May 2014 

NASA/TM–2014-218272 

Mitigation of Crack Damage in Metallic 

Materials 

Patrick E. Leser 

North Carolina State University, Raleigh, North Carolina 

John A. Newman, Stephen W. Smith, William P. Leser, Russell A. Wincheski, 

Terryl A. Wallace, Edward H. Glaessgen, and Robert S. Piascik 

Langley Research Center, Hampton, Virginia 
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May 2014 

NASA/TM–2014-218272 

Mitigation of Crack Damage in Metallic 

Materials 

.

Patrick E. Leser 

North Carolina State University, Raleigh, North Carolina 

John A. Newman, Stephen W. Smith, William P. Leser, Russell A. Wincheski, 

Terryl A. Wallace, Edward H. Glaessgen, and Robert S. Piascik 

Langley Research Center, Hampton, Virginia 
Available from: 

NASA Center for AeroSpace Information 

7115 Standard Drive 

Hanover, MD 21076-1320 

443-757-5802 

Acknowledgments 

The research described in this document was funded by the NASA Vehicle Systems 

Safety Technologies (VSST) Program. The authors wish to express gratitude to Rick 

Ross (VSST, Langley Research Center) for his efforts in supporting this research. One 

author (Patrick Leser) supported this research while participating in the Langley 

Aerospace Research Summer Scholar (LARSS) program as a graduate student at North 

Carolina State University. Another author (W.P. “Paul” Leser) supported this research 

while participating in the NASA Pathways program. Finally, the authors wish to express 

gratitude to the laboratory support provided by William T. “Tommy” Howard (NASA 

retired), Jim Baughman (Analytical Mechanics Association), H.D. “Clay” Claytor 

(Analytical Mechanics Association), Joel Alexa (Analytical Mechanics Association), and 

Greg Shanks (NASA). The work described in this document is covered by a U.S. patent 

(publication number: US8347479 B2, Title “Method for Repairing Cracks in Structures,” 

publication date: January 8, 2013). 
2

Abstract

A system designed to mitigate or heal crack damage in metallic 

materials has been developed where the protected material or 

component is coated with a low-melting temperature film. After a 

crack is formed, the material is heated, melting the film which then 

infiltrates the crack opening through capillary action. Upon 

solidification, the healing material inhibits further crack damage in 

two ways. While the crack healing material is intact, it acts like an 

adhesive that bonds or bridges the crack faces together. After 

fatigue loading damages, the healing material in the crack mouth 

inhibits further crack growth by creating artificially-high crack 

closure levels. Mechanical test data show that this method 

sucessfully arrests or retards crack growth in laboratory 

specimens.

Introduction

Damage tolerance concepts have been applied to aerospace vehicles for decades to 

improve safety. Here, the remaining service life is calculated using fatigue crack growth models 

based on service loads, material, component geometry, and crack size as determined from 

periodic vehicle inspections. If no cracks are detected, the largest crack that could escape 

detection, based on the probability of detection (POD) for the inspection method used, is 

assumed to exist. The remaining useful service life can then be calculated and used to 

establish inspection intervals such that multiple inspections would be performed as a crack 

propagates from an inspection threshold to a critical size (i.e., where component failure would 

occur). This damage tolerance methodology has proven very successful for many applications 

but is not yet practical for all aerospace components, especially components that are difficult to 

inspect and those subjected to high-cycle fatigue loading. For high-cycle fatigue loading, the 

number and severity of load cycles per flight hour is such that the portion of service life with a 

detectable crack is too small to be divided into practical inspection intervals. A novel solution to 

this issue is provided by the authors, where fatigue cracks are “healed” by the introduction of a 

foreign material into the crack mouth, thus slowing or even stopping crack growth and extending 

the portion of service life associated with the propagation of microstructurally-long cracks (ref. 

1). When a crack is detected on an aerospace structure via a diagnostic sensor network or 

through nondestructive evaluation (NDE), the healing material, which has been applied directly 

to the structural material as a thin-film coating, is locally activated by heat, allowing the healing 

material to flow into the crack, as depicted in Figure 1a. Upon solidification, the coating reduces 

the crack driving force by adhering the crack surfaces together, a mechanism termed “crack 

bridging” (see Figure 1b) and by leaving voluminous material in the crack wake, an artificial form 

of crack closure (see Figure 1c). 
3

Figure 1. A schematic of the crack healing procedure is shown. The dotted lines represent the profile of 

the crack without the presence of healing material in the mouth.

