Low Applied Stress Levels Research Articles

Dislocation structure evolution during room temperature dwell loading of a Ti-6Al-4 V alloy

The current work examines the evolution of the dislocation structure, within both the α and β phases, of a Ti-6Al-4 V alloy subjected to room temperature dwell loading performed at about 96% of the macroscopic yield stress level for several durations, as part of a synchrotron in-situ deformation experiment. The obtained findings represent a vital contribution to creating a scientific basis for improving lifing predictions for aero-propulsion components. The dislocation configuration, type and density were examined post-mortem using transmission electron microscopy (TEM). Despite the relatively low applied stress level, well-developed dislocation structures were already formed within the α phase at a dwell duration of 10 min and changed only modestly up to 60 min. The plastic deformation of the above phase took place solely via dislocation slip. To facilitate a comparison between different dwell durations, the TEM investigation focused on the regions oriented for the prismatic glide that represented the main deformation mode due to the starting crystallographic texture. After deformation, the regions oriented for the prismatic slip contained planar slip bands largely consisting of dislocations of a screw type. There was rather limited interaction between different bands, as the dislocation arrays passed through the slip band intersections with only minor difficulty. The screw-type dislocations seemed to preferentially form during straining. Dislocation Burgers vectors experimentally determined within the β phase occasionally departed from the Schmid rule and such dislocations typically showed a prevalent screw component. Some deformation banding and {112} 〈111〉 − type mechanical twin formation were locally detected within the β phase.

Effects of pore fluid type and ionic concentration on electrical conductivity of granular mixtures of electrically conductive graphite and nonconductive sand

This study investigates the effects of pore fluid type and ionic concentration on the electrical conductivity of granular mixtures containing both insulating and conductive particles. Specimens with varying ionic concentrations were prepared, and we measured the electrical conductivity of mixtures of sand with varying graphite contents with increasing vertical stress. Results show that the percolation threshold appears to be at a carbon fraction around 20% and divides the tested mixtures into similar response groups. Normalized particle conduction Kp values reveal negligible effects of pore fluid type and concentration on Kp at a low applied stress level. However, at high stress levels, the Kp increases with increasing ionic concentration of pore water, and the Kp of the dry specimen was comparable to that of a saturated specimen with very low ionic concentration. Finally, the particle conduction estimating method for mixtures is suggested, and the relevance of pore water conduction is investigated.

Effect of interface damage on tensile behavior of fiber-reinforced ceramic-matrix composites after thermal fatigue loading

ABSTRACT In this paper, the effect of interface damage on the tensile damage and fracture of fiber-reinforced ceramic-matrix composites (CMCs) after thermal fatigue loading is investigated. The damage mechanisms of matrix cracking, fiber/matrix interface debonding and wear, interface and fiber oxidation, and fiber failure after thermal fatigue loading are considered in the tensile stress-strain behavior prediction. The relationships between the thermal fatigue temperature, applied cycle number, composite internal damage, and tensile damage and fracture are established. The effects of thermal fatigue temperature, thermal cycle number, fiber volume, interface shear stress, fiber strength, and fiber Weibull modulus on the tensile damage and fracture of fiber-reinforced CMCs subjected to the thermal fatigue loading are analyzed. The experimental tensile stress-strain curves of different C/SiC composites with and without thermal fatigue loading are predicted. When the thermal fatigue temperature and applied cycle number increase, the composite tensile strength and failure strain decrease, the interface debonding ratio, interface oxidation ratio, and broken fibers fraction increase at low applied stress level.

改良9Cr鋼溶接材のクリープ損傷を考慮した多結晶弾粘塑性解析

Modified 9Cr-1Mo steel is used in the high temperature environment such as water supplying pipes in steam power plant because of its low thermal activation to maintain steady rate deformation during long-term creep. In reality, however, in a long term region of, say, 100,000 h or longer, creep rupture lives become much shorter than predicted because of the material degradation which may occur due to constraint among anisotropic crystal grains around a welded joint. This paper describes a polycrystal elasto-viscoplastic finite element code considering the damage mechanics which the present authors have developed in order to analyze long-term creep behavior of a welded joint in modified 9Cr-1Mo steel at 823K in air. The results of the analyses done by the present FE code showed that creep damage preferentially occurred in fine HAZ, which implies internal crack initiation known as Type IV cracking. The developed code reproduced abrupt decrease in the creep rupture lives at lower applied stress levels.

