- Email: [email protected]
Sensors for monitoring early stage fatigue cracking
International Journalof Fatigue
International Journal of Fatigue 29 (2007) 1668–1680
www.elsevier.com/locate/ijfatigue
Sensors for monitoring early stage fatigue cracking John M. Papazian a,*, Jerrell Nardiello a, Robert P. Silberstein a, Greg Welsh b, David Grundy c, Chris Craven c, Leslie Evans c, Neil Goldfine c, Jennifer E. Michaels d, Thomas E. Michaels d, Yuanfeng Li e, Campbell Laird e a
d
Northrop Grumman Integrated Systems, Bethpage, NY 11714, United States b United Technologies Research Center, East Hartford, CT, USA c JENTEK Sensors, Inc., Waltham, MA 02453, United States Georgia Institute of Technology, School of Electrical and Computer Engineering, Atlanta, GA 30332-0250, United States e University of Pennsylvania, Philadelphia, PA 19104, United States Received 8 September 2006; received in revised form 4 January 2007; accepted 8 January 2007 Available online 7 February 2007
Abstract Three sensor systems were evaluated for their ability to detect early stage fatigue cracking in open holes. The sensor systems were (1) a Meandering Winding Magnetometer Array sensor system that induces eddy currents to monitor conductivity changes; (2) a throughtransmission ultrasonic technique that monitors energy loss, and (3) the Electrochemical Fatigue Sensor which detects fatigue-induced changes in a metal surface through their effects on the electrochemical double layer. The sensors were mounted individually or in tandem in samples of 7075-T651 aluminum alloy which were then fatigued using a spectrum loading sequence. The samples were also examined at various stages during fatigue cycling using optical or scanning electron microscopy. Detection thresholds of approximately 100 lm were observed, and calibration curves for crack size in terms of the sensor outputs were obtained. 2007 Elsevier Ltd. All rights reserved. Keywords: Fatigue; Sensors; Early stage cracking; Eddy current; Ultrasonic; Electrochemical
1. Introduction Interest in early stage fatigue has continued to increase with each new generation of observational techniques and with the recognition that short fatigue cracks do not always follow the same laws as long cracks [1]. Likewise, the importance of fatigue cracking to the structural safety of aircraft has become an increasingly serious preoccupation of the world’s aircraft fleet managers, as evidenced by the yearly conferences devoted to this issue (e.g. [2,3]). As part of this interest, the sensing of fatigue cracks for use as a nondestructive inspection technique and for potential use in health monitoring systems is also the subject of much current research [4,5]. Piotrowski et al. [4] recently tested 20 different sensor systems for their ability to detect *
Corresponding author. E-mail address: [email protected] (J.M. Papazian).
0142-1123/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijfatigue.2007.01.023
subsurface cracks in lap joint structures. They chose a threshold value of 3.8 mm at which to compare the techniques, and found that the probability of detection was ‘‘quite low’’ at this value [4]. In the Safe Life approach to aircraft fatigue, the US Navy considers that a primary aircraft structure with a crack of 250 lm has reached the end of its fatigue life [6]. Thus, the Navy threshold is more than an order of magnitude smaller than the threshold used in the Piotrowski study. This comparison is not strictly valid because the Piotrowski study examined cracks in the subsurface layer of lap joints, which are not usually of primary interest in naval aircraft. However, the comparison serves to highlight the gap between current NDI techniques and the need to detect smaller cracks. For naval aircraft, fastener holes are often prime locations for fatigue crack initiation, and some fatigue life tracking is focused on the surveillance of several fastener
J.M. Papazian et al. / International Journal of Fatigue 29 (2007) 1668–1680
1669
hole ‘‘hot spots.’’ Inspection for 250 lm cracks in fastener holes, particularly holes with the fastener in place, is not possible with current technology. However, next generation health monitoring systems, e.g., ‘‘prognosis’’ approaches, will require detailed information about fatigue cracking at this length scale and below. In an attempt to address the need for early stage fatigue crack detection at fastener holes, a series of laboratory experiments was undertaken to evaluate the state-of-theart of currently available sensor systems for cracks in holes. Recent reviews have catalogued most of the available techniques for early stage fatigue crack detection, [7–9] and several of the most applicable and promising of them were selected for evaluation in this study. It was decided to restrict the work to laboratory testing of open-hole 7075T651 aluminum coupons that were designed to be representative of current US Navy interest. The idea was to provide an optimum environment for sensor operation in order to establish the current lower limit for crack detection. The sensor systems were also chosen with an eye toward their potential for eventual use in a fielded system. The three chosen techniques were based on measurements of eddy currents, ultrasonic energy and electrochemical behavior.
designed to mimic the loads encountered by the lower wing covers of a military attack aircraft. The maximum nominal KnF at the holes was approximately 738 MPa, and the spectrum contained nearly negligible compressive loads. Note that the nominal yield stress of this material is approximately 500 MPa, thus local yielding in the high stress region of the fastener hole during the highest load excursions is expected. Sensor data was acquired continuously during the test and the general protocol involved stopping the test when the sensor indicated substantial cracking in the first hole and barely detectable cracking in the second. The samples were then examined in the test machine under a static load using a microscope with oblique illumination (Hirox HiScope), or by removing the sample and observing it in a scanning electron microscope. Some samples were overloaded to failure before removing from the test machine in order to create a fracture surface that contained the largest cracks. However, examination of the bore of the hole generally showed numerous small cracks located above and below the fatal crack.
2. Experimental procedure
Three sensor systems were evaluated. They are described below.
The general approach to these experiments involved mounting the individual sensors on a two-hole coupon of 7075-T651 aluminum alloy and subjecting the coupon to a spectrum fatigue test that resulted in development of fatigue cracks at the 3 and 9 o’clock positions in the holes, where the primary loading direction was along the 12 to 6 o’clock axis. The coupon was a flat, straight sided, rectangular cross section bar, 47.4 mm wide · 350 mm long · 5.7 mm thick. The dimensions were chosen to be representative of a critical area in a lower wing cover of a military aircraft. The coupon was machined from a plate that was originally 50.8 mm thick: the coupon was taken from the near surface region of the plate, its long axis was aligned with the rolling direction and its width and thickness directions were aligned with the width and thickness direction of the starting plate. Two holes, 4.8 mm diameter, were drilled at the centerline, and their centers were 1.27 cm from the edges of the coupon; this spacing resulted in approximately equal local stresses on either side of each hole. Drilling was performed in a milling machine using controlled feeds and speeds and involved making a pilot hole 4.39 mm diameter followed by reaming to final size using a 4.8 mm diameter 6-flute reamer at 450 rpm with a feed rate of 3.8 mm/min. All of the experiments were performed using a single batch of 50.8 mm thick 7075-T651 whose composition (wt. %) was Zn – 5.7, Mg – 2.5, Cu – 1.7, Cr – 0.19, Fe – 0.26, Si – 0.06, Mn – 0.03, Ti – 0.02Ni – 0.003 and V – 0.006. The samples were fatigued in a computer-controlled servo-hydraulic test frame under load control using a 500 h block, flight-by-flight tension-dominated spectrum
3. Sensor systems
3.1. Eddy current (Meandering Winding Magnetometer Array, MWM-Array) The MWM-Array sensor is a product of JENTEK Sensors, Inc., and is based on a refinement of traditional eddy current concepts. The sensors can be miniaturized and can be produced using custom geometries in a thin and flexible configuration. A key aspect of the technology is a rapid data analysis procedure that automates calculation of the two main parameters, lift-off and conductivity. The reader is referred to previous publications for more detailed information [7,10–12]. For this project, a special sensor was designed that fit inside the holes in the fatigue coupon. The sensor consisted of 12 individual sensing elements arranged such that four elements were located in each of the 3, 9 and 12 o’clock regions of the hole, as indicated schematically in Fig. 1a. Only the 3 and 9 o’clock sensing elements are labeled for clarity. Note that four of the sensing elements span the 5.7 mm depth of the hole, thus permitting localization of cracks through the thickness of the coupon. The 12 o’clock sensing elements served as control signals because no cracking was expected in this region. The entire sensor is fabricated on a thin and flexible substrate, and is inserted into the hole by wrapping it into a cylindrical shape and mounted in the hole, as can be seen in Fig. 1b. A soft elastomeric mandrel serves to fix the sensor in position and apply a uniform contact pressure. The raw signal is converted in real time by the JENTEK software and output as curves of conductivity (Fig. 2a) and
1670
J.M. Papazian et al. / International Journal of Fatigue 29 (2007) 1668–1680
Fig. 1. (a) Schematic view of the locations of the sensing elements of the MWM-Array sensor. An additional four sensing elements (not labeled) are located at the 12 o’clock position. (b) MWM-Array sensors installed in the fatigue coupon.
Fig. 2. (a) Conductivity change during a fatigue test for each of four sensing elements on one side of a hole. (b) Lift-off values for the same sensing elements.
lift-off (Fig. 2b). A separate signal is plotted for each of the twelve sensing elements. The changes in the individual values of the conductivity and the lift-off parameters indicate the occurrence of cracking in the local region sensed by the individual sensing elements: conversion of these signals to crack length will be described in a subsequent section. The data sets were synchronized with the test machine cycle count manually for these early tests. This did not result in appreciable errors because 1000 or more data sets were obtained during the 4–6 h tests. Subsequent tests involved faster data accumulation and automated synchronization with the test machine.
detected by monitoring the ratio of the energy received under load (which opens the cracks) to that received under no load (when the cracks are closed). The technique is also capable of measuring the instantaneous load by monitoring changes in the arrival time of the wave, which changes due to both the acoustoelastic effect and dimensional changes
Incident Ultrasound
Re-radiated Creeping Wave
3.2. Ultrasonic sensors The ultrasonic sensor system uses fixed-angle-beam transducers located on each side of the hole in a throughtransmission geometry as shown in Fig. 3. The emitted ultrasonic shear wave is converted to a spiral ‘‘creeping wave’’ at the surface of the hole. Most of the transmitted energy is blocked from reaching the receiver, but the creeping wave ‘‘leaks’’ in the form of a propagating shear wave and thus some energy reaches the receiver. Cracks at or near the surface of the hole attenuate the signal and are
Transmitter
Single “V” Bottom Surface
Receiver
Double “V” Top Surface
Fig. 3. Schematic views of the ultrasonic sensor system showing selected signal paths.
