In many cases, where a high reliability is demanded, such as in nuclear power plants or civil engineering structures, steel components are subject to fatigue damage. The possibility of nondestructively assessing this damage would thus be of great interest, especially in order to evaluate the remaining lifetime and the point when the risk of failure becomes too important.</p

Guyon, Marielle

Mayos, Michel

Digital Repository @ Iowa State University (ISU)

NONDESTRUCTIVE EV ALUA TION OF FATIGUE DAMAGE ON STEELS USING MAGNETIC TECHNIQUES Marielle Guyon and Michel Mayos IRSID 34, rue de la Croix de Fer 78105 St Germain en Laye, France INTRODUCTION In many cases, where a high reliability is demanded, such as in nuclear power plants or civil engineering structures, steel components are subject to fatigue damage. The possibility of nondestructively assessing this damage would thus be of great interest, especially in order to evaluate the remaining lifetime and the point when the risk offailure becomes too important. The study of this problem was the starting point of a three-year contract (1990-1993) sponsored by the French Ministry of Research & Technology (MRT), and involving several laboratories specialized in NDT techniques. Since such an application was a fairly new NDE topic, the aim of this work was to study the basic fatigue behavior of several materials, as related to NDT techniques (directly taking into account industrial problems would have led to confusing results, since other exterior effects can influence the results). This communication will focus on the results IRSID obtained with the Barkhausen noise method, and will refer to other laboratOlies having used the same technique in this contract, such as INSA Lyon and EDF, the results of which have, at least partially, already been published [1] [2]. In a second step, the difficulties to overcome, especially with the instrumentation, in order to achieve a full-industrial testing, will be explored. FATIGUE TEST CONDmONS These tests concel1led two different kinds of steels: - a soft steel A48P2, containing 0.15 % C and displaying a felTito-peariitic structure, used for steam generator pipes. - an alloyed steel 20CDV5 (0.20 % C+ Cr - Mo - V), with a martensitic structure, and mostly encountered in steam turbines clements (e.g. rotors or bolt~). The fatigue tests consisted in tension/compression cycling. Mainly low-cycle fatigue was studied, with a constant applied strain. Two levels of strain were tlied for the A48P2 steel: E = ± 0.2 % and E = ± 0.5 %. As for the alloyed 20CDV5 steel, only one high level was used: E = ± 1 %. Furthermore, some high-cycle fatigue was performed on the 2OCDV5 steel, applying a constant stress at levels lower than the yield strength (685 MPa for this material). Review of Progress in Quantitative Nondestructive Evaluation. Vol. 14 Edited by D.O. Thompson and D.E. Chimenti, Plenum Press, New York, 1995 1717 The unloading mode was seen to be very important in a preliminary study by INSA [1] : it was shown that, for the same numbcr of cycles, this parameter directly affected the Barkhausen responsc. Hence, it was decided to work with a fixed unloading mode, i.e. stopping at a (0 = 0, E = 0) level, coming back from tension. For each matclial, the number Na of cycles leading to crack initiation was detennined using Wohler's curvcs. NDE tests were perfOlmed for several values of the number of cycles N included bctween 0 and Na. The geometry of fatigue specimens was rather peculiar (cf. fig. 1) in order to fit with fatigue machines and the different NDT techniques. The important point is to note the presence of a flat surface of 40 mm x 18 mm. Special attention was given during the machining of these specimens in order to limit surface roughness and residual stresses. LABORATORY RESULTS Experimental Set-up Figure 2 shows a block-diagram of the experimental set-up. The technique was applied with a surface probe of reduced dimensions, so that it was totally included in the flat area of the specimen. A low frequency ([H = 0.1 Hz) field was generated through the magnetizing coil; a constant dHidt was imposed thanks to appropriate electronics. Barkhausen noise (BN) measurements were obtained from the pick-up coil through an IRSID-built equipment, allowing to issue the envelope of the rectified signal, analyzed in a wide frequency band (200 Hz-300 kHz). The BN-curves show the evolution of the envelope amplitude vs the applied field H, measured through a Hall probe connected to a teslameter. Soft Steel (A48P2) During its cycling, the A48P2 steel showed fatigue hardening, i.e. the stress-strain hysteresis curves wcre hroadcning as NINa increascd. FIELD GENERATION dHldt = ct. IV\ fH =O.1Hz 1718 Pick-up coil BARKHAUSEN EQUIPMENT Hall probe FIELD MEASUREMENT Figure I. Geometry of the fatiguc test specimens. BN I· (1)18 ~ 0,02 {ftr-g ~L-5---'.II~ ~ S~ctlon AA Figure 2. 172 .[ Expelimental set-up. As for Barkhausen noise measurements, two velY distinct hehaviors were observed, according to the strain level: - for the high deformation level (E = ± 0.5 %), a very characteristic spreading out of the curves occurs (cf. fig.3) : this results in a decrease of the maximum amplitude BNmax, a decrease of the position of this maximum HDNmax and, ahove all, in the appearance of a second peak at negative values of the applied field. Furthelmore, the degradation of the BN curves is continuous all along fatigue life. This hehavior is characteristic of the intlllence of strain hardening, as it has already heen reported in earlier work [3] : it implies that the major effect on Barkhallsen noise is a growing hindering of domain wall motion due to the increase in dislocation density. 