Key Engineering Materials Vols. 385-387

Abstract: The present paper reports the investigation of a sudden blade fracture leading to a fatal engine failure. The blade sample was subjected to a series of examinations, including visual examination and photographic documentation, optical microscopy, scanning electron microscopy (SEM), using both energy dispersive spectroscopy (EDS) and X-ray mapping. Analysis of all elements show that loss of aluminum from coating due to oxidation and coating phases changing; decreasing of alloy ductility and toughness due to carbides precipitation in grain boundaries; degradation of the alloy gamma prime γ' phase (aging and coarsening) and micro cavities. These were found on fracture surfaces which served as an origin of creeping failure mechanism and are the most important factor for failure of this blade.

Abstract: Fatigue properties of a squeeze cast Al-Si-Mg alloy were examined under rotating bending and the effect of humidity on the crack initiation and propagation behavior was investigated in controlled humidity. The fatigue strength was decreased in high humidity (RH85%) in comparison with the one in low humidity (RH25%). Cracks initiated at the early stage of fatigue life from slip bands and eutectic Si particles in many cases and partially casting defects, and most of fatigue life was occupied by the growth life of a small crack regardless of humidity. The propagation of a crack were promoted by humidity, especially the growth rate of a crack larger than about 100 µm, which propagated in tensile mode, was strongly accelerated.

Abstract: This paper is concerned with the premature failures occurred in the high pressure compressor section of the gas turbine of HESA power plant in Iran. Metallurgical and mechanical properties of the blade alloy were evaluated. Fractography investigations were carried out on the fracture surface of the blade roots using scanning electron microscopy. Stress and fracture simulations were conducted using ANSYS software in both 2D and 3D dimensions under centrifugal, aerodynamic and contact forces. The aerodynamic forces were evaluated using FLUENT software. The results showed no metallurgical and mechanical deviations for the blade material from standards. SEM fractography showed different aspects of fretting fatigue including multiple crack initiation sites, fatigue beach marks, debris particles, and a high surface roughness on the edge of contact (EOC). The simulation results showed that there was a high stress gradient at the EOC of the blade which is one of the most significant characteristics of the fretting fatigue. Another analysis was performed to simulate the fracture by creating an initial crack on the EOC. The stress fields and stress intensity factors for modes I, II and III were evaluated along the crack front. The results indicated a strong stress intensity factor for mode I at the EOC.

Abstract: Turbine vanes and blades are the most intensively loaded elements in that they are subjected to a large variety of mechanical and high temperature loads. The thermal barrier coatings (TBCs) are widely used on different hot components of gas turbines, as blades and vanes, for both, power engineering as well as aeronautical applications. Currently, two methods are used for depositing TBCs on substrate, which are plasma spray (PS) and electron beam-physical vapor deposition (EB-PVD). A typical TBCs system consists of two thin coatings, including a ceramic coating and a metallic bond coat. Despite considerable efforts, the highly desirable prediction of their life time is still a demanding task. The PS coating was focused on in this work. Firstly, the TBCs systems are multiplayer material systems. The material properties are not easily determined, such as Young’s modulus of the top-coating of TBCs. Using the resonant frequency and the composite beam theory, the Young’s modulus of APS TBCs was gotten under from room temperature to 1150°C. Then using a commercial finite-element program, the model geometry is that of a cylinder specimen. The interface region between bond coat and top coating is modeled and meshed with a sinusoidal geometry. The temperature was designed and cycled over a range from room temperature to 1050°C. The force-air-cooling was designed to form temperature gradient across the thickness of TBCs. Finally, the fatigue life of TBCs was predicated. The maximum relative error is 20.1%.

