A fatigue crack initiation model incorporating discrete dislocation plasticity and surface roughness

Although a thorough understanding of fatigue crack initiation is lacking, experiments have shown that the evolution of distinct dislocation distributions and surface roughness are key ingredients. In the present study we introduce a computational framework that ties together dislocation dynamics, the fields due to crystallographic surface steps and cohesive surfaces to model near-atomic separation leading to fracture. Cyclic tension–compression simulations are carried out where a single plastically deforming grain at a free surface is surrounded by elastic material. While initially, the cycle-by-cycle maximum cohesive opening increases slowly, the growth rate at some instant increases rapidly, leading to fatigue crack initiation at the free surface and subsequent growth into the crystal. This study also sheds light on random local microstructural events which lead to premature fatigue crack initiation

A comparison of nanotribology and nanoindentation

Metal friction and wear is the collective contact and interaction of asperities of micrometer dimension. We use a nanoindenter with tangential force measurement to simulate the behavior in engineering contacts and to fundamentally understand friction and wear. This presentation investigates the deformation due to a single stroke scratch of a diamond nanoindenter in austenite base. We find that the elastic and plastic equations for static indentation also apply for the dynamic scratching. Additionally, the friction coefficient is found to be normal force dependent and we observe three domains: microstructure dominated friction, plastic plowing dominated wear and wear particle dominated tribology. Focussing on plasticity, we observe that the local crystal orientation has a significant influence on the development and spread of of plasticity. Additionally, the complex three-dimensional stress state results in the formation of non-obvious plastic slip patterns. Finally, we show crack formation in the scratch track even after a single stroke

Fundamental Differences in Mechanical Behavior between Two Types of Crystals at the Nanoscale

We present differences in the mechanical behavior of nanoscale gold and molybdenum single crystals. A significant strength increase is observed as the size is reduced to 100 nm. Both nanocrystals exhibit discrete strain bursts during plastic deformation. We postulate that they arise from significant differences in the dislocation behavior. Dislocation starvation is the predominant mechanism of plasticity in nanoscale fcc crystals, while junction formation and hardening characterize bcc plasticity. A statistical analysis of strain bursts is performed as a function of size and compared with stochastic models

Using simulations to investigate the apparent fracture toughness of microcantilevers

In the past decade, micrometer cantilevers were frequently used to evaluate the fracture toughness of single phases and the fracture toughness of particular grain and phase boundaries. The calculation of the fracture toughness relies on the cantilever geometry and the experimentally determined maximum force. To quantify the toughness, the geometry and force enter analytical models, which are based on simulations that use an isotropic elastic material, perfect beam geometry and in many cases a two-dimensional configuration. However, the vast majority of materials have a limited amount of plasticity and are anisotropic. Moreover, the intentionally prepared pre-crack is seldom straight due to the focused ion beam (FIB) production method. This study uses thousands of 3D finite element method (FEM) simulations to investigate the influence of anisotropy, imperfect pre-crack shape and plasticity on the apparent fracture toughness of the material. Initially, we will discuss the influence of anisotropy on the apparent fracture toughness. To that end, the ratio of anisotropy between the cantilever axis and the transversal axis is varied to establish the effect on the fracture toughness. More, we investigate the influence of Poisson\u27s ratio and beam geometry because these values interact with the anisotropy. We find that typical metals with an anisotropy ratio less than 3 and typical cantilever geometries with an l/h ratio of more than 4 are only slightly affected by the anisotropy. We present view-graphs that allow the user to determine the influence of anisotropy for other cantilevers and highly anisotropic materials. Secondly, we investigate the influence of material bridges that are frequently used to ensure straight pre-cracks at the start of crack propagation. Additionally, we investigate the influence of pre-crack front rounding, i.e. the material bridges are omitted. We varied the geometry of the material bridge and crack front rounding to establish the influence of the geometry on the apparent fracture toughness. We discuss the difference between load controlled experiments with an internal feedback loop and displacement controlled experiments. We argue about the geometric requirements to ensure stable crack growth and the optimal experiments. We close with a discussion on the influence of plasticity on the fracture toughness and the applicability of the plastic zone size according to Rice for microcantilever experiments. Please click Additional Files below to see the full abstract

How to avoid FIB-milling artefacts in micro fracture? A new geometry for interface fracture

Focused ion beam (FIB) based small-scale fracture studies have been well established in recent years despite the ongoing discussion of possible artefacts caused by FIB milling. Stable crack growth geometries—where the FIB-prepared notch stably propagates through the sample—have the potential to ameliorate some of the FIB-based challenges. In this work, we propose a new sample geometry for testing interface toughness at the micron scale which results in intrinsically stable crack growth. This geometry is straightforward to fabricate using established FIB-based methods and testing setups. We prove the stability of crack growth by finite element modelling and by experimentally applying the approach on a hard coating–silicon interface. We observe that even with small imperfections, the FIB-milled notch propagates towards the interface and the natural crack stably grows along the interface