R-curves determination of ordinary refractory ceramics assisted by digital image correlation method

As a figure-of-merit, the rising ratio of crack propagation resistance to fracture initiation resistance indicates a reduction of the brittleness and enhances the thermal shock resistance of ordinary refractory ceramics. The significant nonlinear fracture behaviour is related to the development of a fracture process zone (FPZ). The universal dimensionless load–displacement diagram method is applied as a promising graphical method for the determination of R-curves for magnesia refractories showing different brittleness. By applying digital image correlation (DIC) together with the graphical method, the problems arisen with accurate determination of the fracture initiation resistance and the crack length are overcome. Meanwhile, the R-curve is subdivided with respect to the fracture processes, viz the fracture initiation, the development of FPZ and the onset of traction free macro-crack. With the simultaneous crack lengths evaluated from DIC, the contribution of each fracture process to the crack propagation resistance at certain loading stage is quantitatively presented.     

Determination of the fracture behaviour of MgO-refractories using multi-cycle wedge splitting test and digital image correlation

Refractories with reduced brittleness show a pronounced deviation from linear elastic behaviour and an enhanced thermal shock resistance. This paper aims to study the influence of microstructure on the fracture behaviour of magnesia refractories. The wedge splitting test(WST), which enables stable crack propagation for quasi-brittle materials, was used to identify the fracture behaviour and evaluate the energy dissipation. The evaluation of the crack lengths of the magnesia and magnesia spinel materials during the entire cyclic WST is based on the localized strain evaluated using the digital image correlation (DIC). A significant fracture process zone develops in the magnesia spinel material. The relationship between the dissipated energy and the actual crack length, which was used to characterize the crack growth resistance, was determined. The refractory materials that showed reduced brittleness consume a small amount of energy for fracture initiation but a large amount of energy for further crack propagation.     

The initiation and propagation of thermal shock cracks in graphite

Thermal shock resistance of a commercial grade of graphite was studied using an arc-discharge test. The thermal shock fracture initiation and crack propagation behaviour of the graphite disks at different input power levels were determined and analysed using fracture mechanics. The temperature gradient was measured experimentally and the profiles were force fitted with an even fourth-order polynomial. The thermal stresses were calculated from the force fits. A radial notch was introduced to the disk specimens to enable calculation of the thermal stress intensity factors. The crack mouth opening displacement was monitored using a special displacement transducer. The thermal stress and stress intensity factors were found to increase with increasing input current (and hence increasing thermal gradient). The thermal shock fracture toughness determined using the arc-discharge technique was found to increase from 0.8 to 1.4 MPa m^1/2 at temperatures from 220 to 420 °C. The longer the notch length, the shorter the time to crack, the smaller the crack mouth opening displacement jump and the shorter the unstable crack growth.     

Assessment of viscoelastic crack bridging toughening in refractory materials

Viscoelastic bridges can be formed in refractory ceramics while cooling from high temperatures. Such bridges can shield crack tips, thus reducing the effective crack tip stress intensity factors leading to higher resistance to creep and thermal shock. The extent to which the crack tip stress intensity is reduced can be estimated from fracture mechanics models that include experimental measurement of crack bridging and microstructural parameters. In this paper a novel approach is proposed for the assessment of the effective crack bridging toughening from combining destructive and non-destructive test methods. Fracture toughness values were determined applying chevron notched specimen technique and surface damage of the specimen was monitored by image analysis. Different cordierite–mullite compositions characterized by different microstructure morphologies and crack propagation behaviour were investigated. A brief discussion about the correlation between thermo-mechanical properties, microstructure, crack propagation behaviour and thermal shock resistance is presented. Moreover, an empirical model able to determine the presence and effectiveness of the viscoelastic crack bridging ligaments acting in the microstructure under thermal shock conditions and their degradation with increasing thermal shock cycles from parameters measured at room temperature is presented.     

