NASA Technical Memorandum 100877 

Characterization of Fatigue Crack Initiation 

and Propagation in Ti-6A1-4V With 

Electrical Potential Drop Technique 

Sreeramesh Kalluri 

Sverdr-up Technology, Inc. 

NASA Lewis Research Center Group 

Cleveland, Ohio 

and 

Jack Telesman 

National Aeronautics and Space Administration 

Lewis Research Center 

Cleveland, Ohio 

July 1988 

188-25337 

IN 

20K Uncfas 

G3/39 0351323 
CHARACTERIZATION OF FATIGUE CRACK INITIATION AND PROPAGATION IN 

Ti -6A1-4V WITH ELECTRICAL POTENTIAL DROP TECHNIQUE 

Sreeramesh Kalluri 

Sverdrup Technology, Inc. 

NASA Lewis Research Center Group 

Cleveland, Ohio 44135 

and 

Jack Telesman 

National Aeronautics and Space Administratlon 

Lewis Research Center 

Cleveland, Ohio 44135 

ABSTRACT 

Electrical potential methods have been used in the past primarily to monitor crack length in long-crack specimens subjected to fatigue loading. In 

this study an attempt was made to develop test procedures for monitoring the 

fatigue crack initiation and the growth of short fatlgue cracks in a turbine 

disk a1 loy with the electrical potential drop technique (EPDT). In addition, 

the EPDT was also applied to monitor the fatigue crack growth in long-crack 

specimens of the same alloy. The experimental program was conducted as part 

- of an AGARD-sponsored roundrobin program on titanium alloy engine disk mate- 

,- d rial. The resolution of the EPDT for different specimen geometries was deteru1 I mined. Factors influencing the EPDT are identified and the applicability of 

EPDT in implementing damage-tolerant design concepts for turbine disk materials 

is discussed. The experimental procedure adopted and the results obtained are 

presented. No substantial differences were observed between the fatigue crack 

growth data of short- and long-crack specimens. 

INTRODUCTION 

During the last 10 years the electrical potential drop technique (EPDT) 

has gained a substantial level of acceptance as a tool to measure fatigue 

crack growth (FCG) rates of long-crack specimens. This technique is based on 

the changing direct-current potential as a function of crack length (refs. 1 

to 5). The two main advantages of the EPDT over optical surface measurements 

are (1) the technique lends itself to full automation and (2) the crack length 

data measured by the EPDT reflect the average crack profile through the entire 

thickness of the specimen whereas the optical crack length measurement technique measures only the intersection of the crack front with the specimen 

surface. 

The use of the EPDT has been almost exclusively limited to FCG testing of 

long-crack specimens. However, other fatigue research ar*eas might benefit from 

the use of this technique, if proper methodology for its application can be 

identified. The areas of research that could benefit from the application of 

the EPDT are as follows: (1) fatigue crack growth of short cracks, (2) crack 

initiation under low cycle fatigue, and (3) measurement of damage under creep 
at elevated temperatures. 

tive enough to be used in these applications. If the answer to this question 

is affirmative, then development of procedures for optimizing data generation 

with EPDT i s needed. 

The question arises as to whether the EPDT is sensiThe need to understand FCG of short cracks as well as early crack detection under low cycle fatigue (LCF) is driven by recent efforts to implement 

the damage-tolerant design concept for turbine engine components. The damage 

tolerance philosophy is based on the assumption that crack-like defects exist 

at every critical location and that the gro th of these defects under actual 

operating conditions can be predicted. Inspection at regular intervals will 

screen out the components that have insufficient remaining lffe. The inspection interval is a function of the maximum allowable crack size, the minimum 

reliably detectable crack size, and the available crack growth data base. 

Since most of the critical engine components are subjected to high stresses, 

only very small cracks can be tolerated before the fracture toughness of the 

material is reached. Thus, understanding of crack initiation and the early 

stages of crack growth is needed before damage tolerance methodology can be 

fully implemented. At present, application of this methodology is hindered by 

the inability to reliably predict the FCG behavior of short cracks. 

on short-crack behavior as well as on crack initiation is considerably hindered 

by labor-intensive data collection methods, such as replication. A successful 

application of the easily automatable EPDT to study both crack initiation and 

short-crack growth behavior would provide a substantial impetus to the implementation of the damage tolerance methodology for the design of engine 

components. 

Research 

A test program was conducted to determine the feasibility of using EPDT 

to study crack initiation and short-crack growth behavior of T1-6A1-4V turbine 

disk forgings. This program was conducted as part of a cooperative roundrobin 

program sponsored by AGARD (refs. 6 to 8). 

