Fracture Strain Decrease Research Articles

Deformation and fracture of emulsion-filled gels: Effect of oil content and deformation speed

The large deformation properties of gelatine, κ-carrageenan and whey protein isolate (WPI) gels filled with bound and unbound oil droplets were studied as a function of compression speed. The rheological properties of the gel matrices controlled the compression speed-dependency of the gels containing oil droplets. Polymer gels (gelatine and κ-carrageenan gels) showed a predominantly elastic behaviour. Their Young's modulus was not affected by the compression speed. The increase of fracture stress and strain observed with increasing compression speed was related to friction between the structural elements of the gels and, for gelatine, to the unzipping of physical bonds. Particle gels (WPI gels) showed a more viscoelastic behavior. Their Young's modulus and fracture stress increased with compression speed. This was attributed to the viscous flow of the matrix and friction phenomena between structural elements of the gel. The effect of an increase in the oil volume fraction ( φ) on the Young's modulus was for all gels according to the Van der Poel theory. In addition, oil droplets embedded in the gel matrix acted as stress concentration nuclei and increased friction. The relative impact of these two effects was related to the viscoelastic properties of the gels and to droplet–matrix interaction. For polymer gels and gels with bound droplets, stress concentration phenomena played a relatively larger role. For particle gels and gels with unbound droplets, friction phenomena were relatively more important, increasing the viscoelastic character of the gels. As a result, an increase in φ resulted in a decrease of both fracture stress and fracture strain for polymer gels and in an increase of the fracture stress and a decrease of fracture strain for particle gels.

Fabrication and characterization of high strength Nylon-6/Si 3N 4 polymer nanocomposite fibers

Alignment of silicon nitride nanorods in Nylon-6 polymeric matrix has been achieved through melt extrusion process. Tensile properties of as-fabricated single fibers show that the strength and modulus of a nanocomposite fibers increased by 273% and 610%, respectively, as compared to the neat Nylon-6 fibers. Field emission scanning electron microscopy (FE-SEM) studies have confirmed the successful alignment of nanorods along the filament length. Larger increase in modulus of the nanorods reinforced Nylon-6 was at a cost of about 35% decrease (reduced from 65% to 30%) in fracture strain. In an attempt to retain the ultimate strain, nanorods were replaced by spherical Si 3N 4 (∼30 nm dia) particles and it was demonstrated that the improvement in tensile properties is still by 179% for strength and 276% for modulus practically without any loss of fracture strain.

Lattice defects dominating hydrogen-related failure of metals

The factor that plays the essential role in hydrogen-related failure has been examined for Inconel 625 and iron by means of tensile testing with interposed unloading and reloading with/without hydrogen charging. Aging at 30 °C or annealing at 200 °C was conducted during the unloaded stage in order to diffuse out hydrogen or to anneal out strain-induced defects. Hydrogen thermal desorption analysis was used to evaluate strain-induced defects that act as trapping sites of hydrogen. Fracture strain decreased in the initially hydrogen-charged specimens even though hydrogen was absent at the late stage of straining. Annealing at 200 °C at the unloaded stage almost completely recovered the decrease in fracture strain. Enhancement of strain-induced defects by hydrogen and their involvement in degradation were revealed by means of hydrogen thermal desorption analysis. The results provide direct evidence of the primary role of vacancies rather than hydrogen itself in hydrogen degradation, and agree well with the hydrogen-enhanced strain-induced vacancy model with respect to the mechanism of hydrogen-related failure.

The Crystal Structure and Mechanical Property of AZ80-2%Li Mg Alloy

In this study, the crystal structure and mechanical property of AZ80-2%Li Mg alloy are investigated. Experimental results show that the density of AZ80 Mg alloy can be reduced to be 1.71 g/cm3 by addition of 2wt% Li. The as-extruded AZ80-2%Li Mg alloy exhibits a two-phase microstructure of α and β phases, with fine strips along the extrusion direction. The fracture strain increases significantly with increasing deformation temperature, and the tensile strengths hardly change at below 100oC. The tensile strength increases, but fracture strain decreases, with increasing strain rate. The 400°C solid-solution treated AZ80-2%Li Mg alloy will produce the AlLi precipitate after aging at 170°C, and produce the Mg17Al12 precipitate after aging at 250°C. The AZ80-2%Li Mg alloy with aging at both 170°C and 250°C will exhibit the phenomenon of precipitation hardening.

