Richard L. Burguete

Tightening characteristics for screwed joints in osseointegrated dental implants

The significance of tightening abutment screws and gold cylinders to osseointegrated fixtures with the correct torque is demonstrated, and a simple relationship between applied torque and screw preload is derived by use of mechanical engineering principles. The principles of a number of tightening methods are outlined and assessments made of their accuracy. The difference between optimum and design torque is highlighted. The necessity and means of achieving optimum torque to ensure a reliable joint in clinical practice is discussed.

A photoelastic study of contact between a cylinder and a half-space

Three-dimensional photoelasticity was employed to study a cylinder in contact with a half-space. Both bodies were modeled in epoxy resin. Three loading cases were examined, namely, the cylinder lying on its side subject to a load normal to the plane, the cylinder on its side subject to both normal and tangential loads and the cylinder standing on its end and subject to a normal compressive load, i.e., as a circular punch. The cylinders and the half-space, which was represented by a large block, were stress frozen with a known coefficient of friction and using relatively small loads so that the strain levels were low. After slicing the cylinders, which resulted in lower fringe orders than could be readily analyzed manually, an automated system based on phase stepping was used to record and process the data. Distributions of maximum shear stress and Cartesian shear stress were obtained for a large area of the slice. Stress separation was performed, using the shear difference method, to obtain the Cartesian stress components in the plane of symmetry of the half-space. These results provide confirmation, by experiment, of the theoretical and numerical models of this type of contact obtained by other investigators.

Shape and displacement measurement of discontinuous surfaces by combining fringe projection and digital image correlation

We describe how a shape-measurement system (SMS) based on fringe projection can be combined with a two-dimensional digital image correlation (DIC) technique to accurately measure both surface profile and displacement fields at the same time. Whereas the measurement of all three displacement components by traditional DIC requires the use of at least two cameras, the approach presented here provides the full three-dimensional (3-D) displacement field from a single-camera, single-projector SMS with no additional hardware requirements. Furthermore, the single-pixel spatial resolution of the fringe projection technique can be exploited to prevent the correlation peak-splitting phenomenon that occurs when a DIC subimage straddles a global geometrical discontinuity. Thus, unlike traditional 3-D DIC techniques, the proposed method can measure displacement fields on discontinuous surfaces as easily as on smooth ones. Details of the algorithm are given together with experimental results of a rigid-body translation test. Measurements made during a routine fatigue test on a part of a wing panel are also presented.

Investigation into the Effect of the Nut Thread Run-Out on the Stress Distribution in a Bolt Using the Finite Element Method

By means of comparing results from finite element analysis and photoelasticity, the salient characteristics of a finite element model of a nut and bolt have been established. A number of two-dimensional and three-dimensional models were created with varying levels of complexity, and the results were compared with photoelastic results. It was found that both two-dimensional and three-dimensional models could produce accurate results provided the nut thread run-out and friction were modeled accurately. When using two-dimensional models, a number of models representing different positions around the helix of the thread were created to obtain more data for the stress distribution. This approach was found to work well and to be economical.

The Effect of Mean Stress on the Fatigue Limit of High Tensile Bolts

Currently engineers use modifications to empirical rules such as the Goodman line or the Gerber parabola, including ideas about notch tip plasticity, to predict the fatigue limit of notched specimens (including bolts) at varying mean stresses. This study compares a large data set obtained from tests on bolts at many different levels of mean stress with these empirical formulations. The data show that the empirical rules are adequate for limited ranges of mean stress. Some new ideas are also presented, which improve the ability of the empirical rules to predict behaviour at high mean stress and for improved design procedures.

Large-scale full-field metrology using projected fringes: some challenges and solutions

The application of optical techniques to the measurement of shape and deformation of structures in the aerospace industry poses unique challenges resulting from the large length scales involved, which are typically in the 1-10 m range. For example, the relative immobility of large samples requires a network of sensors to be linked into a common global coordinate system; traceable calibration requires the development of new types of calibration artefact; and traditional interferometric techniques for displacement field mapping are frequently too sensitive to observe the physical effect of interest. We describe a system designed to address some of these problems based on the projected fringe technique combined with temporal phase unwrapping. Multiple cameras and projectors are linked into a common coordinate system using calibration concepts borrowed from the photogrammetry field. Traceable calibration is achieved through the use of reference spheres separated by a bar of known length. Traditional two-dimensional image processing techniques for recognizing circles (Hough transforms) have been extended to the automatic detection of spheres within the measured 3-D point clouds. Bundle adjustment software has been developed to refine the camera and projector calibration parameters as well as the rigid body translation and rotation coordinates defining the poses of the calibration artefact. An overview of all these aspects of the developed techniques is given in the paper. Typical results from a compression test on a large scale aluminium structure, performed on-site at Airbus UK using the developed system, are also presented.

Fast Hough transform for automated detection of spheres in three-dimensional point clouds

The calibration of 3-D optical sensors normally requires the use of a calibration artifact of known dimensions. By labeling regions within the measured point clouds as belonging to a known region of the artifact, camera and projector parameters can be optimized. A novel 3-D Hough transform has been developed to extend the well-known strategy for detecting circles in 2-D images to detecting spheres in a 3-D point cloud. In its standard form, the Hough transform suffers from excessive memory storage requirements for the intermediate Hough accumulator space, which can make its application to 3-D problems impractical. We describe an accumulator implementation using an optimized sparse 3-D matrix model that provides compact data storage and efficient data access. Application of this method to experimental shape data for spheres is discussed, demonstrating its memory-saving benefits, computational efficiency, and 3-D feature detection capability.

The effect of eccentric loading on the fatigue performance of high-tensile bolts

The effect of eccentric cyclic loading on the fatigue life of bolts has been investigated. It was found that eccentric loading reduced the fatigue life of bolts by an amount proportional to the increase in the local stress amplitude produced by the eccentricity. The increase in local mean stress due to eccentricity was found not to affect the fatigue life, and eccentric loading was found not to affect significantly the shape of fatigue cracks.

A photoelastic analysis of crescent-shaped cracks in bolts:

Three-dimensional photoelasticity was used to analyse the effect of crack shape on the stress intensity factors (SIFs) at the tips of cracks in threaded connectors under axial and eccentric loads. The results show that the shape of the crack does not affect the maximum SIF occurring at the middle of the crack front. However, it does influence the distribution around the crack front, with the KI/ Ko values decreasing more rapidly towards the end of the crack front for more crescent-shaped cracks. Despite the complexity of the geometry, the SIFs were found to be predominantly mode I. Eccentric loading was found to increase the SIFs at the centre of the crack front slightly. The crack shape employed in this study was different from those used by earlier investigators but was based on evidence from fatigue tests.

Calibration and assessment of full-field optical strain measurement procedures and instrumentation

There are no international standards or norms for the use of optical techniques for full-field strain measurement. In the paper the rationale and design of a reference material and a set of standarized materials for the calibration and evaluation of optical systems for full-field measurements of strain are outlined. A classification system for the steps in the measurement process is also proposed and allows the development of a unified approach to diagnostic testing of components in an optical system for strain measurement based on any optical technique. The results described arise from a European study known as SPOTS whose objectives were to begin to fill the gap caused by a lack of standards.