Previous crack healing efforts

The alleviation of crack damage in structures is mostly limited to invasive and costly 

maintenance methods (e.g., patching). If the damage is too severe or too many repairs have 

been made previously, total component replacement is required. These types of repairs 

contribute to aircraft empty weight since excess material is almost always required to perform a 

safe repair (ref. 2). It is for these reasons that investigations into new, low-weight methods to 

treat or “heal” damage with minimal invasiveness have begun to emerge. Many advances have 

already been made in self-healing composites and polymers (refs. 3-8), but the primary 

methods used in these fields to deliver the healing agent to the damaged area are not 

applicable to metals or alloys due to the difficulties and impracticalities of embedding in a 

metallic matrix (ref. 9). As a result, it was not until recently that researchers began to focus on 

the self-healing of metallic materials. Manuel, et al., developed a method that involved the 

introduction of shape memory alloy (SMA) wires into a metallic matrix (refs. 10-12). Upon crack 

propagation, the SMA wires were used to induce crack closure, pulling the crack surfaces 

together and applying a compressive force. The entire specimen was then heated until limited 

surface melting occurred; after cooling, the matrix crack had been essentially removed. While 

the method seems promising, it is currently not applicable to “commonly used” metals and 

(b) Crack bridging (c) Crack closure
4

alloys, such as aerospace-grade aluminum and titanium (refs. 10-12). Another proposed 

healing method includes the delivery of a healing agent via embedded ceramic tubes that are 

exposed by crack propagation and activated through heating (ref. 13). As mentioned 

previously, the complexity of an embedded network to deliver the healing agent presents 

complications, primarily through degradation to the overall metallic material properties as well as 

manufacturing challenges. 

Solid state self-healing concepts have also been proposed. One such method involves the 

self-healing of underaged aluminum through dynamic precipitation (i.e., solute transported to 

open volumes, such as internal cracks and voids) during creep and fatigue (refs. 14-16). 

Further contributions were made with positron annihilation spectroscopy studies, which allowed 

for a deeper understanding of how solute atoms are driven to areas where crack initiation may 

occur (refs. 17, 18). However, a study by Wanhill (ref. 19) suggests that precipitate-based solid 

state healing is not practical for real-world aerospace applications. He argues that most aircraft 

damage will either be too large to be healed by dynamic precipitation (which is mostly limited to 

healing slip band cracks on the scale of 1.4 μm or smaller) or will be located on the surface of 

the component, where environmental effects could potentially hinder crack healing (ref. 19). 

Proposed healing agent

The proposed crack healing process described previously (Figure 1) is novel in that it is 

designed for metallic materials and, because it relies on a surface coating, is readily applicable 

to existing aerospace components. Since this method is intended for aerospace metallic 

materials (e.g., aluminum and titanium), the healing material is to have a melting temperature 

well below that which would cause aging of the protected structural material over any potential

healing times. For this study, the selected healing film was a near-eutectic composition of 

Indium-Tin (In-Sn), which was roughly 60% Indium and 40% Tin by weight (ref. 20). As seen in 

the phase diagram of the In-Sn system, given in Figure 2, the melting temperature of the target 

healing material is approximately 124°C. Since the sputtering process is based on momentum 

transfer and In and Sn vary in atomic weight by 2%, a slight variation in initial and deposited 

chemistry is expected. The 60/40 In-Sn chemistry, which is slightly Indium rich of the eutectic 

composition, was selected to reduce the possibility of creating a local Tin-rich chemistry, which 

would increase the melting point significantly as a function of composition. In contrast, the 

Indium-rich side of the eutectic composition has a shallow liquid-solid slope and is generally 

more forgiving. 

