The Effects of Microstructure and Confinement on the Compressive Fragmentation of an Advanced Ceramic

We investigate the rate‐dependent compressive failure and fragmentation of a hot‐pressed boron carbide, under both uniaxial and confined biaxial compression, using quantitative fragment analysis coupled with quantitative microstructural analysis. Two distinct fragmentation regimes are observed, one of which appears to be more sensitive to the microstructural length scales in the material, while the second is more sensitive to the structural character and boundary conditions of the compressed sample. The first regime, which we refer to as “microstructure‐dependent,” appears to be associated with smaller fragments arising from the coalescence of fractures initiating from graphitic inclusions. This regime appears to become more dominant as the strain rate is increased and as the stress‐state becomes more confined. The second regime generates larger fragments with the resulting fragment size distribution dependent on the specific structural mechanisms that are activated during macroscopic failure (e.g., buckling of local columns developed during the compression). The average fragment size in the latter regime appears to be reasonably predicted by current fragmentation models. This improved understanding of the effects of microstructure and confinement on fragmentation then provides new insights into prior ballistic studies involving boron carbide.     

Residual Stresses in Polycrystalline Diamond Compacts

The effects of residual stress on the integrity of polycrystalline diamond compact (PDC) cutters was investigated. When the compact cooled from the sintering temperature to room temperature, very high radial compressive stresses were induced in the diamond table, and (generally) much lower radial tensile stresses were induced in the cemented tungsten carbide backing. The magnitudes of these residual stresses were not affected very much by the diameter of the compact. However, the residual stresses were affected significantly by the thickness ratio of the carbide layer to the diamond layer. The higher this ratio the greater was the radial compressive stress in the diamond and the lower was the radial tensile stress in the carbide. This same effect was obtained by sintering a relatively thin layer of tungsten carbide on top of the diamond table.     

Prediction of ring crack initiation in ceramics and glasses using a stress–energy fracture criterion

Crack initiation in brittle materials upon spherical indentation is associated with the tensile radial stresses during loading. However, location of crack onset often differs (offset) from the site of maximal stress. In addition, experiments reveal a strong dependency of crack initiation forces on geometrical parameters as well as the surface condition of the sample. In this work, a coupled stress–energy fracture criterion is introduced to describe the initiation of ring cracks in brittle materials, which takes into account the geometry of the contact and the inherent strength and fracture toughness of the material. Several experiments reported in literature are evaluated and compared. The criterion can explain the location offset of the ring crack upon loading, as observed in various ceramics and glasses. It also predicts the ring crack initiation force upon contact loading, provided that surface compressive stresses, introduced during grinding or polishing processes, are taken into account. Furthermore, the stress–energy criterion may be employed to estimate the surface residual stress of ceramic parts, based on simple contact damage experiments.     

Effects of Combined Stresses on Fracture of Alumina and Graphite

A unique method was developed for mechanical testing of brittle materials to create an unlimited number of stress ratios in the tension-tension and tension-compression quadrants. The stress states are achieved by internal and external pressurization of tubular specimens in a special pressure vessel. Failure envelopes were determined for polycrystalline alumina and fine-grained isotropic graphite. The modified maximum strain energy and Coulomb-Mohr theories fit the data best; these theories should be plotted as bands representing probability of failure. The biaxial tensile strengths of alumina and graphite are lower than their uniaxial tensile strengths; the tensile strengths decrease as the compressive stress in the orthogonal directions increases.     

