Ian W. Johnston

Pregnancy following intrauterine implantation of an embryo obtained by in vitro fertilization of a preovulatory egg.

A normally progressing pregnancy has been established by implanting an eight-cell embryo into the uterine cavity of an infertile woman. The embryo was obtained by in vitro fertilization of preovulatory egg aspirated at laparoscopy 28 hours after the beginning of the luteinizing hormone surge during the patients natural menstrual cycle. Elevated levels of serum human chorionic gonadotropin and progressively increasing levels of total urinary estrogens and pregnanediol confirmed the presence of an early pregnancy. Ultrasound examination revealed an intrauterine gestational sac at 5 weeks, a 1-cm fetus with fetal heartbeat at 7 weeks, and an active, normally growing fetus at 13 and 16 weeks. Chromosome analysis at this time revealed no abnormality, and the alpha-fetoprotein level was within normal limits. The patient is due to be delivered on or about June 28.

Stress intensity factors for semi-circular specimens under three-point bending

Abstract The semi-circular specimen under three-point bending technique is a versatile, cost-effective and reliable method for determining the mixed-mode (I and II) fracture toughness envelope of rocks. However, existing numerical results on the stress intensity factor variation with specimen geometry are very limited. This study extends previous numerical work to cover a wide range of possible specimen geometries of experimental interest. From this, a number of observations relevant to the practical use of this technique are discussed. Analytical functions are then provided as an approximation to the mode I variation of the stress intensity factors.

A mechanistically-based model for rough rock joints

Abstract Over the past twenty years, many models have been formulated to predict the behaviour of rough rock joints. While these models have substantially improved our understanding of rock joint behaviour, their limitations must also be recognized. The models currently available are questionable in their application either because they are too simplistic, rely too heavily on empiricism or require complex input parameters that are typically well beyond current capabilities of normal site investigation practice or laboratory procedures. In many cases, the predictive capabilities of the models are limited to peak shear stress determination, giving little or no indication of the displacement to peak. Also, most models are incapable of reproducing even the simplest mechanisms of movement leading to shear displacement. This paper presents a new model that addresses these limitations. It is demonstrated that this new joint model is capable of capturing the basic mechanisms of movement and makes reasonably accurate predictions of shear displacement behaviour. The model relies only on a knowledge of the joint roughness and a few easily determined, basic rock properties.

Shear behaviour of irregular triangular rock-concrete joints

Abstract This paper presents the results of an experimental investigation into the shear behaviour of rock-concrete joints with roughness in the idealised forms of regular and irregular triangles. The tests were carried out in a large direct shear machine under conditions of constant normal load and constant normal stiffness using rock specimens which were typical of a soft mudstone. For both the regular and irregular joints, failure was accompanied by shearing and crushing of the rock asperities. However, the principal difference between the responses of the two types of joint was that the irregular joints, particularly under conditions of constant normal stiffness, showed a much greater relative ductility than the regular joints.

Comparison between various displacement-based stress intensity factor computation techniques

The computation of the stress intensity factor at a crack tip can be determined from the nodal displacements along the crack face. Amongst the existing techniques available are the Displacement Correlation Technique (DCT), the Quarter-Point Displacement Technique (QPDT) and the Displacement Extrapolation Technique (DET). As each of these techniques are popular in general LEFM analysis, an evaluation of their relative performances would seem appropriate. Previously, only limited comparisons have been made. In this paper the comparison is made on the basis of extensive numerical analysis. In addition two new variants to the DET are introduced and shown to be more efficient computationally.The results indicate that the QPDT is generally more accurate and consistent in performance than the DCT. The DET, however, exhibited some erratic characteristics. Detailed examinations revealed that the linear regression analysis employed in the DET for the extrapolation is highly sensitive to the nodal displacement distribution. Both the new variant DETs exhibited much more consistent behaviour.

