by Masuda-Jindo, K.; Hung, Vu; Menon, M.

The crack nucleation and propagation processes in nanoscale materials are studied using the ab initio constraint molecular dynamics method and the lattice Green’s function method. We investigate the strength and fracture behaviors of carbon related nanoscale materials, especially the graphen sheets in comparison with those of carbon nanotubes. The linear elastic parameters, non-linear elastic instabilities, thermal lattice expansion and fracture behaviors are studied in detail. We will show that the thermodynamic and strength properties of the nanoscale materials exhibit characteristic features and they are different from those of the corresponding bulk materials.

DOI: 10.1007/s10704-006-0022-6
Print publication date: 6/1/2006
View article on SpringerLink

by Michot, Gérard; Kirchner, Helmut

DOI: 10.1007/s10704-006-8448-4
Print publication date: 6/1/2006
View article on SpringerLink

by Gupta, Himadri S.; Wagermaier, Wolfgang; Zickler, Gerald A.; Hartmann, Jürgen; Funari, Sérgio S.; Roschger, Paul; Wagner, H. Daniel; Fratzl, Peter

The nanoscale deformation and fracture mechanisms of parallel fibered bone are investigated using a novel combination of in-situ tensile testing to failure combined with high brilliance synchrotron X-ray scattering. The technique enables the simultaneous measurement of strain at two length scales – in the mineralized collagen fibrils (~100 nm diameter) along with the macroscopic strain (~1 mm diameter). Under constant rate tensile loading, we find that fibril strain saturates beyond the macroscopic yield point of bone at ~0.5 %, providing a correlation between the failure mechanisms at the nanoscale and the bulk structural properties. When bone stretched beyond the yield point is unloaded back to zero stress, the fibrils are contracted relative to their original state. We examine the findings in the context of a fiber – matrix shearing model at the nanometer level.

DOI: 10.1007/s10704-006-6635-y
Print publication date: 6/1/2006
View article on SpringerLink

by Peterlik, Herwig; Roschger, Paul; Klaushofer, Klaus; Fratzl, Peter

The critical energy release rate of human bone was determined for different crack propagation directions with three-point-bending tests using controlled crack extension. The local structure was characterised by small-angle X-ray scattering, SEM and polarised light microscopy and related to the energy required for crack extension. It turns out the collagen angle is decisive for switching the fracture behaviour of bone from brittle to quasi-ductile. A significant increase in the critical energy release rate as well as a change of the appearance of the crack path from straight and smooth to deflected and zig-zag is observed.

DOI: 10.1007/s10704-006-6634-z
Print publication date: 6/1/2006
View article on SpringerLink

by Zioupos, P.; Kaffy, C.; Currey, J. D.

The mechanical characteristics of human bone, especially those relating to age, are of immense interest to everyone. A great amount of information has already been accumulated on the macromechanical/phenomenological aspects of bone behaviour and while some aspects, such as stiffness and strength, have been attributed to effects at the architectural/compositional level, some others like toughness have been related to events at the molecular/biophysical level. It is not always easy to unravel the intimate relationship between the architectural and remodelling changes at the macroscale to the biophysical/chemical effects occurring at the ultrastructural level. There is however, the mesostructural level (fractography), which is commonly overlooked or has been approached in a purely qualitative manner. In this article we concentrated primarily on the variation of toughness of ageing bone with age and then examined the fracture profile morphology of the various samples by fractal analysis. The results show that the way bone actually fractures, in either slow/ductile or fast/brittle fracture, has an underlying connection to the architectural status of each individual and the way ageing bone changes as a ‘material’ as well as a ‘collection’ of heterogeneous elements and structures. Of course, fracture morphology cannot simply and uniquely be described by one fractal dimension, but fracture nevertheless is determined by the intrinsic architecture of the bone structure and its material.

