Crack Branching Mechanisms Research Articles

The influence of oxidation on mechanical and fracture behaviour of an air plasma-sprayed NiCoCrAlY bondcoat

The influence of isothermal oxidation on room-temperature mechanical and fracture behaviour of an air plasma-sprayed Ni-23Co-17Cr-12Al-0.5Y bondcoat was investigated by the miniaturised disc bending test (MDBT) technique. Disc specimens were extracted from the bondcoat region of both as-received and oxidised thermal barrier coating (1000 °C, 1000 h). Microstructure analysis revealed that the non-oxidised bondcoat consisted mainly of γ-phase (Ni-structure) and β-NiAl. After 500 h of oxidation no NiAl remained in the bondcoat, an effect due to internal as well as external oxidation of Al. The former resulted in the formation of an extensive oxide network and the latter in the formation of an oxide scale between the topcoat and the bondcoat. The crack propagation behaviour of the bondcoat, both in non-oxidised and oxidised condition can be characterised as intergranular with stable growth. The crack propagation resistance is substantial due to the lamellar grain (splat) orientation and the extensive intergranular oxide network, acting as crack deflection and crack branching mechanisms. As an effect of oxidation, crack propagation resistance of the bondcoat increases but the strain to crack initiation decreases.

Modelling of initial fatigue crack growth and crack branching under fretting conditions

Crack propagation during Stage I, in terms of crack initiation sites and growth directions and crack branching mechanisms under fretting conditions, is investigated using both experimental and theoretical approaches. Fretting tests were conducted on an aeronautical aluminium alloy. Two crack types are observed during Stage I corresponding, respectively, to specific mode I and II conditions. Transition from Stage I to Stage II is characterized for both crack types by a crack branching towards a new propagation direction of ≈65° to the specimen surface. Specific parameters linked to mode I and II propagation driving forces are proposed. Crack location and initial growth directions during Stage I are predicted in accordance with these parameters, and are in very good agreement with experimental observations. The conditions governing the transition from Stage I to Stage II are then identified. It is shown that under fretting conditions, cracks branch along a new direction, thereby maximizing the crack‐opening amplitude.

Basic Studies on the Governing Criterion for Dynamic Crack Branching Phenomena.

Branched cracks are often observed in brittle materials and structures. However the mechanism of crack branching is not fully clarified. In this paper, first, dynamic crack branching phenomena in Homalite-911 and Homalite-100 were experimentally investigated. Caustic patterns during dynamic crack branching were recorded by an ultrahigh-speed camera. Next, on the basis of the dynamic J integral, energy concepts for dynamic crack branching are presented. Special attentions are focused on the energy flow into dynamically propagating crack tips. Then four possible fracture parameters associated with the dynamic J integral are proposed. The dynamic J integral values in a dynamic crack branching test are directly evaluated by the caustic patterns taken by the high-speed camera. The variations of four fracture parameters during dynamic crack propagation including at the instant of crack branching are presented. The experimental results reveal that the energy flux per a unit time into a propagating crack tip governs the crack branching.

A study of shear fatigue crack mechanisms: Hu, Z. Ma, L. and Cao, S. Fatigue Fract. Eng. Mater. Struct. June 1992 15, (6), 563–573

The effects of cryorolling (CR) on high cycle fatigue (HCF) and fatigue crack growth rate behaviour of Al 7075 alloy have been investigated in the present work. The Al 7075 alloy was rolled for different thickness reductions (40% and 70%) at cryogenic (liquid nitrogen) temperature and its tensile strength, fatigue life, and fatigue crack growth mechanism were studied by using tensile testing, constant amplitude stress controlled fatigue testing, and fatigue crack growth rate testing using load shedding (decreasing ΔK) technique. The microstructural characterization of the alloy was carried out by using Field emission scanning electron microscopy (FESEM). The cryorolled Al alloy after 70% thickness reduction exhibits ultrafine grain (ufg) structure as observed from its FESEM micrographs. The cryorolled Al 7075 alloys showed improved mechanical properties (Y.S, U.T.S, Impact energy and Fracture toughness are 430 Mpa, 530 Mpa, 21 J, 24 Mpa m1/2 for 40CR alloy) as compared to the bulk 7075 Al alloy. It is due to suppression of dynamic recovery and accumulation of higher dislocations density in the cryorolled Al alloys. The cryorolled Al alloy investigated under HCF regime of intermediate to low plastic strain amplitudes has shown the significant enhancement in fatigue strength as compared to the coarse grained (CG) bulk alloy due to effective grain refinement. Fatigue crack growth (FCGR) resistance of the ufg Al alloy has been found be higher, especially at higher values of applied stress intensity factor ΔK The reasons behind such crack growth retardation is due to diffused crack branching mechanism, interaction between a propagating crack and the increased amount of grain boundaries (GB), and steps developed on the crack plane during crack-precipitate interaction at the GB due to ultrafine grain formation.

Crack branching and arrest in environmental cracking of polyethylene

At intermediateK values a region where the crack speed is constant, that is, independent ofK, is observed in detergent cracking of low density polyethylene. This region is terminated at highK by one of two processes: in thin specimens the crack arrests, and in thick specimens it branches. The mechanism of crack branching involves the crack front twisting and then the initiation of a new crack at the centre of the specimen. A model has been proposed to explain this mechanism and also the fact that crack arrest or branching occur at approximately the sameK value. After branching the cracks continue to propagate at constant speed along paths whereK| ≈ 0 and if the specimen is wide enough, can branch again.

A mechanism of crack branching in polymethyl methacrylate and the origin of the bands on the surfaces of fracture

At low crack velocities the fracture of high molecular weight polymethyl methacrylate occurs by the separation of a thin craze layer ahead of, and coplanar with, the propagating crack tip. Above some critical velocity, about 400 m sec−1 at room temperature, craze branching or bifurcation is initiated. The craze branching does not cause any detectable surface roughening of the fracture surface until the crack tip stress is sufficient to initiate cracks in the craze branches. At this stage the formation of the branching craze-cracks causes surface roughening (bands or striations), a deceleration of the main fracture and a drop in the stress amplitude around the crack tip which is below that necessary to initiate branching crazes. The fracture then reverts back to the simpler mechanism, with no surface roughening. The repetition of this process gives rise to the banded appearance of the fracture surface.