Softening Region Research Articles

A new flow stress model based on Arrhenius equation to track hardening and softening behaviors of Ti6Al4V alloy

The conventional Arrhenius-type model was adopted to identify the deformation characteristic of Ti6Al4V (TC4) titanium alloy based on the stress–strain curves of isothermal compression test. A new flow stress model based on Arrhenius equation was proposed for TC4, which is composed of peak flow stress (PFS) prediction and strain compensation. The predicted PFS is set as a reference to derive the flow stress model at any strain ranging from approximately 0 to 0.7. The predictability and efficiency among the proposed model, conventional model, and an existing physical-based model of TC4 were comparatively evaluated. It is found that the newly proposed model can simultaneously track the hardening and softening behaviors of TC4 through a single expression while the other existing models are only valid in the softening region. Besides, the wider application range and acceptable accuracy of the new model have been achieved by fewer material constants with much-simplified modeling procedure than the other models.

Learning the deformation mechanism of poly(vinylidine fluoride-co-chlorotrifluoroethylene): an insight into strain-induced microstructure evolution via molecular dynamics.

Learning the micro-mechanisms of fluorinated polymers during mechanical response is more difficult than that of common polymers due to the unique intrinsic characteristics of the fluorine element. In this paper, we applied molecular dynamics simulations to study deformation mechanisms of poly(vinylidine fluoride-co-chlorotrifluoroethylene) during uniaxial tension. We analyzed the variations of individual energy components and structural distribution curves versus strain in addition to the commonly used stress-strain curves and microstructure evolutions during stretching. The elastic limit is ɛ = 0.02, ɛ = 0.06 is the yield point, ɛ = 0.24 is the termination of the softening, necking occurs at 0.24 < ɛ < 0.5, strain hardening occurs at 0.5 < ɛ < 2.6, and ɛ = 2.6 is the damage or brake point. The elastic behavior of the material does not rely on strain rate, the obvious effect of strain rate can be seen at the yield region and strain softening region, and the stress values are not influenced by strain rates at the softening and hardening stages. Overall, total potential energy is mainly correlated with non-bonded energy, and the proportion of ΔEcoul overwhelms all the others. The energy components are ordered: ΔEcoul > ΔEvdwl > > ΔEangle > ΔEdihed > ΔEbond. The chain conformation at yield point is almost unchanged compared with the pre-stretching conformation. The chain conformations at the end of strain softening changes more obviously than that at yield point. The molecular chains maintain random coil structure before strain hardening, and switch into a stretch chain conformation gradually during strain hardening. The maximum change in bond angle during the stretching process is F-C-H, the largest change in bond length is the C-Cl bond, and the largest change in dihedral angle is H-C-C-H. The change of non-bonded interaction in the poly(VDF-co-CTFE) system is much larger than the bonding interaction, and the main factor affecting bonding interaction is the change of angles. Graphical abstract Poly(vinylidine fluoride-co-chlorotrifluoroethylene) during tensionᅟ.

Effects of Overlapping Distances on Steel Microstructure and Properties After Multi-track Laser Quenching

In this study, multi-track laser quenching with different overlapping distances was used to assess SM45C microstructure and properties. Two overlapping distance types were evaluated, including overlapping distances in the transition and martensite regions, respectively. After multi-track quenching, hardness showed a threefold increase. With the overlapping distance in the transition region, surface hardness varied from 600 to 700 HV. There was a region of low hardness in each track. Hardness in the low hardness region was from 300 to 400 HV. The average hardness was basically over 500 HV with the overlapping distance in the martensite region, although there was a softening region. The increased hardness was mainly caused by the martensite. Microstructure in the overlapping region was self-tempered martensite, retained austenite and ferrite. Tempered sorbite and lowly tempered martensite were observed in heat-affected zones. Friction coefficients of quenched specimens varied between 0.275 and 0.325, with a 14% reduction relative to the base metal. The wear rate of the quenched specimen was only 40.8% of the base metal. With time, mass reduction in the quenched specimen was much smaller than that of the base metal. For specimens treated with laser quenching, slight fatigue wear was dominating.