Mechanical and fracture investigation of magnesia refractories with acoustic emission-based method

In this study, the fracture behaviour of magnesia, magnesia chrome and magnesia spinel (MgAl₂O₄ and FeAl₂O₄) refractories under wedge splitting test are qualitatively and quantitatively investigated with the acoustic emission (AE) and digital image correlation (DIC). First of all, the concepts of characteristic widths are proposed for estimating the brittleness of refractory materials according to the shape of load-displacement curve and validated by their good correlation with the characteristic length. Besides, the AE data are analyzed with AE parameter-based approaches and offer new insight into the fracture behaviour of refractory materials, including the classification of the cracking events in grains and in matrix, the distinction between the tensile mode and shear mode damage, and the visualization of the fracture process zone development. It confirms that the pre-existing micro-crack networks in refractories are favourable for the brittleness reduction, which enhance their nonlinear fracture behaviour and thermal shock resistance.     

Fracture behavior of low carbon MgO–C refractories using the wedge splitting test

The fracture behavior of low carbon MgO–C refractories containing various carbon sources were investigated by means of the wedge splitting test and microscopic fractographic analysis to evaluate quantitatively their thermal shock resistance in the present work. The results showed that the addition of various nanocarbons in MgO–C specimens can lead to more tortuous crack propagation path during the wedge splitting test and much better thermal shock resistance compared to the specimen with flaky graphite as carbon source; particularly, the specimen containing carbon nanotubes had the most outstanding thermal shock resistance. Also, it was suggested from the correlation analysis that the increase of the specific fracture energy and interface crack propagation as well as the decrease of the modulus of elasticity, coefficient of thermal expansion and transgranular crack propagation can contribute to an improvement of thermal shock resistance of MgO–C refractories.     

Tensile behaviour of magnesia-spinel refractories: Comparison of tensile and wedge splitting tests

In some industrial applications, the need to improve the thermal shock resistance of refractories by optimisation of their microstructural design is of major importance. Refractories with enhanced thermal shock resistance usually present a rather low resistance to crack initiation but high resistance to crack propagation (rising R-curves), as well as a mechanical behaviour deviating from pure linear elastic fracture mechanics (LEFM), often qualified as nonlinear. The present work aimed at studying the influence of thermal micro-damage within the microstructure released during the cooling process on the nonlinearity of the mechanical behaviour in tension. The two-phase composites considered were magnesia-spinel refractories with different spinel inclusions content allowing to modulate the micro-damage level. Two different destructive mechanical tests, namely tensile and wedge splitting tests, were performed and their results were compared. The influence of thermal damage on different relevant mechanical parameters was investigated, and a quantitative correlation analysis between these parameters was proposed.     

Effect of the phase transformation on fracture behaviour of fused silica refractories

In this paper, the influence of phase transformation on the properties and fracture behaviour of fused silica refractory was investigated. The virgin fused silica refractory is amorphous, and possible failure is attributed to the propagation of a single crack in the structure. Due to the crystallization and phase transformation of low-/high- temperature cristobalite subpolymorphs occurring during the heat treatment, microcracks are formed especially in the matrix and at the grain boundary. This microcracking enables the development of sizable fracture process zone, which is responsible for the increase of specific fracture energy even with the decrease of strength. Therefore, the heat-treated specimens exhibit lower brittleness and higher strain tolerance before failure compared with the virgin fused silica refractory. All of these properties represent a better thermal shock resistance. Furthermore, microcracking causes a characteristic temperature dependence of Young’s Modulus due to phase transformation and partial crack closure at increased temperatures.     

Observation and quantification of the fracture process zone for two magnesia refractories with different brittleness

In this paper, the formation of the fracture process zone (FPZ) of industrially produced magnesia spinel and magnesia refractories was analysed using digital image correlation (DIC). Compared to pure magnesia materials, the magnesia spinel materials exhibited a higher amount of microcracks, causing a larger FPZ. A critical displacement, where the cohesive stress between the crack faces decreases to zero, is determined by analysing the development of the localized zone. Critical displacement determined from the changes of the FPZ width and length is used to determine the onset of macro-cracking and locate the crack tip. The development of the fracture process zone for a magnesia spinel initiates before reaching the maximum load, and the onset of the macro-crack is in the post-peak region. The FPZ size increases until the formation of a macro-crack takes place, but decreases afterwards. For the magnesia refractory, no pronounced FPZ could be detected.     

