Remaining life prediction of cracked stiﬀened panels under constant and variable amplitude loading A. Rama Chandra Murthy * , G.S. Palani, Nagesh R. Iyer Scientists, Structural Engineering Research Centre, CSIR Campus, Taramani, Chennai 600 113, India Received 2 November 2005; received in revised form 30 August 2006; accepted 10 September 2006 Available online 13 November 2006 Abstract This paper presents the methodologies for remaining life prediction of stiﬀened panels under constant and variable amplitude loading (CAL & VAL). Concentric and eccentric stiﬀeners have been considered. Stress intensity factor (SIF) has been computed using the para- metric equations developed by conducting fracture analysis of stiﬀened panels utilizing numerically integrated modiﬁed virtual crack clo- sure integral (NI-MVCCI) technique. Wheeler residual stress model has been employed to represent the retardation eﬀects due to tensile overloads. Eﬀect of various stiﬀener sizes and stiﬀener type (concentric and eccentric stiﬀeners) on remaining life has been studied under CAL & VAL. From the studies, it has been observed that the predicted life is signiﬁcantly higher with concentric and eccentric stiﬀener cases compared to the respective unstiﬀened cases. Further, it has also been observed that the percentage increase in life is relatively more in the case of concentric stiﬀener case compared to eccentric stiﬀener case for the same stiﬀener size and moment of inertia. Expressions for remaining life have been proposed considering various loading conditions, type of stiﬀener and number of overloads. These expres- sions will be useful for designers to design the structural components/structures against fatigue and fracture. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Remaining life; Stiﬀened panels; Stress intensity factors; Fatigue; Fracture 1. Introduction The strength of structural components used in most of the industrial structures and other major infrastructures such as nuclear containments, reactor vessels, ﬂyovers, high-rise buildings, aerospace structures, ship hulls and oﬀ- shore structures are generally improved by providing stiﬀ- ening members such as stiﬀeners or stringers. Stiﬀeners are mainly provided to improve the strength and stability of the structure and to provide a means of slowing down or arresting the growth of cracks in the panel. Remaining life prediction of the cracked structural components in these structures is a major problem facing the engineering community in this era of aging infrastructure facilities. This is due to the fact that strength assessment of the aging structures is necessary for their in-service inspection, plan- ning, repair, retroﬁtting, rehabilitation, requaliﬁcation and health monitoring. In view of these, it is essential to use the damage tolerant design concepts for designing these struc- tural components. Fracture mechanics is a tool employed for investigation of the crack growth and fracture behav- iour of structural components that are subjected to fatigue loading or static overloading. Stiﬀened panels are often subjected to fatigue loading and in most of the cases, the loading may be either constant amplitude loading (CAL) or variable amplitude loading (VAL). In the case of VAL, a major inﬂuencing parameter to be considered is the inﬂuence of load history. Crack growth analysis and remaining life prediction of stiﬀened panels under VAL involves consideration of load interac- tion eﬀects, which inﬂuence the remaining life signiﬁcantly. A superimposed single overload during CAL is the simplest case of VAL. The application of single overload will cause signiﬁcant decrease in the crack growth rate for a large 0142-1123/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijfatigue.2006.09.016 * Corresponding author. Tel.: +91 44 22549206; fax: +91 22541508. E-mail addresses: murthyarc@sercm.org, murthyarc@sercm.csir.res.in (A.R.C. Murthy). www.elsevier.com/locate/ijfatigue International Journal of Fatigue 29 (2007) 1125–1139 International Journalof Fatigue