Stress Reduction Factor Research Articles

Semi-empirical procedures for evaluating liquefaction potential during earthquakes

Semi-empirical procedures for evaluating the liquefaction potential of saturated cohesionless soils during earthquakes are re-examined and revised relations for use in practice are recommended. The stress reduction factor ( r d), earthquake magnitude scaling factor for cyclic stress ratios (MSF), overburden correction factor for cyclic stress ratios ( K σ ), and the overburden normalization factor for penetration resistances ( C N) are discussed and recently modified relations are presented. These modified relations are used in re-evaluations of the SPT and CPT case history databases. Based on these re-evaluations, revised SPT- and CPT-based liquefaction correlations are recommended for use in practice. In addition, shear wave velocity based procedures are briefly discussed.

Fatigue life duration prediction for welded spots by volumetric method

The mechanical behavior of spot-welds under tensile-shear for one, three and five spot-welds is investigated in detail. Volumetric approach is applied to find fatigue life duration of welded spots. The three-dimensional finite element models for different geometries, including gap effects and non-linear features of materials are applied to obtain fatigue life. The elastic–plastic stress distribution at edge of hot spot-weld is considered and effective stress, effective distance and notch strength reduction factor are obtained. The comparison between experimental and volumetric approach reveals good agreement with real fatigue life.

Durability of FRP in concrete ? current specifications and a new approach

The existing design codes and guidelines in Japan, Canada, the USA and Norway are summarised and compared. A new approach to durability specification for FRP in concrete, emerging from the work of fib Task Group 9.3 is also presented. This has been designed to take account of specific environmental aggressivities, a concept that is already familiar to engineers designing concrete for durability. The philosophy adopted identifies the main aggressive situations and introduces a series of stress reduction factors to account for potential deterioration of FRP in these environments. The factors also allow for the relative resistance of different FRP types to the various aggressive environments and the desired design life of the structure. This approach is therefore considerably more flexible and less conservative than existing methodologies in current international design codes.

Quantifying stresses and support requirements in the undercut and production level drifts of block and panel caving mines

A numerical modelling strategy has been developed in order to quantify the magnitude of induced stresses at the boundaries of production level and undercut level drifts for various in situ stress environments and undercut scenarios. The results of the stress modelling were in line with qualitative experiential guidelines and a limited number of induced stress measurements documented from caving sites. A number of stress charts were developed which quantify the maximum boundary stresses in drift roofs for varying in situ stress regimes, depths and undercut scenarios. This enabled many of the experiential guidelines to be quantified and bounded. A limited number of case histories of support and support performance in cave mine drifts were compared to support recommendations using the NGI classification system, The stress charts were used to estimate the Stress Reduction Factor for this system. The back-analyses suggested that the NGI classification system might be able to give preliminary estimates of support requirements in caving mines with modifications relating to rock bolt length and the support of production level intersections. (C) 2002 Elsevier Science Ltd. All rights reserved.

Creep behavior of a continuous strand, swirl mat reinforced polymeric composite in simulated automotive environments for durability investigation: Part I: experimental development and creep-rupture

Experimental methods were developed to simulate automotive environments and investigate the creep-rupture behavior of an isocyanurate base polyurethane matrix with a continuous strand, swirl mat E-glass reinforcement. The material under stress was exposed to various simulated automobile service conditions to test its creep-rupture properties. Results showed that environment had substantial effects on its creep strength durability. The resulting data were analyzed for deriving experimental based equations. Proposed guidelines and stress reduction factors were developed for automotive structural design applications.

Creep Behavior of a Large Full-Size Welded Austenitic Steel Plate

The high-temperature design codes are presently considering the use of stress reduction factors for designing welded structures submitted to creep. These reduction factors are derived from creep tests which are generally made on small specimens and are not necessarily representative of large-size geometries. These codes are very likely overconservative, consequently uneconomical and need to be improved; an investigation to assess and quantify the supposed size effect is required. This paper presents an experimental and numerical study on creep behavior at 600°C of full-size welded joints taking into account real full-thickness of weldings. The material investigated is the austenitic stainless steel 316L(N) with manual metal arc welds using the 19 Cr 12 Ni 2 Mo electrode grade. The creep laws used in calculations are those obtained from tests using small specimens, but some coefficients of their theoretical formulation have been modified to obtain a better coherence with fullsize specimen data. Between small and large full-size specimens, experimental results show no significant difference in time to rupture, and the same location of fracture, at the center of the weldment, is observed. Finite element simulations performed for full-size welded joints provide rupture times that are consistent with measured values. The calculated percentage of the damaged volume in the weld metal as a function of load levels and of creep-time duration is studied; it shows that the creep-rupture times for high stress loading are determined with higher accuracy than for low stress loading.

