OO Gio 6 676 DYNES os Gio dh 6, AO ONO Oo GG VOTO. “Cag Mon ls a MEASUREMENT ACCURACY REQUIREMENTS ...... PO TEN UAT CEE CHNTOWE Sr rome tel giclee. orlve nie: co Mette Bassavie) Sond emlesitimg, i tis) 1 tier ve) ie. fo) elon le Low Frequency Ultrasonics ....... ASSESSMENT OF POTENTIAL TIMBER NDT TECHNIQUES Indirect Ultrasonic Analysis ...... Limp actA alley /SalSivermer (iol let pewiessirorancy fermesh lice KATA yVAhOMO REAP MY; tyewee ese son sr elt ole elon ie Real Time X-ray Imaging ......... /MOOW Ese WH UbUSSBLOy IG Goa oO C=O Ou GGG 1 Dielectric Measurement 3. si5: «0 3s os EVALUATION OF TIMBER INSPECTION TECHNIQUES . . INOMOLGRVNENE. 6 oo od 6) BGG oO oo B to Bo GG INDIRECT ULTRASONEG@ TESTING 203 2 2. 3 1. 5 3s Commercial’ Demonstratvon, "253 5 2. et ls NCEL Ultrasonic Laboratory Testing ... NCEL Ultrasonic Field Testing ...... MPA C Ties SUN Gametarenmenteuitonite ote: ten vel eli cala ce arcanteiinie ts ENC O Tsyamre en toversinnce rel Val Get sisal conus: elute wrsr valve hy pesmot WimpaGes TEStim gir, verte us ei ee 8 EGUe PMC TINE Emmet Mme reeset tei oils! yeni si Seiieh) sikiiei ie Laboratory sProceduTre ys ty) il e's, cet wel Laboratory Test Results ......... ImpactrbaevlidwMestam ees) ee les) es) 6) site PCIE TSS tMRESUIES Met cre elite tic) sllenes Summary of Impact Test Results ..... CONCLUSIONS AND RECOMMENDATIONS ....... REBEREN GES Mrommeu emer aueiicie or komm teliniotee ofl est shits: Cocarorir sie Page ie + Serer tn ila sai Ad an BS: INTRODUCTION Accurate assessment of the condition of Naval shore facilities is a vital aspect of Fleet readiness. More than two-thirds of the Navy's waterfront structures were built before 1950 and are rapidly deteri- orating. Thirty-five percent of Navy piers are wooden superstructures on wooden piles (Ref 1). An economical maintenance management program for these structures requires development of reliable and accurate under- water timber inspection techniques. In 1979, the Naval Civil Engineering Laboratory (NCEL), under the sponsorship of the Naval Facilities Engineering Command (NAVFAC), initi- ated a project to improve the Navy's ability to inspect and assess the soundness of the underwater portion of wooden waterfront facilities. The state-of-the-art of underwater nondestructive testing (NDT) and the application of existing or potential NDT techniques were to be evaluated. This report presents the results of laboratory and field evaluation of several potential techniques, particularly acoustic NDT techniques. BACKGROUND Natural materials, such as wood, often vary inherently to a large degree, and prediction of their properties is considerably more difficult than with man-made materials. Distinguishing the natural property varia- tions from any internal damage of the wood under water has been a major effort at NCEL, and several approaches have been investigated. The types of timber damage, the measurement accuracy requirements for timber piles, and the initial concepts for inspecting wooden waterfront structures are discussed in this report. DAMAGE Structural damage of timber waterfront structures generally falls into one of two categories: mechanical or biological (Ref 1). Mechanical damage usually results from accidental overloads or abrasion. Accidental overloads can occur during construction from excessive pile driving forces or after construction from large impact loads, such as docking ships. Abrasion typically occurs in the intertidal zone and depends upon the amount and type of material or debris in the water. Biological damage to wooden waterfront structures results from the activities of living organisms such as fungi, insects (e.g., termites, ants), and marine borers. Fungi, the cause of wood rot, are low forms of plant life that depend on organic materials for food. Rot damage usually occurs above water in the splash zone and near the pile cap. Insect damage also occurs above water in the atmospheric and splash zones. The most severe type of damage to timber waterfront structures is caused by marine boring organisms because this damage often cannot be detected visually until extensive damage has been done. In the United States alone, marine borers and fungi annually cause an estimated $500 million in damage to wooden waterfront structures (Ref 2). Marine borers are of two types: crustaceans and mollusks (Figure 1). Of the crustaceans, Limnoria or Woodgribbles are of primary importance. The shrimp-related Limnoria attack and damage wood at the piling surface. These tiny animals average 1/8 to 1/4 inch in length and burrow shallow tunnels which are then eroded away by wave action, exposing new wood to attack. Limnoria eventually narrow the pile diameter usually at the waterline (or the mudline), resulting in an hourglass shape. The molluskan type of marine borers are teredines and pholads. Teredines are commonly referred to as Shipworms and include Teredo and Bankia. Shipworms settle into the wood substrate when they are very young and barely visible. Their clamlike shells begin digging into the wood leaving a pinhole entrance. They burrow inwards and eventually turn to tunnel along the soft wood grain. Teredines can cause severe loss of structural integrity and leave essentially no externally visible signs. The average size of adult Teredo is 1/2 to 1 inch in diameter and 1 to 2 feet long. Unlike Limnoria damage, Teredo or Bankia damage usually cannot be detected by visual inspection. Pholads or Martesia are approximately 2 inches in length and 1 inch