Showing papers on "Residual stress published in 2007"

Residual stress and its role in failure

Abstract: Our safety, comfort and peace of mind are heavily dependent upon our capability to prevent, predict or postpone the failure of components and structures on the basis of sound physical principles While the external loadings acting on a material or component are clearly important, There are other contributory factors including unfavourable materials microstructure, pre-existing defects and residual stresses Residual stresses can add to, or subtract from, the applied stresses and so when unexpected failure occurs it is often because residual stresses have combined critically with the applied stresses, or because together with the presence of undetected defects they have dangerously lowered the applied stress at which failure will occur Consequently it is important that the origins of residual stress are understood, opportunities for removing harmful or introducing beneficial residual stresses recognized, their evolution in-service predicted, their influence on failure processes understood and safe structural integrity assessments made, so as to either remove the part prior to failure, or to take corrective action to extend life This paper reviews the progress in these aspects in the light of the basic failure mechanisms

The influence of Johnson-Cook material constants on finite element simulation of machining of AISI 316L steel

Abstract: In literature, five different sets of work material constants used in the Johnson–Cook's (J–C) constitutive equation are implemented in a numerical model to describe the behaviour of AISI 316L steel. The aim of this research is to study the effects of five different sets of material constants of the J–C constitutive equation in finite-element modelling of orthogonal cutting of AISI 316L on the experimental and predicted cutting forces, chip morphology, temperature distributions and residual stresses. Several experimental equipments were used to estimate the experimental results, such as piezoelectric dynamometer for cutting forces measurements, thermal imaging system for temperature measurements and X-ray diffraction technique for residual stresses determination on the machined surfaces; while an elastic–viscoplastic FEM formulation was implemented to predict the local and global variables involved in this research. It has been observed that all the considered process output and, in particular the residual stresses are very sensitive to the J–C's material constants.

A literature survey on the stability and significance of residual stresses during fatigue

Abstract: Many manufacturing processes can induce residual stresses in components. These residual stresses influence the mean stress during cyclic loading and so can influence the fatigue life. However, the initial residual stresses induced during manufacturing may not remain stable during the fatigue life. This paper provides a broad and extensive literature survey addressing the stability of surface and near-surface residual stress fields during fatigue, including redistribution and relaxation due to static mechanical load, repeated cyclic loads, thermal exposure and crack extension. The implications of the initial and evolving residual stress state for fatigue behaviour and life prediction are addressed, with special attention to fatigue crack growth. This survey is not a critical analysis; no detailed attempt is made to evaluate the relative merits of the different explanations and models proposed, to propose new explanations or models or to provide quantitative conclusions. Primary attention is given to the residual stresses resulting from four major classes of manufacturing operations: shot peening and related surface treatments, cold expansion of holes, welding and machining.

Domain wall contributions to the properties of piezoelectric thin films

Abstract: In bulk ferroelectric ceramics, extrinsic contributions associated with motion of domain walls and phase boundaries are a significant component of the measured dielectric and piezoelectric response. In thin films, the small grain sizes, substantial residual stresses, and the high concentration of point and line defects change the relative mobility of these boundaries. One of the consequences of this is that thin films typically act as hard piezoelectrics. This paper reviews the literature in this field, emphasizing the difference between the nonlinearities observed in the dielectric and piezoelectric properties of films. The effect of ac field excitation levels, dc bias fields, temperature, and applied mechanical stress are discussed.

Residual stresses in thermoplastic composites - a study of the literature. Part III: Effects of thermal residual stresses

Abstract: This paper is the third part in a series of review papers on residual stresses in thermoplastic composites. The first two parts were dedicated to the formation of thermal residual stresses and experimental techniques to detect these stresses, respectively. This third paper discusses the effects that thermal residual stresses have on the material properties of thermoplastic composites. Composite laminates as well as composite structures were considered. Residual stresses were found to affect the composites’ matrix-dominated, mechanical and durability properties, as well as to inflict damage and dimensional instability. Several mechanisms to relieve thermal residual stresses are proposed.