EXPERIMENTAL PROCEDURE AND RESULTS 

The material properties of the Ti-6A1-4V alloy turblne disk forging used 

in this study are shown in table I. A representative micrograph of the alloy 

is shown in figure 1. The microstructure consists of equiaxed a grains 

(light) in a transformed 13 matrix (dark) containing coarse acicular a. The 

average size of the cc gralns is approximately 25 pm. 

The test program consisted of four types of experiments, each using a different specimen machined from the turbine disk forging. The geometry of each 

of the four types of speclmens is illustrated in figure 2. The specimens were 

supplied by Rolls Royce Ltd. Low-cycle fatigue and crack in’ltiation data were 

generated by using smooth cylindrical and flat double-edge-notched specimens, 

respectively. FCG data in the short and long crack regimes were obtained by 

using corner crack specimens and ASTM compact tension specimens, respectively. 

The test matrix used in the program is shown in table 11. The EPDT was used 

to monitor crack initiation and propagation for all types of testing with the 

exception of low-cycle fatigue experiments, which were conducted to obtain 

baseline fatigue life data. 

All the experiments were conducted at room temperature in the ambient 

environment on servohydraullc fatigue testing machines. A trapezoidal load 

2 
waveform with R = (minimum load/maximum load) = 0.1 was used in each of the 

experiments. A schematic illustration of the load waveform is shown in 

figure 3. The frequency of the load waveform was 0.25 Hz unless indicated 

otherwise. The experimental procedure and the results of each type of test 

are presented in the following sections. 

Low-Cycle Fatigue Experiments 

Smooth cylindrical specimens (fig. 2(a)> were used to conduct the LCF 

experiments. No attempt was made to monitor crack initiation or propagation in 

these experiments. 

measured to be less than 2 percent. Tests were conducted under load control. 

Axial strain was monitored with a 12.7-mm-gage-length extensometer. Stressstrain hysteresis loops as well as load and strain versus time data were 

recorded throughout each LCF test on X-Y recorders and strip charts, respectively. Tensile strain ratchetting was observed in all of the experiments. 

The LCF data are tabulated in table 111 and plotted in figure 4. The solid 

line represents the least-squares fit to the data points. 

Prior to testing, the bending strains of the test rig were 

Crack Initiation Experlments 

Flat double-edge-notched specimens (fig. 2(b)) with a nominal stress concentration factor Kt of 2.2 were used in these experiments. The deviation 

from the loading axiality was measured under elastic conditions to be 3.7 percent, which was less than maximum allowable value of 5 percent (ref. 7) and 

hence was considered acceptable. EPDT was used to establish crack initiation. 

One set of potential probes (0.5-mm-diameter titanium wire) was welded diagonally at opposite corners of each notch. The reference voltage was generated by 

welding titanium wire reference leads across the notch of an identical doubleedge-notched specimen made of Ti-6A1-4V alloy. This procedure ensured that the 

reference voltage signal was approximately equal to the notch voltage signal. 

The instrumented double-edge-notched specimen is shown in figure 5. The electrical current was supplied to the specimen by a constant dc current source. 

The reference voltage Vref and the notch voltages Vn1 and Vn2 were 

recorded throughout each test on strip chart recorders. 

was defined in the AGARD roundrobin (ref. 7) as the number of cycles Nix at 

which the normalized notch voltage Vn/Vref was 1 percent above the initial 

value. The normalized notch voltage versus number of cycles was plotted for 

the notch that exhibited a more rapid increase in the voltage with cycles. A 

typical normalized voltage versus cycles plot is depicted in figure 6. It is 

evident from this figure that fatlgue damage had actually started well before 

the normalized notch voltage increased by 1 percent. The experimental data 

generated by using the double-edge-notched specimens are tabulated in table IV. 

The dashed and solid lines in figure 7 represent least-squares fits to the 

crack initiation and failure data, respectively. The crack initiation life 

determined by the €POT for the 1 percent normalized voltage increase criterion 

was typically about 92 percent of the failure life of the double-edge-notched 

specimen. 

Crack initiation life 

3 
Fatigue Crack Growth of Short Cracks 

The fatigue crack growth behavior of short cracks was investigated with 

corner crack specimens (fig. 2(c>). The crack growth was monitored both by an 

optical traveling microscope and EPDT. Electrical potential across the corner 

crack was calibrated by using surface crack length as measured with the trave 1 i ng opt i cal ml croscope . 

Two electrical titanium wire probes (0.05-mm diameter) were spot welded on 

either side of the corner crack. The details of an instrumented corner crack 

specimen are shown in figure 8. Because of the strong influence of the location of probes on the calibration, caution was taken in locating the probes 

right on the edges of the corner notch. The influence of the distance between 

the electrical probes on the crack length calibration is illustrated in figure 

9 (from ref. 7). The reference voltage was provided by welding a separate set 

of electrical probes on another corner crack specimen. A dc current of 10 A 

was supplied in series to the reference specimen as well as to the FCG corner 

crack specimen. A fatigue crack was initiated in the corner crack specimen at 

a frequency of 5 Hz. The actual crack propagation test was conducted at 

0.25 Hz uslng the waveform shown in figure 3. The stress intensity factor was 

calculated by the formula shown in appendix A. 