in diameter as adults. Typically, Martesia burrow less than 2-1/2 inches into the piling but leave an entry hole large enough to detect visually. MEASUREMENT ACCURACY REQUIREMENTS The extent and severity of boring damage, coupled with the large number of wooden waterfront structures, necessitate development of quick and effective timber inspection techniques. These techniques must be capable of evaluating remaining structural strength or remaining cross- sectional area. If the timber pile sustains internal damage, then a parameter other than diameter must be used as an indication of struc- tural condition. Inspection data criteria and accuracy requirements were established, based upon structural analyses (see Ref 1). Table 1 lists the accuracy requirements as a function of (1) type of deterioration (internal or external); (2) load capacity of the column; and (3) length of the damaged section (with respect to the total length of the pile) for various degrees of damage. Thus, Table 1 defines the physical and material parameters to be measured and the level of accuracy to which they must be measured. The accuracy is given in terms of coefficient of variation (%), which, in statistical terms, is the standard deviation divided by the mean. With more than one-half the original cross-section remaining, the requirements for accurate measurement are as follows: ie For extensive external damage to the piles, 14% (most stringent requirement) Die For internal damage from Teredo and Bankia, 20% Therefore, the test and evaluation of potential underwater timber inspec- tion techniques were based upon the 14 and 20% accuracy requirements. 2 Popular and generic names Gnbbles Limnoria lignorum (Rathke) Limnoria quadripunctata Holthuis Limnoria tripunctata Menzies Shipworms Teredo navalis its Linne Bankia setacca Tryon Pholads Martesia siriata Linne Appearance 1/8 to 1/4 inch (3 to 6 mm) long; no tubercles. 1/8 to 1/4 inch (3 to 6 mm) long; 4 tubercles, 1/8 to 1/4 inch (3 to 6 mm) long; 3 tubercles. ne Adults can grow 1 to 2 feet (30.5 to 70 cm) long; 4-inch (12 mm) diameter. = —_ shells B at head A Teredo pallet (spadelike) wormlike body siphons Ws Bankia “y C pallet 4 ) (featherlike Adults can grow 5 to 6 feet (1.5 to 1.8 m) long; 7/8 inch (22 mm) diameter, 2 to 2% inches (50 to 63 mm) long; 1-inch (25.4-mm) diameter Damage Characteristics Pile Cap = _MHW & 2 v A Wood Destroyed Wee MLW me ——S Mudline Section B-B Unlike the shipworm's, the size of the entrance hole increases to about % inch (6mm), making it possible to notice their presence, Figure 1. Characteristics of marine borers. Table 1. Accuracy Requirements for Timber Piles {Nomenclature at end of table.] Structural Evaluation Type and Extent of BRIG) WEIEE EERE ES SOSUISESY/ - - . a Requirement riteria Criteri Deterioration Parameter Range (4%) Material Strength Internal Damage f= P/A 1. Magnitude - cross- sectional area: Ap/Ay 22 0515) 50-150 in.? Ap/Ay SS 0)55) O= 75min Instability Length/location >1-2 ft Short Column: 1 Q[x 4 External Damage Wyre 1. Confined damage region: applied where f, $ (2/3)F, L/L § 0.2 i-By Et Ap/Ao 22 055) 5O=il5 Opin] 8-14 in. Long Column: 2 T E é Bee ith Ap/Ay 0.5 0-75 in. € (R/r)2 Location 21-2 ft Extended damage region: Lg/L ZiOe2 78-10 ft Ap/Ay 21035 50-150 in.? 8-14 in. Ap/Ay < 0.5 0-75 in.? Location ?1-2 ft A = cross-sectional area L = total length of structural element Ag = original cross-sectional area La = length of damaged section AR = remaining cross-sectional area & = unsupported length dp = diameter of remaining cross-sectional P = axial load area f. = axial stress BS = critical buckling stress f. = critical stress for a column r = radius of gyration Ha = location of damaged section along pile EL = modulus of elasticity length, distance from pile cap to seabed or to midpoint of damaged section The accuracy needed could be established based on the criteria for maintenance. The degree of damage determines the method of repair. Piling is wrapped when damage is between 5 and 15% of the cross-sectional area. When damage is between 15 and 50%, the piling is repaired with grout or concrete. When damage exceeds 50%, the piling or the damaged area is replaced with wood or concrete (Ref 3). For both economic and safety purposes, the accuracy required should be between 10 and 15%. After 15% cross-sectional area loss, the strength of the pile is affected and the cost for repair increases. Current methods of inspecting waterfront structures do not meet the accuracy required to prevent unexpected or catastrophic failures, par- ticularly in critical waterfront facilities that directly impact Fleet operational readiness. Current methods of inspection include visual surveys, incremental coring, resistance probing, and hammer sounding. In addition, ultrasonic inspection of wood piles is currently being used by Agi and Associates, a consulting firm located in Vancouver, British Columbia, which has often inspected Navy facilities. Visual inspection, the most common method of inspecting underwater structures, is an essential part of any structural survey and can provide information on defects and external condition. However, numerous defects are not visually detectable, particularly in timber waterfront structures. Pilings that appear to be sound may suffer over a 50% loss in cross-—- sectional area from marine borer infestation. In core sampling one or more small diameter cores are removed for examination to determine the internal condition of the piling. Core