Modelling the effects of tool-edge radius on residual stresses when orthogonal cutting AISI 316L

Abstract: Tool-edge geometry has significant effects on the cutting process, as it affects cutting forces, stresses, temperatures, deformation zone, and surface integrity. An Arbitrary-Lagrangian–Eulerian (A.L.E.) finite element model is presented here to simulate the effects of cutting-edge radius on residual stresses (R.S.) when orthogonal dry cutting austenitic stainless steel AISI 316L with continuous chip formation. Four radii were simulated starting with a sharp edge, with a finite radius, and up to a value equal to the uncut chip thickness. Residual stress profiles started with surface tensile stresses then turned to be compressive at about 140 μm from the surface; the same trend was found experimentally. Larger edge radius induced higher R.S. in both the tensile and compressive regions, while it had almost no effect on the thickness of tensile layer and pushed the maximum compressive stresses deeper into the workpiece. A stagnation zone was clearly observed when using non-sharp tools and its size increased with edge radius. The distance between the stagnation-zone tip and the machined surface increased with edge radius, which explained the increase in material plastic deformation, and compressive R.S. when using larger edge radius. Workpiece temperatures increased with edge radius; this is attributed to the increase in friction heat generation as the contact area between the tool edge and workpiece increases. Consequently, higher tensile R.S. were induced in the near-surface layer. The low thermal conductivity of AISI 316L restricted the effect of friction heat to the near-surface layer; therefore, the thickness of tensile layer was not affected.

Welding Residual Stresses in Ferritic Power Plant Steels

Abstract: Many of the degradation mechanisms relevant to power plant components can be exacerbated by stresses that reside within the material. Good design or structural integrity assessments require therefore, an accounting of residual stresses, which often are introduced during welding. To do this it is necessary to characterise the stresses, but this may not be possible in thick components using non-destructive methods. These difficulties, and a paucity of relevant engineering data, have led to an increasing emphasis on the development and validation of suitable modelling tools. Advances are prominent in the estimation of welding residual stresses in austenitic stainless steels. The progress has been less convincing in the case of ferritic alloys, largely due to the complexities associated with the solid state phase transformations that occur in multipass welding. We review here the metallurgical issues that arise in ferritic steel welds, relate these to the difficulties in calculating residual stresses,...

Residual magnetic field sensing for stress measurement

Abstract: The evaluation of both applied and residual stresses in engineering structures to provide early indications of stress status and eventual failure is a fast growing area in non-destructive testing. Much work has been done in recent years in the development of magnetic stress measurement techniques for ferromagnetic materials using applied magnetic fields to monitor changes in the magnetic properties of materials, such as variations in the hysteresis curve or Barkhausen emission. But the area of passive field measurement is relatively unexplored. When magnetic metals are strained, they irreversibly transformed from a non-magnetic state to a magnetic state, this is referred to as metal magnetic memory (MMM) or the residual magnetic field (RMF). This paper investigates the phenomena under different circumstances and applies the residual magnetic field technique to stress measurement. A three axis magneto-resistive magnetic field sensor is used to measure the residual magnetic fields parallel to the applied stress and the material surface (Bx) and perpendicular to the material surface (Bz), generated by the magneto-mechanical effect without the application of an external field, using steel samples exposed to stresses. The test results show that without using an applied field, the stresses in a sample can be measured using magnetic field sensing, with Bx showing particularly good correlation. The work concludes that the novel passive field technique including analysis of the magnetic field pattern and magnetic field variation rate, would prove advantageous in certain circumstances, for example in-service inspection of structures with complex geometries. Further research directions are also highlighted.

Methods for obtaining the strain-free lattice parameter when using diffraction to determine residual stress

Abstract: The determination of residual stress by diffraction depends on the correct measurement of the strain-free lattice spacing d_{hkl}^0, or alternatively the enforcement of some assumption about the state of strain or stress within the body. It often represents the largest uncertainty in residual stress measurements since there are many ways in which the strain-free lattice spacing can vary in ways that are unrelated to stress. Since reducing this uncertainty is critical to improving the reliability of stress measurements, this aspect needs to be addressed, but it is often inadequately considered by experimenters. Many different practical strategies for the determining of d_{hkl}^0 or dref have been developed, some well known, others less so. These are brought together here and are critically reviewed. In practice, the best method will vary depending on the particular application under consideration. Consequently, situations for which each method are appropriate are identified with reference to practical examples.

Finite element modeling the influence of edge roundness on the stress and temperature fields induced by high-speed machining.

