Mechanical Stress Analysis Research Articles

Structure evolution and properties in a rigid polyimide prepared from its flexible poly(amic acid) precursor

AbstractSoluble poly(p‐phenylene biphenyltetracarboxamic acid) precursor was converted to the polyimide by thermal imidization at various conditions. The structure evolution being occurred during its thermal imidization over 25–400°C was investigated as a function of imidization temperature and time by X‐ray scattering with synchrotron radiation sources of Pohang Accelerator Laboratory in Korea and with conventional radiation sources. In addition, properties in the polyimide films were investigated by dynamic mechanical thermal analysis, stress‐strain analysis, prism coupling, and residual stress analysis.

Magnetic in-plane anisotropy in sputtered FeCo films and multilayers

Recent publications report a magnetic in-plane anisotropy in FeCo films and multilayers. This property can be exploited for example to polarize neutrons in a small external field and the remanent magnetization can serve to use the mirrors as a spin selective device. In this work a study on Fe 0.50Co 0.48V 0.02 films and multilayers in combination with Ti:N has been performed with regard to the connection between stress in the coating and the in-plane anisotropy using SQUID and MOKE magnetometry as well as mechanical and X-ray stress analysis. The results prove the magnetostrictive nature of the anisotropu resulting from the condensation of sputtered particles arriving at the surface under oblique incidence. The dependence of the remanence on the thickness of the deposited layer is accentuated in this work. For samples as old as 2 years we have observed a small degree of stress relaxation.

Distortion and Residual Stress in Quenched Aluminum Bars

Predictions of distortion and residual stress for rectangular bars, quenched on one surface, are compared with experimental measurements. The heat flow is unidirectional, and the dominant stress component acts along the length of the bars. Thermocouples measure the transient temperature distribution, and displacement transducers monitor the distortion of the bars into circular arcs. The thermal history inferred from an inverse heat transfer calculation drives the mechanical stress and deformation analysis. The numerical analyses demonstrate the sensitivity of the quench distortion to the thermal boundary conditions, the thermophysical properties and the constitutive model. Agreement between the predicted and measured distortion is only qualitative. The predicted residual stress field compares very well with the residual stress inferred from layer removal techniques.

Mechanical forces simulation and stress analysis of the TEXTOR vacuum vessel during plasma disruption under 3d eddy currents load

The TEXTOR vacuum vessel represents a steel torus shell with numerous radial and vertical ports. The induced eddy currents as well as electromagnetic forces in the vessel during plasma disruption have been calculated using the TYPHOON code. For the purposes of the stress analysis the vessel shells are modeled with shell elements. The bellows and flanges are built with 3D anisotropic solid elements. To apply the calculated electromagnetic forces to this model a special interface code has been developed. Stress analysis has been performed in two steps of loading in reference to symmetry and antisymmetry boundary conditions and the results have been superimposed.

Mechanical stress analysis of an LDD MOSFET structure

This paper presents a mechanical stress study of the LDD MOSFET structure. The stresses are successively evaluated through the forming steps and topological modifications are proposed in order to minimize the residual substrate stress. The analysis is based on a two dimensional finite element method. It includes the cumulative effects from the thermal oxidation, the thermal cycles and the intrinsic stresses, allowing the estimation of the evolution of the stress field during the manufacturing. A nonlinear viscoelastic approach is used to model the nitride, thermal and deposited oxides rheological behaviors. For those oxides, a complete calibration of the mechanical properties is proposed.

Dynamics of a mechanical monoleaflet heart valve prosthesis in the closing phase: effect of squeeze film.

An analysis of the dynamics of a mechanical monoleaflet heart valve prosthesis in the closing phase is presented. The backflow velocity of the fluid and the pressure distribution on the occluder during the closing phase were computed using a control volume approach in the unsteady state. Using moment equilibrium principles on the occluder motion and the squeeze film dynamics of the fluid between the occluder and the guiding strut at the instant of impact, the velocity of the occluder tip and the impact force between the occluder and the guiding struts were computed. The dynamics of fluid being squeezed between the occluder and the guiding struts are accounted for by Reynolds' equation. The effect of the fluid being squeezed between the occluder and the guiding strut was to reduce the velocity of the occluder tip at the instant of valve closure as well as to dampen the fluttering of the occluder before coming to rest in the fully closed position. The squeeze film fluid pressure changed rapidly from a high positive value (10 MPa) to a relatively large negative value (-15 MPa) in < 1 msec. The results of this study may be extended for the analysis of cavitation inception and mechanical stresses on the formed elements and valve components, as well as to estimate the endurance limits of prosthetic valves.