Thermal shock behaviour of unidirectional silicon carbide fibre reinforced calcium aluminosilicate

Unidirectional silicon carbide fibre-reinforced calcium aluminosilicate (CAS) has been subjected to a variety of thermal regimes. Microscopy has been used to assess the degree of matrix damage. Thermal shock induced matrix cracking was first seen on the end faces of the composite, perpendicular to the fibre direction at a temperature differential of 400°C. At more severe thermal shocks the next damage was observed on faces parallel to the fibre direction in the form of cracking in the matrix perpendicular to the fibre direction. Matrix cracking damage increased, initially, with increasing severity of thermal shock, but then became less extensive at the highest temperature differentials (800°C) used. Thermal shock-induced crack densities were correlated with literature data for cracking under quasi-static loading using a simple thermal shock analysis incorporating a stress reduction factor. The suitability of applying a modified Aveston, Cooper and Kelly (ACK) model [1] to predict critical temperature differentials for matrix cracking onset in the unidirectional composite has also been tested. The method was found to be valid for the unidirectional material providing that some key parameters were determined independently.

A new monitoring system for piping systems in fossil fired power plants

A CEC-funded project has been performed to tackle the problem of producing an advanced Life Monitoring System (LMS) which would calculate the creep and fatigue damage experienced by high temperature pipework components. Four areas were identified where existing Life Monitoring System technology could be improved: 1. 1. the inclusion of creep relaxation 2. 2. the inclusion of external loads on components 3. 3. a more accurate method of calculating thermal stresses due to temperature transients 4. 4. the inclusion of high cycle fatigue terms. The creep relaxation problem was solved using stress reduction factors in an analytical in-elastic stress calculation. The stress reduction factors were produced for a number of common geometries and materials by means of non-linear finite element analysis. External loads were catered for by producing influence coefficients from in-elastic analysis of the particular piping system and using them to calculate bending moments at critical positions on the pipework from load and displacement measurements made at the convenient points at the pipework. The thermal stress problem was solved by producing a completely new solution based on Green's Function and Fast Fourier transforms. This allowed the thermal stress in a complex component to be calculated from simple non-intrusive thermocouple measurements made on the outside of the component. The high-cycle fatigue problem was dealt with precalculating the fatigue damage associated with standard transients and adding this damage to cumulative total when a transient occurred. The site testing provided good practical experience and showed up problems which would not otherwise have been detected.

Simplified method for evaluating the seismic buckling capacity of unstiffened steel containment structures

A simplified method is presented for evaluating the seismic buckling capacity of unstiffened, free-standing steel containment structures. The method is consistent with current US Nuclear Regulatory Commission seismic design standards and with containment buckling interaction equations given in the American Society of Mechanical Engineers Boiler and Pressure Vessel Code which includes the influence of geometrical imperfections of the shell on buckling. Stresses to be considered in the interaction equations are determined from beam theory using standard response spectrum analysis. An empirical correction factor is developed to account for hoop stresses that are not explicitly represented in the beam theory. As the results of these analyses are very sensitive to the damping that is assumed, the extensive three-dimensional finite element analyses that were performed to develop the hoop stress reduction factor were also used to study the sensitivity of containment buckling to the assumed damping. Experiments on model containment structures were then performed to further investigate the damping properties exhibited by these structures. The study in concluded by showing that the simplified method reasonably predicts seismic buckling capacities when compared with independently determined predictions from detailed finite element analyses.

Stress reduction factor associated with saddle support with extended top plate

A chart, which consists of a series of parametric curves, is provided to determine the stress reduction factor associated with the use of a saddle support with extended top plate. The theoretical basis for the parametric curves and dimensional parameters is briefly described. It is found that a saddle support with an extended top plate could reduce the peak stress at the support by as much as 50% or more. An example is shown on the use of the chart. The chart will be a useful design aid to vessel designers in specifying suitable extended plate dimensions.

Early Childhood Education Majors: Studying Stress and Stress Reduction Factors

Early Childhood Education Majors: Studying Stress and Stress Reduction Factors

Evaluation of the influence of welding on creep resistance

An evaluation is presented of the influence of welding on the uniaxial creep rupture behaviour of some ferritic, martensitic and austenitic creep resistant materials. After a description of evaluation the most reliable figures for the effect of welding will be presented in the form of two parameters. One is the strength reduction factor (SRF) for design conditions. The other is the lifetime reduction factor (LRF) for judging the lifetime of welded components at normal design stresses. Two criteria will be introduced to ludge whether the introduction of a stress reduction factor for design of welded components is advisable. The needs for further research in this field are indicated

Optimization of Coating Layers in the Design of Ceramic Fiber Reinforced Metal Matrix Composites

The potential of using coating layers to reduce thermal stresses in the matrix of composites with a mismatch in coefficients of thermal expansion (CTE) of fiber and matrix is investigated. Two thermoelastic solutions based on a 3-cylinder model are developed and a comprehensive sensitivity study is conducted. It is shown that the performance of the layer can be defined by the product of its CTE and thickness, and that a compensating layer with a sufficiently high CTE can reduce the thermal stresses in the matrix significantly. A practical procedure offering a window of coating layer candidates is proposed. Easy-to-use contours of matrix stress reduction factors are produced.

