Wear 246 (2000) 134–148 Experimental observations of contact fatigue crack mechanisms for austempered ductile iron (ADI) discs L. Magalhães a , J. Seabra b,∗ , C. Sá c a Departamento de Engenharia Mecânica, Instituto Superior de Engenharia, Instituto Politécnico do Porto, Rua de S. Tomé, 4200 Porto, Portugal b Departamento de Engenharia Mecânica e Gestão Industrial, Faculdade de Engenharia da Universidade do Porto, Rua dos Bragas, 4050-123 Porto, Portugal c Centro de Microscopia da Universidade do Porto, Rua do Campo Alegre 823, 4150 Porto, Portugal Received 9 December 1999; received in revised form 24 July 2000; accepted 24 July 2000 Abstract Collected from several twin-disc contact fatigue tests, images concerning crack development at ADI surfaces and sub-surfaces are presented. Several auxiliary surface analysis techniques were used (electronic scanning microscopy, video imaging and perfilometry). Defects found at these ADI samples are characterised and related to operating conditions. Elastohydrodinamic theory was used to evaluate surface tribological performance, involving parameters such as lubrication, roughness and general operating conditions. Results concerning a quick procedure to allow the evaluation of fatigue resistance properties among different ADI sample discs are also presented. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Fatigue crack mechanisms; Austempered ductile iron; Surface tribological performance 1. Overview Being a relatively recent material, some ADI properties are still not well characterised, namely material behaviour under severe contact conditions, crack initiation and foundry characteristic defects influence. Several authors [1–4] proposed mechanisms to justify crack behaviour in ADI parts, but no model corresponding to a global response to heavily loaded contacts has yet been published. Although that is not the aim of this work, the comprehension of related phenomena may be enhanced by studying results obtained from performed tests. Contact fatigue tests need to be performed during ex- tended periods of time until the first related phenomena oc- cur. At reasonable stress levels surface fatigue cracks appear only after millions of stressing cycles on steels (10 6 –10 8 ), meaning that many hours or days are necessary to complete the mechanical part only of one of these tests. Some differ- ent methods [3–6] have been proposed to accelerate the out- come of surface fatigue and revealed to be effective when testing steel components. Although these modified tests do not allow the evaluation of the material fatigue resistance in ∗ Corresponding author. Present address: INEGI, CETRIB, Rue do Bar- roco 174, 4465-591 Leca do Balio, Portugal. Tel.: +351-229-578-712; fax: +351-229-537-352. E-mail address: jseabra@fe.up.pt (J. Seabra). a conventional sense they allow a direct comparison between different specimens submitted to the same surface stress and operating conditions. The use of artificial indentations is a way to produce quicker contact fatigue response from steel parts. This method was used to test ADI discs in severe operating con- ditions, similar to those that real parts are supposed to be able to attain during service life (gears, namely). Five pairs of ADI discs were tested in a twin-disc ma- chine and exhaustive video-microscopic analysis was done, following the evolution of cracks at the surface of ADI samples during contact fatigue tests. Optical and electronic microscopes were used to perform sub-surface post-test analysis, gathering images of different forms of cracks and other metallurgical aspects. Results of performed tests, con- clusions about surface resistance of the tested ADI and the effect of the use of artificial indentations are presented in the following sections. 2. Austempered ductile iron (ADI) 2.1. General properties ADI has a growing field of application since manufactur- ers realised the advantages of the use of this material. Hav- ing a lower weight than steel and a higher capacity to absorb 0043-1648/00/$ – see front matter © 2000 Elsevier Science S.A. All rights reserved. PII:S0043-1648(00)00493-2