Subcritical Crack Growth Behavior Research Articles

Failure Diagram and Chemical Driving Forces for Subcritical Crack Growth

Kitagawa-Takahashi diagram that is modified for fatigue is now extended to the subcritical crack growth behavior under stress-corrosion crack growth. The analogy with the fatigue helps us to identify several regimes of interest from both the point of understanding of the material behavior as well as quantification of the failure process for structural design of components that are subjected to stress-corrosion and corrosion fatigue crack growths and failure. In particular, the diagram provides a means of defining the mechanical equivalent of chemical stress concentration factor and the chemical crack-tip driving forces to crack growth or its arrest. In addition, threshold stresses, crack arrest, and nonpropagating crack growth conditions can be defined, which help in developing sound design methodology against stress corrosion and corrosion fatigue. Chemical crack driving forces under corrosion fatigue can be similarly defined using the inert behavior as a reference.

Subcritical crack growth behavior and life data analysis of two types of dental Y-TZP ceramics

Objectives The aim of this study was to evaluate the mechanical properties and the subcritical crack growth behavior of a presintered dental Y-TZP (Kavo Everest ZS) and a hot isostatic pressed Y-TZP (Kavo Everest ZH) and to perform life data analysis. Methods For each material 150 bending bars were produced. The initial fracture strength was determined in a four-point bending test. The subcritical crack growth parameters n and A were determined in a dynamic fatigue method at four decreasing loading rates from 110 MPa/s to 0.11 MPa/s in distilled water at 38 °C. For each loading rate Weibull statistics were performed and the Weibull moduli m and characteristic strengths σ 0 were calculated. Using these data, strength–fracture probability–life time (SPT) predictions were derived for 1 day, 1 year, 5 years and 10 years, based on a static crack growth mechanism. Results The “hipped” Y-TZP ceramic exhibited a higher initial strength ( σ c = 1618.18), characteristic strength ( σ 0 = 837.15) and fracture toughness ( K IC = 4.52 MPa/m 1/2) than the pre-sintered ceramic ( σ c = 1431, σ 0 = 745.46 and K IC = 3.17 MPa/m 1/2, respectively). Fatigue parameters, n and A, were 28.5 and 7.97 × 10 −24 for Everest ZH and 30.15 and 5.47 × 10 −20 for Everest ZS. The predicted fracture stress at 5% failure probability for a lifetime of 10 years was 259.34 MPa for Everest ZH and 263.2 MPa for Everest ZS. Conclusions Although the “hipped” Y-TZP showed favorable initial mechanical properties, no significant difference could be found in the susceptibility of both ceramics to subcritical crack growth and their long-term strength.

Subcritical crack growth behavior of dispersion oxide ceramics

Zirconia (Y-TZP) is used as material for components of implants and prostheses because of its high short-term strength. The mechanical long-term reliability, however, is limited for Y-TZP because of hydrothermal aging effects and a pronounced tendency for subcritical crack growth. The hypothesis of this study was that a substantial amount of alumina in a zirconia matrix can help to significantly suppress subcritical crack growth and thereby improve the mechanical long-term reliability. The Weibull parameters as well as the parameters of the subcritical crack growth were determined for Alumina, Y-TZP, and two dispersion ceramics, that is Alumina Toughened Zirconia (ATZ, 20% alumina/80% Y-TZP), and Zirconia Toughened Alumina (ZTA, 75% alumina/25% Y-TZP). The long-term failure probability as a function of service time was predicted for the four ceramics. The parameter n of the subcritical crack growth was approx. 80% higher for ATZ compared to Y-TZP. In consequence, the estimated lifetime revealed a significant better mechanical long-term reliability for ATZ. It can be concluded that tailored dispersion oxide ceramics can address the aging problem of monolithic zirconia. This makes ATZ very interesting for components of joint replacement as well as for dental prostheses and implants.

