NASA CASE NO.

PRINT FIG. 1

NOTICE

/

LAR 14816-1

The invention disclosed in this document resulted from research in aeronautical and

space activities performed under programs of the National Aeronautics and Space

Administration. The invention is owned by NASA and is, therefore, available for

licensing in accordance with the NASA Patent Licensing Regulation (14 Code of

Federal Regulations 1245.2).

To encourage commercial utilization of NASA-owned inventions, it is NASA policy to

grant licenses to commercial concerns. Although NASA encourages nonexclusive

licensing to promote competition and achieve the widest possible utilization, NASA

will provide the necessary incentive to the licensee to achieve early practical

application of the invention.

Address inquiries and all applications for license for this invention to NASA Patent

Counsel, Langley Research Center, Code 143, Hampton, Virginia 23681-0001.

Serial No.: 07/988,084

12/08/92 LaRC

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(NASA-Case-LAR-14816-I-S_) HIGH

SPEED THIN PLATE FATIGUE CRACK

MONITOR Patent Application (NASA)

36 p

G3/39

N93-19329

Unclas

0149907
HIGH SPEED THIN PLATE FATIGUE CRACK MONITOR 

NASA Case No. LAR 14816-1-SB 

Present methods of fatigue crack detection and 

characterization include eddy current, ultrasonic, dye penetrant, 

acoustic emission, and magnetic particle testing. These techniques 

are generally unsuitable for rapid nondestructive testing of many 

critical structures because they are localized techniques, may 

require surface preparation, and, in the case of acoustic emission, 

promote further crack growth. 

The present invention consists of a non-contacting apparatus 

to produce resonant vibrations in thin metallic plates. Crack 

lengths in sample plates are determined by correlating the 

resonance frequency of the sample plate to the resonance 

frequencies previously determined for plates of similar size and 

composition with known crack lengths. Crack geometry in sample 

plates is characterized by correlating acoustic emissions detected 

at the resonance frequency of the sample plate to the acoustic 

emissions detected for plates of similar size and composition 

containing cracks with known geometries. The novelty of the 

present invention encompasses it ability to determine crack length 

and geometry by determining resonance frequencies and the acoustic 

emissions produced at resonance frequencies. Use of resonant 

vibrations to produce acoustic emissions does not promote crack 

growth and produces reproducible signals. In addition, a noncontacting vibrational apparatus is used. Therefore, this method 

and device permits rapid, nondestructive testing for fatigue cracks 

over relatively large areas. 

Inventors: Buzz A. Wincheski Joesph S. Heyman 

4022 Midlands Drive 130 Indian Springs Road 

Williamsbur VA 23188 Williamsburg, VA 23185 

Employer: 

SSN: 

AS&M Employer: 

SSN: - 

NASA LaRC 

Min Namkung James P. Fulton 

112 Villa Way 142 Marcella Road 

Hampton, VA 23666 

SSN: 

Employer: NASA LaRC Employer: AS&M 

.Initial Evaluator: Eric Madaras 

Serial No.: 07/988,084 

Filed: December 8, 1992 
LAR 14816-1-SB -1- PATENT APPLICATION

HIGH SPEED THIN PLATE FATIGUE CRACK MONITOR

Origin of 1;he Invention

5 The invention described herein was jointly made in the performance of

work under a NASA contract and an employee of the United States

Government. In accordance with 35 U.S.C. 202, the contractor elected not

to retain title.

10 Background of the Invention

1___. Technical Field of the Invention

15

The present invention relates generally to the detection of cracks and

more particularly to the nondestructive detection and characterization of

cracks.

2.__. Discussion of the Related Art

20 The rapid detection and characterization of fatigue cracks are of major

importance for many critical structures. Recent developments in the airline

industry have initiated a rethinking of conventional nondestructive evaluation

(NDE) methods. As the commercial airline fleet continues to age, the NDE

community is being challenged to develop rapid nondestructive testing

25 techniques which can ensure the structural integrity of aging aircraft while

keeping aircraft down time to a minimum.

Present methods of fatigue crack detection include eddy current,

ultrasonic, dye penetrant, magnetic particle, and acoustic emission (AE)

testing. Eddy current, ultrasonic, dye penetrant and magnetic particle testing

30 are all localized techniques, that is, only a small area of the structure is
LAR 14816-1 -SB -2- PATENT APPLICATION

examined at a time. In addition, the dye penetrant technique requires a paint

free surface. Both ultrasonic and dye penetrant testing depend upon the

application of foreign substances to the surface of the specimen which allows

for possible contamination. All of the present techniques also depend heavily

5 on the inspector, requiring a skilled operator in order to be effective. Acoustic

emission testing has the advantage of being able to monitor large structures,

but yields difficult to interpret results. It has been most successfully used in

the past to monitor crack growth. See C.B. Scruby, Quantital;ive Acoustic

Emission Techniques, AERE R 11202, July 1984.

10 U.S. Patent 4,975,855 to Miller et al. discloses comparing the natural

frequency responses of a vibrational shaft to an analytical model to determine

crack presence and severity, i.e., to perform modal analysis. Particularly, FIG.

