Fracture strength of all-ceramic crowns

This paper compares the fracture strength of three types of all-ceramic crown shape systems (In ceram, OPCTM and IPS Empress) cemented with either a commercial resin cement, zinc phosphate or glass ionomer. Twenty test crown shapes with 8 mm diameter and 8.5 mm height were fabricated for each type of ceramic. Ten In ceram crown shapes were luted on the die using zinc phosphate, while ten OPCTM and IPS Empress were luted using resin cement specified for the particular system. Another ten specimens each, of In ceram, OPCTM and IPS Empress, were luted on the die using a glass ionomer. The crown shapes were fractured in a mechanical testing machine (Instron) using a steel ball, 4 mm diameter, that contacted the occlusal surface and the resulting data were statistically analysed using a Mann–Whitney test. The results showed that: (1) In ceram crown shapes luted with zinc phosphate were significantly stronger than IPS Empress crown shapes luted with resin cement (p < 0.05), but no difference was observed compared with OPCTM crown shapes luted with resin cement. No statistical difference was found between OPCTM and IPS Empress crown shapes. (2) When the three ceramics were luted with glass ionomer, the In ceram was significantly stronger than OPCTM (p < 0.05) and IPS Empress (p < 0.05). OPCTM was significantly stronger than IPS Empress (p < 0.05).     

Effect of convergence angle and luting agent on the fracture strength of In ceram crowns

This study compared the fracture strength of In ceram crown shapes fabricated with either 8 or 16° total occlusal convergence; and attached with either a commercial zinc phosphate cement or a glass ionomer cement. Thirty crown shapes (8 mm diameter and 8.5 mm high) were fabricated for each preparation design on a brass master die with approximately the same dimensions as a premolar. In ceram crown shapes were luted on to the die using zinc phosphate or glass ionomer. The crown shapes were fractured in a testing machine (Instron) using a steel ball, 4 mm in diameter, that contacted the occlusal surface and the resulting data were statistically analyzed using a Mann–Whitney test. The results indicate that there is no statistical difference in the fracture strength values between preparations with 8° total convergence compared with 16° using the same cement. However, crown shapes luted with zinc phosphate on preparations with 8 and 16° total occlusal convergence were significantly stronger than those luted using glass ionomer cements plt; 0.05).     

Strength and Fracture Toughness of Ceramic Materials for Metal-Ceramic Prosthetic Dentistry

The strength in biaxial flexure and the fracture toughness on indentation of the specimens of ceramic materials for the coverage of metal-ceramic prostheses were studied. The strength was established to be mainly determined by their microstructure and sintering conditions. Comparative studies demonstrated that Ultropaline and VITA OMEGA 900 ceramic compositions exhibited more uniform distribution of leucite microcrystals and higher strength and fracture toughness than Duceram Plus and IPS Classic materials.__________Translated from Problemy Prochnosti, No. 3, pp. 128 – 139, May – June, 2005.     

Effect of grinding conditions on the fatigue life of titanium 5Al-2.5Sn alloy

The principal factors in the performance of aerospace materials are strength-to-weight ratio, fatigue life, fracture toughness, survivability and of course reliability. Machining processes, and in particular grinding under adverse conditions, have been found to cause damage to surface integrity and affect the residual stresses distribution in the surface and sub-surface region. These effects have a direct bearing on the fatigue life. In this investigation the effect of grinding conditions on the fatigue life of titanium 5Al-2.5Sn was studied. This alloy is used in ground form in the manufacturing of some critical components in the space shuttle's main engine. It is essential that materials for such applications be properly characterized for use in severe service conditions. Flat sub-size specimens 0.1 in. (2.5 mm) thick were ground on a surface grinding machine equipped with a variable-speed motor at speeds of 2000 to 6000 fpm (10 to 30 m sec^–1) using SiC wheels of grit sizes 60 and 120. The grinding parameters used in this investigation were chosen from a separate study. The ground specimens were then fatigued at a selected stress and the resulting lives were compared with that of the virgin material. The surfaces of the specimens were examined under a scanning electron microscope and the roughness and hardness were measured using a standard profilometer and microhardness tester, respectively. The fatigue life of the ground specimens was found to decrease with an increase in speed for both dry and wet conditions. For both the grit sizes, the fatigue life was lower than that of the virgin material for the dry condition. The fatigue life of specimens ground under wet conditions showed a significant increase at the wheel speed of 2000 fpm (10 msec^–1) for both grit sizes, and thereafter decreased with increase in speed to below that of the virgin material. The results of the investigation are explained using profilometry, microhardness measurements and scanning electron microscopic examination.     

