Fracture toughness of austempered chilled ductile iron

The structure and properties of ductile iron are highly dependent on the solidification mechanism and chills are used to promote directional solidification to get sound castings. A series of fracture toughness experiments were carried out involving austempered chilled ductile iron containing 3.42% C, 1.8% Si and other alloying elements. By using copper chills of different thickness, the fracture toughness of varying the chill rate was also examined. The fracture toughness tests were carried out using three-point bend specimens, each with a chevron notch, as per ASTM E 399 1990 standards. It was found that austempered chilled ductile iron is highly dependent on the location on the casting from where the test samples are taken and also on the Ni and Mo content of the material. Chill thickness, however, also affects the fracture toughness of the material.     

Fracture toughness of austempered chilled ductile iron

Abstract

The structure and properties of ductile iron are highly dependent on the solidification mechanism, and chills are used to promote directional solidification to obtain sound castings. A series of fracture toughness experiments was carried out, involving austempered chilled ductile iron containing 3·42%C, 1·8%Si, and other alloying elements. By using copper chills of different thicknesses, the fracture toughness with varying chill rate was examined. Fracture toughness tests were carried out using three point bend specimens, each with a chevron notch, according to ASTM Standard E399 : 1990. It was found that the fracture toughness of austempered chilled ductile iron is highly dependent on the location in the casting from where the test specimens are taken and also on the nickel and molybdenum contents of the material. Chill thickness, however, also affects the fracture toughness of the material.     

Fracture of slightly plasticized polyvinyl chloride

Impact tests and fracture toughness tests using compact tension specimens were carried out on a number of slightly plasticized PVC compositions. These measurements, together with calculations from the craze thickness profile were used to determine the fracture mechanisms operating in the various tests. The marked decrease in the impact strength of PVC on addition of small amounts of a conventional plasticizer was found to be due to the plasticizer decreasing the stress intensity necessary to nucleate a craze at the notch tip of the impact specimen. The fracture toughness of the compact tension specimens which failed by a crazing mechanism increased with increasing plasticizer content. It is thought that the fracture of these specimens is controlled by the stress intensity necessary to propagate the pre-existing crack/craze system through the material.     

Mechanical properties of SG-iron with different matrix structure

Spheroidal graphite (SG) irons with a variety of matrix-structure have been produced. The correlation between tensile properties, impact toughness, hardness and pearlite content is investigated. The pearlite content is varied from 0 to about 95 per cent by the use of different heat treatment processes. The apparent variation in the properties with the pearlite level reveals the remarkable consistency in the relationships between mechanical properties and pearlite content. The study of the tensile properties showed that the yield and ultimate tensile strengths are increased with increasing pearlite level in the matrix structure. For matrix structure with 94.6% pearlite, the increases are about 91% and 98%, respectively, compared with those of the ferritic matrix material. The impact toughness of SG-iron is influenced significantly by matrix microstructure. Energy of about 230 × 10³J/m² is required to fracture a ferritic matrix SG-iron. On the other hand, when the matrix structure approaches a fully pearlitic matrix the fracture energy is decreased by an amount of 75.5%. The Brinell hardness value is found to increase with increasing pearlite content in the matrix structure of the present material. It increases from about 128 for a fully ferritic matrix to about 258 as the matrix structure approaches a fully pearlitic condition. This change in the hardness value reflects the change in the mechanical properties presented in this study.     

Effect of rhenium addition on fracture toughness of tungsten at elevated temperatures

Fracture toughness tests of tungsten and tungsten-rhenium alloy specimens were carried out at elevated temperatures. Temperature dependence of fracture toughness and effect of rhenium content on fracture toughness were investigated. Although fracture toughnesses of three kinds of specimens with rhenium contents of 0, 5 and 10 wt% were almost identical at room temperature, fracture toughness at elevated temperatures increased with increasing rhenium content. The brittle-ductile transition, similar to steels, and subsequent transition of the fracture mode from ductile dimple to intergranular were observed for all three kinds of specimens. With increasing rhenium content, the transition temperatures increased. A significant grain growth was found, not for tungsten-rhenium alloy specimens, but for a tungsten specimen without rhenium in a temperature range higher than the recrystallizing temperature, which resulted in transition of the fracture mode from dimple to intergranular.     

