ELSEVIER Int. J. Fatigue Vol. 18, No. 4, pp. 227-233, 1996 Copyright © 1996 Elsevier Science Limited Printed in Great Britain. All rights reserved 0142-1123/96/$15.00 PII:SO142-1123(96)00001-1 Fatigue and creep in titanium grade 2 J.A.M. Ferreira, J.D.M. Costa and P.N.B. Reis* Department of Mechanical Engineering, University of Coimbra, 3000 Coimbra, Portugal *Department of Electromechanical Engineering, UBI, 6200 Covilh~, Portugal (Received 21 July 1995; revised 23 November 1995) The titanium grade 2 is currently used in high-pressure heat exchanges, piping systems in sea water desalination plants, offshore technology and chemical plants. In many of the applications fatigue and/or creep loading conditions occur. In this paper are presented the results of the mechanical behaviour, low cycle fatigue parameters and fatigue crack propagation at room temperature and at 400°C. The influence of temperature, frequency and the shape of wave loading was studied. The creep behaviour was also studied. Curves of the strain and the stress against the time were obtained to 400°C and 500°C. Also the Larson-Miller parameter was determined. Tests to the creep crack propagation study were carried out. Crack propagation in the creep tests conditions used was not verified. INTRODUCTION Commercial titanium contains as the main element titanium and a residual element such as: iron, carbon, oxygen, nitrogen, hydrogen, and others associated with manufacturing techniqueL The purity of commercial titanium is classified in four grades according to the ASTM B 265 standard 2 and depending on the residual elements. About 75% of the world production of titanium is used in the aerospacial industry, but recently the use of titanium has significantly increased in other indus- tries. The main applications of the titanium grade 2 are high-pressure heat exchangers and piping systems in sea water desalination plants, salt production plants, offshore technology, chemical and petrochemical plants 3,4. Also it is used in condensers in nuclear power plants. In some of these applications the environment tem- perature is elevated and then creep of the material may occur. Fatigue can also occur due to the effect of temperature or dynamic external loads. The fatigue at elevated temperature will be the failure mode for some situations. In this paper we obtain the material behav- iour for low cycle fatigue, creep and fatigue crack propagation at room temperature and at 400°C. A tentative study of the creep/fatigue interaction behav- iour was tried. During the low cycle fatigue the strain is predominantly plastic. For dynamic loading the relation between the stress and the strain is designed by the cycle curve of the material and expressed by the Morrow equation 5 cr~ = K'(~-e) -" ( 1) & Z where tra is the stress amplitude, Aep/2 is the plastic strain amplitude, K' and n' are the cyclic hardening coefficient and the cyclic hardening exponent, respect- ively. When the stress level is high, the strain is predomi- nantly plastic and the number of cycles to failure is reduced. The plastic strain and the number of rever- sions are related by the Coffin-Manson equation 6 Aep _ ef'(2Nf)c (2) 2 where Nf is the number of cycles to failure, e~ is the fatigue ductility coefficient and c the fatigue ductility exponent. If Ne is enough high, the stress level is below the yeld point, and the strain is purely elastic. In this case the plastic component of the strain can be ignored and the elastic strain Ae~/2 is related to the number of reversals by the equation proposed by Basquin 7 Aee - cr'f.(2Nf) b (3) 2 where ~ and b are the fatigue strength coefficient and the fatigue strength exponent, respectively. In this paper were obtained the fatigue parameters: K', n', e~, c, ~- and b. The creep behaviour of the material was studied using the conventional curves for the strain rate against the time. These curves (deldt-t) were carried out at 400°C and at 500°C. The de/dt-t data were obtained from the e-t curve by derivation. From the e-t curves, the curves of the stress against the time to fixed values of strain and to failure were also obtained. 227