Comparative study on local and global mechanical properties of bobbin tool and conventional friction stir welded 7085-T7452 aluminum thick plate

7085-T7452 plates with a thickness of 12mm were welded by conventional single side and bobbin tool friction stir welding (SS-FSW and BB-FSW, respectively) at different welding parameters. The temperature distribution, microstructure evolution and mechanical properties of joints along the thickness direction were investigated, and digital image correlation (DIC) was utilized to evaluate quantitatively the deformation of different zones during tensile tests. The results indicated that heat-affected zone (HAZ), the local softening region, was responsible for the early plastic deformation and also the fracture location for SS-FSW samples, while a rapid fracture was observed in weld nugget zone (WNZ) before yield behavior for all BB-FSW specimens. The ultimate tensile strength (UTS) of SS-FSW joints presented the highest value of 410MPa, 82% of the base material, at a rotational speed of 300rpm and welding speed of 60mm/min, much higher than that of BB-FSW joints, with a joint efficiency of only 47%. This should be attributed to the Lazy S defect produced by a larger extent of heat input during the BB-FSW process. The whole joint exhibited a much higher elongation than the slices. Scanning electron microscopic (SEM) analysis of the fracture morphologies showed that joints failed through ductile fracture for SS-FSW and brittle fracture for BB-FSW.

A Multitechnique Characterization of Lignin Softening and Pyrolysis

The understanding of lignin softening and pyrolysis is important for developing lignocellulosic biorefinery in order to produce carbon fibers, polymers additives, green aromatics, or biofuels. Protobind lignin (produced by soda pulping of a wheat straw) was characterized by thermogravimetry, calorimetry (for glass transition temperature and heat of pyrolysis reactions), in situ 1H NMR (for the analysis of the mobility of protons upon lignin thermal conversion), and solution-state 13C and 31P NMR (determination of functional groups in lignin). In situ rheology reveals the real-time viscoelastic behavior of lignin as a function of temperature. Upon heating, lignin undergoes softening, through glass transition overlapped with depolymerization, and is followed by the solidification of the softened material by cross-linking reactions. The lignin residues were quenched within the rheometer at the midpoint temperatures of softening and solidification regions and were further analyzed by elemental analysis, GPC-U...

Phase transitions during compression and decompression of clots from platelet-poor plasma, platelet-rich plasma and whole blood.

The objective of this paper is to measure and mathematically model the compression behavior of various human blood clots. We show by a combination of confocal and scanning electron microscopy that compression proceeds by the passage of a front through the sample that separates a densified region of the clot from a rarefied region, and that the compression/decompression response is reversible with hysteresis. These observations form the basis of a model for the compression response of clots based on the continuum theory of phase transitions. Our studies may reveal how clot rheology under large compression in vivo due to muscle contraction, platelet retraction and hydrodynamic flow varies under various pathophysiological conditions and could inform the design of fibrin based biomaterials.

Microstructure and Mechanical Properties of Laser Welded Dual Phase and Mild Steel Joints for Automotive Applications

Recently, the laser welding technology of carbon steel is being widely used compared with arc welding technology for its better welding characteristics. In the present study, the influence of welding conditions of both laser beam welding (LBW) and gas metal arc welding (GMAW) as a comparative study on the weld joint microstructures, hardness distribution and fatigue properties crosses the butt-welded joints of dual phase (DP) steel and mild steel are investigated. The results show that LBW produced narrow welds with complete penetration while GMAW produces wide fusion and heat affected zones. The microstructure of the fusion zone of laser welded DP steels contains mainly bainite, martensite, and a few amount of acicular ferrite phases. Hardness values of the heat-affected zone (HAZ) for dual-phase (DP) steels showing lower values for both LBW and GMAW processes due to the tempering action of the martensite phase. A narrow softening region was clearly observed in the HAZ welded for LBW compared with GMAW. In general, the fatigue life of the welded joints is improved by using laser welding technique.

Evidence of multimodal vibration damping using a single piezoelectric patch with a negative capacitance shunt

In this work, a piezoelectric patch shunted with a negative capacitance circuit has been used to simultaneously damp several modes of a square aluminum plate at low frequencies. The active nature of such electromechanical system leads to regions of instabilities in which the highest vibration attenuation performance appears in the softening region. Once the geometry is fixed, the system has two degrees of freedom, dominated by the electrical parameters of the circuit: the resistance and the negative capacitance. We tune both the value of the negative capacitance, in order to place the structure close to the instability in the softening region, and the resistance of the circuit in such that control the losses of the system. This work shows an optimal design to simultaneously damp several modes with non-zero electromechanical coupling factors using a single shunted patch at low frequencies. The problem is solved numerically and tested experimentally with good agreement. The results show the possibility of controlling the modal response of the system, opening prospects to improve the acoustic comfort with systems using piezoelectric shunted damping circuits with small additional mass with a high tunability by only adjusting the properties of the shunt.