DOI: 10.1007/s10704-006-6581-8
Print publication date: 6/1/2006
View article on SpringerLink

by Sigrist, Christian; Schweizer, Jürg; Schindler, Hans-Jakob; Dual, Jürg

Before a dry snow slab avalanche is released, a shear failure along a weak layer or an interface has to take place. This shear failure disconnects the overlaying slab from the weak layer. A better understanding of this fracture mechanical process, which is a key process in slab avalanche release, is essential for more accurate snow slope stability models. The purpose of this work was to design and to test an experimental set-up for a mode II fracture test with layered snow samples and to find a method to evaluate the interfacial fracture toughness or alternatively the energy release rate in mode II. Beam-shaped specimens were cut out of the layered snow cover, so that they consisted of two homogeneous snow layers separated by a well defined interface. In the cold laboratory 27 specimens were tested using a simple cantilever beam test. The test method proved to be applicable in the laboratory, although the handling of layered samples was delicate. An energy release rate for snow in mode II was calculated numerically with a finite element (FE) model and analytically using an approach for a deeply cracked cantilever beam. An analytical bilayer approach was not suitable. The critical energy release rate G
c was found to be 0.04 ± 0.02 J m−2. It was primarily a material property of the weak layer and did not depend on the elastic properties of the two adjacent snow layers. The mixed mode interfacial fracture toughness for a shear fracture along a weak layer estimated from the critical energy release rate was substantially lower than the mode I fracture toughness found for snow of similar density.

DOI: 10.1007/s10704-006-6580-9
Print publication date: 6/1/2006
View article on SpringerLink

by Saenger, Erik H.; Krüger, Oliver S.; Shapiro, Serge A.

We present a new technique for static computations of effective elastic properties of multiple fractured rocks. This approach is based on the viscoelastic rotated staggered finite-difference (FD) grid wave propagation technique. Our simulations are used to explain discrepancies of some recent numerical studies. The focus is on scale effects of a so-called representative volume element (RVE). From the point of view of classical micromechanics we review different numerical techniques: Static as well as dynamic numerical experiments. We show that the differential effective medium theory (DEM) is capable of producing satisfactory predictions of effective elastic moduli. For non-dilute crack densities this is not the case for the non-interacting approximation (NIA).

DOI: 10.1007/s10704-006-0105-4
Print publication date: 6/1/2006
View article on SpringerLink

by Motoyashiki, Yasuko; Brückner-Foit, Angelika; Englert, Lars; Haag, Lars; Wollenhaupt, Matthias; Baumert, Thomas

Since small crack propagation behavior is strongly affected by microstructure, very small artificial notches with a length in the submillimeter range are needed for a systematic study of microcrack behavior. Laser processing technique with ultrashort pulses is a micromachining tool which will not cause any serious mechanical damage in metallic materials. Small artificial starter notches were manufactured in medium carbon steels with this technique and some fatigue tests were carried out. Laser affected zones could be observed at the notch boundary but cracks were initiated from the notch tips and propagated steadily. The crack paths were very tortuous like natural small cracks. The experimental results showed that the femtosecond laser processing technique is useful to introduce a small notch and allows systematic investigation of microcrack behavior.

DOI: 10.1007/s10704-006-0104-5
Print publication date: 6/1/2006
View article on SpringerLink

by Ekwaro-Osire, S.; Khandaker, M. P. H.; Gautam, K.; Lolge, G.

In calculating the stress intensity factors (SIFs) using the weight function method, the accuracy of the result depends upon proper selection of the set of reference loadings. The objective of the present work is to find a unique set of reference loadings to calculate the SIF at the tip of a crack of any configuration. Additionally, the universality of these reference loadings with respect to calculating the SIF for various crack configurations is examined. Two sets of crack configurations were considered, with and without a pre-existing crack. In each set, a horizontal crack and a slanted crack were analyzed.

DOI: 10.1007/s10704-006-0103-6
Print publication date: 6/1/2006
View article on SpringerLink

by Yasniy, P.; Maruschak, P.; Lapusta, Y.

Fatigue and fatigue-creep crack growth rate in a bimetal of continuous caster rolls for different loading frequencies and waveforms is investigated with the use of stress, deformation and energy criteria of fracture mechanics.

DOI: 10.1007/s10704-006-0102-7
Print publication date: 6/1/2006
View article on SpringerLink