Modeling impact in down-the-hole rock drilling

In this work a study of impact in Down-the-Hole (DTH) rock drilling is carried out. We present an alternative to a method previously introduced by Lundberg and his co-workers. Our model is formulated in terms of the impulse–momentum principle while Lundberg’s method is based in solving the one-dimensional wave equation. In the case of DTH drilling, the study of the subject becomes easier because the handling of many bodies interacting dynamically is simplified, and different boundary conditions, such as constant body forces, distributed forces and initial strains, can be directly included. The rock–bit interaction is modeled using both a non-linear spring and a variable gap using experimental parameter data obtained by other researchers and by a normalized quasi-static penetration test described in this work. The simulation results are in good agreement with results in previous publications as well as with experimental validation measurements carried out by the authors.     

PREDICTING THE EFFECTS OF LOAD RATIO ON THE FATIGUE CRACK GROWTH RATE AND FATIGUE THRESHOLD

The effect of the load ratio, R, on fatigue crack growth behaviour is analysed on the basis of the recently proposed inelastic discrete asperities model. A wide range of load ratios, both positive and negative, are examined. Particular emphasis is placed on compressive excursions, i.e. negative R loadings. The inelastic discrete asperities model is a micro-mechanical analysis based on the plastic crushing of a single asperity (or multiple asperities) located on the crack face close to the crack tip and under dominantly plane strain conditions. Experimental data have indicated that the primary crack face contacts which obstruct closure are immediately adjacent to the crack tip, although segments of the crack face more distant from the crack tip are not neglected. However, the more distant asperities are a part of the past crack advance history which does not influence current behaviour. By use of this model, it is shown that the effect of the load ratio can be adequately predicted once some baseline information on mechanical material properties and surface roughness is provided. The model also provides useful trend information and explains many of the observed phenomena, e.g. the ‘saturation’ of the compressive underload effects. For a constant applied nominal stress intensity factor range, ΔK_nom , it is shown that the effective stress intensity factor range, ΔK_eff , initially decreases as the positive R decreases (corresponding to the increasing influence of closure), reaches a minimum around R = 0, and then starts increasing with negative R (corresponding to the plastic crushing of the asperities which reduces closure), eventually reaching a saturation level below ΔK_nom . Conversely, for an assumption of a constant ΔK_eff , the applied ΔK_nom increases as the positive load ratio decreases, reaching a maximum around R = 0, and then decreases with more negative R values, eventually reaching again a saturation level (above ΔK_eff ). It is also shown that the effect of material hardness can be directly analysed based on this model.     

Optimum strain gage location for evaluating stress intensity factors in single and double ended cracked configurations

A general methodology has been proposed for calculation of optimum radial strain gage locations for measurement of the stress intensity factors using strain gage technique. The upper bound (r_max) of strain gage locations for complex single ended and double ended cracked configurations has been determined using the proposed method. Further, dependency of the r_max on the crack length to width ratio and on the state of stress is investigated. Numerical results obtained from the present investigation are observed to be in accordance with the theoretical predictions. Using the proposed approach the correctness of strain gage locations used by the earlier researchers is also verified.     

Determination of stress fields in the elastic lithosphere by methods based on stress orientations

Theoretical reconstruction of the stress fields in tectonic plates or particular tectonic regions is an important geophysical problem. Conventional approaches for solving this vital issue are based on classical formulations of boundary value problems of elasticity. In these approaches, stress fields are calculated for diverse boundary conditions defined on the margins of the region in order to fit the experimentally observed orientations of principal stresses inside the region.The present article identifies the major flaw in the conventional approach, which is the impossibility of obtaining a unique and reliable stress field, and suggests alternative methods based on the analysis of orientations of principal stresses. Three methods for determining the elastic state of stress in relatively stable blocks of the lithosphere are described and applied to particular tectonic domains. All of the methods are based on the direct use of experimental data on the stress orientations as input information. The first method exploits direct integration of the equations of elasticity when the field of principal stress trajectories is prescribed within a region. The second one utilizes the non-classical boundary value problem of elasticity, which uses experimentally obtained stress orientations at the region margins as boundary conditions. The third method is aimed at the numerical determination of the stress field from a given set of spatially discrete principal stress orientations.In contrast to the conventional approach, the methods suggested here do not require knowledge of the boundary stress magnitudes. As a consequence, the general solution of the problem becomes non-unique. However, in the case of an elastic medium, only a certain (finite) number of arbitrary parameters control the general solutions. These parameters can be determined from in situ stress measurements within the region under study. Therefore, for the selected spatial scale, the unique stress field can be singled out. In the second method, the number of parameters (and, thus, the minimum number of field measurements) is determined from an analysis of boundary stress orientations alone. In other methods, this number depends upon the harmonicity or non-harmonicity of inclination of the prescribed stress trajectories (for the second method) and the calculated stress trajectories (for the third method).To illustrate the essence of the proposed methods, they are applied to the determination of first-order stress fields in the West European and Australian platforms. These platforms represent two basically different types of stress domains. The stress field for the West European platform reflects nearly homogeneous stress orientations throughout the extent of the region, whereas the Australian platform is characterized by rotation of the principal stress axes while traversing the region margins. In the case of Australia, an important result is the existence (at the chosen spatial scale) of a singular point inside the Australian continent at which the curvature of the stress trajectories is infinite. The local state of stress near such a point has important geophysical and engineering implications.The proposed approaches can be applied not only for the determination of regional stress fields but also at other spatial scales, depending on the scale at which the stress indicators have been characterized.     

