Crack Nucleation Mechanism Research Articles

Fracturing of ice under compression creep as revealed by a multifractal analysis

Fracturing of freshwater granular ice up to failure under uniaxial compression creep was investigated from series of interrupted creep tests and from a multifractal analysis of the corresponding fracture patterns. At the early stages of damage corresponding to primary and secondary creep, the fracturing process is dominated by the nucleation of microcracks from stress concentrations within the material (unlike rocks, artificial freshwater ice does not contains starter flaws). Because of the crack nucleation mechanisms, the microstructure of the material (e.g., the nonfractal grain size distribution) strongly influences the organization of fracturing which is therefore nonfractal. As fracturing proceeds during tertiary creep, a hierarchical (fractal) organization of the fracturing emerges progressively over a wider scale range. At failure, this fractal organization is fully developed without detectable lower or upper bound, and the role of the initial microstructure has completely disappeared. Similarly, cracks are preferentially oriented along the compression axis at the early stages of damage, but this anisotropy vanishes as failure is approached. The simultaneity between the onset of tertiary creep and the emergence of fractal organization suggests that the acceleration of the deformation during tertiary creep is due to the cataclasis of a material which becomes granular. An important consequence of the fractal organization of fracturing is that homogenization procedures, as well as damage mechanics, developed to study the behavior of damaged materials, cannot be used to describe tertiary creep and failure.

Nucleation of feather cracks in columnar freshwater ice: Experimental observations

Biaxial compression experiments were carried out to investigate the mechanism of feather crack nucleation in columnar freshwater ice. The tests were performed with a strain rate of 10−3 s−1 at −10°C. It is shown that the nucleation of feather cracks corresponds to a transition when the last of the remaining ligaments bonding the two grains fractures and the grain boundary transforms into a traction‐transmitting frictional crack. This is in contrast to the nucleation of wing cracks, proposed earlier, where the nucleation was due to the onset of viscous sliding of grains. In addition to the mechanism, the nucleation stress for feather cracks was measured for different confinement ratios. A fracture‐mechanics‐based model for the nucleation process is presented by V. Gupta and R. C. Picu [Nucleation of feather cracks in columnar freshwater ice: Theoretical modeling, manuscript in preparation].

Improvement of the performance of working surfaces of materials susceptible to cracking

We study thermomechanical stresses in components of steels and alloys that are caused by finishing and lead to defects such as high-temperature-induced structural transformations (called “burnings”) and cracking. In doing so, we apply phenomenological theories based on a set of equations that represent the thermal and stress-strain state of a workpiece surface as a thermoelastic body given by a sectionally homogeneous half-plane. We accounted for local physical and mechanical properties of the workpiece material by means of the respective boundary conditions. This allows us to use criteria of fracture mechanics to study the mechanism of crack nucleation at the sites of accumulated process-induced discontinuities in the surface layer. We determine the critical values of heat flows in a machining zone whereby the process-induced discontinuities are kept at equilibrium and thus the workpiece surface remains free from crack-type defects. We consider the influence of cutting forces on the process of cracking. Taking advantage of the capabilities of a stochastic model of workpiece materials, we clarify the causes for cracking in grinding. We suggest technological methods to reduce the intensity of cracking in finishing operations, in particular, in grinding, of steels and alloys.

Shell Modeling of Fretting in Riveted Lap Joints

Fretting is a contact damage mechanism arising from microslip associated with small-scale oscillatory motion of nominally clamped structural members. Fretting has heen ohserved near aircraft skin fastener holes. Shell finite elements are used to model the contact at a typical skin/rivet interface, with emphasis on fretting as a crack nucleation mechanism. Contact elements implementing the Coulomh friction law keep track of contact status between interacting surfaces. The model accounts for bending, contact hetween the skin panels, and rivet head clamping pressure. Elastic supports control the load transferred and simulate various rivet configurations. No interference is considered, leading to loss of contact between the skin and rivet on loading. The distinct stick-slip zones, combined with high tensile stresses at the edge of contact, are indicative of fretting, resulting in crack nucleation at the edge of contact. Away from the edge of contact, the tensile stress decays rapidly. The slip displacement has values typically associated with fretting fatigue. The interface between the two skin panels is also a region of fretting damage. Crack nucleation lives are predicted using a multiaxial fatigue theory. The top row of rivets has the smallest predicted life. For low remote stresses, increase in friction coefficient increases life, whereas for high remote stresses, life decreases with increase in coefficient of friction. Increasing rivet head clamping pressure increases the life. Plasticity blunts the effects of friction coefficient and clamping pressure. An approximate solution that does not require finite element analysis estimates the crack nucleation life for any rivet configuration quickly.