Reducing injury from speed related road crashes

Towards the achievement of a population based preventive strategy While speed management is a key element in the road safety strategies of many regional and national governments, there is little consensus and much controversy over the specific speed reduction interventions that have been employed. Considerable heat—and relatively little light—has accompanied the public debates. This paper reviews the evidence for a population based speed reduction strategy, based predominantly on the widespread mismatch between extant speed limits and levels of infrastructure safety. It then explores how jurisdictions have inflamed public opposition to their interventions with flawed public policy decision making and concludes, hopefully, by putting the baby firmly back in the bath. There can be no doubt that decreasing vehicle travel speeds reduces stopping distances and impact speeds, and thus the incidence of serious casualties and fatalities. While individual studies can be criticised, the weight of evidence, both within and across methodological paradigms, is overwhelming: 1. Numerous studies, across many countries, have evaluated the injury outcomes of changes in prevailing speed limits, both urban and rural. Almost invariably, fatalities and serious casualties have fallen when speed limits have been lowered and have increased when speed limits have been raised.1,2 On interstate highways in the United States, for example, the limit was reduced in the 1970s, restored to its original level in the late 1980s then further increased, in numerous states, in the mid to late 1990s. Fatalities fell, rose, and then rose again correspondingly.3,4 Such is the consistency of results from these types of study that some writers have posited predictive relationships between shifts in speed distribution and crash injury outcomes. 2. Case-control studies, of which there are few, show that both the probability of involvement in a casualty crash and the severity of injury when a crash occurs increase …

Appropriate techniques for triaxial testing of saturated soft rock

Abstract The triaxial test technique represents a convenient form of laboratory testing for the evaluation of both soil and rock parameters for engineering design purposes. Although soil mechanics literature examines in considerable detail the many factors which influence the results obtained, the same depth of understanding cannot be claimed for rock materials. A detailed investigation was undertaken to evaluate the influence of a number of important factors on the results obtained during triaxial testing of a saturated soft rock, namely the Silurian mudstone of Melbourne. Consideration is given to the appropriate triaxial equipment and associated measuring systems for the determination of stresses, strains and pore-water pressures. The preparation of test specimens and their geometry are also examined. Finally, the rates of strain application are investigated in some detail to show that this variable can have a significant influence on the soft rock response and therefore the parameters measured. By considering the influence of these factors in conjunction with the basic soft rock characteristics, it is demonstrated that engineering properties independent of procedures may be derived from the triaxial test.

A finite element code for fracture propagation analysis within elasto-plastic continuum

A new code for the simulation of mixed-mode fracture propagation within a body deforming elasto-plastically is presented in this paper. The analysis is fully automated in that it requires no user interaction. An automatic local remeshing technique is developed based on the window remeshing technique of Murti [Numerical fracture mechanics using finite element methods. Ph.D.Thesis, University of New South Wales (1986)]. This algorithm ensures that proper element geometries are maintained in the crack tip vicinity and that reasonable shaped elements are preserved further away from the crack tip. A new approach was developed to predict the fracture trajectory under load or displacement control. This new approach is particularly suited to applications where the applied loads or displacements are the controlling factors rather than the crack extension. The code incorporates recently developed profile reduction and mesh rezoning techniques for increased efficiency. The capability of this new code is demonstrated by application to a number of practical problems.

An improved numerical inverse isoparametric mapping technique for 2D mesh rezoning

Abstract Mesh rezoning is essential in some finite element analyses. Of the existing techniques, inverse isoparametric mapping ( x, y ) → ( eη ) is a mathematically sound and accurate approach. A survey of existing techniques reveals that the predefined line technique proposed by Murti and Valliappan [Comput. Structures22, 1011–1021 (1986)] is the most efficient for quadratic elements. A new, robust and more efficient implementation of the predefined line technique is presented in tins paper. Extensions of the technique to encompass triangles and collapsed quadrilaterals are described. A detailed examination of its solution convergence rate and its dependence on element distortion is conducted to better understand the techniques performance. Its implementation for other than quadratic elements is also outlined. A numerical example is included to demonstrate the efficiency and effectiveness of this improved technique.

The use of transition elements

Abstract Since the introduction of transition elements for crack tip singularity modelling, a number of opinions have been expressed regarding their application and performance. In particular, conflicting proposals concerning optimal transition element size have been suggested. In order to resolve the anomaly, a critical examination of these proposals was made. Consequently, several inconsistencies with regard to the derivation of the mid-node location of these transition elements were revealed. The effects of these inconsistencies were examined numerically and the accuracy of the computed stress intensity factors was compared with the original formulation of Lynn and Ingraffea. The results of the numerical analyses appear to show no discernible optimal transition element size.