TY - THES

T1 - Numerical Studies on Crack Arrays in Aluminum Pressure Die Casting Molds

AU - Raninger, Peter

N1 - embargoed until null

PY - 2009

Y1 - 2009

N2 - Pressure casting dies are exposed to harsh service conditions consisting of thermal and mechanical loading and thus undergo thermo-mechanical fatigue. Due to cyclic plastic deformation of the material near the surface of the dies the loading conditions gradually change because of the formation of tensile residual stresses which add to the stress field from external loading. This change in the stress field influences the nucleation and growth of cracks. Typically after a few thousand casting cycles a network of heat checks forms. Within a crack network crack shielding has a big influence on the evolution of the crack array. The crack spacing within the network of heat checks, the length at which the cracks stop growing and the crack growth rate are influenced by the thermo-physical and mechanical properties of the die material. In the present study the influences of the material properties on the formation and growth of heat check networks are investigated in a numerical study on the evolutions of crack arrays. In order to obtain the loading conditions for the numerical study on crack arrays a finite element (FE) model of a flat surface area of the pressure casting die is generated to calculate the transient temperature field and the evolution of residual stresses over 100 casting cycles. The temperature field and the residual stress field of cycle 100 are used in script based FE-models with linear elastic material behavior investigating the formation and evolution of crack arrays. By the aid of a Python script FE-models are created, computed and evaluated automatically. In this way a modeling chain is formed in which data obtained from the previous model are used to create the subsequent one. For this purpose, the script compares the maximal stress intensity factor Kmax to a user defined criterion. In this criterion Kmax is converted, based on experimental data and parallel computation of stress ratios, to the effective stress intensity factor DeltaKeff and compared to a threshold value DeltaKth, which is a measure for crack arrest. In this way an initial crack configuration and the evolution of crack arrays can be modeled in an automated manner for different materials. The focus in this work is put on the interplay of material parameters and crack shielding and on the interconnection between hardness, thermal conductivity and thermal expansion. The ultimate goal in this work is to provide mathematical relations linking material parameters and crack propagation that can be used for material selection and material development.

AB - Pressure casting dies are exposed to harsh service conditions consisting of thermal and mechanical loading and thus undergo thermo-mechanical fatigue. Due to cyclic plastic deformation of the material near the surface of the dies the loading conditions gradually change because of the formation of tensile residual stresses which add to the stress field from external loading. This change in the stress field influences the nucleation and growth of cracks. Typically after a few thousand casting cycles a network of heat checks forms. Within a crack network crack shielding has a big influence on the evolution of the crack array. The crack spacing within the network of heat checks, the length at which the cracks stop growing and the crack growth rate are influenced by the thermo-physical and mechanical properties of the die material. In the present study the influences of the material properties on the formation and growth of heat check networks are investigated in a numerical study on the evolutions of crack arrays. In order to obtain the loading conditions for the numerical study on crack arrays a finite element (FE) model of a flat surface area of the pressure casting die is generated to calculate the transient temperature field and the evolution of residual stresses over 100 casting cycles. The temperature field and the residual stress field of cycle 100 are used in script based FE-models with linear elastic material behavior investigating the formation and evolution of crack arrays. By the aid of a Python script FE-models are created, computed and evaluated automatically. In this way a modeling chain is formed in which data obtained from the previous model are used to create the subsequent one. For this purpose, the script compares the maximal stress intensity factor Kmax to a user defined criterion. In this criterion Kmax is converted, based on experimental data and parallel computation of stress ratios, to the effective stress intensity factor DeltaKeff and compared to a threshold value DeltaKth, which is a measure for crack arrest. In this way an initial crack configuration and the evolution of crack arrays can be modeled in an automated manner for different materials. The focus in this work is put on the interplay of material parameters and crack shielding and on the interconnection between hardness, thermal conductivity and thermal expansion. The ultimate goal in this work is to provide mathematical relations linking material parameters and crack propagation that can be used for material selection and material development.

KW - thermo-mechanical fatigue crack shielding pressure die casting hot work tool steels finite element simulation

KW - Thermische Ermüdung Rissabschirmung Aluminium Druckguss Warmarbeitsstahl Finite Elemente Simulation

M3 - Diploma Thesis

ER -