Showing papers on "Residual stress published in 2005"

Photoinduced Plasticity in Cross-Linked Polymers

Abstract: Chemically cross-linked polymers are inherently limited by stresses that are introduced by post-gelation volume changes during polymerization. It is also difficult to change a cross-linked polymer's shape without a corresponding loss of material properties or substantial stress development. We demonstrate a cross-linked polymer that, upon exposure to light, exhibits stress and/or strain relaxation without any concomitant change in material properties. This result is achieved by introducing radicals via photocleavage of residual photoinitiator in the polymer matrix, which then diffuse via addition-fragmentation chain transfer of midchain functional groups. These processes lead to photoinduced plasticity, actuation, and equilibrium shape changes without residual stress. Such polymeric materials are critical to the development of microdevices, biomaterials, and polymeric coatings.

Residual capacity of corroded reinforcing bars

Abstract: This paper presents an experimental investigation into the residual capacity of corroded reinforcing bars. By performing both accelerated and simulated corrosion tests on bare bars and on bars embedded in concrete, the mechanism of the reduction of the capacity of corroded reinforcement was investigated. The influence of type and diameter of reinforcement on its residual capacity is discussed. The experimental results show that, due to local attack penetration, the residual cross-section of a corroded bar is no longer round and varies considerably along its circumference and its length. Although the force–extension curves of corroded bars are similar to those of non-corroded bars for up to 16% corrosion, their residual yield and ultimate forces decrease more rapidly than their average cross-sectional area and, therefore, their residual strength decreases significantly. Even though the residual capacity of corroded small diameter and/or plain bars reduces more than that of large diameter or ribbed ones, di...

Growth and instability in elastic tissues

Abstract: The effect of growth in the stability of elastic materials is studied. From a stability perspective, growth and resorption have two main effects. First a change of mass modifies the geometry of the system and possibly the critical lengths involved in stability thresholds. Second, growth may depend on stress but also it may induce residual stresses in the material. These stresses change the effective loads and they may both stabilize or destabilize the material. To discuss the stability of growing elastic materials, the theory of finite elasticity is used as a general framework for the mechanical description of elastic properties and growth is taken into account through the multiplicative decomposition of the deformation gradient. The formalism of incremental deformation is adapted to include growth effects. As an application of the formalism, the stability of a growing neo-Hookean incompressible spherical shell under external pressure is analyzed. Numerical and analytical methods are combined to obtain explicit stability results and to identify the role of mechanical and geometric effects. The importance of residual stress is established by showing that under large anisotropic growth a spherical shell can become spontaneously unstable without any external loading.

Residual stresses in thin polymer films cause rupture and dominate early stages of dewetting.

Abstract: Residual stresses in thin polymer films cause rupture and dominate early stages of dewetting

Plane-strain Bulge Test for Thin Films

Abstract: The plane-strain bulge test is a powerful new technique for measuring the mechanical properties of thin films. In this technique, the stress–strain curve of a thin film is determined from the pressure-deflection behavior of a long rectangular membrane made of the film of interest. For a thin membrane in a state of plane strain, film stress and stain are distributed uniformly across the membrane width, and simple analytical formulae for stress and strain can be established. This makes the plane-strain bulge test ideal for studying the mechanical behavior of thin films in both the elastic and plastic regimes. Finite element analysis confirms that the plane-strain condition holds for rectangular membranes with aspect ratios greater than 4 and that the simple formulae are highly accurate for materials with strain-hardening exponents ranging from 0 to 0.5. The residual stress in the film mainly affects the elastic deflection of the membrane and changes the initial point of yield in the plane-strain stress–strain curve, but has little or no effect on further plastic deformation. The effect of the residual stress can be eliminated by converting the plane-strain curve into the equivalent uniaxial stress–strain relationship using effective stress and strain. As an example, the technique was applied to an electroplated Cu film. Si micromachining was used to fabricate freestanding Cu membranes. Typical experimental results for the Cu film are presented. The data analysis is in good agreement with finite element calculations.

The effect on fatigue life of residual stress and surface hardness resulting from different cutting conditions of 0.45%C steel

Abstract: The affected layer is generated within the machined surface layer through the cutting process. Cutting conditions such as the nose radius of the tool, feed rate and shape of cutting edge at the finishing operation affect the residual stress, surface hardness, and surface roughness. In this paper, it is shown that such machined surface property could be controlled by the setting of the cutting conditions to some extent. Then the effect of the machining conditions on the fatigue life was investigated through a fatigue test using the specimen finished under various cutting conditions. It was shown that it is possible to get longer fatigue life for machined parts than the virgin material or the carefully finished material without affected layer, only by setting the proper cutting conditions. Such a situation was realized when the generated residual stress was small and the induced surface hardness was high. A longer fatigue life for the machined components can be obtained by applying such cutting conditions as a low feed rate, a small corner radius and a chamfered cutting edge tool.

