{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,4,25]],"date-time":"2026-04-25T14:43:34Z","timestamp":1777128214157,"version":"3.51.4"},"reference-count":57,"publisher":"MDPI AG","issue":"16","license":[{"start":{"date-parts":[[2020,8,6]],"date-time":"2020-08-06T00:00:00Z","timestamp":1596672000000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"DOI":"10.13039\/100006285","name":"Federal Highway Administration","doi-asserted-by":"publisher","award":["DTFH61-13-H-00010"],"award-info":[{"award-number":["DTFH61-13-H-00010"]}],"id":[{"id":"10.13039\/100006285","id-type":"DOI","asserted-by":"publisher"}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Sensors"],"abstract":"<jats:p>This article describes research that investigated the ability of a carbon nanotube (CNT) sensor to detect and monitor fatigue crack initiation and propagation in metal structures. The sensor consists of a nonwoven carrier fabric with a thin film of CNT that is bonded to the surface of a structure using an epoxy adhesive. The carrier fabric enables the sensor to be easily applied over large areas with complex geometries. Furthermore, the distributed nature of the sensor improves the probability of detecting crack initiation and enables monitoring of crack propagation over time. Piezoresistivity of the sensor enables strains to be monitored in real time and the sensor, which is designed to fragment as fatigue cracks propagate, directly measures crack growth through permanent changes in resistance. The following laboratory tests were conducted to evaluate the performance of the sensor: (1) continuous crack propagation monitoring, (2) potential false positive evaluation under near-threshold crack propagation conditions, and (3) crack re-initiation detection at a crack-stop hole, which is a commonly used technique to arrest fatigue cracks. Real-time sensor measurements and post-mortem fractography show that a distinguishable resistance change of the sensor occurs due to fatigue crack propagation that can be quantitatively related to crack length. The sensor does not show false positive responses when the crack does not propagate, which is a drawback of many other fatigue sensors. The sensor is also shown to be remarkably sensitive to detecting crack re-initiation.<\/jats:p>","DOI":"10.3390\/s20164383","type":"journal-article","created":{"date-parts":[[2020,8,6]],"date-time":"2020-08-06T09:41:21Z","timestamp":1596706881000},"page":"4383","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":23,"title":["Performance Evaluation of a Carbon Nanotube Sensor for Fatigue Crack Monitoring of Metal Structures"],"prefix":"10.3390","volume":"20","author":[{"given":"Shafique","family":"Ahmed","sequence":"first","affiliation":[{"name":"Echem Consultants LLC, Poughkeepsie, NY 12601, USA"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-0118-9119","authenticated-orcid":false,"given":"Thomas","family":"Schumacher","sequence":"additional","affiliation":[{"name":"Civil and Environmental Engineering, Portland State University, Portland, OR 97201, USA"}]},{"given":"Erik T.","family":"Thostenson","sequence":"additional","affiliation":[{"name":"Mechanical Engineering and Materials Science, University of Delaware, Newark, DE 19716, USA"}]},{"given":"Jennifer","family":"McConnell","sequence":"additional","affiliation":[{"name":"Civil and Environmental Engineering, University of Delaware, Newark, DE 19716, USA"}]}],"member":"1968","published-online":{"date-parts":[[2020,8,6]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","unstructured":"Campbell, F.C. (2008). Elements of Metallurgy and Engineering Alloys, ASM International.","DOI":"10.31399\/asm.tb.emea.9781627082518"},{"key":"ref_2","doi-asserted-by":"crossref","first-page":"468","DOI":"10.1016\/j.ymssp.2013.12.001","article-title":"Reference-free fatigue crack detection using nonlinear ultrasonic modulation under various temperature and loading conditions","volume":"45","author":"Lim","year":"2014","journal-title":"Mech. Syst. Signal Process."