9 of this patent shows that as the severity of the crack increases, the natural

frequency splits and the difference between the two new frequencies

15 increases. The shaft is vibrated by a directly contacting shaker which induces

extraneous frequencies which would complicate any attempt to combine this

system with an acoustic emission crack detection system. U.S. Patent

4,188,830 to Mason et al. discloses measuring acoustic emission emitted

from opposite crack faces rubbing against each other as the structure is

20 vibrated. Once again, a direct contact vibrator is used, thereby complicating

signal analysis by introducing extraneous signals.

In conventional acoustic emission techniques, it is difficult to obtain

stress information at the time of emission events under passive loading

conditions. Such conventional techniques require a separate sensor technique

25 to determine the stress as a function of time. Although static or linear

loading techniques can produce a record of stress versus the acoustic

emission event, such techniques have the drawback of requiring further

damage to the sample in the form of crack growth or micro-growth as a

source of the emission event.

3O
Y

LAR 14816-1-SB

_Q_bjects

-3- PATENT APPLICATION

It is accordingly an object of the present invention to nondestructively

detect the presence of cracks in a structure.

5 It is another object of the present invention to nondestructively

characterize the length and nature of cracks in a structure.

It is a further object of the present invention to detect and characterize

cracks with a technique that is not localized.

It is another object of the present invention to detect and characterize

I0 cracks in a specimen without contaminating the surface with foreign

substances such as ultrasonic couplants or penetrating dyes.

It is another object of the present invention to detect and characterize

cracks in a specimen in a relatively straightforward, easily interpreted manner.

It is a further object of the present invention to detect and characterize

15 cracks in a specimen without requiring significant operator interpretational

skills.

It is another object of the present invention to detect and characterize

cracks with a technique that uses a non-contacting source of vibration.

It is a further object of the present invention to nondestructively detect

20 and characterize cracks in a specimen without requiring access to both

surfaces of the specimen.

Summary of 1;he Invenl;ion

25 It is a primary object of the present invention to nondestructively detect

and characterize cracks in thin metallic plates. Additional objects and

advantages of the present invention are apparent from the specifications and

drawings which follow.

The foregoing objects are achieved by vibrating the test plate at its

30 resonance frequency by using a non-contacting vibrating apparatus. Crack
5

10

LAR 14816-1 -SB -4- PATENT APPLICATION

lengths in the test plate are then determined by correlating resonance

frequencies for test plates to resonance frequencies for uncracked plates and

plates with known crack lengths. Additional information concerning the

nature of the cracks can be determined by recording variations in the acoustic

emission waveforms produced at the resonance frequency of the test plate

as a result of crack face interactions.

Use of resonance frequencies coupled with acoustic emission permits

accurate determinations of the length and character of cracks without

requiring further crack growth in the test plate. In addition, use of a noncontacting vibration apparatus prevents the introduction of extraneous noise

into the test plate and permits rapid evaluation of test samples in the field.

Brief Description qf the Drawings

15 FIG. 1 is a schematic representation of the present invention as used

to test thin plates.

FIG. 2 is a block diagram depiction of the electronic equipment which

controls sample vibration and records frequency and waveform.

FIG. 3 is a plot illustrating the frequency of the second and fourth

20 vibrational modes of a 0.040 in. thick 2024-T3 sample plate containing a 7/8

in. long fatigue crack.

FIG. 4 is a plot illustrating the frequency shift of the second vibrational

mode as a function of crack size.

FIG. 5 is a drawing of the mesh used for finite element modeling of the

25 uncracked plate.

FIG. 6 depicts the finite element modeling results for a plate with a 3/4

in. long fatigue crack.

FIG. 7 is a plot of the frequency of the second vibrational mode as a

function of crack length.

30 FIG. 8 is a bar graph illustrating the amplitude of crack emissions as a
LAR 14816-1 -SB -5- PATENT APPLICATION

function of crack length.

FIG. 9a depicts the acoustic emission waveform captured at the

resonance frequency of the second vibrational plate mode for a plate

containing a 7/8 in. fatigue crack.

5 FIG. 9b depicts the driving function for the acoustic emission waveform

of FIG. 9a.

FIG. l Oa depicts the acoustic emission waveform captured at the

resonance frequency of the second vibrational plate mode for a plate

containing a 1/2 in. fatigue crack.

10 FIG. 10b depicts the driving function for the acoustic emission

waveform of FIG. 10a.

FIG. 11a depicts the acoustic emission waveform captured at the

resonance frequency of the second vibrational plate mode for a plate with no

cracks.

15 FIG. 11b depicts the driving function for the acoustic emission

waveform of FIG. 1 la.

FIG. 12a is a schematic drawing of the cross section of a plate

containing an unlinked 7/8 in. crack.

FIG. 12b is a schematic drawing of the cross section of a plate

20 containing a linked 1/2 in. crack.

FIG. 13 is a plot of amplitude as a function of frequency for a O. 1 cm.

thick plate containing a 2.7 cm. fatigue crack.