Specimen Preparation

Fatigue crack growth testing was done using the eccentrically-loaded single-edge notch 

tension (ESE(T)) specimen (refs. 21, 22). A schematic of the ESE(T) specimen is shown in

Figure 3. Testing described in this document was done with specimens made from Titanium 

alloy 6-2-2-2-2 sheet material approximately 0.080 inches (2.03 mm) thick. After machining by 

EDM wire, specimen surfaces were mechanically polished to a near-mirror finish on both sides 

before a back-face strain gage was applied to the specimen. Back-face strain gages were used 

to monitor crack length during fatigue crack growth testing (refs. 23, 24). The In-Sn healing 

material was then deposited on either one or two sides of the specimen using a radio frequency 
5

(RF) sputter depostion technique. A deposition rate of ~1 μm/hr for 5-15 hours was used with a 

power of 250 W in 1.1 Pa (8 mTorr) Argon environment, resulting in film thicknesses on the

order of ~5-15 μm. Upon completion of the film deposition, the specimens were annealed at 

135°C for thirty minutes to help consolidate the film. A more detailed description of the sputter 

coating process and characterization efforts of the resulting healing film is given in the 

Appendix.

Figure 2. Phase diagram for Indium-Tin system (ref. 2). The target composition (60/40 In-Sn) used in this 

study is indicated by a dashed line.

Fatigue Crack Growth Test Procedure 

Fatigue crack growth testing was done in accordance with ASTM Standard E647 (ref. 22), 

with a computer-controlled servo-hydraulic test machine, where a back-face strain gage was 

used to determine crack length. Applied loads were continuously adjusted, based on crack 

length, such that programmed stress intensity factors were achieved. To best characterize the 

effects of crack healing, constant-K testing was performed; here, the crack-tip driving force 
6

parameter, K, was held constant as the crack propagated across the specimen. Fatigue 

loading was performed in this study at a load ratio (ratio of minimum to maximum applied loads) 

of R = 0.1. For homogenous materials, this test method is expected to produce constant crack 

growth rates such that a plot of crack length versus applied load cycle count should be a straight 

line (refs. 25, 26). Even subtle variations in crack growth rate can be determined using 

constant-K testing. Due to oxidation concerns of the In-Sn healing material, testing was 

performed in an evacuated vacuum chamber (<1.3·10-6 Pa, <10-8 Torr). After crack healing, the 

In-Sn material in the crack mouth affects the compliance-based crack length such that the 

automated test system perceives a reduction in crack length to have occurred. Should a 

constant-K be programmed at this time, a significant increase in load would occur (due to the 

shorter compliance-based crack length). Since the actual crack length has not decreased, this 

is not desirable; therefore, fatigue loading is resumed at the loads corresponding to that just

before crack healing was initiated. Constant-K testing was resumed after the compliancebased crack length grew to the pre-healed value. A photograph of the test setup is shown in

Figure 4.

Figure 3. Schematic of the ecentrically-loaded single-edge notch tension (ESE(T)) specimen 

configuration. All dimensions in millimeters (mm).
7

Figure 4. Photograph of the test setup.

Crack healing tests were run in multiple steps. First, a steady-state fatigue crack growth rate 

was achieved over a crack growth increment of at least 1.3 mm (0.050 inches). Then, the 

fatigue loading was stopped while the specimen was held at a constant load of approximately 

90% of the maximum cyclic load and heated to 135oC (275oF) for approximately 1 hour. It is not 

believed that transport of the healing material into the crack mouth requires a great deal of time, 

but an hour was allowed to ensure that the entire specimen reached a thermal steady state 

greater than the melting temperature of the healing film. This process will be investigated further 

in future studies. Small, molten metallic spheres observed on the specimen surface during 

heating were believed to be evidence that melting of the film (or at least a significant portion of 

it) had occurred. A photograph of these metallic spheres is shown in Figure 5. After heating for 

1 hour, the specimen was allowed to cool to ambient temperature, and then fatigue loading was 

resumed.