CDM prediction of creep failure modes in thin and thick section Cr‐Mo‐V Butt‐welded pipes under combined internal pressure and end loads at temperatures in the range 540–620 °C

ABSTRACTThe paper examines the dependence of creep failure modes of butt‐welded pipes upon: temperatures in the range 540–620 °C; the ratio of axial to pressure loadings (loading condition); and, load levels which result in failure times in the region 2000–250 000 h. Continuum damage mechanics (CDM) analyses have been used to analyse two extreme geometries: thin section pipes (4.52 mm) and thick section steam pipes (46 mm) of the type found in power generation plant. The thick pipes have been selected to allow through‐thickness constraint to be developed. Two predominant modes of failure have been predicted: Fusion Boundary failure and Type IV failure. For the thin section pipes the modes of failure are not strongly dependent on applied loading levels, and loading condition, but are dependent on temperature: at 540 °C failure is in the Fusion Boundary and at 620 °C it is in the Type IV region. Failure mode in the thick pipes is dependent on temperature, and applied loading levels, but is essentially independent of loading condition. At 540 and 565 °C failure is in the Fusion Boundary; whereas at 575 °C, failure is in the Type IV region at higher applied stress levels, and in the Fusion Boundary at lower applied stress levels.

Effect of NiCr and NiCrAl coatings on the creep resistance of a Ni alloy

The effect of plasma sprayed NiCr and NiCrAl coatings on the creep resistance of Nickel alloy 690 at temperature of 1033K was investigated. Experimental results showed that the coatings had a beneficial effect on the improvement of the creep resistance of substrate. However, there was almost no difference in the creep lives between the NiCr and NiCrAl coated specimens at a given stress level. The relation between the applied stress and time to rupture of the coated specimens can be estimated by using Larson-Miller equation. For the coated specimens tested at low applied stress levels, the product of the minimum creep rate and the time to rupture was a constant value. The θ projection method can be used to accurately characterize the creep behavior of the coated specimens. The variation of the creep strain along with time predicted by using θ projection method agreed well with the experimental results.

Numerical simulation of aggregate dispersion in different flow fields using discrete element method

AbstractAn attempt was made to study the aggregate dispersion process in three different flow fields namely; steady shear, elongation flow, and combined shear and elongational flows using the discrete element method. The simulation was performed on two aggregate structures characterized by their fractal dimensions. The predicted results showed that the aggregate break‐up process evaluated in terms of weighted average fragment size 〈w〉 follows a power–law type relation as 〈w〉 = kt−m in all the three flow fields. The dispersion performance of different flow fields evaluated by dispersing rate and a final steady‐state fragment size was found to be dependent upon the extent of applied stress and flow fields such that at low applied stress levels much smaller steady state values of 〈w〉 could be obtained for the elongational flow. The aggregate structure, characterized by its fractal dimension, was found to have different effects on the aggregate dispersion process depending on the flow field and applied stress level. The results predicted from this simulation could be explained in terms of ability of flow fields in rotating the aggregates and fragments in appropriate position to be broken up and the fractal dimensions of aggregates. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Variations of Fatigue Damage Growth in Cross-Ply and Quasi-Isotropic laminates Under High-Cycle Fatigue Loading

The behavior of transverse crack growth and delamination growth under high-cycle fatigue loadings was investigated with cross-ply CFRP laminates, [0/902]s and [0/906]s, and quasi-isotropic CFRP laminates, [45/0/-45/90]s. As a result, it was observed that the behavior of damage growth was different depending on the applied stress level. The growth of local or edge delamination was exacerbated under the test conditions of a low applied stress level and high-cycle loadings, because the areas of stress concentration were applied with high-cyclic loadings. On the other hand, when the fatigue tests were conducted under the applied stress level of 40% of the transverse crack initiation, the growth of transverse cracks was hardly observed until 108 cycles with [0/902]s, [0/906]s and [45/0/-45/90]s laminates.