A New Experimental Approach for In Situ Damage Assessment in Fibrous Ceramic Matrix Composites at High Temperature

High‐temperature applications of ceramic matrix composites necessitate a rigorous understanding of the fracture and damage mechanisms that occur under thermomechanical loading, requiring the development of advanced small‐scale characterization approaches. In this work, fiber‐reinforced SiC/SiC tensile specimens were loaded in a scanning electron microscope at 800°C, and full‐field deformation maps at the constituent length scale were generated using digital image correlation (DIC). A colloidal system containing mechanically milled titanium nanopowder, bicine, and water was developed for use as a DIC tracking pattern that is stable at 795°C. The resultant full‐field strain maps provide a constituent level characterization of damage evolution from crack initiation through final fracture. An analysis of strain along fiber lengths indicated that fiber mean strain and standard deviation reached a minimum at fiber fracture. In addition, multiple matrix cracks in the process zone ahead of a notch/crack tip were apparent and could falsely appear as a continuous region of high strain in DIC fields. Relatively large displacement (strain) error was attributed to noise and bias at these small length scales and small strain values, and approaches for mitigating this error are discussed.     

聚乙烯醇纤维混凝土动态断裂过程试验研究

为探究纤维体积掺量对聚乙烯醇纤维增强水泥基复合材料(PVA-ECC)断裂过程的影响,基于50 mm的分离式霍普金森压杆(SHPB)装置对不同纤维体积掺量(0%、0.75%、1.50%、2.25%、3.00%)的PVA-ECC中心切槽半圆盘弯曲(NSCB)试件进行冲击试验,同时结合超高速数字图像(DIC)相关试验系统对PVA-ECC材料的动态断裂过程进行试验研究,得到了预制裂纹尖端张开位移的变化规律以及各组试件的临界裂缝尖端张开位移(CTOD_C)。结果表明,当不添加PVA纤维或添加较少(小于1.50%)时,裂尖宏观裂纹基本出现在裂尖荷载的峰值时刻处,而随着PVA纤维掺量的增加,裂尖宏观裂纹的出现显著早于裂尖荷载的峰值时刻,并且纤维体积掺量越大,裂尖宏观裂纹出现得越早,裂纹扩展至完全断裂的时间也显著增加。添加聚乙烯醇纤维可以显著提高混凝土试件的CTOD_C值,提高试件的阻裂能力,相同冲击荷载下,体积掺量为2.25%的聚乙烯醇纤维试件具有较大的CTOD_C值。     

Theoretical prediction of temperature-dependent fracture strength for ultra-high temperature ceramic composites considering the evolution of damage and thermal residual stress

Based on the maximum storage energy density criterion of material fracture, a model of temperature-dependent fracture strength for ultra-high temperature ceramic composites is established. The combined impacts of the evolution of damage and thermal residual stress with temperature are considered. The model predictions are highly consistent with available experimental values. Besides, the critical crack sizes of ZrB₂–30 vol%SiC in air from 1400 to 1600 °C are predicted using the proposed model, which agree well with the total oxidation thickness of the reported literature at 1400 and 1500 °C, and a more reasonable definition of critical crack size at 1600 °C are given. Moreover, the quantitative effect of crack size on the fracture strength is analyzed under different environment temperature, and a useful conclusion is obtained that decreasing crack size is more effective to improve the fracture strength of the composites at low temperatures. This study not only provides a feasible and convenient method to predict the fracture strengths at different temperatures, but also offers a theoretical support for the design of ultra-high temperature ceramic composites.     

