Solveig Melin

A tool to model short crack fatigue growth using a discrete dislocation formulation

A method is presented that combines the modelling of cracks by distributed dislocation dipoles with developing plasticity represented by discrete dislocations moving along slip bands. Crack growth is due to the emission of dislocations from the crack tip along preferred slip planes. Eventual annihilation of dislocations occurs by reunion with the corresponding displacement steps of the crack surface. Crack surface overlap is not allowed. The equilibrium state for each load increment is solved iteratively, allowing various crack geometries. The method is applied to the problem of a short edge crack growing in mode I due to fatigue loading. It is shown that the development of a local plastic zone and the propagation of the crack can be monitored in detail.

Directional stability of an originally straight crack

The directional stability of an originally straight crack under symmetric remote loading is studied after introduction of an infinitesimal disturbance at one or both tips of the crack. The crack is assumed to grow slowly under vanishing mode II stress intensity factor. Directional stability is defined to prevail if the angle formed by the straight line between the crack tips and the original crack direction eventually decreases during crack growth. This is shown to be the case if, and only if, the principal stress perpendicular to the original crack is the largest of the two in-plane stresses.Another candidate for definition of directional stability is also discussed, even though it appears less logical, since it concentrates on the position of the crack tips rather than on the main direction of the crack. It assumes that directional stability prevails if the crack tips eventually move closer toward the line along the original crack. This definition leads to directional instability when the principal stress in the original crack direction is larger than the fraction 1-π/4 of the other in-plane stress.

Numerical investigation of powder compaction of gear wheels

A method to judge the porosity distribution within complex powder compacted 3D structures using a dynamic 3D dilatant finite strain finite element program is presented. The method is demonstrated for a gear wheel, using a combined FKM Gurson model with parameters calibrated from experiments to model a ferrous powder. Compaction is pursued until a final average porosity of 3% in the gear. The method is shown successful in judging the influence on local as well as average properties from change in geometrical parameters and compaction speed.

On the directional stability of wedging

The directional stability of the crack path during wedging of a strip is investigated, using finite element methods. Linearly elastic material and plane conditions are considered. No dynamic effects are included. Stable crack growth in the direction of maximum mode I stress intensity factor at the tip of the crack is assumed. It is found that directional stability seems to prevail if the thickness of the wedge at the foremost point of contact between the wedge and the crack surfaces is less than about 1.69 KIc% MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGak0Jf9crFfpeea0xh9v8qiW7rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaOaaaeaaca% WG3bGaai4laiaadweaaSqabaaaaa!3A93!\[\sqrt {w/E}\], where KIc is the fracture toughness, w half the width of the strip and E the modulus of elasticity.

Long crack behavior in a thermal barrier coating upon thermal shock loading

The behavior of macroscopic long cracks in the ceramic top coat of a thermal barrier coating (TBC) system subjected to thermal shock loading and the influence of the cracks on the coating durability were investigated experimentally and numerically. Thermal shock testing was conducted until coating failure. Comparisons were made with coating samples without macroscopic cracks. The experimental results revealed that the presence of macroscopic cracks reduces the life of the TBC. The finite-element method, with a fracture mechanics approach, was applied to analyze preexisting long cracks, and the calculations correlate well with the experimental findings. It was found that the life of the coating is reduced with crack length as well as with maximum cycle temperature. It was also found that the stress-intensity factors for long cracks are initially high and decrease with the number of temperature cycles, which indicates that rapid crack growth occurs during the first number of cycles.

Numerical Modeling of short crack behavior in a thermal barrier coating upon thermal shock loading

The behavior of microstructurally short inherent cracks within a preoxidized thermal barrier coating system upon thermal shock loading is considered. A thin alumina oxide layer holding residual stresses was induced at the ceramic/metal interface to simulate thermally grown oxide on the bond coat. Undulation of the oxidized bond coat was modeled as a sinusoidal surface. The variations of the stress-intensity factors of inherent centrally located cracks and of edge cracks were calculated during the thermal cycling. The instant crack shapes during the first thermal cycle and at steady state were investigated. It was found that oxide layer thickness, crack tip location, as well as interfacial undulation are factors influencing the risk of crack propagation. It was also found that an edge crack constitutes a greater threat to the coating durability than a central crack. The propagation of an edge crack, if it occurs, will take place during the first load cycle, whereas for a central crack, crack tip position decides the risk of crack propagation.