Essential work of fracture assessment for thin aluminium strips using finite element analysis

The fracture toughness of a commercial thin aluminium sheet (1.2mm) is measured by the essential work of fracture (EWF) method using a double-edge notch tension specimen. The EWF method is implemented at room temperature at a deformation speed of 2mm/min. The basics of the EWF method and its relation with J-integral are described in this paper. Ligament yielding is observed to occur at the peak of the load-displacement curve. Following this, necking and tearing occur in the softening region. The EWF for the thin aluminium sheet is measured as 51.5kJ/m2. However, the non-essential work of fracture is dissipated in the tearing process after yielding, causing expansion of the plastic region. In this work, two advanced finite element models, namely, non-linear and linear J-Integral finite element models, are implemented to simulate the essential work of fracture test. The results for the essential and non-essential fracture indicate good agreement with EWF fitting. The linear extended finite element (XFEM) model yields more accurate results than the J-integral method. Crack opening displacement is measured by the EWF method, and is compared with the results of the studied models.

Glass and carbon geogrid reinforcement of asphalt mixtures

Application of geogrids for asphalt layers in road and airport pavements has been studied since early 1980s. This type of geosynthetics is feasible to mitigate reflective cracking and to prolong fatigue life, that is, to reinforce asphalt layers within pavement structure. Up to now, many laboratory and field studies demonstrated significant benefits of using geogrids but still several aspects of their working mechanism within adjacent asphalt layers remain not fully understood and need to be further explored. This paper presents an initial part of the long-term study initiated in Poland in order to fully assess the benefits of using geogrids within asphalt layers. The first part of this paper presents the state-of-the-art review on geogrid applications in asphalt layers. The next part demonstrates and discusses the results obtained on the geogrid-reinforced beams under monotonic and cyclic loading conditions. Three types of large beams were prepared in the laboratory conditions. One group comprises reference beams without any reinforcement and the other two were reinforced, respectively, with glass and carbon geogrids. Specimens were evaluated in a custom-made 3PB/4PB fixture at 13°C in order to observe their behaviour under force-controlled fatigue test and displacement-controlled fracture test. Results demonstrated a beneficial impact of fibre reinforcement on the development of beam deflections in the fatigue tests and significantly higher fracture energy in the post-peak softening region. These results will be used to prepare a new experimental campaign to closely investigate the reinforcement mechanism of geogrids within asphalt layers. The paper concludes with short- and long-term plans necessary to develop an official design procedure for the pavements with geogrid-reinforced asphalt layers in Poland.

Microstructure, mechanical properties and failure mechanisms of resistance spot welding joints between ultra high strength steel 22MnB5 and galvanized steel HSLA350

Resistance spot welding of 22MnB5/HSLA350 and its weldability are investigated. From base material to the nugget, the microstructure of 22MnB5 side can be expressed as M/tempered M+ granular carbides/F+M/fine M/lath M, and on HSLA350 side, F+M/F+flocculent P+tempered M/F+flocculent P+M/F+M/lath M are observed. The heat-affected zone (HAZ) of 22MnB5 exhibits obvious softening region and strengthening region caused by the tempered M and quite fine M, respectively. The peak load of welded joints undergoes two stages of a dramatic increase and a subsequent decrease with increases in the welding current. The maximum value (13.82 kN) of peak load is achieved at a welding current of 8 kA. The failure modes involving interfacial failure (IF), pullout failure from the galvanized steel (PFG), pullout failure from boron steel and tearing of the galvanized steel (PFB-TG) and pullout failure from the boron steel (PFB) are discussed in detail.

