Key Engineering Materials Vols. 385-387

Abstract: It is well established that within the lower-shelf temperature range of Fe2-3Si polycrystalline steels, the brittle fracture occurs predominately by transgranular cleavage, unless subject to embrittling heat-treatments. The cleavage fracture develops on the well established {001} planes of the bcc structure. In this paper we revisit the growth, of these cleavage cracks by considering crack propagation in single crystals of Fe2Si steel. Three point bend specimens manufactured from oriented crystals have been tested by impact loading at a temperature of -196°C. High spatial resolution focused ion beam imaging combined with ion milling is used to examine in detail the crack propagation path and has provided a new insight into the mechanisms involved. In particular it has been established that within the process zone of the propagating cracks local strain is accommodated by the formation of {112} twins. The results are discussed with respect to the overall crack propagation mechanism.

Abstract: Reliable prediction of fatigue crack growth rates in aerospace materials and components underpins the so-called defect-tolerant approach to lifing. In this methodology the presence or appearance of defects and cracks in components is accepted. However, safe operation is guaranteed by regular inspections and health monitoring, and ensuring (by means of reliable modelling) that no crack may grow far enough to reach the critical size in the interval between inspections. Under such circumstances it is clear that particular attention has to be paid to the development and validation of predictive modelling capabilities for fatigue crack propagation. The situation is complicated by the fact that it is often a challenge to represent correctly the in-service loading experienced by a cracked component. In practice, on top of the major cycles associated with each flight (LCF component), cycles of higher frequency and lower amplitude are also present (HCF component). Sensitivity to dwell at maximum load is also often observed. Furthermore, it is well established that complex load sequences involving overloads and underloads result in fluctuations of fatigue crack growth rates (retardation and acceleration) that must be accounted for in crack growth calculations. In the present study we consider the application of an approach due to Noroozi et al. [1] to the analysis of R-ratio effects in Ti-6Al-4V material, on the basis of the experimental crack growth rate data collected under the auspices of AGARD programme [2]. The approach shows promising results, and has the capacity to capture loading sequence effects.

Abstract: Cotterell and Rice theory (1980) on the kinking of a crack submitted to a biaxial loading in a homogeneous material has been recently revisited (Leguillon and Murer 2008). The mixed criterion for fracture which involves both an energetic and a stress condition (Leguillon 2002) allows defining a positive threshold of the T-stress below which no branching can occur (Selvarathinam and Goree 1998). This analysis enters within a more general mixed-mode analysis (I+II+T-stress). Despite the complex terms and the oscillations, results extend to interfacial cracks. The assumption of a crack jump as a consequence of the energy balance allows getting rid of the problem brought by the oscillations due to these complex terms. This approach brings a new insight on the prediction of crack kinking out of a bimaterial interface.

Abstract: This paper presents applications where the DBEM formulations presented by Dirgantara and Aliabadi [3, 4] is combined with the multi region BEM presented recently by Baiz and Aliabadi [2], for the analysis of cracked shear deformable plates and shallow shell assemblies. Stress intensity factors are obtained using the CTOD technique. Several examples are solved to demonstrate the capabilities of the proposed technique. Comparing DBEM with FEM, it was clear that good accuracy and e􀀡ciency can be achieved with the present multi region DBEM.

Abstract: Triaxiality function (Rv) has been known as one of the important factors that responsible for damage initiation in adhesive bonding. Damage evolution law for low cycle fatigue (LCF) is function of Rv, von Mises equivalent stress (Seqv) and number of cycles (N). From previous research, it was found that the Rv values of two cases: bulk adhesives and single lap joint (SLJ), were close to unity. Those values are uncontrollable. Meanwhile, the damage equation for general solution contains Rv as an independent variable. There is need to choose another joint type that can characterise Rv as an independent variable. This paper presents the choice of scarf joint as specimen that can simulate variation of Rv. Several types of adhesive joints have been modelled and analysed using ANSYS as finite element analysis (FEA) tool. In ANSYS, Rv values were calculated directly from direct output results: von Misses equivalent stress and Hydrostatic stress. From FEA, it was shown that Rv changed as a function of adhesive bondline angle of the scarf joint. The values of Rv are constant along adhesive line except at the free edges. This choice is better than Cleavage joint where the values of Rv are not constant along adhesive line due to the presence of bending moment.

Abstract: Concrete is a type of engineering composites which is widely used in military and civil structures. The static performance of concrete can not be used to estimate the variation of concrete under high strain rates under dynamic loads. By introducing the three point bending test under static and dynamic loads, the plane strain fracture toughness KIC and KId were measured. Using the application of experimental-numerical method a three-dimensional model has been established by FEM software Ansys/LS-DYNA, to carryout transient analysis, and the response curves of stress intensity factor-time through the opening displacement of fracture. Thus, with obtaining the cracking time by the method of strain-gauge, the dynamic plane strain fracture toughness KId can be determined. By comparing KIc and KId we could conclude the result which meant concrete was more liable to crack under dynamic load.

Abstract: A very large number of variables affect the response of bolted or riveted joints typically used in aerospace applications: geometry of the joint, characteristics of the sheets, friction between sheets, geometry of head and kind of fastener, amplitude of clearance before assembly, mounting axial load, pressure effects after manufacture. It must be also recalled that these parameters influence the many failure modes existing for such joints, among which a relevant importance is attributed to bearing. The present paper deals with the study of the influence of assembly parameters on the joint operational behaviour and in particular with the analysis, performed through numerical simulations, of the influence of the residual stress-strain state coming from the riveting operation on the bearing resistance of an aluminium alloy joint. This work has been developed within the FP6 research project called MUSCA (Non linear static multiscale analysis of large aero-structures).

Abstract: In this study failure analysis of a mini van diesel engine intake valve is analyzed in detail. Failure occurred at the groove of the valve. First mechanical and chemical properties of the valve are determined. After visual investigation of the fracture surface it is concluded that fracture occurred due to fatigue. Damaged valve, valve seat, cylinder block and piston surface is investigated and construed. Failure constituted 300 kilometers after the rectification of the engine. Therefore rectification and assembly steps of valve and engine parts are analyzed in detail. Considering assembly, design and working conditions of the engine, stresses occurring at the failure surface are calculated. Endurance limit and fatigue safety factor is determined. Because of high fatigue safety factor, reasons for failure are investigated and effects causing stress increase at groove of the valve are established. These effects are faulty assembly of the valve and valve seat causing a high impact effect. In conclusion effect of impact ratio on stress increase is explained.