Mechanical strength and subcritical crack growth under wet cyclic loading of glass-infiltrated dental ceramics

Objectives Evaluate the flexural strength ( σ) and subcritical crack growth (SCG) under cyclic loading of glass-infiltrated alumina-based (IA, In-Ceram Alumina) and zirconia-reinforced (IZ, In-Ceram Zirconia) ceramics, testing the hypothesis that wet environment influences the SCG of both ceramics when submitted to cyclic loading. Methods Bar-shaped specimens of IA ( n = 45) and IZ ( n = 45) were fabricated and loaded in three-point bending (3P) in 37 °C artificial saliva (IA 3P and IZ 3P) and cyclic fatigued (F) in dry (D) and wet (W) conditions (IA FD, IA FW, IZ FD, IZ FW). The initial σ and the number of cycles to fracture were obtained from 3P and F tests, respectively. Data was examined using Weibull statistics. The SCG behavior was described in terms of crack velocity as a function of maximum stress intensity factor ( K Imax). Results The Weibull moduli ( m = 8) were similar for both ceramics. The characteristic strength ( σ 0) of IA and IZ was and 466 MPa 550 MPa, respectively. The wet environment significantly increased the SCG of IZ, whereas a less evident effect was observed for IA. In general, both ceramics were prone to SCG, with crack propagation occurring at K I as low as 43–48% of their critical K I. The highest σ of IZ should lead to longer lifetimes for similar loading conditions. Significance Water combined with cyclic loading causes pronounced SCG in IZ and IA materials. The lifetime of dental restorations based on these ceramics is expected to increase by reducing their direct exposure to wet conditions and/or by using high content zirconia ceramics with higher strength.

Effects of ultraviolet radiation on ultra-low-dielectric constant thin film fracture properties

The effects of ultraviolet (UV) radiation on ultra-low-kdielectric (ULK) thin film fracture toughness were studied. This work discusses both critical and subcritical crack growth behavior under different environments. The critical fracture toughness was measured as a function of applied phase angle by using the four-point bend flexure and mixed-mode double cantilever beam techniques. Results of critical fracture toughness obtained under different loading configurations and phase angles were found to increase with the UV treatment time. In contrast, mode I subcritical fracture toughness thresholds and the crack propagation velocity appeared to be insensitive to UV curing processes. This study revealed that subcritical fracture toughness values reduced with the moisture concentration in the environment.

V-K Curves from Lifetime Tests with Reloaded Survivals

Abstract Lifetime tests under static load are often carried out in order to determine the relation v(K) of subcritical crack growth. Depending on the scatter behavior of the most serious cracks in a specimen, a number of specimens will not fracture within a certain maximum lifetime limit. In order to get additional information on subcritical crack growth behavior, the authors recommend performing lifetime tests at an increased load level. An appropriate procedure for the evaluation of such tests is proposed and applied to static lifetime measurements on SiAlON in distilled water.

Determination of v–K curves from lifetime tests with reloaded survivals

Abstract Lifetime tests under static load are often carried out in order to determine the relation v(K) of subcritical crack growth. Depending on the scatter behaviour of the most serious cracks in a specimen a number of immediate failures can occur during load application and some specimens will not fracture within a certain maximum lifetime limit. In order to get additional information on subcritical crack growth behaviour, the authors recommend performing lifetime tests at an increased load level. Appropriate procedures for the evaluation of such tests will be outlined. First, the procedure is explained for the simple case of the power-law crack growth relation. Then, a modified lifetime evaluation is proposed that needs no special crack growth law. The methods described are applied to static lifetime measurements on SiAlON in distilled water at 20 °C as the environment.

Mechanical behavior of La 0.8Sr 0.2Ga 0.8Mg 0.2O 3 perovskites

This paper examines the important mechanical properties of commercially purchased La 0.8Sr 0.2Ga 0.8Mg 0.2O 3 at room temperature and 800 °C. Sr and Mg-doped lanthanum gallates (LSGM) are strong candidates for use as solid electrolytes in lower temperature solid oxide fuel cells operating at or below 800 °C. The material was found to be phase pure with a Young's modulus value of ∼175 GPa. The four point bending strength of the LSGM samples remained almost constant from 121 ± 35 MPa at room temperature to 126 ± 20 MPa at 800 °C. The fracture toughness, as measured by the single edge V notch beam (SEVNB) method, was 1.22 ± 0.06 MPa√m at room temperature, 1.04 ± 0.09 MPa√m at 700 °C followed by a small increase 1.31 ± 0.16 MPa√m at 800 °C. We also report, for the first time, the static subcritical (or slow) crack-growth (SCG) behavior of natural cracks in LSGM performed in four point bending tests at room temperature. The exponent of a power-law representation in the SCG tests was found to be n = 15, a rather low value showing LSGM to be highly susceptible to room temperature SCG.