The crack length at the specimen surface as was monitored throughout the 

experiment with the optical traveling microscope. Both the corner crack voltage V, and the reference voltage were recorded throughout the experiment on 

strip chart recorders. The final average crack length was obtained by averaging five radlal measurements taken at Oo, 22.5O, 4S0, 67.SQ, and 90° from the 

crack corner. The average calculated crack length a was 1.078 times as. A 

master calibration curve was first generated for the corner crack specimens by 

plotting t h e o p t i c a l l y m e a s u r e d s u r f a c e c r a c k length a, against the parameter 

Vc/VrefW at several different crack lengths, where w is t h e w i d t h of the 

specimen. The master calibration curve for the corner crack specimens is shown 

in figure 10. 

bration curve was used to normalize the obtained test data. This normalization 

was necessary to account for small variations in the distance between the electrical probes of the corner crack specimens. 

this procedure as well as the equation fitting the data are shown in figure 11. 

In subsequent corner crack growth experiments the master caliThe FCG data obtalned through 

Representative fractographs of a corner crack specimen at the early and 

intermediate stages of the FCG curve are shown in figure 12. In the low AK 

(stress intensity range) region, when the crack was short, the failure had a 

crystallographic appearance (fig. 12(a)>. In the intermedlate AK region the 

f a i l u r e o c c u r r e d m o s t l y t h r o u g h a striation-forming process (fig. 12(b)). 

Fatigue Crack Growth of Long Cracks 

Fatigue crack growth investigation of long cracks was performed with compact tension (CT) specimens (fig. 2(d>>. In order to provide current input to 

the specimen, two holes were drilled and tapped on the top and bottom surfaces 

of the specimen (fig. 13). Two electrical potential probes (0.5-mm-diameter 

titanium wire) were welded to the notch edge at diagonally opposite corners as 

indicated in figure 13. An identical compact tension specimen also made of 

Ti-6A1-4V alloy was used to provide the reference voltage. Direct electric 

current was supplled to the CT specimen and the reference specimen in series. 

4 
All the CT specimens were precracked at a frequency of 5 Hz. 

gation, however, was conducted at 0.25 Hz by using the trapezoidal waveform 

shown in figure 3. 

lated per ASTM E647 (ref. 9). 

The crack propaThe stress intensity factor for the CT specimens was calcuThe notch and reference potential values were recorded throughout the 

experiment on strip chart recorders. The crack was propagated under cyclic 

loading until a/w was approximately equal to 0.65. At this stage the specimen was separated into two pieces and the final crack length before fracture 

was measured optically. The final average crack length was determined by averaging five measurements taken at 0, 25, 50, 75, and 100 percent of the specimen 

width. The calibration equations used to calculate the crack lengths are shown 

in appendlx B. The da/dM versus AK plot for the compact tension specimen 

is shown in figure 14. The FCG data appear to fall into two regimes. As a 

result, two equatlons were used to fit the data. 

DISCUSSIOl4 

The driving force behind this work is the need for a test methodology to 

be used in conjunction with the implementation of damage-tolerant design concepts for turbine engine components. In particular, screening tests are 

required for studying crack Initiation and short-crack propagation in the candidate materials. 

testing methodology have sufficient resolution for the detection and study of 

small flaws. 

These particular requirements make it imperative that the 

Resolution 

Resolution is defined as the smallest increment in the crack length that 

can be accurately measured with a given crack length measurement technique. 

The results of this work show that the resolution of the EPDT depends on the 

following parameters: (1) the magnitude of the dc current used, (2) the geometry of the specimen, and (3) the dlstance between the electrical potential 

probes. In general the resolution of the EPDT increases with an increase in 

the dc current. However, large dc currents can also induce undesirable specimen heating. This phenomenon limits the extent to which the dc current can be 

increased and hence the resolution. In this program, at a dc current of 10 A, 

crack length resolutions of 0.025 and 0.020 mm were obtained for the corner 

crack and compact tension specimens, respectively. The influence of the location of potential probes on the calibration for a corner crack specimen 

(fig. 9) indicates that resolution of the crack length measurement depends on 

the distance between the probes. 

Besides the location of the leads, other factors can have a substantial 

effect on the EPDT output. Substantial effects on the crack length resolution 

were caused by small drifts in the current supplied to the specimens. These 

small drifts would have no appreciable effect on long-crack specimens, but for 

the double-edge-notched and corner crack specimens the small drifts result in 

changes in the output voltage that can mask the crucial initial fatigue damage. 