Molybdenum coatings produced by vacuum arc deposition: B.B.Straumal et al. (Inst. of Solid State Physics, Moscow, Russia.)

Temperature dependence of fracture toughness of molybdenum sheet was studied in the temperature range — 196–300 °C. It was found that nonmonotonous behaviour of this dependence is due to the change in cleavage crack nucleation mechanism.

Shell modeling of fretting in riveted lap joints

Fretting is a contact damage mechanism arising from microslip associated with small-scale oscillatory motion of nominally clamped structural members. Fretting has heen ohserved near aircraft skin fastener holes. Shell finite elements are used to model the contact at a typical skin/rivet interface, with emphasis on fretting as a crack nucleation mechanism. Contact elements implementing the Coulomh friction law keep track of contact status between interacting surfaces. The model accounts for bending, contact hetween the skin panels, and rivet head clamping pressure. Elastic supports control the load transferred and simulate various rivet configurations. No interference is considered, leading to loss of contact between the skin and rivet on loading. The distinct stick-slip zones, combined with high tensile stresses at the edge of contact, are indicative of fretting, resulting in crack nucleation at the edge of contact. Away from the edge of contact, the tensile stress decays rapidly. The slip displacement has values typically associated with fretting fatigue. The interface between the two skin panels is also a region of fretting damage. Crack nucleation lives are predicted using a multiaxial fatigue theory. The top row of rivets has the smallest predicted life. For low remote stresses, increase in friction coefficient increases life, whereas for high remote stresses, life decreases with increase in coefficient of friction. Increasing rivet head clamping pressure increases the life. Plasticity blunts the effects of friction coefficient and clamping pressure. An approximate solution that does not require finite element analysis estimates the crack nucleation life for any rivet configuration quickly.

Microscopical Clarification of Crack Nucleation Mechanism under High-cycle Fatigue with Atomic Force Microscope. 1st Report. Intergranular Crack in Polycrystalline BCC Metal.

Microscopical Clarification of Crack Nucleation Mechanism under High-cycle Fatigue with Atomic Force Microscope. 1st Report. Intergranular Crack in Polycrystalline BCC Metal.

Relation between Mechanical Twinning and Cracking in Stoichiometric Lithium Niobate Single Crystals

The relationships between cracking and mechanical twinning, and between ferroelectric polarization and mechanical twinning, in stoichiometric LiNbO3 were investigated in the present study. Three sets of mechanical twins crossed one another, and the crossed points of those twins provided preferred sites for the nucleation of cracks. Mechanical twins of LiNbO3 revealed a head‐to‐tail arrangement of ferroelectric polarization, and complicated polarization states were observed at the crossed points. Some possible mechanisms of crack nucleation and propagation are proposed in this paper.

Mean stress sensitivity in fatigue of α, (αβ) and β titanium alloys

The effect of the stress ratio on high cycle fatigue (HCF) strength has been investigated on α, (αβ) and β titanium alloys. While α and β titanium alloys show a normal mean stress sensitivity, (αβ) alloys can behave anomalously, i.e. their HCF strength at low tensile mean stresses can be much lower than expected from a linear interpolation between fully reversed HCF strength ( R = −1) and tensile strength. This anomalous mean stress dependence of the HCF strength is discussed in terms of fatigue crack nucleation mechanisms.

Crack nucleation due to dislocation pile-ups at I-, U- and amorphized triple lines

A mechanism of intergranular wedge crack nucleation in polycrystalline materials is the piling up of extrinsic grain boundary dislocations around triple lines. In this paper, an approximate theoretical analysis is carried out to compare nucleation at the I-, U- and amorphized triple lines due to edge dislocations having Burgers vectors tangential to the grain boundary planes. The analysis is based on an energy approach of fracture mechanics. Numerical results for aluminium show that: (l) the overall pile-up equilibrium configurations of the dislocations are insignificantly affected by the type of triple line, (2) two types of cracks, sub-microscopic (G 0.05 μm) and microscopic (10–20 μm), are predicted, depending on the strength of the disclination defects at the triple line, and (3) the pile-up populations and the Burgers vectors of the dislocations can significantly influence the nucleation characteristics.

