True Axial Strain Research Articles

Synthesis and Workability Behavior of Cu-X wt.% TiC (x = 0, 4, 8, and 12) Powder Metallurgy Composites.

In this work, copper (Cu) matrix composite reinforced with titanium carbide (TiC) was fabricated by powder metallurgy (PM) method with the varying TiC content from 0% to 12% by weight in the step of 4%. The required weight percentage of powders was milled in an indigenously developed ball milling setup. Green compacts were made using a computer-controlled hydraulic press (400 kN) and sintered in a muffle furnace at a temperature of 950°C. Scanning electron microscope (SEM) was used to analyze the distribution of TiC particles in Cu matrix in as-sintered conditions. X-ray diffraction (XRD) analysis resulted in the existence of respective phases in the produced composites. The structural characteristics such as stress, strain, dislocation density, and grain size of the milled composites were evaluated. Cold upsetting was conducted for the sintered composites at room temperature to evaluate the axial (σz), hoop (σө), hydrostatic (σm), and effective (σeff) true stresses. These stresses were analyzed against true axial strain (εz). Results showed that the increase in the inclusion of weight percentage of TiC into the Cu matrix increases density, hardness, (σz), (σө), (σm), (σeff), and stress ratio parameters such as (σz/σeff), (σθ/σeff), (σm/σeff), and (σz/σθ) of the composites.

A Conical Striker Bar to Obtain Constant True Strain Rate for Kolsky Bar Experiments

Many split Hopkinson pressure bar (SHPB) or Kolsky bar techniques use pulse shaping methods to obtain constant engineering strain rate for the specimen response. However, constitutive models for numerical simulations use the axial rate of deformation which is the true axial strain rate. In this study, we present an equation for the incident bar strain produced by a truncated conical striker bar. These incident bar strains are shaped so that we can obtain constant true strain rate for the specimen.

Effect of titanium in aluminium matrix on densification and forming limit of P/M composites during upsetting process

The present work investigates the densification and forming limit of sintered porous aluminium–titanium preforms for various relative densities and titanium particles. The compacts of 1 aspect ratio with 2–6% of titanium contents in aluminium matrix for various relative densities were prepared through powder metallurgy route by applying suitable powder forming pressures. A sequence of the cold upsetting test has been carried out using 0.5 MN capacity of hydraulic press machine at 0.1 s−1 strain rate. The relation between true axial strain and preforms relative density was plotted, and the influence of titanium and relative density on the densification curve was investigated at various compaction pressures. The specimen properties like apparent strain hardening exponent (na) and apparent strength coefficient (Ka) was determined through stepwise incremental deformation tests. The critical transition density (CTD) vide forming limit diagram was determined from the plot of na and Ka concerning the preforms relative density, and it was found to be 85.9%, 87.7% and 88% for 2%, 4% and 6% of titanium in the aluminium matrix. The CTD values of porous aluminium–titanium preforms were compared with those of the previous work of porous aluminium–copper preforms under the same compositions. Also, the average grain size of the components was determined and it is observed that 2% of titanium contains a larger grain size compared to other compositions. Hence, a higher forming limit was obtained in 2% of titanium reinforced with aluminium matrix.

Synthesis, characterization and forming behavior of hybrid copper matrix composites produced using powder metallurgy

AbstractHybrid copper matrix composites containing 5 wt.% of titanium dioxide and varying graphite content (0 wt.%, 2 wt.% and 4 wt.%) were synthesized using powder metallurgy. Metallurgical studies were carried out to examine the presence and distribution of reinforcements in the copper matrix. To investigate the forming behavior of the sintered composite preforms, cold upset tests were conducted from which the true axial stress, the true hoop stress, the true hydrostatic stress and the true effective stress were evaluated and their relationship with the true axial strain was analyzed and presented. It is observed that the increase in addition of weight percentage of graphite into the copper matrix increases the true axial, the true hoop, the true hydrostatic and the true effective stresses. The variation of hardness, strength coefficient and strain hardening with respect to the addition of graphite content is also evaluated and reported.

