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International audience; Open-pore replicated microcellular 99.99% pure aluminium is tested in tensile creep, varying the temperature from 150 to 450 degrees C, the applied stress from 0.15 to 0.5 MPa, and the relative density from 0.14 to 0.28. Tensile creep curves are of classical shape, with a well-defined secondary stage of steady-state creep, for all except a few samples that were tested at higher temperature and lower stress; these display signs of extensive oxidation along the pore surface. All other samples crept at 250 degrees C or above exhibit a steady-state creep rate with an activation energy of 141 kJ mol(-1), a stress exponent of 7.2 +/- 0.4, and a dependence on relative density to a power near -21. The substructure of microcellular aluminium crept in this regime consists in subgrains that straddle, as in a bamboo structure, individual struts making the foam. This observation, coupled with the stress exponent near 7.5, shows that fine-scale microcellular pure aluminium can creep >250 degrees C under substructure-invariant conditions. At 150 degrees C, creep data are more scattered and show a steeper dependence on applied stress, suggesting power-law breakdown. The Monkman-Grant correlation describes well the tensile failure of this material. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Lifetime models for both creep and thermo-mechanical fatigue are presented for P23 steel. The models are the result of the joint European RFCS project ALoAS. The creep model, developed for robust creep strain modeling, here adjusted to describe the strain response of the ALoAS data, i.e. P23 pipe material fabricated by TenarisDalmine and plate material heat fabricated by Industeel. The creep modeling range covers 550–660 °C in temperature and 80–180 MPa in stress. The thermo-mechanical fatigue model is adjusted to the acquired data on thick walled P23 pipe material fabricated by TenarisDalmine. The load range covers 20–625 °C and up to 16,000 cycles to failure. A good description of the data has been achieved for both creep and fatigue models.

18Cr–9Ni–3Cu–Nb–N steel has the highest creep strength in the austenitic heat-resistant steels. It is expected as the component of the future thermal power plants. In order to clarify the mechanism of creep strengthening and creep rupture for the 18Cr–9Ni–3Cu–Nb–N steel, the creep test and the microstructural observations using electron microscopy were carried out. Seamless pipe was used as the sample because the initial microstructure for the sample conformed to the one for steels in the plants. From the TEM observation, fine NbX and Cu particles were formed in the austenite grains. The pinning effect by these particles is effective for the creep strengthening. In the regard to the microstructural change during the creep deformation, the crystal orientation rotations were observed using SEM-OIM. In the crystal grains rotated during the creep deformation, ‹001› or ‹111› was oriented to the tensile direction. Many boundaries were cracked due to these crystal rotations. This means that the creep rupture is occurred by the difference of the plastic deformation behavior between the grains oriented ‹001› to the tensile direction and the grains oriented ‹111› to it.

Current fire induced burnthrough models of aluminum typically rely solely on temperature thresholds and cannot accurately capture either the occurrence or the time to burnthrough. This research experimentally explores the fire induced burnthrough phenomenon of AA6061-T651 plates under multiple sized exposures and introduces a new burnthrough model based on the near melting creep rupture properties of the material. Fire experiments to induce burnthrough on aluminum plates were conducted using localized exposure from a propane jet burner and broader exposure from a propane sand burner. A material melting mechanism was observed for all localized exposures while a material rupture mechanism was observed for horizontally oriented plates exposed to the broader heat flux. Numerical burnthrough models were developed for each of the observed burnthrough mechanisms. Material melting was captured using a temperature threshold model of 633 deg C. Material rupture was captured using a Larson-Miller based creep rupture model. To implement the material rupture model, a characterization of the creep rupture properties was conducted at temperatures between 500 and 590 deg C. The Larson-Miller curve was subsequently developed to capture rupture behavior. Additionally, the secondary and tertiary creep behavior of the material was modeled using a modified Kachanov-Rabotnov creep model. Thermal finite element model accuracy was increased by adapting a methodology for using infrared thermography to measure spatially and temporally varying full-field heat flux maps. Once validated and implemented, thermal models of the aluminum burnthrough experiments were accurate to 20 deg C in the transient and 10 deg C in the steady state regions. Using thermo-mechanical finite element analyses, the burnthrough models were benchmarked against experimental data. Utilizing the melting and rupture mechanism models, burnthrough occurrence was accurately modeled for over 90% of experiments and modeled burnthrough times were within 20% for the melting mechanism and 50% for the rupture mechanism. Simplified burnthrough equations were also developed to facilitate the use of the burnthrough models in a design setting. Equations were benchmarked against models of flat and stiffened plates and the burnthrough experiments. Melting mechanism burnthrough time results were within 25% of benchmark values suggesting accurate capture of the mechanism. Rupture mechanism burnthrough results were within 60% of benchmark values. Ph. D.

