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MAT — 176 3



CUt fifH




TNNO: N-1703



tit I p . EVALUATION OF NONDESTRUCTIVE UNDERWATER
' TIMBER INSPECTION TECHNIQUES





AUTHOR : C A. Keeney and S. E. Pollio



DATE: August 1984



SPONSOR." Naval Facilities Engineering Command



PROGRAM NO: YF60.534.091.01.202B




NAVAL CIVIL ENGINEERING LABORATORY
PORT HUENEME, CALIFORNIA 93043

Approved for public release ; distribution unlimited.



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Unclassified



CLASSIFICATION OF THIS PAGE (When Doto Fn



REPORT DOCUMENTATION PAGE



REPORT NU

TN-1703



2 GOVT ACCESSION NO

DN044010



EVALUATION OF NONDESTRUCTIVE UNDERWATER
TIMBER INSPECTION TECHNIQUES



= E OF REPORT & PERIOO COVERED



Not final; Jun 1981 - Aug 1983



4G ORG. REPOF



CONTRACT



C. A. Keeney and S. E. Pollio



PERFORMING ORGANIZATION NAME AND ADDRESS

NAVAL CIVIL ENGINEERING LABORATORY

Port Hueneme, California 93043



ROJECT. TASK



YF60.534.091.01.202B



I. CONTROLLING OFFICE NAME AND ADDRESS

Naval Facilities Engineering Command
Alexandria, Virginia 22332



August 1984



58



■IBER OF PAGES



16 AGENCY NAME » ADDRESSf.



Unclassified



15« DECLASSIFICATION DOWNGRADING
SCHEDULE



16 DISTRIBUTION STATEMEN



Approved for public release; distribution unlimited.



ION STATEMEI-



Block 20. II dtller,



Underwater inspection, nondestructive testing, waterfront facilities, ultrasonic testing,
impact testing, computerized axial tomography, timber inspection



20 ABSTRACT (Can



nd Identity 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DD | j°J M 73 1473 EDITION OF I NOV 65 IS OBSOLETE



Unclassified



(continued)



SECURITY CLASSIFICATION OF THIS PAGE (When Date Entered)



MBL/WHOI



Q 03D1 OOMQElb D



Unclassified



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 58ppillus 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
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.



Unclassified

CLASSIFICATION OF THIS PAGErW/ion Da



CONTENTS

Page

INTRODUCTION 1

BACKGROUND 1

DAMAGE 1

MEASUREMENT ACCURACY REQUIREMENTS 2

POTENTIAL TECHNIQUES 5

Passive Sonic Testing 5

Low Frequency Ultrasonics 6

ASSESSMENT OF POTENTIAL TIMBER NDT TECHNIQUES 8

Indirect Ultrasonic Analysis 8

Impact Analysis 9

X-ray Tomography 10

Real Time X-ray Imaging 10

Acoustic Emission 10

Dielectric Measurement 11

EVALUATION OF TIMBER INSPECTION TECHNIQUES 11

TOMOGRAPHY 11

INDIRECT ULTRASONIC TESTING 15

Commercial Demonstration 15

NCEL Ultrasonic Laboratory Testing 16

NCEL Ultrasonic Field Testing 31

IMPACT TESTING 35

Theory 35

Types of Impact Testing 36

Equipment 37

Laboratory Procedure 39

Laboratory Test Results 41

Impact Field Testing 43

Field Test Results 47

Summary of Impact Test Results 50

CONCLUSIONS AND RECOMMENDATIONS 50

REFERENCES ' 51



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:

1. For extensive external damage to the piles, 14% (most stringent
requirement)

2. 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.



Popular and
generic names

Gribbles
Limnoria
lignorum

(Rathke)



Limnoria
quadripun



Limnoria
tripunctata



Damage CharacterUtics





Ship worms
Teredo



Bankia
setacca
Tryon



Adults can grow
feet (30.5 to 70 c
long; '/S-inch (12 i




Adults can grow 5 l
feet (1.5 to 1.8 m)
long; 7/8 inch (22 ir
diameter.



Pholads
Martesia



i 2'A inches (50 to
n) long; 1-inch
(25.4-n





Unlike the shipworm's, the size of the
entrance hole increases to about V* inch
(6mm), making it possible to notice



Figure 1. Characteristics of marine borers,
3



Table 1. Accuracy Requirements for Timber Piles
[Nomenclature at end of table.]



Structural Evaluation
Criteria


Type and Extent of
Deterioration


Field Data


Required


Accuracy

Requirement

(±%)


Parameter


Range


A. Material Strength


Internal Damage








f a = P/A


1. Magnitude - cross-
sectional area:








or


V A o " °- 5


A R


50-150 in. 2


20




A R /A < 0.5


A R


0- 75 in. 2


25


B. Instability
Short Column:


2. Length/location


L d> H d


>l-2 ft


20


f c - 3 [l/r'J


External Damage

1. Confined damage








applied where f < (2/3)F c

Long Column:

TT E T

f = —

C («7r) 2


region:
L d /L < 0.2
V A > °' 5


L d
A R
d R


1-8 ft

50-150 in. 2

8-14 in.