An experimental technique for determining biaxial fatigue lifetimes in biomedical elastomers

A novel test apparatus was developed to determine fatigue lifetimes of elastomeric materials. Thin elastomeric sheets (membranes) are biaxially fatigued while contained within a temperature controlled liquid environment. Membranes are displaced in a smooth sinusoidal motion by pumping the surrounding fluid through the equipment's inner cells. An electrical signal (AC) passes through the membrane during fatigue and is used to monitor changes in the elastomer's physical characteristics. In addition, the test cell is designed to allow optical observation of the membrane throughout the fatiguing process. Thus the design results in a test rig apparatus that permits failure determination by both material property changes and obvious fracture. Cyclic fatigue frequencies range from 0.2 to 1.2 Hz.     

Study of Dry and Wet Cement Mortar Dynamic Properties

We performed dynamic tests and studied the dynamic properties under compression of dry and wet cement mortar specimens using the Hopkinson split pressure bar method. As a result, we determined the fracture stresses and their dependence on the applied loading growth rate. It is noteworthy that the strength characteristic of wet material under study is less by 10–15% as compared to the dry one.     

Fracture mechanisms of the Strombus gigas conch shell: implications for the design of brittle laminates

Flexural strength, crack-density evolution, work of fracture, and critical strain energy release rates were measured for wet and dry specimens of the Strombus gigas conch shell. This shell has a crossed-lamellar microarchitecture, which is layered at five distinct length scales and can be considered a form of ceramic plywood. The shell has a particularly high ceramic (mineral) content (99.9 wt%), yet achieves unusually good mechanical performance. Even though the strengths are modest (of the order 100 MPa), the laminated structure has a large strain to fracture, and a correspondingly large work of fracture, up to 13 kJ m^–2. The large fracture resistance is correlated to the extensive microcracking that occurs along the numerous interfaces within the shell microstructure. Implications of this impressive work of fracture for design of brittle laminates are considered.     

The fatigue behaviour of dispersed-oxide-strengthened lead in air and in vacuum

Specimens of dispersed-oxide-strengthened lead containing 4.5 wt % oxide were fatigued at room temperature ( 0.5 T _m where T _m is the melting point in degrees K) in air and in vacuum (< 2×10^–4 torr). Metallography of the damage during fatigue and after fatigue fracture showed that the improved fatigue resistance of dispersed-oxide-strengthened lead (DS lead) over that of pure lead was due mainly to the mechanical strengthening effects of the dispersed oxide rather than an increase in the resistance to atmospheric corrosion fatigue. The ratio of the fatigue life of DS lead in vacuum to that in air was 8.5 at a strain of ± 0.145%. In specimens fatigued in air, failure occurred at grain boundaries and in those fatigued in vacuum it occurred by a mixture of intercrystalline and transcrystalline modes.     