J.M. Papazian et al. / International Journal of Fatigue 29 (2007) 1668–1680
3.3. Electrochemical fatigue sensor (EFS) The electrochemical fatigue sensor was developed at the University of Pennsylvania by Yuanfeng Li and Campbell
Fig. 4. Ultrasonic sensors mounted above and below the holes in a fatigue coupon. MWM-Array sensors are in the holes.
Specimen S4-0464, Hole #2 1
0.8
Ultrasonic Energy Ratio
of the propagation path with load. Further, indications of crack closure are also obtained from the variation of the signal during each individual load cycle, and techniques for self-calibration, sensor integrity and bond quality monitoring have been developed. These issues are dealt with in separate publications, and will not be discussed further here [8,9,13,14]. The transducers are commercially available miniature wedges with a center frequency of 10 MHz and a nominal refracted angle of 70. Fig. 4 shows the ultrasonic transducers mounted above and below the two holes in the fatigue coupon. A fast curing epoxy (Loctite E-05CL) was used as a bonding agent and a mounting process was developed that optimized sensor placement and orientation [15]. At the beginning of this work, the ultrasonic data were acquired by periodically stopping the test and acquiring data under a static load and comparing it to no-load data. The ratio of the energy in the received signal under the two loading conditions was then plotted, as seen in Fig. 5. A reduction in energy ratio is generally indicative of the presence of a crack (or cracks) blocking transmission of part of the signal. This hypothesis was verified and quantified by correlating the total area of cracks (as observed by microscopy) with the changes in energy ratio. The development of the calibration curve is discussed in a subsequent section of this paper. More details of the data analysis procedures can be found in [8,9,13–17]. Further refinement of the ultrasonic technique during the experiential program resulted in the ability to make all of the measurements in real time during the fatigue test, eliminating the need to pause the test and apply a static load for data collection.
1671
0.6
0.4
0.2
0
0
10.000
20.000
30.000
40.000
Cycles
Fig. 5. Typical ultrasonic data plotted as an energy ratio versus cycles. The decrease in energy ratio indicates the presence of fatigue cracks.
Laird. It consists of an electrochemical cell created by attaching a small chamber containing electrolyte and an electrode to the article to be monitored. During fatigue cycling, an oscillating voltage is observed in the cell; this signal has been deconvolved into elastic, plastic and cracking components by a series of controlled experiments [18,19]. The electrochemical cell can be designed in a variety of ways, including simple patch type flexible units. The geometry chosen for these experiments is shown schematically in Fig. 6a. The photo in Fig. 6b shows two cells mounted on the two-hole fatigue coupon. The response of the cell during fatigue cycling is shown in Fig. 7. Fig. 7a shows the expected cell current response associated with elastic strain during cycling at a stress low enough to produce insignificant cyclic plasticity. Fig. 7b shows the expected response produced by cyclic plasticity when fatiguing at a higher stress. The onset of cracking is observed to cause distortions of the signal, thus the three components, elastic deformation, plastic deformation and cracking are usually separable in the composite signal using Fourier analysis. For this work, the very early stages of fatigue damage were of the greatest interest, therefore attention was focused on the plastic portion of the EFS signal. Fig. 8a is a plot of the instantaneous value of the plastic component of the current in the two EFS cells during a fatigue test, and Fig. 8b is the same data plotted as a running average over the prior 2519 cycles. This number was chosen because it is the number of cycles in a fatigue block, so the average was always taken over the same spectrum of loads. The running average was found to facilitate data interpretation by smoothing out sources of electrical and electrochemical noise in the individual signals excited by the load spectrum. The EFS current behavior seen in Fig. 8b is typical; there is an initial decrease as the specimen undergoes plasticity during initial cycling and as passivation develops,
1672
J.M. Papazian et al. / International Journal of Fatigue 29 (2007) 1668–1680
Fig. 6. (a) Schematic view of the electrochemical fatigue sensor. (b) Two EFS cells mounted on a fatigue coupon. The coupon is 5.7 mm thick · 47 mm wide, and the sensors are covering the holes.
b Tension
Time
Neutral
Plastic strain
Elastic strain
a
Tension
Time
Neutral Compression
Time
Current response
Current response
Compression
Time
Fig. 7. (a) Schematic showing the relationship of the observed EFS (elastic) current to the elastic deformation of the specimen during fatigue cycling. (b) Schematic showing the relationship of the observed EFS (plastic) current to the plastic deformation of the specimen during fatigue cycling.
Fig. 8. (a) Instantaneous value of the plastic current during a fatigue test. Data from two cells are shown. (b) Running average of the plastic current for the same two cells. The average was taken over the prior 2600 cycles (one block).
then the curve flattens as both hardening and passivation saturate but damage has not yet had an opportunity to develop significantly, and finally the current increases with gradually increasing rate as fatigue damage, dominated by cracking, accumulates.
4. Fatigue testing Several series of fatigue tests were performed. The initial tests were run with only one type of sensor installed in the test coupon, while subsequent tests used two types of
J.M. Papazian et al. / International Journal of Fatigue 29 (2007) 1668–1680
sensors mounted on the same coupon concurrently. The objective of these tests was to establish calibration curves for crack length versus sensor signal for each sensor and to get, in a qualitative manner, an understanding of the validity of each sensor in terms of its rate of false positive and false negative indications. 4.1. Eddy current (Meandering Winding Magnetometer Array, MWM-Array) A typical test involved installation of a sensor in each of the two holes in the fatigue coupon, imposing a flight-byflight block loading spectrum on the coupon, and monitoring the sensor signals. Some tests were continued until all four high-stress locations (the inboard and outboard sides of the two holes) showed some indications of cracking, while others were stopped when the second or third holeside reached a predetermined conductivity change. In this manner, a range of fatigue cracks was observed in the four high-stress regions, with some regions being significantly more advanced than others. In general, the least advanced regions were of interest because they provided the most stringent test of sensor sensitivity while the more advanced
1673
regions provided indications of which crack characteristics were being measured by the sensor. Typically, the bore of the hole was monitored using a microscope with oblique illumination and a rotating mirror (Hirox HiScope), however this was not possible while the MWM-Array sensor was in the hole. Removal of the sensor would necessitate recalibration and loss of the prior baseline. Therefore, no observations of cracks were possible until the test was stopped and the sensor was removed. At that time, the hole was examined for cracks using optical microscopy. Subsequently, the sample was sectioned into four ‘‘matchsticks’’ each of which contained one of the high stress regions, as can be seen in Fig. 9. Fig. 10 shows a SEM micrograph of the 1.2 · 5.7 mm high-stress region of the outboard side of hole #2 in specimen S4-0346 after 28,073 fatigue cycles. The largest crack was approximately 400 lm and is shown in the inset. Close examination of the coupon showed numerous smaller cracks and that the cracks appear to have initiated at constituent particles. This is the usual situation for 7075, and was a consistent observation in this work. The MWM-Array data from the four sensing elements on this side of the hole are shown in
Fig. 9. Schematic view of the preparation of ‘‘matchstick’’ specimens and the mounting device for examination under load. When mounted in the SEM, this permits examination of cracking in the high stress surface region of the hole.
Fig. 10. Scanning electron micrograph of the 9 o’clock region of a hole after fatigue cycling. Dotted lines indicate the approximate regions sensed by the four MWM-Array sensing elements. The main image covers the entire 5.7 mm thickness of the sample. The inset at the bottom left is a magnified view of the 400 lm crack indicated in the main figure.
1674
J.M. Papazian et al. / International Journal of Fatigue 29 (2007) 1668–1680
Fig. 11. MWM-Array conductivity data for the four regions during the fatigue test of the coupon in Fig. 10.
Fig. 11, and the approximate region sampled by each of the four elements is indicated in Fig. 10. It can be seen that a significant decrease in the conductivity of region 15 was observed, and that this corresponds to the region with the largest crack seen in the SEM photo. Using this test and others, an approximate calibration of total crack length versus conductivity decrease was obtained, and this is shown in Fig. 12a, with an enlarged version of the small crack data in Fig. 12b. These curves represent data from at least 10 two-hole coupons, each of which had 16 active sensing channels. Using these calibration curves, a conversion algorithm was incorporated in the data analysis software that converts the observed change in conductivity into total crack length in the region sampled by the sensor. The algorithm considers conductivity changes less than 0.3% as noise, and does not assign a crack length until this threshold is passed. This noise level was chosen because it was found that the false alarm rate when using this threshold was essentially zero. Fig. 13 shows the data in Fig. 11
a
45
Fig. 13. Conductivity data of Fig. 11 converted to crack length using the calibration algorithm.
converted into crack length data. Exceedance of the noise threshold is apparent in the abrupt jump from 0 to 125 lm between 17,000 and 20,000 cycles for three of the curves. This is not physically realistic, but simply a consequence of the 0.3% threshold. Note that the signal for channel 13 (blue curve), only exceeded the threshold near the end of the test with one indication at 28,000 cycles. 4.2. Ultrasonic sensors Similar calibration experiments were performed using the ultrasonic sensors arranged as shown in Figs. 3 and 4. In this case, optical observations were possible, and the tests were generally run until a large crack (2– 3 mm) was observed in one hole. The specimen was then overloaded to failure so that the crack area could be measured on the fracture surface. The observed crack size characteristics were then compared to the ultrasonic signal, and the best correlation was found for plots of energy ratio
b Percent Reduction in Conductivity
Percent Reduction in Conductivity
40 35 30 25 20 15 10 5 0 -5 0
No Cracks All Crack Data
500 1000 1500 2000 Crack length, microns
2500
3.5 3 2.5
No Cracks Small Cracks Regression Line Regression Line ±50 microns Regression Line ±100 microns
2 1.5 1 0.5 0 -0.5 0
100 200 Crack length, microns
300
Fig. 12. (a) Calibration curve of conductivity change versus crack size for the MWM-Array. (b) Enlarged view of the small crack region of the calibration.
