Browsing by Author "Chahardehi, Amir Ebrahim"

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    The effect of residual stresses arising from laser shock peening on fatigue crack growth
    (Elsevier, 2010-07) Chahardehi, Amir Ebrahim; Brennan, Feargal P.; Steuwer, Axel
    Residual stresses have in the past been introduced to manipulate growth rates and shapes of cracks under cyclic loads. Previously, the effectiveness of shot peening in retarding the rate of fatigue crack growth was experimentally studied. It was shown that the compressive residual stresses arising from the shot peening process can affect the rate of crack growth. Laser shock peening can produce a deeper compressive stress field near the surface than shot peening. This advantage makes this technique desirable for the manipulation of crack growth rates. This paper describes an experimental program that was carried out to establish this effect in which steel specimens were partially laser peened and subsequently subjected to cyclic loading to grow fatigue cracks. The residual stress fields generated by the laser shock peening process were measured using the neutron diffraction technique. A state of compressive stress was found near the surface and tensile stresses were measured in the mid-thickness of the specimens. Growth rates of the cracks were observed to be more affected by the tensile core than by the compressive surface stresses.
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    Experimental determination of the overturning moment and net lateral force generated by a novel vertical axis wind turbine: experiment design under load uncertainty
    (Blackwell Publishing Ltd, 2013-01-31T00:00:00Z) Kolios, Athanasios J.; Chahardehi, Amir Ebrahim; Brennan, Feargal P.
    Recent developments in harnessing wind energy propose new, radically different designs to alleviate some of the difficulties associated with conventional wind turbines. New designs however require testing for a variety of reasons ranging from gaining confidence in the analytical models used in the design and development through to satisfaction of certification requirements. Medium-scale prototype testing of large-scale concepts, where parameters such as the response of the structure and the loading conditions are often highly uncertain demand special consideration. This article presents the design of a special test rig and calculation methodology for the experimental determination of the overturning moment and net force generated by the NOVA Vertical Axis Wind Turbine using a field experimental setup. The design of the experimental model involves dealing with modelling uncertainties as loads in operation and therefore the response of the structure are largely unknown before testing has been carried out. The variability in the wind speed and direction also need to be accommodated for.
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    Fatigue Crack Growth in Complex Stress Fields
    (Cranfield University, 2008-08) Chahardehi, Amir Ebrahim; Brennan, Feargal
    Fatigue crack growth has been traditionally modelled using LEFM through the use of the Paris law. This requires an accurate method for stress intensity factor (K) calculation. Weight functions have been developed for one-dimensional cracks (e.g. edge and through cracks); these are functions that enable separation of the loading and geometry and considering the effect of each one of these two factors on the stress intensity factor (SIF) separately. They have been proven to be useful for arbitrary stress distributions where an accurate empirical formula for the stress intensity factor does not exist. Such cases include residual stress fields due to surface treatments or welds. However, in the case of surface cracks, or part-through cracks, the problem of modelling the growth of these cracks poses two main questions, namely, how should the Paris law be generalised to suit the two-dimensional scenario, and under arbitrary loadings, how can the SIFs be calculated for these cracks. Current solutions involve tedious mathematical calculations and are complicated functions. In this thesis, the concept of root mean square (RMS) SIF is examined and by drawing mathematical analogy with the one-dimensional case, a novel weight function is derived which enables calculation of RMS SIF values for a range of semi-elliptical surface cracks under arbitrary loadings. The accuracy of the weight function is verified through comparisons with finite elements results for a variety of loadings/geometries. The simplicity of the weight function construction method makes it a useful tool for fatigue life predictions where incremental recalculations of SIF is required as the crack grows. Surface treatments such as shot peening and laser peening are used for crack growth retardation. It is generally believed that it is through the introduction of what is termed ‘beneficiary compressive residual stresses’ that crack retardation occurs. The compressive residual stresses are superimposed on the ‘detrimental tensile stresses’ due to loading and hence lead to a lower SIF level. By having such a strong tool as weight functions, this general belief can be put to test. To this end, a set of experiments were carried out to study the behaviour of cracks in residual stress fields arising from laser peening. Edge cracks were grown in partially-peened specimens. Neutron diffraction stress measurements were taken and stress profiles were obtained for these specimens. Measurements of strain fields near the crack show the interaction between the crack and the stress field induced by the peening process. The effect of laser peening on crack growth is discussed and recommendations for future work are proposed. Overall the thesis proposes a weight function for surface cracks the uniqueness of which is in its simplicity, and develops an understanding of the nature of induced and transient stresses in laser-peened components. The concept of ‘effective fatigue stress’ is introduced and its calculation is described, and conclusions are drawn from the nature of this stress distribution.
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    A novel weight function for RMS stress intensity factor determination in surface cracks
    (Elsevier, 2010-01) Chahardehi, Amir Ebrahim; Brennan, Feargal P.
    This paper discusses the problem of stress intensity factor determination in surface cracks. In particular, the concept of root mean square stress intensity factors (RMS SIF) is discussed for the general class of semi-elliptical surface cracks. The weight function SIF derivation method is considered problems with the existing techniques are highlighted, and a novel technique for the derivation of the RMS SIF weight functions for surface cracks is presented and results are compared with numerical solutions for a variety of loadings and geometries.
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    Surface Crack Shape Evolution Modelling using an RMS SIF approach
    (Elsevier, 2010) Chahardehi, Amir Ebrahim; Brennan, Feargal P.; Han, S. K.
    The majority of fatigue cracks in thick plate and tubular sections in structural components are two-dimensional surface cracks having significant propagation lives before becoming critical. The modelling of surface crack propagation life is important across a range of industries from power generation to offshore so that inspection, maintenance and repair strategies can be developed. Linear Elastic Fracture Mechanics based predictions are commonplace, however, unlike thin sections with associated one dimensional cracks frequently encountered in aerospace industries, crack shape or aspect ratio has a profound effect on crack front Stress Intensity Factor and any resulting Paris Law based life prediction. The two most commonly used approaches are to calculate the crack growth rate at a number of points around the crack front and to consider only surface and deepest points, calculating the relative crack growth rates. Experience using these approaches has shown that the Paris Law co-efficient as determined from plane stress specimens appears not to be applicable to crack growth in both length and depth directions. This paper examines this apparent anomaly, explaining why this discrepancy exists and suggests a practical solution using an RMS SIF approach for surface cracks that negates the need to correct the plane stress Paris law constants.