Critical Stretch Research Articles

Effect of turbulence on extinction of counterflow diffusion flame

A laminar counterflow diffucion flame established in the forward stagnation region of a porous cylinderhas been used widely for the laminar flame studies. In the present study, this flame was applied to investigating the extinction of a diffusion flame due to air stream turbulence. Propane or methane was ejected from a 30-mm porous cylinder. The turbulence was given to the counterflowing air stream by perforated plates. The results of this study show that the air stream turbulence causes large-scale distortions with small amplitude on the apparently laminar diffusion flame, and the time-averaged thickness of this flame is three times as large as the purely laminar flame. Even if the air stream becomes turbulent, there exists a critical stagnation velocity gradient beyond which the flame can never be stabilized, however large the fuel ejection velocity is. Local extinction near the stagnation region always leads to global extinction of the whole flame. As expected, the critical velocity gradient decreases as the turbulence intensity increases. This flame is subjected to the sum of the bulk stretch rate exerted by the mean flow and the turbulent stretch rate exerted by small eddies of Kolmogorov scale. The critical total stretch rate at which the extinction occurs is nearly constant for each fuel for all turbulence conditions tested in the present study and coincides with the critical stagnation velocity gradient of the laminar diffusion flame, that is, the total stretch rate without turbulence. This fact suggests that large-scale eddies are not so effective for the local extinction and that the chemical reaction that occurs in molecular scale is not affected by small eddies of the Kolmogorov scale.

Evidence for intramolecularly folded i-DNA structures in biologically relevant CCC-repeat sequences.

The structural behaviour of repetitive cytosine DNA is examined in the oligodeoxynucleotide sequences of (CCCTAA)3CCCT (HTC4), GC(TCCC)3TCCT(TCCC)3 (KRC6) and the methylated (CCCT)3TCCT(CCCT)3C (KRM6) by circular dichroism (CD), gel electrophoresis (PAGE), and ultra violet (UV) absorbance studies. All the three sequences exhibit a pH-induced cooperative structural transition as monitored by CD. An intense positive CD band around 285 nm develops on lowering the pH from 8 to slightly acidic condition, indicative of the formation of base pairs between protonated cytosines. The oligomers are found to melt in a fully reversible and cooperative fashion, with a melting temperature (Tm) of around 50 degrees C at pH 5.5. The melting temperatures are independent from DNA concentration, indicative of an intramolecular process involved in the structural formation. PAGE experiments performed with 32P-labeled samples as well as with normal staining procedures show a predominantly single band migration for all the three oligomers suggestive of a unimolecular structure. From pH titrations the number of protons required for generating the structures formed by HTC4, KRC6 and KRM6 results to be around six. These findings strongly suggest that all the three sequences adopt an intramolecular i-motif structure. The demonstration of i-motif structure for KRC6, a critical functional stretch of the c-ki-ras promoter proto-oncogene, besides the human telomeric sequence HTC4, may be suggestive of larger significance in the functioning of DNA.

Expression of TGMV antisense RNA in transgenic tobacco inhibits replication of BCTV but not ACMV geminiviruses

Transgenic tobacco plants expressing an antisense RNA targeted against tomato golden mosaic virus (TGMV) show reduced/no symptoms and viral DNA accumulation upon TGMV infection [5]. The targeted region includes the AL1 gene, encoding an essential viral replication protein. This DNA sequence is conserved in various other geminiviruses, suggesting they too might show inhibition of replication in these plants. We infected leaf material with African cassava mosaic virus (ACMV) and beet curly top virus (BTCV) and saw a 4-fold reduction of BCTV, but not ACMV, DNA accumulation, compared to controls. The equivalent regions of BCTV and ACMV show similar overall homology to the TGMV target (63% and 64% respectively), but within this, BCTV displays a 280 nucleotide region of high homology (82%). In contrast, for ACMV, the homology is more dispersed. This indicates that a critical stretch of good complementarity is needed to block expression of the target mRNA, that is effective even within along antisense transcript. These studies indicate the potential for developing a multifunctional antisense cassette.