J.M. Papazian et al. / International Journal of Fatigue 29 (2007) 1668–1680
1675
S4-0029, Hole 1
Hole #1
0.8
0.6
IB
OB 2 mm
Ultrasonic Energy Ratio
1
0.4
0.2
0 0
10.000
20.000
30.000
Cycles S4-0029, Hole 2
Hole #2
0.8
0.6
OB
0.4
0.2
0 0
IB 2 mm
Ultrasonic Energy Ratio
1
10.000
20.000
30.000
Cycles
Fig. 14. Ultrasonic energy loss data and corresponding fractographs from a sensor calibration test.
versus crack area. An example of this is shown in Fig. 14 where the ultrasonic energy ratio and fractographs from specimen S4-029 are shown. Using data such as this, a calibration curve that is valid for small cracks was generated and is shown in Fig. 15. Each data point in Fig. 15 represents ultrasonic and fractographic data from an individual hole. From this curve, an algorithm for converting energy loss into crack area was developed. It was also noted that
4.3. Electrochemical fatigue sensor (EFS)
Total Crack Area (sq. mm)
Measurements Linear Fit 2
1
0.27 0
0.6
0.8 Ultrasonic Energy Ratio
the minimum detectable crack area using this technique is approximately 0.35 mm2; for a half-penny shaped crack this corresponds to a surface crack length of 0.95 mm. It was also determined that the energy ratio must fall below 0.95 before a crack can be detected with reasonable certainty. Subsequent experiments have refined these parameters, filled in the calibration curve and established reasons for observation of the outlying points in Fig. 14, details can be found in [9].
0.95
Fig. 15. Ultrasonic energy loss versus crack size calibration curve.
1
Calibration of the EFS sensor was performed using several methods. In one of these, a single edge notch coupon was used in order to facilitate the collection of surface replicas at various stages of the fatigue test without disrupting the test coupon. A cell was fabricated that enclosed the notch, and cycling was continued until a change in the EFS current was observed. At that point the cell was removed and a plastic replica of the surface was made. The test was continued until large cracks were detected. Using replicas from later stages of the test, it was possible to work backward and document the progression of crack growth during the test. Fig. 16 shows the images from a particular crack superimposed on the final image. Fig. 17 shows the evolution of the current density in the ‘‘plastic’’ peak as cycling proceeded. Black arrows mark the points
1676
J.M. Papazian et al. / International Journal of Fatigue 29 (2007) 1668–1680
Fig. 16. Superimposed scanning electron micrographs of replicas showing the evolution of a crack. Image 1 – 100 lm at 56,600 cycles. Image 2 – 300 lm at 68,800 cycles. Image 3 – 900 lm at 83,100 cycles.
where the test was stopped and replicas were made. Generally more than one crack was observed, so several parameters that characterized the observed cracking were examined and plotted against the current density. The most well behaved relationship was observed when the length of the largest crack was plotted against the plastic current density, as shown in Fig. 18. The data in Fig. 18 were obtained from SEM observations of 24 matchstick specimens taken from 6 two-hole tensile coupons. Based on these observations, an algorithm for conversion of the current density in the plastic peak into the maximum crack length was written and implemented in the software. The minimum detectable crack size indicated in this figure is approximately 100 lm. However, as can be seen from the enlarged (lower) scale EFS current plot of Fig. 17, smaller cracks can easily be detected by the electrochemical fatigue sensor. For a crack 100 microns deep, the rate of false positive and false negative indications is essentially zero.
Fig. 17. EFS plastic current versus number of cycles for the sample shown in Fig. 16. The arrows denote times where the test was stopped and replicas taken. The lower curve is an enlarged view of the upper curve.
4.4. Summary The calibration process showed that each of the sensor systems was sensitive to a different geometric characteristic of the small fatigue cracks generated in this study. The MWM-Array (eddy current) conductivity signal correlates best with the total surface lengths of the cracks in the localized measuring area. This can be understood as a consequence of the physical basis of the measurement; the resistance to the flow of eddy currents in the near-surface region of the hole which is disrupted by the presence of cracks. In contrast, the ultrasonic energy loss technique was sensitive to the total area of cracks on both sides of the hole. This can be understood as the interruption of the propagation of ultrasonic energy in the vicinity of the hole by the debonded surfaces of the cracks. Further evidence for this interpretation is provided by monitoring the change in energy loss during individual fatigue loading cycles. The energy loss increases systematically with increasing load during one cycle, and the shape of the curve is indicative of partial crack closure during the early part of the loading cycle. This is discussed more fully in [8,16]. The behavior of the electrochemical fatigue sensor appeared to
Fig. 18. Calibration curve of maximum observed crack length versus plastic current.
J.M. Papazian et al. / International Journal of Fatigue 29 (2007) 1668–1680
5. Multiple sensor tests Several series of fatigue tests were performed in which two types of sensors were mounted on the coupon at the same time, such as can be seen in Fig. 4. The sensor pairs that were examined included the ultrasonic and eddy current sensors as seen in Fig. 4, and the ultrasonic and electrochemical sensors. It was not possible to use the eddy current and electrochemical sensors together because they both need access to the bore of the hole. Note that the various methods were calibrated individually. The ultrasonic sensors are not expected to affect either the eddy current or electrochemical fatigue sensors, but the converse is not true. Since the ultrasonic method relies upon the interaction of the surface creeping wave with the crack, the presence of the other two sensors, both of which make intimate contact with the inner surface of the hole, could have a considerable affect on the ultrasonic data. 5.1. Ultrasonic and eddy current A typical test involved installation of MWM-Array eddy current sensors in the two holes and ultrasonic transducers above and below the holes as shown in Fig. 4 (test 466). The crack lengths measured by the eddy current sensors are shown in Fig. 19 and the crack areas measured by the ultrasonic sensors are shown in Fig. 20. Note that the eddy current data from the four separate windings on each side of each hole have been combined into a composite signal representative of all of the cracks, thus there are four curves in total, one each for each side of the two holes.
Crack Length, microns
5.000
Hole 2, Side A
4.000
3.000
Hole 1, Side B Hole 1, Side A
2.000
Hole 2, Side B 1.000
0 0
10.000
20.000
30.000
40.000
50.000
60.000
Cycles
Fig. 19. MWM-Array data of crack length versus cycles for the joint sensor test 466. Note that each curve represents the sum of crack lengths from one side of one hole.
Specimen 466 1
Hole #1
2.5
Hole #2 2
Crack Area (mm2)
be best correlated with the size of the largest crack in the hole. Since this sensor integrates the effects of cyclic plasticity, it actually responds to the total crack length on both sides of the hole, but its signal correlates well with the size of the largest crack because the crack density increases along with the size of the largest crack, and the longest crack has the largest plastic zone at its tip.
1677
2
1.5
1
0.5
0 0
10.000
20.000
30.000
40.000
50.000
Cycles
Fig. 20. Ultrasonic data of crack area versus cycles for joint test 466.
The data in Fig. 19 indicate that the eddy current sensors first detected cracking on side A of hole 1 at about 24,000 cycles. Shortly thereafter, cracks appeared on the other side of hole 1. Cracking was not noted in hole 2 until approximately 34,000 cycles. The ultrasonic sensor did not indicate measurable cracking until 40,000 cycles in hole 2 and 42,000 cycles in hole 1. The fatigue test was stopped at 53,496 cycles and the sample was overloaded to failure. Optical fractographs of the fracture surfaces are shown in Fig. 21, where the cracks at the sides of the holes are visible. For each image, the lengths of the individual cracks and their areas were measured: they are tabulated in Table 1 where ‘‘2a total fract, mm’’ represents the total length of the cracks intersecting the bore of the hole, ‘‘2a max fract, mm’’ represents the length of the longest crack, and ‘‘A fract, mm2’’ represents the total area of the cracks. These numbers are reported for each side of each hole. Also shown in Table 1 are the equivalent values measured by the sensors. In this case, the eddy current sensor measures ‘‘2a EC, mm’’ which represents the total length of crack on one side of one hole and the ultrasonic sensor measures the sum of the area of cracks on both sides of the holes ‘‘A UT, mm2’’. Comparison of the crack lengths measured by the eddy current sensors and fractography show excellent agreement between the two. Similar comparison between the crack areas measured by fractography and by the ultrasonic sensor shows a less satisfactory agreement; the ultrasonic sensor underestimated the crack area for hole 1 and overestimated the area for hole 2. It is expected that the ultrasonic sensor readings will be less accurate because they are somewhat dependent upon the relative shapes and sizes of the cracks on the two sides of the hole [8]. In addition, the effect of the tight contact of the eddy current sensor with the hole inner diameter on the ultrasonic data is not calibrated or quantified: the expectation is that the
1678
J.M. Papazian et al. / International Journal of Fatigue 29 (2007) 1668–1680
Fig. 21. Fractographs from joint sensor test 466.
Table 1 Crack characteristics from joint sensor tests Hole and side
2a total fract, mm
2a EC, mm
2a max fract, mm
466-1 466-1 466-1 466-2 466-2 466-2
A B A+B A B A+B
4.2 2.0 6.3 1.7 3.0 4.7
4.3 1.5 5.8 1.5 2.9 4.4
4.2 0.9
560-1 560-1 560-1 560-2 560-2 560-2
A B A+B A B A+B
3.9 3.3 7.2 2.3 2.9 5.2
A fract, mm2
2a EFS, mm
4.5 0.6 5.1 0.6 0.8 1.4
1.5 2.3 3.9 2.8
4.8
1.2 1.5
ultrasonic response to a crack would be smaller with the eddy current sensor in place than without it by providing an alternate transmission path for waves which would otherwise be blocked by cracks.