Fracture energy evaluation of refractories in wedge splitting tests from notch opening displacements

The work of fracture of refractories is commonly calculated from crack mouth opening displacements (CMODs) in wedge splitting tests (WSTs). This paper proposes a methodology for estimating the fracture energy from notch opening displacement (NOD) measurements, which is useful for setups where CMOD is not accessible. NODs and CMODs are calculated for both faces of two WSTs experiments on a castable refractory via digital image correlation (DIC) and finite element simulations. A quadratic function fits well the non-linear CMOD vs. NOD behavior in the crack initiation regime, while an affine trend describes the propagation regime. Although the nonlinearity associated with crack initiation is more complex, the crack propagation energy can easily be estimated from NOD data when CMODs cannot be measured.     

Evaluation of crack-Growth resistance curve for a particulate ceramic–Metal composite

A technique is described to evaluate the crack growth resistance behaviour in brittle ceramic-base materials. In this method, the crack increment measurements during the stable crack propagation process are not required. The crack growth resistance curves are studied for a particulate ceramic–metal composite in the system lanthanium chromite–chromium. Experiments were performed with standard fracture mechanics single-edge notched beam specimens in a temperature range from room temperature up to 1100°C. Effect of temperature on crack growth resistance behaviour is discussed.     

A lifetime prediction method based on Cumulative Flaw Length Theory

Ultrasonic pulse velocity testing (UPVT) was carried out to perform non-destructive quality control of refractory plates. Used in conjunction with fracture mechanics, ultrasonic velocity measurements have proved a powerful technique for detecting, positioning and sizing internal voids and cracks in the samples, originated from the manufacturing process. Two cordierite-mullite refractory compositions exhibiting different microstructure and crack propagation behaviour were characterized through their lifetime during which they were subjected to thermal shock loading. In this paper, a new statistical method is proposed which allows to estimate the lifetime when the stress state that will be applied in service (loading) and the scattering of the ultrasonic velocity data in the as-received state are known. Since this lifetime prediction method is based on a non-destructive technique, it could be implemented into a code in an automatic quality control device for continuous lifetime estimation. The correlation between crack propagation behaviour and thermal shock resistance is discussed and semi-empirical models were developed to predict the service life of refractory plates from the measured values of ultrasonic velocities on as-received samples.     

Comparison of human enamel and polymer-infiltrated-ceramic-network material “ENAMIC” through micro- and nano-mechanical testing

The mechanical behaviours of a polymer-infiltrated-ceramic-network (PICN) material and human enamel were compared using micro- and nano-mechanical testing. The Vickers hardness and fracture resistance of the two materials were evaluated by the micro-indentation technique. A comparison of the microstructure and crack propagation of both materials was performed by scanning electron microscopy (SEM). From the micro-mechanical level, the Vickers hardness of PICN (3.31±0.11 GPa) was similar to that of human enamel (3.43±0.16 GPa), and the micro-creep behaviours of both materials were also alike. However, the average indentation fracture resistance (K_C) of the enamel (1.26±0.05 MPa·m^1/2) was significantly lower than that of the PICN (1.81±0.08 MPa·m^1/2). Cracks in human enamel are prone to propagate along the rod sheath, while within the PICN, these mainly extend through the ceramic matrix, and are deflected at the polymer–ceramic interfaces. From the nano-mechanical testing, the nanohardness, elastic modulus and nanoindentation creep response of PICN were significant different from that of human enamel. Furthermore, the creep capacity of the PICN was mainly influenced by the infiltrated polymer matrix. Overall, PICN has been found to be more effective than human enamel in crack growth resistance at the microscopic level. The mutually chimeric network structure improves PICN's nano-mechanical behaviour, and the infiltrated polymer matrix facilitates time-dependent deformation that helps dissipate strain energy and prevent fracture.     

