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.

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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

On polypropylene-cellulose fibre-cement hybrid composites

It is suggested that the unnotched impact test is suitable for quantitative quality control of unplasticized poly(vinyl chloride) pipe. The relative level of processing is assessed by the probability that failure is nucleated at a surface flaw. Tests show that this assessment is very sensitive to the level of processing.

Unnotched impact bend test assessment of the degree of gelation in unplasticized poly(vinyl chloride) pipe

A systematic experimental and theoretical study on the crack growth behaviour of a sintered Al2O3/ZrO2 ceramic under cyclic loading is presented. It is found that in the cyclic fatigue experiments conducted on the single-edge-notched beam (SENB) geometry, for similar testing conditions, the crack growth rates are significantly faster than those under static fatigue. Al2O3/ZrO2 therefore suffers genuine mechanical fatigue. Further experiments with the compact tension (CT) geometry show that the mechanical fatigue effect arises mainly from the degradation of the bridging mechanism. A theoretical analysis based on the compliance technique and a power law relationship between the crack-wake bridging stress and the crack-face separation is developed to evaluate quantitatively the degradation of the bridging stress due to cyclic fatigue.

Cyclic fatigue of a sintered Al2O3/ZrO2 ceramic

The effects of processing of unplasticized polyvinyl chloride (uPVC) pipes on fatigue-crack growth in terms of mean stress, frequency and fatigue-crack initiation, as well as on creep rupture, were investigated. No significant processing effect was observed on fatigue crack growth rates and fatigue crack initiation. However, creep rupture with three-point bending tests was significantly affected by the processing level. Two orders of magnitude difference in time to failure were found between well and poorly processed pipes caused by the large difference in the stress intensity factor at fracture instability between these pipes.

Effects of processing on fatigue crack growth and creep rupture in unplasticized polyvinyl chloride (uPVC)

Synopsis It is shown that it is comparatively simple to include a fracture process zone in a fictitious crack model for crack growth in fibre-reinforced cementitious materials if it is assumed that the crack faces remain straight. This model can he used to predict the crack-growth resistance curve for a particular specimen geometry and size if the stress-displacement relationship for the material is known. This curve is somewhat different to the curve obtained if the fracture process zone is not modelled. However, if the fracture parameters are obtained from a “master” experimental crack-growth resistance curve, there is little difference between curves obtained with and without modelling the fracture process zone.

Brian Cotterell

Papers

On polypropylene-cellulose fibre-cement hybrid composites

Unnotched impact bend test assessment of the degree of gelation in unplasticized poly(vinyl chloride) pipe

Cyclic fatigue of a sintered Al2O3/ZrO2 ceramic

Effects of processing on fatigue crack growth and creep rupture in unplasticized polyvinyl chloride (uPVC)

The effect of a fracture process zone on a model for crack growth in fibre-reinforced cementitious materials