samples indicate the pile condi- tion in the exact location of the core. The major disadvantage of incre- mental core inspection is the small probability of intersecting a mollusk tunnel unless the infestation has reached advanced stages. Resistance probing and hammer sounding give only gross indication of internal condi- tion and are typically only successful in identifying extensive deteri- oration. The ultrasonic equipment used by Agi and Associates was developed by B.C. Research of Vancouver, British Columbia. B.C. Research studies revealed that the remaining cross-sectional area could be correlated with the ultrasonic measurement only to within 25%. This is due to the inherent variations in wood strength and the effects that differing eccentricities of the damage in the cross-sectional area have on the buckling and bending moments for the pile (Ref 4). The detailed capabil- ities of the ultrasonic inspection technique used by Agi and Associates are discussed in this report in the section on Commercial Ultrasonic Capabilities Demonstration. POTENTIAL TECHNIQUES Passive Sonic Testing During research to determine growth rates of Bankia and Teredo, Professor E.C. Haderlie of the Naval Postgraduate School, Monterey, Calif., found he could detect the presence and location of borers in timber laboratory test panels. The borers were detected by listening with a sensitive transducer for the rasping sound produced by the organ- isms as they bored into the wood. As a follow-on to this work, NCEL sponsored a project to determine the feasibility of using this technique to detect the presence of marine borers in timber piling. In the first phase of the work, isolated specimens of Bankia and Limnoria were col- lected and their characteristic sound spectra recorded in the laboratory. It was hoped that the sounds made while boring would be unique in spec- tral content and could, therefore, be distinguished from ambient back- ground noise present in all waterfront environments. Test results revealed that the sonograms of isolated mollusks and gribbles were almost identical to the ambient noise recorded in Monterey Harbor. In his final report (Ref 5), Professor Haderlie concluded that ",.,.the natural sound of barnacles and other foulers on a piling are so diverse and complicated that they mask any borer sounds coming from within the piling and we have been unable to filter out the extraneous sounds which might make it possible to detect borers in a wooden harbor structure." Low Frequency Ultrasonics In an early state-of-the-art survey (Ref 6) low frequency ultrasonic NDT was identified as having the greatest potential for improving the Navy's ability to accurately evaluate the integrity of wooden waterfront structures. This NDT method was selected for further evaluation because it is known to penetrate through wood, is not hazardous to work with, and can be readily used in an underwater environment. Low frequency ultrasonic inspection is based upon the influence of the test specimen on the propagation of a known sound wave. In flaw detection, the transit time of an ultrasonic pulse traveling through a test specimen with a fixed path length is measured. Dividing the path length or the separation distance between two transducers by the transit time determines the acoustic velocity. Solid homogeneous materials have a constant acoustic velocity. Therefore, uncharacteristic changes in the pulse velocity in these types of materials are due to defects, such as cracks or voids, which either delay or accelerate the received signal. In nonhomogeneous materials, acoustic velocity varies locally due to natural changes in the microstructure such as grain orientation in wood. Although nonhomogeneous materials do not have a constant acoustic velocity, an average acoustic velocity can be obtained, for instance, for a given wood grain direction in a given specimen. A deviation from the average acoustic velocity greater than the deviations caused by the nonhomogeneity of the material itself signifies an "uncharacteristic" change in pulse velocity and, therefore, material properties. Use of ultrasonics is based on relating the uncharacteristic sonic signal to the condition of the structure. Low frequency ultrasonic inspection of nonhomogeneous materials (wood) uses two transducers in a through-transmission mode, with one transducer acting as the transmitter and the other as a receiver. In contrast, high frequency ultrasonic inspection of homogeneous materials (metals) uses one transducer that acts as both transmitter and receiver in a pulse echo mode. In initial low frequency ultrasonic laboratory tests signals from a solid specimen were compared to signals from a specimen with a known amount of cross-sectional wood loss. The laboratory test procedures, data analysis and test results are explained in detail in Reference 7. The laboratory evaluation of direct and indirect ultrasonic inspection indicated the following: 1. Ultrasonic time-of-flight and attenuation measurements do not consistently correlate with voids smaller than 25% of the wood cross-— section. 2. Direct time-of-flight measurements cannot detect water-filled voids (marine borer tunnels) because the acoustic velocity of wood across the grain is very close to that of the acoustic velocity of seawater (Figure 2). 3. A digital readout of the time-of-flight or transit time alone is not an accurate or reliable measure of cross-sectional wood loss with either direct or indirect transmission modes. j