The effect of specimen thickness and stress ratio on the fatigue behavior of polycarbonate

The cyclic residual stresses ahead of fatigue cracks are known to play an important role in the fatigue fracture response of engineering materials. The size of these residual stresses are directly affected by the stress state of the component. In this paper, we examine the effect of stress state for fully compressive and fully tensile cyclic loading of polycarbonate. The role of stress state is studied using two different specimen thicknesses, one thickness will represent a near-plane stress condition and the other will represent a near-plane strain condition. In cyclic compressive loading, it will be shown that the near-plane stress specimen with its larger zone of residual tension will exhibit enhanced crack saturation lengths. While for cyclic tensile loading, the larger zone of residual compression upon unloading will result in crack retardation for the same unloading stress intensity. A series of systematic experiments on the effects of mean stress on fatigue fracture is reported, and the results of the experiments are rationalized with the aid of scanning electron microscopy of the fracture surfaces. The results of this study have strong implications for both constant amplitude fatigue loading and variable amplitude fatigue loading as well as applicability to other engineering materials.     

Application of Raman spectroscopy to determine stress in polycrystalline diamond tools as a function of tool geometry and temperature

Polycrystalline diamond (PCD) tools commonly consist of a PCD layer sintered onto a cobalt-tungsten carbide (Co-WC) substrate. These tools are used in diverse applications and both the magnitude and distribution of the stresses in the PCD layer affect tool behavior. These stresses in sample drill-bits were investigated by means of micro-Raman spectroscopy in which the properties of the diamond Raman peak reveal both the nature of the stress present (compressive or tensile) and its magnitude. It was found that the surface preparation techniques influenced the average stress present in the PCD surface layer which was in compression in all cases investigated. The largest stresses were encountered in the roughly lapped sample (1.4 GPa) with the stress values decreasing for fine lapping (0.8 GPa) and polishing (0.1 GPa). Small areas with low tensile stresses were found in some polished samples. Measurements of stress as a function of temperature for roughly lapped sample drill-bits indicated a linear trend of decreasing stress values with increasing temperature, although the stress remained compressive. Cyclic annealing of a sample drill-bit to 600 °C shows that the tool properties are retained after 5 cycles, while similar cycling to 800 °C resulting in a permanent degradation of the tool properties.     

Hertzian Indentation Stress Field Equations

Indentation with hard spherical indenters (so called Hertzian indentation) is a well‐known technique to probe the mechanical properties of ceramics and other brittle materials. Using contact mechanics, it is possible to calculate the stress distribution in the sample and use it to express maximum tensile or compressive stresses as a function of load and contact radius. The equations describing the full stress distribution are not usually mentioned in the literature, and surprisingly most of the classical references disclosing them are either mistaken or incomplete. We recovered the full and detailed equations for the stress field distribution and numerically implemented them using the computer language Python. We point out the relationships between the different expressions that have been published, check the consistency with existing results, and discuss the sources of confusion.     

Stresses near the edges of ring-loaded plates

Strength testing of brittle materials often employs beams or plates loaded in bending. Solutions are provided for the in-plane stresses between the support ring and edge of ring-loaded circular plates (i.e., disks) having a diameter larger than the support ring. The region overhanging the larger support ring stiffens the plate and reduces the edge tangential stress. Its consideration is thus crucial to an accurate solution. For typical ceramic disks with little overhang, the edge stress is less than ~50% of the maximum, central stress. With a large overhang, the edge stress can be reduced to ~25% of the central stress. The radial stress reverses the sign outside the support ring, whereas that of the tangential stress remains the same, and thus tensile failure can occur from either face of the disk.     

Effect of Polyaxial Stress States on Failure Strength of Alumina Ceramics

The effect of polyaxial stress fields on the brittle fracture strength of polycrystalline alumina was investigated through the use of thin-walled cylinders. Combinations of internal pressure, external pressure, and axial loads produced stress states of tension-compression, tension-tension, and compression-compression. The failure envelope was generated for these stress states. The results indicated that biaxial tensile stresses reduced the strength of the material; however, the tensile strength increased at least 50% when a compressive stress existed normal to the tensile direction. Compression strengths as high as 640,000 psi were measured for a biaxial compressive stress state.     

