Coalescence Of Cavities Research Articles

Crack propagation behavior in rubber materials

Within the linear viscoelastic theory, crack tip fields are calculated at various crack tip velocities. A transition from rubbery to glassy material behavior in the vicinity of the crack tip can be observed. Shear and bulk behavior is analyzed separately. Whereas the increase of tearing energy at higher crack tip velocities can be ascribed to the shear behavior, bulk behavior influences the fracture mechanism. The results support experimental investigations that the instability separating stable from unstable crack propagation is related to a change in the fracture mechanism. At low crack tip velocities, material separation is the result of formation, growth, and coalescence of cavities. At high crack tip velocities, cavitation is suppressed and fracture is driven by a rather brittle mechanism resulting in a decreased amount of energy to propagate the fracture process zone.

Modeling ductile to brittle fracture transition in steels—micromechanical and physical challenges

Both scientists and engineers are very much concerned with the study of ductile-to-brittle transition (DBT) in ferritic steels. For historical reasons the Charpy impact test remains widely used in the industry as a quality control tool to determine the DBT temperature. The transition between the two failure modes, i.e. brittle cleavage at low temperature and ductile fracture at the upper shelf occurs also at low loading rate in fracture toughness tests. Recent developments have been made in the understanding of the micromechanisms controlling either cleavage fracture in BCC metals or ductile rupture by cavity nucleation, growth and coalescence. Other developments have also been made in numerical tools such as the finite element (FE) method incorporating sophisticated constitutive equations and damage laws to simulate ductile crack growth (DCG) and cleavage fracture. Both types of development have thus largely contributed to modeling DBT occurring either in impact tests or in fracture toughness tests. This constitutes the basis of a modern methodology to investigate fracture, which is the so-called local approach to fracture. In this study the micromechanisms of brittle cleavage fracture and ductile rupture are firstly shortly reviewed. Then the transition between both modes of failure is investigated. It is shown that the DBT behavior observed in impact tests or in fracture toughness specimens can be reasonably well predicted using modern theories on brittle and ductile fracture in conjunction with FE numerical simulations. The review includes a detailed study of a number of metallurgical parameters contributing to the variation of the DBT temperature. Two main types of steels are considered : (i) quenched and tempered bainitic and martensitic steels used in the fabrication of pressurized water reactors, and (ii) modern high-toughness line-pipe steels obtained by chemical variations and optimized hot-rolling conditions. An attempt is also made to underline the research areas which remain to be explored for improving the strength-toughness compromise in the development of steels.

A theoretical approach of strain localization within thin planar bands in porous ductile materials

Propagation of cracks in ductile materials is well known to occur through two possible mechanisms: coalescence of cavities and formation of shear bands (‘void sheet mechanism’). The classical Gurson–Tvergaard–Needleman (GTN) homogenized model for such materials incorporates some phenomenological modelling of coalescence, but not of formation of shear bands assisted by the presence of microvoids, and this generates a number of shortcomings. In order to solve these difficulties, this paper presents a unified model of both coalescence and formation of shear bands in porous plastic solids, including the possible couplings between the two. Both phenomena are viewed as expressions of the same basic effect, namely strain localization within thin planar bands, the only difference being the mode of deformation. The model is first developed assuming a periodic distribution of cavities, then critically assessed through comparison with some micromechanical numerical simulations based on the same assumption, and finally extended to the case of a random distribution of voids. To cite this article: J.-B. Leblond, G. Mottet, C. R. Mecanique 336 (2008).

Failure analysis and creep remaining life of hydrogen reformer outlet pigtail tubes

A failure analysis and creep remaining life assessment on hydrogen reformer outlet pigtail tubes in a methanol plant was conducted. The tube material was manufactured to ASME SB-407 (UNS N08810) material specifications. The observed evolution and progression of grain boundary cavity nucleation, coalescence, microcrack formation and eventual cracks were indicative of long-term creep failure, which occurred before the design life of the tubes. The remaining life evaluation was performed based on metallurgical evaluation of the outer diameter (OD), middle of wall thickness, and inner diameter (ID) percentage creep cavities versus tube OD. Very high statistical correlation was achieved by fitting creep data points to the classic creep strain versus time curve by using third order polynomial equations. The onset of tertiary creep was found to occur at 9.3% increase in the strain of the tube OD. This work affirms that using percentage area of creep cavities and the correlation to increase in OD is a robust technique for the reliable and accurate determination of the creep remaining life of superalloy tubes.

Do plastic zones form at crack tips in silicate glasses?

