Realistic Residual Stress Research Articles

Influence of residual stress on the free vibrations of an elastic solid; application to resonant ultrasound spectroscopy (RUS)

Resonant ultrasound spectroscopy (RUS) is a standard technique for experimentally measuring the elastic constants of anisotropic elastic materials. Traditionally, the influence of internal or residual stresses on the measurement of elastic constants is not taken into consideration. However, it is well-known that residual stress can impart a small variation in the phase velocity of ultrasonic bulk waves, which is known as the acoustoelastic effect. Thus, it is plausible that residual stress will influence the resonances of samples measured during RUS. In this presentation, the elastodynamic theory for describing free vibrations is reformulated by using a stress-dependent constitutive relationship. The model culminates in an eigenvalue problem with a stiffness matrix being dependent on the spatially varying residual stress field within the domain of the sample. The model is evaluated by considering a realistic residual stress distribution. Results show the degree at which frequencies of resonance depend on the residual stress. While accounting for residual stress is expected to improve the estimates of elastic constants, there is also potential to use RUS as a tool to assess and characterize the residual stress present in samples.

Measuring the Casimir Forces with an Adhered Cantilever: Analysis of Roughness and Background Effects

Technological progress has made possible precise measurements of the Casimir forces at distances less than 100 nm. It has enabled stronger constraints on the non-Newtonian forces at short separations and improved control of micromechanical devices. Experimental information on the forces below 30 nm is sparse and not precise due to pull-in instability and surface roughness. Recently, a method of adhered cantilever was proposed to measure the forces at small distances, which does not suffer from the pull-in instability. Deviation of the cantilever from a classic shape carries information on the forces acting nearby the adhered end. We calculate the force between a flat cantilever and rough Au plate and demonstrate that the effect of roughness dominates when the bodies approach the contact. Short-distance repulsion operating at the contact is included in the analysis. Deviations from the classic shape due to residual stress, inhomogeneous thickness of the cantilever, and finite compliance of the substrate are analysed. It is found that a realistic residual stress gives a negligible contribution to the shape, while the finite compliance and inhomogeneous thickness give measurable contributions that have to be subtracted from the raw data.

Evaluation of Residual Stresses and Their Influence on Cavitation Erosion Resistance of High Kinetic HVOF and HVAF-Sprayed WC-CoCr Coatings

Thermal spray processes have been developing toward lower particle temperature and higher velocity. Latest generation high-velocity oxygen-fuel (HVOF) and high-velocity air-fuel (HVAF) can produce very dense coating structures due to the higher kinetic energy typical for these thermal spray processes. Thermally sprayed coatings usually contain residual stresses, which are formed by a superposition of thermal mismatch, quenching and, in case of high kinetic energy technologies, peening stresses. These stresses may have a significant role on the mechanical response and fatigue behavior of the coating. Understanding these effects is mandatory for damage tolerant coating design and wear performance. For instance, wear-resistant WC-CoCr coatings having high compressive stresses show improved cavitation erosion performance. In this study, comparison of residual stresses in coatings sprayed by various thermal spray systems HVOF (Thermico CJS and Oerlikon Metco DJ Hybrid) and HVAF (Kermetico AcuKote) was made. Residual stresses were determined through thickness by utilizing Tsui and Clyne analytical model. The real temperature and deposition stress data were collected in the coating process by in situ technique. That data were used for the model to represent realistic residual stress state of the coating. The cavitation erosion and abrasion wear resistance of the coatings were tested, and relationships between residual stresses and wear resistance were discussed.

Full in-plane strain tensor analysis using the microscale ring-core FIB milling and DIC approach

Microscale Full In-plane Strain Tensor (FIST) analysis is crucial for improving understanding of residual stress and mechanical failure in many applications. This study outlines the first Focused Ion Beam (FIB) milling and Digital Image Correlation (DIC) based technique capable of performing precise, reliable and rapid quantification of this behaviour. The nature of semi-destructive FIB milling overcomes the main limitations of X-Ray Diffraction (XRD) strain tensor quantification: unstrained lattice parameter estimates are not required, analysis is performed in within a precisely defined 3D microscale volume, both amorphous and crystalline materials can be studied and access to X-ray/neutron facilities is not required.The FIST FIB milling and DIC experimental technique is based on extending the ring-core milling geometry to quantify the strain variation with angle and therefore benefits from the excellent precision and simple analytical approach associated with this method. In this study in-plane strain analysis was performed on sample of commercial interest: a porcelain veneered Yttria Partially Stabilised Zirconia (YPSZ) dental prosthesis, and was compared with the results of XRD.The two methods sample different gauge volumes and mechanical states: approximately plane stress for ring-core milling, and a through-thickness average for XRD. We demonstrate using complex analysis methods and Finite Element (FE) modelling that valid comparisons can be drawn between these two stress states. Excellent agreement was obtained between principal stress orientation and magnitudes, leading to realistic residual stress estimates that agree well with the literature (σAv≈460MPa). As a measure of validity of the matching approach we report the upper and lower bounds on the (101) interplanar spacing of YPSZ that are found to correspond to the range 2.9586−2.9596Å, closely matching published values.