Effect of strain rate on compressive behavior of Ti-based bulk metallic glass at room temperature

Compressive deformation behavior of the Ti 40Zr 25Ni 8Cu 9Be 18 bulk metallic glass was investigated over a wide strain rate ranging from 10 −4 to 10 3 s −1 at room temperature. Fracture stress was found to increase and fracture strain decrease with increasing applied strain rate, which were due to the decrease of the density of shear bands. Serrated flow was observed at lower strain rate. The appearance of a large number of shear bands was probably associated with flow serration observed during compression. At high strain rates, the rate of shear band nucleation was not sufficient to accommodate the applied strain rate and thus caused an early fracture of the test sample. The positive strain rate dependence of compressive strength might be associated with the different microstructure on the atomic scale in the Ti-based amorphous matrix.

F4-3 金属材料中の水素存在状態と水素脆化(F-4 水素脆化研究の新展開(シミュレーションと実験との対話),フォーラム)

The factor that plays the essential role in hydrogen degradation has been examined for Inconel 625 and iron by means of tensile testing with interposed unloading and reloading with/without hydrogen charging. Aging at 30℃ or annealing at 200℃ was conducted at the unloaded stage in order to diffuse out hydrogen or to anneal out strain-induced defects. Hydrogen thermal desorption analysis was used to evaluate strain-induced defects that act as trapping sites of hydrogen. Fracture strain decreased in the initially hydrogen-charged specimens even though hydrogen was absent at the late stage of straining. Annealing at 200℃ at the unloaded stage almost completely recovered the decrease in fracture strain.

Dynamic Deformation Behaviour and Microstructural Evolution of High-Strength Weldable Aluminum Scandium (Al-Sc) Alloy

A compressive split-Hopkinson pressure bar is used to investigate the dynamic deformation behaviour, fracture characteristics and microstructural evolution of high-strength weldable aluminum scandium (Al-Sc) alloy at strain rates ranging from 1.3 x 10 3 s -1 to 5.9 x 10 3 s -1 under room temperature conditions. The stress-strain curves reveal that the dynamic mechanical behaviour response of the Al-Sc alloy is highly sensitive to both the strain and the strain rate. As the strain rate increases, the flow stress, work hardening rate and strain rate sensitivity all increase, but the fracture strain and activation volume decrease. The Zerilli-Armstrong FCC constitutive model is applied to describe the high strain rate plastic behaviour of the Al-Sc alloy. A good agreement is found between the predicted flow response and the actual response at all values of the applied strain rate. The Al-Sc alloy specimens fracture primarily as a result of a localised shearing effect. Scanning electron microscopy observations indicate that the fracture features are characterised by a transgranular dimple-like structure. The density and depth of the dimples decrease with increasing strain rate. Transmission electron microscopy observations reveal the presence of Al 3 Sc particles in the matrix and at the grain boundaries. These particles prevent dislocation motion and therefore prompt a significant strengthening effect. The microstructural observations also reveal that the dislocation density and degree of dislocation tangling increase with increasing strain rate. The variations observed in the dislocation cell structure reflect differing degrees of strain rate sensitivity and activation volume and are found to be consistent with the observed stress-strain response of the Al-Sc alloy.

Tensile Properties and Deformation-Fracture Behavior of Ti-6Al-4V Alloy at Cryogenic Temperature

Tensile properties and deformation-fracture behavior at temperatures ranging from 123K to 293K of a Ti-6Al-4V alloy sheet with a thickness of 1.5mm has been studied, and the effects of testing temperature, specimen orientation and heat-treatment were investigated. An increase in strength and a decrease in fracture strain were found with decreasing tension temperature, and the anisotropy in tensile properties was observed at room and cryogenic temperatures both in the annealed and solution treated and aged (STA) specimens. TEM examinations indicated that plastic deformation occurred within both α and β phases in the STA specimens testing at either room or cryogenic temperature. The dominant deformation mechanism varied from dislocation slip at room temperature to twinning at 123K. SEM observations showed that the amount of dimples and tearing grains on fractured surfaces of the specimens decreased as testing temperature was decreased.