The use of a reference specimen completely compensated for the small drifts in 

the dc current, thus significantly improving the resolution of the system. 

5 
Addi tlonal Factors Affecting EPDT 

Hartman and Johnson (ref. 10) identified three additional factors that 

can influence the dc electrical potentlal read'ing: (1 1 material resistivity, 

(2) thermally induced electric potential, and (3) crack closure. 

The resistivity of the test speefmen materlal can change as a result of 

either plastic deformation at the crack tip or a change in the test specimen 

temperature during the fatfgue crack growth test. If large plastic deformations occur at the crack tlp, the electrical potential readings could lead to 

erroneous crack length measurements. However, thls problem can be minimized by 

properly designing the test specimen geometry. Thermally induced resistivity 

changes in the test specimen can be minimized by carefully controlling the temperature of the testing environment. 

In general, the probes used in the potentlal drop measurements are made of 

a different material than the test specimen itself. As a result, a thermally 

induced potential (due to a thermocouple effect) can exist between the electrical potential probes (ref. 10). This thermally induced potential can affect 

the change in potential ar'lsfng from the growth of the fatigue crack. However, 

the thermally induced potentfal can be minimized by maintaining the junctions 

of the two different metals at the same temperature (fig. 5(b)). 

Fatlgue crack closure can also affect the dc potential measured by the 

probes. 

faces of the crack and lower the resistance of the specimen. When the current 

is constant, lower resistance can reduce the electrical potential measured by 

the electrical probes. Hence, crack closure can lead to periodic fluctuations 

in the electrical potential measured by the probes under cyclic loadlngs. However, electr'ical potential corresponding to the actual crack length can be 

obtained by measuring the potentlal at the peak tensile load, where there is no 

crack closure (refs. 1 and 10). The effect of crack closure on the measured 

electric potential has been observed to be more pronounced Sn vacuum than in 

air because of the lack of nonconductlng oxlde layer formatton in vacuum 

(ref. 11). 

During crack closure electrical contact can occur between the two 

FCG of Short-Crack Specimens 

A corner crack specimen was used to simulate the fatigue behavior that 

could occur around the turblne disk bolt holes, where the fatigue cracks are 

short. It has been sho n by researchers (e.g., refs. 12 to ) that cracks 

short enough In length exhlbit accelerated fatfgue crack gro h rates when compared with long, well developed cracks at equivalent stress tenslty. The 

FCG results obtained In thls study with the EPDT, ho ever, do not exhibft the 

short-crack effect and show no substantial difference from the long-crack data 

(flg. 15). The most 'likely explanatlon for thls behavior is that the shortcrack effect is usually noticeable when the crack length is either smaller than 

or equal to the grain stze. In this study the inltlal crack length for the 

corner crack specimen was 0.25 mm whSle the graln slze was 25 pm. Thus the 

crack front was embe~de~ in many dlfferent gralns, and the short-crack behavior 

was not observed. Gangloff (ref. 31, also usfng EPDT and specimens containlng 

small inlttlal flaws, could not detect a short-crack growth regime. In this 

instance again, the initial flaw mlght have been larger than the graln size. 

6 
The corner crack specimen is capable of capturing short-crack effects In a 

coarse-grained material (ref. 8). 

As mentioned earlier, the resolution of the EPDT system was measured to be 

0.025 mm for the corner crack specimens. It is possible to accurately measure 

the early stages of crack growth In corner crack specimens with such a resolution. However, short-crack behavlor can be successfully studled in this type 

of specimen only if the grain size of the alloy is larger than the initlal 

notch. Another set of problems arises in speclmens where the cracks initiate 

naturally, such as the double-edge-notched specimen. One of the problems Is 

that the leads are located further from the crack, thus reducing the system 

resolution. Another important factor is that for both the notched and smooth 

specimens a number of different cracks can inltlate. The resulting voltage 

change is then indicatlve of the cumulative effect of a number of cracks and 

thus cannot be used to calculate the length of a given crack. However, the 

EPDT can be used for these types of speclmens In conjunction with another technique for measurlng crack length, such as replication. For example, EPDT can 

be used to detect crack initiation and then replication can be used to measure 

the surface crack length. 

Damage Tolerance Approach 

The results presented in this report show that EPDT can play a significant 

role as part of the test methodology needed for implementing damage-tolerant 

design concepts. In spite of its shortcomings the EPDT has a role as an inexpensive screening test to evaluate the small-defect tolerance of candidate 

turbine engine materials. Indeed, Van Stone et a1 . (ref. 4) have used EPDT as 

such a screening tool in evaluatlng the damage tolerance of nickel-base superalloys for turbine disk applications. At its present stage the EPDT is not the 

ultimate methodology for the damage tolerance approach; however, its inexpensive cost as well as its compatibility with other techniques makes it a very 

useful tool. 