Influence of Temperature on the Workability and Hardness of Sintered Al-4%B4C in Upsetting Test

The present investigation has been made on the influence of temperature on the workability behavior, stresses and hardness of sintered Al-4%B4C at a strain rate of 0.1s-1 during upsetting. Al-4% B4C preforms were prepared with 90% of initial preform density and sintered at 550±10oC about 60min. These sintered preforms were compressed in a hydraulic press at a strain rate of 0.1 s-1 until the initiation of the cracks appears on the free surface. The various stresses (σz, σθ, σm, σeff), formability stress index (β) and stress ratio parameters (σz/σeff, σm/σeff, σθ/σeff) were calculated and correlated with the true axial strain (ɛz). Also the effect of different deformation temperatures on the hardness of Al-4%B4C preforms was studied in detail.

Hot Upset Studies on Sintered (Al–TiO2–Gr) Powder Metallurgy Hybrid Composite

Aluminum hybrid composites were synthesized using powder metallurgy process from ball milled powders to yield the compositions like: Al+0%TiO2, Al+5%TiO2, Al+5%TiO2+2%Gr and Al+5%TiO2+4%Gr. SEM and EDAX analysis of sintered preforms show the uniform distribution of the reinforcements. Hot upset process was carried out after heating the preforms at 450°C, and the densification and deformation characteristics were studied by correlating the true axial stress with the true axial strain, lateral strain, and theoretical density. Addition of TiO2 to the unreinforced Al and Gr reinforcements to the Al–TiO2 composite increase the induced stresses during hot upset process. The microstructures of hot upset samples are analyzed.

Synthesis and forming characteristics of Al–TiO2 powder metallurgy composites during cold upsetting under plane stress state conditions

Aluminum-based metal matrix composites with various weight percentages of TiO2 reinforcement were synthesized by powder metallurgy technique and their forming characteristics were completely studied during cold upsetting under plane stress state conditions. The milled powders were characterized by scanning electron microscope, X-ray diffraction (XRD), and energy dispersive analysis using X-ray (EDAX) analysis. Green cylindrical compacts were made using a 400 kN hydraulic press using suitable punch and die, sintered at (590 ± 10)℃ for a period of 3 h, furnace cooled, and machined to obtain samples with 0.5 aspect ratio. The cold upset tests were carried out in steps of 10 kN and the true axial stress (σ z), the true hoop stress (σθ), the true hydrostatic stress (σ m), and the true effective stresses (σeff) were determined, and their behavior against the true axial strain (ɛ z) was thoroughly analyzed. It was observed that, the addition of TiO2 to the Al matrix material increases the strength coefficient ( K) and decreases the strain hardening index ( n). The decrease in densification and deformation characteristics was observed for the addition of more weight percentages of TiO2. The microstructure analysis was done for the cold upset samples and reported.

Workability studies on Al+2.5%TiO2+Gr powder metallurgy composites during cold upsetting

Al+2.5%TiO2+Gr composites were synthesized through powder metallurgy technique and their workability behavior has been studied under tri-axial stress state condition. The powders were compacted using suitable punch and die in 400 kN hydraulic press and sintered at a temperature of about 590 ºC for a period of 3 hours in an electric muffle furnace. The sintered preforms were characterized by XRD and SEM analysis. Then the preforms were subjected to incremental compressive loading of 10 kN until the cracks found at the free surface. The true axial stress (σz), true hoop stress (σ}), true hydrostatic stress (σσm) true effective stress (σeff), formability stress index (β) and stress ratio parameters [(σz/σeff), (σm/σeff), (σ}/σeff)] were calculated for all the preforms and these are correlated with the true axial strain (e z). It is inferred that, the addition of TiO2 and Gr to the Al matrix material increases the stress ratio parameters and formability stress index (β). A hardness test was also conducted for sintered and cold forged preforms and results are reported.

