A simple methodology to visualize crack propagation for ceramic materials

Analysis of the crack growth behavior is critical for fracture and thermal shock assessment. The current work presents a simple methodology to visualize cracks in ceramic materials. The procedure is exemplified for refractory materials on the basis of images obtained during mechanical loading using a wedge splitting test. Complementary in situ crack growth observation verify that for one of the materials growth and opening displacement of the main crack is accompanied by pronounced micro-cracking, branching, and bridging processes. Apparent fracture resistance and thermal shock resistance parameters are discussed.     

On the Fracture Toughness of Advanced Materials

Few engineering materials are limited by their strength; rather they are limited by their resistance to fracture or fracture toughness. It is not by accident that most critical structures, such as bridges, ships, nuclear pressure vessels and so forth, are manufactured from materials that are comparatively low in strength but high in toughness. Indeed, in many classes of materials, strength and toughness are almost mutually exclusive. From a fracture‐mechanics perspective, the ability of a microstructure to develop toughening mechanisms acting either ahead or behind the crack tip can result in resistance‐curve (R‐curve) behavior where the fracture resistance actually increases with crack extension; the implication here is that toughness is often developed primarily during crack growth and not for crack initiation. Biological materials are perfect examples of this; moreover, they offer microstructural design strategies for the development of new materials for structural applications demanding combinations of both strength and toughness.     

Analytical and numerical modeling of R curves for cracks with bridging zones

At the onset of fracture in materials with process zones, the fracture resistance, or R curve, rises as the process zone develops. After process zone development, crack propagation proceeds by steady state growth. By considering J integral contours inside and outside the process zone, the available energy can be partitioned into crack tip energy release rate and process zone energy. To model the rising R curve, however, required assumptions about damage mechanisms in the process zone and partitioning of its energy into released and recoverable energy. By considering process zones that are elastic fiber-bridging zones with softening regions caused by fiber breakage or damage, equations for rising R curves were derived as a function of crack tip toughness and bridging zone mechanics. The new methods were implemented into the Material Point Method for generalized numerical crack propagation simulations with bridging zones. The simulation method includes pure fracture mechanics and pure cohesive zone models as extreme special cases. The most realistic simulations for many materials will likely fall between these two extremes. The results guided comments on interpretation of experimental R curves.     

Microstructure variation effects on room temperature fatigue threshold and crack propagation in Udimet 720Li Ni-base superalloy

An assessment of the effects of microstructure on room temperature fatigue threshold and crack propagation behaviour has been carried out on microstructural variants of U720Li, i.e. as‐received U720Li, U720Li‐LG (large grain variant) and U720Li‐LP (large intragranular coherent γ′ variant). Fatigue tests were carried out at room temperature using a 20 Hz sinusoidal cycling waveform at an R‐ratio = 0.1 on 12.5 mm × 12.5 mm square cross‐section SENB specimens with a 60° starter notch. U720Li‐LG showed the highest threshold ΔK (ΔK_th), whilst U720Li‐LP showed the lowest ΔK_th value. U720Li‐LP also showed higher crack growth rates in the near‐threshold regime and at high ΔK (although at higher ΔK levels the difference was less marked). Crack growth rates of U720Li and U720Li‐LG were relatively similar both in the near‐threshold regime and at high ΔK. The materials showed crystallographic stage I type crack growth in the near‐threshold regime, with U720Li showing distinct crystallographic facets on the fracture surface, while U720Li‐LG and U720Li‐LP showed mostly microfacets and a lower proportion of large facets. At high ΔK, crack growth in the materials becomes flat and featureless indicative of stage II type crack growth. The observed fatigue behaviour, which is an effect of the combined contributions of intrinsic and extrinsic crack growth resistances, is rationalized in terms of the microstructural characteristics of the materials. Enhanced room temperature fatigue threshold and near‐threshold long crack growth resistance are seen for materials with larger grain size and higher degree of planar slip which may be related to increased extrinsic crack growth resistance contributions from crack tip shielding and roughness‐induced crack closure. Differences in the deformation behaviour, either homogeneous or heterogeneous due to microstructural variation in this set of materials may provide approximately equivalent intrinsic crack growth resistance contributions at room temperature.     

