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

Fatigue of ceramics due to cyclic loading as distinct from static fatigue due to environmental effects was until recently thought impossible because of the absence of appreciable crack tip plasticity (Evans, 1980). A substantial and growing body of evidence now indicates that true cyclic fatigue may be possible in a wide range of ceramics, for example, Guiu (1976), Ko (1986), Dauskardt et al (1987, 1989, 1990), Hoshide et al (1988), Sylva and Suresh (1989), and Davidson et al (1991). Evidence for true cyclic fatigue has also come from tests performed under cyclic compression (Ewart and Suresh, 1986, Ewart and Suresh, 1987). However, a significant portion of crack growth under cyclic loading in air or other reactive atmospheres is due to environmentally-assisted slow crack growth. For true cyclic fatigue there must be some form of hysteresis whereby energy must be dissipated during cycling. Hysteresis effects have been demonstrated for several transformation-toughened ceramics (Bowman et al, 1986). Such behaviour is not confined to transformation-toughened ceramics, but is likely to be more marked in the tougher ceramics.

Crack Growth in Zirconia Bearing Ceramics under Cyclic Loading

Mixed mode ductile fractures in thin sheets are shown to be possible for two low carbon steels and two aluminium alloys. The staggered deep edge notch tension specimen allows the mixed mode JC to be measured and to be partitioned into its component modes JIC and JIIC by extrapolating the specific work of fracture to zero ligament length. For the low carbon steels JC is shown to be independent of the mode of fracture but for the aluminium alloys JC is dependent on the stagger angle (θ). It is suggested that if necking is intensive and the strain hardening exponent (n) is low JC will vary with θ. If however necking is moderate and n is comparatively large JC is approximately constant. The experimental results obtained from these thin sheet metals seem to support this hypothesis although no theoretical grounds can be offered.

Further Observations on Mixed Mode Plane Stress Ductile Fracture

The fatigue fracture behavior of a high impact polystyrene is investigated in air and in a series of alcohols. The fatigue data are analyzed using a modified Williams model which gives lower crack tip craze stresses in the liquid environments than in air. The fatigue thresholds are, however, larger in the alcohols due to crack tip blunting. For a given alcohol, the fatigue crack growth rates and crack tip craze stresses are independent of stress ratios and frequencies.

Environmental Effect on Fatigue Fracture in High Impact Polystyrene

Fatigue Behavior of an Al 2 O 3 /ZrO 2 Ceramic under Cyclic Loading

Since the Lanxide process was advanced for forming of Al2O3 ceramic composite by directed oxidation of Al alloys, much work has been done with various mechanisms being proposed. The mechanisms have claimed that only certain dopants are essential to the growth process. Nevertheless, no united consensus has yet been reached. In the present work, Al alloy containing 5% Mg was oxidised in air for 12 hours at 1150°C with or without surface dopants of MgO or Pd. The resultant composites showed very different microstructures. Without any surface doping, the alloy did not develop any portion of composite as the initial intimate oxide film stops further oxidation. This intimate oxide film can either be broken off by mechanical means or penetrated by reaction with surface dopants, so that the composite can grow and develop. The results show that the previously reported incubation time is not only related to reaction processes but also to the initial mechanical disturbances. Doping with Pd made the composite darker in colour as the grains of the alumina ceramic matrix and inclusions of Al metal are finer. This shows that Pd may make the top oxide layer less intimate, and more nucleation sites are therefore available for oxidation. A new model is presented for oxide sustained growth based on the existence of oxygen active top surface layer and the capillary flow of molten metal around ceramic phase.

Brian Cotterell

Papers

Crack Growth in Zirconia Bearing Ceramics under Cyclic Loading

Further Observations on Mixed Mode Plane Stress Ductile Fracture

Environmental Effect on Fatigue Fracture in High Impact Polystyrene

Fatigue Behavior of an Al 2 O 3 /ZrO 2 Ceramic under Cyclic Loading

Microstructure and Growth Mechanism of Al2O3/Al Ceramic Matrix Composite Made by Directed Oxidation of Al Alloys