FIG. 14 is a plot of amplitude as a function of frequency for several

samples of O. 1 cm. thick 6061-T6 plate.

25 FIG. 15 illustrates the correlation between finite element and

experimental data for frequency as a function of crack length.

FIG. 16a depicts the acoustic emission waveform captured at the

resonance frequency of the second vibrational plate mode for a plate with a

2.8 cm. crack using a PVDF transducer.

30 FIG. 16b depicts the driving function for the acoustic emission
5

10

LAR 14816-1-SB -6- PATENT APPLICATION

waveform of FIG. 16a.

FIG. 17a depicts the acoustic emission waveform captured at the

resonance frequency of the second vibrational plate mode for an uncracked

plate, using a PVDF transducer.

FIG. 17b depicts the driving function for the acoustic emission

waveform of FIG. 17a.

FIG. 18 is a schematic drawing of the nonconacting vibration device.

FIG. 19 depicts the induced eddy current and its interaction with the

static magnetic fields.

Detailed Description of the Invention

An embodiment of an apparatus for detecting and characterizing cracks

according to the present invention is shown in FIG. 1. Such an apparatus and

15 method have a variety of important applications including the study of aging

aircraft discussed in the Background section. As a part of the effort to meet

these needs, low frequency resonant modal analysis is being explored as a

means of fatigue crack detection and characterization in airframe structures.

Modal analysis is based on the excitation of free standing vibrational

20 waves in a structure. The presence of structural defects alters both the

shape of the modal patterns and the resonance frequency at which the modes

will stand. The vibrational status of the structure can be predicted using a

finite element approach so that the inversion of the acquired data is a less

formidable task than for other NDE techniques.

25 To determine crack length, a test plate is vibrated using a noncontacting apparatus. The vibration apparatus consists of rare earth

permanent magnets attached to the surface of the test sample and an

electromagnet positioned over the rare earth magnets at some distance from

the sample surface. An alternate embodiment of the vibration apparatus uses

30 a ferrite-core electromagnet with rare earth permanent magnets attached at
LAR 14816-1-SB -7- PATENT APPLICATION

each end. This assembly is positiond at some distance from the sample

surface. The resonance frequency of the test plate is determined by tracking

the peak output voltage from a transducer attached to the sample surface.

The crack length in the sample is then determined by correlating the

5 resonance frequency in the test sample to the resonance frequency

determined for plates of similar size and composition containing cracks of

known length.

To determine crack geometries, the test sample is vibrated at the

resonance frequency and the acoustic emissions produced by the interactions

10 of crack faces are detected. These acoustic emissions are correlated to

acoustic emissions determined for cracks with known geometries in thin

plates of similar size and composition in order to determine crack geometries

in the test sample.

Several inherent properties of low frequency resonant modal analysis

15 lend themselves directly to the aging aircraft problem. First of all, resonant

vibrations are relatively easy to produce in the thin aluminum alloys used for

aircraft skins. Also, the technique provides information on the integrity of a

structure over a large area and has potential to be relatively easily converted

into a non-contact rapid scan instrument. Another advantage of the

20 technique is the ability to predict responses from various structure/flaw

combinations, using the finite element approach, so as to increase the

confidence level of flaw detection while holding false calls to a minimum.

Tests were performed on aluminum alloy 2024-T3 sample plates S

with dimensions of 12" x 7" x 0.040". The samples were notched in the

25 center of the long edge and then cyclicly fatigued in order to induce fatigue

crack growth by tension-tension loading at maximum and minimum

amplitudes of 3000 Ibs. and 500 Ibs., respectively, at 15 Hz for 15,000 to

60,000 cycles. Four samples contained fatigue crack lengths of 1/8", 1/2",

7/8", and 1-3/4" respectively. A sample S with a slit of 7/8" as well as an

30 unflawed sample were also included in the experiment. All flaws were grown
LAR 14816-1 -SB -8- PATENT APPLICATIO N

from the center of the front edge of the sample and projected toward the rear

edge. A clamped boundary was enforced along three sides of the sample S

by a sample holder 12, leaving the cracked edge free to vibrate. The

dimensions of the clamped portion of the plates were 10" x 1-3/4".

5 In one embodiment shown in FIG. 1, rare earth permanent magnets 14

and 16 were attached directly to the sample S on either side of the fatigue

crack FC. The magnets were placed 1/4" back from the sample front edge

and 1-1/2" apart, centered along the centerline of the sample plate S. A

sinusoidal mechanical driving force was applied to the samples through the

10 use of U-shaped electromagnets 18 and 20 placed directly above and below

the permanent magnets such that the oppositely charged poles of the

electromagnets were aligned with each other and an associated permanent

magnet along an axis. Common coil wire 19 is wrapped around each

electromagnet in a different direction at each aligned axis. The poles of the

15 permanent magnets 14 and 16 were aligned such that the force on opposite

sides of the plate would be in opposite directions so that only even number

modes, i.e., those with a minimum displacement along the centerline of the

plate, would be excited. The driving current for the electromagnets 18 and

20 was supplied via common coil wire 19 by amplifying the output of a

20 function generator 22 with a bipolar operational amplifier 24, as illustrated in

FIG. 2. The frequency and amplitude of the driving signal were controlled by

connecting the function generator 22 to a microprocessor 26. The

microprocessor 26 is used to sweep the frequency of the function generator

22 over a given range, as discussed below in reference to FIG. 3, while

25 keeping the amplitude constant at 3 volts. Accordingly, the plate and

attached permanent magnets are "air-coupled" to the driving electromagnets,

i.e., are driven by electromagnetic and not mechanical force. The plate is

accordingly free from extraneous noise.