Turbine disks of aircraft engines in operation are subjected to alternating thermocyclic deformation under high temperatures. Operation gives rise to sufficiently high stresses and subsequent creep damaging effects.In this paper we suggest an approach for describing creep rupture of a simple rotating disk due to the accumulation of creep induced microdefects. The dissipated energy of the creep deformation is considered as the measure of material damage.The lifetime of the disk can be predicted on the basis of this approach. Both crack initiation and damage propagation to complete rupture of a disk are calculated. The analytic results can be compared to known experimental data. The derived algorithm can be applied to estimate the lifetime of real machinery components when creep deformations are dominant.

Elevated temperature behaviour of 2.25 Cr-1 Mo steel (UW — unwelded) and weldments, namely, manual metal arc (MMA) and induction pressure (IP) welded joints, was assessed. The creep rupture time was found to follow the order: IPW>UW>MMAW. The nature, size and distribution of carbides, as evidenced by transmission electron microscopic (TEM) studies, were found to have a bearing on the rupture time of the specimens.

Technical report contains research results of weld defect influence, type lack of fusion and lack of penetration, on strength of but weld joint of pipe. Analysis took in to consideration a static and dynamic influence of load. The problem of creep at increased temperature was considered.

According to the technical documentation received from client, weld defect in butt weld according to the EN ISO 6520-1, Series 400 is considered. It is found that there are several joints with the same weld defect, but only the most critical is analysed. Model is analysed for calculating and testing conditions, whereas for testing conditions two cases are analysed: testing conditions according to the EN and testing conditions with maximal allowable pressure. Mentioned weld defect is considered as non-standard crack (failure), so the crack tip is defined as stress concentration. Furthermore, creep diagram is constructed according to the values of creep rupture strength for 200 000 hours from EN 10216-2:2002+A2:2007 (E). Based on that norm, for material used in construction and for temperature conditions, it is found that creep does not occur, but anyway, trend line is constructed in order to find the value of creep rupture for considered conditions.

Damage during installation (DDI), creep and creep rupture behavior of geosynthetics have been subjected to extended studies. To contribute to the comprehension on the effect of damage during installation on the long-term mechanical behavior of geosynthetics, a research program was established. On this paper the results available for one of the geosynthetics studied are presented. This material has been subjected to field damage tests, using two different soils and two compaction energies. To characterize the effect of the damage induced in long-term mechanical behaviour, tensile creep tests and creep rupture tests were carried out, ac-cording with the procedures described in EN ISO 10319 and EN ISO 13431, respectively. The results ob-tained are compared and discussed. The main conclusions of the study are presented. The author’s acknowledgements to the project PTDC/ECM/67547/2006 and the project PTDC/ECM/099087/2008.

We study the creep rupture of fiber composites subjected to uniaxial constant load within the framework of the fiber bundle model. Our numerical studies show that in the primary creep, the deformation rate exhibits a power law relaxation followed by a power law acceleration in the tertiary creep. We also investigate the behavior of the failure time and the avalanche size distribution. A good agreement is found between experiments and the model prediction, particularly for deformation power-laws scaling in the primary-creep regime and in the tertiary-creep neighbourhood of rupture., Proučavamo klizni lom vlaknastog kompozita opterećenog stalnom jednoosnom silom primjenom modela snopa vlakana. Naša numerička istraživanja pokazuju da deformacije u početnom klizanju slijede potencijalni zakon, a u tercijalnom ubrzanje s drugim potencijalnim zakonom. Ispitujemo također vrijeme loma i raspodjelu jakosti lavine. Nalazimo dobro slaganje eksperimentalnih podataka s predviđanjima modela, posebice za sumjerenost potencijalnog zakona u početnim kliznim uvjetima i blizu tercijalnog kliznog loma.