Abstract: High-speed machining (HSM) may produce parts at high production rates with substantially higher fatigue strengths and increased subsurface micro-hardness and plastic deformation, mostly due to the ploughing of the round cutting tool edge associated with induced stresses, and can have far more superior surface properties than surfaces generated by grinding and polishing. Cutting edge roundness may induce stress and temperature fields on the machined subsurface and influence the finished surface properties, as well as tool life. In this paper, a finite element method (FEM) modeling approach with arbitrary Lagrangian Eulerian (ALE) fully coupled thermal-stress analysis is employed. In order to realistically simulate HSM using edge design tools, an FEM model for orthogonal cutting is designed, and solution techniques such as adaptive meshing and explicit dynamics are performed. A detailed friction modeling at the tool–chip and tool–work interfaces is also carried out. Work material flow around the round edge cutting tool is successfully simulated without implementing a chip separation criterion and without the use of a remeshing scheme. The FEM modeling of the stresses and the resultant surface properties induced by round edge cutting tools is performed for the HSM of AISI 4340 steel. Once FEM simulations are complete for different edge radii and depths of cut, the tool is unloaded and the stresses are relieved. Predicted stress fields are compared with experimentally measured residual stresses obtained from the literature. The results indicate that the round edge design tools influence the stress and temperature fields greatly. An optimization scheme can be developed to identify the most desirable edge design by using the finite element analysis (FEA) scheme presented in this work.

Adhesion improvements of Thermal Barrier Coatings with HVOF thermally sprayed bond coats

Abstract: Thermal barrier coatings (TBC) are an effective engineering solution for the improvement of in service performance of gas turbines and diesel engine components. The quality and further performance of TBC, likewise all thermally sprayed coatings or any other kind of coating, is strongly dependent on the adhesion between the coating and the substrate as well as the adhesion (or cohesion) between the metallic bond coat and the ceramic top coat layer. The debonding of the ceramic layer or of the bond coat layer will lead to the collapse of the overall thermal barrier system. Though several possible problems can occur in coating application as residual stresses, local or net defects (like pores and cracks), one could say that a satisfactory adhesion is the first and intrinsic need for a good coating. The coating adhesion is also dependent on the pair substrate-coating materials, substrate cleaning and blasting, coating application process, coating application parameters and environmental conditions. In this work, the general characteristics and adhesion properties of thermal barrier coatings (TBCs) having bond coats applied using High Velocity Oxygen Fuel (HVOF) thermal spraying and plasma sprayed ceramic top coats are studied. By using HVOF technique to apply the bond coats, high adherence and high corrosion resistance are expected. Furthermore, due to the characteristics of the spraying process, compressive stresses should be induced to the substrate. The compressive stresses are opposed to the tensile stresses that are typical of coatings applied by plasma spraying and eventually cause delamination of the coating in operational conditions. The evaluation of properties includes the studies of morphology, microstructure, microhardness and adhesive/cohesive resistance. From the obtained results it can be said that the main failure location is in the bond coat/ceramic interface corresponding to the lowest adhesion values.

Process Maps for Predicting Residual Stress and Melt Pool Size in the Laser-Based Fabrication of Thin-Walled Structures

Abstract: Thermomechanical models are presented for the building of thin-walled structures by laser-based solid freeform fabrication (SFF) processes. Thermal simulations are used to develop quasi-non-dimensional plots (termed process maps) that quantify the effects of changes in wall height, laser power, deposition speed, and part preheating on thermal gradients, with the goal of limiting residual stresses in manufactured components. Mechanical simulations are used to demonstrate the link between thermal gradients and, maximum final residual stresses. The approach taken is analogous to that taken in previous research by the authors in developing process maps for melt pool length, for maintaining an optimal melt pool size during component fabrication. Process maps are tailored for application to the laser engineered net shaping process; however, the general approach, insights, and conclusions are applicable to most SFF processes involving a moving heat source, and to other laser-based fusion processes. Results from the residual stress simulations identify two mechanisms for reducing residual stresses and quantify maximum stress reductions achievable through manipulation of all process variables. Results from thermal gradient and melt pool length process maps are used to identify a manufacturing strategy for obtaining a consistent melt pool size while limiting residual stress in a thin-walled part.

