Thermography Detection on the Fatigue Damage of the Specimen Alloy - Essay Example

Thermography detection on the fatigue damage of the specimen alloy Fatigue damage occurs when a material is ed to periodic stress. Modern structures such as aeroplanes, oil rigs, ships, etc. are routinely subjected to periodic stress. The chance of fatigue failure is very high and the results can be catastrophic. The ability to detect a crack and estimate its propagation speed is important in order to have timely replacement of materials before fatigue failure. The first manifestation of fatigue in a material is development of cracks. Thermography has become the preferred method for non-destructive detection of cracks in materials. The stress intensity factor, K, can be used for predicting crack propagation rate. 2 Main Scope Fatigue occurs when a material is subjected to periodic stress which is below its tensile breaking or yield stress but which is enough to cause permanent damage. The formal definition of fatigue as given by Wikipedia (n.d.1) is: [F]atigue is the progressive, localized, and permanent structural damage that occurs when a material is subjected to cyclic or fluctuating strains at nominal stresses that have maximum values less than (often much less than) the static yield strength of the material. It is because fatigue happens so quietly and insidiously that makes it very dangerous even resulting in loss of life. Sadananda et al (2003) assert that "Fatigue is the principal cause of premature failure of engineering components". Many structures such as aeroplanes, oil rigs and bridges, to name just a few, are exposed to fluctuating stresses. The engineering approach falls into two broad categories of dealing with stress induced fatigue. The first part is prediction of the lifetime of a material under stress. This model will provide recommendations on how frequently materials have to be replaced. The second approach is to predict how far a crack can grow before failure happens. Detection and prediction of failure of cracks can mean the difference between life and death of users of these facilities. S-N Curves "The basic method of presenting engineering fatigue data is by means of the S-N curve, a plot of stress S against the number of cycles to failure N." ( Key to Steel, n.d.) The S-N curves enable prediction of how long a material will last in terms of cycles of loading. Figure 1. A S-N Plot for an aluminum alloy (Kelly, 1997) Kelly (1997) explains that cracks go through three stages of formation, propagation and failure. Stress Intensity factor K "Stress Intensity, K, is a parameter that amplifies the magnitude of the applied stress that includes the geometrical parameter Y (load type)" (Wikepedia, n.d.2). This factor measures the degree to which stress is magnified around a crack. The loading around a crack falls into three modes I, II and III. Figure 2. Three loading modes (Key to Steel, n.d.) The three modes are: Mode 1: opening or tensile mode (the crack faces are pulled apart) Mode 2: sliding or in-plane shear (the crack surfaces slide over each other) Mode 3: tearing or anti-plane shear (the crack surfaces move parallel to the leading edge of the crack and relative to each other) (Key to Steel, n.d.) The most common mode is mode I and this is what is used in most calculations. The intensity factor, K, determines the rate at which a crack will propagate and hence the lifetime of the material. The mathematical relationship is defined by Callister (1994, cited by Kelly(1997)) as: This equation relates the rate of growth of a crack to the change in intensity factor K. In this equation A and m are dependent on the materials and da is the change in crack length while dN is the change in number of stress cycles. The change in K is defined by: Where Kmax and Kmin are the maximum and minimum intensity factors respectively, Y is a constant dependent geometry of the material and is the applied stress on the material. When this equation is re-arranged and integrated it becomes: This equation gives Nf, the estimated number of cycles before failure. Non-Destruct Evaluation (NDE) NDE is a technique for testing materials for faults without destroying them. It is sometimes known as non-destruct testing (NDT). Evaluation of cracks requires NDE techniques to assess the extent and thereby help predict expected life before failure for the material. There are many NDE techniques such as Acoustic Emissions, Ultrasonic testing, Infrared and thermal testing, computer tomography, etc. Of late the methods used for fatigue crack evaluation that are generally used are the infrared and ultrasonic thermography. Both methods involve excitation of the target material