Evaluating the efficacy of a local tree protection policy using LiDAR remote sensing data

This study evaluated the efficacy of a tree removal permit regulation that the city of Lakeway adopted in 2002 to protect trees in private lands by comparing the levels of tree protection in Lakeway with those in the village of The Hills, a neighboring municipality without such a regulation. The level of tree protection was assessed by mean canopy height (MCH) and percent canopy cover (PCC) of a parcel derived from light detection and ranging (LiDAR) data and aerial photography, respectively. MCHs and PCCs were estimated for single family residential parcels where houses were constructed for four years before (1998-2001) and after (2003-2006) the adoption of the tree removal permit regulation by Lakeway. Holding parcel size and vegetation condition before housing construction constant, MCHs of the parcels developed in 2003-2006 were statistically significantly higher in Lakeway than Hills. Because the difference in MCHs was not found in the parcels developed in 1998-2001, it is concluded that Lakeway's tree removal permit regulation has been successful to protect trees in private lands. Unlike MCHs, PCCs between the two municipalities were not statistically significantly different in the parcels developed in each of the two study periods. This result may indicate that PCC is not as accurate in assessing the level of tree protection as MCH due to the inability of PCC to distinguish existing mature trees from young ones planted after construction. Based on these findings, it is recommended that urban planners use LiDAR data to study urban forests.     

浙江南浔医院的创作思维

设计着力于环境品质的优化与提高,体现了“以人为中心”的设计思考,并充分尊重地方文脉,从全新的视角诠释了水乡建筑的内在本质。     

Failure of a 17-4 PH stainless steel sailboat propeller shaft

In this study, a 17-4 PH precipitation hardening stainless steel propeller shaft failed in use when installed in a sailboat working in a marine environment. Failure analysis was conducted on the propeller shaft. Results indicate that the failure was caused by the fracture of the propeller shaft by torsional fatigue and stress corrosion cracking (SCC). SCC progressed transgranulary in the martensitic matrix.     

Stress analysis of perforated composite plates

Thin-walled plates and panels of various constructions find wide use as primary structural elements in simple and complex configuration. In aerospace structures, panels with variously shaped cutout are often used. The understanding of the effects of cutout on the load bearing capacity and stress concentration of such plates is very important in designing of complex structures. An analytical investigation is used to study the stress analysis of plates with different central cutout. Particular emphasis is placed on flat square plates subjected to a uni-axial tension load. The results based on analytical solution are compared with the results obtained using finite element methods. The main objective of this study is to demonstrate the accuracy and simplicity of presented analytical solution for stress analysis of composite plates with central cutout. The effect of cutout geometry (circular, square, or special cutouts), material properties (isotropic and orthotropic), fiber angles, and cutout curvature are considered. The results presented herein, indicated that the presented method can be used to determine accurately the stresses and stress concentration in composite plates with special shape cutouts.     

A weight function methodology for the assessment of embedded and surface irregular plane cracks

A low cost numerical tool for the calculation of mode I stress intensity factors K in embedded and surface irregular cracks is presented in this paper. The proposed tool is an extension of the O-integral algorithm due to Oore and Burns for the assessment of embedded plane cracks using the weight function methodology. The performance of the O-integral is assessed first by comparing its K results to exact solutions for embedded elliptical and rectangular cracks. From the analysis of this data it is found that the error in the K results systematically depends on the crack aspect ratio and the local crack front curvature. Based on this evidence a corrective function is derived in order to remediate the limitations of the O-integral. Solutions due to Newman and Raju are used to account for the effects of free surfaces and finite thickness. The accuracy of the proposed procedure is assessed by solving a number of examples and by comparing the obtained results to those available in the literature.