Mechanisms of fatigue failures

Fatigue failures account for 90% of strength failures in engineering components. In order to analyse such failures and learn to prevent them in future, it is necessary to understand how and why fatigue failures occur. In this paper, an attempt has been made to contribute to such understand¬ing. The concept of the weakest link in a structure initiating failure is discussed. It is shown that the weakest link principle can be extended to explain multiple origin of fatigue cracks. The micromechanisms that are responsible for the initiation of a fatigue crack and those that come into play during the growth process are highlighted. The formation of typical features of fatigue fractures, like striations and beachmarks, are dis-cussed in relation to the mechanisms operative during fatigue crack growth. For the choice of materials for providing service under fatigue loading conditions, and to assess the integrity of components and structures already in service under such conditions, the fatigue resistance of material has to be quantified. The conventional approach to such quanti-fication and the more recent fracture mechanics based differential approach to representing the fatigue resistance of materials are discussed

Advances in computer-aided crack length measurement during fatigue crack growth testing

The accurate measurement of crack length is one of the most important aspect of fatigue crack growth rate (FCGR) testing. Of the various methods available for crack length measurement, compliance technique is very popular due to the facilities it provides for easy automation. In the compliance technique, compliance crack length (CCL) rela-tions are used for correlating the compliance, computed from measurements of displacements & loads during fatigue cycling, to the crack length contained in the specimen. CCL relations are specific not only to the specimen geometry, but also to the location on the specimen body at which displacements are measured. This specificness is not very conducive to the experimentalist as it introduces errors in the measured crack length if the location of displacement measurement is not accurately maintained. With variations in specimen geometry and size, the accu-rate positioning of displacement measurement transducers is not an easy task. In order to provide greater flexi-bility in the use of the compliance technique, a new scheme has been proposed in this paper. Modelling the defo-rmation of a fracture mechanics specimen during fatigue cycling as rotation of two rigid hinge about a hringe point, the relationship between the location of the hinge-point with crack length has been established using finite element analysis for the single-edge notched three point bend specimen. Further -an iterative method has been developed which can be implemented in the background software for on-line crack length measurement. It has been shown that the iterative method converges rapidly to give the crack length with high accuracy

Recent Advances in Heat-Treat Technology

In the last ten to fifteen years, heat treating techno-logy has witnessed a lot of advances. The introduction of new alloys, like duplex stainless steel, micro- alloyed steel, HSLA steels, low-cobalt maraging steels, austempered ductile iron, directionally solidified and single crystal superalloys, aluminium-lithium alloys, various metal matrix composites etc., have called for new heat treatments based on structure-property corre-lations. There have also been changes in heat treatment processes, including improvements in continuous annealing, induction heating, and surface hardening operations using laser or electron beams, establishment of the commer-cial viability of plasma-assisted case-hardening processes, and advances in thermo-mechanical processing. However, the most dramatic advances have taken place in the area of newly developed tools for improving process character-ization and process control. Notable amongst such devel-opments are the improved instrumentation for controlling furnace temperature, furnace atmosphere, and surface carbon content, the practical application of statistical process control (SPC), the application of computer mode-lling for prediction of hardness profiles, quantitative modelling of tempered on case hardened properties, and the computer-assisted object oriented selection of materials and their heat treatment processing. Many of the advances listed above are being covered in some of the other lect-ures in this workshop. A selection of the items mentioned above are discussed in a general manner in this paper

Analysis of potential flow around two-dimensional body by finite element method

The paper presents a numerical method for analyzing the potential flow around two dimensional body such as single circular cylinder, NACA0012 hydrofoil and double circular cylinders by finite element method. The numerical technique is based upon a general formulation for the Laplace’s equation using Galerkin technique finite element approach. The solution of the systems of algebraic equations is approached by Gaussian elimination scheme. Laplace’s equation is expressed in terms of both steam function and velocity potential formulation. A finite element program is developed in order to analyze the result. The contours of stream and velocity potential function are drawn. The contour of stream function exhibits the characteristics of potential flow and does not intersect each other. The calculated pressure co-efficient shows the pressure decreasing around the forwarded face from the initial total pressure at the stagnation point and reaching a minimum pressure at the top of the cylinder

Compliance crack length relations for the four-point bend specimen

Compliance crack length relations for the four-point bend specimen geometry have not been reported in the literature in spite of this geometry being one of the popularly used specimens for fatigue crack growth studies. An effort has been made in the present work to fill this gap. Accordingly, the finite element technique was employed to simulate loading and calculate displacements at various locations in a four-point bend specimen. The load-displacement data thus obtained were processed to yield compliance crack length relations. These relations were employed to calculate the crack length during fatigue testing of four-point bend specimens in which the crack length was also measured by optical means. A good correlation was observed between the predicted crack length and that measured optically

Evaluation of Mechanical Properties and Mechanisms of Deformation and Fracture

Determination of mechanical properties, together with metallographic examination of microstructures, is the basis for evaluating the quality of metallic materials and their suitability for application, particularly for engineering purposes. To the engineer, mechanical prop-erties are important for selection of materials and for the design of components and structures made from them. Mechanical properties determine the "range" of useful- ness of a material, and establish the "service" that can be expected from it

Analysis of damage in high strength steels

In continuum damage mechanics (CDM) approach, damage accumulation takes place through void initiation, growth and coalescence. In this study, a Bayesian Neural Network based model has been developed to calculate the complex relationship between the extent of damage accumulation and its influencing parameters for a variety of high strength low alloy steels. The model has been applied to confirm that the predictions are reasonable

Corrosion of stressed components

Many engineering components operate under the combined influence of service stresses and deleterious environment. For better life prediction and to prevent failures, it is necessary to understand the mechanical behaviour of materials under conjoint action of stress and environment. Depending upon the metal-stress-environment system, the operating mechanism that would affect the life of the component may differ. Some of these mechanisms are discussed in this paper. Typically, the life limiting factor of engineering materials in corrosive environments have been identified as stress corrosion cracking, damage in components due to the presence of hydrogen, corrosion fatigue and fretting corrosion. In this paper, each of these phenomena has been discussed with respect to their mechanisms, methods to quantify the damage and ways to prevent or control them. Developments in fracture mechanics have been used as an effective tool in assessing the materials resistance to fracture. Application offracture mechanics techniques to predict the fracture behaviour of materials in aggressive environments has been discussed along with the conventional techniques

Component Integrity Evaluation

The increasing demand for higher output efficiency coupled with rising material costs have forced man-made structures to be very critically designed.Components are now pushed to their limits, having to operate at higher stress levels and in more severe environments. Moreover, the conseque-nces at stake should a component fail are now greater than ever. Hence their design has to conform to the higher stan-dards of safety demanded

Microstructural damage evaluation in Ni-based superalloy gas turbine blades by fractal analysis

Ni-based superalloys are used as turbine disc and blade material in which creep, fatigue and creep-fatigue are the important damage mechanisms. Mechanical properties of these alloys depend upon the amounts of gamma-gamma-prime present in the microstructure as well as precipitation of carbides along the grain boundaries. The distribution of gamma-prime depends on the chemical composition, operating temperature and the length of service exposure. During service exposure, as damage accumulates progressively, the morphological characteristics of microstructure change which needs to be assessed using metallographic technique. Conventionally, the extent of damage resulting in deterioration of mechanical properties is quantified by hardness measurement. The variation in hardness is correlated with the morphological features in the metalographic images by identifying precipitation of carbides, presence of cuboidal gamma-prime and the structural changes that occur in the matrix. In this paper, we report fractal dimensions of the insitu metallographic images which can correlate the progressive damage accumulation at various locations of the blades