0 Introduction quantification of various phenomena near the fatigue threshold,e.g.,crack closure effects under plane strain conditions [10,11].These concepts were also able to provide a physically clear background of the size effect observed in micro and nanocomponents [12]. During 1975-1985,many experimental results verified an anomalous frac- ture behaviour of ultra-high-strength low-alloy(UHSLA)steels.The fracture toughness of these steels significantly increased with coarsening of the mean prior austenite grain size (raising austenitizing temperature).On the other hand,the absorbed energy in impact tests revealed the opposite trend.These contradictions could be elucidated by an extreme geometrical shielding of the crack tip that was produced by decohesion of coarse grain boundaries dur- ing the fracture toughness tests.A multiscale model that coupled methods based on topology,stereology and metallography with finite element analy- sis quantitatively predicted fracture behaviour in very good agreement with experimental data.A similar model could quantitatively reproduce a steep increase in fracture toughness of borosilicate glasses with increasing concen- tration of reinforced particles (see also Chapter 2). Besides a survey of our work,this book aims to present a rather consistent overview of fracture micromechanisms.Indeed,all the fundamental kinds of fracture processes and related micromechanisms are considered except for high-temperature creep damage.The description of fracture processes ranges from atomistic up to macroscopic levels.Therefore,the multiscale context can be easily found in many models of fracture processes that couple such approaches by means of sequential integration.Although emphasis is given to metallic materials,the fracture behaviour of ceramics and composites is also discussed. The topic of the first chapter is the deformation and fracture of perfect crystals.Their mechanical behaviour under various kinds of monotonously increasing (static)loading is preferentially investigated by means of ab ini- tio (first principles)methods based on electronic structure calculations.It is our strong belief that engineers can also learn a lot from the results of these apparently academic studies.They provide a clear distinction between in- trinsic lattice properties and those induced by defects and secondary phases in engineering materials.They also constitute physical benchmarks for en- gineering multiscale models such as upper and lower limits of fracture and fatigue characteristics (ideal strength and fracture toughness).Three-scale models coupling electronic structure with atomic arrangements and crystal- lography are applied to calculate the ideal strength of crystals and nanocom- posites under various loading conditions.Multiscale models predicting intrin- sic ductile/brittle behaviour of crystals are also presented.These models cou- ple atomistic,crystallographic and fracture-mechanics approaches.Finally,a multiscale model of nanoindentation is presented in order to quantify pop-in effects observed in load-penetration diagrams.This model links numerical methods and results embracing all spatial scales from nano to macro.6 0 Introduction quantification of various phenomena near the fatigue threshold, e.g., crack closure effects under plane strain conditions [10, 11]. These concepts were also able to provide a physically clear background of the size effect observed in micro and nanocomponents [12]. During 1975–1985, many experimental results verified an anomalous frac￾ture behaviour of ultra-high-strength low-alloy (UHSLA) steels. The fracture toughness of these steels significantly increased with coarsening of the mean prior austenite grain size (raising austenitizing temperature). On the other hand, the absorbed energy in impact tests revealed the opposite trend. These contradictions could be elucidated by an extreme geometrical shielding of the crack tip that was produced by decohesion of coarse grain boundaries dur￾ing the fracture toughness tests. A multiscale model that coupled methods based on topology, stereology and metallography with finite element analy￾sis quantitatively predicted fracture behaviour in very good agreement with experimental data. A similar model could quantitatively reproduce a steep increase in fracture toughness of borosilicate glasses with increasing concen￾tration of reinforced particles (see also Chapter 2). Besides a survey of our work, this book aims to present a rather consistent overview of fracture micromechanisms. Indeed, all the fundamental kinds of fracture processes and related micromechanisms are considered except for high-temperature creep damage. The description of fracture processes ranges from atomistic up to macroscopic levels. Therefore, the multiscale context can be easily found in many models of fracture processes that couple such approaches by means of sequential integration. Although emphasis is given to metallic materials, the fracture behaviour of ceramics and composites is also discussed. The topic of the first chapter is the deformation and fracture of perfect crystals. Their mechanical behaviour under various kinds of monotonously increasing (static) loading is preferentially investigated by means of ab ini￾tio (first principles) methods based on electronic structure calculations. It is our strong belief that engineers can also learn a lot from the results of these apparently academic studies. They provide a clear distinction between in￾trinsic lattice properties and those induced by defects and secondary phases in engineering materials. They also constitute physical benchmarks for en￾gineering multiscale models such as upper and lower limits of fracture and fatigue characteristics (ideal strength and fracture toughness). Three-scale models coupling electronic structure with atomic arrangements and crystal￾lography are applied to calculate the ideal strength of crystals and nanocom￾posites under various loading conditions. Multiscale models predicting intrin￾sic ductile/brittle behaviour of crystals are also presented. These models cou￾ple atomistic, crystallographic and fracture-mechanics approaches. Finally, a multiscale model of nanoindentation is presented in order to quantify pop-in effects observed in load-penetration diagrams. This model links numerical methods and results embracing all spatial scales from nano to macro