Effects Of Slow Crack Growth Research Articles

On the feasibility of the Chevron Notch Beam method to measure fracture toughness of fine-grained zirconia ceramics

ObjectiveThe fracture toughness determination of fine-grained zirconia ceramics using the chevron notched beam method (CNB) was investigated to assess the feasibility of this method for quality assurance and material characterization. MethodsCNB tests were performed using four different yttria-stabilized zirconia ceramics under various testing modes and conditions, including displacement-controlled and load-rate-controlled four point bending to assess the influence of slow crack growth and identify most suitable test parameters. For comparison, tests using single-edge V-notch beams (SEVNB) were conducted. ResultsIt was observed that the CNB method yields well-reproducible results. However, slow crack growth effects significantly affect the measured KIC values, especially when slow loading rates are used. To minimize the effect of slow crack growth, the application of high loading rates is recommended. SignificanceDespite a certain effort needed for setting up a sample preparation routine, the CNB method is considered to be very useful for measuring and controlling the fracture toughness of zirconia ceramics.

Effect of microstructure and slow crack growth on lifetime prediction of monolithic silicon carbide

The lifetime of silicon carbide (SiC) based materials is strongly dependent on the presence of pre-existing flaws or cracks and their extension under an applied load during their service life. The purpose of this work is to determine the long term strength of SiC based materials with different grain morphologies and grain boundary chemistry. Solid state (SS) sintering of SiC with carbon and boron and liquid phase (LP) sintering of SiC using alumina and yttria as additives were used to produce fine and coarse grained materials to clarify the role of chemistry and grain morphology respectively. Fracture toughness, strength and slow crack growth (SCG) data were used to determine lifetime diagrams to accurately evaluate the long term strength behaviour for natural and artificial defects. The LP-SiC materials have more susceptibility to SCG compared to SS-SiC. However, the LP-SiC with coarse grains has a higher toughness and can be used at higher stresses after a large defect has been accidentally introduced. This indicates that the effect of the slow crack growth as a result of introducing oxides on the grain boundaries is not sufficient to alter the ranking between materials in terms of their deterministic allowable stress after such a damage event. On the other hand, the allowable stress in terms of the natural defect population revealed different results for using a low probability of failure (5%) and a much higher probability of failure (63.2%): the ranking of the materials alters when the stress level at which it is to be used changes.

Micro- and macro-mechanical testing of transparent MgAl2O4 spinel

Advanced transparent ceramics with high chemical and thermal stability are gaining increasing interest as replacement of glass-based materials in technical window applications. The mechanical reliability and performance of transparent MgAl2O4 with a grain size of 5 μm has been characterized at ambient temperature using micro-mechanical indentation and macroscopic bending tests. The measurements focused on elastic modulus, fracture toughness, crack kinetics, and strength, the latter analyzed with Weibull statistics. The effect of slow crack growth is assessed using a strength–probability–time plot. Complementary fractography by optical, confocal and scanning electron microscopy provided a correlation between failure origin and fracture stress. The results and reliability aspects are discussed in terms of linear elastic fracture mechanics.

Mechanical properties and lifetime predictions for Ba 0.5Sr 0.5Co 0.8Fe 0.2O 3− δ membrane material

The mixed ion–electron conductor Ba 0.5Sr 0.5Co 0.8Fe 0.2O 3− δ has a strong application potential as high-temperature gas separation membrane. However, for real components the mechanical integrity of this brittle perovskite ceramic will be challenged by the boundary conditions of transient and stationary temperature exposure. In particular, long-term failure mechanisms such as static fatigue at room temperature and creep rupture at operation temperature may occur. The relevance of both effects is assessed. The effect of slow crack growth at room temperature has been investigated using fracture stresses obtained in biaxial bending under different loading rates. The provided data permit to assess the fracture stresses for different loading rates. Furthermore, a strength–probability–time plot is derived that permits a prediction of the lifetime under static loading conditions and hence the long-term reliability at room temperature. The creep rupture at typical operating temperatures was analysed using three-point bending tests permitting a determination of the failure stress in this application-related combined tensile–compressive mode. The creep rupture data are described by a modified Monkman–Grant relationship.

Fatigue crack propagation under variable amplitude loading in PMMA and bone cement

Fatigue failure of PMMA bone cement is an important factor in the failure of cemented joint replacements. Although these devices experience widely varying loads within the body, there has been little or no study of the effects of variable amplitude loading (VAL) on fatigue damage development. Fatigue crack propagation tests were undertaken using CT specimens made from pure PMMA and Palacos R bone cement. In PMMA, constant amplitude loading tests were carried out at R- ratios ranging from 0.1 to 0.9, and VAL tests at R = 0.1 with 30% overloads every 100 cycles. Palacos R specimens were tested with and without overloads every 100 cycles and with a simplified load spectrum representing daily activities. The R- ratio had a pronounced effect on crack propagation in PMMA consistent with the effects of slow crack growth under constant load. Single overloads caused pronounced crack retardation, especially at low da/dN. In Palacos R, similar overloads had little effect, whilst individual overloads at low da/dN caused pronounced acceleration and spectrum loading retarded crack growth relative to Paris Law predictions. These results demonstrate that VAL can have dramatic effects on crack growth, which should be considered when testing bone cements.