Abstract A number of recent studies claim that silicate glasses fracture by the formation, growth and coalescence of cavities at crack tips, in the same way as metals, but at a much smaller scale. Evidence for cavity formation comes from the examination of side surfaces of fracture mechanics specimens, at the point where the crack tip intersects the free surface. Such measurements exhibit small depressions in regions that are supposedly located in front of moving crack tips. These depressions were interpreted as cavities. In this paper, we summarize experimental results obtained using an atomic force microscope to characterize the fracture surfaces. The experimental results demonstrate an absence of residual damage on fracture surfaces that could be interpreted as cavity formation. We also observe cracks moving in glass and show that the features reported as cavities actually occur behind and not in front of the moving crack. A simulation of an atomic force microscope probe passing over the emerging tip of a crack in glass suggests that the features identified as cavities are in fact due to the roughness of the specimen surface. Our results support the view that cracks in glass propagate by brittle fracture. We find no evidence for nanoscale ductility in silicate glasses.

Cavity of Al-Cu-Mg Alloy during Superplastic Deformation

According to the characteristic of appearing cavitation in the metals during superplastic deformation, the influence of strain rate on cavity evolvement, the influence of cavity on superplastic deformation capability, and the formation, development process of cavity were investigated for Al-Cu-Mg alloy (i.e. coarse–grained LY12). The results show that: ①The pore nucleation occurs not only at triangle grain boundaries, but also along nearby the second phase particles, and even within grains. The cavities at the triangle grain boundaries are present in V-shape, others near the second phase particles and within grains are present in O-shape. These cavities may result from disharmony slippage of grain boundaries. ②The tendency of cavity development decreases with increasing of strain-rate. In lower strain-rate condition, though Al-Cu-Mg alloy has better superplasticity, many big cavities in the specimen may reduce the room temperature properties of the alloy. In higher strain-rate condition, Al-Cu-Mg alloy has certain superplasticity and room temperature properties as well as few cavities forming. By analyzing, viscous layer on grain boundaries is very thin and grain sizes can be refined during their extruding and rotating each other in higher strain-rate superplastic deformation condition. ③Growth and coalescence of cavity are the main reason of the superplastic fracture of Al-Cu-Mg alloy. And small and a certain amount of cavities with dispersion and independence state are very useful to crystal boundary slippage.

Development of a New Wrought Magnesium-Aluminum-Manganese Alloy AM30

A new wrought magnesium alloy, AM30 (Mg-3 pct Al-0.4 pct Mn), has been developed. Compared to the current workhorse commercial wrought magnesium alloy AZ31 (Mg-3 pct Al-1 pct Zn), the new AM30 alloy has 20 to 50 pct higher maximum extrusion speed, up to 50 pct ductility improvement at temperatures up to 200 °C, with similar yield and tensile strengths. The tensile behavior of AM30 and AZ31 magnesium alloy tubes suggests a moderate temperature forming range of 100 °C to 175 °C in which strain hardening offsets plastic instability until fracture by cavity coalescence occurs. The AM30 alloy has slightly better formability than AZ31 at room and moderate temperatures, due to higher strain-hardening rates (dσ/de) and exponents (n). Microstructural evaluation indicates that twinning is the major deformation mechanism for these alloys at room and moderate temperatures in addition to slip mechanisms. Dynamic recrystallization (DRX) is observed at temperatures greater than ∼150 °C, which causes local instabilities to occur, leading to failure.

Creep and strain-dependent microstructures of synthetic anorthite–diopside aggregates

We investigated plastic deformation of fine-grained synthetic diopside and two-phase anorthite–diopside aggregates in triaxial compression and torsion (up to γ ∼ 5). Temperature, confining pressure and stress ranged between 950–1180°C, 200–400 MPa and 5–500 MPa, respectively. Water content of samples ranged between ∼0.005 ± 0.002 and 0.075 ± 0.025 wt% H 2O. All samples deformed in linear-viscous creep with a stress exponent of 1.0 ± 0.2. The activation energy ranged between 571 ± 53 and 290 ± 28 kJ/mol, depending on mineralogy and water content. Sample strength depended on water fugacity with an exponent of 1.55 ± 0.25. Samples deformed in torsion and coaxial compression gave similar flow laws, in spite of significant differences in the corresponding microstructures. Scanning and transmission electron microscopy of two-phase samples deformed in torsion showed phase mixing, cavitation and dislocation processes. We suggest that linear-viscous creep of fine-grained two-phase aggregates involved grain boundary sliding accommodated by grain boundary diffusion and significant dislocation accommodation at high strains. We also observed that cavity coalescence and microcracking led to sample failure. Hence, dynamic instabilities may exist in high-strain shear zones accommodating viscous deformation in the lower continental crust.

Mechanical properties of individual phases, deformation and fracture in mechanically alloyed Al-Al<SUB align=right>4C<SUB align=right>3 composite

Using a Depth Sensing Indentation technique (DSI), the Martens hardness HM, indentation modulus EIT and deformation work W for Al matrix and Al4C3 particles have been measured. It has been shown that the hardness of Al4C3 particles is 57 times higher than the hardness of matrix. The change in fracture for the Al-12Al4C3 system was investigated and analysed at different temperatures and strain rates. With increasing tensile load, local cracks are formed by rupture of large particles and by decohesion of smaller particles from the matrix. Further increase of load leads to the crack growth by coalescence of cavities.