Large scale testing and statistical analysis of dynamic fracture toughness of ductile cast iron

The present paper deals with the experimental determination and statistical analysis of dynamic fracture toughness values of ductile cast iron. K_Id data from 140 mm thick single edge bend specimens of two dynamic fracture toughness test series on ductile cast iron from heavy-walled castings were analysed.At first, the statistical analysis of data at −40 °C was done based on ASME Code Case N-670 using a two-parameter Weibull distribution function. Weibull analyses of three samples covering different pearlite contents (?4%, ?9%, ?20%) were performed and characteristics of the distribution functions as well as two-sided confidence intervals were calculated. The calculated characteristics show that K_Id of ductile cast iron decreases with increasing pearlite content.In a second step, the applicability of the Master curve procedure according to ASTM E 1921 to ductile cast iron materials was investigated and it was formally used for statistical analysis of ductile cast iron dynamic fracture toughness data. Although the Master curve method was originally introduced for static fracture toughness data of ferritic steels, the successful individual analyses performed here support the engineering way taken to apply the method to ductile cast iron materials too. The results of both methods, the Master curve procedure and the ASME Code Case N-670, show acceptable congruity. At the same time, it is concluded from the present study that further investigations and experiments are required to improve precision and for verification before the results could be applied within component safety analyses.     

Estimation of fracture toughness of steel using chevron notched round bar specimens

A generalized methodology has been outlined in this paper for estimating the minimum normalized stress intensity factor (Y*_min) of chevron notched round bar specimens, subjected to three‐point bend loading. Using such specimens, a series of fracture toughness tests have been carried out for the first time on two steels. The major inferences drawn from this investigation are: (i) reproducible fracture toughness values can be achieved using chevron notched rod specimens of identical configuration and (ii) the estimated magnitudes of fracture toughness obtained by using chevron notched rod specimens are in good agreement with those achieved by using chevron notched rectangular bar specimens of the same material.     

Structure,fracture toughness,and fatigue of two aluminium matrix compositesproduced by vertical squeeze casting technique

Abstract

Composite materials produced from ceramic reinforcement of aluminium alloys have some properties that are better (higher modulus and strength, lower thermal expansion coefficient and density, and good creep and wear resistance) than those of the conventional monolithic aluminium alloys. However, they have a poor fracture toughness. The aim of the present work was to characterise the structure and mechanical properties of two different aluminium matrix composites (AS9C1G/20%(Al₂O₃-SiO₂) and 2014/20%(Al₂O₃-SiO₂)) manufactured using the vertical squeeze casting technique. Tensile, plane strain fracture toughness, and fatigue crack growth rate tests were carried out. In particular, the influence of specimen geometry on the toughness tests was examined. It was found that chevron notched short bar specimens gave toughness values ～ 40% higher than other types of specimens. Fatigue crack growth rate data were interpolated using some semiempirical models. An accurate metallographic investigation of both the structures and the fatigue fracture surfaces was carried out using optical microscopy and energy dispersive spectroscopy with SEM.     

High-temperature strength and fracture toughness in γ-phase titanium aluminides

High-temperature strengths and fracture toughnesses of -phase titanium aluminides were estimated at room and elevated temperatures. The effects of chromium on these mechanical properties were investigated. It was found that addition of chromium substantially improved the room- and high-temperature strength and toughness of the binary titanium aluminides. The transition temperature at which the strength drops was found to increase due to the addition of chromium. The fracture behaviour of binary and chromium-alloyed titanium aluminides were investigated. The fracture mechanism was affected by the addition of chromium. Ductile tearing was observed for the ternary material at 800 °C, and this was delayed for the binary material.     

