Decrease In Yield Stress Research Articles

Tensile and energy density properties of 2024, 6013, 8090 and 2091 aircraft aluminum alloy after corrosion exposure

Evaluation was made for the corrosion susceptibility of aircraft structure aluminum alloys 2024 T351, 6013 T6, 8090 T81 and 2091 T84. Tensile and energy density data were obtained. Stereoscopic and metallographic corrosion analysis were made as well. The specimens were pre-corroded using accelerated laboratory corrosion tests or out-door atmospheric conditions before testing. Noticeable decrease of yield and ultimate tensile stress were detected when the specimen surface was corroded. Dramatic volumetric embrittlement was observed even after short exposure times that were not sufficient for the appreciable development of surface corrosion attack. Observed material degradation behavior is attributed to hydrogen penetration and absorption.

Effects of the degree of graft on the tensile and dynamic behavior of high impact polystyrene

High Impact Polystyrene (HIPS) polymers varying widely in the degree of graft, size of the rubber particles and volume fraction of the rubber phase have been tested in uniaxial tension. From these tests, it is shown that the yield stress and tensile modulus decrease as the degree of graft increases up to about 120%, and then increase. Increasing the rubber volume fraction and the size of the rubber particles also reduces the yield stress.A dynamic test is conducted and it is verified that the glass transition temperature of the rubber phase increases with increasing the degree of graft. In addition to this, the maximum tanδ increases with increasing the degree of graft up to about 120%, and then decreases.It is also shown that the yield stress and tensile modulus of HIPS have a relationship with the dynamic behavior of the rubber phase, i.e. the yield stress and tensile modulus decrease almost linearly as the maximum tanδ increases.

Influence of SiC particles on mechanical properties of Mg based composite

AZ91 magnesium alloy reinforced with different sizes of SiC particulates has been fabricated using powder metallurgy route. Mechanical properties of the specimens have been studied. Yield and ultimate tensile stresses show a decrease with the increase in the size of SiC particulates. The influence of thermal shock between 400°C and 30°C on the mechanical properties was also investigated. The results show a decrease in yield stress and elongation to fracture with the number of thermal shock cycles.

Vanishing elasticity for wet foams: Equivalence with emulsions and role of polydispersity

We present an experimental study of the rheology of polydisperse aqueous foams of different gas volume fractions φ. With oscillatory deformation at fixed frequency, we determine the behavior of the maximum stress as a function of the strain amplitude. At low strain, the maximum stress increases linearly, defining a shear modulus G. At progressively higher strains, the response eventually becomes nonlinear, defining the yield strain and the yield stress. While φ decreases toward φc=0.635±0.01, G goes to zero, and the yield stress decreases by many orders of magnitude with a quadratic behavior. The yield strain, which can be extrapolated to 0.18±0.02 at φ=1, has a minimum value of 0.045±0.010 at φc. This behavior shows the occurrence of a melting transition located at φc, which can be correlated to the random close packing of spheres. We compare these results to similar ones obtained previously for monodisperse and polydisperse emulsions. Our new experiments clarify the rheological similarities between emulsions and foams, as well as the role of polydispersity. We find that as long as polydispersity is moderate, it does not play a crucial role in the elastic response of foams and emulsions.

Polypropylene–rubber blends: 2. The effect of the rubber content on the deformation and impact behaviour

The effect of the rubber content on the deformation and impact behaviour of polypropylene–EPDM rubber blends is studied. The blends are made on a twin screw extruder. The rubber content ranged from 0 to 40 vol.%. The tensile modulus and the yield stress decrease linearly with increasing rubber content. The crystallinity of the PP phase as measured with differential scanning calorimetry did not change with rubber content. The fracture behaviour was studied with a notched Izod impact test and with an instrumented single edged notched tensile test at 1 m/s and 1 mm/s. The blends were studied in temperature range from −80 to 120°C. The brittle–ductile transition temperature ( T bd) decreases with increasing rubber content from 85°C for pure polypropylene to −50°C for a 40 vol.% blend, a shift of 135°C. The T bd with the notched Izod test and the SEN at 1 m/s are very comparable. The T bd for the SEN at 1 mm/s if compared to the 1 m/s are at a 30°C lower temperature. The brittle–ductile transition at the low test speed is gradual, while at high test speeds the transition is abrupt, discontinuous. A good criterion for the onset of ductility is the crack propagation displacement.