FATIGUE CRACK GROWTH BEHAVIOUR AND FRACTURE RESISTANCE IN CERAMICS

Fatigue crack growth and the fracture resistance curve (R-curve) were investigated in a polycrystalline alumina (AD90) and a silicon carbide whisker-reinforced alumina composite (Al₂O₃-SiC_w) at room temperature in air using a combined loading technique for stabilizing crack growth, and a surface film technique for monitoring crack length. Fatigue crack growth was evaluated successfully with those experimental techniques. Load shedding tests were performed until the crack became dormant, in order to determine the threshold stress intensity factor K_th. Subsequently, the specimens were used for quasi-static crack growth tests under a monotonic loading condition. The R-curves were determined in this experiment; however, fracture resistance did not increase markedly with crack growth. Detailed observations of the crack growth behaviour revealed that the flat R-curve was attributed to the shielding effect of the fatigue crack tip wake. Thus, the fatigue precrack introduced by the load shedding test was not regarded as an ideal crack for determining the R-curve. Fractographic observations were performed to investigate the mechanistic difference between fatigue and quasi-static crack growth. It was found that the cyclic loading produced fretting damage in the wake region and it reduced the shielding effect of the fatigue cracks. Based on the experimental results, the relationship between the fatigue crack growth and the R-curve is discussed as is the significance of K_th as a material parameter.     

The effect of microstructures on fatigue crack growth in Q345 steel welded joint

It is a traditional that the fatigue crack growth behavior is sensitive to microstructure in threshold regime, while it is sensitive to R‐ratio in Paris regime. Fatigue test is carried out for welded joints of a Q345 steel where the compact tension specimens with 3.8 and 12.5 mm thickness are used, and comparisons of fatigue crack growth behavior between base metal and a few different locations in the welded joint are considered in Paris regime. Welding residual stresses are removed by heat treatment to focus the study on the microstructural effect. It is shown that fatigue crack growth rate (FCGR) in the base metal is not sensitive to R‐ratio, but the FCGR increases in the overheated zone, the fusion zone and the weld metal zone with R‐ratio increasing. To the low R‐ratio, FCGR in the three zones is smaller than that in the base metal, but they approximate the same with base metal under the high R‐ratio. The mechanism of fatigue crack growth is analyzed through crack path in microstructures and SEM fractograph. The coarse‐grained ferrite in the base metal is of benefit to relaxation of the average stress at the crack tip, and the fatigue crack growth predicts branching and deflection within above different locations in the welded joint. These tortuous crack paths with crack branching and deflection will promote crack closure as well as crack‐tip stress shielding and then resulted in higher crack growth resistance.     

氧化铝基陶瓷复合材料的阻力曲线行为及其抗热震性能

采用压痕-弯曲强度法获得了Al₂O₃-SiC_W和Al₂O₃-TiC_P陶瓷基复合材料的裂纹扩展阻力曲线(R-曲线),并测试了材料的抗热震性能,分析了材料的阻力曲线行为与其抗热震性能之间的内在联系。结果表明:材料的阻力曲线行为与抗热震性之间存在明显的相关性。热震引起材料强度的下降幅度与其阻力曲线的陡峭程度及上升幅度有关。阻力曲线越陡峭,上升幅度越大,抗热震性也越好。其中Al₂O₃-SiC_W复合材料显示出更为优越的抗裂纹扩展能力与抗热震性能。扫描电镜观察及理论分析显示:晶须的拔出与桥联补强增韧机制是产生这一现象的主要原因。     

Crack resistance curves of alumina and zirconia at room temperature

Ceramic three-point bend specimens were pre-cracked in a displacement-controlled test in air at room temperature to form sharp cracks of different lengths. Critical stress intensity factors (K _IC were then measured as a function of sharp crack length in a fast-fracture, load-controlled test. Crack resistance curves (K _IC against crack length) were determined for three commercially pure aluminas of different grain size, a debased alumina containing a glassy phase, and a partially stabilized zirconia (PSZ) material. The crack resistance curves proved to be flat for the finer-grained and the debased alumina. A steeply rising crack resistance curve was, however, observed for a pure coarse-grained alumina material which is explained by friction effects of the cracked microstructure behind the measured crack front. The effect is influenced by the test procedure itself. Though crack branching takes place the crack resistance curve of PSZ is completely flat, which is attributed to fast fracture testing where only the most dangerous flaw is activated.     

A fracture evolution procedure for cohesive materials

The present paper deals with the problem of the evaluation of the softening mechanical response of cohesive materials under tensile loading. A nonlinear fracture mechanics approach is adopted. A new numerical procedure is developed to study the evolution of the crack processes for 2D solids. The proposed algorithm is based on the derivation and use of the fracture resistance curve, i.e., the R-curve, and it takes into account the presence of the process zone at the crack tip. In fact, assuming a nonlinear constitutive law for the cohesive interface, the procedure is able to determine the R-curve, the process zone length and hence the mechanical response of any material and structure. Numerical applications are developed for studying the damage behavior of a infinite solid with a periodic crack distribution. Size effects are investigated and the ductile-brittle transition behavior for materials characterized by the same crack density is studied. The results obtained adopting the proposed procedure are in good accordance with the results recovered through nonlinear step by step finite element analyses. Moreover, the developed computations demonstrate that the procedure is simple and efficient.     