Densification and deformation studies on powder metallurgy Al–TiO2–Gr composite during cold upsetting

Abstract

Effect of TiO2 in aluminum matrix on workability behavior of powder metallurgy composites during cold upsetting

Abstract Aluminum-based metal matrix composites with various weight percentages (0, 2.5, 5, 7.5, and 10) of TiO2 reinforcement were synthesized by means of powder metallurgy and their workability behavior under triaxial stress-state conditions was completely studied during cold upsetting. Green cylindrical compacts were made using a 400 kN hydraulic press with suitable punch and die, sintered at (590 ± 10) °C for a period of 3 h, furnace cooled and machined to obtain samples with 0.5 aspect ratio. The cold upset tests were carried out in steps of 10 kN and the results were studied for the workability behavior of the composites. The formability stress index (β) and stress ratio parameters (σz/σeff), (σm/σeff), and (σθ/σeff) were calculated for all composites and correlated with true axial strain (∊ z ). It was observed that the addition of TiO2 to the Al matrix material increases the stress ratio parameters and formability stress index (β).

Al–TiO2–Gr powder metallurgy hybrid composites with cold upset forging

The cold upsetting studies were carried out for the aluminium metal matrix hybrid composites in the present study. Aluminium metal matrix hybrid composites were synthesised through powder metallurgy route from ball-milled powders to yield the following compositions: Al + 2.5 wt% TiO2 + 2 wt% Gr, Al + 2.5 wt% TiO2 + 4 wt% Gr, Al + 5.0 wt% TiO2 + 2 wt% Gr and Al + 5.0 wt% TiO2 + 4 wt% Gr. The compaction process was carried out using suitable punch and die in 40 kN hydraulic press, and sintering was done in an electric muffle furnace at the temperature of 590 °C for 3 h. The sintered preforms were subjected to incremental compressive loading of 10 kN until the cracks were found at the free surface. The true axial stress, true hoop stress, true hydrostatic stress and true effective stress were calculated for all the preforms, and all these stresses are correlated with the true axial strain. The stress ratio parameters (σ z/σ eff, σ θ/σ eff, σ z/σ m and σ θ/σ m) of the all preforms were correlated with true axial strain. The maximum true axial stress, true hoop stress, true effective stress and hydrostatic static stress are obtained for the composite containing 5 wt% of TiO2 and 4 wt% of graphite and the minimum ones are obtained for composite containing 2.5 wt% of TiO2 and 2 wt% of graphite.

Synthesis and forming behavior of aluminium-based hybrid powder metallurgic composites

Aluminium-based metal matrix composites were synthesized from Al-TiO2-Gr powder mixtures using the powder metallurgy technique and their forming characteristics were studied during cold upsetting. Green cylindrical compacts of pure Al, Al-5wt%TiO2, Al-5wt%TiO2-2wt%Gr, and Al-5wt%TiO2-4wt%Gr were made using a 400-kN hydraulic press equipped with suitable punch and die and by sintering at (590 ± 10)°C for 3 h. Cold upset forging tests were carried out, the true axial stress (σ z ), the true hoop stress (σ θ ), and the true hydrostatic stress (σ m) were evaluated and, their behavior against the true axial strain (ɛ z ) was also analyzed. It is observed that the addition of 5wt% TiO2 into the Al matrix increases σ z , σ θ , and σ m. The addition of both TiO2 and Gr reinforcements reduces the densification and deformation characteristics of the sintered preforms during cold upsetting. Microstructure analyses of the as-sintered and cold upset forged specimens also were carried out to substantiate the experimental results.

True intrinsic mechanical behaviour of semi-crystalline and amorphous polymers: Influences of volume deformation and cavities shape

A model enabling the determination of the intrinsic mechanical constitutive equations of uniaxially stretched polymers is presented. This model takes into account the cavitation-induced volume strain which can occur during the deformation of such materials. In particular, the true intrinsic axial stress and strain depends on the overall volume strain and a form factor depicting the evolution of the voids shape. Based on our model, the true intrinsic behaviour of high-density polyethylene (HDPE), polypropylene/ethylene-propylene rubber (PP/EPR), and polyethylene terephtalate (PET) was assessed in tension. Compared to the overall true behaviour, the intrinsic true behaviour of the materials did not exhibit anomalies at large strain levels with changing experimental parameters (strain rate and temperature), and can be accurately predicted by means of phenomenological constitutive equations as the one proposed by G’sell and Jonas (1979).