破壊力学 遷移温度域の弾塑性破壊じん性に及ぼすひずみ速度の影響

Static and dynamic fracture toughness tests at three strain rates were performed on two ASTM A508Cl. 3 steels by using 1T-CT specimens and fatigue pre-cracked instrumented Charpy specimens. The KJC converted from JC of the small specimens indicated a wide scatter. When the strain rate increased, the fracture toughness transition curves shifted to a higher temperature region. An increase in strain rate reduced the scatter of KJC dramatically, especially in the high temperature region, and decreased the lower bound fracture toughness. Fractographic examination of the fractured specimen surfaces indicated that the relationships between KJC and the stable crack growth, Δa0, distance from stable crack front to trigger point, X, or distance from fatigue crack front to trigger point, Δa0+X, were expressible by a single curve, respectively. The scatter of KJC was caused by the varience of Δa0, X, and Δa0+X. With increasing strain rate, Δa0, X, and Δa0+X decreased significantly, leading to a small scatter of KJC. The KJC value at Δa0=0, X=0, and Δa0+X=0 was proposed as the lower bound fracture toughness of a steel and labeled KJCi. The shape of KJCi versus temperature curve was controlled by the critical stretch zone width, SZWc. The Weibull slope m of KJC became larger with increasing strain rate and decreasing temperature. Higher toughness data with larger stable crack extension than 100μm violated the linearity of Weibull plots. In the statistical approach to determine the lower bound fracture toughness in the transition region, much more analytical development is needed. The KJC value with 3% fracture probability coincided with the KJCi value in the low temperature region even in a high strain rate.

Experimental investigation of stretched premixed flames burning mixtures of methane and methyl chloride in air and comparison with numerical simulations

Experimental measurements of the laminar burning velocities of flames burning methane/methyl chloride mixtures, and methyl chloride in air, are made in a counterflow burner. The flame speeds are observed to decrease with increasing chlorine loading, from 40 cm s −1 for a stoichiometric methane-air flame to 4.9 cm s −1 for a stoichiometric methyl chloride-air flame. Good agreement is found between flame speeds deduced from experiments and those predicted by numerical simulations at the periphery of the domain, i.e., for methane-air and methyl chloride-air flames. The agreement between experiments and calculations at intermediate regions, that is, for varying methanemethyl chloride-air mixtures is at best reasonable. Sources for the discrepancies in the kinetic mechanism are identified. The critical extinction stretch rate for flames burning methyl chloride in air is measured, as is the stretched flame speed close to extinction.

Fatigue crack propagation in dispersion-strengthened P/M aluminium alloys at room and elevated temperatures: Bray, G.H. Proc. Conf. on P/M in Aerospace and Defense Technologies, Tampa, Florida, USA, Mar. 1991

An experimental study was performed to analyze the effect of the fatigue precracking stress intensity range (ΔK) on the formation of the stretch zone, a potential measure of JIC, in AISI 4340 steel. Compact tension specimens were fatigue precracked at 62%, 100% and 147% of the ΔK limit prescribed by ASTM E813-93, then loaded to levels of J ⩽ JIC. The resulting stretch zone widths (SZW) on the post-fracture surfaces were measured using scanning electron microscopy and plotted against J calculated from load vs load-line displacement. Precracking was found to reduce the SZW formed at a given value of J in proportion to the maximum applied J during precracking. Blunting line equations were fitted to experimental data for J < 0.65 JIC, revealing a constraint factor of m = 1.08. At J > 0.65 JIC, a transition from blunting to ductile crack propagation occurred. Extrapolation of the blunting line proposed in ASTM E813 to the critical stretch zone width yielded values of JIC that were erroneously high compared with extrapolation from experimental data.

Void nucleation and growth for compressible non-linearly elastic materials: An example

In this paper, we carry out an explicit analysis of a bifurcation problem for a solid sphere, composed of a special class of compressible non-linearly elastic materials, and subjected to prescribed radial stretch λ > 1 at its boundary. One solution to this problem, for all values of λ, is that of pure homogeneous stretching in which the sphere expands radially. However, for sufficiently large values of λ, a second configuration is possible where an internal traction-free spherical cavity forms at the origin. The critical stretch λ = λcr at which this solution bifurcates from the trivial homogeneous solution is determined. The trivial solution is shown to become unstable at λ = λcr. It is also shown how the bifurcation model may be interpreted as describing sudden rapid growth of a pre-existing microvoid. The analogous issues for axisymmetric plane strain deformations of a cylinder are briefly discussed.