},{"key":"ref_3","doi-asserted-by":"crossref","first-page":"153","DOI":"10.1177\/1475921716666532","article-title":"Development of a \u201cstick-and-detect\u201d wireless sensor node for fatigue crack detection","volume":"16","author":"Liu","year":"2017","journal-title":"Struct. Health Monitor."},{"key":"ref_4","doi-asserted-by":"crossref","unstructured":"Reed, R.P. (1983). The Economic Effects of Fracture in the United States.","DOI":"10.6028\/NBS.SP.647p1"},{"key":"ref_5","doi-asserted-by":"crossref","first-page":"171","DOI":"10.1016\/1350-6307(94)90016-7","article-title":"The importance of the management of structural integrity","volume":"1","author":"Milne","year":"1994","journal-title":"Eng. Fail. Anal."},{"key":"ref_6","unstructured":"Dowling, N.E. (2013). Mechanical Behavior of Materials: Engineering Methods for Deformation, Fracture, and Fatigue, Pearson."},{"key":"ref_7","unstructured":"Van Leeuwen, H.P., Schra, L., and Meulman, A.E. (1970). The Repair of Fatigue Cracks in Low-Alloy Steel Sheet, National Aerospace laboratory NLR. Report No. NLR-TR 70029 U."},{"key":"ref_8","doi-asserted-by":"crossref","first-page":"1333","DOI":"10.1016\/j.ijfatigue.2004.04.009","article-title":"Stop drilling procedure for fatigue life improvement","volume":"26","author":"Song","year":"2004","journal-title":"Int. J. Fatigue"},{"key":"ref_9","doi-asserted-by":"crossref","first-page":"187","DOI":"10.1016\/0143-974X(94)90042-6","article-title":"Fatigue life prediction of repaired welded structures","volume":"28","author":"Castro","year":"1994","journal-title":"J. Constr. Steel Res."},{"key":"ref_10","doi-asserted-by":"crossref","first-page":"38","DOI":"10.1016\/j.jcsr.2016.12.026","article-title":"Behaviour of retrofitted steel structures using cost effective retrofitting techniques","volume":"131","author":"Mirza","year":"2017","journal-title":"J. Constr. Steel Res."},{"key":"ref_11","doi-asserted-by":"crossref","first-page":"331","DOI":"10.1061\/(ASCE)1090-0268(2003)7:4(331)","article-title":"Application of Fiber Reinforced Polymer Overlays to Extend Steel Fatigue Life","volume":"7","author":"Jones","year":"2003","journal-title":"J. Compos. Constr."},{"key":"ref_12","doi-asserted-by":"crossref","first-page":"81","DOI":"10.1680\/stbu.2007.160.2.81","article-title":"Strengthening a steel bridge with CFRP composites","volume":"160","author":"Moy","year":"2007","journal-title":"Proc. Inst. Civ. Eng. Struct. Build."},{"key":"ref_13","doi-asserted-by":"crossref","first-page":"119","DOI":"10.1016\/j.compstruct.2017.06.056","article-title":"Mode I Fatigue Crack Arrest in Tensile Steel Members Using Prestressed CFRP Plates","volume":"178","author":"Hosseini","year":"2017","journal-title":"Compos. Struct."},{"key":"ref_14","doi-asserted-by":"crossref","unstructured":"Haggag, F.M., and Richardson, A.K. (1988). Precracking and Computerized Single-Specimen J|c Determination for Irradiated Three-Point Bend Specimens. Fracture Mechanics: Eighteenth Symposium, ASTM International.","DOI":"10.1520\/STP23254S"},{"key":"ref_15","first-page":"40","article-title":"Monitoring Fatigue Crack Initiation and Propagation in Cruciform Joints Using Resistance-Type Gages","volume":"32","author":"Thomas","year":"2004","journal-title":"J. Test. Eval."},{"key":"ref_16","doi-asserted-by":"crossref","first-page":"331","DOI":"10.1080\/14786430310001612157","article-title":"Electrochemical fatigue sensor response to Ti-6 wt% Al-4 wt% V and 4130 steel","volume":"84","author":"Witney","year":"2004","journal-title":"Philos Mag."},{"key":"ref_17","doi-asserted-by":"crossref","first-page":"1668","DOI":"10.1016\/j.ijfatigue.2007.01.023","article-title":"Sensors for monitoring early stage fatigue cracking","volume":"29","author":"Papazian","year":"2007","journal-title":"Int. J. Fatigue"},{"key":"ref_18","first-page":"852","article-title":"Eddy current sensor networks for aircraft fatigue monitoring","volume":"61","author":"Goldfine","year":"2003","journal-title":"Mater. Eval."