The origin of the “crack tip” mode of failure in boron filaments

A study has been made of the crack tip mode of failure in boron filaments. Filaments produced by a technique that ensures a high percentage of this type of flaw were subjected to tensile testing and fracture characterization. These filaments were split longitudinally and etched to expose possible fracture-causing defects. A high density of voids was detected within the bulk boron coinciding with the residual stress neutral axis, which is also the location of the tip of the radial crack in the filament. A model is proposed for this type of failure that is consistent with experimental observations and theoretical predictions.     

Investigations of fracture instability in crack growth for several metals—Part II: An energy approach for fracture instability analysis of crack growth

An energy approach is presented to predict fracture instability using elastic-plastic internal energy in structures, the physical behaviour of crack growth, dynamic effects in the process of unstable crack growth and intense strain region near the crack tip are considered. The practical forms of the approach have predicted correctly fracture instability of tests with three-point bending specimens (3PB specimen) and compact specimens (CT specimen) under highly stiff supporting system, but tearing modulus criterion gave incorrect estimations which were shown in part I [1].     

A crack in a viscoelastic functionally graded material layer embedded between two dissimilar homogeneous viscoelastic layers – antiplane shear analysis

A crack in a viscoelastic functionally graded material (FGM) layer sandwiched between two dissimilar homogeneous viscoelastic layers is studied under antiplane shear conditions. The shear relaxation modulus of the FGM layer follows the power law of viscoelasticity, i.e., = ₀ exp (y/h) [t₀ exp (y/h) /t]^q, where h is a scale length, and ₀,t ₀,, and q are material constants. Note that the FGM layer has position-dependent modulus and relaxation time. The shear relaxation functions of the two homogeneous viscoelastic layers are =₁(t ₁/t)^q for the bottom layer and =₂(t ₂/t)^q for the top layer, where ₁ and ₂ are material constants, and t ₁ and t ₂ are relaxation times. An elastic crack problem of the composite structure is first solved and the `correspondence principle' is used to obtain stress intensity factors (SIFs) for the viscoelastic system. Formulae for SIFs and crack displacement profiles are derived. Several examples are given which include interface cracking between a viscoelastic functionally graded interlayer and a viscoelastic homogeneous material coating. Moreover, a parametric study is conducted considering various material and geometric parameters and loading conditions.     

The strengthening effect caused by an elastic contrast��part II: stratification by a thin stiff layer

To quantify the gain in strength of a layered heterogeneous structure caused by the elastic contrast between the layers, especially if no crack deflection is observed at the interface, two original mechanisms baptized ??step-over?? and ??jump-through?? were proposed in Part I. They addressed the ability of a crack to pass through an interface and were applied to a bimaterial structure; whereas part II is dedicated to a homogeneous beam embedding a thin stiff film. The asymptotic expansions differ significantly since the small parameter is now the layer thickness. Unlike the first part where it was difficult to evidence the gain in toughness due to the superposition of two effects: a simultaneous increase in stiffness and in toughness, here it is possible to characterize the strength enhancement in using a single parameter. A discussion of the possibility to repeat the effect by multiplying the thin films is presented at the end.     

On fast fracture in an elastic-(plastic)-viscoplastic solid Part II – The motion of crack

The motion of a crack in an elastic-(plastic )-viscoplastic medium is studied in terms of an energetic analysis. Combined with the stress and velocity fields obtained in Part 1, Kishimoto's energy integral, , is used as a crack driving force to determine its motion. The major results obtained are: (1) dependence of crack speed on a modified near-field parameter, K _I tip, (or equivalently, a modified dynamic energy release, G _I tip), which is different from the usual stress intensity factor K _I of an elastic crack-tip field but is related to it; (2) influence of inelastic effect, such as the viscoplastic exponent n, on the motion of the crack; and (3) stability condition of crack motion. In particular, for the last point, it has been found that, for a given loading and material coefficients, there exist two possible motions of the crack: one is stable crack growth and the other is unstable fracture. The lower and upper bounds of crack motion are also discussed. It is finally shown that the maximum crack velocity is lower than the Rayleigh wave speed, and is dependent on the viscoplastic exponent of the material.     

The effects of corrosion on fatigue crack propagation in 2024‐T3 Aluminium

Abstract

The effects of salt water with 0.5 percent nitric acid on fatigue crack propagation in 2024‐T3 aluminium alloy are studied experimentally. First, the plastic zone size ahead of the crack tip is measured. It is found that the plastic zone size as well as the closure stress in corrosion fatigue is smaller than that in ordinary fatigue. Secondly, the crack propagation rate of corrosion fatigue is much higher than that of ordinary fatigue due to the creation of microcracks ahead of the main crack. Thirdly, the effective stress range governs the crack propagation rate in both ordinary and corrosion fatigue.     

