Expansion Strain Research Articles

Stable and unstable growth of crack tip precipitates

A model is established that describes stress driven diffusion, resulting in formation and growth of an expanded precipitate at the tip of a crack. The new phase is transversely isotropic. A finite element method is used and the results are compared with a simplified analytical theory. A stress criterium for formation of the precipitate is derived by direct integration of the Einstein-Smoluchowski law for stress driven diffusion. Thus, the conventional critical concentration criterium for precipitate growth can be replaced with a critical hydrostatic stress. The problem has only one length scale and as a consequence the precipitate grows under self-similar conditions. The length scale is given by the stress intensity factor, the diffusion coefficient and critical stress versus remote ambient concentrations. The free parameters involved are the expansion strain, the degree of anisotropy and Poisson’s ratio. Solutions are obtained for a variation of the first two. The key result is that there is a critical phase expansion strain below which the growth of the new phase is stable and controlled by the stress intensity factor. For supercritical expansion strains, the precipitate grows even without remote load. The anisotropy of the expansion strongly affects the shape of the precipitate, but does not have a large effect on the crack tip shielding.

Material Selection for Turbine Seal Strips using PROMETHEE-GAIA Method

Material selection plays a very vital role in the designing of a product. Designers often face with the problem of choosing the best material for a product under conflicting criteria. A variety of Multi-Criteria Decision Making (MCDM) tools have been developed by the researchers to aid the designers in making the best choice for material for product. One of the most popularly used MCDM tool is PROMETHEE (Preference ranking organization method for enrichment evaluation). The visual aid developed using PROMETHEE is the PROMETHEE-GAIA software. In the present work material selection for seal strips used in turbines has been done. Five potential materials used for the purpose were: AISI 304, 310, 316, 321 and 430 stainless steels. The criteria on which the selection was made were: Creep strength, Resistance to oxidation, Coefficient of thermal expansion, Yield strength, Toughness and Limit strain. AISI 321 was found to be the optimum material for the seal strips. The Results shows that AISI 316 stainless steel is found to be the most suitable material for the turbine seal strips.

Study on Mechanics‐Based Cracking Resistance of Semiflexible Pavement Materials

Semiflexible mixture is a composite paving material combining the advantages of both asphalt and cement concrete materials. It consists of matrix asphalt skeleton and cement mortar. Due to the different volume characters between asphalt structure and cement mortar, stress concentration always happens in this semiflexible mixture, leading to internal cracking. The objective of this study is to alleviate the internal cracking concern of the semiflexible mixture by adjusting the material components. To this end, optimal material design and numerical simulation have been conducted. Matrix asphalt structures with four different air voids were incorporated with different dosages of cement mortar. The contraction strain and expansion strain of cement mortar as well as the indirect tensile strength of matrix asphalt structure were measured. The results were input into ABAQUS for numerical simulation. The results indicated that (1) the internal stress in this semiflexible mixture is mainly determined by the contraction of cement mortar, rather than expansion; (2) larger air void of matrix asphalt structure and less volumetric variation of cement mortar reduce the internal stress; (3) once the air void of matrix asphalt structure is decided, both maximum contraction and expansion deformations of cement mortar should meet specific requirement to ensure less internal cracking. This is a practical‐ready paper that provides reference for the anticracking design of semiflexible pavement.

Estimation of the Restrained Strains and Self-stresses under Two-way Restraint Conditions

Models for the restrained expansion strains under two-way restraint conditions both with their advantages and disadvantages are presented. An implementation of the modified strains development model (MSDM) for the assessment of the two-way restrained expansion strains is proposed. The reliability of the proposed model for the two-way restraint conditions is confirmed by the experimental results based on the tests of the expansive concrete plane specimens with the mesh reinforcement in the mid-depth of the cross section. DOI: http://dx.doi.org/10.5755/j01.sace.20.3.19082

Effects of curing conditions and addition of fine blast furnace slag powder on the initial performance of concrete using expanding material

In this study was investigated the effect of steam curing in the case of adding expandable material to powdery high fluidity concrete containing blast furnace slag fine powder on two kinds of expansive materials, ettringite type inflation material and lime type expansion material. As a result, the constraint expansion strain increases with the ettringite type due to the increase in the steam curing temperature. However, it was confirmed that it decreased with the lime type expansion material.