},{"key":"ref_19","doi-asserted-by":"crossref","unstructured":"Weiss, V., Goldfine, N., and Natishan, M. (2000). Early stage fatigue damage characterization in aluminum alloys and stainless steels with Meandering Winding Magnetometer technology. Fatigue and Fracture Mechanics: 30th Volume, ASTM International.","DOI":"10.1520\/STP13418S"},{"key":"ref_20","doi-asserted-by":"crossref","first-page":"8088","DOI":"10.3390\/s150408088","article-title":"Detection of steel fatigue cracks with strain sensing sheets based on large area electronics","volume":"15","author":"Yao","year":"2015","journal-title":"Sensors"},{"key":"ref_21","doi-asserted-by":"crossref","first-page":"075602","DOI":"10.1088\/0957-0233\/25\/7\/075602","article-title":"Sensing sheet: The sensitivity of thin-film full-bridge strain sensors for crack detection and characterization","volume":"25","author":"Tung","year":"2014","journal-title":"Meas. Sci. Technol."},{"key":"ref_22","doi-asserted-by":"crossref","first-page":"43","DOI":"10.1016\/j.jsv.2012.07.049","article-title":"Fiber Bragg grating sensor fatigue crack real-time monitoring based on spectrum cross-correlation analysis","volume":"332","author":"Bao","year":"2013","journal-title":"J. Sound Vib."},{"key":"ref_23","doi-asserted-by":"crossref","first-page":"04018116","DOI":"10.1061\/(ASCE)AS.1943-5525.0000940","article-title":"Thin-Film Sensor for Fatigue Crack Sensing and Monitoring in Steel Bridges under Varying Crack Propagation Rates and Random Traffic Loads","volume":"32","author":"Kong","year":"2018","journal-title":"J. Aerosp. Eng."},{"key":"ref_24","doi-asserted-by":"crossref","first-page":"263","DOI":"10.1016\/0025-5416(77)90180-X","article-title":"Acoustic emission analysis during fatigue crack growth in steel","volume":"28","author":"Sinclair","year":"1977","journal-title":"Mater. Sci. Eng."},{"key":"ref_25","doi-asserted-by":"crossref","first-page":"1254","DOI":"10.1016\/j.jcsr.2011.03.005","article-title":"Prediction of fatigue crack growth in steel bridge components using acoustic emission","volume":"67","author":"Yu","year":"2011","journal-title":"J. Constr. Steel Res."},{"key":"ref_26","doi-asserted-by":"crossref","first-page":"107","DOI":"10.1016\/j.jcsr.2016.07.007","article-title":"Study of fatigue crack growth in RAFM steel using acoustic emission technique","volume":"126","author":"Babu","year":"2016","journal-title":"J. Constr. Steel Res."},{"key":"ref_27","doi-asserted-by":"crossref","first-page":"351","DOI":"10.1784\/insi.2016.58.7.351","article-title":"Innovative technology for continuous fatigue crack condition monitoring of bridges: Long-term electrochemical fatigue sensor (LTEFS)","volume":"58","author":"Miceli","year":"2016","journal-title":"Insight Non Destr. Test. Cond. Monit."},{"key":"ref_28","doi-asserted-by":"crossref","unstructured":"Moshier, M.A., Nelson, L., Brinkerhoff, R., and Miceli, M. (2016). Continuous fatigue crack monitoring of bridges: Long-term electrochemical fatigue sensor (LTEFS). Active and Passive Smart Structures and Integrated Systems 2016, Proceedings of SPIE\u2014The International Society for Optical Engineering, SPIE.","DOI":"10.1117\/12.2219633"},{"key":"ref_29","doi-asserted-by":"crossref","first-page":"1387","DOI":"10.1002\/stc.1655","article-title":"Crack detection and characterization techniques\u2014An overview","volume":"21","author":"Yao","year":"2014","journal-title":"Struct. Control Health Monit."},{"key":"ref_30","doi-asserted-by":"crossref","first-page":"477","DOI":"10.1016\/S0142-1123(01)00154-2","article-title":"MWM eddy current sensors for monitoring of crack initiation and growth during fatigue tests and in service","volume":"23","author":"Zilberstein","year":"2001","journal-title":"Int. J. Fatigue"},{"key":"ref_31","unstructured":"Vishay Micro-Measurements (2007). Fatigue Characteristics of Vishay Micro-Measurements Strain Gages, Vishay Precision Group."},{"key":"ref_32","doi-asserted-by":"crossref","first-page":"621","DOI":"10.1088\/0964-1726\/13\/3\/021","article-title":"Detection and monitoring of hidden fatigue crack growth using a built-in piezoelectric sensor\/actuator network: II. Validation using riveted joints and repair patches","volume":"13","author":"Ihn","year":"2004","journal-title":"Smart Mater. Struct."