Subcritical Crack Growth Rate Research Articles

Experimental investigation of surface energy and subcritical crack growth in calcite

[1] Subcritical cracking behavior and surface energies are important factors in geological processes, as they control time-dependent brittle processes and the long-term stability of rocks. In this paper, we present experimental data on subcritical cracking in single calcite crystals exposed to glycol-water mixtures with varying water content. We find upper bounds for the surface energy of calcite that decrease with increasing water concentration and that are systematically lower than values obtained from molecular dynamics simulations. The relation of surface energy to water concentration can explain water weakening in chalks. The rate of subcritical crack growth in calcite is well described by a reaction rate model. The effect of increasing water on crack velocity is to lower the threshold energy release rate required for crack propagation. The slope of the crack velocity curve remains unaffected, something which strongly suggests that the mechanism for subcritical cracking in calcite does not depend on the water concentration.

Nano-mechanics based modeling of lifetime distribution of quasibrittle structures

The statistics of structural lifetime under constant load are related to the statistics of structural strength. The safety factors applied to structural strength must ensure failure probability no larger than 10 - 6 , which is beyond the means of direct verification by histogram testing. For perfectly brittle materials, extrapolation from the mean and variance to such a small tail probability is no problem because it is known that the Weibull distribution applies. Unfortunately, this is not possible for quasibrittle materials because the type of cumulative distribution function (cdf) has been shown to vary with structure size and shape. These are materials with inhomogeneities and fracture process zones (FPZ) that are not negligible compared to structural dimensions. A probabilistic theory of strength of quasibrittle structures failing at macro-crack initiation, which can be experimentally verified and calibrated indirectly, has recently been deduced from the rate of jumps of atomic lattice cracks governed by activation energy barriers. This paper extends this nano-mechanics based theory to the distribution of structural lifetime. Based on the cdf of strength and a power law for subcritical crack growth rate, the lifetime cdf of quasibrittle structures under constant loads is derived. The lifetime cdf is shown to depend strongly on the structure size as well as geometry. It is found that, for the creep rupture case, the mean structural lifetime exhibits a very strong size effect, much stronger than the size effect on the mean structure strength. The theory also implies temperature dependence of the lifetime cdf. For various quasibrittle materials, such as industrial ceramics and fiber composites, it is demonstrated that the proposed theory correctly predicts the experimentally observed deviations of lifetime histograms from the Weibull distribution.

Effect of frequency on ambient temperature fatigue crack growth in a silicon carbide reinforced silicon nitride composite

A detailed study on a silicon nitride reinforced with silicon carbide whiskers has been undertaken on room temperature fatigue during static and dynamic loading at constant Δ K. It is shown that sub-critical crack growth rates are lower when the material experiences sustained far field loading than during cyclic far field loading. The increased crack growth rate during cyclic loading is attributed to a wedging effect within the crack wake causing an increase in the tensile stress and resultant increased micro-cracking ahead of the crack tip. This additional micro-structural damage leads to enhanced sub-critical crack growth rates during cyclic loading. The asperities that are responsible for the wedging effect are attributed to the isolation of small portions of material due to branching of small cracks and by degradation of the bridging SiC whiskers and Si 3N 4 grains within the crack wake.

Sub-critical crack growth rate of soda-lime-silicate glass and less brittle glass as a function of fictive temperature

Sub-critical crack growth rates of soda-lime-silicate glass and less brittle glass with different fictive temperatures were compared using the DCDC method under both dry and humid atmospheres in order to investigate the origin of the unique mechanical features of the less brittle glass developed by Ito and his collaborators. In both dry and humid atmospheres, the crack velocity of the soda-lime-silicate glass was slower than that of the less brittle glass. For both glasses, the glass sample with higher fictive temperature showed a slower crack growth rate under both dry and humid atmospheres. These observations can be explained by the tendency for the plastic flow at the crack tip; the soda-lime-silicate glass is expected to show easier plastic flow under tension than the less brittle glass, and also the samples with higher fictive temperatures are expected to show easier plastic flow, leading to greater fracture toughness, KIC, and slower crack growth rate.