As the sample vibrates, stress waves will propagate to a polyvinyldene

30 (PVDF) thin film transducer 28 attached to the sample and which outputs a
LAR 14816-1 -SB -9- PATENT APPLICATION

voltage proportional to the amplitude of the stress waves. As discussed

below, transducer 28 is also used to record the waveform of acoustic

emission bursts to permit further flaw characterization. The transducer

output is amplified by a large bandpass high gain amplifier 30 and then sent

5 through a peak detect circuit 32 which tracks the peak transducer output.

The output of circuit 32 is digitized by a voltmeter 34 which is read by the

microprocessor 26. A table of the peak transducer output at each driving

frequency of the electromagnetic is accordingly stored in computer memory.

A digital oscilloscope 36 is used to monitor the waveform of the transducer

10 output at the resonance frequency and the waveform is recorded with the

microprocessor. Crack lengths in test plates are then determined by

correlating the observed resonance frequency of the test plate to resonance

frequencies for plates of similar size and composition containing known crack

lengths. This correlation is preferably accomplished by the microprecessor.

15 It was found that a PVDF transducer 28 of dimensions

2.54cm x 2.54cm with a 28pm thickness could be attached directly to the

sample surface without appreciably affecting the sample resonance

frequency. At the resonance frequency of the plate a large increase in the

amplitude of the plate vibration occurs. When the PVDF transducer is in

20 direct contact with the sample surface, the amplitude of the plate vibrations

will directly correlate to the voltage output of the transducer. Therefore the

determination of resonance modes was accomplished by recording the peak

amplitude of the transducer output as a function of the driving frequency of

the electromagnets. A resonance mode would then be recorded as a peak in

25 the amplitude vs. frequency data.

For example, FIG. 3 shows typical experimental results of the

procedure described above. The plot displays the data obtained for a plate

containing a fatigue crack 7/8" long. The large peak in the transducer output

at approximately 280-290 Hz signifies the frequency of the second vibrational

30 plate mode for the sample. The graph in FIG. 3 also shows some indication
LAR 14816-1 -SB -10- PATENT APPLICATION

of the fourth mode occurring near 725 Hz, although this experiment focused

solely on tracking the location of the second mode. The data for FIG. 3 was

acquired by rapidly stepping the driving frequency of the electromagnets 18

and 20 across the entire range of 100-1000 Hz in two Hz steps.

5 Two procedures were employed to more accurately pinpoint the

resonance frequency of the plates. Specifically, the range over which the

frequency was scanned was limited to the area of interest for the second

mode, and the current to the electromagnets was interrupted between each

of the frequency shifts. FIG. 4 displays the results obtained when the two

10 steps mentioned above are added to the experimental procedure. This plot

combines the data obtained for test specimens on the same graph. The

frequency shift caused by increasing the crack size is clearly visible in this

figure, dropping from approximately 370 Hz for the uncracked plate to under

300 Hz for the plate with a fatigue crack 7/8" long, i.e., increasing flaw

15 length reduces the frequency of the second plate vibrational mode. The graph

also displays data obtained from a plate containing a slit of length 7/8". The

close overlap of the peaks of the two data sets should be noted. The

resonance frequency of these two different type flaws of the same length is

nearly equal. This fact was used in developing the finite element model used

20 to simulate the experimental conditions.

The finite element model used in the computer simulations discussed

immediately below was built entirely from first order quadrilateral plate

elements. FIG. 5 displays the area discretization used to mesh the problem.

The completed mesh for the uncracked plate contained 328 nodes and 280

25 elements. The clamped boundary conditions were enforced by holding the

amplitude of all the degrees of freedom to zero for nodes lying along the

edges of the mesh corresponding to the clamped sides and are represented

by circled nodes. The permanent magnets 14 and 16 were added to the

model by adjusting the mass of the elements over which the magnets were

30 attached and are represented by the "X" hatching. Although this approach
LAR 14816-1 -SB -11- PATENT APPLICATION

of handling the permanent magnets neglected any stiffness the magnets

would add to the plate, it did account for all the mass of the system and

allowed for the use of a simple two-dimensional model. The method

incorporated for modeling the fatigue cracks took advantage of experimental

5 results showing the similarity in resonance frequencies for plates with fatigue

cracks as compared to plates with simple slits. The fatigue cracks were

therefore modeled as zero width slits in the plate. The connectivity between

adjacent quadrilateral elements was disconnected along the path the fatigue

crack traversed, from the center of the front edge toward the rear edge. This

10 modeling technique required only the use of additional nodes in the model.