Effects of combined plasma-carburizing and shot-peening on fatigue and wear properties of Ti–6Al–4V alloy

The effects of combined plasma-carburizing and shot-peening on fatigue and wear properties of Ti–6Al–4V alloy specimen were investigated. Surface morphology and roughness, microstructure, compressive residual stress, work hardening state, and micro-hardness on the surface modified layer in the Ti–6Al–4V alloy specimen were measured and analyzed. The shot-peening effectively induces highly compressive residual stress and work hardening states on the surface layer of the plasma-carburized Ti–6Al–4V alloy specimen. Consequently, the fatigue life of plasma-carburized Ti–6Al–4V alloy specimen has been significantly improved by subsequent shot-peening. The cracks of both shot-peened and shot-peened carburized specimens initiated on the surface at higher applied stress levels. On the contrary, interior-originating fractures occurred at lower applied stress levels. Corresponding to this behavior, the S– N curves show the shape of two-step stages. The wear resistance of Ti–6Al–4V alloy specimen was also significantly improved by the hardness increase on the surface layer by combination of plasma-carburizing and shot-peening.

Creep deformation and crack growth rates of a super α 2 titanium aluminide alloy

The influences of stress and temperature on creep deformation behavior and the creep crack growth rates of the super α 2 Ti 3Al alloy were investigated with respect to its safe application at high temperatures. In a temperature range of 1033–1093 K at low applied stress levels, the stress exponent was equal to 1.5. At an intermediate stress range (10 −3 < σ/ E < 3 × 10 −3), a stress exponent of 3.3 was observed. As the applied stress was increased, the stress exponent changed from 3.3 to 4.4. The high temperature crack growth rate of the Ti 3Al alloy can be correlated with stress intensity factor K rather than C∗ at 1033 K due to environmental embrittlement.

Effect of surface roughness on step-wise S– N characteristics in high strength steel

Rotary bending fatigue tests have been performed in laboratory air at ambient temperature using specimens with different surface roughness of very high strength steel (Ni–Cr–Mo steel). The effects of surface roughness on crack initiation mechanism and associated step-wise S– N characteristics were studied. Regardless of surface roughness, step-wise S– N curves were seen. In short life regime where cracks initiated at the specimen surface, fatigue life decreased with increasing surface roughness because surface roughness acted as a small notch. On the contrary, in long life regime where subsurface crack initiation occurred, surface roughness exerted no influence on fatigue life because cracks initiated at an inclusion and grew inside the specimens. A fish-eye was always observed in subsurface crack initiation, which had the same morphological features independent of surface roughness. The transition stress, i.e. the conventional fatigue limit, at which the crack initiation site changed from surface to subsurface was significantly affected by surface roughness and the specimens with rougher surface showed lower transition stress. This was also attributed to the notch effect of surface roughness, which caused crack initiation at the specimen surface more readily even at lower applied stress levels. The transition stress could be predicted reasonably by using the area parameter model in which surface roughness was assumed as a small defect.

Evaluation of the fatigue behavior of ductile irons with various matrix microstructures

The stress-failure (S-N) curves for ferritic irons, pearlitic irons, and austempered ductile irons (ADIs) have been determined under tension-tension loading with a stress ratio of 0.1. The effects of Ti contents of up to 0.10 wt pct (resulting from the deliberate use of Ti-containing steel scrap) on fatigue behavior were investigated. It was found that ferritic and pearlitic ductile irons can contain up to 0.10 wt pct Ti without any adverse effect on fatigue behavior. In ADIs, fatigue properties deteriorate at such high Ti contents. Tests were also conducted to investigate the effects of microstructural features on fatigue properties. It was found that the effect of the graphite nodule count (the number of graphite particles on a unit area of a polished surface) on the fatigue limit is significant only in ADIs. Scanning electron microscope (SEM) analysis has shown that cracks usually initiate from surface dross-type defects. However, in ADIs, fatigue cracks can also initiate at shrinkage cavities and at surface or subsurface locations. An offset bilinear S-N curve behavior (the linear S-N curve at higher stress levels is separated from the linear S-N curve at lower stress levels) has been observed in ADIs. This is attributed to surface residual compressive stresses, which prohibit fatigue crack initiation from surface positions at lower applied stress levels. In ferritic and pearlitic ductile irons, the offset bilinear S-N curve behavior is not observed because of the rapid relaxation of the residual compressive stresses.