Design and evaluation of coils for a 50 mm diameter induction coilgun launcher

Coilguns have the ability to provide magnetic pressure to projectiles which results in near constant acceleration. The authors have developed coils to produce an effective projectile base pressure of 100 MPa (1 kbar) as a step toward reaching base pressures of 200 MPa. The design uses a scalable technology applicable to the entire range of breech to muzzle coils of a multi-stage launcher. This paper presents the design of capacitor-driven coils for launching nominal 50 mm, 350 g projectiles. Design criteria, constraints, mechanical stress analysis, launcher performance, and test results are discussed. >

Considerations of the Competent Authority Concerning the Assessment of a Brittle Fracture Safe Ductile Cast Iron (DCI) Cask Design

AbstractThe assessment of a brittle fracture safe ductile cast iron (DCI) cask design must ensure the integrity of transport and storage casks for radioactive materials under the most damaging accident conditions. Based upon the determination of the mechanical impact behaviour and stress analysis, for cask design sufficient material properties like fracture toughness must be ensured in series cask production by accepted quality assurance measures. The current BAM safety assessment concept has been established in the 1980s and is mainly based upon approval design tests with large cylindrical CASTOR and TN casks equipped with impact limiters. This concept needs only a reduced fracture mechanics analysis because of stress limitation at a level of approximately 50% of materials yield strength and appropriate quality assurance measures which ensure only tolerable crack-like defects within the cask structure in connection with guaranteed fracture toughness down to −40°1C. In other cases a detailed fracture mech...

Analysis of General Electric Model 7001 First-Stage Nozzle Cracking

Analyses of first-stage nozzle cracking in General Electric Model 7001B and 7001E industrial gas turbines are presented. Empirical algorithms are developed to predict the maximum extent of cracking that is visible on these nozzles as a function of engine cyclic history and the number of fired hours. It is shown that the algorithms predict this cracking to within a factor of two. Metallurgical analyses of nozzles show that crack growth follows the carbide-matrix interface, environmental attack occurs at the crack tip, and that the microstructure changes by increasing the amount of carbide precipitation, which increases the hardness. These metallurgical results, along with mechanical test data and stress analyses from the literature, are used to understand the nature of nozzle cracking. The maximum extent of cracking coincides with locations of maximum thermal stresses as determined by finite element analyses of similar nozzle designs. This location is at the airfoil-shroud junction on the middle vanes of multivane castings. The use of these algorithms as a predictive maintenance tool and the ability to inspect nozzles visually in the engine also are discussed.

Magnetotelluric analysis using Mohr circles

The Mohr circle, most commonly met in the analysis of mechanical stress, is used to depict magnetotelluric impedance information, taking the real and quadrature parts of magnetotelluric tensors separately. The magnetotelluric concepts of two‐dimensionality, three‐dimensionality, skew and anisotropy are then all given quantitative expression on a diagram, as are various magnetotelluric invariants. In particular, a new invariant, the “central impedance,” becomes evident in a discussion of effective impedances. Some insight is gained into impedance rotations, and an anisotropy angle is defined, analogous to skew angle. Mohr circles are also tested to depict the effects of the shear and twist operations on a regionally two‐dimensional structure. Generally, the application of shear or twist results in an impedance tensor with a Mohr circle of typical three‐dimensional form.

On the phenomenon of curved microcracks in [formula omitted] laminates: Their shapes, initiation angles and locations

A variational analysis of the stress state in microcracked cross-ply laminates has been used to investigate the phenomenon of curved microcracking in [( S)/90 n] s laminates. Previous investigators proposed that the initiation and orientation of curved microcracks are controlled by local maxima and stress trajectories of the principal stresses. We have implemented a principal stress model using a variational mechanics stress analysis and we were able to make predictions about curved microcracks. The predictions agree well with experimental observations and therefore support the assertion that the variational analysis gives an accurate stress state that is useful for modeling the microcracking properties of cross-ply laminates. An important prediction about curved microcracks is that they are a late stage of microcracking damage. They occur only when the crack density of straight microcracks exceeds some critical value-the critical crack density for curved microcracking. The predicted critical crack density for curved microcracking agrees well with experimental observations.