Fatigue Crack Growth Results

The results of two fatigue crack growth tests performed at constant-K values of (a) 6.6 

MPam and (b) 13.2 MPam are shown in Figure 6. When constant-K test data are plotted as 

crack-length-versus-cycle-count, the slope of the data is the fatigue crack growth rate. Consider 

the test run at the lower K value (K = 6.6 MPam); the steady state fatigue crack growth rate 

data (for the inert environment) are shown as solid circular symbols. These data form a nearlystraight line indicating that steady-state crack growth occurred between crack lengths of a = 6.9 

mm (0.27 inches) and a = 9.9 mm (0.39 inches). The crack growth rate of da/dN = 1.96 x 10-9

m/cycle (7.7x10-8inch/cycle) is indicated in the figure. At a crack length of a = 9.9 mm (0.39 

inches), cyclic loading was paused while the healing film was activated (heated and cooled as 

previously described). After the specimen cooled to ambient temperature and solidification of 

the healing material occurred, cyclic loading was resumed at constant load. The crack length, 
8

based on compliance data, appeared to have decreased to a value of approximately a = 9.9 mm 

(0.30 inches).1 After approximately 1 million load cycles, the compliance-based crack length 

increased slightly to approximately a = 8.4 mm (0.33 inches) (open circular symbols). 

Presumably, this increase in compliance-based crack length is a result of damage to the healing 

material in the crack mouth. Further cyclic loading resulted in negligible crack growth; the nearhorizontal line between 3 million and 4.5 million load cycles indicates that the crack growth rate 

was nearly zero (da/dN = 0). This suggests that no further damage to the healing material 

occurred and crack arrest was achieved. If the healing material was not introduced to the crack, 

the crack would have continued to propagate at a constant rate as indicated by the dashed line 

in Figure 6a.

.

Figure 5. Photograph of the In-Sn healing film after thermal cycling. The metallic spheres, which suggest 

that initial melting of the film has occurred, are indicated.

Next, consider the test run at the higher K value (K = 13.2 MPam, twice the crack-tip 

loading of Figure 6a). The steady state fatigue crack growth rate data (for the inert 

environment) are shown as solid circular symbols. These data form a nearly-straight line 

indicating that steady-state crack growth occurred between crack lengths of a = 12.2 mm (0.48 

inches) and a = 14.5 mm (0.57 inches). The crack growth rate of da/dN = 5.08x10-8 m/cycle 

(2.0x10-6inch/cycle) is indicated in the figure. At a crack length of a = 14.5 mm (0.57 inches), 

cyclic loading was paused while the healing film was activated (heated and cooled as previously 

described). After the specimen cooled to ambient temperature, and solidification of the healing 

 

1

 Recall that crack length is determined from compliance data measured from a strain gage on the 

specimen back face. The reduction in crack length is due to the presence of the healing material in the 

crack mouth affecting the specimen compliance. It is believed that the crack length nearly returning to 

the original value where the crack started suggests that the healing material has adhered to nearly the 

entire crack surface that was produced in the inert environment. Considering that the crack is not truly 

“healed,” and that the crack-tip loading is a function of applied load and crack length, the applied load 

corresponding to the crack length at the initiation of the healing process (here, a = 9.91 mm (0.39 

inches)) was maintained.
9

material occurred, cyclic loading was resumed. Without healing, the crack would have 

continued to propagate as indicated by the red dashed line in the figure; instead, the crack 

length, based on compliance data, appeared to have decreased to a value of approximately a = 

8.1 mm (0.32 inches). It should be noted here that the tests associated with Figures 6a and 6b 

were run on the same specimen.2 After approximately 8,000 load cycles, the compliance-based 

crack length returned to the crack length before the healing material was heated (a = 14.5 mm

(0.57 inches)). Presumably, this rapid increase in compliance-based crack length (open circular 

symbols) is a result of damage to the healing material in the crack mouth, destroying the crack 

bridging capability of the healing material. Further cyclic loading resulted in negligible crack 

growth for approximately 200,000 additional load cycles, as indicated by the near-horizontal line 

between 100,000 and 300,000 load cycles (i.e., temporary crack arrest was attained). After 

300,000 load cycles, the crack began to propagate, but did so at about 45% of the original crack 

growth rate (da/dN = 2.26x10-8 m/cycle (8.9x10-7inches/cycle)). High crack closure loads 

measured during the test strongly suggest that the decrease in crack growth rates were a result 

of the artificially-high crack closure effect of the residual healing material in the crack wake. 