Residual stresses in LENS® components using neutron diffraction and contour method

Abstract: During manufacturing of components by laser engineered net shaping (LENS®), a solid freeform fabrication process, the introduction of residual stresses causes deformation or in the worst case, cracking. The origin is attributed to thermal transients encountered during solidification. In the absence of reliable predictive models for the residual stresses, measurements are necessary. Residual stresses were measured in LENS® samples of 316 stainless steel and Inconel 718 having simple geometrical shapes by both neutron diffraction and the contour methods. The results by the two methods are compared and discussed in the context of the growth direction during the LENS® process. Surprisingly, the residual stresses are practically uni-axial, with high stresses in the growth direction.

Thermo-mechanical model with adaptive boundary conditions for friction stir welding of Al 6061

Abstract: Thermo-mechanical simulation of friction stir welding can predict the transient temperature field, active stresses developed, forces in all the three dimensions and may be extended to determine the residual stress. The thermal stresses constitute a major portion of the total stress developed during the process. Boundary conditions in the thermal modeling of process play a vital role in the final temperature profile. The heating and cooling rates with the peak temperature attained by the workpiece determine the thermal stress. Also, predicting realistic peak temperature becomes important as the operating temperature at the interface of tool-workpiece is very close to the solidus temperature of the aluminum workpiece. The convection heat-transfer coefficients of the surfaces exposed to air can be theoretically determined using Newton's law of cooling. Contact conductance depends on the pressure at the interface and has a non-uniform variation. The actual pressure distribution along the interface is dependent on the thermal stress from local temperature and non-linear stress–strain state. Therefore, applying an adaptive contact conductance can make the model more robust for process parameter variations. A finite element thermo-mechanical model with mechanical tool loading was developed considering a uniform value for contact conductance and used for predicting the stress at the workpiece and backplate interface. This pressure distribution contours are used for defining the non-uniform adaptive contact conductance used in the thermal model for predicting the thermal history in the workpiece. The thermo-mechanical model was then used in predict stress development in friction stir welding.

Effect of feed rate, workpiece hardness and cutting edge on subsurface residual stress in the hard turning of bearing steel using chamfer + hone cutting edge geometry

Abstract: Residual stress on the machined surface and the subsurface is known to influence the service quality of a component, such as fatigue life, tribological properties, and distortion. Therefore, it is essential to predict and control it for enhanced performance. In this paper, a newly proposed hardness based flow stress model is incorporated into an elastic–viscoplastic finite element model of hard turning to analyze process variables that affect the residual stress profile of the machined surface. The effects of cutting edge geometry and workpiece hardness as well as cutting conditions, such as feed rate and cutting speed, are investigated. Numerical analysis shows that hone edge plus chamfer cutting edge and aggressive feed rate help to increase both compressive residual stress and penetration depth. These predictions are validated by face turning experiments which were conducted using a chamfer with hone cutting edge for different material hardness and cutting parameters. The residual stresses under the machined surface are measured by X-ray diffraction/electropolishing method. A maximum circumferential residual stress of about 1700 MPa at a depth of 40 μm is reached for hardness of 62 HRc and feed rate of 0.56 mm/rev. This represents a significant increase from previously reported results in literatures. It is found from this analysis that using medium hone radius (0.02–0.05 mm) plus chamfer is good for keeping tool temperature and cutting force low, while obtaining desired residual stress profile.

Numerical analysis of cure temperature and internal stresses in thin and thick RTM parts

Abstract: Resin transfer molding (RTM) is a widely used manufacturing technique of composite parts. Proper selection of processing parameters is critical in order to produce successful molding and to obtain a good part. Notably, when thermosetting resins are processed, the shrinkage that results from resin polymerization increases the complexity of the problem. Numerical prediction of internal stresses during composite manufacturing has three objectives: (1) to improve knowledge about the process; (2) to analyze the effects of processing parameters on the mechanical integrity of the part; and (3) to validate the principles of thermal optimization. This investigation aims to predict residual stresses and part deformation (i.e. warpage) in thin and thick composites. Accurate characterization of materials is essential for effective numerical analysis of phenomena which determine the generation of processing stresses. For this purpose, a reaction kinetics model of the resin is presented, together with a description of mechanical properties as a function of the degree of polymerization and glass transition temperature. A linear model is used to predict volume changes in glass–polyester composites. A finite difference analysis is used to simulate the effect of thermal and rheological changes during the processing of sample plates. Classical laminate theory is applied to calculate the internal stresses that result from processing conditions. These stresses are compared to determine different curing strategies for thick composite parts. Finally, a thermal optimization algorithm is applied to demonstrate the advantages of transient heating and cooling, to minimize processing stresses and avoid thermal degradation of the material or composite delamination.