Effects of loading rate on the deformation and cracking of dental multilayers: Experiments and models

The paper presents the results of combined experimental, analytical, and computational studies of contact-induced deformation and cracking in dental multilayers. These include studies of individual layers and composite structures that consist of tri-layers of glass bonded to ceramic-filled polymer foundation with an acrylate-based join material. Loading-rate-dependent Young's moduli of the join and foundation materials were obtained from monotonic compression tests. Critical loads were also determined for the tri-layers from Hertzian contact tests at different loading rates. The fracture onset (sub-surface radial cracking) was detected using an in situ telescope. The measured rate-dependent Young's moduli were then incorporated into a finite element model that was used to predict the rate-dependent critical loads in the tri-layer system. Finally, the paper shows that the observed loading rate-dependence of the critical load (for radial cracking) is due to the combined effects of slow crack growth in glass and rate-dependent Young's moduli in the join and foundation layers.

Influence of aggregate size on fracture toughness of concrete

Employing an extension of the splitting tensile by using a notched cylinder specimen, we have studied effects of initial notch length and maximum aggregate size on fracture toughness of concrete. Experimental results show that maximum aggregate size does influence ductility, with increasingly ductile behavior associated with increasing aggregate size. The results also support previous work in that initial notch length and maximum load do not yield a constant value for fracture toughness, whereas maximum linear load and initial notch length minimize the effects of slow crack growth and do produce a more constant value.

The fracture toughness testing of ceramics and acoustic emission

Measurements of the fracture toughness of commercial sintered alumina were monitored using acoustic emission to detect subcritical crack growth. Double-cantilever-beam specimens gave higher values of toughness than single-edge-notched bend (SENB) or double-torsion specimens did, even after correction for slow crack growth. The SENB test with a sharp crack appears to give the most reliable results, requires the easiest specimens to prepare and suffers the least slow crack growth. In view of this, detailed studies of the damage made by sawing notches, of the effects of the a/W ratio and of the cross-head displacement rate were made for SENB specimens. Sharp cracks with small a/W ratios and high cross-head speeds minimize the effects of slow crack growth.

Thermal fracture studies in ceramic systems using an acoustic emission technique

An acoustic emission technique for measuring the failure time in thermal shock experiments is described. The technique offers a unique opportunity to measure the heat-transfer coefficient of the test system and hence, to obtain a fully quantitative measure of the peak surface stress generated during the test. Measurements on soda-lime glass have demonstrated that rapid thermal fracture in the material occurs when the surface stress attains a critical value equal to the propagation stress for the most deleterious surface flaw. The effects of slow crack growth on the failure time are also investigated and correlated with recently developed theory.

Proof testing — the effects of slow crack growth

When slow crack growth occurs during proof testing, the extent of the crack growth while unloading from the proof stress has a substantial effect on the minimum predicted in-service failure time. It is essential that this crack growth is minimized to achieve effective failure prediction. Analyses are presented which show that the detrimental effects can be negated by environmental control during proof testing and/or rapid unloading from the proof stress; the requisite combination of unloading rate and environmental control is determinated separately for each system.

ChemInform Abstract: EFFECT OF SLOW CRACK GROWTH ON THE THERMAL‐STRESS RESISTANCE OF AN NA2O‐CAO‐SIO2 GLASS

AbstractDer Einfluß langsamen Rißwachstums auf die Einwirkung eines thermischen Schocks (Abschrecken in Wasser) wird an einem Na‐Ca‐silicatglas untersucht.

Effect of Slow Crack Growth on the Thermal‐Stress Resistance of an Na2O‐CaO‐SiO2 Glass

The effect of slow crack growth on the thermal‐shock resistance of an Na2O‐CaO‐SiO2 glass was studied. An analytic and numerical technique was developed to calculate the critical quenching temperature difference, ΔTc, of circular rods quenched in water from known crack velocity data. For rods of a specific radius and crack depth, ΔTc, was calculated to be 147°C, in favorable correspondence with experimentally observed values of 155° to 160°C. In the absence of crack growth, ΔTc was estimated to be 238°C, well in excess of the observed value and indicative of the significant effect of slow crack growth on thermal‐stress resistance. It is also shown that crack growth significantly extends the time‐to‐failure to a value much greater than the time of maximum thermal stress. A form of subcritical crack instability is predicted at stress intensity factors well below the critical stress intensity factor, at which catastrophic failure becomes inevitable over the duration of the transient thermal stress. It is suggested that, when all other factors are equal, the effect of slow crack growth on thermal‐stress resistance can be minimized by maximizing thermal diffusivity. It is also argued that surface‐compression strengthening will be more effective than reduction in flaw size in increasing thermal‐stress resistance. Recommendations are made for the design and selection of brittle materials subjected to thermal stress in stress‐corrosive environments.