Effect of Tungsten Substitution on Precipitation Retardation and Stress Corrosion Cracking Properties of 25%Cr-6%Ni Duplex Stainless Steel

Abstract The effect of substituting W for Mo in duplex stainless steel (DSS) was investigated with respect to microstructure and stress corrosion cracking (SCC) behavior. The homogenizing treatment was performed at 1,100°C for 10 min while the aging treatment was performed at 900°C at different holding times. In the homogenized condition, regardless of W substitution, all the specimens had about an equal ratio of ferrite to austenite. In the early stage of aging, the beneficial effect of W modification on the suppression of secondary phase precipitation was very notable. The total amount of secondary phase precipitates was greatly reduced in the W-modified DSS. However, its effect decreased rapidly as the aging time increased. SCC was examined in boiling 42% magnesium chloride (MgCl2) solution by slow strain rate test (SSRT) and constant load test (CLT) methods. In the homogenized condition, the beneficial effect of W was clearly observed at the low applied stress levels, where an electrochemical reaction...

Permanent Deformation In Idealised “Sand Asphalt” Bituminous Mixtures

ABSTRACT This paper investigates the quasi-static permanent deformation (rutting) behaviour of two idealised bituminous mixtures (sand asphalts). The permanent deformation behaviour is assessed using simplified quasi-static uni-axial and tri-axial laboratory tests performed over a range of temperatures, stress levels and strain rates. The results are compared to results from a standard repeated load laboratory test and a more simulative laboratory scale wheel tracking test performed at different temperatures and load levels. Results from the quasi-static uni-axial testing show that the deformation behaviour of the idealised mixtures follows the same pattern as the deformation behaviour of the bitumen used in the idealised mixtures (previously measured). At high stress levels the response is non-linear with a power law creep exponent of approximately 2.4. At lower stress levels the effective power law exponent decreases and the behaviour becomes more linear. The effect of the aggregate is to reduce the strain rate corresponding to a particular applied stress level (i.e. stiffening effect). Results from the tri-axial testing show that this stiffening effect depends on the ratio of mean stress to deviator stress as well as mixture volumetrics and temperature. Analysis of results from a standard repeated load uni-axial test often used as a practical tool to assess the permanent deformation resistance of bituminous mixtures shows that, due to the lower applied stress levels, the non-linear behaviour of the mixture is reduced resulting in lower values of the effective creep exponent. Results from more realistic laboratory scale simulative wheel tracking tests show a similar value of creep exponent to that measured in the quasi-static uni-axial and tri-axial tests due to the higher applied stress levels under the tyre in this test.

Effect of flowing slurry on the delayed failure behavior of polycrystalline mullite

The delayed failure behavior of a normally sintered polycrystalline mullite in flowing slurry was examined with static fatigue tests. A brief discussion was conducted on the experimental data by comparing them with those measured in water. It was concluded that flowing slurry plays an important role in the delayed failure of the present mullite, especially at low applied stress level.