The number of elements was held constant and two-dimensional analysis

could be used to solve the problem

The COSMIC NASTRAN finite element software package was used to

solve the eigenvalue equation for resonance mode determination. The

15 NASTRAN package is available from the Computer Software Management and

Information Center (COSMIC), Computer Services Annex, University of

Georgia, Athens, Georgia 30602. NASTRAN uses a basic quadratic format

in performing dynamic analysis. The quadratic equation governing the motion

of a linear discretized system is

2O

÷ + {u} -

(1)

25

where [M] is the mass matrix, [B] is the damping matrix, [K] is the stiffness

matrix, {P(t)} is the time dependent applied force vector, {u} is the vector of

grid point displacements, and a dot denotes differentiation with respect to

time. For the calculation of the natural frequencies of the plate, damping

terms are neglected and the applied force vector is set to zero. As discussed

in William Weaver, Jr. and P. R. Johnston, Finite Elements for Structural
LAR 14816-1 -SB -12- PATENT APPLICATION

Analysis, Prentice-Hall, Inc., Englewood Cliffs, N. J., 1984, pp. 279-81. The

equation of motion then reduces to

5

[_ {_} + [xj{.} - {0}

(2)

It is next assumed that all parts of the system are vibrating sinusoidally with

the same phase and frequency, such that

10

{u} - {u}c0swt

(3)

{u} - e]'{u}c0twt

(4)

15

where {0} represents the amplitudes of the degrees of freedom and _ is the

circular frequency. Substituting equations (3) and (4) into equation (2) yields

the linear eigenvalue equation

_ - w'I_}{_} - {0})

(5)

2O Equation (5) is satisfied for the nontrivial case of nonzero {u } only when the

determinant ([K] - w 2 [M]) vanishes. For each eigenvalue o:_there corresponds

an eigenvector {u }_ which is a natural mode of vibration of the plate.
LAR 14816-1-SB -13- PATENT APPLICATION

Finite element modeling results are displayed in FIG. 6 for a plate

modeled with a 3/4" long fatigue crack. The frequency corresponding to the

second vibrational mode is graphed in FIG. 7 as a function of defect size.

This figure shows both modeling and actual experimental results. The close

5 agreement between the results illustrates the effectiveness with which low

frequency resonant modal analysis can characterize fatigue cracks in simple

structures with well defined boundary conditions.

Finite element modeling predicts the mechanical behavior of the test

piece under vibration. For certain vibrational modes the faces of fatigue

I0 cracks present in the structure will interact. The interaction of the crack face

walls releases energy into the sample which can be recorded with acoustic

emission sensors. Emission events caused by crack wall interactions are

identified by the frequency, amplitude, and phase of the signal with respect

to that of the driving function on the plate. FIG. 8 displays the results of an

15 emission waveform analysis. The amplitude of the high frequency stress

waves associated with crack face rubbing increases as the crack length

increases. A larger crack will develop a greater separation between crack

face walls. This leads to a larger impact force between the crack faces and

therefore a stronger output signal.

20 The acoustic emission (AE) signals emitted at the resonance frequency

of the plates were next examined. The waveforms of the AE signals were

captured by bypassing the peak detect circuitry 32 and the voltmeter 34 in

favor of a digitizing oscilloscope 36 which monitored the acoustic emission

signals, which were also recorded by the microprocessor 26. Initial results

25 showed that a large portion of the signal being received by the transducer 28

was due to permanent magnet vibration on the sample surface. This noise

was eliminated from the experiment by placing a thin piece of porous rubber

38 between the permanent magnets and the sample surface. This succeeded

in eliminating the excess noise, but also caused a shift in all resonance

30 frequencies since the stiffness of the system was changed. All acoustic
LAR 14816-1-SB - 14- PATENT APPLICATION

emission waveform data was acquire at these shifted resonance frequencies

for the second vibrational mode of the samples.

FIGS. 9(a), 10(a) and 11(a) display acoustic emission waveforms

captured at the resonance frequency of the second vibrational plate mode for

5 three different samples. The voltage-time plates relate directly to energy

arrivals at the transducer surface. FIGS. 9(b), 10(b) and 11(b) are the

respective driving functions for the acoustic emission waveforms of FIGS.

9(a), 10(a) and 11(a). FIG. 9(a) displays a large burst of high frequency

energy occurring at a frequency equal to that of the driving force on the plate.

10 This plate contained a relatively large fatigue crack with a length of 7/8".

When the crack size was reduced to 1/2" a distinctively different emission

waveform was recorded in FIG. 10(a) showing two emission events per cycle

of the driving frequency. FIG. 11(a) displays the acoustic emission data

captured for an uncracked plate, for which the high frequency acoustic

15 emission bursts are completely absent.

The information contained in these plots, combined with knowledge

gained from finite element modeling, can be used to develop a simple model

of the interaction of crack face walls during vibration. A first approximation

is given by the finite element crack model, approximating the crack as a thin

20 slit. This choice is supported by the relationship between the modeled results

and the experimentally determined resonance values, and the comparison

between the experimentally determined resonance values for the two plates

containing flaws of 7/8", one being a fatigue crack and the other being a thin

slit.