Ductile fracture toughness of polycrystalline armco iron of varying grain size

The influence of polycrystalline grain size on the ductile fracture toughness ( J ic) of Armco iron has been studied over the grain size range of 38–1050 μm. Experimental evidence for the various stages of ductile fracture during fracture toughness testing was obtained through scanning electron metallography. J ic decreases with coarsening of grain size and follows a parabolic relation with d −1/2. The critical stretch zone width in fracture mechanics specimens decreases with increasing grain size; at 1050 μm grain size, the stretch zone is not detectable and the material fails by cleavage fracture. The cleavage fracture behaviour at room temperature has been explained in terms of a stress concentration ahead of the crack tip that reaches the level of the cleavage fracture stress. The variation of fracture toughness J ic with grain size was studied in terms of the plastic zone size. Indirect methods based on K jc and P j fail to estimate the plastic zone size correctly. Microhardness measurements were found to be quite suitable for estimation of the plastic zone size. At finer grain sizes, the plastic zone size encompasses a substantial number of grains while at the coarsest grain size (1050 μm it is confined to a single grain, a feature that corresponds with the occurrence of cleavage fracture. The present measurements on Armco iron are shown to be consistent with the J ic data of Klasen et al. [ Mater. Sci. Engng 80, 25 (1986)] on microalloyed steels once one has accounted for inclusions and the higher carbon content.

Effect of hydrostatic pressure on fracture toughness (2nd report, Elastic-plastic fracture toughness)

To investigate the effect of hydrostatic pressure on the fracture toughness of boron steel and aluminum alloy, the compact tension test based on ASTM standard is carried out under pressure up to 400 MPa. The displacement at the crack initiation is detected by an electric potential difference method under pressure. The elastic-plastic fracture toughness JIC increases with increasing pressure. The phenomenon of the increase of fracture toughness due to pressure is confirmed by measuring a critical stretch zone width of boron steel. The experimental JIC values are compared with the J integral values analyzed by FEM.

Effect of Hydrostatic Pressure on Fracture Toughness : 2nd Report, Elastic-plastic Fracture Toughness

To investigate the effect of hydrostatic pressure on the fracture toughness of boron steel and aluminum alloy, a compact tension test based on ASTM Standard is carried out under pressure up to 400 MPa. The displacement at the crack initiation is detected by an electric potential difference method under pressure. The elastic-plastic fracture toughness JIC increases with an increasing increasing pressure. The phenomenon of the increase of fracture toughness due to pressure is confirmed by measuring a critical stretch zone width of boron steel. The experimental JIC values are compared with the J-integral values analyzed by FEM.

Influence of specimen size on the crack-opening stretch zone

An investigation has been carried out to find the influence of specimen size (thickness and width) on the blunting line in the J Ic test procedure (where J Ic is the critical value of the elastic-plastic toughness parameter). Compact tension specimens prepared from aircraft quality AISI 4340 steel heat treated to a yield strength level of 1000 MPa were used. The J Ic values were determined by two different methods, i.e. (i) multiple-specimen resistance curve and (ii) by measurement of the critical stretch zone width on the fracture surface. This investigation shows that the stretch zone size ahead of the crack tip during blunting as well as the post-crack-initiation stage is independent of specimen size. The slope of the blunting line was found to be 1.2, compared with 2.0 (eqn. (2)) in the standard J Ic test procedure. J Ic values obtained by measurement of the critical stretch zone size were found to be in good agreement with the J Ic value obtained by the multiple-specimen resistance curve method. J, the crack-opening displacement COD and the stretch zone size ( Δα) sz were found to be related by the expressions J = 0.6 σ 0 (COD) and COD ≈1.86 ( Δα) sz (where σ 0 is the flow stress).

Surface instabilities on an equibiaxially stretched elastic half-space

The existence of non-homogeneous modes of deformation in an equibiaxially stretched elastic half-space is investigated. The surface of the half-space is assumed to be traction-free.It is shown that three different eigenmodal deformations are possible, according as the cube of the critical stretch is greater than, less than, or equal to ¾.For the class of strain-energy functions due to Ogden (9), necessary conditions for bifurcation are deduced-for equibiaxial tension. Results are given and discussed for single-term, as well as two- and three-term Ogden strain-energy functions, for equibiaxial tension and compression.