Residual Stresses in Machined MgO Crystals

The residual stresses introduced in MgO crystals by grinding on {100} surfaces in 〈100〉 directions were measured using photoelastic techniques. Grinding was conducted with two wheels; a 100-grit diamond wheel removed material by brittle fracture, and a 46-grit alumina wheel caused plastic flow and burnishing. Both wheels introduced a discrete, highly deformed layer adjacent the machined surface. In all cases the machined surfaces were under a residual tensile stress which became compressive within the deformed region. Beneath the deformed layer the residual stress patterns were distinctly different. In crystals ground with the alumina wheel the stresses became tensile again within 0.5 mm of the ground surface, whereas the subsurface stresses in crystals ground with the diamond wheel remained compressive to distances ≥1 mm. These residual stress distributions are discussed in terms of a simple model based on the superposition of mechanically and thermally induced stresses.     

Cr3+ microspectroscopy measurements and modelling of local variations in surface grinding stresses in polycrystalline alumina

Surface residual stresses caused by grinding and polishing of alumina are thought to influence materials properties but have previously been measured only by low spatial resolution techniques which sample average stresses. In this work confocal Cr³⁺ fluorescence microscopy has been used to investigate the spatial distribution of the residual stresses. A model for the residual stresses, accounting for both surface plastic deformation and “pullout” of material from the surface by brittle fracture, was developed to help in analysing the results. After coarse diamond grinding, the results showed that the residual stresses fluctuate greatly with position. Large tensile stresses (up to ～600 MPa) were found below the plastically deformed surface layer in regions between the “pullouts”. These tensile stresses are expected to aid crack propagation and further surface pullout. They arise because pullout removes parts of the plastically deformed surface layer. The stresses beneath the pullout sites themselves were compressive, but the largest compressive stresses (≈?1.5 GPa) were within the plastically deformed surface regions and extended to a depth of 1.3 μm. The plastically deformed surface layer was much shallower following polishing with 3 μm diamond paste but the compressive stress within it was of similar magnitude to that in the plastically deformed surface layer caused by grinding.     

Physical and mechanical characterization of oriented polyoxymethylene produced by die-drawing and hydrostatic extrusion

The mechanical properties and structure of uniaxially oriented polyoxymethylene (POM) produced by two solid-state processes, hydrostatic extrusion and die-drawing, are compared. In the former process there is no net component of tensile stress whereas in the latter case the sample is subjected to axial tensile stresses at the die-exit. The tensile nature of the stresses in die-drawing causes void formation and growth in the oriented sample whereas, in the case of hydrostatic extrusion, voids are suppressed due to the compressive stress fields. The mechanical properties of the oriented samples are compared together with relevant structural data, and their differences discussed.     

Cavitation wear of structural oxide ceramics and selected composite materials

The usage of ceramic materials in the applications endangered by intensive cavitation could limit erosion phenomena. In the presented work, cavitation erosion resistance of commonly used (in structural application), oxide phases (α-alumina, tetragonal zirconia) were investigated. Additionally, the behaviour under cavitation conditions of two composite materials, based on alumina and zirconia matrices, was tested.Significant difference in cavitation wear mechanisms for alumina and tetragonal zirconia materials was observed. Alumina was degraded by removing the whole grains from the large surface subjected to cavitation. Degradation of zirconia proceeded locally, along ribbon-like paths of removed grains. Cavitation wear of composites was strongly dependent on the residual stress state in the material. Alumina/zirconia composite with compressive stresses in the matrix showed a significant improvement of cavitation resistance. The zirconia/tungsten carbide composite with relatively high level of tensile stresses in the matrix was the worst of all investigated materials.     

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.     

Residual Stress Measurements in Alumina and Silicon Carbide

Residual stresses were measured in three types of ceramic components. Stresses were measured using X-ray diffraction and an advanced X-ray instrument. Measured stresses in alumina rods were shown to correlate well with breaking strength, and stress variations in an alumina tile were hypothesized to result from inhomogeneous cooling. The compressive stresses induced in a silicon carbide tube, by an outer steel sleeve, were seen to be balanced by tensile stresses in the sleeve.     