Stability of retained austenite in martensitic high carbon steels. Part I: Thermal stability

Thermal stability of retained austenite in 1C-1.5Cr steels with two Si and Mn contents is studied. Time-resolved high resolution synchrotron X-ray radiation and dilatometry are employed. The threshold transformation temperatures, decomposition kinetics, associated transformation strain, as well as the influence of Si and Mn were investigated. The coefficients of linear thermal expansion for both the bulk materials and individual phases are also obtained. The results indicate that an increase in the Mn and Si contents show little influence on the onset of retained austenite decomposition, but result in more thermally stable austenite. The decomposition is accompanied by a simultaneous increase in ferrite content which causes an expansive strain in the order of 10−4, and subsequent cementite development from 300 to 350°C which causes a contraction that helps to neutralise the expansive strain. During decomposition, a continuous increase in the carbon content of austenite, and a reduction in that of the tempered-martensite/ferrite phase was observed. This process continued at elevated temperatures until full decomposition was reached, which could take less than an hour at a heating rate of 0.05°C/s. Additionally, the observation of austenite peak splitting on samples with high Mn and Si contents suggests the existence of austenite of different stabilities in such matrix.

Non-destructive fiber Bragg grating based sensing system: Early corrosion detection for structural health monitoring

Steel corrosion is known as one of the major structural defects in steel structures such as pipelines. The high resolution of fiber Bragg grating (FBG) sensor and its ability to provide real-time monitoring make it a potential candidate for steel corrosion detection. Nevertheless, fiber optic sensor is now been widely used in civil engineering for structural health monitoring purpose. In this study, a non-destructive corrosion detection approach is developed using FBG sensors. The sensor comprises FBG coated with mixed pH-sensitive hydrogel and PDMS strain-sensitive coating. FBG sensors were embedded on the specimen to monitor the expansion strain caused by rebar corrosion. Specimens were placed in different environment conditions namely air, acidic and alkaline conditions, to experience different corrosion rate. The sensing principle is based on a Bragg wavelength shifts resulted from the induced strain on the FBG due to mechanical expansion and swells, in response to the changes in pH of the coating materials. A significant wavelength shift occurred at about day 20, inducing wavelength shift of 0.27nm, 0.06nm and 0.011nm under acidic, air and alkaline conditions respectively. The relationship between wavelength shift, corrosion rate and strain induced is investigated and validated through this experiment. These sensors can be installed for real-time monitoring and early corrosion detection due to its non-destructive and highly sensitivity performance.

Landau theory and giant room-temperature barocaloric effect in MF3 metal trifluorides

The structural phase transitions of MF$_3$ (M=Al, Cr, V, Fe, Ti, Sc) metal trifluorides are studied within a simple Landau theory consisting of tilts of rigid MF$_6$ octahedra associated with soft antiferrodistoritive optic modes that are coupled to long-wavelength strain generating acoustic phonons. We calculate the temperature and pressure dependence of several quantities such as the spontaneous distortions, volume expansion and shear strains as well as $T-P$ phase diagrams. By contrasting our model to experiments we quantify the deviations from mean-field behavior and found that the tilt fluctuations of the MF$_6$ octahedra increase with metal cation size. We apply our model to predict giant barocaloric effects in Sc substituted TiF$_3$ of up to about $15\,$JK$^{-1}$kg$^{-1}$ for modest hydrostatic compressions of $0.2\,$GPa. The effect extends over a wide temperature range of over $140\,$K (including room temperature) due to a large predicted rate $dT_c/dP = 723\,$K GPa$^{-1}$, which exceeds those of typical barocaloric materials. Our results suggest that open lattice frameworks such as the trifluorides are an attractive platform to search for giant barocaloric effects.