},{"key":"ref_33","doi-asserted-by":"crossref","first-page":"609","DOI":"10.1088\/0964-1726\/13\/3\/020","article-title":"Detection and monitoring of hidden fatigue crack growth using a built-in piezoelectric sensor\/actuator network: I. Diagnostics","volume":"13","author":"Ihn","year":"2004","journal-title":"Smart Mater. Struct."},{"key":"ref_34","doi-asserted-by":"crossref","first-page":"640","DOI":"10.1002\/adma.200306310","article-title":"Carbon nanotube film sensors","volume":"16","author":"Li","year":"2004","journal-title":"Adv. Mater."},{"key":"ref_35","doi-asserted-by":"crossref","first-page":"379","DOI":"10.1088\/0957-4484\/15\/3\/026","article-title":"Nanotube film based on single-wall carbon nanotubes for strain sensing","volume":"15","author":"Dharap","year":"2004","journal-title":"Nanotechnology"},{"key":"ref_36","doi-asserted-by":"crossref","first-page":"737","DOI":"10.1088\/0964-1726\/15\/3\/009","article-title":"A carbon nanotube strain sensor for structural health monitoring","volume":"15","author":"Kang","year":"2006","journal-title":"Smart Mater. Struct."},{"key":"ref_37","doi-asserted-by":"crossref","first-page":"429","DOI":"10.1088\/0964-1726\/16\/2\/022","article-title":"Multifunctional layer-by-layer carbon nanotube-polyelectrolyte thin films for strain and corrosion sensing","volume":"16","author":"Loh","year":"2007","journal-title":"Smart Mater. Struct."},{"key":"ref_38","doi-asserted-by":"crossref","first-page":"17728","DOI":"10.3390\/s150717728","article-title":"Processing and characterization of a novel distributed strain sensor using carbon nanotube-based nonwoven composites","volume":"15","author":"Dai","year":"2015","journal-title":"Sensors"},{"key":"ref_39","doi-asserted-by":"crossref","first-page":"2837","DOI":"10.1002\/adma.200600977","article-title":"Carbon Nanotube Networks: Sensing of Distributed Strain and Damage for Life Prediction and Self Healing","volume":"18","author":"Thostenson","year":"2006","journal-title":"Adv. Mater."},{"key":"ref_40","doi-asserted-by":"crossref","first-page":"1113","DOI":"10.1016\/j.compscitech.2010.02.022","article-title":"Delamination detection with carbon nanotube thread in self-sensing composite materials","volume":"70","author":"Abot","year":"2010","journal-title":"Compos. Sci. Technol."},{"key":"ref_41","doi-asserted-by":"crossref","first-page":"4085","DOI":"10.1080\/14786430903352649","article-title":"Damage monitoring in fiber-reinforced composites under fatigue loading using carbon nanotube networks","volume":"90","author":"Gao","year":"2010","journal-title":"Philos. Mag."},{"key":"ref_42","doi-asserted-by":"crossref","first-page":"2557","DOI":"10.1016\/j.compscitech.2008.05.016","article-title":"Carbon nanotube-based health monitoring of mechanically fastened composite joints","volume":"68","author":"Thostenson","year":"2008","journal-title":"Compos. Sci. Technol."},{"key":"ref_43","doi-asserted-by":"crossref","first-page":"1183","DOI":"10.1016\/j.compscitech.2010.10.009","article-title":"Damage sensing of adhesively-bonded hybrid composite\/steel joints using carbon nanotubes","volume":"71","author":"Lim","year":"2011","journal-title":"Compos. Sci. Technol."},{"key":"ref_44","doi-asserted-by":"crossref","first-page":"386","DOI":"10.1016\/j.compositesb.2016.01.020","article-title":"Damage sensing and fracture detection of CNT paste using electrical resistance measurements","volume":"90","author":"Kwon","year":"2016","journal-title":"Compos. Part B Eng."},{"key":"ref_45","doi-asserted-by":"crossref","first-page":"04016202","DOI":"10.1061\/(ASCE)ST.1943-541X.0001697","article-title":"Development of a Novel Integrated Strengthening and Sensing Methodology for Steel Structures Using CNT-Based Composites","volume":"143","author":"Ahmed","year":"2017","journal-title":"J. Struct. Eng."},{"key":"ref_46","doi-asserted-by":"crossref","first-page":"182","DOI":"10.1016\/j.compstruct.2018.07.005","article-title":"Integration of carbon nanotube sensing skins and carbon fiber composites for monitoring and structural repair of fatigue cracked metal structures","volume":"203","author":"Ahmed","year":"2018","journal-title":"Compos. Struct."