Compliance-based data suggested that the crack closure levels were initially high, with crack 

closure loads greater than 50% of the maximum load, and tended to decrease as the crack 

propagated. Typical crack closure loads before healing were nearly 25% of the maximum load.

 (a) ΔK = 6.6 MPa√m (b) ΔK = 13.2 MPa√m

Figure 6. Plots of fatigue crack growth data for titanium specimens during healing tests.

 

2 After precracking in air, both tests were run in vacuum without exposing the fresh crack surfaces to an 

oxygen-rich environment. Here, the crack appears to have healed to a compliance-based crack length 

corresponding to the end of specimen precracking in air, covering the crack surfaces of both tests 

generated in an inert environment.
10

Discussion of Results

The results shown in Figure 6 indicate that the crack healing method developed by the 

authors is capable of arresting fatigue crack growth at low K (i.e., K = 6.6 MPam). Although 

the results show that crack arrest is temporary at higher K values (i.e., K = 13.2 MPam), 

there was still a significant benefit from the crack healing process. Presumably, at low K the 

healing material remained intact, preserving the crack bridging capability of the crack-mouth 

healing material. At higher K values, the crack-mouth healing material was ruptured such that 

crack growth eventually resumed; however, the subsequent crack growth occurred at a much 

lower crack growth rate. The reduction in crack growth rate coinciding with high crack closure 

levels strongly suggests that the residual healing material in the crack wake created artificially 

high crack closure levels. Considering that the fracture toughness of aerospace titanium alloys 

is nearly 50 MPam, the inabililty to arrest cracks above 13 MPam might suggest that this 

crack healing method is of limited use; however, the nature of crack growth is that cracks 

typically spend the majority of their lives slowly propagating small cracks at low K condidtions

(i.e., near-thresold fatigue crack growth). Furthermore, since a single specimen was healed 

twice, as demonstrated in Figure 6, repeatability has been demonstrated in that a healing film is 

reusable even after the first healing attempt.

The evidence is clear that the crack healing material flows into the crack mouth based on 

X-ray Computed Tomography (CT) and Energy Dispersive Spectroscopy (EDS) scans, which 

are presented in Figure 7a and 7b, respectively, for a typical healed crack growth specimen. An 

X-ray CT scan of a healed specimen is shown in Figure 7a; here, the In-Sn healing material can 

be seen as the light colored region where the specimen is viewed normal to the plane of the 

crack. In this case, the healing material appears to coat nearly the entire crack surface, from 

one surface to the other and all the way to the crack tip. After healing, some specimens were 

loaded to failure, exposing the crack surfaces for study. EDS scans were conducted on the 

exposed crack surfaces producing the color-coded maps shown in Figure 7b. Coverage of 

Indium (In) and Tin (Sn) on the crack growth surface produced during the test (in an inert 

environment) and the absence of these elements on the crack surfaces produced as the 

specimen was overloaded also suggests that the material infiltrated the crack mouth 

successfully during the healing process. However, careful examination of the In and Sn 

coverage reveals that the crack surface was less-than-uniformly coated with the In-Sn healing 

material, perhaps because this material seems to have infiltrated the crack opening only at a 

few discrete locations; these locations can be identified as points were the healing material on 

the surface of the specimen meets the material on the crack surface.

.
11

Figure 7. Evidence of healing material in crack mouth. (a) CT scan image of healed crack. (b) EDS image 

of material on crack faces. Note that (a) and (b) show results from two different specimens.

Summary

A crack healing system for metallic aerospace alloys is proposed where the protected 

material or component is coated with a low-melting-temperature metallic material. After crack 

damage occurs, the component is heated to melt the healing film, which then flows into the 

crack opening. Upon solidification, the healing material serves to mitigate crack damage in two 

ways. First, while the healing material is intact, tensile forces are transmitted across the crack 

faces as if the crack faces were glued or welded together. This mechanism, termed “crack 
12

bridging” reduces the crack-tip driving force (K) which in turn reduces the crack growth rates. 

After the healing material is damaged or ruptured, the presence of residual healing material in 

the crack mouth causes premature contact of crack faces. This process is an artificial crack 

closure mechanism.