This thesis presents the results of testing and modeling conducted to characterize the performance of 3M™ VHB™ structural glazing tape in both shear and tension. Creep rupture testing results provided the failure time at a given static load and temperature, and ramp-to-fail testing results provided the ultimate load resistance at a given rate of strain and temperature. Parallel testing was conducted on three structural silicone sealants to compare performance. Using the time temperature superposition principle, master curves of VHB tape storage and loss moduli in shear and tension were developed with data from a dynamic mechanical analyzer (DMA). The thermal shift factors obtained from these constitutive tests were successfully applied to the creep rupture and ramp-to-fail data collected at 23°C, 40°C, and 60°C (73°F, 104°F, and 140°F), resulting in master curves of ramp-to-fail strength and creep rupture durability in shear and tension. A simple linear damage accumulation model was then proposed to examine the accumulation of wind damage if VHB tape is used to attach curtain wall glazing panels to building facades. The purpose of the model was to investigate the magnitude of damage resulting from the accumulation of sustained wind speeds that are less than the peak design wind speed. The model used the equation derived from tensile creep rupture testing, extrapolated into the range of stresses that would typically be generated by wind loading. This equation was applied to each individual entry in the data files of several real wind speed histories, and the fractions of life used at each entry were combined into a total percentage of life used. Although the model did not provide evidence that the established design procedure is unsafe, it suggested that the accumulation of damage from wind speeds below the peak wind speed could cause a VHB tape mode of failure that merits examination along with the more traditional peak wind speed design procedure currently recommended by the vendor. Master of Science

Creep rupture time data of discontinuously reinforced 6061Al–15 vol.% SiCw metal–matrix composite (MMC) have been analyzed. Well-known phenomenological models, usually applied to high-temperature structural metals, such as those described by the Monkman–Grant and Larson–Miller equations, have been used. Consistent results are obtained when data are analyzed in the context of the effective stress borne by the metallic matrix. Such analysis supports further the relevance of a load transfer mechanism during creep of these MMCs, as previously suggested., Financial support from MEC (Ministerio de Educacio ´n y Ciencia), Spain, under project MAT05-00527 is acknowledged.

A simulation model which incorporates the statistical- and numerical-based Lattice Green Function Local Load Sharing Model and a Fracture Mechanics-based Residual Strength Model has been developed. The model simulates creep rupture by imposing a fixed load of constant stress on the composite over the simulation duration. Simulation of the fatigue of glass fiber-reinforced composites is achieved by replacing the constant stress parameter in the model with a sinusoidal wave function. Results from the creep rupture model using fused silica fiber parameters, compare well with S-2 glass/epoxy systems. Results using Mandell's postulate that fatigue failure in glass fiber-reinforced polymeric composites is a fiber-dominated mechanism, with a characteristic slope of 10 %UTS/decade are consistent with available experimental data. The slopes of fatigue curves for simulated composites for three frequencies namely: 2, 5 and 10 Hz are within 12-14 %UTS/decade compared with that of 10.6-13.0%UTS/decade for unidirectionl glass reinforced composites (epoxy and vinyl ester) obtained from Demers' [40] data. Master of Science

[No abstract available]

The objectives of this thesis are to study time-dependent fracture behaviour in concrete. The thesis consists of an experimental study, constitutive modelling and numerical analysis.

Experimental investigations on the creep rupture behaviour of type 304, 304L and 316 stainless steels, a nickel-base superalloy SuperNi 600 and a turbine disc alloy (equivalent to MAR M 200) have been carried out. Stainless steels 304L and 316 have been tested with and without weldment, and materials 304 SS and SuperNi 600 have been tested with and without corrosive coatings. MAR M 200 was tested in air. A parametric method is suggested for obtaining the master curves for these alloys under test conditions. The applicability of the parameter to 25-20 stainless steel and a modified nuclear grade 316 type stainless steel has been verified.

