NASA CASE NO. 

PRINT FIG. f 

NOTICE 

The invention disclosed in this document resulted from 

research in aeronautical and space, activities performed under 

programs of the National Aeron3,utics 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 consider the granting of a limited exclusive license, 

pursuant to the NASA Patent Licensing Regulations, when such a 

license 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, Mail 

Code 279, Langley Station, Hampton, VA 23665. Approved NASA 

forms for application for nonexclusive or exclusive license are 

available from the above address. 

LaRC 

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AWARDS ABSTRACT! 

ULTRASONIC METHOD AND APPARATUS 

FOR DETERMINING CRACK OPENING LOAD 

NASA CASE NO. LAR-13889-1 

This invention relates generally to materials 

testing, and more particularly, to an ultrasonic 

apparatus for determining crack opening load in a 

specimen having a crack. 

As illustrated in Figs. 7-9, a specimen 10 having 

a crack 12 is provided with a transmit transducer 22 

and a receive transmitter 24. An ultrasonic signal 

passing between the two transducers is mechanically 

rectified by the crack to produce a second harmonic of 

the input signal. By measuring the peak harmonic 

amplitude while increasing the tension load on the 

specimen 10, a crack opening load is determined. 

Inventor: 

Home Address : 

William T. Yost 

797 Fawn Lane 

Newport News, VA 23502 

S.S. #:. 

Employer : NASA 

Initial Evaluator: Joseph S. Heyman 

Serial No.: 

Filed : 

210,277 

June 23, 1988 
FIG. I 

(PRIOR ART) 

FIG. 2 

- - 

DISPLACEMENT 

IG. 3 

CRACK 

OPEN IN^ 

REDUCED DISPLACEMENT 
FIG. 4 

I 

DISPLACEMENT 

CRACK _*' 

OPENING 

1 

SLOPE INCREASE % 

C 

FIG. 6 
FIG. 8 

' Oscilloscope \ 

Receiver 

- 

\ 

36' 34) 
LAR 13889-1 PATENT 

Title of the Invention 

ULTRASONIC METHOD AND APPARATUS 

FOR DETERMINING CRACK OPENING LOAD 

Oriqin of the Invention 

The invention described herein was made by an 

employee of the United States Government and may be 

manufactured and used by or for the government for 

5 governmental purposes without payment of any royalties 

thereon or therefore. 

Background of the Invention 

Field of the Invention 

The present invention relates generally to 

10 the testing of materials, and more specifically, to an 

ultrasonic method and apparatus for determining a crack 

opening load. 

Description of the Related Art 

It is known to test compact tension specimens 

15 placed under a load to determine the condition of full 

crack opening. Referring to Fig. 1, a compact tension 

specimen 10 has a crack 12 and a notch opening 14 

formed at the outer end portion of the crack 12. An 

extensiometer 16 is placed at the notch opening A4 in 

20 order to measure displacement of the notch, and hence, 

the crack opening. A tension load is applied to the 

specimen 10 by pulling clevis grips 18 and 20 in 

opposite directions as indicated by the directional 

arrows. 

25 Using the apparatus schematically illustrated in 

Fig. 1, known techniques have been employed for 
LAR 13889-1 - 2 - PATENT 

determining crack opening load. The first technique is 

illustrated in Figs. 2 and 3 and is known as the 

"load-reduced displacement" method. In this method, 

the extensiometer 16 placed at the notch opening 14 

5 measures displacement. By plotting the load on the 

Y-axis against displacement on the X-axis, a 

determination of reduced displacement leads to a 

determination of the load at which the crack is 

actually open. In Fig. 2, displacement is 

10 determined by the difference between a straight line 

extrapolation of the upper (straight) data and the 

measured or actual (curved) data. The extrapolated 

portion is shown as a broken line and the reduced 

displacement is indicated by the distance between the 

15 broken line and the curved portion of the measured 

data. The load at which crack opening occurs is 

illustrated in Fig. 3 as the point at which the reduced 

displacement does not change with increasing load. As 

indicated in Fig. 3, this occurs at the vertical 

20 portion of the curve. 

The second technique is illustrated in Figs. 4 and 

5 and is known as the llload-slope changev1 method of 

determining crack opening load. In Fig. 4, load is 

plotted against displacement as in the load-reduced 

25 displacement technique. Changes in slope are measured 

from regions S1 to S6. Slope increase is plotted in 

Fig. 5 based on the measured slope changes from Fig. 4. 

Crack opening is indicated in Fig. 5 where the slope 

increase changes from perpendicular. 

30 A problem associated with the above-described 

techniques results from the fact that extensiometers 

and other similar strain sensors are electrically 
LAR 13889-1 -3- PATENT 

noisy, and thus, the signals generated by the 

extensiometers lack the required degree of certainty 

for precise determination of crack opening load. 

Another problem is that it is difficult to decide at 

5 what point the plot becomes tangent to a line or 

crosses an axis. The problem is illustrated in Fig. 6 

where load is plotted against slope increase. As is 

evident from Fig. 6, the plotted points of slope 

increase move to both sides of zero, thereby making 

10 determination of an exact crack opening load difficult. 