Roles of precipitates on corrosion fatigue crack growth of high-strength steels in corrosive environments

The effect of microstructures on resistance to corrosion fatigue cracking and fracture surface morphology for age-hardened steels were investigated in a 3.5% NaCl aqueous solution under a cathodic potential of –0.85 V (Ag/AgCl). The free corrosion was about –0.63 V (Ag/AgCl). The resistance to corrosion fatigue cracking of materials containing coherent precipitates in the matrix (underaged conditions) was less than that of materials containing incoherent precipitates (reheated conditions) at equal strength levels. Accelerated fatigue crack growth rates of the underaged material in the aqueous solution were followed by cracking along prior-austenite grain boundaries, due to hydrogen embrittlement, while the overaged material did not show accelerated fatigue crack growth rates and had fracture surfaces similar to those in air. The difference in the fracture surfaces of both materials in air and in the aqueous solution was considered to depend on the ease of diffusion of hydrogen to the prior-austenite grain boundaries. It is concluded that incoherent precipitates in the matrix made hydrogen accumulation at prior-austenite grain boundaries much slower than for coherent precipitates.     

Preparation,microstructure and mechanical properties of dense polycrystalline hydroxy apatite

Hydroxy apatite ceramic blocks of varying density have been prepared from a commercial powder. The elastic properties, fracture toughness, strength and sub-critical crack growth of these materials have been investigated. Young's modulus for the nearly fully dense material is 112 GPa while the compressive strength is about 800 MPa. For the same material the strength and fracture toughness under dry conditions are 115 MPa and 1.0 MPa m^1/2, respectively. Substantial slow crack growth was found under these conditions. Under wet conditions the values for strength and fracture toughness drop to about 75% of their “dry” values. In this case very serious slow crack growth is present.     

Strengths of C/C composites under tensile, shear, and compressive loading: Role of interfacial shear strength

Various strengths of carbon–carbon composites (C/Cs) are comprehensively reviewed. The topics reviewed include tensile, shear, compressive, and fatigue strength as well as fiber/matrix interfacial strength of C/Cs. When data are available, high temperature properties, including creep behavior, are presented. Since C/Cs have extremely low fiber/matrix interfacial strength τ_d, the interfacial fracture plays important roles in all of the fracture processes dealt in this review. The low τ_d was found to divide tensile fracture units into small bundles, to seriously degrade both shear and compressive strength, and to improve fatigue performance. In spite of the importance of the interfacial strength of C/Cs, techniques for its evaluation and analysis are still in a primitive stage.     

Strength wearout of adhesively bonded joints under constant amplitude fatigue

The behaviour of adhesively bonded lap joints subjected to fatigue loading is still not well understood. In this paper strength degradation of joints during fatigue cycling is measured experimentally and related to damage evolution. Strength wearout (SW) measurements carried out under constant amplitude fatigue loading of single lap joints are presented and correlated with in situ measurements of back-face strain (BFS) and estimations of damage progression from fracture surfaces and sectioning of partially fatigued samples. Residual strength was found to decrease non-linearly with respect to the number of fatigue cycles and this corresponded to non-linear increases in the BFS and damage measurements. In particular it was noted that fatigue damage accelerated very quickly towards the end of the fatigue life of a joint. A non-linear SW model is proposed and was found to agree well with the experimental results. This model can be used to predict the residual strength of a joint after a period of fatigue loading once a single empirical constant has been determined.     

Fracture of filaments and its influence on critical current and residual strength of fatigued Nb-Ti/Cu superconducting composite

Fatigue behavior at room temperature and its influence on critical current at 4.2 K and residual strength at room temperature of multifilamentary Nb-Ti/Cu superconducting composite wire with a filament volume fraction of 0.49 (copper ratio of 1.04) were studied. The fatigue crack nucleated in the copper in the circumferential region and propagated stably into the inner region, causing fracture of the Nb-Ti filaments in the late stage of the fatigue life. Once the fracture of the filaments started, the number of the fractured filaments increased steeply with increasing number of stress cycles, and correspondingly, the current-transportable and stress-carrying capacity of the composite decreased steeply. In this process, both the critical current and residual strength of the fatigued composite decreased nearly linearly with decreasing fraction of surviving filaments. Thus, the critical current of the fatigued composite was proportional to residual strength as a first approximation.     