CONCLUSIONS 

The EPDT technique was successfully appl fed In determining the fatlgue 

crack initiatlon and monftoring the fatigue crack growth in two different specimens of Ti-6A1-4V alloy. 

observed between the FCG data of corner crack and compact tension specimens. 

In this study no substantlal differences were 

The resolution of the crack length obtained at a dc current of 10 A was 

0.025 mm for a corner crack specimen. 

crack length resolutlon observed for a compact tension specimen was 0.020 mm. 

Even though EPDT is not the ultimate methodology for the damage tolerance 

approach, it can be used as an inexpensive screenlng tool to assess the damage 

tolerance of turbine disk alloys. 

For the same magnitude of dc current the 

7 
APPENDIX A 

STRESS INTENSITY CALCULATIONS FOR CORNER CRACK SPECIMEN 

The stress intensity factor KI at the specimen surface was calculated 

for the corner crack specimen by the following formulas (ref. 7): 

KI = 1.16 ($) Q for a/w < 0.2 

For higher values of a/w the following equation was used: 

KI = [1.12 - 0.13 (i) + 1.84 (Zr + 0.11 (ir + 0.8 (q] ($) u 

where 

a length of crack, m 

u nominal stress, MPa 

L 

w width of specimen, m 

KI stress intensity factor, MPaG 

8 
APPENDIX B 

CRACK LENGTH CALCULATION FOR COMPACT TENSION SPECIMEN 

The crack length a of the compact tension specimen was calculated from 

the notch voltage Vn by using a normalized calibration curve. Hicks and 

Pickard (ref. 2) and Mom (ref. 7) reported such a normalized calibration curve 

(fig. 16) for a compact tension specimen. The normalized calibratlon curve, 

developed using finite element analysis, establishes a relationship between 

the crack parameter a/w and the voltage parameter V"/Vr,f. The relationship corresponding to the calibration curve can be mathematically expressed as 

follows (from ref. 7): 

- vn = A + Al (i) + A2 (ir + A3 (ir 

vO 

where 

Vn notch voltage 

Vo = Vref reference voltage 

and 

A = 0.5766 

A1 = 1.9169 

A2 = -1.0712 

A3 = 1.6898 

A1 ternatively 

- a 9 B + B1 re) + B2 (c) 'n + B3 

W 

where 

B = -0.5051 

B1 = 0.8857 

82 = -0.1398 

B3 = 2.398~10'4 

The final value (before the specimen was separated into two pieces under 

tensile loading) of the parameter Vn/Vref was calculated from the measured 

values of Vn and Vref for each specimen. The final value of the same 

parameter was also calculated from the normalized calibration curve (eq. (B1)) 

by using the measured average final crack length. In general, these two values 

9 
would not be the same, since at the beginning of the experiment it would be 

difficult to obtain a notch voltage that is identical to tile reference voltage. 

Hence the following ratio was computed for each specimen: 

- .. 

‘vref’fi nal ,normal i zed 

where NP is the normalizing parameter. 

The calibration was verified for each specimen 

(1) The measured initial voltage ratio Vn/Vref 

normalizing parameter NP. 

n the following manner: 

was normalized with the 

= (k) (L) ()i,tial ,normalized i ni ti a1 ,measured 

(2) The normalized initial voltage ratio was then used together with equation (62) to calculate the initial crack length. This calculated initial crack 

length was compared with the measured initial crack length to verify the accuracy of the calibration. 

puted, and these ratios were normalized for each specimen with its normalizing 

parameter. 

malized calibration curve (eq. (82)) to compute the crack length a. The ASTM 

E647 standard (ref. 9) was used to truncate the initial crack propagation data. 

At regular intervals of cyclic lives the measured voltage ratios were comThe normalized voltage ratio was then used together with the nor10 
REFERENCES 

.. 

. 

1. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

9. 

10. 

11. 

12. 

Wei, R.P.; and Brazill, R.L.: An Assessment of A-C and D-C Potential 

Systems for Monitoring Fatigue Crack Growth. 

urement and Data Analysis, ASTM STP-738, S.J. Hudak, Jr., and R.J. Bucci, 

eds., ASTM, 1981, pp. 103-119. 

Fatigue Crack Growth MeasHicks, M.A.; and Pickard, A.C.: A Comparison of Theoretical and Experimental Methods of Calibrating the Electrical Potential Drop Technique for 

Crack Length Determination. Int. J. Fract., vol. 20, no. 2, 1982, 

pp. 91-101. 

Gangloff, R.P.: Electrical Potential Monitoring of Fatigue Crack Formation and Growth From Small Defects. General Electric, Report 

No. 79CRD267, Jan. 1980, pp. 1-21. 