Measurement and Modeling of Hydrogen Environment–Assisted Cracking of Ultra-High-Strength Steel

Modern precipitation-hardened ultra-high-strength AERMET 100 steel (Fe-Co-Ni-Cr-Mo-C) is susceptible to severe transgranular hydrogen environment–assisted cracking (HEAC) in neutral 3.5 pct NaCl solution. The threshold stress intensity for HEAC, K TH , is reduced to as low as 10 pct of K IC , and the stage II subcritical crack growth rate, da/dt II, is up to 0.5 μm/s. Low K TH and high da/dt II are produced at potentials substantially cathodic, as well as mildly anodic, to free corrosion. However, a range exists at slightly cathodic potentials (–0.625 to –0.700 VSCE), where the crack growth rate is greatly reduced, consistent with reduced crack-tip acidification and low cathodic overpotential for limited H uptake. Short crack size (250 to 1000 μm) does not promote unexpectedly severe HEAC. High-purity AERMET 100 is susceptible to HEAC because martensite boundary trapping and high crack-tip stresses strongly enhance H segregation to sites that form a transgranular crack path. The stage II da/dt is H diffusion rate limited for all potentials examined. A semiquantitative model predicts the applied potential dependence of da/dt II using reasonable input parameters, particularly crack-tip H uptake reverse calculated from measured K TH and a realistic critical distance. Modeling challenges remain.

Fictive temperature dependence of subcritical crack growth rate of normal glass and anomalous glass

Subcritical crack growth rates of soda–lime–silicate glass, which is a typical normal glass, and silica glass, which is a typical anomalous glass, with different fictive temperatures were measured by the double-cleavage-drilled-compression (DCDC) fracture mechanics technique under both dry and humid atmospheres in order to clarify the effect of the fictive temperature on mechanical strength and fatigue. In the humid atmosphere, the soda–lime–silicate glass with a higher fictive temperature showed a slower crack growth rate than the same glass with a lower fictive temperature while the silica glass with a higher fictive temperature showed a faster crack growth rate than the silica glass with a lower fictive temperature. These results imply that normal glass with a higher fictive temperature is expected to show a higher mechanical strength compared with the same glass with a lower fictive temperature and anomalous glass with a higher fictive temperature is expected to show a lower mechanical strength than the same glass with a lower fictive temperature when tested in ambient air if the flaw size is the same. In the dry atmosphere, the fictive temperature effects on the crack growth rate in both glasses were small and within the experimental error.

Testing study of subcritical crack growth rate and fracture toughness in different rocks

Subcritical crack growth of double torsion specimens made of ore, 1herzolite, marble and granite was studied using Instron1342 type electro hydraulic servo test machine. The relations of the mode-I stress intensity factor K 1 versus the subcritical crack growth velocity v and the fracture toughness K IC were obtained by the double torsion constant displacement load relaxation method. The behavior of subcritical crack growth was analyzed for different rocks. The results show that 1g K I—1g v relations of four kinds of rocks measured by this method accord with linear rule, i.e. the relations between subcritical crack growth velocity and stress intensity factor have a power law, which is in good agreement with CHARLES theory. 1g K I—1g v curves move to top left corner with the decrease of the elastic modulus, which implies that the subcritical crack growth velocity speeds up. The maximum subcritical crack growth velocity exhibits negative exponential increase, and mode-I fracture toughness 1g K IC decreases with the decrease of elastic modulus. The testing results provide a basis for time-dependence of rock engineering stability.

