Fatigue and endurance
Summary
The research is focused on fatigue life assessment of mechanical structures operated under complicated load conditions with the aim to assure reliable operation from the fatigue failure point of view. The emphasis is put on probabilistic approach considering the random nature of service loading as well as random nature of material properties of the construction.Objectives
The goal of the research was to develop- methodology of computational estimation of fatigue life of the component regarding the random character of both operational loading and cyclic properties of the material, and
- the system of service reliability assessment of the construction enabling to estimate the risk of premature fatigue fracture and to solve various tasks from the field of fatigue-reliability of the constructions. Practically it means to assess the resistance of the construction against fatigue failure at design stage (fatigue endurance check), at stage of the service (after the change of the load conditions to check ability to guarantee the originally designed fatigue properties), or in the case of modification of the requirements on operational endurance (to assess ability of the construction to operate after expiration of the designed endurance, etc).
Achievements
- A new probabilistic model for computation of the behavioral characteristics of the material under cyclic loading has been developed. The model enables to include inhomogeneity in cyclic material's properties into the fatigue-damage calculation.
- The model of fatigue-life estimation in the form of distribution function has been developed. The model respects the random character of operational loading and cyclic material's properties (Fig.1).
- The model of fatigue reliability evaluation was proposed based on the comparison of fatigue-life distribution function (FLDF) with the requirements on operation of the construction (Fig.2). The model enables to determine probability of premature fatigue failure in respect of the required operational endurance of the construction.
- A method of FLDF prediction has been proposed for the non-proportional random biaxial loading. FLDF calculated for the most damaged plane represent the service fatigue life of the construction, respects the random nature of cyclic material’s propereties, the random nature of the loading process and the random nature of relation between normal (σ) and shear (τ) stress during the loading.
- The methodology of fatigue-life assessment has been extended to the case of welded joint. The effect of weld grinding has been investigated with the aim to reduce the notch effect of the weld. It has been shown that the effect of weld grinding is of a probabilistic character and the result doesn’t need to be always positive.
- The methodology of continuous assessment of fatigue reliability of the construction has been developed. The method is based on real time computation of the current FLDF and its comparison with the requirements for operation of the construction (required service life for allowable probability of premature fatigue fracture occurrence).
- For the purpose of continual monitoring of fatigue damage of the construction the computing program has been developed. The program in real time computes and displays all the indicators needed for assessment of the construction from the point of view of the fatigue-damage (accumulated fatigue damage at current time of operation, immediate probability of survival, immediate residual life for allowable probability of the premature fracture, etc.)
Fig.1 Layout of fatigue life distribution function calculation: c(p)-DF considering the confidence (prediction) interval for material parameters.
Fig.2 Illustration of the verification of fatigue life of construction design. FLDF-fatigue life distribution function of designed construction; Lreq – desired operational endurance of construction; Pa – permissible probability of occurrence of premature fatigue failure; Lg - operational endurance for admissible value of Pa, confirmed by the calculated FLDF; Pg – probability of occurrence of premature fatigue failure, confirmed by the calculated FLDF for the Lreq value.
Additional reading
KLIMAN, Vladimír - JELEMENSKÁ, Jana. Fatigue reliability evaluation of structure under random loading using safety criterion. Part I. Methodology. In Kovové materiály, vol.39, 2001, No.3, p. 199-217 (In Slovak).KLIMAN, Vladimír - JELEMENSKÁ, Jana. Fatigue reliability evaluation of structure under random loading using safety criterion. Part II. Application. In Kovové materiály, vol.39, 2001, No.4, p. 244-262 (In Slovak).
KLIMAN, Vladimír - JÁROŠI, Marián. System of continuous assessment of structure fatigue reliability. Part I. Methodology. In Strojnícky časopis,55, 2004, No. 4, p. 218-234. ISSN 0039-2472 (In Slovak).
KLIMAN, Vladimír - JÁROŠI, Marián. System of continuous assessment of structure fatigue reliability. Part II. Application - experiment . In Strojnícky časopis, 55, 2004, No. 5, p. 293-307. ISSN 0039-2472 (In Slovak).
KLIMAN, Vladimír. Hodnotenie únavovej spoľahlivosti konštrukcií : Fatigue reliability assessment of structure. In ÚFM AV ČR. Životnost materiálů a konstrukcí 2006: sborník semináře. - Brno: ÚFM AV ČR, 2006. ISBN 80-239-6751-7.
KLIMAN, Vladimír. A probabilistic approach to fatigue life estimation under biaxial non-proportional random loading. Part I. Methodology. In Strojnícky časopis, 58, 2007, No. 3, p. 147-167. ISSN 0039-2472 (In Slovak).
KLIMAN, Vladimír. A probabilistic approach to fatigue life estimation under biaxial non-proportional random loading. Part II. Model experiment. In Strojnícky časopis, 58, 2007, No. 3, p. 168-184. ISSN 0039-2472 (In Slovak).
KLIMAN, Vladimír - GIBA, Vladimír - JELEMENSKÁ, Jana - ŠIŠKO, Róbert. Probabilistic approach to fatigue reliability assessment. Part I. Fatigue-life estimation model. In Strojnícky časopis, 61, 2010, No. 4, p. 191-202. ISSN 0039-2472.
KLIMAN, Vladimír - GIBA, Vladimír - JELEMENSKÁ, Jana - ŠIŠKO, Róbert. Probabilistic approach to fatigue reliability assessment. Part II. Example of application . In Strojnícky časopis, 61, 2010, No. 4, p. 203-214. ISSN 0039-2472.
KLIMAN, Vladimír - KEPKA, Miroslav - VÁCLAVÍK, Jaroslav. Influence of scatter of cyclic properties of material on operational endurance of construction. In Kovové materiály, 48, 2010, p. 367-378. (2010 - Current Contents). ISSN 0023-432X. DOI: 10.4149/km_2010_6_367.
CHMELKO, Vladimír - KLIMAN, Vladimír - GARAN, Martin. In-time monitoring of fatigue damage. Ed. J. Papuga, M. Ružička. In Procedia Engineering : Special Issues, 2015, vol. 101, p. 93 - 100. ISSN 1877-7058.
KLIMAN, Vladimír - CHMELKO, Vladimír. - MARGETIN, Matúš. Analysis of the notch effect of welded joint and of grinding effect. In Kovové materiály, 53, 2015, p. 429-441. ISSN 0023-432X.