Periodic corrugation on dynamic fracture surface in brittle bulk metallic glass

Dynamic crack propagation in a model brittle bulk metallic glass (BMG) is studied. Contrary to other brittle glassy materials, the authors find nanometer scale out-of-plane periodic corrugations along the crack surface of the BMG. The nanoscale periodicity remains nearly constant at different loading rates. An interpretation is presented to explain the evolution and the periodic coalescence of the nanometer scale cavities along the crack surface. The observation sheds light on the origin of dynamic fracture surface roughening in brittle materials and could be generally applicable to brittle materials.

Amethyst-bearing lava flows in the Paraná Basin (Rio Grande do Sul, Brazil): cooling, vesiculation and formation of the geodic cavities

Size distribution data obtained from detailed field study of bubbles and amethyst-geodes in the basaltic lava flows of the Serra Geral Formation (Ametista do Sul, Rio Grande do Sul, Brazil) are used in cooling and vesiculation models to infer the origin and the formation processes of the geodic cavities. Coupled field observations and modelling results emphasize that (1) the formation of geodes in the studied lava flow can be explained, qualitatively and quantitatively, by the exsolution of magmatic gas from the supersaturated melt with no need for external surface water supply; (2) the vertically elongated habits of the geodes result from higher cooling rate of the magma in contact with the accumulating bubbles; and (3) the abnormal metre-sized geodes with their branching habits result not only from the diffusive/decompressive bubble growth but also from the coalescence of inwards-progressing tubular cavities.

Experimental investigation of damage and fracture in glassy materials at the nanometre scale

Slow crack advance is observed in real time via an Atomic Force Microscope in a minimal glass, amorphous Silica, under stress corrosion. Fracture proceeds through the nucleation, growth and coalescence of damage cavities, as recently reported in an aluminosilicate glass. The crack growth velocity as observed at the continuum scale is shown to be dominated by accelerating phases corresponding to the cavity coalescence with the main crack front. The process zone at the crack tip is then determined, and shown to increase with time when both the average crack growth velocity and the mechanical stress are kept constant. Transport of water molecules within the process zone is conjectured to be the dominant mechanism responsible for this time evolution. Migration of alkali Na ions in more complex Silicate glass is finally evidenced at the sub-mocrometric scale by observing through AFM the crack propagation in binary Na2O/SiO2 glasses.

A Study on the Creep Cavitation in an Ni3Al Alloy Using the Hydrogen Embrittlement Technique

Abstract In this study, the hydrogen embrittlement technique was employed in order for the direct observation of entire cavities on exposing grain boundary facets in a Ni3Al alloy. Also, stereo pair SEM micrographs were used to investigate the effects of inclined angle of grain boundary on cavity growth, by comparing relative positions of features on the stereo pair. Many of cavities at 65% of the rupture time are on the verge of coalescing with other cavities. Cavities on inclined boundaries had grown for longer time than those on transverse boundaries. Thus, the time to cavity coalescence on transverse facets are lower bound of the material lifetime. And, overall damage was found to have the value of about 0.6 at rupture.

Theoretical modeling and numerical study of coalescence of cavities in porous ductile viscoplastic solids

One presents a model for coalescence of cavities in porous ductile viscoplastic solids. The representative volume element considered is schematized as a ‘sandwich’ consisting of a central porous layer surrounded by two external sound layers, the stress and strain rate tensors being considered as homogeneous in each layer. The sound layers obey the classical Norton model and the porous one some specific homogenized model for porous viscoplastic solids accounting for void shape. An important feature is the description of the peculiar evolution of this shape during coalescence. The model predictions are successfully compared to the results of some finite element micromechanical simulations. To cite this article: L. Flandi, J.-B. Leblond, C. R. Mecanique 333 (2005).

Effect of State of Stress on the Cavitation Behavior of Al 5083 Superplastic Material

In this paper we address the controversial issue of nucleation of cavities in Al 5083 alloys and their subsequent growth to coalescence and failure. We focus on the origin and growth of cavities not only during the primary processing of Al 5083 in sheet forms, but also during the manufacture of these sheets into SPF (superplastic forming) components. Experimental observations of pre-existing cavities in this alloy are made using optical and electron microscopy. The role of sheet rolling direction, and the state of stress during superplastic deformation on the cavity formation and coalescence are also discussed. The effect of the state of stress (uniaxial, plane strain, balanced biaxial, and tri-axial) on the growth characteristics of cavitation is also examined. It is found that the uniaxial model based cavitation cannot directly be extended to predict the behavior of more complex stress states, unless great care is taken to identify the right strain measure for the mapping process.