It is generally known to employ acoustic signals 

to determine the presence of a crack in an object. For 

example, U.S. Patent No. 3,911,734 to Mehdizadeh 

discloses a method of detecting incipient fatigue 

15 damage in metal using acoustic emission characteristics 

obtained during application of a load. 

U.S. Patent No. 4,265,120 to Morris et al. 

discloses a method of detecting fatigue using acoustic 

harmonics. A surface acoustic wave is generated at a 

20 first position on the object and a harmonic of a 

generated wave is detected at a second position on the 

object. The testing method involves relating the 

characteristics of the detected wave to the remaining 

useful life of the object. ., 25 U.S. Patent No. 4,522,064 to McMillan discloses an 

ultrasonic method and apparatus for determining the 

depth of cracks in pipe or other conduit. 

U.S. Patent No. 4,534,219 to Nadeau et al. 

discloses a device which relates the frequency of an 

30 acoustic wave to an indication of a crack within a test 

piece. 
LAR 13889-1 PATENT 

Sumaary of the Invention 

An object of the invention is to provide an 

apparatus for determining precisely when a crack in a 

compact tension specimen begins to open. 

5 Another object of the invention is to provide a 

method and apparatus for determining crack opening load 

without generating signal-interfering noise. 

Another object of the invention is to provide an 

apparatus for determining crack opening load which is 

10 relatively simple to operate, inexpensive, and highly 

accurate. 

In a preferred embodiment of the invention, an 

ultrasonic monitor for determining crack opening load 

in a specimen having a crack includes an ultrasonic 

15 generator outputting a series of tone bursts, a 

transmit transducer for placement on the specimen and 

receiving the output of the ultrasonic generator, a 

receive transducer for placement on the specimen 

opposite the transmit transducer and receiving acoustic 

20 signals passing through a portion of the specimen which 

includes the crack, and outputting a fundamental output 

signal and a harmonic output signal, means for 

converting the transmit transducer output signals into 

electrical signals, and means for measuring a hasmonic 

25 amplitude of the converted harmonic signal as an 

increasing tension load is applied to the specimen to 

determine a peak harmonic amplitude which indicates 

crack opening load. 

By measuring peak harmonic amplitude, a crack 

30 opening load is identified when displacement occurs on 

the atomic level, meaning that when the crack moves 

about one Angstrom, this displacement will be 
LAR 13889-1 -5- PATENT 

identified. Therefore, the present invention provides 

a highly precise measurement and determination of crack 

opening load. 

In another embodiment of the invention, the 

5 received transducer is placed on the same side of the 

test specimen as the transmit transducer, whereby a 

wave transmitted by the transmit transducer reflects or 

bounces off the crack and is received by the receive 

transducer after interacting with the crack as a second 

10 harmonic signal. The receive transducer may be 

positioned in proximity to the transmit transducer, or 

it may be stacked on top of the transmit transducer. 

These objects, together with other objects and 

advantages which will be subsequently apparent reside 

15 in the details of construction and operation of the 

ultrasonic monitor for determining crack opening load 

as more fully hereinafter described and claimed, 

reference being had to the accompanying drawings 

forming a part hereof, wherein like reference numerals 

20 refer to like parts throughout. 

Brief Description of the Drawinqs 

Fig. 1 is a schematic view of a known apparatus 

for determining crack opening load; 1 

Fig. 2 is a graph plotting load against 

25 displacement and illustrating reduced displacement 

ac'cording to a known method of determining crack 

opening load; 

Fig. 3 is a graph plotting load against reduced 

displacement and illustrating the level of crack 

30 opening load of the Fig. 2 method; 
LAR 13889-1 -6- PATENT 

Fig. 4 is a graph plotting load against 

displacement, illustrating areas of measurement for 

determining change of slope according to another known 

method of determining crack opening load; 

5 Fig. 5 is a graph plotting load against slope 

increase and illustrating the crack opening load level 

of the Fig. 4 method; 

Fig. 6 is a graph showing load plotted against 

slope increase based on an actual test of crack opening 

10 load for a compact tension specimen according to the 

Fig. 4 method; 

Fig. 7 is a schematic view of a compact test 

specimen and a portion of the ultrasonic monitor for 

determining crack opening load according to the present 

15 invention; 

Fig. 8 is a schematic, block diagram illustrating 

an ultrasonic monitor for determining crack opening 

load according to the present invention; and 

Fig. 9 is a graph plotting harmonic amplitude 

20 against tension load, as measured by the ultrasonic 

monitor of Fig. 8. 

Detailed Description of the Preferred Embodiments 

Referring now to Fig. 7, a compact test speci..en 

10 has a crack 12 and a notch opening 14. Test 

25 specimens to be tested are generally standardized in 

terms of length and thickness. Clevis grips 18 and 20 

apply a tension force to the compact tension specimen 

10 by moving in opposite directions as indicated by the 

directional arrows. A transmit transducer 22 receives 

30 and transmits an ultrasonic signal into the test 

specimen 10. A receive transducer 24 is located on an 
LAR 13889-1 -7- PATENT 

opposite side of the compact tension specimen 10 from 

the transmit transducer 22. Acoustic signals passing 

between transducer 22 and 24 pass through the cracked 

region of crack 12. 