Fracture Toughness of 4130 Quenched and Tempered Steel

The fracture-energy criterion JIc was determined by the single specimen method on quenched and tempered 4130 steel used for tower anchor bars of high voltage transmission lines. A thermodynamic approach was used to rationalize JIc as a stable crack initiation criterion. Slow-bend testing was performed on three-point bend specimens of different dimensions. The effect of microstructure (martensite and bainite + ferrite) on the value of JIc is not significant. Specimen dimensions (1.5 × 3 × 15 cm; 2 × 4 × 20 cm) seem not to affect JIc values. Critical stretch zone width, WSZc, was evaluated on broken specimen halves using scanning electron microscopy. With some assumptions, relationships between WSZc, CTOD and JIc were established. Values of JIc calculated via WSZc compare favorably with JIc determined from a single specimen.

A thermodynamic approach to the stress‐induced crystallization in cross‐linked rubbers

AbstractThe thermodynamic treatment of crystallization phenomena in a prestretched rubber was undertaken. Emphasis was put on defining conditions for the thermodynamic stability of the extendedor folded‐chain crystal structure. The extended‐chain structure is found to be stable thermodynamically at temperatures higher than the isotropic melting point of un‐cross‐linked polymer T in the stretched state, while the folded chain one is not. Below T, the stretch ratio of the network structure determines which crystal structure is more stable. The relation among the critical stretch ratio for the extended/folded crystalline structure transition, temperature, and molecular weight is also discussed. The crystallinity predicted by this work becomes zero at a temperature of T, the isotropic melting point of a cross‐linked system. The value of T decreases with increasing cross‐link density, and this is consistent with the experimental data reported in the literature.

Ductile fracture theories for pressurised pipes and containers

The probable mechanisms of fracture which may be encountered in various components may generally be classified in two main groups. In the first the fracture is of ‘plane strain’ type which may occur in components ( e.g. thick walled pressure vessels and other heavy section components) where prior to and during a possible fracture propagation the material is not expected to undergo large scale plastic deformations. In this case the underlying fracture theory is rather well-understood, and a criterion based on K IC usually provides a highly reliable tool to deal with the problem. The second type of fracture failure falls into the general category of ‘plane stress’ or ‘high energy’ fracture. In a great variety of tubings and containers, due to relatively small wall thickness, large defect size, high material toughness, and high temperature, prior to and during a possible rupture process, around the defect region the material would be expected to undergo large scale plastic deformations. In this case the standard theories of fracture based on the concept of plane strain fracture toughness are not applicable. This type of fracture which is generally accompanied by large inelastic deformations is (somewhat loosely) termed the plane stress fracture for which currently there does not seem to be a universally accepted criterion. In some components an additional complicating factor arises where one is dealing essentially with a shell of given curvature rather than a flat plate. The theories which are currently in use in practice to analyse plane stress type of fracture are those which are based on the concepts of critical crack opening stretch, K R - characterisation , J- integral , and the recently proposed plastic instability. In this paper the application of the fracture criteria based on these concepts to the fracture of shells will be discussed and the concept of plastic instability will be developed in some detail. Since there is no widely accepted standard criterion to deal with this type of fracture, one of the aims of the paper will be to provide an up-to-date critical appraisal of the current theories.

Studies on melt spinning. VI. The effect of deformation history on elongational viscosity, spinnability, and thread instability

The effect of deformation history on the elongational behavior and spinnability of polypropylene melt was investigated by carrying out isothermal melt-spinning experiments. For the study, spinnerettes of different die geometries were used to investigate the effect, if any, of the entrance angle, the capillary length-to-diameter (L/D) ratio, and the reservoir-to-capillary diameter (DR/D) ratio on the elongational behavior of molten threadlines. An experimental study was also carried out to investigate the phenomenon of draw resonance in the extrusion of polypropylene melts through spinnerettes of different die geometries. Draw resonance is the phenomenon which gives rise to pulsations in the threadline diameter when the stretch ratio is increased above a certain critical value. The results of our study show that the critical stretch ratio at which the onset of draw resonance starts to occur decreases as the L/D ratio is decreased, as the entrance angle is increased, as the DR/D ratio is increased, as the melt temperature is decreased, and as the shear rate in the die is increased. Of particular interest is the observation that, at 180°C, the severity of fiber nonuniformity increases as the stretch ratio is increased, whereas at 200°C and 220°C, the severity of fiber nonuniformity first increases and then decreases as the stretch ratio is increased considerably above the critical value. A rheological interpretation of the observed onset of draw resonance is presented with the aid of the independently determined rheological data.