Dynamic Mechanical Response and Microstructural Evolution of High Strength Aluminum–Scandium (Al–Sc) Alloy

The dynamic mechanical properties of high strength aluminum–scandium (Al–Sc) alloy are investigated using a compression split Hopkinson bar. Dynamic impact testing is carried out at nominal strain rates (abbreviated to strain rate hereafter) ranging from 1:2 � 10 3 to 5:8 � 10 3 s � 1 at room temperature. The effects of strain rate on the mechanical properties, microstructural evolution and fracture characteristics are investigated and the relationship between the mechanical properties of the alloy and its microstructure is explored. The measured strain– stress curves reveal that the dynamic mechanical behaviour of Al–Sc alloy is highly dependent on the strain rate. The flow stress, work hardening rate and strain rate sensitivity increase with increasing strain rate, but the fracture strain and activation volume decrease. The Zerilli–Armstrong fcc constitutive law is used to model the shear flow response of the Al–Sc alloy. A good agreement is found between the predicted and measured shear flow responses. The Al–Sc alloy specimens fracture as a result of shear band formation and crack propagation within the shear band. SEM observations indicate that the fracture features are dominated by a transgranular dimple-like structure. The density and depth of the dimples decrease with increasing strain rate. TEM microstructural observations reveal that the presence of Al3Sc precipitated particles in the matrix and at the grain boundaries prevents dislocation motion and leads to a significant strengthening effect. An analysis of the dislocation substructure indicates that a higher strain rate increases the dislocation density, thereby reducing the size of the dislocation cells. The variations of the dislocation cell structure reflect different degrees of strain rate sensitivity and activation volume, and correlate well with the impact flow stress– strain response.

Mechanical Properties of Thin Pyrolitic Carbon Interphase SiC–Matrix Composites Reinforced with Near‐Stoichiometric SiC Fibers

Tensile properties of Tyranno™‐SA near‐stoichiometric silicon carbide (SiC)‐fiber–reinforced chemically vapor‐infiltrated SiC‐matrix composites with pyrolytic carbon interphases were experimentally studied. The influence of interphase thickness in a range of 60–300 nm on the tensile properties of the materials appeared to be generally minor. Thin interphase (<100 nm) did not have a significant deteriorating effect on composite properties, which has commonly been reported for conventional SiC‐fiber composites. For very thin interphase (<60 nm) composites, a slight decrease in fracture strain and a substantial increase in interfacial sliding stress were noted. Increases in ultimate tensile strength and fracture strain were observed at a much thicker interphase (>600 nm) at the expense of composite stiffness.

Strain rate-dependent compressive deformation behavior of Nd-based bulk metallic glass

Compressive deformation behavior of the Nd 60Fe 20Co 10Al 10 bulk metallic glass was characterized over a wide strain rate range (6.0×10 −4 to 1.0×10 3 s −1) at room temperature. Fracture stress was found to increase and fracture strain decrease with increasing applied strain rate. Serrated flow and a large number of shear bands were observed at the quasi-static strain rate (6.0×10 −4 s −1). The results suggest that the appearance of a large number of shear bands is probably associated with flow serration observed during compression; and both shear banding and flow serration are a strain accommodation and stress relaxation process. At dynamic strain rates (1.0×10 3 s −1), the rate of shear band nucleation is not sufficient to accommodate the applied strain rate and thus causes an early fracture of the test sample. The fracture behavior of the Nd 60Fe 20Co 10Al 10 bulk metallic glass is sensitive to strain rate.

Characterization of Phytagel® modified soy protein isolate resin and unidirectional flax yarn reinforced “green” composites

The mechanical properties and moisture resistance of the soy protein isolate (SPI) resin and flax yarn reinforced composites were improved significantly by incorporation of a poly-carboxylic acid based modifier, Phytagel®. SPI and Phytagel® were blended to form an interpenetrating network-like cross-linked complex. This complex showed significantly improved tensile and moisture properties as well as higher thermal stability as compared to the unmodified SPI resin. The incorporation of Phytagel® (40% w/w of SPI powder) in SPI resin and subsequent lowering of the amount of glycerol (from 30% to 12.5%) led to an overall 10-fold increase in the tensile fracture stress and a nine-fold increase in the Young's moduli of the SPI resin along with a seven-fold decrease in fracture strain. The dynamic mechanical properties such as storage and loss modulus of the modified resin increased and the glass transition temperature also increased by about 56°C. The unidirectional flax yarn reinforced composites were made using two modified resins with 20% and 40% Phytagel® contents. Both resins contained 12.5% glycerol. The composites fabricated using resin containing 20% Phytagel® showed significantly higher tensile and flexural moduli as well as fracture stress in the axial direction than the composites with resin containing 40% Phytagel®, which was higher than the SPI resin based composites. POLYM. COMPOS., 26:647–659, 2005. © 2005 Society of Plastics Engineers