with radiation and then capturing a thermal image of the target. The weaknesses in the material will be revealed as 'hot spot' areas on the thermal image. Sonic Thermography "Sonic thermography is an emerging technique which to date has shown good prospect for difficult inspection problems like tightly closed cracks in metallic structures and kissing bonds in composite repairs." (Clark, 2005). This technique involves excitation of material under investigation with sonic waves at 20-40kHz. These waves cause heating of material in defective areas and a thermal image of the material under test is taken to reveal "hot spots". Tsoi and Rajic (2004) describe the use of ultrasonic testing. Initially it was feared that this method of testing might permanently damage the material under test but Tsoi and Rajic (2004) found no evidence of permanent damage and therefore this method qualifies as truly NDE. The challenge with sonic thermography is the coupling required for efficiently transmitting the sound waves to the material being tested. Infrared Thermography Cramer and Winfree (n.d.) describe the use of infrared thermography (IRT) to get "images of fatigue cracking, bond integrity and material loss due to corrosion". IRT for NDE is aimed at the discovery of subsurface features (such as subsurface thermal properties, presence of subsurface anomalies/defects), thanks to relevant temperature differences observed on the surface with an infrared (IR) camera. (Madaque, 2004) There are two forms of IRT called passive and active. In passive IRT is used for objects that have temperature higher than ambient temperature. Active IRT involves the use of IR excitation of objects and then taking their thermal pictures. The active vary between the use of a single shot on the object or scanning approach as described by Cramer and Winfree (1992). The first problem with IRT relate to emissivity. "Emissivity is a surface property that specifies the ability of that surface to emit energy, values of are between 0 (for a perfect reflector) to 1 (for a perfect emitter, called a 'blackbody.')" (Madalque, n.d.). Low emissivity creates problem with spurious signals coming from radiation emitted by external projects confounding the results. Materials with low emissivity may need to be coated with a high emissive paint. Shielding the surface that is being examined against spurious sources is another alternative. The second problem identified by Madalque (n.d.) is that a model of heat transfer for the material is required to correctly interpret the results. The heat is being applied at different times, one line at a time. Programs are available to interpret the results. Madalque (n.d.) advances the following advantages for IR thermography. fast inspection rate; no contact; security (no harmful radiation involved, however high power external stimulation - such as powerful flashes - requires screens during flashes); results relatively easy to interpret (image format); wide range of applications (sometimes unique NDE tool). Recent Studies A lot of studies have gone into understanding and developing models for cracks. Improvements in thermography have enabled better ability to detect and model cracks and flaws in materials. IRT is being developed to detect corrosion in materials. Pappalettere and Lamberti (n.d.) assert that "[n]umerical models where neural networks are used as defect classifiers have been also developed in order to handle the high degree of uncertainty in defect class boundaries". This gives more accurate information on the nature of the flaws in the material. Pappalettere and Lamberti (n.d.) further observe that: Lock-in thermography has been utilized for evaluating aspects of industrial interest such as inclusions of spurious materials in both carbon-epoxy and glass-epoxy, impact damage and delaminations occurring around holes during drilling in carbon-epoxy, bonding improvements in Certran((R)) after plasma treatments and steel modifications after welding. The failure of components is a probabilistic event. Pappalettere and Lamberti (n.d.) mention tests that are designed to characterize a material; e.g. Aluminium. "Fatigue life prediction involves analyzing data gathered from a large number of tests." Results from these tests are used to recommend lifetime of components. Future Development of Thermography Refinement of thermographic techniques is ongoing. Recent work by Plekhov et al investigates "fatigue crack initiation and growth in middle-cycle fatigue (105 cycles)." In this work they show that local temperature variations are a good indicator of formation of cracks under fatigue. Due to convenience, the usage of thermography is increasing. A vast array of applications such as imaging for crack detection in machine bearings, aeroplane structures, ships, runways, bridges, etc. has emerged. Work on developing models of crack formation and propagation that give better prediction of an appliance's lifetime is continuing. Influences from fields such as quantitative fractrography to link macro behavior of crack with its micro-structures are being investigated. Theories on crack mechanics are being developed and experimental characterization of material behavior under fatigue are being formulated and tested against experimental facts. 