Influence of carbon content on densification behaviour in forming of sintered plain carbon steel preforms

Complete experimental investigation on the deformation and densification behaviour of sintered plain carbon steel cylindrical preforms with carbon contents of 0, 0.35, 0.75 and 1.1 per cent, under cold upsetting, have been studied in order to understand the influence of various forming parameters such as true axial strain, flow stress, Poisson's ratio, forming limits and formability stress index on the densification process. The above-mentioned powder metallurgy sintered preforms with constant initial theoretical density of 84 per cent and aspect ratio of 0.4 were prepared using a suitable die-set assembly on a 1 MN capacity hydraulic press and sintered for 90 min at 1200 °C. Each sintered preform was cold upset under two different frictional constraints, namely, nil/no and graphite lubricant conditions. Densification–true strain and fracture strain–carbon content relationship was established and presented in this work. Further, attained density is considered to establish flow stress and formability stress index behaviour. The present work provides a guideline for producing P/M components free from open surface pore.

Constitutive analysis on formability of aluminium iron composite preforms using artificial neural network

Cold upsetting experiments were carried out on sintered aluminium iron composite preforms in order to analyse the formability. A Neural Network (NN) model has been developed with a radial basis neural network algorithm to find the effect of particle size and iron content on formability. The training data were collected by the experimental setup in the laboratory for the sintered aluminium iron composite preforms with various preform densities, particle size, iron content and aspect ratios. The network is trained to predict the deformation characteristics such as formability index β, instantaneous strain hardening coefficients ki and ni and instantaneous density coefficients Bi and Ci. It is found that formability index increases rapidly at the earlier stages of deformation and followed by a gradual increase with further increase in true axial strain. It is also observed that coarse particle size and higher addition of iron content exhibits improved formability index values. In addition, instantaneous density coefficients Bi and Ci of the aluminium iron composite preforms were mathematically evaluated and simulated. Regression analysis has confirmed a well coincidence between predicted and experimental data with more accuracy. Hence, this approach helps to facilitate a knowledge base in order to generate advice for the designer at the earlier stages of design by forecasting and preventing the surface defects at the computer design stage itself. It also avoids the cost of prototyping or trialing.

Plastic deformation of low-density polyethylene reinforced with biodegradable polylactide, Part 2: Creep characterization and modeling

The behavior of low-density polyethylene (LDPE) and two blends prepared with polylactide (PLA) was determined by means of a novel video-controlled testing method under stretching at constant true strain rate, under creep at constant true stress, and under creep at constant nominal stress. Most tests were performed at 23°C and 50°C. In this second part, the experimental data are modeled with the G'Sell-Jonas phenomenological law expressing the axial true stress versus axial true strain and axial true strain rate. This model describes correctly the various deformation stages: (i) initial viscoelasticity, (ii) plastic yielding, and (iii) strain hardening up to rupture. It shows clearly the reinforcing effect of the PLA particles that increases the yield stress in stretching experiments and slows down the deformation kinetics under creep. It is shown how the local stress/strain behavior is related to the standard force/extension curves. Consequently, it is proposed that tensile tests at constant true strain rates should be systematically preferred to creep tests for the characterization of constitutive relations because they take much less time to be performed. POLYM. ENG. SCI., 2011. © 2010 Society of Plastics Engineers.