Experimental study of tensile properties of PMMA at intermediate strain rate

Tensile tests of polymethylmethacrylate (PMMA) at quasi-static loading conditions and at intermediate strain rates were performed on MTS810 materials test system and on a self-developed intermediate strain rate materials testing apparatus, respectively. The tested results show that the mechanical properties of PMMA are strongly rate sensitive: When the strain rate increases, the strength and initial modulus increase, while the fracture strain decreases. Scanning electronic microscope (SEM) fractography on the fracture surface indicates that a relationship between the morphology of fracture surface and strain rates exists: When the strain rate increases, the mirror area gets smaller, the ribs and the gaps between ribs get wider, and the number of the ribs gets smaller. At the strain rate of 18.6 s −1, the rib area disappears, and only a large range of brittle shatter fracture morphology remains. This is coincident with the high-rate enbrittlement of PMMA.

Effect of Oxidative Enzymes on Bulk Rheological Properties of Wheat Flour Doughs

The use of enzymes such as peroxidases or glucose oxidase instead of chemical oxidants is a very interesting option for improving breadmaking performance of doughs. In this study the effect of such enzymes on bulk rheological properties of dough was quantified and their influence on the polymer network in dough deduced. Small deformation oscillation and relaxation tests (strain 0·001) are not suitable for discriminating between doughs prepared in the presence of the different enzymes. Flow relaxation tests at high deformation and long relaxation times showed a clear distinction between the effect of peroxidase and glucose oxidase. Peroxidase increases only the number or lifetime of transient bonds, whereas glucose oxidase additionally produced cross-links that were permanent on time scales up to 3 h. Peroxidase probably introduced a second, more transient structure (arabinoxylan network) through the gluten network, whereas glucose oxidase may also have strengthened the gluten network. A higher water addition could not compensate for the effect of peroxidase; on longer time scales the stress remained at a higher level. Similar results were obtained in large deformation biaxial and uniaxial extension tests. Peroxidases only increased stress levels. The addition of glucose oxidase resulted in a higher stress and more intense strain hardening. Only in biaxial extension was an influence of pH observed. An increase in stress level was accompanied by a decrease in fracture strain, making predictions of the effects on bread structure complicated.

An investigation of the plastic failure of spheroidized steels

The mechanical properties, fracture behaviors and micro-mechanism of damage have been studied for three spheroidized steels, respectively. It is shown that void formation and growth induced by decohesion at the interface between the ferrite matrix and non-metallic particles or by particle cracking are the main reasons for the failure of these materials during plastic deformation. Experimental results show that the void volume fractions of these materials increase as strain increases, with the increasing rate being larger for the material with more carbide particles. The strain -hardening exponent and fracture strain decrease with the volume fraction of second-phase particle increasing. Based on the concepts of effective stress and strain softening induced by void evolution during plastic deformation, analytical expressions of strain hardening exponent and fracture strain for plastically damaged materials with second-phase particles are derived, respectively, and shown to be in good agreement with experimental data.

Deformation and Fracture of Rubber under Tensile Impact Loading

Abstract An impact experiment is designed to obtain the deformation and fracture characteristics of SBR specimens under tensile impact loading. The experimental apparatus is capable of achieving very large strains (about 300%) and high strain rates (between 10 and 1000 s−1) in the specimen. Dynamic stress-strain curves reveal that SBR goes through several phases of deformation and fracture as the strain rate increases in the specimen. In the first phase, the initial modulus, yield stress, tensile strength, and fracture strain increase, while the final modulus remains fairly constant with increasing strain rate. Increases in the initial modulus, tensile strength, and strain at fracture with increasing strain rate are due to a lack of stress relaxation in constituents that have longer relaxation times than the load duration. Local scale relaxation times are shorter than the load duration in this phase so that the final modulus is almost insensitive to strain rate. In the second phase, the initial modulus and the yield stress remain roughly constant while the final modulus increases with increasing strain rate. The tensile strength also increases, but the fracture strain decreases as the strain rate increases. Increases in the final modulus and tensile strength are due to a lack of relaxation on a local scale. The tensile fracture strain decreases because convolutions do not have sufficient time to slip completely. In the third phase, all stress-strain curves follow a master curve, but both the tensile strength and fracture strain decrease with increasing strain rate. The decrease in tensile strength as the strain rate increases could be due to stress concentration at the tip of microcracks within the SBR.

