International Journal of Fatigue 22 (2000) 65–73 www.elsevier.com/locate/ijfatigue Thermographic methodology for rapid determination of the fatigue limit of materials and mechanical components  G. La Rosa * , A. Risitano Istituto di Macchine, Facolta ` di Ingegneria, Universita ` di Catania, Viale Andrea Doria 6, 95125 Catania, Italy Received 13 January 1999; received in revised form 10 May 1999; accepted 28 June 1999 Abstract By analysing the temperature of the external surface during the application of cyclic loading, it is possible to evaluate the dynamic behaviour of an element and to determine the fatigue limit. The methodology (Risitano method) does not need any particular testing machine and allows reliable results to be obtained using a very limited number of specimens in a very short time. This methodology also yields information on the energy retained by the specimen and mechanical components. The results of 15 years of research into a new methodology for the determination of the fatigue limit of materials or mechanical components are reported.  1999 Elsevier Science Ltd. All rights reserved. Keywords: Fatigue limit; Experimental (thermographic) methodology; Life prediction; Cumulative damage; Image analysis 1. Introduction and aim The methods used to characterise a material under fatigue are, generally, often affected by large scatter and are often difficult to apply. Wo ¨hler curves themselves show mean square differences which are very high (even for materials with a defined endurance limit, such as steel) with large percentage errors. Thus the character- istics of the material can only be evaluated by general statistical considerations. The long test times and the large number of specimens to be analysed (and, conse- quently, their cost) required for the certification of materials and components discourage the designer from requesting detailed information on dynamic properties. The methods for the rapid determination of the fatigue limit [1,2] do not have reliable theoretical bases, and the methods for analysis of the dynamic parameters of fail- ure mechanics [3–8] require the determination of highly specialised parameters (K I , J-integral) and often are too  This work has never previously been published in any other jour- nal; some partial results were published in the proceedings of confer- ences or congresses. * Corresponding author. Tel. + 39-095-330240; fax: + 39-095- 330258. E-mail address: glarosa@im.ing.unict.it (G. La Rosa) 0142-1123/00/$ - see front matter  1999 Elsevier Science Ltd. All rights reserved. PII:S0142-1123(99)00088-2 closely tied to the micro-mechanics of the material, downgrading the engineering aspects of the problem. In the present paper the authors report on the state of their research using thermography to characterise the dynamic properties of materials and components com- monly used in the industrial sector. The paper describes a method (called the Risitano method, after the researcher who first used thermography to explore the thermal map over the surface of a specimen and thereby determine the fatigue limit), first developed and used by the authors in 1986 [9] and subsequently studied and refined to better exploit its potential and develop further modes of application. Other authors [10–12] have since applied the methodology described in the Risitano method which is already protected by patent licence [32]. 2. Proposed methodology It is known that when a material is subjected to load- ing, its temperature varies. In the case of loading above the fatigue limit, the increase in temperature is signifi- cant and can be detected by various experimental methods [13,14]. However, in order to obtain infor- mation regarding the variation in temperature during tests without the detection system affecting the measure-