Application of Thermography in Analysis of Fatigue Strength of Materials and Structures

Termografija, kao nerazorna metoda, ima sve veću primjenu u istraživanjima i industriji. U radu je prikazana primjena termografije u procjeni koncentracije naprezanja te zamora materijala i konstrukcija. Na primjerima uzoraka s koncentratorima naprezanja prikazana je primjena termografije u analizi metalnih materijala. U tu svrhu uspoređeni su rezultati klasičnih ispitivanja na umarlici s tri termografske metode: termoelastičnom analizom naprezanja, Risitanovom metodom te metodom praćenja širenja zone plastifikacije i loma. Termoelastičnom analizom naprezanja na ciklički opterećenim uzorcima dobiva se raspodjela naprezanja te koncentracije naprezanja na površini materijala. Risitanova metoda na temelju mjerenja porasta temperature ciklički opterećenog uzorka s porastom opterećenja predviđa dinamičku čvrstoću uzorka. Zbog velike disipacije topline u zoni plastifikacije i pukotine, brze srednjovalne kamere omogućuju praćenje tijeka plastifikacije i širenja pukotine. Opisane metode primjerene su za dinamička ispitivanja, nerazorne su i beskontaktne te ne utječu na rezultate ispitivanja.Thermography is becoming more and more relevant method in industry and as a research tool. It is an accepted method in many fields where non-destructive testing is carried out. In this paper focus was on evaluation of stress concentrations and fatigue of metal structures. Three thermographic methods: Thermoelastic stress analysis, Risitano method and acquisition of plastification zone and fracture propagation, are addressed and compared with results of classical cyclic testing of Al2024 alloy specimens. Specimens with three types of stress concentrator are used; 3 mm triangular notch, R3 mm circular notch, and a hole with 6 mm diameter. All thermographic methods showed high level of coincidence with classical fatigue tests. Thermoelastic stress analysis provides first stress invariant field for cyclic loaded sample, revealing stress concentrations near notches. Risitano method, from thermal dissipations at various levels of cyclic load, estimates dynamic strength of materials. Fast cooled middle-wave infrared cameras enable locating and tracing material plastification and fracture propagation. The outcomes of all evaluated methods are in accordance with each other