Experimental results have shown that, during fatigue crack growth of a titanium alloy (Ti 6-2-

2-2-2), crack arrest is possible at crack-tip loads of K= 6.6 MPam. Although no permanent

crack arrest occurs at higher K (i.e., 13.2 MPam), the data show that a significant reduction 

(on the order of 50%) in crack growth rate still occurs. Furthermore, the process is repeatable in 

inert environments in that a single healing film can be reused.

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Society for Testing and Materials, West Conshohocken, Pennsylvania, 1999, pp. 407-425.

(26) J.A. Newman, “The Effects of Load Ratio on Threshold Fatigue Crack Growth in Aluminum Alloys,” 

Ph.D. dissertation, Virginia Polytechnic Institute and State University, Blacksburg, Virginia, 2000.

(27) Y.C. Chen and C.C. Lee, “Indium-copper multilayer Composites for Fluxless Oxidation-free Bonding,” 

Thin Solid Films, Vol. 283 (1996) pp. 243-246.

(28) B.S. Lee, et al., “Piezoelectric MEMS Generators Fabricated with an Aerosol Deposition PZT Thin 

Film,” J. Micromech. Microeng. Vol 19 (2009).

(29) X.Y. Wang, et al., “A Micrometer Scale and Low Temperature PZT Thick Film MEMS Process 

Utillizing an Aerosol Deposition Method,” Sensors and Actuators, A 143 (2007) pp. 469-474.
14

APPENDIX – Healing Film Application and Characterization

Details of the healing film sputter deposition process, as well as some film characterization 

studies are presented in this appendix. For brevity in the main body of the paper, this 

information is presented as an appendix. 

Healing Film Sputter Deposition Process

A radio frequency (RF) sputter depostion technique was utilized for the application of the 

metallic healing film. A deposition rate of ~1 μm/hr for 5-15 hours was used with a power of 250 

W, resulting in film thicknesses on the order of ~5-15 μm. Upon completion of the film 

deposition, the specimens were annealed at 135°C for thirty minutes to help consolidate the 

film. Deposition took place in an inert argon environment with pre-backfill pressures on the order 

of 1.3x10-4 Pa (1x10-6 Torr) and an argon pressure of approximately 1.1 Pa (8 mTorr). After the 

chamber was pumped down and backfilled with inert gas, specimens were etched for 10 

minutes with a power of 125W to remove surface oxides and promote better film adhesion. 

Typically, sputter coating processes are used to deposit films much thinner than the targeted 

range of 5-15 μm of this study. New deposition methods better suited for thick films have been 

developed in the Microelectromechanical Systems (MEMS) field. Of notable mention is the 

aerosol deposition technique, which is currently being investigated for its utility in applying the 

healing material quickly and in relatively large thicknesses (refs. 28, 29). Since sputter times 

tended to be long, overheating was a constant concern. Therefore, the sputtering deposition 

had to be done in layers for two hours or less, with at least one hour of cooling time. Due to 

these time constraints, approximately one healing coating could be deposited per week, with the 

possibility of coating up to four specimens per deposition along with witness samples. Typical 

runs consisted of only two single-edge notch tension (ESE(T)) specimens along with witness 

samples as shown in Figure A1. The witness samples were small coupons of material with the 

same healing film as the crack growth specimens. Scanning electron microcope (SEM) analysis 

and scratch tests were conducted to evaluate the quality of the In-Sn deposited film.

A diagram highlighting the basic coating process can be seen in Figure A2. The “target” is 

essentially a disk of material that is to be deposited on the substrate, which sits on the stage as 

shown. A plasma is initiated in the chamber which propels ions at the target, dislodging atomicscale particles of, in this case, 60/40 In-Sn. The particles then travel back toward the substrate, 

coating it uniformly over time. The target and the gun assembly are the main concern when 

dealing with overheating. Due to the low melting temperature of the eutectic alloy (as is required 

for practical healing of many aerospace components), overheating can cause the target to melt, 

potentially ruining any specimens in the chamber and, more seriously, causing direct etching of 

the copper contact on the gun itself once the target is spent. The gun’s copper contact is all that 

stands between the high vacuum chamber and the water cooling system within the gun; 

damage to the copper plating and an ensuing water leak could result in mechanical failure of the 

vacuum system.
15

Figure A1. Typical specimen configuration for sputter deposition process.