Van Stone, R.H.; Krueger, D.D.; and Duvelius, L.T.: Use of a d-c Potential Drop Crack Monitoring Technique in the Development of Defect Tolerant 

Disk Alloys. Fracture Mechanics: Fourteenth Symposium, Vol. 2, Testing 

and Applications, ASTM STP-791, J.C. Lewis and G. Sines, eds., ASTM, 1983, 

pp. 11-553 to 11-578. 

Van Stone, R.H.; and Richardson, T.L.: Potential-Drop Monitoring of 

Cracks in Surface-Flawed Specimens. Automated Test Methods for Fracture 

and Fatigue Crack Growth, ASTM STP-877, W.H. Cullen, et al., eds., ASTM, 

1985, pp. 148-166. 

Mom, A.J.A.; and Raizenne, M.D.: AGARD Cooperative Programme on Titanium 

Alloy Engine Disc Material. Damage Tolerance Concepts for Critical Components, AGARD CP-393, 1985, pp. 9-1 to 9-9. 

Mom, A.J.A.: Working Document for the AGARD Cooperative Test Programme 

on Titanium Alloy Engine DISC Material. National Aerospace Laboratory, 

The Netherlands, NLR TR 84022 L, 1984. 

Mom, A.J.A.; and Raizenne, M.D.: AGARD Engine Disc Cooperative Test Programme - Core Programme Results. National Aerospace Laboratory, The 

Netherlands, NLR MP 86045 U, 1987. 

Standard Test Method for Constant-Load-Amplitude Fatigue Crack Growth 

Rates Above 10-8 mlCycle. E647-86, Metals Test Methods and Analytical 

Procedures, Annual Book of ASTM Standards, Section 3, Vol. 03.01, ASTM, 

1986, pp. 714-736. 

Hartman, G.A.; and Johnson, D.A.: DC Electr 

to Thermal/Mechanical Fatigue Crack Growth. 

vol. 27, no. 1, Mar. 1987, pp. 106-112. 

Bachmann, V.; and Munz, D.: Fatlgue Crack C 

Potential Method. Eng. Fract. Mech., vol. 1 

c-Potenti a1 Method Appli ed 

Experimental Mechanics, 

osure Eva1 uation With the 

, no. 1, 1979, pp. 61-71. 

Pearson, S.: Initiation of Fatigue Cracks in Commercial Aluminum Alloys 

and the Subsequent Propagatlon of Very Short Cracks. Eng. Fract. Mech., 

vol. 7, no. 2, 1975, pp. 235-247. 

11 
13. Morris, W.L.; Buck, 0.; and Marcus, H.L.: Fatigue 

Early Propagation in AL 2219-T851. Metall. Trans. 

Aug. 1976, pp. 1161-1165. 

Type of test 

Crack Initiation and 

A., vol. 7, no. 8, 

LCF life/crack initiation Crack propagation 

14. Telesman, J.; Fisher, D.M.; and Holka, D.: Variab es Controlling Fatigue 

Crack Growth of Short Cracks. Fatigue Life: Analysis and Prediction, 

V.S. Goel, ed., American Society for Metals, 1986, pp. 53-67. 

Number of specimens 

Crack EPDT 

detection 

Optical 

Goal 

TABLE I. - MINIMUM MEASURED ROOM-TEMPERATURE 

MATERIAL PROPERTIES OF Ti-6A1-4V ALLOY DISK 

[From ref. 8.1 

6 6 3 3 

J J r/ 

3 

Baseline fatigue Only initial "Short" crack Total da/dN-AK 

life data crack formation range curve 

Tensile strength, MPa . . . . . . . . . 969 

0.2-Percent yield strength, MPa . . . . 865 

Reduction in area, percent . . . . . . . 26 

Fracture toughness, MPap . . . . . 52.3 

Specimen Stress range, Number of cycles 

Aa amax - amin, to failure, 

MPa Nf 

I Test specimen Smooth Ooubl e-edge- I cylindrical I notched (Kt - 2.2) I 

LCF 9 

LCF 8 

LCF 7 LCF 12 LCF loa 

LCF Ilb 

809.2 2 679 

808.6 2 350 

778.9 5 510 

765.9 11 016 

750.6 8 156 

746.7 11 327 

TABLE 111. - Ti-6A1-4V LOW-CYCLE-FATIGUE DATA 

[R = 0.1.1 

2 ;2 

Kt 8 

K: 10 

Kt 7 

K+ 11 

775 2 575 2 890 

775 4 050 4 340 

625 8 950 9 810 

625 9 200 9 660 

500 25 100 27 120 

500 27 000 29 350 

aSpecimen failed outside the gage section. 

bSpecimen failed near one of the knife edges of 

the extensometer. 