Crack Nucleation in ion Beam Irradiated Magnesium Oxide and Sapphire Crystals

ABSTRACTMonocrystals of magnesium oxide and sapphire have been subjected to ion implantation with 86 keV Si− ions to a dose of 5×1016 cm−2 and with 3 MeV H+ ions with a dose of 4.8×1017 cm−2 prior to thermal stress testing in a pulsed plasma. Fracture and deformation characteristics of the surface layer were measured in ion implanted and unimplanted samples using optical and scanning electron microscopy. Ion implantation is shown to modify the near-surface structure of samples by introducing damage, which makes crack nucleation easier under the applied stress. The effect of ion dose on the thermal stress resistance is investigated and the critical doses which produce a noticeable change in the stress resistance is determined for sapphire crystals implanted with 86 keV Si−. In comparison with 86 keV Si− ions the high energy implantation of sapphire and magnesium oxide crystals with 3 MeV H+ ions results in the formation of large-scale defects, which produce a low density crack system and cause a considerable reduction in the resistance to damage. Fracture mechanics principles are applied to evaluate the size of the implantation-induced microcracks which are shown to be comparable with the ion range and the damage range in the crystals tested. Possible mechanisms of crack nucleation for a low and high energy ion implantation are discussed.

Fracture toughness of molybdenum sheet under brittle-ductile transition

Temperature dependence of fracture toughness of molybdenum sheet was studied in the temperature range — 196–300 °C. It was found that nonmonotonous behaviour of this dependence is due to the change in cleavage crack nucleation mechanism.

Effect of specimen size on ductile-brittle transition of recrystallized molybdenum: Y.Hiraoka, T.Hoshika (University of Science, Okayama, Japan). J. Japan Soc. Powder Powder Metall., vol 42, no 1, 1995, 108–112. (In Japanese.)

Temperature dependence of fracture toughness of molybdenum sheet was studied in the temperature range — 196–300 °C. It was found that nonmonotonous behaviour of this dependence is due to the change in cleavage crack nucleation mechanism.

Effect of saline solution environment on the cyclic deformation of Ti-6Al-4V alloy

The present investigation studies the effect of physiological solution at 37°C on the cyclic deformation behaviour of a Ti-6Al-4V alloy, with a microstructure corresponding to that obtained in the substrate when a sintered metallic porous coating is produced. Cyclic deformation tests have been carried out up to fracture and the fatigue crack nucleation mechanisms have been analysed. Since fatigue is a phenomenon related with plastic deformation, which is enhanced at corrosion and/or at stress concentration sites, cyclic deformation tests conducted at a level of stress above the elastic limit can provide a clear picture of the crack nucleation mechanisms involved.

A simulation of the reaction bonding of Si3N4 to generate porous microstructures

A stochastic numerical technique is presented that has been used to study the formation of reaction-bonded porous silicon nitride (RBSN). Like many reaction-bonded materials, RBSN is a structural ceramic that has a naturally occurring porosity of approximately 25% resulting from the unique kinetics of formation. Since such porosity is usually unavoidable, it is of interest to discover its relevant measures, such as the size distribution and degree of homogeneity of pores, and the factors that govern the formation of these pores, in order to understand and optimize the crack propagation resistance of such materials. The probing of the fracture toughness of the generated microstructures by computational means is described in the companion paper [E G. Haubensak, V V Bulatov and A.S. Argon, J. Comput.-Aided Mater. Design, 2 (1995) 205]. The primary purpose of the simulated reaction was the generation of realistic microstructures incorporating the major features of reaction bonding. In this manner it is possible to generate objective, systematic variations of similar microstructures. With such microstructures, crack nucleation and propagation mechanisms can be observed, and processing parameters, which control fracture toughness, can be identified. In the present study, we outline the details of the reaction kinetics model, demonstrate its ability to generate realistic microstructures, and report some additional observations on some topological phenomena resulting from the reaction, such as those controlling the formation of large pores in the final structure. In yet a separate communication of a related experimental study we present the microstructures of the actual RBSN. We will use the findings of the latter study here for making some comparisons.