Characterization of interfacial and mechanical properties of green composites with soy protein isolate and ramie fiber

Environment-friendly fiber-reinforced composites were fabricated using ramie fibers and soy protein isolate (SPI) and were characterized for their interfacial and mechanical properties. Ramie fibers were characterized for their tensile properties and the parameters for the Weibull distribution were estimated. Effect of glycerol content on the tensile properties of SPI was studied. Interfacial shear strength (IFSS) was determined using the microbond technique. Based on the IFSS results and fiber strength distribution, three different fiber lengths and fiber weight contents (FWC) were chosen to fabricate short fiber-reinforced composites. The results indicate that the fracture stress increases with increase in fiber length and fiber weight content. Glycerol was found to increase the fracture strain and reduce the resin fracture stress and modulus as a result of plasticization. For 10% (w/w) of 5 mm long fibers, no significant reinforcement effect was observed. In fact the short fibers acted as flaws and led to reduction in the tensile properties. On further increasing the fiber length and FWC, a significant increase in the Young's modulus and fracture stress and decrease in fracture strain was observed as the fibers started to control the tensile properties of the composites. The experimental data were compared to the theoretical predictions made using Zweben's model. The experimental results are lower than the predicted values for a variety of reasons. However, the two values get closer with increasing fiber length and FWC.

Room temperature mechanical behavior of silicon-doped TiAl alloys with grain sizes in the nano- and submicron-range

Nano- and submicron-grained intermetallic compounds consisting of γ-TiAl and ξ-Ti 5(Si,Al) 3 were produced by high energy milling and hot isostatic pressing. Owing to the pure and controlled processing conditions, the mechanical properties may be indubitably related to the microstructure. Both yield strength and hardness show a Hall–Petch-type dependence on grain size, resulting in extremely high flow and fracture stresses under compression of up to 3 GPa. With a reduction of grain size, the coefficient of strain hardening as well as the compressive fracture strain decrease and drop to zero for alloys with grain sizes of about 150 nm. Deformation at room temperature is accomplished by dislocation glide and mechanical twinning, with twinning attaining more and more importance as the grain size is further reduced. Diffusion-controlled deformation mechanisms can be ruled out even for intermetallics with crystallite sizes as small as 50 nm. A room temperature ductilization of intermetallic compounds by switching to a nanocrystalline microstructure seems to be rather unlikely.

急冷凝固Al-Fe-Y合金の焼鈍による相変化過程と機械的特性

The utilization of crystallization reaction of an amorphous phase is a useful method for the formation of nanocrystalline structure. However, degassing and extrusion processes at elevated temperatures are necessary for the consolidation of the rapidly solidified powders. Consequently, the thermal history must be taken into consideration in the control of the structure and characteristics of the consolidated alloys. In this paper we examined the structure and mechanical properties of as-quenched amorphous and annealed nanocrystalline states of Al-Fe-Y alloys with high crystallization temperatures. The Fe and Y contents are located near the low solute concentration range of the amorphous phase region. The structure of the as-quenched alloys is composed of amorphous, amorphous+fcc-Al and fcc-Al solid solution and the grain size of the Al phase is about 5 nm. The annealing of the Al90Fe5Y5 amorphous alloy causes an increase of hardness and a decrease of fracture strain. The maximum hardness was obtained after annealing for 3.6 ks at 573 K. With further increase of annealing temperature to 623 and 673 K, the structure changes to Al+intermetallic compounds, accompanied by a rapid decrease in hardness. The embrittlement of the annealed samples having hardness above 200 is suppressed by Al enrichment. The crystallization process for this alloy is similar to that for the Al90Fe5Y5 alloy. The grain size of the crystallized phases is about 100 nm after annealing at 673 K. It is therefore concluded that the Al92Fe5Y3(at%) alloy is an appropriate composition for the production of nanocrystalline Al-based alloys with high elevated temperature strength.

Development of a Three-dimensional Model for Ductile Fracture in Materials Containing Two Types of Microvoids.

There are three stages in ductile fracture process: voids initiation, void growth and void coalescence. In the final stage, ductile fracture occurs due to the void coalescence. Thomason suggested that voids begin to coalesce when a plastic limit-load condition is fulfilled for localized plastic failure of the intervoid matrix. On the basis of this critical void-coalescence condition, we developed a three-dimensional model for ductile fracture considering two types of voids with different sizes, and applied this model to a SA440 steel with and without pre-straining. With the critical strength of particle/matrix interface and the theoretical void growth strain obtained from the model, the reason for the decrease in fracture strain caused by plastic pre-strain was discussed.