Effect of dislocation structure on fracture toughness of strained BCC-metals

Mechanical properties of materials depend on their structure. Examined are the effects of dislocation structure on fracture toughness and mechanisms of fracture of BCC-metals. Fracture toughness was determined by depending specimens with cracks introduced into the plane perpendicular to the plane of rolling. Fracture toughness increases with decreasing yield stress (for ε=15–25%). This is due to instability of slightly misoriented cell structure under repeated loading. The peak of fracture toughness at the temperature 77 K was not observed. The increase of fracture toughness for high strained metals ( ε>60% for Mo, and ε>85% for Cr) corresponds to cell size reduction and the change of fracture mechanisms.

Influence of annealing on mechanical properties of an Fe–28Al–4Cr–0.1Ce alloy

Structure and mechanical properties of a vacuum-cast and hot-deformed Fe–28Al–4Cr–0.1Ce (at%) alloy were studied. Samples were hot-forged or hot-extruded and afterwards annealed above or below the point of the B2–D0 3 order–order transition ( T c ∼540 °C). Mechanical properties were evaluated from Vickers hardness, tensile, compression and three-point bend tests. Structure was examined by optical and electron metallography. Besides large particles in the matrix, which contain Ce, Cr–Fe precipitate has also been found on the grain boundaries. Annealing at 700 and 800 °C leads to recovery of the dislocation structure and to the corresponding decrease of the yield stress. Even during low-temperature annealing in the range between 350 and 510°C the yield stress decreases at each of the annealing temperatures due to the growth of the ordered domains. Room temperature ductility after hot extrusion and annealing for 2 h at 700°C reached 4.3%.

Phase behavior and mechanical properties of blends of cellulose propionate and an alternating propene-carbon monoxide copolymer

Cellulose propionate (CP) and plasticized cellulose propionate (CP16) were blended, both in solution and by extrusion, with high molecular weight alternating propene-carbon monoxide copolymers (P-CO). Tensile testing was performed on injection molded blends with a P-CO content of up to 50 wt.-%. The phase behavior was investigated by means of differential scanning calorimetry and electron microscopy. The blends are miscible at compositions up to about 10 wt.-% P-CO. P-CO lowers the melt viscosity of both CP and CP16 and acts as a polymeric plasticizer. Higher amounts of P-CO result in phase separation with domains of P-CO-rich material in the dispersed phase and a continuous CP/CP16-rich phase with a T g even higher than that of pure CP. In this state toughness decreases slightly with increasing P-CO content, while modulus and yield stress increase. After transition into the bicontinuous phase, occurring at a P-CO-content of around 25 wt.-%, toughness and elongation at break increase significantly, while modulus and yield stress decrease.

On the viscoelastic and plastic behaviour of methylmethacrylate-co-N-methylglutarimide copolymers