The effect of basalt fibres on fracture toughness of asphalt mixture

This paper deals with the effect of basalt fibres on fracture toughness of asphalt mixture. For this purpose, basalt fibres with three different contents (i.e., 0.1%, 0.2%, and 0.3% by weight of asphalt mixture) and lengths (ie, 4, 8, and 12 mm) are incorporated into asphalt mixture to prepare fibre‐reinforced asphalt mixtures. Fracture tests are then carried out on these mixtures under four different modes of loading (i.e., pure mode I, pure mode II, and two mixed modes of I/II) using semicircular bend (SCB) specimens. The results exhibit that the fracture toughness increases with the enhancement of the fibre content. In addition, increase in the length of basalt fibre results in reduction of the fracture toughness of asphalt mixture. However, the asphalt mixture containing 0.3% of basalt fibres with the length of 4 mm shows the highest fracture toughness compared with other mixtures. It is also found that the basalt fibre improves mode I fracture toughness of asphalt mixtures more significantly than mode II one. Statistical analysis is also performed on the experimental data. Analysis of ANOVA demonstrates that all the three factors investigated in this study (i.e., length of basalt fibre, content of basalt fibre, and mode of loading) have significant influence on the fracture toughness of asphalt mixtures.     

The effect of fatigue pre‐cracking forces on fracture toughness

Testing procedures for the determination of the fracture toughness of a material by monotonic loading of fatigue pre‐cracked specimens are well established in standards such as BS 7448, BS EN ISO 15653, ISO 12135, ASTM E1820 and ASTM E1921. However, a review of these standards indicates a wide range of permitted fatigue pre‐cracking forces, whilst the underlying assumption in each standard is that the pre‐cracking conditions do not affect the fracture toughness determined. In order to establish the influence of different fatigue pre‐cracking forces on the fracture toughness, tests were carried out on specimens from an API 5L X70 pipeline steel. Single‐edge notch bend specimens of Bx2B geometry were notched through thickness and tested at temperatures of +20 °C, ?80 °C and ?140 °C to show the fracture behaviour in different regions of the fracture toughness ductile‐to‐brittle transition curve. Fatigue pre‐cracking was conducted on a high‐frequency resonance fatigue test machine over a range of pre‐cracking forces permissible within the various standards and beyond. The results showed that an excessively high pre‐cracking force can result in a significant overestimation of the value of fracture toughness for material exhibiting brittle behaviour, whilst very low fatigue pre‐cracking forces appeared to result in an increase in scatter of fracture toughness. A review of standards indicated that there was a possibility to misinterpret the intention of the ISO 12135 standard and potentially use excessively high pre‐cracking forces. Suggested clarifications to this standard have therefore been proposed to avoid the risk of overestimating fracture toughness.     

Fracture and physical properties of carbon anodes for the aluminum reduction cell

An experimental study with total 504 specimens has been carried out to investigate the fracture and physical properties of the carbon anode materials. The specimens were sampled from anodes produced with machined stub holes. From normal-and Weibull analysis the fracture toughness and the tensile strength showed a clear temperature dependency and orthotropic behavior. It has been found that both the fracture toughness and tensile strength increases with the temperature and are larger for the specimens directed in the horizontal direction than in the vertical direction. The variation in the tensile strength within an anode decreased with the temperature but the variation in the fracture strain increased. The tensile strain appears to be only dependent on the temperature and insensitive to the routine anode properties of the anode material. A multivariate linear regression analyses of the fracture toughness and tensile strength has been conducted and a typical correlation of R² = 0.5 (R is the Coefficient of Determination) to the measured routine anode properties was found. The thermal expansion coefficient is also larger in the vertical anode direction which makes the crack initiation more sensitive to temperatures. The orthotropic studies also showed that the air permeability has a tendency to be larger in the horizontal direction in the upper part of the anode which can induce unnecessary burning from the anode sides. The influence of the processing parameters in the paste plant and baking furnace has not been presented in this paper.     