Stress-corrosion fatigue–crack growth in a Zr-based bulk amorphous metal

Electrochemical and mechanical experiments were conducted to analyze the environmentally influenced cracking behavior of a bulk amorphous metal, Zr41.2Ti13.8Cu12.5Ni10Be22.5. This study was motivated by a scientific interest in mechanisms of fatigue–crack propagation in an amorphous metal, and by a practical interest in the use of this amorphous metal in applications that take advantage of its unique properties, including high specific strength, large elastic strains, and low damping. The objective of the work was to determine the rate and mechanisms of subcritical crack growth in this metallic glass in an aggressive environment. Specifically, fatigue–crack propagation behavior was investigated at a range of stress intensities in air and aqueous salt solutions by examining the effects of loading cycle, stress-intensity range, solution concentration, anion identity, solution deaeration, and bulk electrochemical potential. Results indicate that crack growth in aqueous solution in this alloy is driven by a stress-assisted anodic reaction at the crack tip. Rate-determining steps for such behavior are reasoned to be an electrochemical, stress-dependent reaction at near-threshold levels, and mass transport at higher (steady-state) growth rates.

Influence of the temperature on the strength and the subcritical crack growth rate of alumina

Abstract Dynamic four-point-bending tests between 20 and 900 °C were carried out in laboratory air on an alumina ceramic with a few intergranular glass phases (MgO, SiO 2 , CaO). The inert strength decreases with increasing temperature from 25 to 900 °C. In this range the temperature has no influence on the values of the corrected Weibull modulus m k . This means, that the strength distribution of the alumina investigated did not depend on the test temperature. The subcritical crack growth parameter A increases with increasing temperature from 25 to 500 °C and then decreases with further increasing temperature up to 900 °C. In contrast to this behaviour, for the parameter n the opposite temperature dependence turns out; n decreases between 25 and 500 °C and then increases. From d a /d t - K I plots it can be seen that the subcritical crack growth behaviour changes at a temperature of about 500 °C. For a constant crack propagation rate K I decreases with increasing temperature from 25 to 500 °C and then increases with further increasing temperature.

The structural integrity management of a deaerator feed water vessel approaching the end-of-life condition

The present paper describes the integrity management of a deaerator feedwater vessel over a 20-month service period within which it approached the end-of-life condition. A series of four remnant life assessments were carried out using both Crackwise and the R6 failure assessment programmes which determined critical crack sizes for vessel failure and a leak-before-break event. Interestingly, actual crack growth data obtained from this particular feedwater vessel over a 12 year service period was used in the sub-critical crack growth rate assessments. Remnant life values were reduced from only 25% of the normal guideline period of two years to re-inspect repaired weld cracks to around 12% or only three months within a 30-month service period. Also at each inspection a large amount of cracks, which numbered over one hundred, were found. From this it was clear that the vessel was approaching an end of life condition and the final assessment deemed that the vessel should be removed from service and all the problematic weld runs be replaced with new welds.

Internal hydrogen embrittlement of ultrahigh-strength AERMET 100 steel

Near-peak-aged AERMET 100 is susceptible to severe internal hydrogen embrittlement (IHE) at 23 °C, if a sufficient diffusible hydrogen content is present, compromising the high toughness of this ultrahigh-strength steel (UHSS). Evidence includes the threshold stress intensity for subcritical IHE (K TH ) as low as 10 pct of the plane-strain fracture toughness (K IC ) and a fracture-mode transition from microvoid coalescence to brittle transgranular (TG) cracking, apparently along martensite lath interfaces and cleavage planes. The K TH value decreases from a K IC value of 132 to 143 MPa√m to 12 MPa√m, and the amount of brittle TG fracture increases to nearly 100 pct as the concentration of diffusible H increases from essentially 0 to 8 wppm, with severe embrittlement in the 0 to 2 wppm H regime. The IHE is time dependent, as evidenced by increasing K TH values with increasing dK/dt and K-independent subcritical crack growth rates, and is attributed to diffusional H repartition from reversible trap sites to the stressed crack tip. The partition distance is ∼1 µm, consistent with the fine-scale microstructure of AERMET 100. The causes of the susceptibility of AERMET 100 to TG IHE are very high crack-tip stresses and a reservoir of mobile H trapped reversibly at (Fe,Cr,Mo)2C precipitates. These factors enable repartition of H to misoriented martensite lath interfaces and interstitial sites near cleavage planes, with each prone to decohesion along a connected path. Predissolved H also reduces the ductile fracture toughness of AERMET 100 at high loading rates, perhaps due to reduced void growth caused by H trapped strongly at undissolved metal carbides.