Finite element analysis of temperature distribution in single metallic powder layer during metal laser sintering

Abstract: The metal laser sintering (MLS) is used to make strong or hard metallic models for tools and dies directly from metallic powders. Thermal distortion is the serious problem after cooling of the solidified part rapidly. Uncontrolled temperature distribution in the metallic powder layer leads to thermal distortion of the solidified part. The study of temperature distribution within the metallic layer during MLS is important from the quality of the layer point of view. The high temperature generated in the powder layer leads to thermal distortion of the part and causes thermal as well as residual stresses in the part. In this paper the powder layer is assumed to be subjected to plane stress type of temperature variation and a transient finite element method-based thermal model has been developed to calculate the temperature distribution within a single metallic layer during MLS. A finite element code has been developed and validated with the known results from the literature. The obtained results of temperature distribution show the temperature and temperature gradient variation along X- and Y-axis. The effect of process parameters such as laser power, beam diameter, laser on-time, laser off-time and hatch spacing on temperature distribution within a model made of titanium during MLS is studied. The results computed by the present model agree with experimental results. Temperature increases with increase in laser power and laser on-time but temperature decreases with increase in laser off-time and hatch spacing.

Methods for the obtaining the strain-free lattice parameter when using diffraction to determine residual stress

Abstract: The determination of residual stress by diffraction depends on the correct measurement of the strain-free lattice spacing d(hkl)(0), or alternatively the enforcement of some assumption about the state of strain or stress within the body. It often represents the largest uncertainty in residual stress measurements since there are many ways in which the strain-free lattice spacing can vary in ways that are unrelated to stress. Since reducing this uncertainty is critical to improving the reliability of stress measurements, this aspect needs to be addressed, but it is often inadequately considered by experimenters. Many different practical strategies for the determining of d(hkl)(0) or d(ref) have been developed, some well known, others less so. These are brought together here and are critically reviewed. In practice, the best method will vary depending on the particular application under consideration. Consequently, situations for which each method are appropriate are identified with reference to practical examples.

FEM calculation of residual stresses induced by laser shock processing in stainless steels

Abstract: Laser shock processing, also known as laser shock peening, generates through a laser-induced plasma, plastic deformation and compressive residual stresses in materials for improved fatigue or stress corrosion cracking resistances. The calculation of mechanical effects is rather complex, due to the severity of the pressure loading imparted in a very short time period (in the ns regime). This produces very high strain rates (106 s−1), which necessitate a precise determination of dynamic properties.Finite element techniques have been applied to predict the residual stress fields induced in two different stainless steels, combining shock wave hydrodynamics and strain rate dependent mechanical behaviour. The predicted residual stress fields for single or multiple laser processes were correlated with those from experimental data, with a specific focus on the influence of process parameters such as pressure pulse amplitude and duration, laser spot size or sacrificial overlay.Among other results, simulations confirmed that the affected depths increased with pulse duration, peak pressure and cyclic deformations, thus reaching much deeper layers (> 0.5 mm) than with any other conventional surface processing. To improve simulations, the use of experimental VISAR determinations to determine pressure loadings and elastic limits under shock conditions (revealing different strain-rate dependences for the two stainless steels considered) was shown to be a key point.Finally, the influence of protective coatings and, more precisely, the simulation of a thermo-mechanical uncoated laser shock processing were addressed and successfully compared with experiments, exhibiting a large tensile surface stress peak affecting a few tenths of micrometres and a compressive sub-surface stress field.

Comparison of the fatigue behavior and residual stress stability of laser-shock peened and deep rolled austenitic stainless steel AISI 304 in the temperature range 25–600 °C

Abstract: Laser-shock peening and deep rolling were applied to an austenitic stainless steel AISI 304 and the effects on the fatigue life were investigated. Isothermal push–pull fatigue tests were performed in the temperature range 25–600 °C. The results of the investigations, e.g. s/n curves, cyclic deformation curves as well as the residual stress stability of the laser-shock peened condition were compared to untreated and deep rolled conditions. Near-surface regions were characterized using X-ray diffraction methods, transmission electron microscopy (TEM) and focused ion beam microscopy (FIB). The different near-surface microstructures as well as residual stress stabilities of laser-shock peened and deep rolled specimens were investigated and discussed.