2.5
2.0
3.0 1.9 4.9 0.3 0.7 1.0
1.7
A UT, mm2
1.5
1.0
underestimate for hole 1. The effect of the electrochemical fatigue sensor on the ultrasonic data is also not calibrated or quantified. The expectation is that the gel inside the hole would provide an alternate path of wave propagation and would thus cause the ultrasonic crack size estimates to be
5.2. Ultrasonic and electrochemical fatigue sensor Specimen 560 1.5
1
Hole #1 Hole#2
Crack Area (mm2)
Another joint test paired the ultrasonic sensor with the electrochemical fatigue sensor. The ultrasonic data are shown in Fig. 22, the EFS data in Fig. 23, and the fractographs in Fig. 24. In this case, the EFS sensor indicated the onset of cracking at about 18,000 cycles in hole 1 and 23,000 cycles in hole 2. The ultrasonic sensors first detected cracking at 35,000 cycles in hole 2 and 40,000 cycles in hole 1. When both sensors showed significant indications of cracking, the test was stopped and the coupon was overloaded to failure (at 50,597 cycles) and photographed. Analysis of the fractographs yielded the crack characteristics listed in Table 1. The sensitivity of the EFS extends to the longest crack provided the crack does not become so long as to outstrip the available volume of gel to be sucked into the crack tip; comparison of the EFS measurement to the fractographic one shows good agreement for hole 2 and reasonable agreement for hole 1. The ultrasonic measurements of total crack area were very good for hole 2, but a significant
2
1
0. 5
0 0
10.000
20.000
30.000
40.000
50.000
Cycles
Fig. 22. Ultrasonic data of crack area versus cycles for joint test 560.
J.M. Papazian et al. / International Journal of Fatigue 29 (2007) 1668–1680
1679
capable of detecting 100–125 lm cracks with good reproducibility and good reliability. This represents a significant advance over current NDI capabilities. 6. Discussion
Fig. 23. EFS ‘‘plastic’’ current density plotted on two scales as a function of number of cycles for the joint sensor test 560. The crack length can be determined from a calibration curve such as Fig. 18. For example, at a current density of 0.02 lA/cm2, the maximum crack length is already 150 microns.
understated. Additionally, as the gel penetrates into the crack, the crack would have less of an effect on the ultrasonic response because ultrasonic energy could propagate through the gel-filled crack. 5.3. Multiple sensor test summary The fatigue tests with multiple sensors mounted on the same coupon permitted comparison of the various sensors and illustrated the different fatigue crack characteristics measured by the three sensing techniques. The eddy current sensor measured the total length of cracks on one side of each hole. In general, these measurements agreed well with the actual values. The electrochemical fatigue sensor was quite sensitive to the onset of cracking and was reasonably accurate in its estimate of the size of the largest crack in the hole. The ultrasonic approach was the least sensitive of the three and also appeared to be the least accurate, at least when used in conjunction with the other two sensors. Both the eddy current and the electrochemical sensors were
These tests showed that under laboratory conditions with unrestricted access to a hole and with the ability to monitor cracking continuously during cycling from the beginning of a fatigue test, both the MWM-Array eddy current and the electrochemical sensor systems can detect fatigue cracks in the 100–125 lm range. With the appropriate calibration, these sensors are reasonably accurate, and can follow the increase of crack length as the test progresses. The ultrasonic technique employed in these tests was not as sensitive as the MWM-Array or EFS sensors, but it was also the only sensor that did not have direct access to the internal surface of the hole where the cracks originated. The ultrasonic measurements were also the least accurate of the three systems, almost certainly due to both inadequate dual-sensor calibration and variability in response when used in conjunction with the other sensors inside the hole. When used alone, the ultrasonic method provides reasonable accuracy [9], although not the sensitivity of the other two methods; it has the advantage of not requiring that the sensors be in or near the hole. These experiments also illustrated the different parameters measured by the techniques. In a prognosis system currently under development [20], the different parameters are used to advantage by the reasoning module to provide more accurate information about the state of the material and to select between several plausible model scenarios [20]. For example, while the ultrasonic technique provides a measure of total crack area, it cannot tell whether all of the cracking is on one side of the hole or if it is on both sides. Likewise, the EFS cannot determine if the cracking is on one or both sides of the hole. Combining the ultrasonic data with the MWM-Array data can establish whether the reasoning system should assign all of the crack area to one side of the hole or if it should split it. Knowledge of the crack area in conjunction with knowledge of the
Fig. 24. Fractographs from joint sensor test 560.
1680
J.M. Papazian et al. / International Journal of Fatigue 29 (2007) 1668–1680
crack length can provide an estimate of the crack aspect ratio, an important parameter in some fatigue models. Likewise combining information about the total crack length in a hole with knowledge of the longest crack can also help select between several plausible modeling scenarios. The idealized situation employed in these tests is not generally applicable to depot or field inspection scenarios. The need for access to the inside of a fastener hole (at least for the MWM and EFS sensors) and the requirement that the sensor be in place from the beginning of fatigue exposure eliminates these approaches from consideration. However, they are useful in a laboratory environment as part of a combined sensor-modeling-reasoning approach to development of a prognosis system. Acknowledgements This work is partially sponsored by the Defense Advanced Research Projects Agency under contract HR0011-04-C-0003. Dr. Leo Christodoulou is the DARPA Program Manager. It is a pleasure to acknowledge the valuable contributions of Robert Fidnarick, and Robert Christ, Jr. of Northrop Grumman Corp. References [1] Suresh S. Fatigue of materials. Cambridge, UK: Cambridge University Press; 2001. p. 541. [2] ASIP 2006. Aircraft Structural Integrity Program, San Antonio (TX); 2006. [3] Aging Aircraft 2006. Joint FAA/DoD/NASA Aging Aircraft Conference, Atlanta (GA); 2006. [4] Piotrowski D, Bohler J, Bode M, Moore D, Bakuckas J, Gallella D, et al. Assessment of capabilities and readiness of conventional and emerging NDI methods for detecting subsurface cracks in lap joint structure. In: 9th Joint FAA/DoD/NASA aging aircraft conference, Atlanta (GA), Session 34, March 9; 2006. p. 1–14. [5] Ramakrishnan R, Jury D. Characterization of defects and damage in rivet holes in a crown lap joint of a commercial aircraft at design service goal. In: 9th Joint FAA/DoD/NASA aging aircraft conference, Atlanta (GA), Session 5, March 7; 2006. p. 1–26. [6] Hoffman ME, Hoffman PC. Current and future fatigue life prediction methods for aircraft structures. Naval Res Rev 1998;50(4):4–13.
[7] Zilberstein V, Schlicker D, Walrath K, Weiss V, Goldfine N. MWM eddy current for monitoring of crack initiation and growth during fatigue tests and in service. Int J Fatigue 2001;23:S477–85. [8] Mi B, Michaels JE, Michaels TE. An ultrasonic method for dynamic monitoring of fatigue crack initiation and growth. J Acoust Soc Am 2006;119(1):74–85. [9] Michaels JE, Michaels TE, Mi B. An ultrasonic angle beam method for in situ sizing of fastener hole cracks. J Nondestruct Eval 2006;25(1):3–16. [10] Goldfine N, Zilberstein V, Washabaugh A, Schlicker D, Shay I, Grundy D. Eddy current sensor networks for aircraft fatigue monitoring. Mater Eval 2003;61(7):852–9. [11] Goldfine NJ, Schlicker DE, Washabaugh AP, Zilberstein VA, Tsukernik V. Surface mounted and scanning spatially periodic eddy-current sensor arrays. United States of America Patent no. 6,952,095 B1: JENTEK Sensors, Inc.; 2005. p. 1–86. [12] Zilberstein V, Grundy D, Weiss V, Goldfine N, Abramovici E, Newman J, et al. Early detection and monitoring of fatigue in high strength steels with MWM-Arrays. Int J Fatigue 2005;27:1644–52. [13] Cobb AC, Michaels JE, Mi B, Michaels TE. Ultrasonic monitoring of fastener holes using load modulated energy algorithms for early detection of fatigue cracks. In: Thompson DO, Chimenti DE, editors. Review of progress in quantitative nondestructive evaluation B, vol. 25. American Institute of Physics; 2006. p. 1664–71. [14] Michaels JE, Michaels TE, Mi B, Cobb AC, Stobbe DM. Selfcalibrating ultrasonic methods for in-situ monitoring of fatigue crack progression. Review of progress in nondestructive evaluation B, vol. 24. American Institute of Physics; 2005. p. 1765–72. [15] Cobb AC, Michaels JE, Michaels TE. The effect of transducer placement on the monitoring of fatigue cracks emanating from fastener holes. In: 33rd Annual review of progress in quantitative nondestructive evaluation, vol. 26. Portland (OR); 2006, in press. [16] Michaels JE, Michaels TE, Cobb AC, Kacprzynski GJ. Ultrasonic sensing and life prediction for the DARPA structural integrity prognosis system. In: 33rd Annual review of progress in quantitative nondestructive evaluation, vol. 26. Portland (OR); 2006, in press. [17] Mi B, Michaels TE, Michaels JE. In-situ monitoring of crack growth under static and dynamic loading conditions. In: Schull PJ, Gyekenyesi AL, Mufti AA, editors. SPIE, Health monitoring and smart nondestructive evaluation of structural and biological systems III, vol. 5767. Bellingham (WA); 2005. p. 1–8. [18] Li Y, Laird C. Methods and devices for electrochemically determining metal fatigue status. United States of America Patent No. 5,419,201; 1995. p. 13. [19] Witney A, Li Y-F, Wang J, Wang MZ, DeLuccia JJ, Laird C. Electrochemical fatigue sensor response to Ti–6 wt%Al–4 wt%V and 4130 steel. Phil Mag 2004;84(3–5):331–49. [20] Papazian JM, Anagnostou EL, Engel SJ, Hoitsma D, Madsen J, Silberstein RP, et al. A structural integrity prognosis system. Eng Fract Mech [submitted for publication].