Effect of strengthening mechanisms on cold workability and instantaneous strain hardening behavior during grain refinement of AA 6061-10 wt.% TiO 2 composite prepared by mechanical alloying

The mechanical alloying (MA) of AA 6061 alloy reinforced with 10 wt.% fine anatase-titania composites powder milled with different timings (1, 5, 10, 20, 30, and 40 h) was cold consolidated and sintered. The main purpose of this study is to investigate the effect of microstructure and the various strengthening mechanisms such as solid solution, grain size, precipitate, dislocation and dispersion strengthening during grain refinement of AA 6061-10 wt.% TiO 2 composite via MA on cold working and strain hardening behavior. The sintered composite preforms were characterized by X-ray diffraction, scanning electron microscope, and transmission electron microscope. The strengthening mechanisms were estimated by using simplified models available in the literatures. The evaluation of cold deformation behavior under triaxial stress condition through room temperature cold-upsetting tests (incremental loads) was studied by correlating the strengthening mechanisms. Among the developed strengthening mechanisms the grain size and dislocation strengthening mechanisms diminished the deformation capacity of the composites. The strain hardening behavior was also examined by proposing instantaneous strain hardening index ( n i ). The value of maximum instantaneous strain hardening index for ultrafine-grained composite was to be about twice that of coarse-grained composite at lower true axial strain value. Also, the coarse grained 5 h composite was observed to be the best one as it exhibited a better strain hardening or deformation behavior.

Constitutive equations for thermomechanical deformations of glassy polymers

Poly-Carbonate (PC) and Poly-Methyl-Methacrylate (PMMA) are lightweight and mechanically tough transparent glassy polymers. Their mechanical behavior at low to moderate strain rates has been well characterized; however, that at high strain rates needs additional work. We propose two modifications to existing pressure-dependent viscoplastic constitutive equations that enable one to simulate better mechanical deformations of PC and PMMA at high strain rates. First, the elastic moduli are taken to depend upon the current temperature and the current effective strain rate. Second, two internal variables are introduced to better characterize the strain softening of the material at high strain rates. A technique to find values of newly introduced material parameters is described. We compute the local temperature rise due to energy dissipated during plastic deformations. The true axial stress vs. the true axial strain curves in uniaxial compression from numerical simulations of the test configurations at high strain rates using the proposed constitutive equations are found to agree well with the experimental results available in the literature.

An analysis of formability of aluminium preforms using neural network

Cold upsetting experiments were carried out on sintered aluminium preforms in order to model and analyze the formability of aluminium preforms by using Neural Network (NN). A NN model has been explored with a radial basis neural network algorithm. The training data used was obtained from the experimental set up in the laboratory for the sintered aluminium with various preforms densities and different aspect ratios by using MoS 2 as lubricant. The network was trained to predict the formability index ( β), triaxial stress ratio parameters and axial strain. It is deduced that a lower aspect ratio and higher initial fractional density preforms exhibits improved formability index values compared to that of higher aspect ratio preforms. In addition to that, instantaneous stress ratio parameter coefficients B i and A i of the aluminium preforms were mathematically evaluated and simulated. It is found that it increases rapidly at the earlier stages of deformation and followed by a gradual increase with further increase in true axial strain. Regression analysis has confirmed a good coincidence between predicted and experimental data with small error and hence this approach helps to facilitate a knowledge base in order to generate advice for the designer at the earlier stages of design in the metal forming industries by forecasting and preventing the surface defects at the computer design stage itself and avoids the cost of prototyping or trailing.

Experimental characterization of the volume strain of poly(vinylidene fluoride) in the region of homogeneous plastic deformation

AbstractDamage to poly(vinylidene fluoride) was studied under uniaxial tension testing at different temperatures and strain rates. A satisfactory accuracy was reached with an original procedure for the assessment of the principal strains and volume strain. The semicrystalline polymer exhibited an increase in the volume strain with the true axial strain caused by material damage. This damage depended on the temperature and strain rate and was related to the whitening phenomenon generally observed. The use of the general ideas of Bucknall's model, based on the decomposition of the whole volume strain into three components, allowed the extraction of the component of the volume strain corresponding to microstructural mechanisms of plastic deformation other than shearing. Therefore, nucleation and growth stages of cavitation were revealed. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2516–2522, 2002