Abstract: The study of bi-material notches is becoming a topical problem as they can model geometrical or material discontinuities efficiently. Assessing the conditions for crack initiation in bimaterial notches makes it necessary to calculate the generalized stress intensity factors H. In contrast to the determination of the K factor for a crack in an isotropic homogeneous medium, for the ascertainment of a generalized stress intensity factor (GSIF) there is no procedure incorporated in the calculation systems. The calculation of these fracture mechanics parameters is not trivial and requires certain experience. Nevertheless, the accuracy of the H-factor calculation directly influences the reliability of the assessment of the singular stress concentrators. Direct methods of the estimation of H factors usually require choosing the length parameter entering into the calculation. Two types of direct methods of calculating the GSIFs are presented, tested and mutually compared. Recommendations for reliable estimation of H factors are suggested.

Abstract: Welding residual plastic strains and residual stresses are the product of complicated nonlinear behaviors of the structures during welding. The residual strains and stresses for stiffened stainless steel structure and welding fatigue analysis are studied in this paper. The three-dimensional transient temperature field is analyzed by FEM first. Doing the welding experiment for fundamental model is to verify the numerical results, the infrared thermograph is used to follow and record the whole process. The results of FEM are supported by experimental data. Then, the elastic-plastic-model is established to analyze the residual strains and stresses. Based on the stresses, loading and fatigue factors, according to Miner fatigue damage law, some fatigue behaviors such as fatigue life and remaining life are obtained.

Abstract: The stress distributed in the mid-bondline of the joints made of aluminium alloy and an epoxy adhesive was determined with the ANSYS software. The results from the FEA showed that the values of the peak stresses of the all the stress components (including the longitudinal stress Sx, the peel stress Sy, the shear stress Sxy, the 1st principal stress S1 and the von Mises equivalent stress Seqv) distributed in the mid-bondline are changed a little as the notch distance L was increased while the notch depth d was not great than 0.6mm. But the evidently changes occurred when the notch depth d was great than 0.9 mm for the stress Sx, Sy and S1 distributed in both the mid-bondline and the interface of the lap zone. When taken the stress distributed in the middle part of the lap zone into account, the peak stress at the point in the mid-bondline corresponding to the edge of the notch decreased firstly and then increased again as the notch distance L was increased from 0 mm to 8 mm. The proper geometry of the notch in the specimen was chosen by finite element analysis.

Abstract: In this paper, the experimental investigation into the enhancement of mechanical strength in shape memory alloy (SMA) fiber composites is made by using knotted fiber at the two ends instead of straight fiber. TiNi SMA fiber with both ends knotted is used for purpose of better ensuring stress transfer from the matrix to the fiber than straight fiber. Tension test is carried out above the austenitic finish temperature in air. Specimens are heated by means of electrical resistive lamplight heating. The results indicate that the mechanical strength is larger in the knotted fiber composite than in the straight fiber composite. Knotted fiber exerts the superiority of TiNi SMA fiber composite.

Abstract: The effect of nitrogen on creep properties of 22Cr-25Ni austenitic heat-resistant steels with different nitrogen content was studied. The tensile and creep strength increased with increasing nitrogen content. The tensile strength increased due to grain refinement by Nb-rich carbonitride. The creep strength increased with increasing nitrogen content because nitrogen retarded the formation and growth rate of Cr-rich carbide and the growth rate of Nb-rich carbonitride during creep.

Abstract: Impact properties of solder ball joints with Sn-9mass%Zn and Sn-3mass%Ag- 0.5mass%Cu lead-free solders were investigated under aging at 423 K. In the case of the joints with Cu electrodes, both Sn-9Zn and Sn-3Ag-0.5Cu joints have similar impact forces before aging. For the Sn-3Ag-0.5Cu joint, fracture occurred in an intermetallic compound (IMC) layer formed in a joint interface regardless of aging, and thus the impact force was stable upon aging at 423 K for 500 h. For the Sn-9Zn joint, fracture mode changed from solder fracture to other fracture upon aging and thus this change led a decrease in the impact force. In the Sn-9Zn joint with an electroless Ni/Au plated electrode, fracture occurred in the solder regardless of aging, and thus impact properties improved compared with the joint using the Cu electrode. The impact properties are superior to those of the Sn-3Ag-0.5Cu joints with the Cu and the electroless Ni/Au electrodes.