5 An ultrasonic system 26 of the present invention 

is illustrated in Fig. 8. A signal generator 28 

outputs a series of tone bursts of a specific 

frequency, such as 5 MHz, and is connected to an 

amplifier 30 where the tone bursts are amplified. The 

10 transmit transducer 22 is connected to the amplifier 30 

for receiving the amplified tone bursts. The 

transducer 22 is connected to the specimen 10 and 

excites an acoustic tone burst in the compact tension 

specimen 10 in the form of a wave which propagates 

15 towards the cracked region of crack 12. When the crack 

is closed, the acoustic wave is transmitted across the 

cracked region by the portions of the two surfaces that 

are in intimate contact. The crack alters the 5 MHz 

signal (f) to generate a second harmonic of the 5 MHz 

20 signal due to dislocations in the crystalline structure 

of the specimen. Thus, a 5 MHz (f) signal and a 10 MHz 

(2f) signal are simultaneously received by the receive 

transducer 24, which is connected to the specimen 

opposite transducer 22. The generation of the second 

25 harmonic signal (2f) results from the fact that as the 

surface of the crack just begins to separate, the wave 

will only transmit during a portion of the cycle, 

giving rise to a mechanical "rectification" of the 

acoustic signal. The effect of a barely opened crack 

30 is to enrich the relative portion of the 2f signal. 

The acoustic tone burst, as altered by the crack 

12, is received by a 10 MHz transducer 24. The output 
LAR 13889-1 -8- PATENT 

is received by the transducer 24 at both the f and 2f 

frequencies. Both frequencies pass through a circuit 

32 which allows selective switching of the f and 2f 

signals to a receiver 34. The 10 MHz 2f signal is 

5 switched to receiver 34 where it is amplified. The 5 

MHZ f signal is switched through an attenuator 33 

before passing to the receiver 34 which is connected to 

the circuit 32. The receiver 34 is first manually 

tuned to the f signal, and then after switching, is 

10 retuned to the 2f signal. The output of the receiver 

34 is monitored by an oscilloscope 36, which is 

connected to the receiver and synchronized with 

generator 28. 

While holding the received fundamental output 

15 fixed (5 MHz) an increasing load is applied to the 

compact tension specimen 10 through clevis grips 18 and 

20. 

Retuning of the receiver 34 at different loads 

generates a measurable harmonic output (10 MHz) which 

20 is plotted as a function of load in Fig. 9. 

As seen in Fig. 9, harmonic amplitude increases at 

an increasing rate until the curve peaks, the peak 

indicating the point of crack opening; thereafter, 

harmonic amplitude falls sharply. - 

25 Referring again to Fig. 7, another embodiment of 

the invention involves placing the transmit and receive 

transducers on the same side of the compact tension 

specimen 10. A receive transducer 23 is shown in 

broken lines on the same side as transmit transducer 

30 22. The transmit and receive transducers are arranged 

such that a wave transmitted by the transmit transducer 

reflects or bounces off the crack 12 and is received by 
LAR 13889-1 PATENT 

the receive transducer 23. The wave transmitted by 

transmit transducer 22 interacts with the crack to 

produce the same second harmonic as which is produced 

by the wave passing through the cracked region. 

5 Frequencies other than those mentioned above can 

be used. Also, other frequencies generated by the 

acoustic rectification described above could be 

monitored. The harmonically generated ultrasonic 

signal generated by acoustic rectification determines 

10 when the crack is fully open. By using an ultrasonic 

signal, a favorable signal to noise ratio is achieved 

while maintaining substantial noise immunity from the 

operation of the load frame and its associated parts. 

The many features and advantages of the present 

15 invention are apparent from the detailed specification, 

and thus, it is intended by the appended claims to 

cover all such features and advantages of the 

ultrasonic monitor which fall within the true spirit 

and scope of the invention. Further, since numerous 

20 modifications and changes will readily occur to those 

skilled in the art based upon the disclosure herein, it 

is not desired to limit the invention to the exact 

construction and operation illustrated and described. 

Accordingly, all suitable modifications and equivalents 

25 may be resorted to falling within the scope and the 

spirit of the invention. 

Whht is claimed is: 
LAR 13889-1 PATENT 

ULTRASONIC METHOD AND APPARATUS 

FOR DETERM3CNING CRACK OPENING WAD 

Abstract of the Disclosure 

An ultrasonic apparatus determines crack opening 

load by placing a transmit transducer on one side of 

the crack and a receive transducer on the opposite side 

of the crack. An acoustic signal passing through a 

region of the crack is mechanically rectified to 

produce a second harmonic of an input signal. A 

harmonic output signal of the receive transducer is 

converted into an electrical signal and the peak 

harmonic amplitude is determined while increasing a 

tension load on the specimen. The peak harmonic 

amplitude indicates crack opening load.