Amir Malakizadi

Inverse identification of flow stress in metal cutting process using Response Surface Methodology

In this study, a methodology was presented to determine the flow stress behaviour of the work material within the range of strain, strain rate and temperature encountered during chip formation process by means of inverse modelling of orthogonal cutting operations. This approach was based on the concept of Design of Experiments (DOEs) and Response Surface Methodology (RSM). Initially, an extension of Oxleys machining theory incorporating the Johnson-Cook material model was integrated with RSM to accomplish a fast assessment of the material parameters. Having provided the material parameters by Oxleys machining theory, the optimum set of friction coefficients were determined through evaluation of the Finite Element (FE) simulation results. The final step involved direct integration of 2D FE models incorporating the optimum frictional boundary conditions with RSM to reassess the optimum set of material parameters. This approach was implemented to determine the constitutive parameters for wide range of materials including Inconel 718 in aged condition, AISI 1080 plain carbon steel and AA6082-T6 aluminium alloy. The calibration of material models using the presented inverse methodology led to a significant improvement in simulation results. The reasons for the robustness of the proposed inverse methodology were discussed.

Thermo-Mechanical Fatigue Life Prediction of a Heavy Duty Diesel Engine Liner

Engine designers are increasingly using more advanced simulation techniques to reduce design time and costs and at the same time to improve the accuracy of the work to limit the number of validation tests required. In recent years, the demand for higher specific power has enforced higher operating temperatures in engine parts, so thermo-mechanical fatigue (TMF) analysis is becoming more important. Liners are one of the challenging parts in heavy duty diesel engines which are exposed to high temperature differential between cooling jacket and combustion chamber and also they are in frictional contact with piston rings, therefore liners are subjected to complicated multiaxial thermal and mechanical stresses. In this study, a detailed analysis is conducted on a centrifugal gray cast iron liner using different CAE tools to have a more accurate estimate of thermo-mechanical loads. The coolant flows inside the liner jacket and combustion process within the cylinder are simulated using 3D CFD methods. Besides the linear FE analysis which is known as an oversimplified method, a multilinear kinematic model is used to simulate the material response more accurately. Using experimental results obtained from cylinder liner material, the different TMF approaches have been investigated and the differences between linear and nonlinear FE simulation in lifetime prediction have been revealed. It is shown that modified Manson-Coffin criterion is the promising relation for lifetime prediction of liner material that correlate better with experimental results.Copyright

A Fast Coupled CFD-Thermal Analysis of a Heavy Duty Diesel Engine Water Cooling System

In recent years, the design of an efficient cooling system together with good thermal efficiency for a new engine is becoming a critical task and therefore the need for an accurate and fast thermo-fluid simulation of engine cooling system is of vital importance. In this study, a detailed CFD and thermal FE simulation of a 12 cylinders V-type medium speed heavy duty diesel engine cooling system has been carried out using ANSYS-CFX commercial code. At first, a global model, for one bank with six cylinders, has been simulated using appropriate mesh density which ensures the accuracy of the results together with reasonable computational time. At this stage, the worst cylinder has been selected based on the wall temperature and the cooling flow rate. Later, using the inlet and outlet boundary conditions extracted from the global model, a series of detailed thermo-fluid analyses have been conducted for the worst cylinder with a finer mesh. The subcooled nucleate boiling heat transfer computation is carried out using the boiling departure lift-off (BDL) model, in which the total heat flux is assumed to be additively composed of a forced convective and a nucleate boiling component. In order to obtain the temperature field for components under consideration, a comprehensive thermal analysis has been preformed coupling with the detailed CFD analyses to reach an accepted value through transferring data between the CFD and FEA software. This method leads to an accurate prediction of the wall temperature and heat flux. It is observed that at hot spots, nucleate boiling occurs for low coolant flow regions specifically around the cylinder head’s exhaust port and liner coolant side wall. Also a considerable increment in the Heat Transfer Coefficient (HTC) has been observed on the superheated regions where the boiling is initiated.Copyright

High cycle fatigue life assessment of a heavy duty diesel engine cylinder head

Among all critical components in heavy duty diesel engines, cylinder head is generally counted as the most challenging part due to the complex loading conditions. This component experiences both thermal and mechanical stresses. Temperature difference between combustion gases and cooling water induces significant thermal stress in the flameface, while the firing pressure builds up cyclic mechanical stresses superimposed on the former thermal stress. On the other hands, cylinder heads are geometrically complex and generally contain many small fillets and blends. These small geometrical features and complex loading condition cause multiaxial stress state which should be considered during High Cycle Fatigue (HCF) life assessment. In this study, a detailed finite element analysis has been conducted on a heavy duty diesel engine cylinder head, which is followed by HCF life assessment. In order to predict the HCF safety factor, a post-processing routine has been developed using ANSYS Parametric Design Language (APDL) based on a normal stress critical plane approach. This method is shown as a promising approach for multiaxial HCF life assessment of a wide range of engineering metals. Moreover, normal stress critical plane approach is known as a computationally time efficient approach for the huge finite element models. Notch effect has been considered by introducing the relative stress gradient parameter, which is obtained from the best fit of fatigue test data for different notch geometries. Finally, the effect of HCF on Low Cycle Fatigue (LCF) crack growth has been discussed.