International Journal of Fatigue 29 (2007) 1668–1680
www.elsevier.com/locate/ijfatigue
Sensors for monitoring early stage fatigue cracking John M. Papazian a,*, Jerrell Nardiello a, Robert P. Silberstein a, Greg Welsh b, David Grundy c, Chris Craven c, Leslie Evans c, Neil Goldfine c, Jennifer E. Michaels d, Thomas E. Michaels d, Yuanfeng Li e, Campbell Laird e a
d
Northrop Grumman Integrated Systems, Bethpage, NY 11714, United States b United Technologies Research Center, East Hartford, CT, USA c JENTEK Sensors, Inc., Waltham, MA 02453, United States Georgia Institute of Technology, School of Electrical and Computer Engineering, Atlanta, GA 30332-0250, United States e University of Pennsylvania, Philadelphia, PA 19104, United States Received 8 September 2006; received in revised form 4 January 2007; accepted 8 January 2007 Available online 7 February 2007
Abstract Three sensor systems were evaluated for their ability to detect early stage fatigue cracking in open holes. The sensor systems were (1) a Meandering Winding Magnetometer Array sensor system that induces eddy currents to monitor conductivity changes; (2) a throughtransmission ultrasonic technique that monitors energy loss, and (3) the Electrochemical Fatigue Sensor which detects fatigue-induced changes in a metal surface through their effects on the electrochemical double layer. The sensors were mounted individually or in tandem in samples of 7075-T651 aluminum alloy which were then fatigued using a spectrum loading sequence. The samples were also examined at various stages during fatigue cycling using optical or scanning electron microscopy. Detection thresholds of approximately 100 lm were observed, and calibration curves for crack size in terms of the sensor outputs were obtained. 2007 Elsevier Ltd. All rights reserved. Keywords: Fatigue; Sensors; Early stage cracking; Eddy current; Ultrasonic; Electrochemical
1. Introduction Interest in early stage fatigue has continued to increase with each new generation of observational techniques and with the recognition that short fatigue cracks do not always follow the same laws as long cracks [1]. Likewise, the importance of fatigue cracking to the structural safety of aircraft has become an increasingly serious preoccupation of the world’s aircraft fleet managers, as evidenced by the yearly conferences devoted to this issue (e.g. [2,3]). As part of this interest, the sensing of fatigue cracks for use as a nondestructive inspection technique and for potential use in health monitoring systems is also the subject of much current research [4,5]. Piotrowski et al. [4] recently tested 20 different sensor systems for their ability to detect *
Corresponding author. E-mail address: [email protected] (J.M. Papazian).
0142-1123/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijfatigue.2007.01.023
subsurface cracks in lap joint structures. They chose a threshold value of 3.8 mm at which to compare the techniques, and found that the probability of detection was ‘‘quite low’’ at this value [4]. In the Safe Life approach to aircraft fatigue, the US Navy considers that a primary aircraft structure with a crack of 250 lm has reached the end of its fatigue life [6]. Thus, the Navy threshold is more than an order of magnitude smaller than the threshold used in the Piotrowski study. This comparison is not strictly valid because the Piotrowski study examined cracks in the subsurface layer of lap joints, which are not usually of primary interest in naval aircraft. However, the comparison serves to highlight the gap between current NDI techniques and the need to detect smaller cracks. For naval aircraft, fastener holes are often prime locations for fatigue crack initiation, and some fatigue life tracking is focused on the surveillance of several fastener
J.M. Papazian et al. / International Journal of Fatigue 29 (2007) 1668–1680
1669
hole ‘‘hot spots.’’ Inspection for 250 lm cracks in fastener holes, particularly holes with the fastener in place, is not possible with current technology. However, next generation health monitoring systems, e.g., ‘‘prognosis’’ approaches, will require detailed information about fatigue cracking at this length scale and below. In an attempt to address the need for early stage fatigue crack detection at fastener holes, a series of laboratory experiments was undertaken to evaluate the state-of-theart of currently available sensor systems for cracks in holes. Recent reviews have catalogued most of the available techniques for early stage fatigue crack detection, [7–9] and several of the most applicable and promising of them were selected for evaluation in this study. It was decided to restrict the work to laboratory testing of open-hole 7075T651 aluminum coupons that were designed to be representative of current US Navy interest. The idea was to provide an optimum environment for sensor operation in order to establish the current lower limit for crack detection. The sensor systems were also chosen with an eye toward their potential for eventual use in a fielded system. The three chosen techniques were based on measurements of eddy currents, ultrasonic energy and electrochemical behavior.
designed to mimic the loads encountered by the lower wing covers of a military attack aircraft. The maximum nominal KnF at the holes was approximately 738 MPa, and the spectrum contained nearly negligible compressive loads. Note that the nominal yield stress of this material is approximately 500 MPa, thus local yielding in the high stress region of the fastener hole during the highest load excursions is expected. Sensor data was acquired continuously during the test and the general protocol involved stopping the test when the sensor indicated substantial cracking in the first hole and barely detectable cracking in the second. The samples were then examined in the test machine under a static load using a microscope with oblique illumination (Hirox HiScope), or by removing the sample and observing it in a scanning electron microscope. Some samples were overloaded to failure before removing from the test machine in order to create a fracture surface that contained the largest cracks. However, examination of the bore of the hole generally showed numerous small cracks located above and below the fatal crack.
2. Experimental procedure
Three sensor systems were evaluated. They are described below.
The general approach to these experiments involved mounting the individual sensors on a two-hole coupon of 7075-T651 aluminum alloy and subjecting the coupon to a spectrum fatigue test that resulted in development of fatigue cracks at the 3 and 9 o’clock positions in the holes, where the primary loading direction was along the 12 to 6 o’clock axis. The coupon was a flat, straight sided, rectangular cross section bar, 47.4 mm wide · 350 mm long · 5.7 mm thick. The dimensions were chosen to be representative of a critical area in a lower wing cover of a military aircraft. The coupon was machined from a plate that was originally 50.8 mm thick: the coupon was taken from the near surface region of the plate, its long axis was aligned with the rolling direction and its width and thickness directions were aligned with the width and thickness direction of the starting plate. Two holes, 4.8 mm diameter, were drilled at the centerline, and their centers were 1.27 cm from the edges of the coupon; this spacing resulted in approximately equal local stresses on either side of each hole. Drilling was performed in a milling machine using controlled feeds and speeds and involved making a pilot hole 4.39 mm diameter followed by reaming to final size using a 4.8 mm diameter 6-flute reamer at 450 rpm with a feed rate of 3.8 mm/min. All of the experiments were performed using a single batch of 50.8 mm thick 7075-T651 whose composition (wt. %) was Zn – 5.7, Mg – 2.5, Cu – 1.7, Cr – 0.19, Fe – 0.26, Si – 0.06, Mn – 0.03, Ti – 0.02Ni – 0.003 and V – 0.006. The samples were fatigued in a computer-controlled servo-hydraulic test frame under load control using a 500 h block, flight-by-flight tension-dominated spectrum
3. Sensor systems
3.1. Eddy current (Meandering Winding Magnetometer Array, MWM-Array) The MWM-Array sensor is a product of JENTEK Sensors, Inc., and is based on a refinement of traditional eddy current concepts. The sensors can be miniaturized and can be produced using custom geometries in a thin and flexible configuration. A key aspect of the technology is a rapid data analysis procedure that automates calculation of the two main parameters, lift-off and conductivity. The reader is referred to previous publications for more detailed information [7,10–12]. For this project, a special sensor was designed that fit inside the holes in the fatigue coupon. The sensor consisted of 12 individual sensing elements arranged such that four elements were located in each of the 3, 9 and 12 o’clock regions of the hole, as indicated schematically in Fig. 1a. Only the 3 and 9 o’clock sensing elements are labeled for clarity. Note that four of the sensing elements span the 5.7 mm depth of the hole, thus permitting localization of cracks through the thickness of the coupon. The 12 o’clock sensing elements served as control signals because no cracking was expected in this region. The entire sensor is fabricated on a thin and flexible substrate, and is inserted into the hole by wrapping it into a cylindrical shape and mounted in the hole, as can be seen in Fig. 1b. A soft elastomeric mandrel serves to fix the sensor in position and apply a uniform contact pressure. The raw signal is converted in real time by the JENTEK software and output as curves of conductivity (Fig. 2a) and
1670
J.M. Papazian et al. / International Journal of Fatigue 29 (2007) 1668–1680
Fig. 1. (a) Schematic view of the locations of the sensing elements of the MWM-Array sensor. An additional four sensing elements (not labeled) are located at the 12 o’clock position. (b) MWM-Array sensors installed in the fatigue coupon.
Fig. 2. (a) Conductivity change during a fatigue test for each of four sensing elements on one side of a hole. (b) Lift-off values for the same sensing elements.
lift-off (Fig. 2b). A separate signal is plotted for each of the twelve sensing elements. The changes in the individual values of the conductivity and the lift-off parameters indicate the occurrence of cracking in the local region sensed by the individual sensing elements: conversion of these signals to crack length will be described in a subsequent section. The data sets were synchronized with the test machine cycle count manually for these early tests. This did not result in appreciable errors because 1000 or more data sets were obtained during the 4–6 h tests. Subsequent tests involved faster data accumulation and automated synchronization with the test machine.
detected by monitoring the ratio of the energy received under load (which opens the cracks) to that received under no load (when the cracks are closed). The technique is also capable of measuring the instantaneous load by monitoring changes in the arrival time of the wave, which changes due to both the acoustoelastic effect and dimensional changes
Incident Ultrasound
Re-radiated Creeping Wave
3.2. Ultrasonic sensors The ultrasonic sensor system uses fixed-angle-beam transducers located on each side of the hole in a throughtransmission geometry as shown in Fig. 3. The emitted ultrasonic shear wave is converted to a spiral ‘‘creeping wave’’ at the surface of the hole. Most of the transmitted energy is blocked from reaching the receiver, but the creeping wave ‘‘leaks’’ in the form of a propagating shear wave and thus some energy reaches the receiver. Cracks at or near the surface of the hole attenuate the signal and are
Transmitter
Single “V” Bottom Surface
Receiver
Double “V” Top Surface
Fig. 3. Schematic views of the ultrasonic sensor system showing selected signal paths.