異材接合体におけるき裂近傍の赤外線画像応力場と破壊強度

An explosion clad plate composed of copper and mild steel is dealt with as a typical example of the bonded dissimilar plate. Tensile tests are carried out using rectangular plate specimens extracted from the clad plate. An artificial through-the-thickness edge crack or parallel cracks are made in each specimen. The cracks are close and perpendicular to the explosive interface. When a tensile load is applied perpendicularly to the crack plane, stress field near cracks, stress intensity factor, crack opening displacement and fracture strength are examined by experiment as well as elasto-plastic finite element analysis. The stress field and the stress intensity factor are evaluated by an infrared stress imaging system. The effects of the material inhomogeneity, the residual stress, the change of material characteristics and the interaction of parallel cracks on the stress field, the stress intensity factor, the deformation behavior and the fracture strength are revealed. The lower strength material ahead of the bonded interface increases the stress intensity factor and the crack opening displacement. The higher strength material ahead of the bonded interface decreases the stress intensity factor and the crack opening displacement. The effects of the material inhomogeneity and the interaction of parallel cracks on the crack opening displacement can be explained from the square of stress intensity factor at the low applied stress level. The brittle fracture strength of the inhomogeneous specimen can be evaluated from the stress intensity factor and linear elastic fracture mechanics criterion.

Steady-state creep behavior of a Ti-25al-10Nb-3v-lMo alloy

Creep tests were conducted on Ti-25Al-10Nb-3V-1Mo alloy in the the temperature range of 913 - 1093 K at stresses ranging from 40 to 600 MPa. The creep behavior of the Ti3Al alloy under these testing conditions revealed three different stress exponent regimes. In the temperature range of 1033 to 1093 K at low applied stress levels, the stress exponent was equal to 1.5. At the intermediate stress range (103<σ/E<3x10-3), a stress exponent of 3.3 was exhibited indicating that the creep deformation was controlled by a viscous dislocation glide process As the applied stress increase, the stress exponent changed from 3.3 to 4.4 The activation energy for creep was equal to 288 kJ/mole in the region where viscous dislocation glide was the dominant deformation mechanism (n=3.3) In view of the diffusion data, the rate-controlling species in the viscous glide region was assumed to be Ti lattice diffusion

A Viscoelastic Analysis of the Creep Behaviour of a Chopped Strand Mat E-Glass Fibre Reinforced Vinylester Resin

The viscoelastic response of a chopped strand mat E-glass fibre reinforced vinylester resin has been studied over a wide range of applied stress levels. At low applied stress levels, the material exhibited a linear viscoelastic response well represented by Schapery's power law model with constant C and n terms. At higher stresses nonlinear behaviour was observed which apparently is caused by the multiplicity of complex local phenomena associated with and preceding damage development (plastic deformation of the matrix, interfacial slippage, fiber-matrix debonding). The limits of linear viscoelastic behaviour and of damage initiation – about 0.48% strain or 42 MPa – coincide for this material. However, for successful modelisation of uniaxial creep strain in the nonlinear range a modified power law is proposed which uses stress-dependent creep parameters C and n.

Deformation near parallel cracks close to interface in bonded dissimilar plate

Tensile tests are carried out by using rectangular plate specimens extracted from the explosion clad plate. Artificial parallel cracks are made perpendicularly to the explosive interface in each specimen. When a tensile load is applied perpendicularly to the crack plane, strain, plastic deformation near cracks and crack opening displacement are examined by experiments as well as elasto-plastic finite element analysis. The effects of the material inhomogeneity, the change of material characteristics, the residual stress and the interaction between parallel cracks on the deformation behavior are revealed. The plastic zone around parallel cracks, at relatively low applied stress level, grows mainly ahead of and between two cracks. The interaction between parallel cracks on the crack opening displacement can be explained from the square of stress intensity factor at the low applied stress level and from the plastic deformation at the high applied stress level.

High temperature creep transitions in single crystalline Ni3Al(Ta, B)

The high temperature (T= 1083–1388 K, 0.65–0.84Tm) creep behavior of single crystalline Ni3Al(Ta, B) was investigated. A change in the creep characteristics as a function of applied stress was observed at the uppermost testing temperatures of 1288 and 1388 K. At low applied stress levels the Norton law stress exponent is equal to 4.3; for higher stresses the stress exponent is equal to 3.2. Different creep curves were observed, depending on the value of the stress exponent. The change in stress exponent and nature of the creep curve correspond to a change in the controlling deformation mechanism from dislocation climb to viscous dislocation glide for Ni3Al(Ta, B). The experimentally observed transition stress values between climb and viscous glide are in good agreement with values predicted from theory, assuming that the major force retarding viscous dislocation glide in Ni3Al(Ta, B) is the antiphase boundary interaction.