1400 . .. .. .. .. - • • 10fl1' I • II/II. • 21' • , -. -·1500 ·1Doo ·500 50D 1000 1500 ·200 RElD W.I , 3.J Figure 3. Evolution of BN curves for A48P2 steel (E = ± 0.5 %). 1719 - for the low deformation level (E = ± 0.2 %), as it can be seen on figure 4, BNmax increases while HBNmax slightly decreases, with bigger sensivity towards the early stages of fatigue life. This behavior, similar to the one observed under increasing tensile stress (e.g. in reference [4]), indicates that the stress state is dominant in this case, and influences Barkhausen noise through the magnetomechanical effect. Alloyed Steel C2OCDV5) Unlike the soft fenito-pearlitic steel, the 20CDV5 steel exhibited fatigue softening during cycling, i.e. a narrowing of the stress-strain hysteresis when NINa increased. At first, Barkhausen noise showed a very low sensivity to high-cycle fatigue, as it can be seen on figure 5, cOITesponding to an applied stress of 550 MPa, confirming that plastic deformation has a greater effect on this technique than elastic phenomena. As for low-cycle fatigue for a high strain level, the behavior is completely different from the one observed with the A48P2 steel (cL fig. 6) : an increase in the maximum amplitude is observed with non significant vatiations of HI3Nmax. Furthelmore, no second peak appears. A greater sensitivity is achieved in the early stages of fatigue life. 1600 :'~I r;""1 L_ f---r-----t---- .~I-----+-·1500 500 1000 1500 RRDIA/Iol/ U Figure 4. Evolution of BN curves for A48P2 steel (E = ± 0.2 %) 1200 :'~I ·1000 ·600 ·200 o 200 400 600 600 1000 Figure 5. Evolution of BN curves for 20CDV5 steel (high-cycle fatigue: (J = 550 MPa) 1720 'NIN. - 2" : I ............... r;~"'" L _I ·1500 ·1000 ·500 1200 • :'o~1 --- .... -....-...., 500 1000 1500 AElDlA/1o'f U Figure 6. Evolution of BN curves for 20CDVS steel (low-cycle fatigue: E = ± 1 %) Possibilities of Fatigue Damage Assessment If we summarize the previous results, we see that Barkhhausen noise is sensitive to low-cycle fatigue and rather unsensitive to high-cycle fatigue. No unique behavior is observed, and matelial structure, as well as strain level, have a strong influence on the Barkhausen noise level, which implies these parameters have to he known plior to any attempt of nondestructive evaluation. The maximum sensivity is ohtained in the case of a soft steel undergoing a defonnation with a high strain level, i.e. the case where dislocation density has a major influence on BN activity. The most charactelistic feature of fatigue evolution being the second peak appearence, it has heen found that a good representation of this occurence was obtained by using the 30 %-width parameter, corresponding to the width of the BN curve for an amplitude heing 30 % of BN max. The evolution of this parameter during fatigue life is shown on figure 7. Its good sensivity to NINa at any stage is confinned, as well as the consistency of results between both sides of the specimen. A very charactelistic inversion of hehavior is observed around NINa = 70 %. A microstructural explanation can be given to this phenomenon: it is most probably linked with the re-arrangement of dislocations in cells as their increasing number reaches a critical value. Figure 7. Evolution of the 30 %-width dllling fatigue life (A48P2 steel-E=±O.S%) 1721 This re-arrangement will result in a diminishing numher of pinning sites, and in an increase of the mean free path for Bloch wall motion, hence in an easier and more intense Barkhausen activity, which will lead to thinner BN curves. Towards the end of fatigue life, as their density continues to increase, newly-appeming dislocations are located inside these cells, in the same way the first dislocations occmed inside the grain structure: the 30 %-width parameter is then caused to increase again. This behavior is very promising for fatigue life monitoling, since the decrease in the 30 %-width, corresponding to values of NINa close to 70 %, could be used as an alarm for failure risk. It should be noted at last that this change of vmiation was not observed by the laboratories who used encircling probes, such as EDF and INSA, while, so far, all the results observed in magnetic techniques had been consistent. A possible explanation of this fact would be that the phenomena involved dUling the last stages of fatigue life preferently excite Barkhausen activity with !lux perpendicular to the surface, to which surface probes are sensitive and encircling ones are not. Anyway, this is the evidence of the sensor specificity during testing with magnetic techniques. STEPS TOWARDS THE INDUSTRIALIZATION The previous study has shown the possibility of assessing fatigue damage, using magnetic NDE methods. Moving to the real industrial application may prove though to be a very long and difficult way to go. The first difliculty is the exact definition of the industrial problem, in terms of material, fatigue sollicitation and environment conditions, since it has been seen previously that these parameters greatly inlluenced the response. Then, more experimental data will be needed on specific samples cOiTesponding to the above defined conditions. The second -and not the least- problem concell1s the instrumentation. So far, magnetic techniques have proven to he mostly effective under laboratory conditions, and applying them in a severe industlial atmosphere can cause many difficulties. IRSID has been working a lot on this subject