Anomalous temperature dependence of sub-critical crack growth in silica glass

The effects of temperature, water vapor, and stress on the rate of sub-critical crack growth (SCG) in fused silica are reported. The crack velocity was measured using the double-cleavage-drilled compression method. In contrast to other inorganic oxide glasses, crack growth velocities (in region I) were found to decrease with increase in temperature. Hence a small temperature rise has the apparent effect of improving the mechanical strength of a stressed-glass part. Despite the anomalous temperature dependence, SCG in fused silica is still likely governed by the established water-enhanced stress–corrosion mechanism; another competing phenomenon is proposed to cause the observed temperature dependence. Measured crack velocities are described using an empirical model (for region I) and a mass-transport model limited by Knudsen diffusion (for region II).

Effect of oxidation treatment on the crack propagation rate of aerogels

Typical silica aerogels obtained by alcohol supercritical drying are hydrophobic. After a low temperature heat treatment below 400°C, they become hydrophilic. The temperature of this heat treatment may increase the network connectivity by establishing new siloxane bonds. On the microscopic scale, this process induces a very small shrinkage during which the silica backbone strengthens and approaches that of pure porous silica. Silica aerogels, whatever the nature of their surface, hydrophobic or hydrophilic, exhibit a stress corrosion effect as previously reported. A comparison of the subcritical crack growth rates of differently heat treated aerogels is given. The experimental investigations are carried out under a controlled moisture of 50% RH using the double cleavage drilled compression (DCDC) technique. The crack length is optically measured within the range 10 −10– 10 −5 m s −1 . For a given stress intensity factor K I, we observe in the stress corrosion domain that hydrophobic aerogels display a very low crack rate. This rate increases with the amount of surface silanols. For higher oxidation temperatures, the number of surface silanols decreases inducing a lowering of the crack rate.

Measurement of sub-critical crack-growth rates in glass fibers by means of acoustic emission

In this paper, a new methodology is proposed to establish the v-K I relationship (v is the crack velocity and K I is the stress intensity factor) from tensile tests using fiber bundles and acoustic emission (AE) measurements.

Time-dependent failure mechanisms in silicon carbide composites for fusion energy applications

Silicon carbide has many properties that are attractive for applications in fusion energy systems. The reliability of monolithic silicon carbide, however, is insufficient for its use in large components. Ceramic matrix composites offer greater flaw tolerance and reliability, but their failure mechanisms are less well understood. This work has focussed on studying potential failure mechanisms in silicon carbide fiber-reinforced, silicon carbide matrix (SiCf/SiCm) composites.In the event of pre-existing cracks, subcritical crack-growth may occur due to creep of fibers that bridge the crack faces. Irradiation-enhanced creep will enhance the subcritical crack-growth rate. The presence of oxygen leads to oxidation of the interphase material and subcritical crack-growth controlled by the rate of interphase recession. In addition, fiber shrinkage or weakening due to exposure to radiation can promote additional failure mechanisms, including embrittlement. These mechanisms, the conditions, under which they occur, and the current state of models of the crack-growth mechanisms will be discussed.

Effects of OH content, water vapor pressure, and temperature on sub-critical crack growth in phosphate glass

The effects of temperature and water vapor pressure on the rate of sub-critical crack growth in meta-phosphate laser glasses containing different OH concentrations (128 and 773 ppmw) are reported. The crack velocity was measured using the double-cleavage-drilled-compression method. When plotted as a function of stress intensity, the samples have the classic region I, II and III crack growth properties similar to that reported for silicate glasses. The glass containing the larger OH content has a 10-fold greater crack velocity in region I; crack velocities is region II are the nearly the same for both glasses. The crack velocities are analyzed using a chemical kinetic and mass-transport limited reaction rate model. At temperatures >150°C and water vapor pressures >10 mmHg, crack tip blunting is observed and the glass containing the larger OH content is more prone to blunting.