3 Conclusion Thermography as an emerging field is having considerable impact on NDE techniques used in situ to evaluate materials that are subjected to stress. The objective is to detect cracks, estimate their propagation rate and decide on when to replace before failure of the material. Modelling of cracks and characterisation of behaviour by specific materials under stress is an important aspect of research to enable more accurate prediction. The stress intensity factor can be used to predict rate of growth of a crack. This enables the responsible person to make a decision on whether a given component needs replacement or not. 4 References Clark, G. (2005).A Review of Australian and New Zealand Investigations on Aeronautical Fatigue During the Period April 2003 to March 2005: Government of Australia, Department of Defence, Air Vehicles Division Platforms Sciences Laboratory [Online]. Available at: http://www.dsto.defence.gov.au/publications/3813/DSTO-TN-0624.pdf. [Accessed February 4, 2005]. Cramer, K.E. and Winfree, W.P. (n.d.): Thermal characterization of defects in aircraft structures via spatially controlled heat application. Thermosense XIV jan. K. EK lund, editor, proc. SPIE 1682,162-170,1992. Kelly, S.M. (1997). Fatigue [Online]. Available at: http://www.sv.vt.edu/classes/MSE2094_NoteBook/97ClassProj/anal/kelly/fatigue.html#S_Ncurve. [Accessed February 3, 2006]. Key to Steel (n.d.). Fatigue of metals:Part One [Online]. Available at: http://www.key-to-steel.com/Articles/Art142.htm [Accessed February 3, 2006] Madalque, X.(n.d.) Applications Of Infrared Thermography In Nondestructive Evaluation. [Online]. Available at: http://www.gel.ulaval.ca/maldagx/r_1123.pdf [Accessed February 3, 2006] Meola C, Carlomagno GM, Squillace A, Giorleo G. (2002) Non-destructive control of industrial materials by means of lock-in thermography. Measurement Science & Technology, 13 (10): 1583-1590, 2002. Marinetti S, Vavilov V, Bison PG, Grinzato E, Cernuschi F. (2003) Quantitative infrared thermographic non-destructive testing of thermal barrier coatings. Materials Evaluation, 61 (6): 773-780, 2003. Pappalettere, C and Lamberti,L. (n.d.) A Brief Review of Expertises and Applications of Experimental Mechanics in Italy. [Online]. Available at: http://www.aiasonline.org/ExperimentalMechanicsinItaly1.pdf [Accessed February 6, 2006]. Plekhov, O., Palin-Luc, T., Saintier, N., Uvarov, S. & Naimark, O. (2005) Fatigue crack initiation and growth in a 35CrMo4 steel investigated by infrared thermography. Fatigue & Fracture of Engineering Materials & Structures28(1-2),169-178 Sadananda, K., Holtz, R.L. and Vasudevan, A.K. (2003). Unified Approach to Fatigue Damage Evaluation Tsoi, K.A. and Rajic, N.(2004). Effect of Sonic Thermographic Inspection on Fatigue Crack Growth in an Al Alloy. Government of Australia Department of Defence. [Online]. Available at: http://www.dsto.defence.gov.au/publications/3447/DSTO-TN-0584.pdf. [Accessed February 3, 2006] Wikipedia (n.d.1). Fatigue (material) [Online]: Available at: http://en.wikipedia.org/wiki/Fatigue_%28material%29 [Accessed February 2, 2006] Wikipedia (n.d.2). Stress Intensity Factor. [Online]. Available at: http://en.wikipedia.org/wiki/Stress_Intensity_Factor. [Accessed February 3, 2006] 5 Bibliography Gyenkesi, A. L. (1998). Isothermal Fatigue, Damage Accumulation, And Life Prediction Of A Woven PMC. NASA/CR-1998-206593 [Online]. Available at: http://gltrs.grc.nasa.gov/reports/1998/CR-1998-206593.pdf [Accessed February 4, 2006] Hancq, D.A. (n.d.). Fatigue Analysis Using ANSYS [Online]. Available at: http://www.caeai.com/papers/Fatigue_ANSYS.pdf [Accessed February 4, 2006] Yang,B., Liaw, P.K., Wang,G., Peter, W.H., Buchanan, R.A., Yokoyama, Y., Huang, J.Y., Kuo, R.C., Huang, J.G., Fielden, D.E, And Klarstrom, D.L. (2004). Thermal-Imaging Technologies for Detecting Damage during High-Cycle Fatigue: Metallurgical And Materials Transactions A Volume 35a, January 2004 p.15-24 [Online]. Available at: http://doc.tms.org/ezMerchant/prodtms.nsf/ProductLookupItemID/MMTA-0401-15/$FILE/MMTA-0401-15F.pdfOpenElement. [Accessed February 3, 2006] Read More