Hardening-softening transition in pre-annealed and slightly deformed Ni70Fe30 nanoalloy

The Effect of slight deformation on microstructure evolution and microhardness variation in pre-annealed Ni70Fe30 nanoalloy has been investigated. An obvious hardening-to-softening transition is observed at the rolling strain of ∼6%. Further x-ray diffraction analysis and transmission electron microscopy observation reveal a accumulation of dislocation, stacking fault and twin fault before the sample deformed to ∼6%, which is considered the dominant contribution to strain hardening. Moreover, despite of rolling the sample at a relative small strain level, obvious grain growth takes place overall deformation process. With the increasing grain size, Ni70Fe30 nanoalloy enters into strain softening region due to a decrease in the quantity of defect densities when the rolling strain exceeds ∼6%.

Estimation of Ct for weld cracks including HAZ softening region

Ct is an important parameter in predicting creep crack growth life from the small-scale creep stage to the extensive steady state creep stage. In this paper, weld interface and non-interface creep cracks for compact tension (CT) specimens with heat-affected zone (HAZ) were studied by finite element method (FEM) to systematically research the estimation method of Ct for weld creep cracks. Simulation results showed that Ct of weld interface cracks could be estimated by Yoon’s method. For weld non-interface cracks, Ct under the small-scale and transition creep stages could be exactly estimated by a proposed method. The accuracy of the proposed method was not affected by HAZ, but Yoon’s method was slightly affected by HAZ.

Microstructural Characteristics and Mechanical Properties of 7050-T7451 Aluminum Alloy Friction Stir-Welded Joints

The ultra-high-strength Al-Zn-Mg-Cu alloy, 7050-T7451, was friction stir welded at a constant tool rotation speed of 600 rpm. Defect-free welds were successfully obtained at a welding speed of 100 mm/min, but lack-of-penetration defect was formed at a welding speed of 400 mm/min. The as-received material was mainly composed of coarse-deformed grains with some fine recrystallized grains. Fine equiaxed, dynamic, recrystallized grains were developed in the stir zone, and elongated grains were formed in the thermomechanically affected zone with dynamic recovered subgrains. Grain sizes in different regions of friction stir-welded joints varied depending on the welding speed. The sizes and distributions of precipitates changed in different regions of the joint, and wider precipitation free zone was developed in the heat-affected zone compared to that in the base material. Hardness of the heat-affected zone was obviously lower than that of the base material, and the softening region width was related to the welding speed. The tensile strength of the defect-free joints increased with the increasing welding speed, while the lack-of-penetration defect greatly reduced the tensile strength. The tensile fracture path was significantly influenced by the position and orientation of lack-of-penetration defect.

Improving the structure-property of aluminum alloy friction stir weld by using a non-shoulder-plunge welding tool

In order to improve the joint performances by controlling the thermal effect of tool shoulder, a non-shoulder-plunge (NSP) welding tool is utilized to conduct friction stir welding (FSW) of an Al-Mg aluminum alloy in this paper. Sound FSW joint is successfully produced by using the NSP welding tool. In contrast to conventional FSW (C-FSW), the thermal effect of tool shoulder is effectively reduced by NSP-FSW, leading to a significant size reduction of shoulder-affected zone in the joint. Besides, the NSP-FSW joint shows finer grain structures in the stir zone and thermo-mechanically affected zone, and exhibits lower extents of grain coarsening, dislocation polygonization, and particle dissolution in the heat-affected zone. Hardness maps show that the softening region width of NSP-FSW joint is only nearly half that of C-FSW joint and the NSP-FSW joint exhibits higher hardness in the heat-affected zone than C-FSW joint. The tensile test results indicate that the joint efficiency is improved from 89 % of C-FSW joint to 97 % by the NSP-FSW process.

Deformation behaviors of a Ti-based bulk metallic glass composite in the dendrite softening region

Deformation behaviors of the Ti50Zr20Nb12Cu5Be13 bulk metallic glass composite in the dendrite softening region have been investigated using uniaxial compressive and tensile tests. All the compressive stress–strain curves show work-hardening phenomenon with the same tendency, the tensile stress–strain curves show work-softening phenomenon. The strain rate sensitivity exponent ranges from 0.061 to 0.072 in the compressive tests, and 0.33 to 0.38 in the tensile tests. The activation volume of the Ti50Zr20Nb12Cu5Be13 bulk metallic glass composite at 710K ranges from 1.6nm3 to 2.5nm3 under compressive tests and 1.0nm3 to 2.7nm3 under tensile tests, indicating that there is a migration of atomic group (about 170–180 atoms) during the deformation. The dendrite of the Ti50Zr20Nb12Cu5Be13 BMGC has been well spheroidized and undergoes severe plasticity deformation after the deformation in the dendrite softening region.