J.M. Papazian et al. / International Journal of Fatigue 29 (2007) 1668–1680
3.3. Electrochemical fatigue sensor (EFS) The electrochemical fatigue sensor was developed at the University of Pennsylvania by Yuanfeng Li and Campbell
Fig. 4. Ultrasonic sensors mounted above and below the holes in a fatigue coupon. MWM-Array sensors are in the holes.
Specimen S4-0464, Hole #2 1
0.8
Ultrasonic Energy Ratio
of the propagation path with load. Further, indications of crack closure are also obtained from the variation of the signal during each individual load cycle, and techniques for self-calibration, sensor integrity and bond quality monitoring have been developed. These issues are dealt with in separate publications, and will not be discussed further here [8,9,13,14]. The transducers are commercially available miniature wedges with a center frequency of 10 MHz and a nominal refracted angle of 70. Fig. 4 shows the ultrasonic transducers mounted above and below the two holes in the fatigue coupon. A fast curing epoxy (Loctite E-05CL) was used as a bonding agent and a mounting process was developed that optimized sensor placement and orientation [15]. At the beginning of this work, the ultrasonic data were acquired by periodically stopping the test and acquiring data under a static load and comparing it to no-load data. The ratio of the energy in the received signal under the two loading conditions was then plotted, as seen in Fig. 5. A reduction in energy ratio is generally indicative of the presence of a crack (or cracks) blocking transmission of part of the signal. This hypothesis was verified and quantified by correlating the total area of cracks (as observed by microscopy) with the changes in energy ratio. The development of the calibration curve is discussed in a subsequent section of this paper. More details of the data analysis procedures can be found in [8,9,13–17]. Further refinement of the ultrasonic technique during the experiential program resulted in the ability to make all of the measurements in real time during the fatigue test, eliminating the need to pause the test and apply a static load for data collection.
1671
0.6
0.4
0.2
0
0
10.000
20.000
30.000
40.000
Cycles
Fig. 5. Typical ultrasonic data plotted as an energy ratio versus cycles. The decrease in energy ratio indicates the presence of fatigue cracks.
Laird. It consists of an electrochemical cell created by attaching a small chamber containing electrolyte and an electrode to the article to be monitored. During fatigue cycling, an oscillating voltage is observed in the cell; this signal has been deconvolved into elastic, plastic and cracking components by a series of controlled experiments [18,19]. The electrochemical cell can be designed in a variety of ways, including simple patch type flexible units. The geometry chosen for these experiments is shown schematically in Fig. 6a. The photo in Fig. 6b shows two cells mounted on the two-hole fatigue coupon. The response of the cell during fatigue cycling is shown in Fig. 7. Fig. 7a shows the expected cell current response associated with elastic strain during cycling at a stress low enough to produce insignificant cyclic plasticity. Fig. 7b shows the expected response produced by cyclic plasticity when fatiguing at a higher stress. The onset of cracking is observed to cause distortions of the signal, thus the three components, elastic deformation, plastic deformation and cracking are usually separable in the composite signal using Fourier analysis. For this work, the very early stages of fatigue damage were of the greatest interest, therefore attention was focused on the plastic portion of the EFS signal. Fig. 8a is a plot of the instantaneous value of the plastic component of the current in the two EFS cells during a fatigue test, and Fig. 8b is the same data plotted as a running average over the prior 2519 cycles. This number was chosen because it is the number of cycles in a fatigue block, so the average was always taken over the same spectrum of loads. The running average was found to facilitate data interpretation by smoothing out sources of electrical and electrochemical noise in the individual signals excited by the load spectrum. The EFS current behavior seen in Fig. 8b is typical; there is an initial decrease as the specimen undergoes plasticity during initial cycling and as passivation develops,
1672
J.M. Papazian et al. / International Journal of Fatigue 29 (2007) 1668–1680
Fig. 6. (a) Schematic view of the electrochemical fatigue sensor. (b) Two EFS cells mounted on a fatigue coupon. The coupon is 5.7 mm thick · 47 mm wide, and the sensors are covering the holes.
b Tension
Time
Neutral
Plastic strain
Elastic strain
a
Tension
Time
Neutral Compression
Time
Current response
Current response
Compression
Time
Fig. 7. (a) Schematic showing the relationship of the observed EFS (elastic) current to the elastic deformation of the specimen during fatigue cycling. (b) Schematic showing the relationship of the observed EFS (plastic) current to the plastic deformation of the specimen during fatigue cycling.
Fig. 8. (a) Instantaneous value of the plastic current during a fatigue test. Data from two cells are shown. (b) Running average of the plastic current for the same two cells. The average was taken over the prior 2600 cycles (one block).
then the curve flattens as both hardening and passivation saturate but damage has not yet had an opportunity to develop significantly, and finally the current increases with gradually increasing rate as fatigue damage, dominated by cracking, accumulates.
4. Fatigue testing Several series of fatigue tests were performed. The initial tests were run with only one type of sensor installed in the test coupon, while subsequent tests used two types of
J.M. Papazian et al. / International Journal of Fatigue 29 (2007) 1668–1680
sensors mounted on the same coupon concurrently. The objective of these tests was to establish calibration curves for crack length versus sensor signal for each sensor and to get, in a qualitative manner, an understanding of the validity of each sensor in terms of its rate of false positive and false negative indications. 4.1. Eddy current (Meandering Winding Magnetometer Array, MWM-Array) A typical test involved installation of a sensor in each of the two holes in the fatigue coupon, imposing a flight-byflight block loading spectrum on the coupon, and monitoring the sensor signals. Some tests were continued until all four high-stress locations (the inboard and outboard sides of the two holes) showed some indications of cracking, while others were stopped when the second or third holeside reached a predetermined conductivity change. In this manner, a range of fatigue cracks was observed in the four high-stress regions, with some regions being significantly more advanced than others. In general, the least advanced regions were of interest because they provided the most stringent test of sensor sensitivity while the more advanced
1673
regions provided indications of which crack characteristics were being measured by the sensor. Typically, the bore of the hole was monitored using a microscope with oblique illumination and a rotating mirror (Hirox HiScope), however this was not possible while the MWM-Array sensor was in the hole. Removal of the sensor would necessitate recalibration and loss of the prior baseline. Therefore, no observations of cracks were possible until the test was stopped and the sensor was removed. At that time, the hole was examined for cracks using optical microscopy. Subsequently, the sample was sectioned into four ‘‘matchsticks’’ each of which contained one of the high stress regions, as can be seen in Fig. 9. Fig. 10 shows a SEM micrograph of the 1.2 · 5.7 mm high-stress region of the outboard side of hole #2 in specimen S4-0346 after 28,073 fatigue cycles. The largest crack was approximately 400 lm and is shown in the inset. Close examination of the coupon showed numerous smaller cracks and that the cracks appear to have initiated at constituent particles. This is the usual situation for 7075, and was a consistent observation in this work. The MWM-Array data from the four sensing elements on this side of the hole are shown in
Fig. 9. Schematic view of the preparation of ‘‘matchstick’’ specimens and the mounting device for examination under load. When mounted in the SEM, this permits examination of cracking in the high stress surface region of the hole.
Fig. 10. Scanning electron micrograph of the 9 o’clock region of a hole after fatigue cycling. Dotted lines indicate the approximate regions sensed by the four MWM-Array sensing elements. The main image covers the entire 5.7 mm thickness of the sample. The inset at the bottom left is a magnified view of the 400 lm crack indicated in the main figure.
1674
J.M. Papazian et al. / International Journal of Fatigue 29 (2007) 1668–1680
Fig. 11. MWM-Array conductivity data for the four regions during the fatigue test of the coupon in Fig. 10.
Fig. 11, and the approximate region sampled by each of the four elements is indicated in Fig. 10. It can be seen that a significant decrease in the conductivity of region 15 was observed, and that this corresponds to the region with the largest crack seen in the SEM photo. Using this test and others, an approximate calibration of total crack length versus conductivity decrease was obtained, and this is shown in Fig. 12a, with an enlarged version of the small crack data in Fig. 12b. These curves represent data from at least 10 two-hole coupons, each of which had 16 active sensing channels. Using these calibration curves, a conversion algorithm was incorporated in the data analysis software that converts the observed change in conductivity into total crack length in the region sampled by the sensor. The algorithm considers conductivity changes less than 0.3% as noise, and does not assign a crack length until this threshold is passed. This noise level was chosen because it was found that the false alarm rate when using this threshold was essentially zero. Fig. 13 shows the data in Fig. 11
a
45
Fig. 13. Conductivity data of Fig. 11 converted to crack length using the calibration algorithm.
converted into crack length data. Exceedance of the noise threshold is apparent in the abrupt jump from 0 to 125 lm between 17,000 and 20,000 cycles for three of the curves. This is not physically realistic, but simply a consequence of the 0.3% threshold. Note that the signal for channel 13 (blue curve), only exceeded the threshold near the end of the test with one indication at 28,000 cycles. 4.2. Ultrasonic sensors Similar calibration experiments were performed using the ultrasonic sensors arranged as shown in Figs. 3 and 4. In this case, optical observations were possible, and the tests were generally run until a large crack (2– 3 mm) was observed in one hole. The specimen was then overloaded to failure so that the crack area could be measured on the fracture surface. The observed crack size characteristics were then compared to the ultrasonic signal, and the best correlation was found for plots of energy ratio
b Percent Reduction in Conductivity
Percent Reduction in Conductivity
40 35 30 25 20 15 10 5 0 -5 0
No Cracks All Crack Data
500 1000 1500 2000 Crack length, microns
2500
3.5 3 2.5
No Cracks Small Cracks Regression Line Regression Line ±50 microns Regression Line ±100 microns
2 1.5 1 0.5 0 -0.5 0
100 200 Crack length, microns
300
Fig. 12. (a) Calibration curve of conductivity change versus crack size for the MWM-Array. (b) Enlarged view of the small crack region of the calibration.