Influence of bias variation on residual stress and texture in TiAlN PVD coatings

Abstract: It is recognized that the quality and performance of PVD coatings on tungsten carbide based cutting tools are strongly affected by the level of residual stress as it may cause the coatings to delaminate from the substrate and affect the tool life. Here we report on the substrate bias as an important parameter to affect the compressive stress and the texture in TiAlN PVD coatings on WC/Co cutting inserts as analyzed by X-ray diffraction. Approximately 3 μm TiAlN coatings were deposited by arc evaporation with constant process parameters except for the negative substrate bias, which was varied between 40, 70, 100, 140 and 200 V. The residual stress was analyzed using X-ray diffraction and the sin 2 ψ method was used to evaluate the data. The compressive stress increased with increasing bias from − 1.7 GPa for the coating deposited at 40 V to about − 5 GPa for the 70–200 V coatings. As seen in SEM-images, the coatings prepared at higher bias (140 and 200 V) showed a glassy-like type of structure, with {111} texture and with a lattice parameter close to TiN. For the 40 V coating, a columnar type of structure with pronounced {200} texture was observed. Nano-indentation measurements show an increase in hardness by 15% in the bias range studied. Cutting tests show a correlation between the increase of the substrate bias and the edge line coating delamination, and also give an indication of a critical residual stress level for these coatings.

Behavior of Cold-Formed High Strength Stainless Steel Sections

Abstract: This paper presents the behavior of cold-formed high strength stainless steel sections. The test specimens were cold-rolled from flat strips of duplex and high strength austenitic stainless steel. The material properties of high strength stainless steel square and rectangular hollow sections were determined. Tensile coupons at different locations in cross section were tested. Hence, the distributions of 0.2% proof stress and tensile strength measured in the cross section of cold-formed high strength stainless steel sections were plotted. The material properties of the complete cross section in the cold-worked state were also obtained from stub column tests. Detailed measurements of initial local geometric imperfections of the sections were obtained. The initial local plate imperfection profiles were plotted. Residual stress measurements of the high strength stainless steel sections were also conducted. The membrane and bending residual stress distributions in the cross section of the specimens were obtained. Furthermore, the stub column test strengths were compared with the design strengths.

Measurement of the mechanical properties of electroplated gold thin films using micromachined beam structures

Abstract: We have measured the Young’s modulus, residual stress, and stress gradient of electroplated gold thin films using surface micromachined beam structures. Cantilever and bridge beam structures of different lengths were fabricated using UV-LIGA surface micromachining and dry-release methods. The Young’s modulus and residual stress of the fabricated beams were determined from the resonance frequencies of electrostatically excited beams, and the stress gradient was evaluated from the self-deformation of released cantilevers. The observed Young’s modulus was smaller than the bulk Young’s modulus, and showed small changes depending on the deposition current density. The average residual stress was found to be tensile in nature, and the observed residual stresses showed no differences, regardless of the current density. However, the stress gradient increased with increasing current density. The deformation of the cantilever beam after release was dependent on the plasma ashing time used. This result implies that additional thermal effects from the post-deposition process may have an influence on the final performance of fabricated micro electro mechanical systems (MEMS) devices.

An experimental method to study the frictional processes during composites manufacturing

Abstract: Composite parts manufactured by autoclave processes have manufacturing distortions due to unavoidable physical mechanisms that take place during the process. A good understanding of these mechanisms is required to manufacture parts with better dimensional fidelity and reduce the residual stresses locked in parts during manufacturing. One of these mechanisms is the friction between the tool and the part, and this mechanism can have a major influence on the final geometry of flat sections if the thermal expansion coefficients of the tool and the part are different. Tool interaction causes the stretching of the low shear modulus, curing composite and results in in-plane residual stresses with through-the-thickness gradients that cause distortion. In addition to the tool–part interaction, the prepregs laid up to form a corner section in more complex geometries may slip with respect to each other as a result of consolidation, causing stretching of fibres close to the inner surface of the parts manufactured in a female tool. In previous studies, considerable effort has been spent on modelling the tool–part interaction. These simulations are based on semi-empirical models, which should be calibrated according to geometrical deformations observed in manufactured parts. This paper presents an novel experimental method to directly measure the frictional shear stresses at prepreg/prepreg and prepreg/tool interfaces as a function of the development of cure throughout the Manufacturer's Recommended Cure Cycle (MRCC).