Fracture toughness and fatigue crack growth behaviour of an Al2O3-SiC composite

The fracture toughness and fatigue crack growth characteristics of an Al₂O₃-SiC whisker composite were investigated. Quasi static fracture experiments were conducted on double edge-notched tension specimens and on four-point bend specimens containing a through-thickness Mode I crack which was introduced under uniaxial cyclic compression. The toughness results obtained using this procedure are more reproducible than those derived from the indentation technique and the notched bend bar method. The fracture toughness of the composite is about 60% higher than that of the unreinforced matrix material. Crack growth characteristics at room temperature were also investigated in notched plates of Al₂O₃-SiC subjected to fully compressive far-field cyclic loads. In the presence of a stress concentrator, this composite is found to be highly susceptible to fatigue crack growth under cyclic compressive loads.     

Chevronproben zur näherungsweisen Bestimmung der Bruchzähigkeit

Chevron Specimen for the Estimation of Fracture Toughness Fracture toughness is a material property which is presently used in many industrial areas, either as material selection criteria or as material quality requirement. In some areas, nuclear power plants and aerospace, it is also a design parameter for design against catastrophic failures. Determination of the fracture toughness in accordance with ASTM E 399 is relatively elaborate. Depending on the material concerned, a certain minimum material cross section is required to obtain the necessary size of the specimen. Many semi-finished product forms of the different materials can not be tested for fracture toughness due to the specimen size requirements. For these reasons, alternative test methods were sought of which testing of chevron-notched specimens is one method. In the work to be presented, the test method to determine fracture toughness via chevron-notched specimens is briefly described. The most frequently used chevron-notched specimens are shown together with loading grips to be used in conjunctions with universal testing machines. Certain effects associated with some of the chevronnotched specimens are pointed out which result in a large difference between the fracture toughness determined in accordance with ASTM E 399 and that obtained via chevron-notched specimens. The aim of our research effort is to develop a chevron-notched specimen geometry which furnishes fracture toughness values compatible with K_Ic values without complicating the test method. Such a chevronnotched specimen is presented and the fracture toughness values obtained from these specimens of 7475-T 7351 and different Ti-alloys are compared to the K_Ic values obtained in accordance with ASTM E 399 for the same materials.     

Ductile–brittle fatigue and fracture behaviour of aluminium/PMMA bimaterial 3PB specimens

Fracture toughness and fatigue crack growth tests and numerical simulations on 3PB specimens were carried out to study the behaviour of a crack lying perpendicular to the interface in a ductile/brittle bimaterial. Polymethylmethacrylate acrylic (PMMA) and aluminium alloy 2024 T531 were joined together using epoxy resin. A precrack was introduced into the ductile material and tests were carried out to obtain fracture toughness and fatigue properties. The body force method and elastic–plastic finite-element analyses were used to simulate the experimental stress intensity K_I and cracking behaviour under monotonic and cyclic loads. It was found that the bimaterial fatigue crack growth rate is higher than that for monolithic aluminium 2024 but lower than the rate for a monolithic PMMA. This agreed with the trend for the fracture toughness values and was consistent with the numerical method results. The initial Mode I stable ductile cracking in the aluminium appears to ‘jump’ the interface and continues under mixed fracture Mode (I and II) in the PMMA material up to the final failure. A consistency between the simulation methods has indicated that the bimaterial fatigue crack growth is dominantly elastic with a small plastic zone near the crack tip.     

Einfluß der Probengröße auf die bruchmechanischen Eigenschaften von Sinterstahl

Influence of Specimen Size on the Fracture Mechanical Behaviour of Sintered Steel Fracture mechanics testing was carried out with small and big specimens using high-temperature sintered Fe-2%Cu-2.5%Ni-alloys in the densities of ρ = 7.1 and 7.4 g/cm³. These steels are often used in the manufacturing of PM-parts. Due to the different dismensions the crack propagation is for the bigger sizes faster than for the smaller sizes. Also the conditional fracture toughness of the big specimens is superiour to the toughness of the small specimens. But under consideration of a plain strain state for the big specimens and of a plain stress state for the small specimens valid fracture toughness values being independent from the specimen size can be calculated applying linear-elastic fracture mechanics. These results were obtained for both densities investigated. The increase of the density delivers principally better fracture mechanical data. Hereby the relation of strength data with the microstructure is also discussed.     