TABLE IV. - Ti-6A1-4V FATIGUE CRACK INITIATION DATA 

FOR DOUBLE-EDGE-NOTCHED SPECIMENS 

CKt = 2.2; R = 0.1.1 

Stress range, Number of cycles to 1 speclT Aa = a!:: - amin, 1 crack initiation, to failure, 

N1% 

12 
FIGURE 1. - flICROSTRUCTURE OF Ti-6AI-4V ALLOY USED IN THIS STUDY. 

13 
2 

I 

I 

.64&0.03 DIAM 

(a) CYLINDRICAL UNNOTCHED 

< - 

LCF SPECINEN. 

34 60 -a 

REFERENCE CROSS SECTION, 25 m2. 

'1 1.6R 

i 

81to.2R-' 

_- + A' 

-351to.2 

'\ 

\ 

\ 

SECTION A-A' 

(b) FLAT DOUBLE-EDGE-NOTCHED SPECIMEN (NOMINAL STRESS CONCENTRATION FACTOR. Kt - 2.2). 

FIGURE 2. - TEST SPECIMENS. (LINEAR DIMENSIONS ARE IN MILLIPlETERS.) 

14 
SECTION A-A 

(c) CORNER CRACK SPECIRN. NOTCH DEPTH. 0.25 M; NOTCH WIDTH, 0.10 MM. 

CURRENT 

INPUT/ 

(d) COMPACT TENSION SPECIMEN. 

FIGURE 2. - CONCLUDED. 

I 

TIME, SEC 

FIGURE 3. - SCHEMATIC ILLUSTRATION OF LOAD WAVEFORM. R = P,,,/P,, = 0.1. 

15 
. 900r 

0 LCF DATA 

LEAST-SQUARES 

TITANIUN WIRE 

(o.~-MM DIAM) 

NUMBER OF CYCLES TO FAILURE, Nf 

FIGURE 4. - RESULTS OF LCF EXPERIMENTS. 

(a) ATTACHMENT OF POTENTIAL LEADS. 

L ASSURE SAME 

TEMPERATURE 

(b) SETUP OF CURRENT AND VOLTAGE WIRING. 

FIGURE 5. - WIRING OF DOUBLE-EDGE-NOTCHED SPECIMENS. (ALL DIMENSIONS ARE IN MILLIMETERS. 1 

16 
c 

3 W 0 

(1.01 * VflI)/vREF 

0 CRACK INITIATION DATA, Nix 

0 FAILURE DATA. Nf 

v) 

v) W

L 

-96 .97 .98 

400 % 

4 6 810 20 4Ox1O3 

NORMALIZED NOTCH VOLTAGE, VflNREF 

FIGURE 6. - NORMALIZED NOTCH VOLTAGE VERSUS NUMBER 

OF CYCLES FOR DOUBLE-EDGE-NOTCHED SPECIMEN. 

IMEN NUMBER, Kt 9; CRACK INITIATION, NIX = 4050: 

SPECNf = 4340. 

CURRENT 

OUTPUT 

4 

NUMBER OF CYCLES, N 

FIGURE 7. - RESULTS OF CRACK INITIATION EXPERIMENTS ON 

DOUBLE-EDGE-NOTCHED SPECIMENS. 

0.05-DIAM 

PROBE WIRE 

'L NOTCH 

CERAMIC 

ENLARGED SECTION A-A' 

CURRENT 

INPUT 

FIGURE 8. - POTENTIAL DROP LEAD LOCATIONS FOR CORNER CRACK SPECIMEN. (ALL DIMENSIONS ARE IN MILLIMETERS.) 

17 
.8 

.7 

.6 

.5 

=U 

W 

.3 

.i 

.I 

C 

- ~u'~ W .2 

5 .I 

--O-- Z/W = 0 FINITE -*-- Z/W = 0.05 ELEmNT 

- 4-- Z/W = 0.1 RESULTS 

n 

i 

ANALYTICAL SOLUTION 

FIGURE 9. - COMPARISON OF ANALYTIC AND FINITE ELEVENT RESULTS FOR CORNER CRACK SPECIMEN. 

REF. 7.) 

(FROH 

0 

1 

.I .2 .3 .4 

a,/w 

FIGURE 10. - MASTER CALIBRATION CURVE FOR CORNER 

CRACK SPECIMEN. z/w = 0.102. 

da/dN = 9. IIxIO-~~ -,, 

10-8 

10 

3 

20 40 60 80 100 

STRESS INTENSITY RANGE, AK, RPafi 

FIGURE 11. - FATIGUE CRACK GROWTH DATA OBTAINED WITH 

CORNER CRACK SPECIMENS. 

18 
(a) CRYSTALLOGRAPHIC FRACTURE IN LOW AK REGIME. 

(b) STRIATION FORMATION IN INTERMEDIATE AK REGIME. 