25 The thin slit model can be built upon from information contained in the

voltage time plots of FIGS. 9(a), 10(a), and 11 (a). Three distinctly different

emission patterns are observed, those containing one, two, or zero high

frequency bursts per cycle. One documented source of emission events is

unsticking of cracks after closure. See F. Hamel and M. N. Bassim,

30 "Detection of crack propagation in fatigue with acoustic emission," Strength
LAR 14816-1 -SB -15- PATENT APPLICATION

of Metals and Alloys, R. C. Gifkines, ed., Pergamon Press, New york, 1983

pp. 839-44. In this experiment with the forcing function normal to the

surface, examination of the AE signal will provide information concerning the

crack cross-section perpendicular to the growth direction. Emission events

5 will be recorded each time the crack faces interact and then separate. If one

burst is recorded for each cycle of the driving frequency, it can be concluded

that the crack face walls interact once per cycle. Two bursts will similarly

signify two interactions per cycle. The crack face model corresponding to the

recorded AE waveforms is shown in FIGS. 12(a) and 12(b). These figures

10 depict crack cross-sections corresponding to different emission patterns. The

cross-section of the large 7/8" crack of FIG. 12(a) will provide one AE burst

per cycle, as no interaction will occur when the right-hand side of the crack

is forced up and to left-hand side down. The smaller 1/2" crack of FIG. 12(b)

is still linked so that two bursts per cycle will be recorded as the crack faces

15 interact for both maximum and minimum amplitudes of each crack face.

These geometries were confirmed by viewing the crack cross-sections with

an optical microscope.

The present invention has the potential to detect local flaws while

locking on a global scale. Low frequency resonant modal analysis combined

20 with crack emission analysis potentially meets this challenge. The resonance

frequency of a test piece with well defined boundary conditions has been

found to directly correlate to localized flaws in the structure. In addition, an

examination of the AE signal at the resonance frequency can provide

characteristics of flaws presenting the structure. The ability to characterize

25 a fatigue crack by its emission signature under resonant vibration has the

potential to lead to a practical large area crack detection technique not limited

by complex structural designs and boundary conditions.

The preceding discussion focused on edge cracks in a clamped sample.

Fatigue cracks in the central region of aluminum alloy plates clamped on each

30 edge were also examined. In addition, the effect of alternate sensors and the
LAR 14816-1-SB -16- PATENT APPLICATION

response at higher modes were studied. All tests were performed on

aluminum alloy 6061-T6 plates with dimensions of 30.5 cm x 12.7 cm x 0.1

cm. A hole of radius .635 cm was drilled in the center of these plates.

These holes were then notched on one or both sides before the samples were

5 cyclically fatigued in order to induce fatigue crack growth. A sinusoidal

loading cycle between upper and lower load values of approximately 3000

and 500 Ibs. was used. Varying states of fatigue in the samples were

induced by varying the number of cycles to which each sample was fatigued.

Table 1 gives a breakdown of the loading parameters for each sample used

10 in the study.

TABLE 1

LOADING PARAMETERS FOR EXPERIMENTAL SAMPLES

15

20

Sample # Sides Notched # of Cycles Fatigue Crack Length

0 0 0 0 cm

1 1 70,000 1.5 cm

2 2 24,000 2.8 cm, 2.6 cm

3 1 20,000 0.7 cm

4 1 55,000 2.0 cm, 0.6 cm

5 1 30,000 2.7 cm, 1.3 cm

25

The fatigued samples were individually placed into the sample holder

which supplied a clamped boundary along all edges of the plate. Rare earth

permanent magnets were attached directly to the sample on either side of the

fatigue crack. The spacing between the magnets was 3.8 cm, centered along

the centerline of the plate. A sinusoidal mechanical driving force was applied
LAR 14816-1 -SB -17- PATENT APPLICATION

to the samples via the electromagnets 18 and 26 placed directly above and

below the permanent magnets 14 and 16. Microprocessor 26 was used to

step the frequency of the forcing function over the range of interest, while

recording the amplitude of the received signal at each frequency level. The

5 poles of the permanent magnets were aligned such that the force on opposite

sides of the plate would be in opposite directions. This alignment ensured the

maximum shearing force across the fatigue crack face.

Various sensors were evaluated for retrieving information from the

sample. Initial tests focused on the use of a polyvinylidene fluoride (PVDF)

10 thin film transducer. Specifically, a PVDF transducer of dimensions 2.54 cm

x 2.54 cm with a 28/_m thickness as in the foregoing example. For the

present experimental setup however, this transducer was unable to

consistently monitor the low frequency vibrations of the structure. Wide

band commercial acoustic emission sensors were also tested, but produced

15 no improvement in the results. The final pickup sensor that was used was an

audible sound level meter 29 as shown in FIG. 1. This instrument gave an

excellent recording of the low frequency resonant vibration, but was unable

to detect high frequency signals associated with crack face rubbing during

vibration which the PVDF thin film transducer could detect. A combination

20 of these two pickup sensors was therefore used to monitor the vibrational

status of the samples.