Crack propagation behavior of cermets and cemented carbides under repeated thermal shocks by the improved quench test

In this study, an improved quench testing method for thermal shock resistance has been proposed. Repeated thermal shock tests were performed on cemented carbides to show the advantages of the new proposed method that would enable us to estimate an intrinsic relationship between the crack propagation rate and the stress intensity factor under repeated thermal shocks. The cyclic thermal fatigue crack propagation behavior and fracture toughness values were shown to be independent of the specimen heights and the cooling media employed. We then evaluated the thermal crack propagation behavior for cermets and cemented carbides by using this method, and discussed the differences between both materials in the crack growth behavior on the basis of their microstructures.     

R-curve behaviour of PZT ceramics near the morphotropic phase boundary

The mechanical failure of PZT ceramics was characterized by measuring R-curves for compositions near and at the morphotropic phase boundary (MPB) where tetragonal and rhombohedral phases coexist in equal quantities. The R-curve behaviours (an increasing fracture toughness with crack extension) were identified by indentation-fracture testing and they were analysed to determine the key parameters. The fracture toughness of the PZT ceramics consisted of three different terms, representing particular microstructural processes in front of advancing cracks, that is, intrinsic cleavage, 90° domain switching and microcracking. Their relative contributions to an overall crack-extension resistance varied with the length of the advancing crack and, more importantly, with the compositions of the PZT. In the compositional range where the tetragonal phase was dominant, the R-curves were determined by domain switching and microcracking. However, the compositional dependency of the fracture toughness was due to the microcracking mechanism. On the other hand, in regions rich in rhombohedral phases, the R-curves were essentially determined by domain switching in the crack-tip area. The R-curves characterized by the domain-switching mechanism were insensitive to the compositional variation near the MPB. Our results also demonstrated that R-curve analysis could be used to probe further into the microstructural responses of materials in front of advancing cracks and to quantify them particularly in systems like PZT where several different toughening processes compete with each other.     

Ermittlung des Bruchwiderstands an Oxidkeramik und Hartmetallen mit verschiedenen Methoden

Fracture Toughness Determination of Alumina and Cemented Carbide with Different Testing Methods Fracture toughness of a sintered alumina and two tungsten carbidecobalt materials was determined using four-point-bend specimens with straight through and chevron notches and with the short rod specimen. With the specimens with a straight through notch a lower K_Ic was measured for Al₂O₃ and a higher for WC-Co compared to the chevron-notched specimens. This behavior was explained by the different shapes of the crack growth resistance curves and the different critical notch radii. For Al₂O₃ a steeply rising crack growth resistance curve was measured in a controlled fracture test, for WC-Co a flatter curve was obtained. The effect of the shape of the crack growth resistance curve and of notch width on the evaluated toughness is discussed.     

The thermal shock of ß-alumina

The thermal shock of sodiumβ-alumina with relative densities from 60 to 98% theoretical has been investigated over the temperature range 150 to 700° C by quenching into water. The samples were ring segments cut from electrolyte tubes and were subsequently tested in both compression and tension. For relative densities of 75% and below the thermal shock damage was typical of stable crack growth and a steady decline in strength with sintering temperature was observed. For relative densities of 95% and above, thermal shock causes unstable crack growth and a critical value of ΔT was observed in the range 170 to 250° C depending on initial strength. From the linear relationship between observed ΔT _c and the thermal shock resistance parameter,R, it was concluded that the rapid heat transfer during quenching was nucleate water boiling and that cooling from ∼110° C to 0° C was not responsible for damage. The fracture stress after thermal shock above ΔT _c was consistent and showed little dependence on initial strength for relative densities ⩾95%. However, the fractional reduction in strength was related to the damage resistance parameterR‴. An estimate of the energy expended in fracture has been made, based on microscopic observation and compared with estimates of the stored strain energy due to thermal stresses.     

A node release approach to estimate J‐R curve for single‐edge‐notched tension specimen under reversed loading

This paper proposes a node release approach to estimate the fracture resistance curve, often known as the J‐R curve, for monotonic and cyclic fracture tests. The node release approach simulates the crack extension by releasing the constraints imposed on the node at the crack tip and estimates the J‐R curve by coupling the domain integral value with the corresponding crack extension. This proposed node release approach estimates closely the J‐R curve for SE(B) and SE(T) specimens subjected to monotonic loading. For SE(T) specimen under cyclic loading, this study implements the node release analysis in two approaches: (1) an equivalent monotonic analysis corresponding to the envelope of the cyclic load‐CMOD response and (2) a direct cyclic simulation. Both approaches lead to close estimations of the experimentally measured J‐R curve. The numerical analysis also confirms the path independence of the domain integral values in the direct cyclic simulation.     