Fracture mechanics analysis of microcracks in thermally cycled thermal barrier coatings

The effects from thermal shock loading on pre-existing microcracks within thermal barrier coatings (TBCs) have been investigated through a finite element based fracture mechanical analysis. The TBC system consists of a metallic bond coat and a ceramic top coat. The rough interface between the top and bond coats holds an alumina oxide layer. Stress concentrations at the interface due to the interface roughness, as well as the effect of residual stresses, were accounted for. At the eventual closure between the crack surfaces, Coulomb friction was assumed. To judge the risk of fracture from edge cracks and centrally placed cracks, the stress intensity factors were continuously monitored during the simulation of thermal shock loading of the TBC. It was found that fracture from edge cracks is more likely than from centrally placed cracks. It was also concluded that the propagation of an edge crack is already initiated during the first load cycle, whereas the crack tip position of a central crack determines whether propagation will occur.

Which is the most unfavourable crack orientation

The most unfavourable orientation of a straight crack of given length is investigated, assuming mode I growth. It is shown that this orientation is not always the one perpendicular to the largest principal stress. If the smallest in-plane principal stress is compressive and its magnitude more than about one third of the largest (tensile) in-plane stress, then some other orientations are more unfavourable. Crack growth then takes place after kinking.

Atomistic studies of the elastic properties of metallic Bcc nanowires and films

In this paper a systematic study of the surface influence on the elastic properties of nanosized iron and tungsten wires and films is performed. Single crystal defect-free nanowires and nanofilms are examined through molecular statics simulations, and the concepts of surface energy and third order elastic constants are used in an attempt to describe the elastic properties. For structures where the relaxation strains are small in magnitude, reasonable agreements between the continuum mechanical solutions and the simulations are obtained. For structures where the relaxation strains are significant it is shown that third order elastic continuum theory is not sufficient to describe the elastic properties; in fact, sometimes it actually increases the discrepancies between the simulated and predicted results.

Growth from a straight crack subjected to arbirtrary remote loading

Growth from straight cracks is studied, both for loading in pure mode I or II and for mixed mode loading. Small scale yielding and a plane condition are assumed. Growth is assumed to take place in either mode I or mode II. Loss of directional stability sometimes occurs even from cracks that seem destined for continued growth straight forward. A striking example is the case of collinear cracks under symmetric remote loading growing toward each other: before coalescence they deflect from their straight paths and avoid each other instead of running together tip to tip as expected. One single crack growing in mode I under symmetric loading conditions might suffer from loss of directional stability. Directional stability is here defined to prevail if the angle formed by the straight line between the crack tips and the original crack direction eventually decreases during growth. This is shown to be the case if, and only if, the principal stress perpendicular to the original crack is the largest of the two in-plane stresses. Another, although less logical, candidate for definition of directional stability is also discussed. It concentrates on the position of the crack tips rather than on the main direction of the crack and it is in this case assumed that directional stability prevails if the crack tips eventually move closer toward the line along the original crack. This definition leads to directional stability when the principal stress in the original crack direction is smaller than the fraction 1 -π/4 of the other in-plane stress. For a crack under mixed mode loading conditions an abrupt deviation from the straight path is expected. Comparison of theoretically obtained values of the mode I and II stress intensity factors at the tips of the crack, after introduction of an infinitesimal disturbance, with experimental results indicates that mode I growth is preferred before mode II in the absence of a confining pressure, at least if the ratio between the critical mode II and I stress intensity factors is larger than 0.38-0.81, depending on the load situation. Thus also a straight crack, originally directed favourably for extension straight forward in mode II, will exhibit directional instability and mode I growth will take over after kink formation at the tips of the crack. Eventually the growth approaches a direction perpendicular to the largest in-plane principal stress. (Less)