For axi-symmetrically notched tension bars [Dyson, B.F., Loveday, M.S., 1981, Creep Fracture in Nimonic 80A under Tri-axial Tensile Stressing, In: Ponter A.R.S., Hayhurst, D.R. (Eds.), Creep in Structures, Springer-Verlag, Berlin, pp. 406–421] show two types of damage propagation are shown: for low stress, failure propagates from the outside notch surface to the centre-line; and for high stress, failure propagates from the centre-line to the outside notch surface. The objectives of the paper are to: identify the physics of the processes controlling global failure modes; and, describe the global behaviour using physics-based constitutive equations. Two sets of constitutive equations are used to model the softening which takes place in tertiary creep of Nimonic 80A at 750 °C. Softening by multiplication of mobile dislocations is firstly combined, for low stress, with softening due to nucleation controlled creep constrained cavity growth; and secondly combined, for high stress, with softening due to continuum void growth. The Continuum Damage Mechanics, CDM, Finite Element Solver DAMAGE XX has been used to study notch creep fracture. Low stress notch behaviour is accurately predicted provided that the constitutive equations take account of the effect of stress level on creep ductility. High stress notch behaviour is accurately predicted from a normalized inverse cavity spacing d /2l = 6, and an initial normalized cavity radius r hi /l = 3.16 × 10 −3 , where 2l is the cavity spacing, and d is the grain size; however, the constants in the strain rate equation required recalibration against high stress notch data. A void nucleation mechanism is postulated for high stress behaviour which involves decohesion where slip bands intersect second phase grain boundary particles. Both equation sets accurately predict experimentally observed global failure modes.

The influence of nitrogen content on the high temperature mechanical properties of 316LN stainless steel has been studied at nitrogen levels of 0.07, 0.11, 0.14 and 0.22 wt. %. These studies have shown that 316LN SS containing 0.14 wt. % nitrogen has optimum mechanical properties. Detailed creep studies were carried out on 316LN SS at 823 K, 873 K and 923 K at stress levels ranging from 140 to 300 MPa. The longest rupture life in this study was 32,500 hours. Creep strength was found to increase substantially with increase in nitrogen content. Metallographic studies were performed on creep tested specimens to understand the influence of nitrogen content on the evolution of creep damage. The extent of internal and surface creep damage decreased drastically with increase in nitrogen content. Area fraction of internal creep damage was measured and it was observed to decrease significantly with increase in nitrogen content. The steel containing the highest nitrogen content showed a negligible amount of creep damage irrespective of the stress level. Hence, increasing the nitrogen content increases the resistance to creep damage formation leading to improved rupture life. The extent of internal and surface creep damage decreased with increasing temperature and decreasing stress. Fracture mode was found to predominantly intergranular in the steel containing the higher nitrogen content.

The influence of grain boundary sliding on the mechanical behaviour of materials subject to creep at elevated temperatures has been investigated by a number of model analyses. Some models have focussed on the interaction of linearly viscous behaviour in the grain boundaries and power-law creep of the grains. It is found that there is a transition range such that at higher stresses sliding has little influence, while at lower stresses sliding accelerates creep. Other investigations have focussed on the effect of sliding on intergranular creep fracture. Freely sliding grain boundaries give much reduced rupture times, and the behaviour is rather strongly affected by whether the cavitating grain boundary facets are closely-spaced or well-separated.

An experimental procedure is presented for lifetime testing of graphite bundles under constant load. The attributes of the experiment are expedience in implementation and a substantial accumulation of information equivalent to a large number of single filament tests. To achieve the objectives of the experiment a specially adapted Instron machine was used with a digital process/controller. Two trial tests were conducted using Hercules high strength graphite. The preliminary results are presented and the effects of inter-fiber friction evaluated. http://archive.org/details/experimentalproc1094521848 Lieutenant, United States Navy Approved for public release; distribution is unlimited.

Investigations have been carried out to study the effect of hot corrosion on the creep rupture properties of type 304 stainless steel in the temperature range 600–700°C. The corrodents used were 100% Na 2 SO 4 and a mixture of 75% Na 2 SO 4 and 25% NaCl. The creep life decreases in the presence of the corrodents and the mixture of Na 2 SO 4 and NaCl has been found to exert the worst attack in all cases. The corrosion products on the fractured surface has been analysed and, based on the data, a model for creep rupture life has been proposed under the interaction condition.