The SAS proposals for participation in FP6 projects
are listed in blocks according to the FP6 priority theme structure

FP6 priority
1.1.4   Aeronautics and Space
Title of the proposal

Improvement of structural safety estimationsusing quadratic hypothesis of fatigue damage accumulation

Slovak Academy of Sciences, Institute of Materials and Machine Mechanics
Racianska 75, 83102 Bratislava 3, Slovak Republic
Jan KOSUT, Ing. PhD.
+421 2 59412668

Research subject for a potential FP6 project

Fatigue is the most frequent cause of service failure of structures. The vast majority of safety estimates is based on simple Palmgren’s linear hypothesis of fatigue damage accumulation. However, this hypothesis does not account for loading sequence and loading level interactions, which causes considerable inaccuracies. There are also more precise hypotheses but they are complicated for practical use, especially for analytical solutions.

Therefore, a quadratic hypothesis has been proposed [1, 2]. It accounts for loading sequence and loading level interactions, so it is suitable for accounting for both stationary and non-stationary service loadings. The hypothesis is well analyzable [3, 4]. Its quadratic structure enabled very simple derivation of expression for number of repeated blocks to fracture without any additional simplifying assumptions [3], which is not a rule for hypotheses accounting for the loading sequence effects. It appears that the quadratic hypothesis provides useful compromise between accuracy and simplicity [5]. It calls for use in reliability and safety estimates, especially in the structure development stage.

There are two main fields of potential future research:
1. Investigation of physical damage mechanism aspects of the quadratic hypothesis to improve its accuracy, applicability and knowledge of its limitations and physical background.
2. Application of the quadratic hypothesis in reliability and safety models accounting for service random loading.
Experimental requirements of the investigation are on basic level covered by experimental facilities of our institute.
Output of the research can be suitable for integration into design systems.

Recent international cooperation of the research team

Proposer´s relevant publications related to the research subject

1. J. Kosut (2000). A Simple Phenomenological Hypothesis of Cumulative Fatigue Damage, Part I. Half-Cycle Formulation. Kovove Mater. 38, 96-106.
2. J. Kosut (2000). A Simple Phenomenological Hypothesis of Cumulative Fatigue Damage, Part II. General Formulation and Discussion. Kovove Mater. 38, 178-187.
3. J. Kosut (2000). Analysis of Hypothesis of Cumulative Fatigue Damage Based on the Damage Factors in Repeated Block Loading Case. Strojn. Cas. 51, 161-173.
4. J. Kosut (2000). Cumulative Fatigue Damage Hypothesis Based on Immediate and History Damage Factors and Its Basic Loading Sequence Properties. In: Life Assessment and Management for Struct. Components, Proc. of the Conf., Vol. I, IPS NASU, Kiev, pp. 113-118.
5. J. Kosut (2002). History influence exponent in cumulative fatigue damage determined using two-step loading experiments. Fatigue Fract Engng Mat Struct 25, 575-586.