Optimization of electromechanical coupling for a thin-film PZT membrane: II. Experiment

Abstract: In this two-part paper, the optimization of the electromechanical coupling coefficient for thin-film piezoelectric devices is investigated both analytically and experimentally. The electromechanical coupling coefficient is crucial to the performance of piezoelectric energy conversion devices. A membrane-type geometry is chosen for the study. In part I a one-dimensional model is developed for a membrane composed of two layers, a passive elastic material and a piezoelectric material. The lumped-parameter model is then used to explore the effect of design and process parameters, such as residual stress, substrate thickness, piezoelectric thickness and electrode coverage, on the electromechanical coupling coefficient. The model shows that the residual stress has the most substantial effect on the electromechanical coupling coefficient. For a given substrate material and thickness an optimum piezoelectric thickness can be found to achieve the maximum coupling coefficient. The substrate stiffness affects the magnitude of the maximum coupling coefficient that can be obtained. Electrode coverage was found to be important to electromechanical coupling. The model predicts an optimum electrode coverage of 42% of the membrane area. The model developed in part I formed the basis for the parameters studied experimentally in part II.

The finite element simulation of the friction stir welding process

Abstract: Solid mechanics-based finite element models and computational procedures are developed to study the flow patterns and the residual stresses in frictional stir welding (FSW). Two-dimensional results of the material flow patterns and the residual stresses are presented. The flow of metal during FSW is investigated using tracer particles. It is shown that the flows on the advancing side and retreating side are different. After several rotations the material which is rotating around the nib sloughs off in its wake of the pin, primarily on the advancing side. The residual stresses of the welded plate are investigated in this analysis. The distribution of the longitudinal residual stress along the direction perpendicular to the welding line is a double feature curve. With the increase of the translational velocity, the maximum longitudinal residual stress can be increased.

Constrained Densification of Alumina/Zirconia Hybrid Laminates, II: Viscoelastic Stress Computation

Abstract: To analyze the inhibited densification during sintering and differential shrinkage during cooling of Al2O3/ZrO2symmetric and asymmetric laminates, viscoelastic formulations, in which the viscosity and elastic modulus vary with time, have been developed. The viscoelastic mismatch stresses have been numerically computed over the entire processing cycle, including the heating period, the isothermal period, and the cooling period. The viscosity and free sintering rates that are needed for stress computation have been obtained by modifying the parameters that are measured for a normal isotropic densifying compact using cyclic loading dilatometry. The modification is based on the available sintering models to account for the effect of strain history on compact viscosity and sintering rates. The stress calculation shows that, with the exception of the initial heating period, the viscoelastic stress is identical to the viscous stress that is calculated solely from the strain rate mismatch. Sintering damage in the laminates is shown to occur during densification under conditions where the differential sintering stress is smaller than the intrinsic sintering pressure. The magnitude of residual stress in hybrid laminates on cooling is dependent on the cooling rate, and slower cooling rates are capable of almost completely relaxing the expansion mismatch stress at temperatures of >1200°C.

Effect of the composition and sintering process on mechanical properties and residual stresses in zirconia-alumina composites

Abstract: Samples of zirconia-toughened alumina (ZTA) with small amounts of chromia and magnetoplumbite-type crystalline phase (CeMgAl 11 O 19 ) have been prepared and processed under different conditions. Mechanical properties like hardness and fracture toughness were examined as a function of different parameters. As an example, fracture toughness was increased by the chromia addition, whereas platelets reinforcement addition suppressed the tetragonal zirconia (t-zirconia)–monoclinic zirconia (m-zirconia) transformation. In addition, transformability of the tetragonal zirconia and the residual stress in the alumina phase were examined by Raman and fluorescence piezo-spectroscopy, respectively. In particular, the extent to which t-zirconia transforms to m-zirconia was determined by Raman spectroscopy after Vickers indentation and the transformability was correlated to the fracture toughness. It was demonstrated that the monoclinic content and the toughness were correlated linearly and experimental results were compared with models already available for zirconia-based materials. On the other hand, residual stresses originated by transformation toughening mechanism were correlated to the transformability of the tetragonal phase.