TY - BOOK

T1 - Application of the configurational forces concept for the design of tough, strong and damage-tolerant composites

AU - Sistaninia, Masoud

N1 - no embargo

PY - 2017

Y1 - 2017

N2 - This thesis deals with the application of the concept of configurational forces for the design of novel damage-tolerant and fracture-resistant materials and components. Recently, numerical modeling with application of the concept of configurational forces has revealed that multilayer structures with thin, compliant interlayers can have highly improved fracture strength and fracture toughness compared to the homogeneous bulk material, if the composite architecture fulfills certain design rules. The reason for this effect is the strong reduction of the crack driving force, if the crack tip is located in the interlayer with low Young’s modulus, which can lead to crack arrest. The aim of the current thesis is to extend the idea of utilizing the material inhomogeneity effect for the enhancement of the fracture toughness and fracture stress to technical elastic–plastic materials. It will be shown that it is possible to improve the strength and the fracture toughness of inherently brittle matrix materials by the introduction of thin interlayers that have the same Young’s modulus but lower yield stress than the matrix. The reason is that a crack arrests near the interface to the hard matrix material, caused by the strong decrease of the crack driving force. This effect appears without previous delamination of the interlayer. The effectiveness of soft interlayers as crack arrester is quantified by numerical case studies, based on the application of the configurational forces concept. The decisive parameters influencing the effect are the interlayer spacing (the wavelength of the yield stress variation), the interlayer thickness and the ratio of the yield stress between interlayer- and matrix material. Based on numerical simulations, it is demonstrated how to find, for a given matrix material and load, the architectural parameters of the multilayer in order to enhance the fracture stress and the fracture toughness of the material. An iterative procedure is proposed to find the optimum configuration. It is found that the optimum wavelength is inversely proportional to the square of the applied stress. The design concept presented in this thesis can be applied for different types of multilayers and loading scenarios. Experimental results of fracture tests, conducted on compounds made of high-strength steel as matrix and low-strength steel as interlayer material, confirm the findings. This thesis also deals with another type of material inhomogeneity, the thermal expansion inhomogeneity effect, which plays a significant role in the thermal behaviors of refractory materials, such as magnesia-spinel refractories. The concept of configurational forces is applied for the investigation of thermal shock resistance of magnesia spinel products during the cooling from the burning temperature. Evaluations provide valuable insights into the behavior of these composites. It is shown that the pronounced damage initiation, in combination with the low crack driving force, is the main reason for the good thermal shock resistance of magnesia spinel refractories.

AB - This thesis deals with the application of the concept of configurational forces for the design of novel damage-tolerant and fracture-resistant materials and components. Recently, numerical modeling with application of the concept of configurational forces has revealed that multilayer structures with thin, compliant interlayers can have highly improved fracture strength and fracture toughness compared to the homogeneous bulk material, if the composite architecture fulfills certain design rules. The reason for this effect is the strong reduction of the crack driving force, if the crack tip is located in the interlayer with low Young’s modulus, which can lead to crack arrest. The aim of the current thesis is to extend the idea of utilizing the material inhomogeneity effect for the enhancement of the fracture toughness and fracture stress to technical elastic–plastic materials. It will be shown that it is possible to improve the strength and the fracture toughness of inherently brittle matrix materials by the introduction of thin interlayers that have the same Young’s modulus but lower yield stress than the matrix. The reason is that a crack arrests near the interface to the hard matrix material, caused by the strong decrease of the crack driving force. This effect appears without previous delamination of the interlayer. The effectiveness of soft interlayers as crack arrester is quantified by numerical case studies, based on the application of the configurational forces concept. The decisive parameters influencing the effect are the interlayer spacing (the wavelength of the yield stress variation), the interlayer thickness and the ratio of the yield stress between interlayer- and matrix material. Based on numerical simulations, it is demonstrated how to find, for a given matrix material and load, the architectural parameters of the multilayer in order to enhance the fracture stress and the fracture toughness of the material. An iterative procedure is proposed to find the optimum configuration. It is found that the optimum wavelength is inversely proportional to the square of the applied stress. The design concept presented in this thesis can be applied for different types of multilayers and loading scenarios. Experimental results of fracture tests, conducted on compounds made of high-strength steel as matrix and low-strength steel as interlayer material, confirm the findings. This thesis also deals with another type of material inhomogeneity, the thermal expansion inhomogeneity effect, which plays a significant role in the thermal behaviors of refractory materials, such as magnesia-spinel refractories. The concept of configurational forces is applied for the investigation of thermal shock resistance of magnesia spinel products during the cooling from the burning temperature. Evaluations provide valuable insights into the behavior of these composites. It is shown that the pronounced damage initiation, in combination with the low crack driving force, is the main reason for the good thermal shock resistance of magnesia spinel refractories.

KW - Mehrschichtverbundwerkstoffe

KW - Materialinhomogenitätseffekt

KW - Konfigurelle Kräfte

KW - Finite Elemente Simulation

KW - Risstreibende Kraft

KW - Bruchzähigkeit

KW - Multilayer composites

KW - Material inhomogeneity effect

KW - Configurational forces

KW - Finite element modeling

KW - Crack driving force

KW - Fracture toughness

M3 - Doctoral Thesis

ER -