Predicting weld creep strength reduction for 9% Cr steels

In design standards and in post-service life assessment, the cross-weld (CW) creep strength of ferritic steels is nearly universally assumed to be 80% of the corresponding value for the parent material (PH). However, CW data assessment of some 9% Cr steels such as E911 and P91 suggests that this would not hold at least at the high temperature end of the testing range. The resulting weld creep strength factor (WSF) is then attaining values well below 0.8 when extrapolated to typical design life of 100 000 h or more. Under such conditions the conventional value of 0.8 would result in non-conservative (too long) predicted life for structures subjected to CW loading in the creep regime.To accommodate the CW strength data for realistic values of WSF requires appropriate correction based on actual data. For this purpose, an alternative assessment approach, rigidity parameter correction (RPC), is proposed. This approach can be used to predict CW rupture strength from the PM master curves, with any PM rupture model optimized to correspond to the welded materials data.     

Dual-stage tertiary creep of a ceramic matrix composite

The creep behaviour of an alumina fibre/silicon carbide matrix composite has been studied. The creep curves are characterised by a short primary stage followed by one or two tertiary stages. The secondary regime of this composite is limited to a single point. The occurrence of one or two tertiary stages in the creep behaviour is discussed, and some theoretical considerations are invoked to explain this behaviour. Creep in this composite is controlled by two mechanisms, namely viscoplastic creep of the alumina fibres and damage accumulation within the composite. The two tertiary stages differ in the damage mechanisms occurring, the first one being related to fibre–matrix debonding only, whereas successive fibre failure dominates in the second part. The second tertiary regime occurs only at low creep stresses, for which a non-catastrophic rupture of the composite is observed.     

远近围岩含水不同对套管岩压外载的影响

建立了围岩多层性受载模型，按粘弹对应原理，对均匀地应力场下及非均匀地应力场下蠕变引起的岩压外载及因远近含水量不同而引起的变化，从理论上进行了分析。     

Unified Elastoplastic–Viscoplastic Bounding Surface Model of Geosynthetics and Its Applications to Geosynthetic Reinforced Soil-Retaining Wall Analysis

The static and dynamic behaviors of reinforced soil structures are possibly subjected to the effects of creep or stress relaxation due to the time-dependent behavior of geosynthetic inclusions and backfill. To simulate the time-dependent monotonic and cyclic behavior of geosynthetics, an isothermal constitutive model is formulated within the framework of elastoplasticity–viscoplasticity. The concept of bounding surface plasticity is first utilized to formulate a time-independent cyclic model of geosynthetics. In order to capture the hardening stiffness of some polyester geosynthetics, an exponential bounding curve is used in simulating the primary loading. The time-independent version of the model was extended into an elastoplastic–viscoplastic model using overstress viscoplasticity with reference to available experimental data. The model was evaluated using creep, stress relaxation, monotonic, and cyclic loading test results obtained for different geosynthetics. It was then incorporated into a finite-element code and the static and dynamic behavior of a geosynthetic reinforced soil wall was analyzed. The analyzed results, with and without consideration to the time-dependent behavior of the reinforcements, were compared. It was demonstrated that although the end-of-construction behavior of the reinforced soil wall was less influenced by the time-dependent properties of geogrids, the long-term performance was considerably affected. The seismic response was also affected to some extent by the rate-dependent behavior of geogrids. The effects were more significant for short and/or large vertical spacing reinforcement layout.     

Prediction of Creep Stiffness of Asphalt Mixture with Micromechanical Finite-Element and Discrete-Element Models

This study presents micromechanical finite-element (FE) and discrete-element (DE) models for the prediction of viscoelastic creep stiffness of asphalt mixture. Asphalt mixture is composed of graded aggregates bound with mastic (asphalt mixed with fines and fine aggregates) and air voids. The two-dimensional (2D) microstructure of asphalt mixture was obtained by optically scanning the smoothly sawn surface of superpave gyratory compacted asphalt mixture specimens. For the FE method, the micromechanical model of asphalt mixture uses an equivalent lattice network structure whereby interparticle load transfer is simulated through an effective asphalt mastic zone. The ABAQUS FE model integrates a user material subroutine that combines continuum elements with viscoelastic properties for the effective asphalt mastic and rigid body elements for each aggregate. An incremental FE algorithm was employed in an ABAQUS user material model for the asphalt mastic to predict global viscoelastic behavior of asphalt mixture. In regard to the DE model, the outlines of aggregates were converted into polygons based on a 2D scanned mixture microstructure. The polygons were then mapped onto a sheet of uniformly sized disks, and the intrinsic and interface properties of the aggregates and mastic were assigned for the simulation. An experimental program was developed to measure the properties of sand mastic for simulation inputs. The laboratory measurements of the mixture creep stiffness were compared with FE and DE model predictions over a reduced time. The results indicated both methods were applicable for mixture creep stiffness prediction.     