Sub-Critical Crack Growth Behavior of a Low-Temperature Co-Fired Ceramic

The strength of sintered low-temperature co-fired ceramic (LTCC) disks was measured under dry (<2% relative humidity [RH]) and wet (98% RH) conditions, using ring-on-ring, biaxial-flexure testing in the stressing rate range of 0.002–2 MPa/s. The empirical sub-critical crack growth parameters, n and A, were calculated. The values of the parameters in the wet environment were higher than in the dry environment. It is argued that the high RH n value is more representative of material behavior. Results of fractographic examination are reported and expected lifetime and safe design stresses for LTCC are calculated.

Advances in research of stress-assisted corrosion fatigue problem

Ceramic materials are notable for their rigidity, insulation and resistance to hostile environment. Nevertheless, if a stressed ceramic component is exposed to chemical attack, it may suffer from a form of delayed fracture known as static fatigue. From the point of view of a designer, it is clearly desirable to determine the behavior of sub-critical crack growth; the crack path and crack growth rate, as a function of material properties and loading conditions are of particular interest. This paper presents a review of advances in stress assisted corrosion problem in history and its corresponding numerical approaches in the last decades, and finally, comes up with consideration and crucial suggestions for future work.

Relation between subcritical crack growth behavior and crack paths in granite

Relation between subcritical crack growth behavior and crack paths in granite

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.

Cathodic delamination of elastomer-to-metal adhesive joints: Experimental data and empirical modeling

In order to estimate the cathodic delamination rates of metal adhesive/steel adhesive bonds, an empirical model based on crack growth is developed. To this end, a considerable amount of delamination data was experimentally collected under several levels of cathodic environmental harshness (some highly accelerated) and applied peel stress. For a given electrolyte, cathodic delamination was found to depend on cathodic voltage (or current density), temperature, and applied strain energy release rate, G. In order to describe the progress of delamination under various cathodic environments as a function of applied G, the collected, experimental delamination-distance-versus-time data were used to develop characteristic curves of log delamination rates versus log G. For all conditions tested, these log–log plots were dominated by linear regions (commonly referred to as Region II) that follow the Paris law. In this region, it was found that bond delamination rates vary dramatically depending on the environment. Furthermore, and unlike with the critical fracture toughness ( G c), the threshold value of G ( G th) for the degraded bond varies from one set of cathodic conditions to another. To model delamination rates as function of the cathodic environment and G, a nonlinear equation capable of modeling subcritical crack growth behavior was used. The experimental delamination data was fitted to this equation in order to determine the coefficients of this equation. For each environment tested, one equation was determined and which describes the bond delamination rate versus G behavior over the entire G-range. Consequently, these coefficients were consolidated into master, easy-to-use, empirical equations that relate delamination rates to G. The resulting empirical model is applicable over a wide range of cathodic conditions of voltage and temperature.

Cyclic fatigue in water of veneer–framework composites for all-ceramic dental bridges

Objectives Ceramic materials applied in dentistry may exhibit significant subcritical crack growth due to the severe cyclic loading in the aqueous environment encountered in the mouth during mastication. The authors report on the subcritical crack growth behavior of three dental restoration systems (Empress 2/IPS Eris, TZP/Cercon S and Inceram-Zirconia/Vita VM7) under cyclic loading in water, in order to establish guidelines for the use and design of long-lifetime all-ceramic posterior bridges. Methods Inert strength and lifetime tests under cyclic loading in an aqueous environment were performed in a mechanical bending apparatus and evaluated with Weibull statistics. Results Subcritical crack growth occurred predominantly in the outer veneer layer of the veneer–framework composites. The apatite-based veneer (IPS Eris) was more susceptible to subcritical crack propagation compared to the feldspathic glass veneers (Cercon S and Vita VM7). Significance Dental restoration systems containing apatite-based veneers and weak frameworks (Empress 2/IPS Eris) are not recommended for the fabrication of all-ceramic bridges in the molar region. Conversely, veneer–framework systems consisting of feldspathic glass veneers and tough zirconia-based frameworks (TZP/Cercon S and Inceram-Zirconia/Vita VM7) may exhibit lifetimes longer than 20 years if the bridge connector is properly designed.