20
20
10


V A < °' 5
Location


A R
H d


0-75 in. 2
>l-2 ft


25
20




2. Extended damage






-




region:










L d /L > 0.2


L d


>8-10 ft


2




V A o > °- 5


A R


50-150 in. 2


14






d R


8-14 in.


7




A R /A < 0.5


A R


0-75 in. 2


25




Location


H d


>l-2 ft


20



A = cross-sectional area
A n = original cross-sectional area



*R



R



remaining cross-sectional area
diameter of remaining cross-sectional



area

f = axial stress
a

f = critical stress for a column
c

H . = location of damaged section along pile
length, distance from pile cap to seabed
or to midpoint of damaged section



L = total length of structural element
L. = length of damaged section
Jl = unsupported length
P = axial load

F = critical buckling stress

r = radius of gyration
E T = modulus of elasticity



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 tirae-of-f light and attenuation measurements do not
consistently correlate with voids smaller than 25% of the wood cross-
section.

2. Direct time-of-f light 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-f light or transit time alone
is not an accurate or reliable measure of cross-sectional wood loss with
either direct or indirect transmission modes.



w = 5000 fps

WOOD




(a) SOLID WOOD PATH

TIME = 12"/12

5000 FT/SEC



= 200^XSEC



= 4860 fps




(b) 2 WATER FILLED VOID
TIME = 10V12 + 2/12



5000 FT/SEC 4860 FT/SEC
= 201/XSEC



• =800 fps

'air




(c) 2" AIR FILLED VOID
= 10"/12 + 2"/12



5000 FT/SEC 800 FT/SEC
= 188/XSEC



Figure 2. Direct transmission time-of-f light measurements through

(a) solid wood, (b) solid wood with a 2-inch water filled
void, and (c) solid wood with a 2-inch air filled void.



Although consistent correlation between the ultrasonic signal and
the timber specimen had not been identified, further testing was required
to determine what, if any, inspection capability existed. In particular,
the accuracy and reliability of using ultrasonics for the inspection of
timber waterfront structures must be determined. Based upon the labora-
tory test results the following recommendations were made:



1. Utilization of timber specimens with increasing known amounts
of damage or wood loss to identify the minimum amount of damage detec-
table.

2. Incorporation of appropriate state-of-the-art data processing
techniques to correlate digitized acoustic data to the condition of the
timber.

3. Evaluation of ultrasonic parameters besides time-of-f light and
peak-to-peak values.

4. Utilization of multiple transducers to show relative changes in
the acoustic signal at adjacent locations.



ASSESSMENT OF POTENTIAL TIMBER NDT TECHNIQUES

In light of the problems encountered during the initial laboratory
evaluation of ultrasonic inspection of timber piling, a study was con-
ducted to identify new alternative techniques. A contract was awarded
to Southwest Research Institute (SWRI) , San Antonio, Tex., to evaluate
existing and assess new NDT techniques for use by Navy divers (Ref 8).
Potential NDT techniques were analyzed to determine feasibility and to
predict performance capabilities and characteristics. The most promising
techniques were then tested in the laboratory to demonstrate their feasi-
bility.

From the results of the laboratory experiments and feasibility study
conducted in seawater on small-scale wooden models with simulated marine
borer damage along the grain, the contractor determined that the following
six techniques were technically feasible:

• indirect ultrasonic testing

• impact testing

• X-ray tomography

• real time X-ray imaging

• acoustic emission

• dielectric measurement

Each of the six potential timber inspection techniques are discussed
below.

Indirect Ultrasonic Analysis

SWRI evaluated this technique using essentially the same equipment
as during the laboratory tests at NCEL. Two 50-kHz transducers, separated
a distance of 18 inches in the axial direction, generated compressional



waves along the wood grain of timber specimens. Axial damage was simu-
lated by cutting slots in wood blocks of various depths. Laboratory
tests were conducted on wood blocks with slots equal to 0, 25, 50, 75,
and 100% of the block thickness. The blocks were submerged in seawater.

Measurements of the RMS (root-mean-square) of the received ultra-
sonic signal were taken between two fixed times. An increase in the
attenuation of the acoustic waves was expected with an increase in slot
depth because of the impedance mismatch between water and wood in the
axial direction. It was also expected that a portion of the wave energy
would be delayed in time. Test results, summarized in Table 2, show a
decrease in RMS amplitude with an increase in slot depth.

However, the decrease in RMS is not linearly related to the increase
in damage. SWRI concluded "that the RMS of the transmitted signal between
two fixed points in the time window decreases with an increase in slot
depth, but there is no consistent relationship between slot depth and
RMS amplitude."



Table 2. RMS Amplitude of Ultrasonic Signals for Various
Slot Depths


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