Crack opening stretch in a plate of finite width

The problem of a uniaxially stressed plate of finite width containing a centrally located “damage zone” is considered. It is assumed that the flaw may be represented by a part-through crack perpendicular to the plate surface, the net ligaments in the plane of the crack and through-the-thickness narrow strips ahead of the crack ends are fully yielded, and in the yielded sections the material may carry only a constant normal traction with magnitude equal to the yield strength. The problem is solved by neglecting the bending effects and the crack opening stretches at the center and the ends of the crack are obtained. Some applications of the results are indicated by using the concepts of critical crack opening stretch and constant slope plastic instability.

Theoretical Study on the Biaxial Tensile Properties of Plain Tricot Fabric

The structure of the plain tricot knitted fabrics underlies two-bar tricots knitted fabrics. The biaxial tensile properties of the plain tricot fabric are calculated theoretically from the structure of fabric and some of mechanical properties of yarn, applying the method developed by S. Kawabata.The tensile process is divided into two regions, i.e., the bending effective region and the stretching effective region. The border of these two regions can be given by critical stretch ratios λc1 and λc2 where suffixes 1 and 2 indicate wale direction and course direction, respectively.The tensile properties in the two regions are analyzed by using a new model which is different in structure from single dembigh stitch. The model used for plain tricot stitch consists of needle loop and sinker loop, i.e., layer of front yarn and of back yarn. In this model are two yarn crossing points; one is of the front yarn layer and another is of the back yarn layer. It is assumed that these two crossing points are in contact with each other and do not seperate during deformation process before critical ratios, that is, in bending effective region, but, after the critical state, these two points are separated in proportion to λ2. The other analytical method is same as that of single dembigh fabric. Yarn frictional properties between crossing yarns are introduced and the accuracy of the calculation is much more enhanced due to this improvement, compared with the theory of single dembigh fabric.Using some of actual fabrics, the theoretical and experimental values are compared, in all deformation modes of biaxial extension, with good agreements.

Rheological and Stochastical Considerations in Fracture of Elastomers

The rheological and stochastic characteristics of the fracture of elastomers are hereunder discussed to begin with. For the failure process of elastomers, two hypotheses are proposed, viz.(a) The flow of the polymer chains in polymeric solids may be effective to disperse the stress concentration around a flaw in the solid.(b) The probability to grow a catastrophic flaw in the solid may increase by the increase in time interval during the flow.These two hypotheses will bring reverse effects on the strength of the polymer. A theory on fracture mechanism based on the hypothesis (b) is introduced, and, on this theory, the creep failure process of some elastomers are analyzed according to the theory. In this analysis, two material constants on fracture, by which the failure process is characterized, are obtained by the experiment.As another application of the theory, the distribution of the breaking stretch ratios in uniaxial extension of constant rate is estimated by the constants obtained by the creep fracture. There is good agreement between the theoretical and the experimental results, but there are cases where the fracture mechanism assumed by hypothesis (a) is also important. It is considered practically that both the mechanisms occur simultaneously.Secondly, the importance of the research on the strength in multiaxial stressed state is emphasized for the sake of considering the fracture phenomena of elastomers in relation to the micro structures. Some data from a biaxial extension testing of valcanized SBR is introduced and, as a fracture criterion, the following formula is presentedλi≥λB (i=1 or 2)where λi is stretch ratio along the principal axes (axis 1 and 2 have orthogonal direction to each other), λB is a critical stretch ratio and independent value on any combinations of λi (i=1, 2).This simple result is discussed in relation to its mechanical property obtained by the biaxial tensile testing.

Theoretical Study on the Biaxial Tensile Properties of Warp Knitted Fabric of Single Tricot Stitch

The biaxial tensile property of warp knitted fabric of single tricot stitch with close lap has been calculated theoretically from yarn properties and fabric structure, by using the method introduced by Kawabata. In this analysis, two regions, i.e., the deformation regions of bended yarn and yarn stretching, are separated with a critical stretch ratio taken as a border and the properties in these regions are analyzed separately. Then the properties in these two regions are composed. That is:1. Calculating method of critical stretch ratios is introduced theoretically from fabric structure by putting a model in which yarn is straight and not stretched.2. The biaxial tensile properties in the bending effective region is calculated from fabric structure and the equilibrium of forces, assuming that the unit structure consists of three arcs in relaxed form and that friction between yarns can be neglected.3. Next, the biaxial tensile properties in the stretch effective region is calculated from the equilibrium of forces and the tensile property of yarn on assumption that yarn does not move from needle loop to sinker loop.4. Finally, some examples are calculated and examined the effects of some structure constants of the fabric on its tensile property.