Composites reinforced with cellulose based fibres

Publisher Summary This chapter describes the mixed mode cracking in layered materials. There is ample experimental evidence that cracks in brittle, isotropic, homogeneous materials propagate such that pure mode I conditions are maintained at the crack tip. An unloaded crack subsequently subject to a combination of modes I and II will initiate growth by kinking in such a direction that the advancing tip is in mode I. The chapter also elaborates some of the basic results on the characterization of crack tip fields and on the specification of interface toughness. The competition between crack advance within the interface and kinking out of the interface depends on the relative toughness of the interface to that of the adjoining material. The interface stress intensity factors play precisely the same role as their counterparts in elastic fracture mechanics for homogeneous, isotropic solids. When an interface between a bimaterial system is actually a very thin layer of a third phase, the details of the cracking morphology in the thin interface layer can also play a role in determining the mixed mode toughness. The elasticity solutions for cracks in multilayers are also elaborated.

Mixed mode cracking in layered materials

High-entropy alloys are equiatomic, multi-element systems that can crystallize as a single phase, despite containing multiple elements with different crystal structures. A rationale for this is that the configurational entropy contribution to the total free energy in alloys with five or more major elements may stabilize the solid-solution state relative to multiphase microstructures. We examined a five-element high-entropy alloy, CrMnFeCoNi, which forms a single-phase face-centered cubic solid solution, and found it to have exceptional damage tolerance with tensile strengths above 1 GPa and fracture toughness values exceeding 200 MPa·m(1/2). Furthermore, its mechanical properties actually improve at cryogenic temperatures; we attribute this to a transition from planar-slip dislocation activity at room temperature to deformation by mechanical nanotwinning with decreasing temperature, which results in continuous steady strain hardening.

/pdf/a-fracture-resistant-high-entropy-alloy-for-cryogenic-4v6ic2t1x4.pdf

A fracture-resistant high-entropy alloy for cryogenic applications

Graphene based materials: Past, present and future

This paper deals with the general problem of crack extension in a combined stress field where a crack can grow in any arbitrary direction with reference to its original position. In a situation, when both of the stress-intensity factors,k
1,k
2 are present along the crack front, the crack may spread in any direction in a plane normal to the crack edge depending on the loading conditions. Preliminary results indicate that the direction of crack growth and fracture toughness for the mixed problem of Mode I and Mode II are governed by the critical value of the strain-energy-density factor,S
cr. The basic assumption is that crack initiation occurs when the interior minimum ofS reaches a critical value designatedS
cr. The strain-energy-density factorS represents the strength of the elastic energy field in the vicinity of the crack tip which is singular of the order of 1/r where the radial distancer is measured from the crack front. In the special case of Mode I crack extensionS
cr is related tok
1c alone asS
cr = (κ − 1)k

 1
 2
 /8μ. In general,S takes the quadratic forma
1 1
k
1 + 2a
1 2
k
1
k
2 +a
2 2
k
2 whose critical value is assumed to be a material constant. The analytical predictions are in good agreement with experimental data on the problem of an inclined crack in plexiglass and aluminum alloy specimens. The result of this investigation provides a convenient procedure for determining the critical crack size that a structure will tolerate under mixed mode conditions for a given applied stress.

Strain-energy-density factor applied to mixed mode crack problems

The effect of shallow cracks on crack tip opening displacement

A method of fabricating a device comprising mechanically patterning a device layer using a stamp containing the desired pattern. The device layer is formed on a plastic or polymeric substrate. The stamp is pressed against the substrate under a load which patterns the device layer without cracking it in the non-patterned areas.

Mechanical patterning of a device layer

A cloud of microcracks opens plastically around the tip of a long fatigue crack in sintered steel as it propagates. Because of the dilatation caused by the plastic opening of the microcracks, a negative stress intensity factor is induced at the crack tip which reduces the maximum crack tip stress intensity factor. At low values of the stress ratio the minimum crack tip stress intensity factor can be less than zero causing the crack to close and thus reducing the effective crack tip stress intensity factor range and retarding crack growth. Short cracks growing from pores do not have a systematic shield of microcracks and so propagate faster than long cracks at low stress ratios.

Brian Cotterell

Papers

The effect of shallow cracks on crack tip opening displacement

Mechanical patterning of a device layer

Fatigue of sintered steel

Anisotropic elastic and elastic-plastic bending solutions for edge constrained beams

The effect of geometry on the crack opening displacement of a low carbon steel