Figure A2. Schematic of sputter deposition system.
16

Film Analysis and Scratch Testing

As shown in Figure A1, square aluminum specimens were also coated with a thin layer of the 

In-Sn alloy (approximately 5-15 μm thick). These witness specimens were mechanically 

polished before being placed in the sputtering chamber, as was done for the fatigue crack 

growth specimens, in order to ensure that the film quality was consistent. It should be noted that 

aluminum was orignally used in place of titanium in all experiments until a switch to titanium was 

made for temperature robustness. Due to availability, use of the aluminum witness samples was 

continued. However, during the study, it became apparent that wetting characteristics were a 

critical aspect of the healing process, and, therefore, future work will match the substrate 

material of witness samples to that of the crack growth test specimens. In addition to the 

aluminum coupons, silicon witness coupons were also coated for purposes of measuring true 

film thickness. These silicon specimens were notched on the back side so that they could be 

broken post-deposition, exposing a clean film cross-section which could then be dimensioned 

and characterized using an SEM. A typical cross-section of a healing film is shown in Figure A3;

for this particular cross-section, the film thickness was measured to be between 5.28 and 5.88 

μm. Additionally, the quality of the film could be characterized; specifically, the void content 

within the In-Sn film. A quartz gage was available for thickness measurements inside the 

sputtering chamber, but the heavy coatings of metal being applied relatively constantly over the 

course of the experiment made the quartz gage difficult to maintain, so the witness sample was 

deemed more reliable (recall, the films applied in this study are significantly thicker than typical 

applications using sputter deposition.)

Figure A3. Typical cross-section of a sputter deposited In-Sn film.

5.88μm 5.28μm
17

To characterize the performance of the healing film, “scatch tests” were performed on the 

aluminum witness specimens in an SEM equipped with a heating stage. These specimens 

were scratched with a pair of tweezers before being installed in the microscope to expose some 

of the substrate beneath the film. This was an attempt to provide a region for flow to be 

observed, although it should be noted that the scratched region would also oxidize since the 

scratching was performed in air. Specimens were then heated to the melting point of the 

healing material, and the resulting molten material was expected to flow into the scratched 

regions. An example of this can be seen in Figure A4. Here, some of the In-Sn material flowed 

into the scatch in the form of large beads, while other regions of the In-Sn film did not appear to 

melt. Presumably, the area that didn’t melt was oxidized material with a significantly higher 

melting temperature. It is believed that an ideal healing film would allow In-Sn material to flow 

into a scratch or crack uniformly over its area. More work is needed to study the In-Sn film 

characteristics that limit the appearance of molten material to a few discrete locations. 

Eliminating these features should lead to better performance of the healing film.

Figure A4. Results of a scratch test on an In-Sn witness specimen (aluminum substrate).
REPORT DOCUMENTATION PAGE Form Approved

OMB No. 0704-0188

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Technical Memorandum

 4. TITLE AND SUBTITLE

 Mitigation of Crack Damage in Metallic Materials 

5a. CONTRACT NUMBER

 6. AUTHOR(S)

Leser, Patrick E.; Newman, John A.; Smith, Stephen W.; Leser, William P.; 

Wincheski, Russell A.; Wallace, Terryl A.; Glaessgen, Edward H.; 

Piascik, Robert S.

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14. ABSTRACT

A system designed to mitigate or heal crack damage in metallic materials has been developed where the protected material or 

component is coated with a low-melting temperature film. After a crack is formed, the material is heated, melting the film 

which then infiltrates the crack opening through capillary action. Upon solidification, the healing material inhibits further 

crack damage in two ways. While the crack healing material is intact, it acts like an adhesive that bonds or bridges the crack 

faces together. After fatigue loading damages, the healing material in the crack mouth inhibits further crack growth by creating 

artificially-high crack closure levels. Mechanical test data show that this method succesfully arrests or retards crack growth in 

laboratory specimens. 

15. SUBJECT TERMS

Adhesives; Bonding; Coatings; Crack; Crack opening displacement; Damage

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