dUling the past years, and some hints can now be given on what would be a fully-industrial magnetic NDE device. Three needs can be defined for this lahoratOly-industlial transition: - a compact and resistant instrumentation, which can he pOitahle on-site. The solution of functional modules included in a standard 19" rack interfaced with a personal computer is a good way to meet this achievement. It has already been successfully adapted for several years in the AMANDA device, in order to provide a nondestructive evaluation of surface structure gradients [5] - a probe adapted to environment conditions. Every application represents a new problem, but, in general, a lot of the technological knowledge derived from the conception of eddy current probes can be used. - a quick scanning of the product. This generally means a non-contact inspection and a raise in the magnetizing frequency fH. This generally implies an electrical modification of the probe, so that impedances can still be matched. According to this frame, IRSID has lately developed applications of on-line testing of steel mechanical properties using magnetic techniques (in this case, incremental permeability). If the application does not concell1 fatigue damage assessment, the above-described needs are the same, and transposability should apply. 1722 Figure 8. Instrumentation for magnetic techniques on industlial site. A picture of the instrumentation realized for this purpose is shown on figure 8. The functional racks lies on the left, and includes different modules in the form of drawers: power supply, field generation, eddy ClllTent excitation and reception, field or CUlTent measurement, logical functions. It is connected to a PC with an approptiate software, using commercial data acquisition modules. On-line ttials were perfOimed on moving steel sheets after continuous annealing, using shaped probe set above a roll, place where there is almost no up-and-down motion of the product. A lift-off of 2 mm was used. The importance of the ratio fHlproduct speed can be seen on figure 9, representing trials made at 0.3 m/s. A frequency of 1 Hz is clearly insufficient: a given point cannot undergo a complete cycle before completely crossing the probe (in this case, 100 mm long) and curves appear not to be reproducible. This rule-of-thumb indicates that frequencies greater than 3 Hz should be used: it can be effectively observed that for til = 5Hz, curves keep their consistency. Ultra-low carbon steels annealing lines can reach speeds up to 10 m/s. The above-applied ratio gives then a value of 100 Hz for n I. Tlials have clTectively been led at his frequency, implying a complete re-modeling of the probes. As it can be seen on figure 10, the results obtained on-line (in this case, the 50 % width was acquired) show a dramatic agreement with hardness profiles, which were made afterwards on the same sheet (more than 1000 points were taken), and with microstructure: high hardness/low LSO % values correspond to correct recrystallization, low hardness/high LSO % values to non-recrystallization and coarse grains, intermediate values to mixed structures. (NB : the middle section was not tested, necause the probe was removed, due to folds on the sheet surface). All the results prove that instrumentation for NDE llsing magnetic methods is now portable on-line and can work under extreme conditions. All these achievements should normally be transposable to the Barkhausen noise technique (only the receiving module should be modified) and to the pal1icular problem of fatigue damage assessment, provided the fact that plior complementary laboratory data have been obtained on samples specific to the application. fH - 5 Hz fH*lHz 2 _ 2500 -2000 -1 !>!Xl 500 Figure 9. Incremental penneability curves obtained on-line for different magnetizing frequencies (product speed: 0.3 m/s). 1723 900 IICO 700 1 600 ~~, not tested • • ~oo 300 200 100 dlatlnce Figure 10. Result~ of on-line tIials for ultra-low cal'hon steels - VaIiation of the 50 %-width, compaIison with Vickers hardness profile and micrographs (product speed: 10 m/s) ACKNOWLEDGEMENTS The laboratory part of this work was supported by a contract from the French Ministry of Research & Technology (contract # C89047). REFERENCES 1. J. Lamontanara, J. Chicois, P. Fleischmann, R. Fougcres : "Monitoring fatigue damage in industIial steel hy Barkhausen noise", Nondestructive Characterization of Materials V, cds. T. Kishi, T. Saito, C. Ruud and R. Green, Jr (May 1991), pp. 603-614. 2. A. Le Brun, F. Billy: "Nondestructive fatigue damage assessment using ultrasonic and magnetic measurements on metallic mateIials", Review of Progress in QNDE, vol. 13B, eds. D.O. Thompson and D.E. Chimenti (Plenum, New York, 1994), pp. 1833-1840. 3. M. Guehis, S. Scgalini : "Effect of tensile stresses on magnetic properties of strain hardened steel", Soft Magnetic Materials 8 (sept. 1987). 4. R. Rautioaho, L.P. Karjalainen, M. Moilanen: "Variation in stress response curves on the Barkhausen noise method", International Conference on Residual Stresses 2 (Nov. 1988). 5. M. Mayos, J.P. Viale, M. Putignani : "Dispositif AMANDA pour l'evaluation non destructive de la dccarhuration superficielle des acicrs", Revue Pratique de Contr6le Industriel, 164 his (oct. 1990), pp. 72-78. 1724 

Nondestructive Evaluation of Fatigue Damage on Steels using Magnetic Techniques

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Nondestructive Evaluation of Fatigue Damage on Steels using Magnetic Techniques

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