J.M. Papazian et al. / International Journal of Fatigue 29 (2007) 1668–1680
1675
S4-0029, Hole 1
Hole #1
0.8
0.6
IB
OB 2 mm
Ultrasonic Energy Ratio
1
0.4
0.2
0 0
10.000
20.000
30.000
Cycles S4-0029, Hole 2
Hole #2
0.8
0.6
OB
0.4
0.2
0 0
IB 2 mm
Ultrasonic Energy Ratio
1
10.000
20.000
30.000
Cycles
Fig. 14. Ultrasonic energy loss data and corresponding fractographs from a sensor calibration test.
versus crack area. An example of this is shown in Fig. 14 where the ultrasonic energy ratio and fractographs from specimen S4-029 are shown. Using data such as this, a calibration curve that is valid for small cracks was generated and is shown in Fig. 15. Each data point in Fig. 15 represents ultrasonic and fractographic data from an individual hole. From this curve, an algorithm for converting energy loss into crack area was developed. It was also noted that
4.3. Electrochemical fatigue sensor (EFS)
Total Crack Area (sq. mm)
Measurements Linear Fit 2
1
0.27 0
0.6
0.8 Ultrasonic Energy Ratio
the minimum detectable crack area using this technique is approximately 0.35 mm2; for a half-penny shaped crack this corresponds to a surface crack length of 0.95 mm. It was also determined that the energy ratio must fall below 0.95 before a crack can be detected with reasonable certainty. Subsequent experiments have refined these parameters, filled in the calibration curve and established reasons for observation of the outlying points in Fig. 14, details can be found in [9].
0.95
Fig. 15. Ultrasonic energy loss versus crack size calibration curve.
1
Calibration of the EFS sensor was performed using several methods. In one of these, a single edge notch coupon was used in order to facilitate the collection of surface replicas at various stages of the fatigue test without disrupting the test coupon. A cell was fabricated that enclosed the notch, and cycling was continued until a change in the EFS current was observed. At that point the cell was removed and a plastic replica of the surface was made. The test was continued until large cracks were detected. Using replicas from later stages of the test, it was possible to work backward and document the progression of crack growth during the test. Fig. 16 shows the images from a particular crack superimposed on the final image. Fig. 17 shows the evolution of the current density in the ‘‘plastic’’ peak as cycling proceeded. Black arrows mark the points
1676
J.M. Papazian et al. / International Journal of Fatigue 29 (2007) 1668–1680
Fig. 16. Superimposed scanning electron micrographs of replicas showing the evolution of a crack. Image 1 – 100 lm at 56,600 cycles. Image 2 – 300 lm at 68,800 cycles. Image 3 – 900 lm at 83,100 cycles.
where the test was stopped and replicas were made. Generally more than one crack was observed, so several parameters that characterized the observed cracking were examined and plotted against the current density. The most well behaved relationship was observed when the length of the largest crack was plotted against the plastic current density, as shown in Fig. 18. The data in Fig. 18 were obtained from SEM observations of 24 matchstick specimens taken from 6 two-hole tensile coupons. Based on these observations, an algorithm for conversion of the current density in the plastic peak into the maximum crack length was written and implemented in the software. The minimum detectable crack size indicated in this figure is approximately 100 lm. However, as can be seen from the enlarged (lower) scale EFS current plot of Fig. 17, smaller cracks can easily be detected by the electrochemical fatigue sensor. For a crack 100 microns deep, the rate of false positive and false negative indications is essentially zero.
Fig. 17. EFS plastic current versus number of cycles for the sample shown in Fig. 16. The arrows denote times where the test was stopped and replicas taken. The lower curve is an enlarged view of the upper curve.
4.4. Summary The calibration process showed that each of the sensor systems was sensitive to a different geometric characteristic of the small fatigue cracks generated in this study. The MWM-Array (eddy current) conductivity signal correlates best with the total surface lengths of the cracks in the localized measuring area. This can be understood as a consequence of the physical basis of the measurement; the resistance to the flow of eddy currents in the near-surface region of the hole which is disrupted by the presence of cracks. In contrast, the ultrasonic energy loss technique was sensitive to the total area of cracks on both sides of the hole. This can be understood as the interruption of the propagation of ultrasonic energy in the vicinity of the hole by the debonded surfaces of the cracks. Further evidence for this interpretation is provided by monitoring the change in energy loss during individual fatigue loading cycles. The energy loss increases systematically with increasing load during one cycle, and the shape of the curve is indicative of partial crack closure during the early part of the loading cycle. This is discussed more fully in [8,16]. The behavior of the electrochemical fatigue sensor appeared to
Fig. 18. Calibration curve of maximum observed crack length versus plastic current.
J.M. Papazian et al. / International Journal of Fatigue 29 (2007) 1668–1680
5. Multiple sensor tests Several series of fatigue tests were performed in which two types of sensors were mounted on the coupon at the same time, such as can be seen in Fig. 4. The sensor pairs that were examined included the ultrasonic and eddy current sensors as seen in Fig. 4, and the ultrasonic and electrochemical sensors. It was not possible to use the eddy current and electrochemical sensors together because they both need access to the bore of the hole. Note that the various methods were calibrated individually. The ultrasonic sensors are not expected to affect either the eddy current or electrochemical fatigue sensors, but the converse is not true. Since the ultrasonic method relies upon the interaction of the surface creeping wave with the crack, the presence of the other two sensors, both of which make intimate contact with the inner surface of the hole, could have a considerable affect on the ultrasonic data. 5.1. Ultrasonic and eddy current A typical test involved installation of MWM-Array eddy current sensors in the two holes and ultrasonic transducers above and below the holes as shown in Fig. 4 (test 466). The crack lengths measured by the eddy current sensors are shown in Fig. 19 and the crack areas measured by the ultrasonic sensors are shown in Fig. 20. Note that the eddy current data from the four separate windings on each side of each hole have been combined into a composite signal representative of all of the cracks, thus there are four curves in total, one each for each side of the two holes.
Crack Length, microns
5.000
Hole 2, Side A
4.000
3.000
Hole 1, Side B Hole 1, Side A
2.000
Hole 2, Side B 1.000
0 0
10.000
20.000
30.000
40.000
50.000
60.000
Cycles
Fig. 19. MWM-Array data of crack length versus cycles for the joint sensor test 466. Note that each curve represents the sum of crack lengths from one side of one hole.
Specimen 466 1
Hole #1
2.5
Hole #2 2
Crack Area (mm2)
be best correlated with the size of the largest crack in the hole. Since this sensor integrates the effects of cyclic plasticity, it actually responds to the total crack length on both sides of the hole, but its signal correlates well with the size of the largest crack because the crack density increases along with the size of the largest crack, and the longest crack has the largest plastic zone at its tip.
1677
2
1.5
1
0.5
0 0
10.000
20.000
30.000
40.000
50.000
Cycles
Fig. 20. Ultrasonic data of crack area versus cycles for joint test 466.
The data in Fig. 19 indicate that the eddy current sensors first detected cracking on side A of hole 1 at about 24,000 cycles. Shortly thereafter, cracks appeared on the other side of hole 1. Cracking was not noted in hole 2 until approximately 34,000 cycles. The ultrasonic sensor did not indicate measurable cracking until 40,000 cycles in hole 2 and 42,000 cycles in hole 1. The fatigue test was stopped at 53,496 cycles and the sample was overloaded to failure. Optical fractographs of the fracture surfaces are shown in Fig. 21, where the cracks at the sides of the holes are visible. For each image, the lengths of the individual cracks and their areas were measured: they are tabulated in Table 1 where ‘‘2a total fract, mm’’ represents the total length of the cracks intersecting the bore of the hole, ‘‘2a max fract, mm’’ represents the length of the longest crack, and ‘‘A fract, mm2’’ represents the total area of the cracks. These numbers are reported for each side of each hole. Also shown in Table 1 are the equivalent values measured by the sensors. In this case, the eddy current sensor measures ‘‘2a EC, mm’’ which represents the total length of crack on one side of one hole and the ultrasonic sensor measures the sum of the area of cracks on both sides of the holes ‘‘A UT, mm2’’. Comparison of the crack lengths measured by the eddy current sensors and fractography show excellent agreement between the two. Similar comparison between the crack areas measured by fractography and by the ultrasonic sensor shows a less satisfactory agreement; the ultrasonic sensor underestimated the crack area for hole 1 and overestimated the area for hole 2. It is expected that the ultrasonic sensor readings will be less accurate because they are somewhat dependent upon the relative shapes and sizes of the cracks on the two sides of the hole [8]. In addition, the effect of the tight contact of the eddy current sensor with the hole inner diameter on the ultrasonic data is not calibrated or quantified: the expectation is that the
1678
J.M. Papazian et al. / International Journal of Fatigue 29 (2007) 1668–1680
Fig. 21. Fractographs from joint sensor test 466.
Table 1 Crack characteristics from joint sensor tests Hole and side
2a total fract, mm
2a EC, mm
2a max fract, mm
466-1 466-1 466-1 466-2 466-2 466-2
A B A+B A B A+B
4.2 2.0 6.3 1.7 3.0 4.7
4.3 1.5 5.8 1.5 2.9 4.4
4.2 0.9
560-1 560-1 560-1 560-2 560-2 560-2
A B A+B A B A+B
3.9 3.3 7.2 2.3 2.9 5.2
A fract, mm2
2a EFS, mm
4.5 0.6 5.1 0.6 0.8 1.4
1.5 2.3 3.9 2.8
4.8
1.2 1.5
ultrasonic response to a crack would be smaller with the eddy current sensor in place than without it by providing an alternate transmission path for waves which would otherwise be blocked by cracks.
