Jacques Lanteigne

Theoretical model for FCGR near the threshold

A theoretical model for fatigue crack growth rate at low and near threshold stress intensity factor is developed. The crack tip is assumed to be a semicircular notch of radius ρ and incremental crack growth occurs along a distance 4ρ ahead of the crack tip. After analysis of the stress and strain distribution ahead of the crack tip, a relationship between the strain range and the stress intensity range is proposed. It is then assumed that Manson-Coffin cumulative rule can be applied to a region of length 4ρ from the crack tip, where strain reversal occurs. Finally, a theoretical equation giving the fatigue crack growth rate is obtained and applied to several materials (316L stainless steel, 300M alloy steel, 70-30 α brass, 2618A and 7025 aluminum alloys). It is found that the model can be used to correlate fatigue crack growth rates with the mechanical properties of the materials, and to determine the threshold stress intensity factor, once the crack tip radius α is obtained from the previous data.

The mechanical performance of GRP used in electrical suspension insulators

Abstract The work deals with the use of GRP material in suspension electrical insulators. The mechanical behavior of this material is modelized, and the model inserted in a finite element program in order to estimate stress distributions in these composite insulators and describe their failure mode. The material is assumed typically orthothropic with transverse isotropy. A three-dimensional failure criterion, based on the fracture strength along the different axis of the material, is used to predict the location of failure and the distribution of stress components at the occurence of this event. Some elastic properties and strength characteristics of GRP rod were obtained experimentally and used as inputs in the FEM program. The FEM prediction concerning the location and orientation of fracture compares well with the insulator destructive test results. It is found that the fracture propagates in the direction of the equivalent stress gradient vector {∇ - σ eq }.

Failure Analysis Of Synathetic Insulators With Fiberglass Rod Submitted To Mechanical Loads

Four end fitting designs of long rod fiberglass core suspension insulators are studied and the failure mechanisms are analyzed after UTS and creep tests. The fracture occurs in the fiberglass rod due to the stresses induced by the metal part of the endfitting that hold the insulating components.

Numerical Method for the Investigation of Fault Containment and Tank Rupture of Power Transformers

This paper presents an analysis method based on explicit dynamic analysis, enabling detailed and representative investigations of the mechanical effects of low-impedance faults in power equipment. To illustrate the effectiveness of the proposed method, a given transformer design having caused the projection of a heavy chimney/bushing assembly over a significant distance is used as a case study. A comparison about the observed projection distance of this assembly shows that the method provides representative results, thus offering the significant advantage of replacing costly and unpractical tests. The method is then used to study the retrofit measure proposed in this case to retain the chimney, further exemplifying its power to effectively study different design and retrofitting options.

Residual Stresses Induced by Robotized Hammer-Peening

The strains induced by hammer-peening were measured by strain gauges on a mild steel plate during the hammer-peening operation. This process has recently been robotized by the research institute of Hydro-Québec (IREQ), so the hammer-peening performed for this study was done with the help of a Scompi robot. The resulting stresses calculated from the strain measurements were compared with residual stress measurements made with the hole drilling technique. The comparison shows a very good correlation of the two sets of measurements. Residual stress measurements were also made in the hammer-peened zone: as expected we found a highly biaxial compressive state of stress in this zone. An unexpected region of transverse tension was found at the end of the hammerpeened zone. This region can be very critical if hammer-peening is made with the objective of improving fatigue behaviour.

Optimization of Stresses in the End-Fittings of Composite Insulators for Distribution and Transmission Lines

This paper addresses the problem of stress calculation and optimization in a FRP (Fibre-glass Reinforced Plastic) pultruded rod crimped into a metal end-fitting. This type of assembly is used mainly for suspension and line post insulators. We discuss herein some aspects that merit consideration in any given design approach. In many respects FRP pultruded materials are anisotropic. The anisotropy of their elastic constants influences the distribution of stresses while the anisotropy of their mechanical properties (ultimate strengths) limits their load carrying capacity. Finally, the anisotropy of their failure modes governs the progression and the spread of the damage. Each level of anisotropy will be discussed distinctly. Geometrical considerations are also necessary to arrive at an efficient design. Namely, we will consider the frictional interactions between the end-fitting inner wall and the rod, then we will present our approach in selecting the appropriate LID ratio (i.e., the ratio of the end-fitting length over the rod diameter), and finally we will discuss the influence of the compression profile, i.e., the permanent radial displacement imparted to the rod due to the action of the crimping force.

A New Experimental Method for the Introduction of a Predetermined Amount of Residual Stresses in Fatigue Test Specimens

A new experimental method for the incorporation of residual stresses (RS) in standard fatigue steels specimens was developed. RS were introduced by means of high frequency (360 kHz) induction heating. Surface tensile RS resulted from cooling down the specimen subjected to a high thermal gradient. To preserve the mechanical properties of the steel, it was necessary to circulate a coolant at the center of the specimen. The 304L austenitic stainless steel does not undergo phase transformation nor micro structural changes in the solid state and was thus selected for this purpose. Multiphysics finite element (FE) analysis was used to calculate the distributed RS in the fatigue samples. These calculations were compared to XRD measurements and a very good agreement was obtained. It was therefore demonstrated that RS induced with induction heating could be numerically assessed. This conditioning method was then proved efficient to study the only influence of a predetermined amount of RS on the fatigue properties of austenitic 304L steel without undergoing influences of other parameters such as microstructure, surface finish and geometry.