The Potential for Assessing Residual Stress Using Thermoelastic Stress Analysis: A Study of Cold Expanded Holes

Thermoelastic stress analysis (TSA) is a well established tool for non-destructive full-field experimental stress analysis. In TSA the change in the sum of the principal stresses is derived, usually when a component is subjected to a cyclic load. Therefore the mean stress or any residual stress in a component cannot be obtained from the thermoelastic response. However, modifications to the linear form of the thermoelastic equation that incorporate the mean stress may provide a means of establishing the residual stresses. It has also been shown that the application of plastic strain modifies the thermoelastic constant in some materials, causing a change in thermoelastic response, which may also be related to the residual stress. The changes in response due to plastic strain and mean stress are of the order of a few mK and are significantly less than those expected to be resolved in standard TSA. Recent developments in infra-red detector technology have enabled these small variations in the thermoelastic response to be identified, leading to renewed interest in the use of TSA for residual stress analysis in realistic components. The component studied in this work is an aluminium plate that contains a cold expanded hole, hence providing an opportunity to examine any changes in thermoelastic response caused by the residual stress in the neighbourhood of the hole. The variations in thermoelastic response due to residual stress are shown to be measurable and significant; validation of the residual stress field is provided by laboratory X-ray diffraction. The potential for a TSA based approach for residual stress analysis is revisited, and the feasibility of applying it to components containing realistic residual stress levels is assessed.

Effect of residual stresses on the linear-elastic KI–T field for circumferential surface flaws in pipes

This paper describes the effect of four realistic residual stress patterns on the crack-front constraints for circumferential surface flaws in the wall of a pipe, characterized by the linear-elastic K I – T stresses. The numerical representation of the residual stresses combines a thermal-induced eigenstrain field with a local mechanical traction applied over the interface between the heat affected zone and the remaining pipe. The verification study confirms the accuracy of this approach based on the analytical K I solution for a 2-D crack under prescribed residual stresses. This study demonstrates low crack-front constraints near the deepest crack front for three typical residual stresses.

Experimental investigation of residual stresses in roller bent wide flange steel sections

Residual stresses in straight hot rolled wide flange sections are well documented and have been investigated in the recent past. However, to the knowledge of the authors, residual stress measurements have not been published on roller bent wide flange sections. Straight sections are curved into roller bent ones at ambient temperatures by means of the roller bending process. Since roller bent sections underwent severe plastic deformation during the forming process, the well-known residual stress patterns from hot rolling may not be appropriate for the roller bent steel. Roller bent sections can be applied in halls, roofings and bridges, thereby acting as structural arches and it is important that a realistic residual stress pattern is implemented when assessing their load carrying capacity. An experimental program has been carried out to investigate the residual stresses in roller bent wide flange sections bent about the strong axis. Residual stresses were measured with the sectioning method. The experimental technique was investigated with respect to possible temperature influence and repeatability of the measurements. Experimental values revealed that the residual stress pattern and magnitude in roller bent sections is different when compared to their straight counterparts.

Code characterisation of weld residual stress levels and the problem of innate scatter

Economic and safe management of operating nuclear power plant is increasingly dependent upon structural integrity assessments for pressure vessels and piping. The residual stress distribution assumed in defect assessments for welded joints often have a deciding influence on the analysis outcome. Guidance on characterising conservative levels of weld residual stress can be found in structural integrity codes and procedures such as R6, API 579 and BS7910. There is an increasing need to develop more realistic and reliable residual stress distributions that will deliver more accurate integrity assessments. However, future development of such distributions will have to deal convincingly with what is often termed the “innate scatter” of weld residual stresses. This paper first identifies and illustrates some of the origins of apparent innate scatter. The stability of the welding process is examined. The importance of transient weld bead starts/stops and the lay-up of passes in multi-pass welds are demonstrated. Uncertainties associated with two commonly used residual stress measurement techniques are reviewed and simple quantitative studies used to reveal the role of measurement gauge length and, more significantly, errors in spatial location on the level of measured residual stress. The final part of this paper surveys how structural integrity codes and procedures currently characterise welding residual stresses for defect-assessment purposes and discusses the development of more realistic residual stress profiles based on statistical treatment of scatter and uncertainties.