Computational fracture analysis of concrete gravity dams by crack-embedded elements––toward an engineering evaluation of seismic safety

From a point of view to utilize fracture mechanics of concrete for the solution of engineering problems, the problems in crack modeling, computational algorithm, and damping implementation are discussed in conjunction with safety assessment of concrete dam against large earthquakes. It is shown that the formulation for the crack-embedded element has an analogy with that of computational plasticity. This analogy enables us to utilize the return-mapping algorithm well established in computational plasticity for the dynamic analysis of crack growth in concrete. The ways of implementing damping to avoid diffused cracking are presented and simple examples of numerical analysis are shown to demonstrate the effect of damping and the performance of appropriate damping implementation for cracked elements.     

The influence of Zr layer thickness on contact deformation and fracture in a ZrN–Zr multilayer coating

In order to understand the influence of ductile metal interlayer on the overall deformation behavior of metal/nitride multilayer, different configurations of metal and nitride layers were deposited and tested under indentation loading. To provide insight into the trends in deformation with multilayer spacings, an FEM model with elastic-perfect plastic metal layers alternate with an elastic nitride on top of an elastic–plastic substrate. The strong strain mismatch between the metal and nitride layers significantly alters the stress field under contact loading leading to micro-cracking in the nitride, large tensile stresses immediately below the contact, and a transition from columnar sliding in thin metal films to a more uniform bending and microcracking in thicker coatings.     

Determination of material fracture toughness by a computational/experimental approach for rapid crack propagation in PE pipe

Based on an investigation of the Small Scale Steady State (S4) test, an integrated computational/ experimental approach has been developed in order to assess the fracture behaviour of polyethylene (PE) gas distribution pipe material during rapid crack propagation (RCP). This paper describes the use of the results obtained from the S4 test and program modified from PFRAC (Pipeline Fracture Analysis Code) to evaluate the fracture toughness of the material, G _d, which could not be directly obtained from the test, and to predict critical pressure, p _c, for RCP in a full scale PE pipe. The contact algorithms are developed to consider the opening pipe wall impact against a series of containment rings and the capabilities of PFRAC are also extended. Since G _d is evaluated, the investigations are made on it to the effect of temperature and wall thickness. In addition, procedures to evaluate the critical pressure for the S4 test pipe are also discussed.     

The analysis of elastic liner in a cylindrical tunnel subjected to sh‐waves

Abstract

In this paper, we investigate the dynamic stresses of an elastic liner around a cylindrical tunnel subjected to an incident plane SH‐wave in an infinitely extended elastic medium. The investigation is based on the anti‐plane strain approximation of the dynamic theory of elasticity. The solutions are obtained by applying the method of wave functions expansion. Numerical results of dynamic stress concentrations at the outer and inner boundaries of the liner for various parameters are discussed in detail.     

Mechanical analysis of fracture in an automotive radiator head produced from a nylon–glass fibre composite

The fracture during assembly of a radiator head produced from a nylon–short glass fibre composite is studied in the framework of complexities such as determining elastic constants and fracture stresses on the one hand and manufacturing problems such as distortion after moulding and deformations induced during assembly on the other. The combination of in situ measurements, SEM observation of fracture surfaces and reverse modelling of non-linear material properties permitted to make a sufficiently accurate estimate of the real mechanical behaviour of the material, as opposed to the properties mentioned in its data sheet. While the method was applied to solve a particular production problem, its applicability in process evaluation and redesign of injection-moulded short fibre composites is general.     

The problem of substance in chemistry — an approach by means of large deviation statistics

Different substances can phenomenologically be defined by use of Gibbs' phase rule. In quantum mechanics, on the other hand, the notion of a substance is not clearly understood: The thermal density operator D_β corresponding to some Hamiltonian and some chosen inverse temperature β is uniquely defined, which implies that different isomers exhibit the same thermal density operator. Coexistence of isomers, substances or phases at some temperature, on the other hand, would need different thermal density operators, one for each isomer, substance or phase. Here we try to understand this problem for the classical van der Waals gas using large deviation statistics. We show that the gaseous and liquid phase of the van der Waals gas emerge with increasing numbers of particles. Intermediate states — neither gaseous nor liquid — exist but die out with increasing number of particles. Extension of our method to quantum mechanics is not straightforward, but looks promising.     