Determination of interfacial fracture toughness of bone-cement interface using sandwich Brazilian disks

The long-term stability of cemented total hip replacements critically depends on the lasting integrity of the bond between bone and bone cement. Conventionally, the bonding strength of bone-cement is obtained by mechanical tests that tend to produce a large variability between specimens and test methods. In this work, interfacial fracture toughness of synthetic bone-cement interface has been studied using sandwiched Brazilian disk specimens. Experiments were carried out using polyurethane foams as substrates and a common bone cement as an interlayer. Selected loading angles from 0° to 25° were used to achieve full loading conditions from mode I to mode II. Finite element analyses were carried out to obtain the solutions for strain energy release rates at given phase angles associated with the experimental models. The effects of crack length on the measured interfacial fracture toughness were examined. Microscopic studies were also carried out to obtain the morphology of the fractured interfaces at selected loading angles.The implication of the results on the assessment of fixation in acetabular replacements is discussed in the light of preliminary work on bovine cancellous bone-cement interface.     

The influence of pearlite fraction on fracture toughness and fatigue crack growth in nodular cast iron

Fracture toughness and fatigue crack growth data of four nodular cast irons with different pearlite fractions are studied. The influence of temperature on fracture toughness is also investigated. Fracture surfaces are observed using a scanning electron microscope to correlate fracture toughness with the fracture surface and to understand the mechanism of crack growth at different stress intensity factor ranges. It is shown that the upper shelf fracture toughness increases with pearlite fraction. The existence of nodular void regions on the fracture surface plays an important role in fracture toughness. The fatigue crack growth rate is less sensible to stress intensity factor range when pearlite fraction increases.     

Effect of ionomer thickness on mode I interlaminar fracture toughness for ionomer toughened CFRP

Effect of ionomer thickness on mode I interlaminar fracture toughness was investigated for the ionomer-interleaved CFRP. Laminates were fabricated with Toho UT500/111 prepregs. Ethylene-based ionomer, which has high ductility and good adhesion to epoxy resin, was used as an interleaf material in this study. Thickness of the ionomer film selected was 12, 25, 100 and 200 μm. EPMA analysis showed the existence of the interphase region between the interleaf film and the base prepreg lamina where ionomer and epoxy were mixed. Mode I fracture toughness tests were carried out using DCB specimens. Precracks were introduced into all of the specimens. Fracture toughness values were much improved by interleaving the ionomer films. The fracture toughness value increased sharply by inserting thin ionomer film; however, the additional increase with the increase of the ionomer thickness was smaller. The thickness effect of the ionomer interleaf differs from that of the other kinds of thermoplastic-interleaf. Microscopic observation revealed that the crack path depended on the thickness of the ionomer region. Crack propagated in the interphase/ionomer interfaces for thinner-ionomer-interleaved CFRP, and in the interphase region, at the interphase/base lamina interface and interphase/ionomer interface for thicker-ionomer-interleaved CFRP. Ionomer resin deformed largely only near the crack surfaces, and this fact is responsible for the nonlinear increase of the fracture toughness with the increase of the ionomer thickness.     

Mixed-mode cohesive fracture of adhesive joints: Experimental and numerical studies

A broad experimental and analytical effort using fracture mechanics as the prime tool was conducted to investigate and improve the understanding of the mixed-mode cohesive fracture behavior of bonded joints. As a part of experimental efforts, mixed-mode fracture tests were performed using modified Arcan specimens consisting of several combinations of adhesive, composite and metallic adherends with a special loading fixture, in which by varying the loading angle, from 0° to 90°, mode-I, mixed-mode and mode-II fracture data were obtained. Finite element analyses were also carried out on specimens with different adherends. The main objective of this study was to determine the fracture toughness K_IC and K_IIC for a range of substrates under mixed-mode loading conditions. Another goal was to study the relationship between the stress intensity factors and the fracture toughness. Based on those analyses, mixed mode fracture criterion for the adhesively bonded systems under consideration determined. Fracture surfaces obtained at different mixed-mode loading conditions for various adherends were finally discussed.