Analytical Solution for Size-Dependent Elastic Field of a Nanoscale Circular Inhomogeneity

Abstract: Two-dimensional elastic field of a nanoscale circular hole/inhomogeneity in an infinite matrix under arbitrary remote loading and a uniform eigenstrain in the inhomogeneity is investigated. The Gurtin-Murdoch surface/interface elasticity model is applied to take into account the surface/interface stress effects. A closed-form analytical solution is obtained by using the complex potential function method of Muskhelishvili. Selected numerical results are presented to investigate the size dependency of the elastic field and the effects of surface elastic moduli and residual surface stress. Stress state is found to depend on the radius of the inhomogeneity/hole, surface elastic constants, surface residual stress, and magnitude of far-field loading.

Residual stresses in thermoplastic composites—A study of the literature—Part II: Experimental techniques

Abstract: This study forms part of a literature survey on residual stresses in continuous fibre reinforced thermoplastic composites. After identification of the factors responsible for the thermal residual stress build-up in Part I [Parlevliet PP, Bersee HEN, Beukers A. Residual stresses in thermoplastic composites – A study of the literature – Part I: Formation of residual stresses. Composites Part A – Appl Sci Manuf, in press, doi:10.1016/j.compositesa.2005.12.025 ], experimental techniques to determine the magnitude and distribution of these stresses on various mechanical levels are outlined. The techniques are placed in the following categories: techniques utilising the intrinsic composite constituents’ material properties; techniques employing embedded “foreign” stress sensors; techniques based on in-plane and out-of-plane deformations, and destructive techniques.

Glass: Mechanics and Technology

Abstract: INTRODUCTION GLASS, A CERAMIC MATERIAL Four Classes of Materials Ashby's Charts Co-Selecting Materials Performance Indexes Shape Factors in Mechanical Design GLASS PRE- AND HISTORY Natural Glasses Early Glasses First Optical Glasses Modern Glasses APPLICATIONS OF GLASSES Glazing Packaging Optical Glass Glass Fibres for Insulation and Reinforcement Abrasive Tools Glass Manufacturers GLASS STRUCTURE Introduction Silica Glass and Related Glasses Organic and Chalcogenide Glasses Avoiding Crystallization Surface Structure GLASS RHEOLOGY Viscosity Glass Transition and its Observation How to 'Observe' Glass Transition? Viscous Response of Glass Visco-Elastic Response of Glass Tempering of Glass MECHANICAL STRENGHT OF GLASS Theoretical Strength Tensile Resistance of Glass Stress Concentration Linear Elasticity Crack Tip Stress Field Toughness Measurement Influence of Residual Stress on Strength and Fragmentation Statistic Weibull Analysis CONTACT RESISTANVCE OF GLASS Sharp and Blunt Contact Sharp Contact Resistance Abrasion Resistance Cutting of Glass Application to other Materials AGEING OF GLASS Stress Corrosion Charles and Hillig Theory Life Time Under Static Fatigue Applications MECHANICS OF GLASS PROCESSES Introduction Float Process Blow-and-Blow and Press-and-Blow Process Fusion Draw PRODUCTION AND CONTROL OF RESIDUAL STRESSES Introduction Residual Stresses in Flat Glass Basics of Photoelasticity in Flat Glass Stress Meters HIGH-TECH PRODUCERS AND R&D Market Trend Driven R&D Flat Display Panel Thin Film Technology Residual Stresses in Thin Film Applications Conclusion APPENDICES

Fatigue and mechanical characteristics of nano-structured tool steel by ultrasonic cold forging technology

Abstract: Ultrasonic cold forging technology (UCFT) utilizing ultrasonic vibration energy is a method to induce severe plastic deformation to a material surface, therefore, the structure of the material surface becomes a nano-crystal structure from the surface to a certain depth. It improves the mechanical properties; hardness, compressive residual stress, wear and fatigue characteristics. Applying UCFT to a rolling process in the steel industry is introduced in this study. First, the UCFT specimens of a tool steel (SKD-61/equivalent H13) are prepared and tested to verify the effects of the UCFT in a variety of mechanical properties, the UCFT is applied to the trimming knives in a cold rolling process. It has been determined that UCFT improves the mechanical properties effectively and becomes a practical method to improve productivity and reliability by about two times compared with the conventionally treated tooling in the trimming process in a cold rolling line.