Both viscoelastic and plastic properties were investigated on random copolymers of methylmethacrylate (MMA) and N-methyl-glutarimide (GIM) in the range 0–76mol%. All the measurements were performed on samples quenched from the melt in order to break free from physical aging effects. Dynamic mechanical experiments were performed at very low deformation and temperatures ranging from −150°C up to the glass transition temperature (Tg) region. Increase in GIM amount improves the thermomechanical stability of the copolymers, as revealed by the increase of both Tg and α relaxation temperature. In the β relaxation region, the E″ loss peak first decreases in amplitude with increasing GIM content and then broadens further and finally spreads out till the onset of the α peak at the largest GIM amounts. A quantitative analysis of the β relaxation phenomena was performed by considering the loss compliance J″ instead of the loss modulus E″. It turns out that in the low temperature range (−80°C–0°C) the mechanical damping associated with the MMA motions is stronger for MMA–GIM than for MMA–MMA linkages; in addition, the mechanical damping associated with the motions of the GIM units is very low. By contrast, in the high temperature range (30°C to about 100°C), the mechanical damping associated with the MMA motions drops with increasing GIM amount, whereas a significant damping coming from the GIM units is observed. These results suggest that the β relaxation would mainly consist of MMA isolated motions at low temperature and of cooperative motions at higher temperature, involving the MMA units at GIM amounts lower or equal to 58mol% and the GIM units at higher GIM content. The stress–strain curves were determined at low strain rate (2×10−3s−1) and temperatures ranging from −120°C to Tg. Analysis of the plastic deformation region shows that the yield stress decreases with increasing GIM amount at low temperatures. The opposite trend shows up on the high temperature side of the β relaxation, where strain softening peaks at intermediate GIM amounts. As a plausible explanation, the cooperative β motions, whenever they exist, are suspected to be responsible for the decrease of both yield stress and strain softening. These conclusions agree well with those of a previous study on methylmethacrylate-co-maleimide copolymers. They are also consistent with our earlier identification of the microdeformation mechanisms involved in the stretching of methylmethacrylate-co-N-methylglutarimide thin films.

Tensile test of poly(vinyl chloride) filled with ground calcium carbonate particles

The fracture behavior of poly(vinyl chloride) filled with ground calcium carbonate particles during a tensile test was studied. The particles were prepared by crushing natural raw crystalline limestone. For this purpose, 10–50 parts of the particles having two different mean sizes (2 and 8 μm) without further surface treatment were mixed with 100 parts of poly(vinyl chloride) and 3 parts of lead stearate as a stabilizer using a mixing roll. A tensile test was carried out using a dumbbell specimen. As a result, the yield stress decreased with increase in the particle content; however, there was no significant influence of particle size. From scanning electron microscopic observations of the specimen's surfaces during the tensile test, it was found that the particle/matrix interfaces were delaminated and formed voids around the particles when the applied stress approached the yield stress, that is, the particles acted as voids and the matrix around the voids was plastically deformed effectively. These observations appear to be the reason for the decrease of yield stress by the incorporation of the particles. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 311–316, 1998

Effect of the thickness of an electroless Ni–P deposit on the mechanical properties of an AISI 1045 plain carbon steel

Abstract Fatigue tests under rotating bending conditions have been conducted on samples of AISI 1045 plain carbon steel with and without the application of electroless Ni–P (EN) deposits of relatively high P content and thicknesses ranging between 7 and 37 μm. The coated samples have been post-heat treated (PHT) according to two different schedules: 473 K for 60 min (PHT-1) and 473 K for 60 min followed by a second treatment at 673 K for 60 min (PHT-2). The mean microhardness values determined on the surface of the coated samples after both post-heat treatments (PHT) were found to be about 675 and 1044 KHN 50 , respectively. In relation to the tensile properties of the coated and PHT specimens it has been determined that both the yield stress and ultimate tensile stress (UTS) decrease slightly as the deposit thickness increases, whereas the reduction in area is severely decreased, independently of the post-heat treatment applied. In relation to the fatigue behavior of the coated and PHT samples, it has been observed that the substrate with a deposit 7 μm thick after PHT-1 behaves similarly to the uncoated substrate. However, as the thickness deposit increases the fatigue life is reduced. The decrease in fatigue life is observed to be more marked for those materials with PHT-2 in comparison with those with PHT-1.