Stress and Deformation Characteristics of Nonwoven Geotextile Reinforced Sand Under Different Directions of Principal Stress

A series of drained torsional shear tests using a hollow cylinder apparatus HCA was performed to investigate the anisotropic characteristics of sand in terms of the stress–strain and volumetric changes of large scale sand samples reinforced with nonwoven geotextile under different directions of principal stress α. Results revealed that the inclusion of geotextile resulted in a significant increase in the peak deviator stress when α was 0°, 15° and 30°, corresponding with considerable restraint against the expansion strain of the samples. When α was rotated towards the horizontal, α = 60–90°, reinforced samples exhibited clear anisotropic behavior. Variations in the peak deviator stress between the maximum value occurred at α = 0° and minimum at α = 60° was about 30–47% (depending on the number of geotextile layers). This indicates that the ability of layers embedded horizontally resisting the shearing decreased as the principal stress rotated towards the bedding plane. Under the conditions of α = 60–90° tensile strength of the geotextile could not be mobilized to the same extent, compared to α = 0–30°. Larger values of α resulted in less expansion strain of the reinforced samples.

Experimental Study of Thermal Expansion Characteristics of Coaly Mudstone at High Temperatures

This study investigated the thermal expansion characteristics of coaly mudstone specimens at temperatures ranging from room temperature up to 800 °C. The experiments were performed by an MTS810 electro-hydraulic servo system and an MTS653.02 high-temperature furnace. Results revealed that mudstone experienced thermal expansion at high temperature, and that the relationship curve between thermal expansion strain and testing time could be divided into three stages. With increasing temperature, mudstone’s thermal strain and expansion coefficient exhibited an upward trend. With 400 °C as the critical temperature, the curve could be divided into (1) rapid increase stage, (2) flat stage, and (3) rapid increase stage. The major causes behind thermal expansion are changes in internal pores and fissure, as well as the thermochemistry effect of mineral components.

Infrared spectroscopic study of sulfate-bearing calcite from deep-sea bamboo coral

The powder infrared spectra of a sulfate-bearing calcitic coral sample are recorded at room temperature in attenuated total reflectance (ATR) geometry and at low temperature in transmission geometry. The comparison of ATR spectra recorded with diamond or Ge crystal confirms that the width and the shape of the prominent absorption bands, related to CO 3 groups, are dominantly affected by macroscopic electrostatic interactions. In contrast, the temperature-dependence of the position and width of weak and narrow absorption bands, related to CO 3 or SO 4 groups, can be interpreted in terms of thermal expansion, anharmonic coupling, and local strain fluctuations. The three SO 4 stretching bands display a contrasted thermal behavior, the frequency of one of them increasing with temperature. Based on first-principles calculation, this unusual temperature dependence is traced back to the anisotropy of calcite thermal expansion. These results sustain the previously proposed atomic-scale model of sulfate in calcite and attest to a dominantly structural nature of sulfate in the investigated sample. Accordingly, structurally substituted sulfur in deep-sea calcitic bamboo coral could be used as proxy of seawater sulfate.

Effects of foaming parameters on microstructure and compressive properties of aluminum foams produced by powder metallurgy method

Semi open-cell aluminum foams having channels between individual cells were produced using low cost CaCO3 foaming agent and applying the powder compact melting process. To this end, the aluminum and CaCO3 powder mixtures were cold compacted into dense cylindrical precursors for foaming at specific temperatures under air atmosphere. The effects of several parameters including precursor compaction pressure, foaming agent content as well as temperature and time of the foaming process on the cell microstructure, linear expansion, relative density and compressive properties were investigated. A uniform distribution of cells with sizes less than 100 μm, which form semi open-cell structures with relative densities in the range of 55.4%–84.4%, was obtained. The elevation of compaction pressure between 127–318 MPa and blowing agent up to 15% (mass fraction) led to an increase in the linear expansion, compressive strength and densification strain. By varying the foaming temperature from 800 to 1000 °C, all of the investigated parameters increased except compressive strength and relative density. The results indicated the optimal foaming temperature and time as 900 °C and 10–25 min, respectively.

