Key Engineering Materials Vols. 488-489

Abstract: The car body of the bi-modal tram has been designed and built to have low floor to provide the old and the handicapped with easy access considering Korean physical standard, passenger capacity (standee, seated, handicapped), and bus and urban railway regulations in Korea. The car body has been composed of composite materials (light-weight sandwich constructions) with glass epoxy skins, aluminum honeycomb cores and aluminum inner-frames. This paper was aimed to evaluate dynamic characteristics of the car body while the bi-modal tram was running in the test track. Based on the results from vibration accelerations measured on the floor level of centers of car bodies and axle, dynamic characteristics of the composite materials car body were discussed in this paper.

Abstract: The paper deals with the discussion how nominal spur tooth root stress of thin-rimmed gear determined in accordance with the standard ISO 6336-3:2006, method B, agrees with the results of the 3D FEM stress analysis. The procedure proposed by standard ISO for the tooth root stress calculation doesn’t identify the stress behaviour related to mutual affects of characteristic thin-rimmed gear geometrical parameters. Therefore, the 3D FEM analysis of tooth root stress has been performed for gear structures with middle and offset web with various rim and web thickness. The rim thickness has covered and slightly overcome the backup ratio defined by ISO procedure, while the web thickness has covered the range of practical interest.

Abstract: Calibration and exploitation of failure criterion is at present a challenging field in the structural integrity scenario. Calibrated failure criteria allow the simulation/reproduction of damages using virtual approach and eventually further assessment of the residual integrity of the components. Therefore the increase of awareness in failure issues makes the numerical simulation an actual, useful and reliable tool for the analysis of complex structures under extreme loads, especially in aerospace field where full scale tests are often very expensive and difficult to carry out. With this aim, the constitutive relations of an Aluminium Al 6061-T6 alloy have been calibrated with dedicated focus on failure criterion. The results obtained have been discussed considering the crystallographic measurements that permit to point out the dissipative behavior on the basis of texture formation as a function of the load type. The final aim is to confirm and explain the different failure behavior depending on the different stress triaxiality.

Abstract: Components in jet engines are subject to time-dependent multiaxial loading. This creates a time-varying mixed-mode stress state at the crack tip. Mixed-mode loading leads to out-of-plane crack propagation and has been treated in previous articles [1,2]. This paper concentrates on coping with the time-dependent character. Key issues are the crack propagation rate and the crack propagation direction in three-dimensional space. In order to determine the prevalent crack propagation direction the dominant loading case is determined based on its crack propagation rate. Then, a mixed-mode equivalent K-factor is calculated for all other loading cases based on the closeness of their associated crack propagation direction with the dominant one. Subsequently, a cycle extraction is performed on the crack propagation rate for all loading cases. The extracted cycles are processed based on their mimimum and maximum equivalent K-factor and the maximum temperature. The mission crack propagation rate consists of the sum of the rate of all extracted cycles.

Abstract: Purpose: Applications for highly corrosive environments and cyclic loading are often made out of austenitic stainless steels. Corrosion fatigue and crack propagation behaviour has been studied to determine failure processes and damage mechanisms. Approach: CrNiMo stabilized austenitic stainless steel and CrMnN austenitic stainless steel in solution annealed and cold worked condition are compared. S/N curves and crack propagation rate curves are recorded in 43 wt% CaCl₂ solution at 120 °C, which resembles most severe potential service conditions. For comparison these experiments are also performed in inert glycerine. Additionally, the electrochemical behaviour of these materials has been studied. Findings: The CrMnN steels have excellent mechanical properties but are very susceptible to stress corrosion cracking in the test solution. The fatigue limit as well as the threshold for long crack growth are significantly reduced in corrosive environment. Moreover these steels exhibit a remarkable increase in the propagation rate, which is extremely pronounced in the near threshold region. This effect is enhanced by cold working. CrNiMo steels also show a reduction in the fatigue limit, but it is less pronounced compared to CrMnN steels. The threshold is significantly reduced in corrosive environment, but propagation rate is lower in corrosive environment compared to inert glycerine. Possible explanations of this surprising behaviour are discussed.

Abstract: The crack path and growth life of surface initiated rolling contact fatigue was investigated numerically based on the asperity point load mechanism. Data for the simulation was captured from a gear contact with surface initiated rolling contact fatigue. The evolvement of contact parameters was derived from an FE contact model where the gear contact had been transferred to an equivalent contact of a cylinder against a plane with an asperity. Crack propagation criteria were evaluated with practically identical crack path predictions. It was noted that the trajectory of largest principal stress in the uncracked material could be used for the path prediction. The mode I fracture mechanism was applicable to the investigated rolling contact fatigue cracks. The simulated path agreed with the spall profile both in the entry details as in the overall shape, which suggested that the point load mechanism was valid not only for initiation but also for rolling contact fatigue crack growth. Different equivalent stress intensity factor ranges were used to estimate the fatigue life, which agreed with the life of the investigated gear wheels.

Abstract: Friction Stir Channelling (FSC) is a simple and innovative technique of manufacturing integral and continuous channels (also referred as conformal channels) in monolithic plates in a single step. This paper is focused on the mechanical behaviour of integral and continuous FS channels produced in a monolithic plate of the aluminium alloy AA7178‑T6 with 13mm of thickness, typically used in structural aircraft applications. Internal pressure tests were conducted on specimens with a longitudinal friction stir channel. In-plane bending tests were carried out in specimens with longitudinal and transversal channels; different conditions were analysed for each FSC parameters set. Results were analysed and compared concerning the FS channels microstructure and base material mechanical properties. Bending tests results show that tool rotation speed has more influence in the FSC specimens bending strength than tool travel speed and internal pressure tests show that increase tool rotation speed increases the minimum pressure that leaking points arise.

Abstract: This article deals with the development of functional graded materials which is pursued by the collaborative research centre Transregio 30. The functional gradation is produced by a thermo-mechanical production process and is characterized by different fracture mechanical specific values. Because cracks are by all means responsible for failure of structural elements crack growth has to be considered during the development of these materials. In this contribution simulations of crack growth which were carried out with the system ADAPCRACK3D are presented to illustrate the influence of a material gradation on crack growth.

Abstract: This paper deals with calculation procedure for lifetime determination of a large three-row roller slewing bearing. First, the contact force distribution between the rollers and the bearing raceway is determined both analytically and numerically. Then, the position and the magnitude of the maximum contact force are identified. In a separate numerical model finite element analyses are performed to determine the subsurface stress distribution as the result of contact between the raceway and different rollers. These stress fields serve as an input for the fatigue lifetime calculation, which is done in accordance with the stress-life approach (S-N). At the end the influence of roller geometry on the calculated fatigue life is presented.

Abstract: Macroscopic delamination of polymer-metal interfaces is one of the main failure modesobserved in micro-electronic components. Due to the irregularly shaped metal roughness profile, thisdelamination not only consists of interface separation but also bulk cracking at the micro-scale ofthe roughness. In fact, one of the key mechanisms that results in increased adhesion toughness atroughened interfaces is the transition from adhesive to cohesive failure. A semi-analytical approachis discussed in which the competition between adhesive and cohesive cracking is analyzed by meansof the theoretical relation between interface and kinking stress intensity factors. The parameters thatdefine this relation, the solution coefficients, are quantified by finite element (FE) simulations. Accordingly, the crack kinking location and kinking angle into the softer polymer is readily calculated.Furthermore, the geometrical effect of roughness is evaluated by means of FE simulations in whichthe interface topology follows from measured roughness profiles while also including interface delamination using cohesive zone elements.