The production of science in Africa: an analysis of publications in the science disciplines, 2000–2015

The importance of science in Africa has been gaining the interest of scholars and policymakers alike who find that the relationship between science and development is getting stronger than ever. Science in Africa continues to find its own place in the global world of science. However, there have not been any attempts in recent years to study the production of science in all African countries taken as a whole. In this bibliometric study, science in Africa is analyzed using the publications African countries have produced in the last 15 years i.e. from 2000 to 2015. It is not only the number of publications but also the research areas which are of great importance. Not many recent studies have examined the publication productivity of all African countries. The analysis undertaken here shows that a few countries, namely, South Africa, Egypt, Tunisia, Nigeria and Algeria are the leading countries. These produced the highest number of publications during 2000–2015. At the same time, some of the weakest countries in Africa have strengths in specific research areas and are thus able to contribute to knowledge production in those areas. The findings presented in the paper suggest the implications for development in Africa and the measures which could be taken.     

Testing the proficiency of analytical laboratories by means of inter-laboratory comparison tests – an important element in assurance of the uniformity of measurements

The role of proficiency testing of laboratories through inter-laboratory comparison tests in an effort to assure the uniformity of measurements is considered.     

The wear and fracture behaviour of ultra high molecular weight polyethylene subjected to γ-irradiation in an atmosphere of acetylene

The wear and mechanical properties of GUR 1020 (Perplas IMP 2000-2) Ultra High Molecular Weight Polyethylene (UHMWPE) subjected to -irradiation in an atmosphere of acetylene, were evaluated for a range of processing conditions of irradiation, annealing and ageing. The results were compared with those obtained for the virgin UHMWPE material and material processed using conventional -irradiation in nitrogen. Cross-linking produced by irradiation in acetylene, followed by subsequent annealing was found to be significantly more effective in improving the mechanical and wear properties of UHMWPE compared to when the material was irradiated in nitrogen. Gel fraction analysis on its own, while being able to detect the degree of cross-linking, was found to be insufficient in determining the effectiveness of the cross-links and the resulting mechanical properties of the UHMWPE material. The results suggest that -irradiation in an atmosphere of acetylene may provide significant advantages over conventional UHMWPE processing and irradiation cross-linking techniques.     

Spatial–temporal enhancements of material flux analysis methods, with application to cadmium management in an agricultural region in Australia

The accumulation of cadmium (Cd) on agricultural lands and the resultant contamination of the food chain are challenging problems for the agricultural sector in Australia. This paper describes a method to estimate spatial accumulation of Cd on agricultural land, to support policy developments that ensure acceptable limits are not exceeded. This method integrates material flux analysis and geographic information system models. It is demonstrated through application to Cd management within the Murrumbidgee Irrigation Area (MIA). The results of this study include spatially estimated net Cd balance in MIA soils over the 10 years 1992–2000. Combined with initial estimates of spatial Cd contamination in 1992, this permits estimation of soil Cd concentration in the MIA in 2002. The areas that have high soil Cd concentration have seen increases in Cd levels of 1.5–1.6 times of Cd concentration in 1992. The results are used to evaluate the trend of Cd exceeding an acceptable limit on a spatial basis, and development of an effective policy and limit to control Cd input to soil.     

The influence of sputtering and transport mechanisms on target etching and thin film growth in rf systems—I. Target processes

Ion impact sputtering, particularly rf sputtering in a plasma, can be used for changing the composition and profile of a target surface, exposing surface structure and for depositing films of electrical conductors and insulators based on a wide range of substances. These sputtering processes are discussed in relation to the emission phenomena occurring at the target and the mode of transport of liberated material in the plasma and its deposition at the receiver. Studies have been made in the writers' laboratory of each of the latter topics using rf sputtering systems and the results obtained are reviewed together with the published results of other workers. Subjects considered are the influence of the sputtering mechanism on target topography; the reaction of residual gases with growing films; the rate of getter-sputtering and the function of vacuum pumps; and the nature of energetic particles bombarding the substrate. In conclusion the preparation of metal and compound films is discussed. The subjects treated conveniently divide into two parts—I. Target Processes and II. Transport and Deposition Processes which are produced as two consecutive papers.     

Free convection of water–Fe3O4 nanofluid in an inclined cavity subjected to a magnetic field: CFD modeling,sensitivity analysis

Modeling of natural convection heat transfer in an inclined C-shape cavity is studied in this paper. The enclosure is filled with H₂O-Fe₃O₄ nanofluid under the effect of magnetic field. The operating range of parameters used in this study were Hartmann number (Ha) from 0 to 80, Rayleigh number (Ra) from 1E2 to 1E6, nanoparticles volume fraction (φ) from 0 to 0.1, inclination angle (α) from 0 to 90 deg, and aspect ratio (AR) from 0.2 to 0.8. The employed model is solved using CFD tools based on the finite element method. The comparison with reference experimental data indicated the accuracy and generalization capability of the model. In addition, a novel correlation and an artificial neural network (ANN) model were productively developed for predicting Nu number as a function of aforementioned independent variables. The influence of the model parameters on the Nu number is precisely presented and discussed. It is shown that Ra number and aspect ratio have more impact on Nu than the other variables.