FIGURE 12. - REPRESENTATIVE FRACTOGRAPHS OF A CORNER CRACK SPECIMEN AT EARLY AND 

INTERMEDIATE STAGES OF FATIGUE CRACK GRONH. 
CURRENT [- 19.65 4 INET 

I 

0 

C-VREF 

/ 0 

CURRENT [- 19.65 - INPUT 

TITANIUM WIRE 

10.5 

PROBE 

LOCATION 

CURRENT OUTPUT 7\ 

k ;?.6 _.___) 

I 

,- TITANIUM 0 ,‘ WIRE ,r COPPER 

< (I1 

Vn L\’ 

,/ A ,/ CABLE 

\ 

‘-ASSURE SM 0 TEMPERATURE 

TITANIUM WIRE 

10.5 

PROBE 

LOCATION 

CURRENT OUTPUT 7\ 

k ;?.6 _.___) 

CURRENT INPUT -’ 

(a) ATTACHNENT OF CURRENT LEADS AND POTENTIAL PROBES, 

CURRENT 

-5 

CURRENT 

f== 

. 

20 
da/dN = 5.95~10-l~ AK3*51 7, 

L._U 

.- da/dN = 2.35~10-l~ AK7'08 

I I I I IIIII 

40 60 80 100 

10-9 

10 20 

STRESS INTENSITY RANGE, AK, MPafi 

FIGURE 19. - FATIGUE CRACK GROWTH DATA OBTAINED WITH 

CONPACT TENSION SPECIMENS. 

20 40 60 80 100 

10-9 

10 

STRESS INTENSITY RANGE, AK, MPafi 

FIGURE 15. - COMPARISON OF FATIGUE CRACK GROW RATES 

FOR CORNER CRACK AND COMPACT TENSION SPECIMENS. 

21 
a/w 

FIGURE 16. - FINITE ELEMENT POTENTIAL DROP ANALYSIS FOR 

COMPACT TENSION SPECIMEN NORHALIZED TO Vn/Vo = 1 

a/w = 0.244, WHERE Vo IS POTENTIAL DROP AT INITIAL 

AT 

CRACK LENGTH a. AND Vn IS POTENTIAL DROP AT CRACK 

LENGTH a. (FROW REFS. 2 AND 7.) 

22 
NASA National Aeronautics and Report Documentation Page 

Space Administration 

1. Report No. 2. Government Accession No. 3. Recipient’s Catalog No. 

NASA TM-100877 

4. Title and Subtitle 5. Report Date 

July 1988 Characterization of Fatigue Crack Initiation and 

Drop Technique 

Sreeramesh Kall uri and Jack Telesman 

Propagation in Ti-6A1-4V Hith Electrical Potential 6. Performing Organization Code 

7. Author(@ 8. Performing Organization Report No. 

E-41 11 

10. Work Unit No. 

- 505-63-1 B 

9. Performing Organization Name and Address 

11. Contract or Grant No. National Aeronautics and Space Administration 

Lewis Research Center 

Cleveland, Ohio 44135-3191 13. Type of Report and Period Covered 

12. Sponsoring Agency Name and Address Technical Memorandum 

National Aeronautics and Space Administration 14. Sponsoring Agency Code 

Washington, D.C. 20546-0001 

15. Supplementary Notes 

Sreeramesh Kalluri , Sverdrup Technology, Inc., NASA Lewis Research Center Group, 

Cleveland, Ohio 44135; Jack Telesman, NASA Lewis Research Center. 

16. Abstract 

Electrical potential methods have been used in the past primarily to monitor 

crack length in long-crack specimens subjected to fatigue loading. 

study an attempt was made to develop test procedures for monitoring the fatigue 

crack initiation and the growth of short fatigue cracks in a turbine disk alloy with the electrical potential drop technique (EPDT). In addition, the EPDT was 

also applied to monitor the fatigue crack growth in long-crack specimens of the 

same alloy. roundrobin program on ti tani um a1 loy engine di sk material. The resolution of 

the EPDT for different specimen geometries was determined. Factors influencing 

the EPDT are identified and the applicability of EPDT in implementing damagetolerant design concepts for turbine disk materials is discussed. 

tal procedure adopted and the results obtained are presented. No substantial 

differences were observed between the fatigue crack growth data of short- and 

long-crack specimens. 

In this 

The experimental program was conducted as part of an AGARD-sponsored 

The experimen17. Key Words (Suggested by Author@)) 18. Distribution Statement 

Electrical potential drop technique; 

Crack propagation; Damage tolerance 

Unclassified - Unlimited 

’ Fatigue; Crack initiation; Subject Category 39 

I 

19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No of pages 22. Price’ 

24 A02 Unclassified Unclassified 

*For sale by the National Technical Information Service, Springfield, Virginia 22161 NASA FORM 1626 OCT 88 
National Aeronautics and 

Space Administration 

Lewis Research Center 

Cleveland. Ohio 44135 

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