Effects of UV exposure and thermal history on properties of a preimidized photosensitive polyimide

AbstractThe structure and properties of a preimidized photosensitive polyimide (Probimide PSPI, a copolyimide of benzophenonetetracarboxylic dianhydride with alkyl groups substituted aromatic diamines) were studied with variations of UV exposure energy and bake temperature by means of wide angle X‐ray diffraction, dynamic mechanical thermal analysis, stress‐strain analysis, and residual stress analysis. The X‐ray diffraction patterns patterns indicate that the PSPI is amorphous in the solid state. The Tg was 378°C ∼ 410°C, depending upon the thermal history over the range of 350°C ∼ 400°C. At the glass transition region, the dynamic storage modulus E′ was very sensitive to both i‐line exposure energy and thermal history. However, the mechanical stress‐strain behavior at room temperature was primarily dependent on the thermal history. The mechanical properties were 2.6 GPa ∼ 2.9 GPa Young's modulus, 131 MPa ∼ 168 MPa tensile strength, 10% ∼ 12% yield strain, and 16% ∼ 74% elongation at break, depending upon the baking or annealing. These dynamic and static mechanical properties indicate that on the PSPI backbone, crosslinks are formed thermally as well as photochemically. The thermal crosslinks might be formed through thermal liberation of the labile alkyl groups of aromatic diamine moieties and subsequent coupling of the radicals. The thermal degradation was also evidenced in the mechanical properties degraded by baking above 375°C or annealing above 350°C. In addition, during baking and cooling, the residual stress was dynamically measured on Si wafers as a function of temperature. The stress at room temperature was 48 MPa ∼ 52 MPa for the PSPI films baked at 350°C or 400°C, regardless of i‐line exposure.

Adhesion of polymers to reinforcing fibres

AbstractA single fibre pull out technique is presented which makes it possible to measure the strength in the interface between fibre and polymer with high precision even if the embedded length of the fibre is short. The method allows measurements for all kinds of fibres in thermoplastic or thermosetting polymers.The effect of fibre surface treatment can be investigated as well as the effect of morphology or internal stresses in the polymer. Examples are given.The shear strength results are compared with results of tensile shear tests performed on symmetrically notched unidirectional reinforced composite samples. The correlation is good and it is shown that the new single fibre pull out method is able to give a better discrimination between the composites than other methods.An analysis of mechanical stress induced during the pull out demonstrates the use and limitation of this method.

Analysis of service water pump shaft failure: Shaikh, H., Khatak, H.S. and Gnanamoorthy, J.B. Prak. Metall. July 1990 27, (7), 362–368 (in English and German)

Centrifuge rotors are designed for high service life duty and should stand against 2 times the maximum operation speed in a spin tester in terms of the manufacture’s standard. A centrifuge rotor prototype had burst before reaching 2 times the maximum operation speed in an over-speed spin testing, therefore, failure analysis was required to determine the cause of the burst and recommendations for the structure optimization were needed to improve the rotor integrated strength. After the chemical analysis and the microstructure identity, a finite element model of 1/24 rotor under centrifugal load was adopted for mechanical stress analysis. The analysis indicated that local stress peaks occurred on the top round between the receptacle and the hub and on the edges of the tube cavities. By comparing computational and experimental results, it is revealed that the rotor fractured on two regions one after another when the peak stress exceeded the ultimate strength of the material. Consequently, some recommendations were made for the structure optimization to improve the rotor safety performance. The peak stress was reduced below the allowable material strength margin with increasing the fillet radius of the round and decreasing the depth of the liquid gather groove. The burst of the redesigned rotor was eliminated in the later over-speed spin testing.