Influence of cooling rate on room temperature tensile behaviour of thermally oxidised MA 956

Abstract The effect of the cooling rate from the preoxidation temperature on the room temperature tensile behaviour of the alloy MA 956 is analysed for scale free specimens and specimens with scale. It is shown that an increase in the cooling rate causes a decrease in yield stress and tensile strength. For scale free specimens, this effect is caused by less extensive precipitation of the Cr rich α′ phase at about 475°C and higher constraint of the tensile residual stresses associated with the difference in the thermal expansion coefficients of the dispersoids and the matrix. For specimens with scale, yield stress and tensile strength were even lower than those of scale free specimens because of the additional scale induced residual stresses. Moreover, a ductile to brittle transition was observed at the lowest cooling rate as a consequence of the highest precipitation of the Cr rich α′ phase.

Influence of cooling rate on room temperature tensile behaviour of thermally oxidised MA 956

The effect of the cooling rate from the preoxidation temperature on the room temperature tensile behaviour of the alloy MA 956 is analysed for scale free specimens and specimens with scale. It is shown that an increase in the cooling rate causes a decrease in yield stress and tensile strength. For scale free specimens, this effect is caused by less extensive precipitation of the Cr rich α′ phase at about 475°C and higher constraint of the tensile residual stresses associated with the difference in the thermal expansion coefficients of the dispersoids and the matrix. For specimens with scale, yield stress and tensile strength were even lower than those of scale free specimens because of the additional scale induced residual stresses. Moreover, a ductile to brittle transition was observed at the lowest cooling rate as a consequence of the highest precipitation of the Cr rich α′ phase.

Tensile Properties of Poly(vinyl Chloride) Filled with Precipitated Calcium Carbonate

The tensile properties of poly(vinyl chloride) filled with precipitated calcium carbonate particles having two different mean sizes were studied. The particles were porous agglomerates of spherical primary particles. For this purpose, 10–50 parts of the particles were mixed with 100 parts of poly(vinyl chloride) and 3 parts of lead stearate using a mixing roll. The particle size decreased during the mixing process. The measured mean sizes in the resin were about 0.6 and 1.2 μm at filler contents of 10 phr. The tensile tests were carried out using dumbbell specimens. The yield stress decreased only slightly with filler content in the small particle-filled system, whereas it decreased considerably with an increase in filler content in the large particle-filled system. From a scanning electron microscopic observation of the specimen surfaces after test, the larger particles used in this study seemed to be more brittle, therefore, the particles tended to fracture when a stress was applied, and voids formed around them. These observations appear to explain the decrease in yield stress in the large particle-filled system, because the voids made the ductile polymer matrix yield around the particles.

Tensile Properties of Poly(vinyl Chloride) Filled with Precipitated Calcium Carbonate

The tensile properties of poly(vinyl chloride) filled with precipitated calcium carbonate particles having two different mean sizes were studied. The particles were porous agglomerates of spherical primary particles. For this purpose, 10–50 parts of the particles were mixed with 100 parts of poly(vinyl chloride) and 3 parts of lead stearate using a mixing roll. The particle size decreased during the mixing process. The measured mean sizes in the resin were about 0.6 and 1.2 μm at filler contents of 10 phr. The tensile tests were carried out using dumbbell specimens. The yield stress decreased only slightly with filler content in the small particle-filled system, whereas it decreased considerably with an increase in filler content in the large particle-filled system. From a scanning electron microscopic observation of the specimen surfaces after test, the larger particles used in this study seemed to be more brittle, therefore, the particles tended to fracture when a stress was applied, and voids formed around them. These observations appear to explain the decrease in yield stress in the large particle-filled system, because the voids made the ductile polymer matrix yield around the particles.

On the yield stress anomaly in stoichiometric FeAl

Abstract A vacancy-hardening model has recently been proposed to explain the yield stress anomaly in FeAl (George and Baker, Philos. Mag. (1997) in press) in which the yield stress rise with increasing temperature at intermediate temperatures is associated with frictional drag of vacancies on dislocations, whereas its fall at high temperatures is ascribed to dislocation creep. One implication of the model is that the yield anomaly should be observable even in stoichiometric FeAl (where it has not yet been seen) by rapid straining. Here, we report on compression tests on both polycrystals and single crystals of stoichiometric FeAl as a function of temperature and strain rate, and show that this prediction was borne out, i.e. at a strain rate of 1.6 s−1 a yield peak was observed, whereas at a strain rate of 1×10−4 s−1 a slow decrease in yield stress was observed with increasing temperature.