Air pressure vibrations received by the microphone of the sound level

meter are converted into equivalent AC voltage vibrations. When the

microphone is near the sample, surface vibrations of the sample will produce

25 air pressure vibrations at the microphone head. At the resonance frequency

of the plate a large increase in the amplitude of the plate vibrations occurs.

Therefore the determination of resonance modes was accomplished by

recording the peak amplitude of the sound level meter output as a function

of the driving frequency of the electromagnets. A resonance mode would

30 then be recorded as a peak in the amplitude vs. frequency data.
LAR 14816-1 -SB -18- PATENT APPLICATION

FIG. 13 displays the experimental data acquired for sample 5 of Table

1. The directionality of the forcing function caused only even number modes

to be stimulated so that the two sharp peaks in the graph locate the second

and fourth resonance modes of the sample at approximately 430 Hz and

5 approximately 750 Hz, respectively. FIG. 14 shows the results when the

data from several samples are plotted on the same graph. The frequency

scale for this figure has been reduced so as to focus on the second resonance

mode in order to more clearly illuminate the frequency shift caused by fatigue

cracks in the sample.

10 The finite element approach was used again to numerically calculate

the mechanical response of the samples. A uniform mesh of first order

elements was constructed for use in the COSMIC NASTRAN finite element

package previously discussed. A two-dimensional analysis was used where

fatigue cracks were modeled as zero width slits in the structure. FIG. 15

15 displays a comparison of finite element results and experimentally determined

resonance values for the second plate vibrational mode. All of the

experimental data in FIG. 15 are from plates that were notched on one side

of the center hole. The corresponding crack length for these samples was

therefore taken as the length of the crack propagating from the notched edge.

20 Any crack growth from the opposite side of the center hole was ignored for

the purposes of this figure. The finite element results shown in FIG. 13 are

likewise results for fatigue cracks of the given lengths propagating from one

side of the center hole.

A comparison of the finite element and experimental results showed a

25 lower correlation at the fourth resonance mode than is seen in FIG. 15 for the

second mode. This can be explained by the width of the resonance peaks as

seen in FIG. 15. The width of the peak occurring at the fourth mode is much

wider than that of the peak for the second mode. In addition, the normalized

frequency shift of a given flaw does not increase at higher modes. Table 2

30 displays the normalized frequency shift generated by a .5 cm slit for the first
LAR 14816-1 -SB -19- PATENT APPLICATION

nine resonance modes. This simulation data clearly shows that little

advantage in terms of the normalized frequency shift can be obtained by

working at a higher order mode.

TABLE 2

NORMALIZED FREQUENCY SHIFT FOR

FIRST TEN RESONANCE MODES

10

15

Mode #

2

3

4

5

6

7

Unflawed

Resonance

393 Hz

450 Hz

717 Hz

781Hz

1151Hz

1292 Hz

1311Hz

2.5 cm Slit

Resonance

290 Hz

430 Hz

707 Hz

763 Hz

1150 Hz

1236 Hz

1287 Hz

Normalized Shift

1.0%

4.4%

1.4 %

2.3%

0.1%

4.3%

1.8%

8 1546 Hz 1510 Hz 2.3%

9 1608 Hz 1607 Hz 0.1%

20

25

It was previously mentioned that two different sensors were evaluated

for use in this experiment, namely a sound level meter, from which the

previous results were obtained, and a PVDF thin film transducer. The PVDF

transducer, although unable to accurately trace the low frequency resonant

vibrations, held a major advantage in its ability to detect high frequency noise
LAR 14816-1-SB -20- PATENT APPLICATION

associated with crack face rubbing during vibration. FIG. 16(a) is the signal

obtained from the PVDF transducer for sample number 2 and FIG. 16(b) is the

associated driving function. The large high frequency burst in the data

obtained from sample number 2 is a result of fatigue crack face interaction

5 during vibration. It is actually the unsticking of cracks after closure, which

produces the acoustic signature seen in FIG. 16(a), as shown in F. Hamel and

M. N. Bassiu, "Detection of crack propagation in fatigue with acoustic

emission," Strength of Metals and Alloys, R. C. Gifkin, ed., Pergamon Press,

N.Y., 1983. The sinusoidal force on the plate alternately pushes the crack

10 face walls together and then begins to push them apart until the unsticking

occurs, accompanied by a large burst of high frequency acoustic energy

emission. FIG. 17(a) is he signal obtained by the PVDF transducer for

uncracked sample number 0 and FIG. 17(b) is the associated driving force.

The present invention focuses on the detection of fatigue cracks in thin

15 walled structures. The device also has the capability of detecting disbonds

in layered structures and could be used to gain important information on crack

propagation properties.

An alternate embodiment of the invention does not require the

permanent magnets 14 and 16 shown in FIG. 1. Instead, as shown in FIG.