Fatigue studies of high-palladium dental casting alloys: Part I. Fatigue limits and fracture characteristics*

The fatigue limits and fracture characteristics for a Pd–Cu–Ga alloy and a Pd–Ga alloy were studied. The alloys were cast into tensile test bars with gauge diameter of 3 mm and gauge length of 15 mm, and the surfaces of the castings were neither air-abraded nor polished after removal from the investment. Specimens were prepared from all-new metal (not previously melted), a combination of 50% new metal and 50% old metal (previously melted one time) and 100% old metal. The cast bars were subjected to heat treatment simulating the complete firing cycles for dental porcelain, and fatigued in air at room temperature under uniaxial tension-compression stress at 10 Hz and a ratio of tensile stress amplitude to compressive stress amplitude (R-ratio) of –1. The alloy microstructures and fracture surfaces were examined with a scanning electron microscope (SEM). Results showed that the fatigue limits at 2 x 10⁶cycles of the Pd–Cu–Ga and Pd–Ga alloys were approximately 0.20 and 0.15 of their 0.1% yield strength (YS) in tension, respectively. The fatigue resistance for specimens from both alloys containing 50% old metal and 50% new metal was comparable to that of specimens containing all-new metal, although this decreased dramatically for Pd–Cu–Ga alloy specimens containing all-old metal. The fatigue resistance of the Pd–Cu–Ga alloy subjected to heat treatment simulating the porcelain firing cycles was not adversely affected by remnants of the original as-cast dendritic microstructure that remained in the relatively large test specimens. A longer heat treatment than recommended by the manufacturer for the porcelain firing cycles is needed to completely eliminate the as-cast dendritic structure in these specimens. The Pd–Cu–Ga alloy exhibited superior fatigue resistance to the Pd–Ga alloy, which has an equiaxed-grain microstructure and lower yield strength.     

The durability of glass fibre cement-the effect of fibre length and content

The properties of glass reinforced cement composites (grc) containing 2–8 vol % of alkali resistant glass fibres of lengths 10–40 mm have been studied for periods of up to 5 years in various environments. Fibre volume fraction was found to be an important factor influencing the strength of grc at all ages, while fibre length was of decreasing significance as storage periods in wet environments increased. In relatively dry conditions, little change with time of bending, tensile or impact strengths was observed, but the matrix cracking stress was reduced. In wet environments, the cracking stress tended to increase but the ultimate strength to decrease.At 28 days maximum strength was achieved with composites having 6 to 8 vol % fibre 30 to 40 mm long. Composites with similar formulations were found to have the greater strength after 5 years' storage but, after water storage or natural weathering a strength reduction had occurred. Bending strength was approximately 70% to 86% of its 28 day value, tensile strength between 55% and 84% and impact strength 32% to 78%. Young's modulus is largely dependent upon the degree of hydration of the cement matrix and in the long-term was greater for water-stored material than for that stored in dry air.     

Cyclic Crack Resistance of Grey and High-Strength Cast Irons with Increased Phosphorus Content

We investigated the influence of phosphorus (0.02–0.76%) on the microstructure, short-term strength, cyclic crack resistance characteristics, and micromechanism of fatigue fracture of grey and high-strength cast irons. It was established that the low cyclic crack resistance of grey and high-strength cast irons with increased phosphorus contents (0.7–0.8%) is caused by the propagation of a fatigue crack via intergranular cleavage, initiated by a discontinuous or continuous network of precipitates of the ternary fine-grained phosphide eutectic along boundaries of ferrite grains. We showed that, from the viewpoint of cyclic crack resistance, it is admissible to alloy cast irons of the ferritic and ferritic–pearlitic class with phosphorus up to 0.3% when the phosphide eutectic forms in amounts of 3–5% for grey cast irons and 4–7% for high-strength cast irons without significant decrease in their resistance to brittle fracture.     

