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Determination of cyclic properties of a material used for many engineering structures, e.g. airframes, requires preparation of specimens from two-dimensional, shell structures. A specimen must be cut out from the structure component in the way allowing application of a load corresponding to that reflecting service cases. In order to fulfil this requirement, tension-compression testing of the flat specimen is necessary to obtain credible data. Problems associated with such kind of testing are discussed in this paper. In the first part, an extensive review of the anti-buckling fixtures developed up to now is given. Several representative solutions are discussed, with special emphasis on the technique limitations. In the second part of the paper, detailed description of the proposed design is given. Finally, the results of preliminary tests carried out using new fixture to mount the flat specimens cut from steel sheets are presented. Application of the proposed testing technique allowed tension-compression tests to be performed at the displacement amplitude within the range ±5 mm what corresponds to the maximum strain amplitude of ±0.4 for the specimen gauge length to be equal 12.5 mm. Taking into account all data captured by means of new fixture, one can conclude that the technique is promising with respect to providing data for modelling of cyclic deformation behaviour for shell structures.

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Changes in the mechanical characteristics of the structural steel because of fatigue damage accumulation are investigated in this article. Cyclic loading was performed in a complex stress state. Tubular specimens were loaded by axial force and torque, with loading performed along proportional and non‐proportional paths in the strain space. Material characteristics in the form of the stress–strain curve and yield surface were determined in the ‘as received’ state and after cyclic loading. It was found that accumulation of the fatigue damage because of cyclic loading in a complex stress state along proportional and non‐proportional paths in the strain space is manifested by the increase in inelastic response. The rate of damage was found to be higher for non‐proportional (circular) loading paths than that for proportional loading with the same strain amplitude.

A B S T R A C T Two grades of structural steel were subjected to fully reversible, constant stress amplitude cyclic loading. The local strain response of the material was measured and recorded during the test, with the applied testing technique enabling the monitoring of hysteresis loop variation for the narrowest cross-section of the hourglass specimen. Changes in hysteresis loop width, representing the local inelastic response of the material, were recorded in order to monitor the density of structural imperfections. Material ratcheting behaviour was observed as changes in the mean strain for selected load cycles. Ratcheting was attributed to local deformation of the material in the vicinity of imperfections such as voids or inclusions, as well as deformation induced by the propagation of microcracks. Definitions of a damage indicator parameter and damage parameter were proposed. The fatigue behaviour of the two investigated grades of steel was finally illustrated in the form of damage curves for different stress amplitudes and for undamaged and fatigue pre-damaged material.Keywords fatigue damage; inelastic strain; ratchetting; damage accumulation; damage parameter. N O M E N C L A T U R Ea = crack length D = damage parameter N, n = number of load cycle N f = number of cycles to failure R = cycle asymmetry ratio ε i = inelastic strain range for load cycle ε m = mean strain for load cycle φ = damage indicator parameter σ a = stress amplitude I N T R O D U C T I O NIn the two hundred years since fatigue in metal alloys was first investigated, 1 numerous studies have been carried out with the aim of identifying the physical phenomena underlying the accumulation of fatigue damage and leading to final failure of construction components. Although significant progress has been achieved, with the results of investigations published in many papers, the basic procedure for the estimation of fatigue life in construction components is still based on Wöhler's proposition. 1 This is because of the fact that observations of changes in internal structure have revealed the huge complexity of physical phenomena underlying fatigue damage accumulation. Many attempts have been made to relate identified phenomena to changes in the macroscopically observed mechanical properties of the investigated alloys. A summary regarding the elaborated damage models can be found in a number of review books 2 and papers. [3][4][5] Because finding relationships between changes in material mechanical characteristics and the physical phenomena underlying fatigue damage accumulation is very difficult, none of the mathematical models developed in this way have as yet been accepted by the engineering community for the estimation of component fatigue life.One obvious result of fatigue damage accumulation is the formation of dominant crack. The rapid development of fracture mechanics theory and testing methods have Correspondence: G. Socha.

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