The stress–life fatigue behaviour of aluminium alloy foams

The tension–tension and compression–compression nominal stress versus fatigue life responses of Alulight closed cell aluminium alloy foams have been measured for the compositions Al–1Mg–0.6Si and Al–1Mg–10Si (wt %), and for relative densities in the range 0.1–0.4. The fatigue strength of each foam increases with the relative density and with the mean applied stress, and is greater for the transverse orientation than for the longitudinal orientation. Under both tension–tension and compression–compression loading the dominant cyclic deformation mode appears to be material ratchetting; consequently, the fatigue life is highly sensitive to the magnitude of the applied stress. A micromechanical model is given to predict the dependence of life upon stress level and relative density. Panels containing a central hole were found to be notch insensitive for both tension–tension and compression–compression fatigue loading: the net-section strength equals the unnotched strength.     

Techniques for the investigation of the ratchetting behaviour of piping components under internal pressure and simulated seismic loading

The ratchetting behaviour of piping components under internal pressure and simulated seismic loading has been investigated using special fixtures and a standard laboratory universal testing machine fitted with a fatigue module. Experimental arrangements are presented for testing plain pipes with or without thinned sections, elbows and tee branch junctions under in-plane dynamic bending; similar fixtures and arrangements could be used for reducers and flanged connections and other types of loadings. Input accelerations up to 5g have been achieved and the results demonstrate that a great deal can be learnt about piping component behaviour from such simple arrangements and without resorting to sophisticated or expensive experimental techniques. The necessary instrumentation and algorithms for the analysis of results are described. Some sample results are included and discussed in terms of overall piping component behaviour. Two recommendations are suggested to more realistically simulate typical earthquake input spectra.     

Measurement of dynamic response and failure of pipework

Pipework in nuclear power stations is designed to withstand the effects of earthquakes. The current methods of design are known to be very conservative and lead to the inclusion of large numbers of pipework restraints. These restraints are only included to limit stresses in the unlikely event of an earthquake but they can, under some circumstances, increase stresses during comparatively frequent thermal transients and thus their unnecessary inclusion is undesirable.
With the ultimate objective of reducing aseismic design conservatism the experimental work, described in this paper, has been conducted at Berkeley Nuclear Laboratories (BNL) to investigate the response and failure of pipework under high levels of dynamic excitation. The initial work was aimed at response prediction, but it is now clear that the improvement of failure criteria is a preferred route to reduce design conservatism. The measurement of high strains and dynamic yield performance of materials has necessitated the development of novel measurement techniques which have wider application in other areas. This experimental work is now finished, and the results are being used to support a change in the design codes.     

HARDENING INTERACTION IN CYCLIC CREEP OF A TYPE 316 STAINLESS STEEL AT 20°C AND 500°C

Abstract— The development of anisotropic material properties due to torsional cyclic plastic straining of tubes with sustained axial loads was examined for a type 316 stainless steel at room temperature and at 500°C. The effect of the cyclic strains and the cumulative ratchetting strains on the axial tensile properties was determined and the results show a significant increase in the tensile strength at both 20°C and 500°C, with more pronounced hardening at the higher temperature. The cyclic shear stress-strain response of the material is shown to be extremely temperature dependent and the hardening ratio is much greater at 500°C, which is consistent with the dynamic strain ageing observed previously for this material. The ratchetting strains are controlled by the cyclic shear strain hardening, by the axial hardening resulting from the cyclic shear and the cumulative axial strains, and by the ratio of the secondary shear stress to the primary axial stress.