Design against creep failure for weldments in 0·5Cr0·5Mo0·25V pipe

The present paper studies design principles for weldments against creep failure in terms of failure experiences. Constitutive equations are developed for simulation of the creep behaviour of postservice materials. Numerical simulations of damage development in the weldment are then carried out. The damage state is well predicted by the presented procedure. Microcracking or severe cavitation is predicted to occur in the weld metal and the heat affected zone, which is in good agreement with the observations of the damage distribution through the wall thickness. It is found that, in the normal operating condition, the creep lifetime can be reduced by a factor of 3 due to the presence of the weld. Weldment reduction factors are then developed for design considerations. A stress reduction factor of about 0·9 or a strain reduction factor of 0·33 is required for the weldment design. The strain reduction factor is smaller than 0·5, which implies that the rule for the weldment design in the ASME CC-N47 might not be sufficiently conservative.

Optimal support radius of loose-fitting saddle support

A parametric study is presented in this article with regard to the determination of optimal support radius of loose-fitting saddle support for the cylindrical vessel. It is found that highly localized stresses arising from the saddle support can be reduced quite dramatically by using a clearance-fit saddle support. The initial gap between the saddle and vessel relaxes the radial constraint at the saddle horn and thereby results in stress reduction. Through the parametric study, the stress reduction factor ( S g ) and a range of optimal saddle radii are obtained and expressed by parametric formulae. Finally, examples are shown to illustrate the use and validity of the derived formulae.

Effect of hydrostatic retest on the fatigue behaviour of a steel gas cylinder: Bhuyan, G.S., Akhtar, A, and Webster, C.T.L. J. Pressure Vessel Technol. (Trans. ASME)113 4 (Nov 1991) 556–559

The present paper studies design principles for weldments against creep failure in terms of failure experiences. Constitutive equations are developed for simulation of the creep behaviour of postservice materials. Numerical simulations of damage development in the weldment are then carried out. The damage state is well predicted by the presented procedure. Microcracking or severe cavitation is predicted to occur in the weld metal and the heat affected zone, which is in good agreement with the observations of the damage distribution through the wall thickness. It is found that, in the normal operating condition, the creep lifetime can be reduced by a factor of 3 due to the presence of the weld. Weldment reduction factors are then developed for design considerations. A stress reduction factor of about 0·9 or a strain reduction factor of 0·33 is required for the weldment design. The strain reduction factor is smaller than 0·5, which implies that the rule for the weldment design in the ASME CC-N47 might not be sufficiently conservative.

Effectiveness of Wear Plate at the Saddle Support

A theory is presented to analyze the effectiveness of incorporating a wear plate at the saddle support of a cylindrical pressure vessel. The wear plate can be either welded to the vessel or of loose fitting. The theory is based on a contact stress formulation together with the use of a cylindrical shell bending theory for simply supported cylinder. The effectiveness of the wear plate in reducing the peak circumferential saddle-horn stress is quantified by definition of a stress reduction factor. Parametric data are carried out for the saddle angle of 120 deg. From the study, the optimum thickness of the wear plate for three different angular extensions, are obtained.

Correlation between observed support pressure and rock mass quality

Abstract The correlation between rock mass quality and support pressure proposed by Barton et al. (1974) has proven useful, except in cases of squeezing ground conditions. Field data collected systematically from 20 tunnel sections indicate a clear need for correction factors to account for height of overburden and tunnel closure, which do not seem to be adequately accounted for by the stress reduction factor. As expected, the support pressure decreases rapidly with tunnel closure and then increases beyond a limiting closure. The fact that the observed wall support pressures were always close to zero except in squeezing ground conditions has been taken care of by slightly modifying wall factors for Q-wall. A criterion derived from the field data shows that squeezing ground conditions would be encountered where the height of the overburden is greater than 350 Q 1 3 . The data reported herein confirm the earlier findings of Barton et al. (1974) that the support pressure is independent of the tunnel size.

Reduction factor for liquefaction potential analysis

A probabilistic method to evaluate liquefaction potential based on uniform cyclic laboratory tests is developed. In this method, a peak shear stress reduction factor and a number of uniform cycles of load are paired to yield probabilities of initial liquefaction equivalent to that from nonuniform earthquake loading. An illustrative analysis is presented for a hypothetical site. The method is then used to analyze a case study of a site known to have liquefied during the 1981 Westmorland, CA earthquake.

Tension and bending stress concentration factors in ‘U’, ‘V’, and opposed ‘U’–‘V’ notches

An analytical and experimental study has been performed to determine the tension and bending stress concentration factors as a function of notch depth for ‘U'—‘V’, and opposed ‘U'—‘V’ notches. Accurate results are produced using quadrilateral finite elements where the edge-lengths in the notch region are less than 10 percent of the notch radius, and using strain gauges where the active gauge length is less than 1/3 the notch radius. Results are presented for single ‘U’ and ‘V’ notches that are outside the range of previously published studies. A detailed stress analysis of a single notched specimen revealed the presence of a tension stress concentration factor ( Kt > 1) and a bending stress reduction factor ( Kb < 1) on the un-notched side of the bar directly opposite the notch. An approximate method of determining stress concentration factors for opposed ‘U'—‘V’ notch geometries is presented.