Subcritical Crack Growth in a Phosphate Laser Glass

The rate of subcritical crack growth in a metaphosphate Nd‐doped laser glass was measured using the double‐cleavage‐drilled compression (DCDC) method. The crack velocity is reported as a function of stress intensity at temperatures ranging from 296 to 573 K and in nitrogen with water vapor pressures ranging from 40 Pa (0.3 mmHg) to 4.7 × 104 Pa (355 mmHg). The measured crack velocities follow region I, II, and III behavior similar to that reported for silicate glasses. A chemical and mass‐transport‐limited reaction rate model explains the behavior of the data except at high temperatures and high water vapor pressures where crack tip blunting is observed. Blunting is characterized by an arrest in the crack growth followed by the inability to reinitiate slow crack growth at higher stresses. A dynamic crack tip blunting mechanism is proposed to explain the deviation from the reaction rate model.

Fiber creep rate and high-temperature properties of SiC/SiC composites

Optimization of the high-temperature structural properties of continuous silicon carbide fiber, silicon carbide matrix composites (SiC/SiC) requires a fundamental understanding of the relationship between fiber, matrix, and interface properties. Results of a study aimed at relating the fiber creep rate to the subcritical crack growth (SCG) rate and fracture properties of SiC/SiC composites have demonstrated that the crack growth rate in a bulk composite is controlled by the fiber creep rate. This result was demonstrated for Nicalon–CG and Hi–Nicalon fiber reinforced material where a 100°C shift in the creep strength of the fiber resulted in a similar shift in the crack growth rate of the composite. Irradiation enhanced creep of SiC fibers and matrix must also be considered in the performance assessment of SiC/SiC composites. An estimate of the impact of irradiation creep of SiC fibers on the SCG of SiC/SiC will be presented.

Effect of strength on environment-assisted cracking of Ti–8V–6Cr–4Mo–4Zr–3Al in aqueous NaCl: Part 1: Age hardening vs. work hardening

The objective of this research is to understand the effect of yield strength on the environment-assisted cracking (EAC) susceptibility of a β Ti alloy, Ti–8V–6Cr–4Mo–4Zr–3Al (Beta-C), precracked and stressed actively in aqueous NaCl. Isothermal aging renders immune, β-solution treated Beta-C susceptible to severe EAC coincident with strengthening by α precipitates. The threshold for EAC decreases to 40% of the air fracture toughness, subcritical crack growth rate approaches 100 μm s −1, and the crack path becomes fully intergranular for aging times in excess of several hours. In contrast, solution treated and cold-worked single-phase β Beta-C retains immunity to EAC for similar high strengths. For several levels of cold work, Beta-C cracks by transgranular microvoid processes, with equal values of EAC threshold stress intensity and air fracture toughness. Cold work reduces the air fracture toughness of β-phase Beta-C compared to values for β/α microstructures at similar strengths. The strength increase due to α precipitation does not uniquely promote EAC susceptibility.

Effect of Cold Drawing on Microstructure and Corrosion Performance of High-Strength Steel

This paper deals with the effect of cold drawing on a high-strength steel in wire form with pearlitic microstructure. Cold drawing produces a preferential orientation of the pearlite lamellae aligned parallel to the cold drawing direction, resulting in anisotropic properties with regard to fracture behaviour in air and aggressive environments (stress corrosion cracking). While the hot rolled bar has a randomly oriented microstructure in both transverse and longitudinal sections, the fully drawn wire presents a randomly oriented appearance in the transverse cross-section, but a marked orientation in the longitudinal cross-section. These microstructural characteristics affect the time-dependent behaviour of the steels when a crack is present in a corrosive or hydrogen environment and influences both the subcritical crack growth rate, the time to failure and the crack propagation path. It is shown that in the strongly drawn steels the crack changes its propagation path, and a micromechanical model is proposed to explain this behaviour.