Age-related changes in cyclic voltammetry and potentiodynamic studies of normal human dentine

Impedance spectroscopy is one of the non-destructive techniques used by researchers to measure electrical resistance of biological tissues and ceramics. The purpose of this study is to investigate the voltage–current (V–I) characteristics of sound human dentine from young and old teeth, using cyclic voltammetry and potentiodynamic techniques. Dentine samples were prepared from freshly extracted sound third molars. After electrical measurements, dentine samples were characterized using scanning electron microscopy (SEM). Cyclic voltammetric measurements showed that variation of current through sample as a function of applied voltage is linear for dry samples of both age groups. However, for wet samples V–I characteristic were found to be different. The resistivity of dry young dentine is greater than that of old dentine in dry environment, whereas, it was found to be opposite for wet dentine samples. Using the same voltage sweep in potentiodynamic measurements dry samples display similar traces to controls suggesting that the dry dentine acts as an insulator. The number of dentinal tubules and their diameter has been found to decrease with increasing age. We propose that these changes determine the changes in electrical characteristics of sound human dentine. In spite of increasing use of electrical techniques to understand electrical properties of teeth, it is clear from this study that local structural variations and environment have a marked influence. Therefore, this baseline data needs to be considered in any future study or clinical application.     

Advanced elastomer nano-composites based on CNT-hybrid filler systems

Different techniques to disperse multiwalled carbon nanotubes (CNT) in elastomers using an internal mixer are applied and physical properties of the composites are evaluated: stress–strain behavior, dynamic-mechanical, thermal diffusivity, dielectric and fracture mechanical properties. The electrical percolation threshold is found to decrease by using ethanol as dispersion agent, compared to “dry” mixing, correlating with improved optical dispersion. The effect of nanoscopic gaps between adjacent CNTs on the electrical and thermal conductivity of the composites and the missing percolation behavior of the thermal conductivity are discussed. We have found some technically promising synergetic effects of the hybrid filler systems. For all systems one observes significantly steeper stress–strain curves by addition of 1.6 vol.% CNT to the systems with conventional fillers. In natural rubber the fatigue crack propagation resistance, tensile strength and electrical conductivity is found to be improved also for dry mixed CNT-silica hybrid systems.     

Effects of stress waveform and water absorption on the fatigue strength of angle-ply aramid fiber/epoxy composites

The influences of stress waveform and water absorption on the tension–tension fatigue fracture behavior were investigated in ±45° angle-ply laminates of aramid fiber reinforced epoxy matrix composite. For dry specimens, the fatigue strength under negative pulse waveform was higher than that under the positive pulse waveform. Rotation of fibers to the longitudinal direction, which resulted from creep deformation caused by the cyclic loading superimposed on the maximum stress hold time, decreased the compliance, thereby increasing the fatigue life under the negative pulse waveform. Water absorption degraded the fiber/matrix interfacial strength and caused the swelling of the matrix, which resulted in decreases in the static tensile strength and fatigue strength. Although the strength under the negative pulse waveform was slightly higher than that under the positive one, the influence of stress waveform on fatigue strength was smaller in wet specimens.     

The immersed fatigue response of polymer composites

The effect of sea water on carbon/epoxy cross ply specimens is studied by consideration of fatigue data and failure modes. Tests were conducted using dry and saturated coupons fatigued in air, as well as pre-saturated coupons fatigued while immersed in sea water. The saturated coupons fatigued in air had the longest fatigue life, which was attributed to stress relief from sorption induced swelling. Saturated coupons fatigued in an immersed environment exhibited the shortest fatigue life, and also experienced significant deliminations prior to failure. Water trapped inside the transverse cracks during the load cycle provides a physical mechanism explaining this behavior. Analytical and numerical analyses are presented which show how moisture inside the transverse cracks can alter the coupon stress state and enhance delimination.