},{"key":"ref_47","unstructured":"Dai, H. (2017). An Innovative Sensing Approach Using Carbon Nanotube-Based Composites for Structural Health Monitoring of Concrete Structures. [Ph.D. Thesis, University of Delaware]."},{"key":"ref_48","doi-asserted-by":"crossref","unstructured":"Ahmed, S., Schumacher, T., Thostenson, E.T., and McConnell, J. (2017). Simultaneous life extension and crack monitoring of fatigue-damaged steel members using multifunctional carbon nanotube based composites. Health Monitoring of Structural and Biological Systems 2017, Proceedings of SPIE\u2014The International Society for Optical Engineering, SPIE.","DOI":"10.1117\/12.2272206"},{"key":"ref_49","doi-asserted-by":"crossref","first-page":"315501","DOI":"10.1088\/0957-4484\/18\/31\/315501","article-title":"Spatial conductivity mapping of carbon nanotube composite thin films by electrical impedance tomography for sensing applications","volume":"18","author":"Hou","year":"2007","journal-title":"Nanotechnology"},{"key":"ref_50","doi-asserted-by":"crossref","unstructured":"Dai, H., Gallo, G.J., Schumacher, T., and Thostenson, E.T. (2016). A Novel Methodology for Spatial Damage Detection and Imaging Using a Distributed Carbon Nanotube-Based Composite Sensor Combined with Electrical Impedance Tomography. J. Nondestr. Eval., 35.","DOI":"10.1007\/s10921-016-0341-0"},{"key":"ref_51","doi-asserted-by":"crossref","first-page":"085024","DOI":"10.1088\/1361-665X\/aa75ef","article-title":"A large-area strain sensing technology for monitoring fatigue cracks in steel bridges","volume":"26","author":"Kong","year":"2017","journal-title":"Smart Mater. Struct."},{"key":"ref_52","doi-asserted-by":"crossref","first-page":"065004","DOI":"10.1088\/0964-1726\/24\/6\/065004","article-title":"Smart sensing skin for detection and localization of fatigue cracks","volume":"24","author":"Kharroub","year":"2015","journal-title":"Smart Mater. Struct."},{"key":"ref_53","unstructured":"ASTM International (2011). Standard Test Method for Measurement of Fatigue Crack Growth Rates, ASTM E647-15, ASTM International."},{"key":"ref_54","doi-asserted-by":"crossref","first-page":"14","DOI":"10.1016\/j.tafmec.2017.02.008","article-title":"A J-integral approach using digital image correlation for evaluating stress intensity factors in fatigue cracks with closure effects","volume":"90","author":"Castro","year":"2017","journal-title":"Theor. Appl. Fract. Mech."},{"key":"ref_55","doi-asserted-by":"crossref","first-page":"2063","DOI":"10.1016\/j.engfracmech.2010.03.025","article-title":"Quantifying crack tip displacement fields with DIC","volume":"77","author":"Yates","year":"2010","journal-title":"Eng. Fract. Mech."},{"key":"ref_56","doi-asserted-by":"crossref","first-page":"2707","DOI":"10.1016\/j.engfracmech.2011.07.001","article-title":"Back-face strain compliance relation for compact specimens for wide range in crack lengths","volume":"78","author":"Newman","year":"2011","journal-title":"Eng. Fract. Mech."},{"key":"ref_57","doi-asserted-by":"crossref","unstructured":"Newman, J.C., Ziegler, B.M., Shaw, J.W., Cordes, T.S., and Lingenfelser, D.J. (2012). Fatigue Crack Growth Rate Behavior of A36 Steel using ASTM Load-Reduction and Compression Precracking Test Methods. Fatigue and Fracture Mechanics: 38th Volume, ASTM International.","DOI":"10.1520\/STP49542T"}],"container-title":["Sensors"],"original-title":[],"language":"en","link":[{"URL":"https:\/\/www.mdpi.com\/1424-8220\/20\/16\/4383\/pdf","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2025,10,11]],"date-time":"2025-10-11T09:56:59Z","timestamp":1760176619000},"score":1,"resource":{"primary":{"URL":"https:\/\/www.mdpi.com\/1424-8220\/20\/16\/4383"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[2020,8,6]]},"references-count":57,"journal-issue":{"issue":"16","published-online":{"date-parts":[[2020,8]]}},"alternative-id":["s20164383"],"URL":"https:\/\/doi.org\/10.3390\/s20164383","relation":{},"ISSN":["1424-8220"],"issn-type":[{"value":"1424-8220","type":"electronic"}],"subject":[],"published":{"date-parts":[[2020,8,6]]}}}