Kinetics and Mechanisms of High-Temperature Creep in Silicon Carbide: I, Reaction-Bonded

The kinetic characteristics and the controlling mechanism of steady-state creep were determined for NC–430 reaction-bonded silicon carbide which was subjected to high temperatures (1848 to 1923 K) and constant compressive stresses (110 to 220 MN/m²). Both as-received and as-crept materials were studied extensively by transmission electron'microscopy as one means of determining the controlling creep mechanism. Small variations in sample density resulted in large variations in the creep rate. The stress exponent, n in the relation εασⁿ, was found to be 5.7 and the creep activation energy 711 ± 20 kJ/mol. The controlling creep mechanism was determined to be dislocation glide/climb controlled by climb.     

Determination of bulk residual stresses in superhard diamond-SiC materials

Superhard silicon carbide bonded diamond materials can be produced by liquid silicon infiltration of diamond containing preforms. These materials can be produced as bulk materials and as layered materials with the SiC bonded diamond only in areas where it is required. In order to understand the behaviour of the materials it is necessary to know the internal stresses in the different phases and at the interface. These stresses were determined by Raman spectroscopy and in the bulk by neutron diffraction using the SALSA instrument in the ILL. In the SiC bonded diamond material the diamond and the remaining Si are under compressive stresses. The SiC-phase is under tensile stresses up to 500 MPa. The Raman investigations and the neutron diffraction resulted in similar results. At the interface between the SiC-bonded diamond and the SiSiC no significant additional stresses could be observed.     

Physical aging in particulate-filled composites with an amorphous glassy matrix

Physical aging was studied on particulate -filled glassy network polymers by means of mechanical -dilatational, differential scanning calorimetry (DSC) and density measurements on specimens that were aged at room temperature. The composites aged for 0.5 day fractured in a brittle manner at a constant ultimate stress, which is close to the tensile strength of the unfilled material, regardless of the filler content and the presence of a coupling agent. This type of mechanical behavior is caused by the compressive residual stresses that are present due to curing and differential thermal shrinkage. As aging takes place, the compressive residual stresses are relieved; as a result the ultimate tensile strengths of the composites decrease. The 120 -day -old untreated glass bead containing composites exhibited dilatation and yield in mechanical -dilatational testing. This type of behavior is described as “having no adhesion” between the filler and the matrix. The 120 -day -old composites with coupling agent -treated glass beads fractured at a tensile stress which is equal to 1/1.6 the tensile strength of the unfilled material. These materials did not exhibit dilatation and yield in mechanical -dilatational testing. Density and DSC data indicate densification and enthalpy relaxation upon again and support the hypothesis presented for the observed change in the mechanical -dilatational behavior.     

Tensile,compressive and flexural basic creep of concrete at different stress levels

Concrete is brittle and highly sensitive to cracking, which is detrimental to the sustainability of its applications. Although it is well known that cracks occur mainly in tension, research on the mechanical behavior of concrete is usually limited to compression and investigations of creep behavior, a major concern for concrete structures, are no exception in this respect. This paper is intended to help remedy the situation. First, the new experimental set-ups developed to achieve tensile and bending creep are presented. The precautions taken to obtain relevant experimentation are also described. Results for specimens subjected to sustained stresses of 30, 40 and 50% of the tensile or compressive strength are then presented. The final discussion compares basic creep under the different types of loading for the three stress levels.     

A Statistical, Micromechanical Theory of the Compressive Strength of Brittle Materials

A general theory of the compressive strength of brittle materials is presented. This theory proposes that failure is brought about by structural weakening from accumulated crack damage which increases with the stress level. The statistics of the flaw distribution and the mechanism of crack initiation and extension are important. A sample calculation using the theory is given to demonstrate its application.