Experimental Investigation of Cavitation Behavior in AZ61 Magnesium Alloy

The rate of cavitation with superplastic strain was investigated for a superplastic AZ61 magnesium alloy at a strain rate of 2 × 10 - 4 s - 1 and temperature of 648 K, under the conditions of which an elongation of more than 250% has been found. Cavities initiated at grain boundaries. The cavitation showed a growth perpendicular to the applied stress direction after the initial strains. The subsequent growth and coalescence of cavities invariably leads to failure of the material. The experimental growth rates are in good agreement with the rate predicted by the plasticity-controlled growth mechanism.

Internal damage processes in low alloy chromium–molybdenum steels during high-temperature creep service

Results are presented of investigations into structure of low alloy Cr–Mo steels exhibiting internal damage after long-term creep service. It was demonstrated that intercrystalline cavitation cracks were the dominant factor in service damage of power station boiler components operating in creep regimes. Consecutive stages in development of internal damage involving intercrystalline cavitation cracking were discussed and illustrated with micrographs. The results seem to indicate that nucleation of creep cavities in materials under consideration is related to grain boundary slip. Evidence confirming the shear mechanism proposed by Sklenička and Saxl for cavity coalescence was obtained. Occurrence of intercrystalline crevice cracking was demonstrated. Micrographs were used to illustrate wedge crevice nucleation modes on triple junctions depending on the direction of slip according to Chang and Grant. A classification of internal damage in relation to life exhaustion was proposed for materials under consideration. A method used in industrial practice for evaluation and qualification of creep-damaged materials was presented.

Creep mechanism of gas-pressure-sintered silicon nitride polycrystals I. Macroscopic and microscopic experimental study

The creep behaviour and microstructure of two silicon nitride ceramics have been investigated. Compressive creep tests were performed at temperatures between 1450 and 1700°C at stresses between 6 and 90 MPa in an Ar atmosphere. The creep behaviour was characterized by a stress exponent lower than one for both materials, with an average value n ≈ 0.6 over the whole range of stresses and temperatures, and with apparent activation energies between 470 and 530 kJ mol−1. The study of the microstructural evolution revealed the absence of dynamic grain growth and, in some cases, evidence of grain rearrangement. Partial coalescence of cavities was observed only at the highest stress, but this did not result in accelerated creep.

Nano-ductile crack propagation in glasses under stress corrosion: spatiotemporal evolution of damage in the vicinity of the crack tip

Recent experiments have evidenced the existence of a ductile fracture mode at the nanometer scale in Aluminosilicate glass. The present study is designed to check whether such a ductile mode is inherent to the amorphous nature of glass. Therefore, the slow crack advance is observed in real time via an Atomic Force Microscope in a minimal glass, amorphous Silica, under stress corrosion. In this case, the Crack propagation proceeds by the nucleation, growth and coalescence of damage cavities as in the Aluminosilicate glass, but the cavity size is significantly larger. We focus here on the kinematics of crack propagation by looking at the spatio-temporal evolution of both the tip of the main crack and the cavity ahead. It is shown that the velocity of the main crack tip is significantly lower than the one of the cavity edge toward the main crack tip, like in metallic alloys. Moreover, the velocities of the different fronts (main crack, frontward and backward cavity tips) at these nanometric scales is one order of magnitude smaller than the crack tip velocity at the continuum scale. This has important consequences for the modelling of stress corrosion, especially at ultra-slow crack propagation.

Microstructure and creep properties of a cast intermetallic Ti–46Al–2W–0.5Si alloy for gas turbine applications

Abstract The microstructure and creep properties including minimum creep rate, time to 1% creep deformation and creep fracture time of a cast TiAl-based alloy with nominal chemical composition Ti–46Al–2W–0.5Si (at.%) were investigated. The creep specimens were prepared from investment-cast plate and two large turbine blades. Constant load creep tests were performed in air at applied stresses ranging from 150 to 400 MPa in the temperature range 973–1073 K. The microstructure of the specimens is characterised by optical, scanning and transmission electron microscopy before and after creep deformation. The minimum creep rate is found to depend strongly on the applied stress and temperature. The power law stress exponent of minimum creep rate is n = 7.3 and the apparent activation energy for creep is Q a = 427 ± 14 kJ/mol. The initial microstructure of the creep specimen is unstable. The α 2 (Ti 3 Al)-phase transforms to γ(TiAl)-phase and needle-like B2-precipitates during long-term creep testing at all testing temperatures. At lower applied stresses, the creep specimens fail by the growth and coalescence of cavities and small cracks formed along the γ/α 2 interfaces. At the highest applied stresses, the specimens fail by nucleation and propagation of cracks.