Microstructure–property characteristics of a novel non-weld-thinning friction stir welding process of aluminum alloys

Abstract The weld thinning phenomenon occurring in conventional friction stir welding (C-FSW) has negative effects on the formation quality and serviceable reliability of FSW joints. The existing methods for solving the weld thinning problem in FSW exhibit some universal deficiencies, such as low efficiency, low quality and narrow applicable range. Motivated by this challenge, a novel non-weld-thinning (NWT) FSW process, during which a zero shoulder plunge depth is applied to the welding tool, is proposed and investigated in the present paper. The results indicate that the high quality NWT-FSW joints can be efficiently produced by using this novel process. Microstructural analysis implies that the grains in weld nugget of NWT-FSW joints present smaller size and a more uniformly distributed feature than those of C-FSW joints. Compared with the C-FSW joints, the NWT-FSW joints are characterized by narrower softening regions and higher minimum hardness values, leading to an increase in tensile strength during tensile test. The temperature history analysis suggests that the thermal effect of tool shoulder is effectively controlled during NWT-FSW due to the application of zero shoulder plunge depth, which is the intrinsic reason for the grain refinement and strength improvement of NWT-FSW joints.

Chemorheology investigation of a glassy epoxy thermoset on tensile plastic flow and fracture morphology

Reproducible and uncharacteristic tensile stress–strain behavior of cured glassy epoxy-amine networks produces distinctive fracture surfaces. Test specimens exhibiting plastic flow result in mirror-like fracture surfaces, whereas samples that fail during yield or strain softening regions possess nominal mirror-mist-hackle topography. Atomic force microscopy and scanning electron microscopy reveal branched nodule morphologies in the 50-nm size scale that may be responsible for the unusual tensile properties. Current hypothesis is that plastic flow of the glassy thermoset occurs through the existence and deformation of these nodular nanostructures. The thermal cure profile of the epoxy-amine thermoset affects the size and formation of the nodular nanostructure. Eliminating vitrification during thermoset polymerization forms a more continuous phase, reduction in size of the nodules, and eliminates the capacity of the material to yield in plastic flow. This maximizes nanostructure connectivity of the glassy epoxy-amine thermoset and reduces strain to failure significantly. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 1333–1344.

Evaluating the Hot Deformation Behavior of a Super-Austenitic Steel Through Microstructural and Neural Network Analysis

A series of hot compression tests were conducted in the temperature range of 800-1100 °C under the strain rates of 0.001, 0.01, and 0.1 s−1 to assess the flow behavior and microstructure evolution of a super-austenitic stainless steel. The occurrence of dynamic recrystallization has been characterized as the dominant restoration mechanism operating in the investigated range of temperature. This is considered as the main factor affecting the related flow characteristics of the material. To better analyzing the obtained results, an artificial neural network (ANN) model with single hidden layer composed of 20 neurons has been established to simulate the flow behavior of the material. To train the model, a feed-forward back propagation algorithm has been employed. The reliability of the proposed model has been evaluated using standard statistical indices. In addition, the capability of the model has been assessed under the conditions at which the related data were not incorporated in the model. It was found that the developed ANN model employing this algorithm could efficiently track the work hardening and dynamic softening regions of the deforming material.

Fracture process zone and tensile behavior of blended binders containing limestone powder

Higher volumes of limestone can be used as OPC replacement by exploiting its increased reactivity in the presence of aluminous sources, thereby ensuring property-equivalence or improvements (e.g., strength, ionic transport). To examine the improvements in matrix fracture response in blended binder systems, this study characterizes and quantifies the flexural fracture process zone (FPZ) using digital image correlation. For the traditional OPC mortar, the localized strain intensity in the crack vicinity is the highest, FPZ initiates at a lower crack mouth opening displacement (CMOD), and its width is the lowest, while OPC–limestone–metakaolin blends show the lowest localized strain intensity, higher CMOD at FPZ initiation, and the highest FPZ width. The ultimate tensile strain and tensile toughness, extracted via inverse analysis, are found to be the highest for the ternary blends. The tensile modulus and the area under the softening region of the tensile constitutive relationship correlate well to the FPZ parameters.