Analyzing Hysteresis and Thermal Expansion Coefficient of M40/AZ91D during Thermal Cycling

In this work, AZ91D composite reinforced with M40 fiber was prepared by pressure impregnation method. Expansion behaviors of M40/AZ91D composite were studied with thermal expansion instrument in 25-150 °C and 25-150 °C temperature ranges of internal heat circulation, and then analyzed the influence law of hysteresis, residual strain and coefficient of thermal expansion (CTE) by different temperature change rate. The results revealed that residual stress and strain in process of pressure impregnation would lead to strain hysteresis and residual strain of composite in thermal cycling. At the same time, the CTE of the composites decreased with the increase of cycle times. Residual strain of the composite went up with the rise of temperature changed rate of the thermal cycling and CTE decreased with temperature change rate ascending in the 25-150°C temperature range. The CTE of the composites decreased with the increase of temperature during the heating process, which cut down with the increase of temperature in process of heating and cooling in the 25-495°C temperature range.

A chemomechanical coupling model for stress analysis of oxide scale growing between ceramic coating and substrate

The thermal growth of the oxide scale between the ceramic coating and substrate due to high-temperature oxidation is regarded as the dominating cause for the failure of the ceramic/substrate system, for example the thermal barrier coating. Based on the irreversible evolution equations, the growth strain in the oxide scale is formulated by considering the coupling effects of stress and chemical reaction during isothermal oxidation. A model which accounts for the growth strain, thermal expansion strain, and viscoplastic effect is developed for the stress analysis of the oxide scale. Numerical results reveal that there is a significant gradient in the oxide scale with the maximum compressive stress at the oxide/substrate interface and the minimum compressive stress at the ceramic/oxide interface, and the stress in the oxide scale is larger than those in the ceramic coat and bond coat. The effects of the growth strain, viscoplasticity, chemomechanical coupling, and chemical reaction on the stresses are numerically discussed.

Study on stress-strain relation of concrete confined by CFRP under preload

This paper presents the experimental and analytical results of 32 circular-section and 16 square-section concrete columns confined by CFRP sheets under axial compression. The main parameters were: the preload ratio, the concrete strength and the number of CFRP piles. The failure modes, mechanical behaviors, and reasons of the descending of the peak stress and strain of the confined concrete in columns were analyzed. The experiment results indicated that the effectiveness of CFRP confinements were influenced by the preload ratio observably. Both the ultimate stress and strain decreased with the increasing of the preload ratio, and the increasing of CFRP piles will magnify the negative effect. Combined with the previous literatures, this paper concluded formulas about the second stiffness, lateral expansion coefficient, peak stress and strain of the concrete columns confined by CFRP under a certain preload ratio. Furthermore, based on these concluded formulas, a new design-oriented stress-strain model for the circular and square concrete column confined by CFRP sheets under preload was purposed respectively.

A new approach for the cylindrical cavity expansion problem incorporating deformation dependent of intermediate principal stress

The problem of cylindrical cavity expansion incorporating deformation dependent of intermediate principal stress in rock or soil mass is investigated in the paper. Assumptions that the initial axial total strain is a nonzero constant and the axial plastic strain is not zero are defined to obtain the numerical solution of strain which incorporates deformation-dependent intermediate principal stress. The numerical solution of plastic strains are achieved by the 3-D plastic potential functions based on the M-C and generalized H-B failure criteria, respectively. The intermediate principal stress is derived with the Hook\'s law and plastic strains. Solution of limited expansion pressure, stress and strain during cylindrical cavity expanding are given and the corresponding calculation approaches are also presented, which the axial stress and strain are incorporated. Validation of the proposed approach is conducted by the published results.