Effects of heat treatment on stress corrosion cracking of a discontinuously reinforced aluminum (DRA) 7XXX alloy during slow strain rate testing

Discontinuously reinforced aluminum (DRA) alloys are being developed as candidate materials for the automotive and aerospace industry. Although the corrosion and stress corrosion cracking (SCC) susceptibility of aluminum alloys have been extensively studied, comparatively little is known about the corrosion and SCC behavior of DRA materials. The intent of the present work was to study the effects of changes in microstructure/heat treatment on the crack nucleation mechanisms in DRAs and their monolithic atrices on the overall slow strain rate SCC performance in a 3.5% NaCl solution (pH = 3.0). For a given heat treatment, MB78 DRA materials show more susceptibility to stress corrosion cracking than the equivalent monolithic material. For the MB78 composite, the UAII material exhibited the maximum susceptibility to SCC. Both the UAI and UAII material were more susceptible to SCC than the OA material. MB78 DRA and monolithic specimens which have been shown to have a continuous ({eta} and {eta}{prime}) layer along the grain boundaries also showed higher susceptibility to stress corrosion cracking. Significantly more crack coalescence to form larger cracks was observed for the DRA specimens tested in the NaCl solution compared to the DRA specimens tested in dry-air. Monolithic specimens (OA as well as UA)more » did not exhibit visible micro-cracks or significant crack coalescence on the surfaces.« less

Observations of crack nucleation in columnar ice due to grain boundary sliding

Biaxial compression experiments were carried out to investigate the mechanisms of crack nucleation in columnar freshwater ice. For strain rates higher than 10 −3 s −1 at — 10°Cm, it is observed that crack nucleation is preceded by grain boundary sliding which results in local decohesion pockets spread intermittently along the boundary facets. The experimental results are shown to support the crack nucleation model presented in the companion paper.

Indentation induced crack nucleation and propagation in single crystal MgO

A study of indentation induced cracks in single crystal Magnesium Oxide ([001] orientation) was conducted to understand the micromechanisms controlling crack nucleation (and subsequent propagation) in semibrittle solids. Contrary to the expectations based on generally accepted crack nucleation models, no evidence of dislocation pile-ups was observed. However, SEM and TEM study of the deformation structure within the indent impression did reveal the presence of shear fault induced microcracking. This phenomenon was confirmed by examination of cross-sections through the indents using SEM. When the indentation temperature was varied from ambient to 800°C, a direct correlation was observed between the degree of subsurface damage, and the absence or presence of surface radial cracks. Based on the afore-mentioned results, an alternate crack nucleation mechanism is proposed, whereby shear fault induced microcracks act as nuclei for subsequent radial cracking. Further, microdiffraction analysis of the misorientations induced outside the indent suggest that crack propagation parallel to {110} is the result of severe plastic incompatibility across this plane.

A dislocation mechanism of crack nucleation in cubic zirconia single crystals

Abstract The dislocation structure around elevated-temperature indentations on {111} planes of 9·4 and 21 mol.% Y2O3 fully stabilized cubic ZrO2, was investigated using selective etching and transmission electron microscopy (TEM). The temperature dependences of the microhardness and the extent of the plastic zone were measured. Different fracture modes were detected in the two materials. Cracking arising from interaction between slip bands was observed in the 21 mol.% Y2O3 material and direct evidence of the formation of Lomer-type dislocation pile-ups leading to crack nucleation was obtained by TEM. The mechanisms of dislocation plasticity and fracture during indentation of cubic zirconia are discussed.

Crack nucleation mechanism in saline ice

A mechanism for crack nucleation in saline ice is presented, by considering a planar array of hexagonal grains containing the brine pockets as a model of poly crystalline saline ice. It is shown through a thermodynamic analysis that important local stresses arise associated with the internal pressure which builds up inside a brine pocket due to a drop in the temperature. As the temperature drops, the water inside the brine freezes, and because of the variation in the water density on freezing, this results in a buildup of pressure inside the pocket. For typical field conditions, assuming elastic behavior for the matrix, pressures as high as 7 MPa are estimated. Next, using a finite element method, the stress concentration at a grain triple junction is determined under the influence of the stress field associated with a nearby brine pocket. The resulting stress state is used to determine the condition for crack nucleation. The analysis is restricted to only elastic deformation regimes with isotropic grains, albeit with elastic constants corresponding to extreme values in a single freshwater ice crystal. The mechanism discussed here provides an explanation for the widely observed brine channels in sea ice. In addition, the effect of the externally applied stress is also considered, and the resulting stress singularities at the grain triple junctions analyzed by an asymptotic method as well as by a numerical solution. Both the strength and an approximate energy criteria suggest crack nucleation from the brine pocket surface towards the grain triple junction. The results are shown to be consistent with the experimental observations.