Improved relaxation time coverage in ramp-strain histories

Standard methods for deriving relaxation data from measurements invariably involve some form of ramp-type deformation history, the initial portion of which is typically not employed for modulus evaluation. In fact, the “ten-times-rule” or a variant thereof is widely used at the expense of short term data acquisition. This paper suggests a simple if (not) obvious method to extend the range of relaxation data that can be acquired from a single test at a single temperature. The method draws on new computational developments for inverting ill-conditioned systems of equations which allows the determination of relaxation parameters nearly routinely and trouble-free. We demonstrate this process for extraction of relaxation characterization from ramp strain histories through (a) numerical evaluation with a virtual test sequence, as well as through (b) data measured in the laboratory. Limitations regarding the time range over which the relaxation modulus can be extracted from laboratory measurements in terms of equipment resolution and stability are discussed. With these constraints in mind it appears feasible to extend the time range by three to four decades towards shorter times when compared with the application of the “ten-times-rule”. Similar treatments apply to the acquisition of creep compliance data.     

Short- and long-term performance of the “Tecnaria” stud connector for timber-concrete composite beams

The paper presents the results of some experimental tests performed on the shear stud connection system for timber-concrete composite beams manufactured by the Tecnaria Ltd. Some push-out specimens constructed using both normal weight (NW) and light weight (LW) concrete were subjected to collapse and long-term creep tests. During the collapse test, the connector exhibited significant strength and stiffness. In the creep test performed in constant environmental conditions, delayed (creep) deformations took place mainly during the first days after loading. The next part of the creep test, conducted under cycles of environmental relative humidity, was characterized by an increase in delayed deformations (the so-called mechano-sorptive effect) due to the hygroscopic behaviour of timber around the connector. The amount of delayed deformation depended upon the cycle duration and was negligible for short period (less than one week) cycles. The use of LW concrete instead of␣NW concrete was found not to significantly affect␣the performance of the connection system␣neither in the long-term, nor in the collapse tests.     

Feature tests on welded components at higher temperatures—Material performance and residual stress evaluation

A reliable maintenance and service of energy generation plants would be impossible without the professional designing, manufacturing and monitoring of welded joints. The lifetime assessment factors of welded components as implemented in the design codes must be updated accounting for the modern materials and the advanced steam parameters used in the piping construction [Weld Strength for high temperature components design and operation (WELDON). European Project No. GRD2-2000-30363, Gampe U, Seliger P, Creep crack growth testing of P91 and P22 pipe bends. Int J Pressure Vessels Piping 2001;78:859-64, INTEGRITY of repair welds in high temperature plants operating under steady and cyclic loading conditions. European Project No. G5RD-CT-1999-00118].Within the EU 5th Framework RTD project ‘WELDON’ tests at high temperatures are performed on component-like feature test specimens like welded pipes and large tensiles in addition to laboratory specimens to study the geometry and size effects on the damage evaluation methodology. The circumferential welds of the pipes are subjected simultaneously to internal pressure and an axial load. The large tensile specimens manufactured from the welded pipes are subjected to uniaxial loading. These components are made from steel grade P22 and P91 and are equipped with gages for on-line monitoring of temperatures, deformations and strains.Residual stresses are measured on these components in as welded and after post-weld heat treatment (PWHT) conditions and are monitored during creep testing with interruptions and after failure. X-ray and hole drilling techniques are employed in these measurements. These data will be used to validate the FE modelling of residual stresses and damage assessment.The results obtained from long time creep tests, metallographic investigations of damage development in the different zones of the weldment and residual stress measurements are presented in this paper.     

Comparison of creep crack growth rate in heat affected zone of welded joint for 9%Cr ferritic heat resistant steel based on C, dδ/dt, K and Q parameters

Creep crack growth behavior is very sensitive to the materials’ micro-structures such as the heat affected zone of a weld joint. This is a main issue to be clarified for 9%Cr ferritic heat resistant steel for their application in structural components. In this paper, high temperature creep crack growth tests were conducted on CT specimens with cracks in the heat affected zone of weld joints of W added 9%Cr ferritic heat resistant steel, ASME grade P92. The creep crack growth behavior in the heat affected zone of welded joint was investigated using the Q^∗ concept following which the algorithm of predicting the life of creep crack growth has been proposed. Furthermore, three-dimensional elastic-plastic creep FEM analyses were conducted and the effect of stress multiaxiality of welded joint on creep crack growth rate was discussed as compared with that of base metal.     

Interface diffusion-induced creep and stress relaxation in unidirectional metal matrix composites under biaxial loading

Analytical solutions are developed for interface diffusion-induced creep and stress relaxation in unidirectional metal matrix composites under biaxial transverse loading. The driving force for the interface diffusion is the normal stress acting on the interface, which is obtained from rigorous Eshelby inclusion theory. The solutions are a function of the applied stress, volume fraction and radius of the reinforced-fiber, the modulus ratio between the fiber and the matrix, specially, exhibit a strong dependence of creep rate and stress relaxation behavior on the biaxial stress ratio. Moreover, the solution for the interface stress presented in this study also gives some insight into the relationship between the interface diffusion and interface slip. For the application of the solutions in the realistic composites, the scale effect is taken into account by detailed finite element analysis based on a unit cell model.