2.5
2.0
3.0 1.9 4.9 0.3 0.7 1.0
1.7
A UT, mm2
1.5
1.0
underestimate for hole 1. The effect of the electrochemical fatigue sensor on the ultrasonic data is also not calibrated or quantified. The expectation is that the gel inside the hole would provide an alternate path of wave propagation and would thus cause the ultrasonic crack size estimates to be
5.2. Ultrasonic and electrochemical fatigue sensor Specimen 560 1.5
1
Hole #1 Hole#2
Crack Area (mm2)
Another joint test paired the ultrasonic sensor with the electrochemical fatigue sensor. The ultrasonic data are shown in Fig. 22, the EFS data in Fig. 23, and the fractographs in Fig. 24. In this case, the EFS sensor indicated the onset of cracking at about 18,000 cycles in hole 1 and 23,000 cycles in hole 2. The ultrasonic sensors first detected cracking at 35,000 cycles in hole 2 and 40,000 cycles in hole 1. When both sensors showed significant indications of cracking, the test was stopped and the coupon was overloaded to failure (at 50,597 cycles) and photographed. Analysis of the fractographs yielded the crack characteristics listed in Table 1. The sensitivity of the EFS extends to the longest crack provided the crack does not become so long as to outstrip the available volume of gel to be sucked into the crack tip; comparison of the EFS measurement to the fractographic one shows good agreement for hole 2 and reasonable agreement for hole 1. The ultrasonic measurements of total crack area were very good for hole 2, but a significant
2
1
0. 5
0 0
10.000
20.000
30.000
40.000
50.000
Cycles
Fig. 22. Ultrasonic data of crack area versus cycles for joint test 560.
J.M. Papazian et al. / International Journal of Fatigue 29 (2007) 1668–1680
1679
capable of detecting 100–125 lm cracks with good reproducibility and good reliability. This represents a significant advance over current NDI capabilities. 6. Discussion
Fig. 23. EFS ‘‘plastic’’ current density plotted on two scales as a function of number of cycles for the joint sensor test 560. The crack length can be determined from a calibration curve such as Fig. 18. For example, at a current density of 0.02 lA/cm2, the maximum crack length is already 150 microns.
understated. Additionally, as the gel penetrates into the crack, the crack would have less of an effect on the ultrasonic response because ultrasonic energy could propagate through the gel-filled crack. 5.3. Multiple sensor test summary The fatigue tests with multiple sensors mounted on the same coupon permitted comparison of the various sensors and illustrated the different fatigue crack characteristics measured by the three sensing techniques. The eddy current sensor measured the total length of cracks on one side of each hole. In general, these measurements agreed well with the actual values. The electrochemical fatigue sensor was quite sensitive to the onset of cracking and was reasonably accurate in its estimate of the size of the largest crack in the hole. The ultrasonic approach was the least sensitive of the three and also appeared to be the least accurate, at least when used in conjunction with the other two sensors. Both the eddy current and the electrochemical sensors were
These tests showed that under laboratory conditions with unrestricted access to a hole and with the ability to monitor cracking continuously during cycling from the beginning of a fatigue test, both the MWM-Array eddy current and the electrochemical sensor systems can detect fatigue cracks in the 100–125 lm range. With the appropriate calibration, these sensors are reasonably accurate, and can follow the increase of crack length as the test progresses. The ultrasonic technique employed in these tests was not as sensitive as the MWM-Array or EFS sensors, but it was also the only sensor that did not have direct access to the internal surface of the hole where the cracks originated. The ultrasonic measurements were also the least accurate of the three systems, almost certainly due to both inadequate dual-sensor calibration and variability in response when used in conjunction with the other sensors inside the hole. When used alone, the ultrasonic method provides reasonable accuracy [9], although not the sensitivity of the other two methods; it has the advantage of not requiring that the sensors be in or near the hole. These experiments also illustrated the different parameters measured by the techniques. In a prognosis system currently under development [20], the different parameters are used to advantage by the reasoning module to provide more accurate information about the state of the material and to select between several plausible model scenarios [20]. For example, while the ultrasonic technique provides a measure of total crack area, it cannot tell whether all of the cracking is on one side of the hole or if it is on both sides. Likewise, the EFS cannot determine if the cracking is on one or both sides of the hole. Combining the ultrasonic data with the MWM-Array data can establish whether the reasoning system should assign all of the crack area to one side of the hole or if it should split it. Knowledge of the crack area in conjunction with knowledge of the
Fig. 24. Fractographs from joint sensor test 560.
1680
J.M. Papazian et al. / International Journal of Fatigue 29 (2007) 1668–1680
crack length can provide an estimate of the crack aspect ratio, an important parameter in some fatigue models. Likewise combining information about the total crack length in a hole with knowledge of the longest crack can also help select between several plausible modeling scenarios. The idealized situation employed in these tests is not generally applicable to depot or field inspection scenarios. The need for access to the inside of a fastener hole (at least for the MWM and EFS sensors) and the requirement that the sensor be in place from the beginning of fatigue exposure eliminates these approaches from consideration. However, they are useful in a laboratory environment as part of a combined sensor-modeling-reasoning approach to development of a prognosis system. Acknowledgements This work is partially sponsored by the Defense Advanced Research Projects Agency under contract HR0011-04-C-0003. Dr. Leo Christodoulou is the DARPA Program Manager. It is a pleasure to acknowledge the valuable contributions of Robert Fidnarick, and Robert Christ, Jr. of Northrop Grumman Corp. References [1] Suresh S. Fatigue of materials. Cambridge, UK: Cambridge University Press; 2001. p. 541. [2] ASIP 2006. Aircraft Structural Integrity Program, San Antonio (TX); 2006. [3] Aging Aircraft 2006. Joint FAA/DoD/NASA Aging Aircraft Conference, Atlanta (GA); 2006. [4] Piotrowski D, Bohler J, Bode M, Moore D, Bakuckas J, Gallella D, et al. Assessment of capabilities and readiness of conventional and emerging NDI methods for detecting subsurface cracks in lap joint structure. In: 9th Joint FAA/DoD/NASA aging aircraft conference, Atlanta (GA), Session 34, March 9; 2006. p. 1–14. [5] Ramakrishnan R, Jury D. Characterization of defects and damage in rivet holes in a crown lap joint of a commercial aircraft at design service goal. In: 9th Joint FAA/DoD/NASA aging aircraft conference, Atlanta (GA), Session 5, March 7; 2006. p. 1–26. [6] Hoffman ME, Hoffman PC. Current and future fatigue life prediction methods for aircraft structures. Naval Res Rev 1998;50(4):4–13.
[7] Zilberstein V, Schlicker D, Walrath K, Weiss V, Goldfine N. MWM eddy current for monitoring of crack initiation and growth during fatigue tests and in service. Int J Fatigue 2001;23:S477–85. [8] Mi B, Michaels JE, Michaels TE. An ultrasonic method for dynamic monitoring of fatigue crack initiation and growth. J Acoust Soc Am 2006;119(1):74–85. [9] Michaels JE, Michaels TE, Mi B. An ultrasonic angle beam method for in situ sizing of fastener hole cracks. J Nondestruct Eval 2006;25(1):3–16. [10] Goldfine N, Zilberstein V, Washabaugh A, Schlicker D, Shay I, Grundy D. Eddy current sensor networks for aircraft fatigue monitoring. Mater Eval 2003;61(7):852–9. [11] Goldfine NJ, Schlicker DE, Washabaugh AP, Zilberstein VA, Tsukernik V. Surface mounted and scanning spatially periodic eddy-current sensor arrays. United States of America Patent no. 6,952,095 B1: JENTEK Sensors, Inc.; 2005. p. 1–86. [12] Zilberstein V, Grundy D, Weiss V, Goldfine N, Abramovici E, Newman J, et al. Early detection and monitoring of fatigue in high strength steels with MWM-Arrays. Int J Fatigue 2005;27:1644–52. [13] Cobb AC, Michaels JE, Mi B, Michaels TE. Ultrasonic monitoring of fastener holes using load modulated energy algorithms for early detection of fatigue cracks. In: Thompson DO, Chimenti DE, editors. Review of progress in quantitative nondestructive evaluation B, vol. 25. American Institute of Physics; 2006. p. 1664–71. [14] Michaels JE, Michaels TE, Mi B, Cobb AC, Stobbe DM. Selfcalibrating ultrasonic methods for in-situ monitoring of fatigue crack progression. Review of progress in nondestructive evaluation B, vol. 24. American Institute of Physics; 2005. p. 1765–72. [15] Cobb AC, Michaels JE, Michaels TE. The effect of transducer placement on the monitoring of fatigue cracks emanating from fastener holes. In: 33rd Annual review of progress in quantitative nondestructive evaluation, vol. 26. Portland (OR); 2006, in press. [16] Michaels JE, Michaels TE, Cobb AC, Kacprzynski GJ. Ultrasonic sensing and life prediction for the DARPA structural integrity prognosis system. In: 33rd Annual review of progress in quantitative nondestructive evaluation, vol. 26. Portland (OR); 2006, in press. [17] Mi B, Michaels TE, Michaels JE. In-situ monitoring of crack growth under static and dynamic loading conditions. In: Schull PJ, Gyekenyesi AL, Mufti AA, editors. SPIE, Health monitoring and smart nondestructive evaluation of structural and biological systems III, vol. 5767. Bellingham (WA); 2005. p. 1–8. [18] Li Y, Laird C. Methods and devices for electrochemically determining metal fatigue status. United States of America Patent No. 5,419,201; 1995. p. 13. [19] Witney A, Li Y-F, Wang J, Wang MZ, DeLuccia JJ, Laird C. Electrochemical fatigue sensor response to Ti–6 wt%Al–4 wt%V and 4130 steel. Phil Mag 2004;84(3–5):331–49. [20] Papazian JM, Anagnostou EL, Engel SJ, Hoitsma D, Madsen J, Silberstein RP, et al. A structural integrity prognosis system. Eng Fract Mech [submitted for publication].