Fatigue thresholds of holed components under in‐phase and out‐of‐phase torsional and axial loadings

The resistance‐curve (R‐curve) method was applied to the prediction of the fatigue thresholds of notched components under in‐phase and out‐of‐phase combinations of cyclic torsion and axial loadings. The prediction was compared with the experimental data obtained from thin‐walled tubular specimen of medium‐carbon steel with a hole. The stress was completely reversed and the mean stress was zero. The crack was nucleated at the position of the maximum range of the circumferential stress on the periphery of a hole, and propagated almost straight for all cases examined. The experimental data of the thresholds for crack initiation and fracture agreed well with the predictions for in‐phase and for out‐of‐phase loadings with 45° phase difference. For out‐of‐phase loading with 90°, the threshold for fracture was close to the crack initiation limit, because of the reduction of crack closure due to crack face rubbing by mode II shear cycling.     

Short thermal fatigue crack propagation behavior of alumina short fibre reinforced aluminum matrix composites

Thermal fatigue resistance is one of the most important parameters to design engine materials. The thermal fatigue crack growth behavior of alumina short fibre (V _f = 18 vol.%) reinforced AlSi12CuMgNi aluminum alloy composite has been investigated under thermal cycling condition between room temperature and 280 °C. Initiation and propagation of thermal fatigue crack have also been discussed. The results show that in the range of short crack, the fibres play an important role in the path of thermal fatigue crack, and the crack propagation rate of composites is much larger than that of the matrix alloy.     

Thermal shock behavior of nano-sized SiC particulate reinforced AlON composites

Aluminum oxynitride (AlON) has been considered as a potential ceramic material for high-performance structural and advanced refractory applications. Thermal shock resistance is a major concern and an important performance index of high-temperature ceramics. While silicon carbide (SiC) particles have been proven to improve mechanical properties of AlON ceramic, the high-temperature thermal shock behavior was unknown. The aim of this investigation was to identify the thermal shock resistance and underlying mechanisms of AlON ceramic and 8 wt% SiC–AlON composites over a temperature range between 175 °C and 275 °C. The residual strength and Young's modulus after thermal shock decreased with increasing quenching temperature and thermal shock times due to large temperature gradients and thermal stresses caused by abrupt water-quenching. A linear relationship between the residual strength and thermal shock times was observed in both pure AlON and SiC–AlON composites. The addition of nano-sized SiC particles increased both residual strength and critical temperature from 200 °C in the monolithic AlON to 225 °C in the SiC–AlON composites due to the toughening effect, the lower coefficient of thermal expansion and higher thermal conductivity of SiC. The enhancement of the thermal shock resistance in the SiC–AlON composites was directly related to the change of fracture mode from intergranular cracking along with cleavage-type fracture in the AlON to a rougher fracture surface with ridge-like characteristics, crack deflection, and crack branching in the SiC–AlON composites.     

Microstructural aspects of Thermal Stress Resistance of high-strength engineering ceramics. Part I: Data of thermal stress resistance of high-strength engineering ceramics

Based on the criteria for the improvement of thermal shock resistance mainly two microstructural aspects of thermal stress resistance are discussed: First, the influence of microstructure on thermal shock resistance to fracture initiation, and second, the improvement of thermal shock resistance on the basis of microstructural considerations. In this connection, data of thermal stress resistance (thermal shock and thermal cycling) of various engineering ceramic materials are presented. Using laboratory grades with well-defined microstructures the interdependence between various microstructural variables and the mechanical and thermal properties, which control the thermal shock resistance, is discussed and the relation to thermal shock resistance is outlined by the example of the two materials, dense and porous reaction-bonded Si₃N₄. Moreover, the improvement of thermal shock resistance by microstructural optimization is demonstrated. Some examples of the improvement of thermal stress resistance by developing advanced composite materials are given. The paper is divided into three parts: Part I: Data of Thermal Stress Resistance of High-Strength Engineering Ceramics Part II: Influence of Microstructure on Thermal Shock Resistance of High-Strength Engineering Ceramics Part III: Improvement of Thermal Stress Resistance of High-Strength Engineering Ceramics.     

Thermal shock resistance of alumina-sialon composites

The thermal shock behaviour of pure alumina and an alumina-sialon composite was examined. The critical temperature difference of the thermal shock, T _c , had a good relationship with the calculated thermal shock resistance parameter,R. The thermal shock resistance of the composite was remarkably improved by simple modification, such as oxidation at 1400 °C for 100 h. The oxide layer, with porous microstructure, consisted mainly of mullite and a small amount of alumina. The T _c of pure alumina, the composite and the oxidized composite was 210, 225 and 360 °C, respectively. This improvement was considered to be due to the characteristics of the oxide layer, which had high strength as well as a low elastic constant and a low thermal expansion coefficient.