Proposed provisions for the design of steel beam-columns in S16-2001

The provisions for the design of steel beam-columns in CSA Standard CAN/S16.1-94 are reviewed and changes are proposed to provide more consistent reliability in the next edition, CAN/CSA-S16-2001. The in-plane strength interaction equations have been checked using plastic-zone finite element analysis based on realistic residual stress patterns and initial imperfections. The same overall format is maintained. P-δ effects have been found to be negligible for members in sway frame. The interaction equation for checking the in-plane strength of sway beam-columns has been modified by taking the factor U1as 1.0 for members in unbraced frames. For weak-axis bending, a new factor β accounts for the effect of the distributed plasticity on stability. The concept of the notional load for sway buckling, also used in Australia and in Eurocode 3, is now applied for all load combinations and not just as a minimum. The upper limit on U2of 1.4 is removed. Examples are given illustrating these changes. A re-analysis of the local buckling experiments of other researchers on Class 2 sections, using a widely accepted buckling criterion, shows that no distinction need be made between Class 1 and 2 section beam-columns, i.e., Class 2 sections are treated as Class 1 sections. The changes in total do not increase the complexity of the standard, but reflect better the behaviour of beam-columns. The standard is aligned more closely with the best international practice.Key words: analysis, beam-column, buckling, compact, design, interaction equation, resistance, stability, steel, sway.

Modeling the effects of residual stresses on defects in welds of steel frame connections

This work describes an eigenstrain approach to impose realistic residual stress fields on 3-D finite element models for the lower-flange welds of the type found in large, beam-column connections. The 3-D models incorporate the full geometry of a pull-plate specimen designed to reduce the cost of testing full connections while retaining essential features of the fracture prone, weld region. The 3-D analyses examine pull-plate configurations with a variety of geometric parameters for both through-width and semi-elliptical cracks in the weld root pass. The numerical results include tables of values for the non-dimensional “geometry” factors ( Y) needed to compute stress intensity factors. The residual stress fields generated by the proposed eigenstrain functions closely match those computed using 2-D, thermo-mechanical simulation of the welding process. Stress intensity factors, obtained from corresponding J-integral computations, show clearly the significant fracture toughness demand from residual stresses prior to application of the structural loads. An alternative design for a grooved backup bar shows considerable promise to reduce fracture toughness demands by re-positioning the weld root pass below the beam flange and thus outside the primary load path. Analyses here indicate approximately a 65% reduction in K I -values for tension loading in the flange.

The effect of residual stresses on brittle and ductile fracture initiation predicted by micromechanical models

The effect of a realistic residual stress field on the predicted initiation of brittle and ductile fracture in a pressure and axially loaded circumferentially cracked pipe is examined using finite element analysis, micromechanical models of fracture initiation, andJ-Q theory. The study confirms that residual stresses contribute to the driving force and reduce fracture loads early in the loading history. In addition, results show that the residual stresses severely alter theJ-value (i.e., fracture toughness) predicted for the onset of brittle fracture. The reason for this decrease is found to be the increase in constraint generated by the residual stress field. In contrast, the effect of residual stresses on the ductile fracture initiation toughness is shown to be negligible. kw]Key words kw]residual stress kw]fracture initiation kw]micromechanics

Stable crack growth in a surface compression-strengthened hollow cylinder

In this paper the static fatigue problem for a circumferentially cracked hollow cylinder is examined. For this particular configuration, stable crack growth, in the absense of any external forces, is determined for cylinders with axial components of residual stress which are compressive on the inner and outer radial surfaces and tensile in the cylinder wall. An initial surface crack which is deep enough to penetrate the compression strengthened surface region and enters the tensile zone may propagate in a stable manner until either sudden spontaneous failure occurs or the crack arrests. Since a portion of the crack near the cylinder surface will be closed because of the compressive residual stress field, an additional unknown in the problem is the extent of the crack surface contact. This crack surface contact length is determined by iteration on the integral equation which arises in the mathematical derivation for an embedded circumferential crack in a hollow cylinder. As an illustration of stable crack growth for this geometry with a realistic residual stress distribution, numerical results are presented for a hollow, soda-lime glass cylinder, based on crack growth rates in soda-lime glass exposed to water at 25‡ C. Using the fracture toughness and slow crack growth characteristics for soda-lime glass, the conditions for no crack propagation, crack propagation leading to crack arrest, and catastrophic failure are established.