Radial tensile properties of the lumbar annulus fibrosus are site and degeneration dependent.

We conducted an in vitro study of the radial tensile properties of the annulus fibrosus. The stress-strain response was nonlinear, with a mean tangent modulus of 0.19 MPa at zero strain and 0.47 MPa at 70% of the yield strain. We also investigated whether these properties varied as a function of location within the disc and degree of degeneration. Specimens harvested from the middle layers of the annulus were stiffer and failed at smaller strain magnitudes than those from the inner or outer annulus (analysis of covariance, p < 0.05). Differences due to degeneration were evident; degenerated discs had a 30% decrease in yield and ultimate stress compared with normal discs. Similarity between our data and those reported for the annulus in compression suggests that these values reflect the material behavior of the interlaminar matrix and are an order of magnitude smaller than values used in previous analytical representations of this tissue. We expect that awareness of these data will result in improved understanding of the physical behavior and tolerance to injury of the annulus fibrosus.

The dependence of tensile behaviour of L1 2 compound Al 67Ti 25Mn 8 on the strain rate at 1173 K

The tensile behaviour of the forged L1 2 compound Al 67Ti 25Mn 8 was studied as a function of strain rates from 3.34 × 10 −5 s −1 to 6.68 × 10 −4 s −1 at 1173 K. The results showed that strain rate has a substantial effect upon the tensile properties, especially the tensile elongation of the alloy. When the rate decreases from 6.68 × 10 −4 s −1 to 3.34 × 10 −5s −1, the yield stress decreases and the tensile ductility increases from 2.8% to 21%. The good high temperature ductility at low strain rates is attributed to dynamic recovery and dynamic recrystallization during deformation. The fractured surfaces of the alloys at various rates are fully or predominantly intergranular, which suggests that the grain boundaries of L1 2 Al 67Ti 25Mn 8 are intrinsic weak and/or the slip transfer across the grain boundaries is difficult.

Mechanism of the drop in the dependence of yield stress on neutron irradiation dose for low-alloy steel

In the present work, influence of irradiation on tensile and impact properties of Cr-Ni-Mo steel using for WWER-1000 type reactor pressure vessel (RPV) manufacturing was studied. An abnormal behavior of tensile properties of the investigated materials under irradiation was observed. Decrease in yield stress and ultimate tensile stress at the first stage of irradiation was revealed while ductile-to-brittle transition temperature (DBTT) was continuously increasing with damage dose. A model of the phenomenon considering influence of residual impurities and alloying elements on α-Fe lattice tetragonality under irradiation was proposed. Some experimental evidences of the model were found. Nickel was supposed to govern the radiation sensitivity of WWER-1000 type steel.

Effects of stoichiometry, microalloying and aging heat treatments on the compressive behavior of NiAl alloys

The compressive properties of NiAl were found to be a sensitive function of Ni-content, solid solution hardening (by microalloying) and precipitate strengthening. The Ostwald ripening of the precipitates, which supposedly consist of the impurities C, B, O, N, Si, and S together with the extra getter metals Ti, Zr, and Hf or getter traces in the NiAl matrix, results in a specific yield stress decrease during aging at 573--973 K. The total amount of the intrinsic impurities in as-produced alloys has been found to be independent of stoichiometry and microalloying with a getter. Among the extra impurities studied, C was found to change the local electron density in a much more pronounced way compared to B or Si. Static strain-aging at 573 K and strain-rate sensitivity parameters at 473 K can be interpreted adequately by taking into account the less pronounced gettering in NiAl doped with Ti, and more pronounced gettering in NiAl doped with Hf and Zr.