Damage evaluation of concrete based on Brillouin corrosion expansion sensor

To evaluate the damage degree of reinforced concrete due to steel bar corrosion, a damage factor is proposed to quantitatively evaluate the damage degree of concrete before initial cracking and during the development of cracks. Brillouin optical fiber time domain analysis (BOTDA) sensors are fabricated to monitor the expansion strain of steel corrosion. Two concrete specimens embedded with corrosion sensors are cast. An accelerated corrosion experimental program is used to accelerate the process of steel corrosion. The experimental results show that the corrosion sensor can be used to monitor the expansion strain of steel corrosion in real-time. At last, to map the monitoring results with the damage factor, finite element analysis is used to simulate the process of steel corrosion to determine the cracking strain of the interfacial concrete and concrete cover.

Monitoring Concrete Deterioration Due to Reinforcement Corrosion by Integrating Acoustic Emission and FBG Strain Measurements.

Corrosion of concrete reinforcement members has been recognized as a predominant structural deterioration mechanism for steel reinforced concrete structures. Many corrosion detection techniques have been developed for reinforced concrete structures, but a dependable one is more than desired. Acoustic emission technique and fiber optic sensing have emerged as new tools in the field of structural health monitoring. In this paper, we present the results of an experimental investigation on corrosion monitoring of a steel reinforced mortar block through combined acoustic emission and fiber Bragg grating strain measurement. Constant current was applied to the mortar block in order to induce accelerated corrosion. The monitoring process has two aspects: corrosion initiation and crack propagation. Propagation of cracks can be captured through corresponding acoustic emission whereas the mortar expansion due to the generation of corrosion products will be monitored by fiber Bragg grating strain sensors. The results demonstrate that the acoustic emission sources comes from three different types, namely, evolution of hydrogen bubbles, generation of corrosion products and crack propagation. Their corresponding properties are also discussed. The results also show a good correlation between acoustic emission activity and expansive strain measured on the specimen surface.

Evaluation of time-dependent strain loss in CFRP eccentrically reinforced Shrinkage Compensating Concrete (SHCC) flexural members

Eccentrically CFRP enclosed Shrinkage Compensating Concrete (SHCC) flexural members show benefits of easy prestressing, better durability and self-made camber compared with traditional prestressed concrete. This paper presents the experimental and analytical work to evaluate the expansion strain losses in the proposed flexural members. A proposed analytical model to calculate the time-dependent strain losses is established including variables of shrinkage and creep of SHCC concrete based on GL2000 model. The relationship between maximum expansion strain and axial stiffness of FRP (EA) is also constructed. The same procedure was also conducted for Portland Cement Concrete (PCC) for the CFRP-PCC beam specimens. It is indicated that the proposed analytical model produces an accurate prediction in strain losses over time.

Constitutive analysis of electrically-assisted tensile deformation of CP-Ti based on non-uniform thermal expansion, plastic softening and dynamic strain aging

Electrically-assisted (EA) deformation has shown promising effects in increasing formability and reducing springback in sheet metal forming. This work experimentally and analytically investigated the influence of non-uniform and transient Joule heating on the plastic flow stress of titanium which evolved nonlinearity with time and deformation. Three-stage constitutive analysis was carried out in the present study. First, the pure influence of non-uniform Joule heating was investigated in terms of linear elastic thermal expansion, which then explained the measured drops in stress. Second, the Johnson-Cook model was adopted to interpret a plastic thermal-mechanical behavior of the material loaded at a constant quasi-static deformation rate under the uniaxial tension combined with a single pulse of electric current. Finally, it was revealed that the sudden change in strain rate and rapid heating rate due to an electric current pulse could give rise to the transient occurrence of dynamic strain aging (DSA) in materials. This resulted in an accumulated plastic strain as well as transient high-temperature strain hardening, which estimated the experimentally measured data well. The DSA contribution revealed in this work could help to explain many observations in the past studies in the field of EA deformation.