4054 DT法によるセラミックスのき裂進展特性評価 : コンプライアンス法によるき裂長さ測定(G03-12 き裂進展(1),G03 材料力学)

In order to determine the K_I-V curve of brittle materials as ceramics, Double-Torsion (DT) technique was used in combination with a compliance method. In crack propagation test, the compliance of DT specimen was obtained from the relation between the displacement of loading point measured by laser displacement meter and applied load. In addition to that the crack length was measured by using a grid pattern film method. Then, a formula of the relative crack length, a/W, as a function of specimen compliance was derived. The relation between stress intensity factor and crack growth rate obtained from the experiment using soda-lime glass could give the detail in the subcritical crack growth behavior.

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.

花こう岩における応力腐食による亀裂進展現象の異方性および粒径依存性

Subcritical crack growth is considered to be a source of the time-dependent fracture in rocks. Thus, it is very important to know the nature of subcritical crack growth in rocks. Although rocks are generally anisotropic, rocks have been assumed as isotropic materials in many studies. Therefore, anisotropic properties of subcritical crack growth in rocks are not known well. In this study, Double-Torsion test was carried out using three kinds of granite to investigate the anisotropic properties and the grain-size dependency of subcritical crack growth. Double-Torsion specimens were prepared from each granitic rock with considering the propagation direction and the opening direction of the crack.It was shown experimentally that the subcritical crack growth behavior in granite was anisotropic. When the crack advanced parallel to RIFT plane, the stress level at the crack tip was the smallest in all granite used in this study. Additionally, the stress level at the same crack velocity was in good agreement when the opening direction of the crack was the same.It was also indicated that subcritical crack growth in granite was dependent on the grain size. For coarser grained granite, the stress level causing crack growth tended to be lower than that of fine-grained granite.It could be considered that anisotropy and grain-size dependency of subcritical crack growth in granite related to the preferred orientation and the density of pre-existing microcracks, because it was recognized that as the velocity of P-wave propagating in the opening direction of the crack was low, that is, as the crack density of granite was high, the stress level causing subcritical crack growth was low. It can be concluded that the subcritical crack growth behavior in granite is affected strongly by the orientation distribution of pre-existing microcracks.

Interfacial fatigue crack propagation in Ni‐base superalloy protective coatings

It is important to control the interface strength of coatings and composite materials; a feature that has been noted for many years. However, how can interface strength be evaluated and controlled? In order to explore this aspect, subcritical fatigue crack growth behaviour was investigated along the interface of a CoNiCrAlY coated Ni‐Base superalloy. According to the traditional fatigue test methodology, fatigue crack propagation tests were carried out, using double cantilever beam specimens. The resistance to the fatigue crack propagation was successfully evaluated by a fracture mechanics approach. Particular attention was given to the effects of surface finish of the substrate as a coating parameter, the test temperature, and long‐term thermal ageing after the coating.

00/02870 Subcritical crack growth and creep behavior of silicon carbide for heat exchanger applications

00/02870 Subcritical crack growth and creep behavior of silicon carbide for heat exchanger applications

Adsorption and Kinetic Effects on Crack Growth in MnZn Ferrites

The variation of the fracture toughness of MnZn ferrite ceramics with varying loading rate and humidity was determined with the aid of the single edge notched beam (SENB) test. A strong decrease with increasing humidity and decreasing loading rate was observed. A model for subcritical crack growth incorporating kinetic and adsorption effects was formulated to analyze the data. The value of the adsorptioncontrolled fracture toughness was determined independently by double torsion experiments and agreed favorably with the values as determined from the SENB data using the model. The strength of the material was determined, and analysis showed a strength behavior similar to the fracture toughness behavior, as predicted by the model. The analysis presented can be used to assess the subcritical crack growth behavior using a limited number of SENB specimens.