Boundary elements and fracture stress analysis

In this paper we concentrate on the application of the boundary element method to problems of fracture mechanics and the particular difficulties which are germane to this field. As summarised in Atkinson I these occur because a crack is usually modelled as having a planform of zero thickness (e.g. a line crack in two dimensions) and this leads to two distinct kinds of difficulty. These are:(a) The indeterminacy when (in plane strain or stress), the direct BIE formulation of a thin ellipse is used to represent a crack when the ellipse degenerates to a line. The corresponding feature for three-dimensional problems is the BIE formulation of an ellipsoidal defect degenerating to a flat crack with elliptical planform. In Ref. 1 it was suggested that the natural way to treat crack problems of arbitrary planform was to model the crack profile by means of infinitesimal Somigliana dislocations. These dislocations are elementary solutions of the equilibrium elasticity equations where there is a specified jump in the displacement components across an infinitesimal domain. Such a formulation leads to integral equations which are more singular than the standard forms so some care is required in the implementation of this method. Attempts at implementing this approach to stationary crack problems will be reviewed in Section 2. It is noted, however, that the indeterminacy difficulty can be avoided either by subdividing the body into regions or by using special Green's functions which take the crack into account. The first method increases the size of the problem and the second works only for special cases since it requires quite a lot of preliminary analytic work to find the necessary Green's functions. (b) The second distinct feature which arises in fracture mechanics stress analysis is the occurrence of stress singularities. The most common of these being the square root stress singularity associated with a crack in a homogeneous elastic body. In this case special procedures have been developed to take into account the known character of the displacement variation along the crack faces (usually accompanying a simplifying symmetry assumption so difficulty (a) above is avoided). These include the use of special elements, the subtraction of singularities and the application of various path independent integrals which enable information about the near crack tip stress and displacement fields to be

A dislocation based analysis of continuum mechanical and microscopic local stresses during cyclic deformation of copper

A relationship between continuum mechanical internal stress variables, kinetic back stress, isotropic drag stress, and microscopic local stresses in the dislocation cell interior and cell walls, is developed based upon Mughrabi's composite model of deformation of heterogeneous microstructure during cyclic deformation in cell forming metals. The experimental data on the evolution of kinematic back stress and isotropic drag stress during cyclic deformation of Cu along with TEM measurements of cell diameter and cell width are utilized to determine the evolution of mobile and immobile dislocation densities in the cell interior and cell walls, respectively, as a function of the number of cycles. The range of values obtained is in agreement with the available experimental data.

IEE Power Division: Chairman's address. The solution of scientific and engineering problems in the development of electrical generating plant

Developments in large generating plant over the last 25 years are considered and some of the problems encountered during this period are described. These problems were tackled jointly by the operators and the manufacturers and were solved using a wide range of scientific and engineering methods. Different approaches to the design of large 500 MW generators are analysed, and these approaches are correlated to the risks and subsequent operating experience. It is shown how the successful solution of the problems led to the design of a range of larger generators having high reliability. Modern analytical techniques are described such as finite-element analysis of thermal and mechanical stresses, as well as of fields and electromagnetic stresses. Reference is made to the use of fracture mechanics to analyse the integrity of a large generator. Developments in large generator transformers are also considered, and a number of problems and their solution are described. In particular, the analysis of an overheating condition is considered, with the successful use of the information to prevent a further failure.

An Analysis of Mechanical Stress in Solid Dielectrics Caused by Discharges in Voids

The stress waves generated by electrical discharges within a void in polyester resin were analyzed quantitatively. The rise time of the stress transient acting against the void wall was 800 ns. The acoustic energy submitted to the solid was 3 nJ which was 0.002% of total energy released by the discharge. These values were determined by the original wave analysis in which the influence of the transfer functions of a propagation path and a piezoelectric transducer were eliminated from the detected waveforms using a deconvolution technique. The shape of the time function of the source is discussed. It is shown that the pressure waves from large electrical discharges within voids in polyethylene are too small to cause any mechanical damage.

Kinematic analysis of machine keys for attaching drill tubes and casings

This article describes a method for conducting mechanical testing and stress analysis under variable tensile and dynamic load conditions for machine keys used to engage and disengage the threaded joints of oil and gas well drill pipes. The method is implemented in a hydraulic test stand and enables the optimization, from a strength and weight standpoint, of the design of the keys.

Mechanical loading history and skeletal biology

A comprehensive theory which relates tissue mechanical stresses to many features of skeletal morphogenesis, growth, regeneration, maintenance and degeneration is reviewed. The theory considers the repeated or intermittent mechanical forces which consitute the loading history on the chondro-osseous skeleton. The results of numerous mechanical stress analyses indicate that the local tissue stress history plays a major role in controlling connective tissue biology. The strong influence of mechanical energy in ontogenesis implies a comparably strong influence in phylogenesis. The fact that the mechanical stress histories in skeletal tissues are directly related to the force of gravity suggests that the life forms that have evolved on Earth are closely tied to our gravitational field.