20 18, the external vibration source 40 consists of a ferrite-core electromagnet

45 and a pair of rare earth permanent magnets 50, composed of a material

such as cobalt-samarium. This embodiment uses a concentric distribution of

electric currents which is induced by an AC magnetic field perpendicular to

the test plate 55 in order to induce vibration in the plate 55. The ferrite-core

25 electromagnet 45 has common coil wire 60 wrapped around the

electromagnet and provides an AC magnetic field 65 with the field lines

perpendicular to the plate surface 55. As illustrated in FIG. 19, such AC

magnetic fields 65 create circular eddy currents 70 on the surface of a thin

metal plate. The permanent magnets 50 provide static magnetic fields 75

30 which spread out rapidly such that the field lines B are mainly parallel to the
LAR 14816-1 -SB -21- PATENT APPLICATION

plate surface 55. As illustrated in FIG. 19, the static field 75 exerts a force

F which is a special case of the Lorentz force, on the electric current 70 in

the direction perpendicular to the surface of the magnetic field B and eddy

current J, which is the plate surface. By setting the polarity of the permanent

5 magnets properly, the device can induce two different modes of local plate

displacement as shown in FIGS. 20(a) and (b). For purposes of the present

invention, the sinusoidal displacement mode shown in FIG. 20(a) is preferred.

The end surfaces of the electromagnet 45 are concave shaped to

maintain uniform magnetic flux lines 65 in the region of the plate 55. The

10 surfaces of the permanent magnets 50 are rounded to enhance the parallelism

of the static magnetic fields 75 at the tip of the permanent magnets 50 and

to ensure rapid spread of the static magnetic field lines 75. The optimized

geometrical configurations of the electromagnet 45 and permanent magnets

50 is determined using finite element modeling techniques.

15 This alternate embodiment of a vibration source is particularly useful

in the testing of hollow plate structures such as aircraft wings, where the

interior of the wing cannot be accessed to attach permanent magnets as

discussed in the first embodiment of FIG. 1. Since permanent magnets do

not need to be attached to the surface of the test section, this embodiment

20 provides a much faster method of crack detection. In addition, this

noncontacting vibration source can be used to excite displacement modes for

the application of other NDE techniques such as laser shearography and

acoustic emission tests.

This invention provides a clear indication of fatigue cracks in the

25 sample over a global scale. It is not limited to point by point testing like eddy

current, ultrasonic, dye penetrant and magnetic particle techniques. Use of

the non-contacting, sinusoidal driving mechanism of the present invention is

unique in that all mechanical noise associated with the driver is removed from

the system. The test piece, with magnets attached, is air coupled to the

30 remainder of the driving circuitry, i.e., electromagnets, amplifier and function
LAR 1481 6-1-SB -22- PATENT APPLICATION

generator. This method eliminates extraneous noise associated with sample

conditions. The present invention also produces a flaw signature which is

reproducible and easy to interpret in terms of crack length and connectivity

between crack face walls by monitoring both the frequency at which peak

5 amplitude signals are obtained (modal analysis) and the waveform of the

emitted signals (acoustic emission analysis).

Unlike conventional acoustic emission techniques, the present invention

operates at the resonant frequency of the thin plate being tested and

therefore does not require crack growth, which will further damage the

10 sample. In conventional acoustic emission techniques, it is difficult to obtain

stress information at the time of emission events under passive loading

conditions. Such conventional techniques require a separate sensor technique

to determine the stress as a function of time. Although static or linear

loading techniques can produce a record of stress versus the acoustic

15 emission event, such techniques have the drawback of requiring further

damage to the sample in the form of crack growth or micro-growth as a

source of the emission event. Since the main source of acoustic emission

during resonant vibration is from crack face rubbing, crack growth during

testing is avoided.

20 An important characteristic of using resonant vibration as a means of

stimulating acoustic emission is that the stress distribution on the plate at the

time of the emission burst can be determined from the phase of the emission

bursts with respect to the driving function. This information is very important

in determining the degree of closure or tightness of a fatigue crack. In

25 addition, spurious signals due to background noise can be discriminated

against since it is unlikely that they will repeatedly occur at the same

frequency as the resonant vibrations. Emission events due to defects in the

structure, however, will occur repeatedly at this frequency. Finally,

resonance modal patterns can be modeled with a computer to allow accurate

30 contemporaneous comparisons with test results.
LAR 14816-1-SB -23- PATENT APPLICATION

Many modifications, substitutions, and improvement will be apparent

to the skilled artisan without departing from the spirit and scope of the

present invention as described herein and defined in the following claims.
• J

LAR 14816-1-SB _ PATENT APPLICATION

HIGH SPEED THIN PLATE FATIGUE CRACK MONITOR

Abstract of the Disclosure

5 A device and method are provided which non-destructively detect crack

length and crack geometry in thin metallic plates. A non-contacting vibration

apparatus produces resonant vibrations without introducing extraneous noise.

Resulting resonant vibration shifts in cracked plates are correlated to known

crack length in plates with similar resonant vibration shifts. In addition,

10 acoustic emissions of cracks at resonance frequencies are correlated to

acoustic emissions from known crack geometries.
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