Deformation mechanisms and evolution of mechanical properties in damaged advanced ceramics

We investigated the uniaxial compressive behavior of damaged and intact alumina using quantitative X-ray computed tomography (XCT) analysis coupled with digital image correlation (DIC) for mechanical characterization. Internal three-dimensional crack characteristics such as crack surface area and orientation were quantified using XCT to assess the level of damage. From the quasi-static and dynamic stress–strain results, the primary effects of crack damage are to reduce the initial stiffness and rate of lateral expansion in damaged alumina. With increasing axial strain, crack closure was found to lead to a recovery of elastic properties, in some cases to intact levels, in the damaged specimens. Localized deformation mechanisms related to the crack structure, including lateral crack closure, axial crack opening and closing, and inclined crack sliding, were visualized in-situ and connected to XCT reconstructions. High-speed imaging also revealed a mixed fracture mode for damaged alumina that included axial splitting and failure along pre-existing cracks.     

Critierional evaluation of thermal destruction of corundum concretes

Conclusions The resistance of corundum concretes to thermal destruction was studied using the experimentally plotted deformation diagrams and the standard thermal cycling method. The correlation between the values of thermal shock resistance obtained according to these methods was established for the corundum concretes.It was established that as compared to the concrete produced using a binder based on the Talyum cement, the concretes incorporating a hydraulically hardening (water-setting) binder prepared from the VTs-70 grade high-alumina cement exhibit better resistance to thermal stresses. With increasing strength and density of the concretes, their thermal destruction occurs catastrophically (abruptly) since the elements of a stronger structure (the concretes produced using the Talyum cement-based binder) are incapable of effectively hindering the growth of a potential crack.Modification of the structure of the concrete by introducing Cr₂O₃ additive leads not only to an increased thermal shock resistance in the high-temperature range due to an increased critical crack length, but also to an increased resistance ot crack displacement (opening).Translated from Ogneupory, No. 5, pp. 3–8, May, 1988.     

Damage law identification of a quasi brittle ceramic from a bending test using Digital Image Correlation

Although ceramics are generally considered to be elastic brittle solids, some of them are quasi brittle. These ceramics show a non-linear mechanical behaviour resulting most of the time in a difference between their tensile and compressive stress–strain laws. The characterization of their fracture strengths might be biased if linear elastic formulae are used to analyze classical tests like bending tests. Based on Digital Image Correlation (DIC), an efficient technique to measure full field displacements, a methodology is proposed to characterize and model materials with dissymmetric behaviours between tension and compression. Applying specific basis functions for DIC displacement decompositions for bending, compressive and tensile tests, a stress–strain model and its damage law are identified and then validated for aluminium titanate, a damageable micro-cracked ceramic at room temperature. This identification method using DIC can obviously be applied to other quasi brittle materials.     

CRACK GROWTH OF STRUCTURAL ADHESIVE JOINTS IN HUMID ENVIRONMENTS

The adhesive fracture energy, G_c, of aluminiumalloy and steel joints bonded with a rubber-toughened epoxy adhesive has been measured using monotonicallyloaded tests. Such tests have been conducted at different levels of relative humidity, and two surface pretreatments have been employed for the substrates prior to bonding: a simple grit-blast and degrease (GBD) pretreatment or a silane primer (GBS) pretreatment. When G_c was plotted against the crack velocity, three regions of fracture behaviour could be distinguished. At low rates of displacement the crack grew in a stable manner, visually along the interface, and relatively low crack velocities could be readily measured. This was termed “Region I”, and here the value of the adhesive fracture energy was relatively low and decreased steadily as the relative humidity was increased. On the other hand, at relatively high rates of displacement the crack grew in a stick-slip manner mainly cohesively in the adhesive layer at approximately 20 km/min. This was termed “Region III”, and here the value of G_c was relatively high and independent of the relative humidity. In this region the crack was considered to grow faster than the water molecules were able to reach the crack tip, which explains the independence of G_c upon the test environment. In between Region I and Region III a transition region was observed, which was designated “Region II”. The major effect of the GBS pretreatment, compared to which the GBD pretreatment, was to increase the value of G_c both in Regions I and III, although the presence of the silane primer had the greater effect in Region I.