Constant Tensile Force Research Articles

Permanent effect of a cryogenic spill on fracture properties of structural steels

Fracture analysis of a standard construction steel platform deck, which had been exposed to a liquid nitrogen spill, showed that the brittle fracture started at a flaw in the weld as a consequence of low-temperature embrittlement and thermal stresses experienced by the material. In the present study, the permanent effect of a cryogenic spill on the fracture properties of carbon steels has been investigated. Charpy V-notch impact testing was carried out at 0 °C using specimens, from the platform deck material. The average impact energy appeared to be below requirements only for transverse specimens. No pre-existing damage was found when examining the fracture surfaces and cross sections in the scanning electron microscope. Specimens of the platform deck material and a DOMEX S355 MCD carbon steel were tensile tested immersed in liquid nitrogen. Both steels showed a considerable increase in yield- and fracture strength and a large increase in the Lüders strain compared to the room temperature behavior. A cryogenic spill was simulated by applying a constant tensile force to the specimens for 10 min, at -196 C. Subsequent tensile tests at room temperature showed no significant influence on the stress-strain curve of the specimens. A small amount of microcracks were found after holding a DOMEX S355 MCD specimen at a constant force below the yield point. In a platform deck material tensile tested to fracture in liquid nitrogen, cracks associated with elongated MnS inclusions were found through the whole test region. These cracks probably formed as a result of the inclusions having a higher thermal contraction rate than the steel, causing decohesion at the inclusion-matrix interface on cooling. Simultaneous deformation may have caused formation of cracks. Both the microcracks and sulphide related damage may give permanently reduced impact energy after a cryogenic exposure.

Inhomogeneous Computed Tomographic Densities in Lungs in Acute Respiratory Distress Syndrome: Stress Multipliers Leading to Ventilator-induced Injury?

Inhomogeneous Computed Tomographic Densities in Lungs in Acute Respiratory Distress Syndrome: Stress Multipliers Leading to Ventilator-induced Injury?

THERMO - MECHANICAL MODIFICATION OF AMORPHOUS POLYESTER FIBRES: I. DEFORMATION BEHAVIOUR

The deformation behaviour of as-spun amorphous poly (ethylene terephthalate) (PET) yarns subjected to simultaneous thermal and mechanical treatments has been studied. The samples heat mechanical treatments were realized using constructed and produced in the author’s laboratory device. Thermal deformation experiments were carried out on conditions of linear samples heating from room temperature up to 200 0C while they were subjected to a well-defined constant tensile force. It was observed very interesting and seemingly strange dependence of the final fibre length from the values of the applied tensile stress. The results obtained show the role of the strain stress on the causing samples deformation and in the control of the rival processes of the fluid like deformation and stress-induced crystallization.

Molecular Mechanisms, Thermodynamics, and Dissociation Kinetics of Knob-Hole Interactions in Fibrin

Polymerization of fibrin, the primary structural protein of blood clots and thrombi, occurs through binding of knobs 'A' and 'B' in the central nodule of fibrin monomer to complementary holes 'a' and 'b' in the γ- and β-nodules, respectively, of another monomer. We characterized the A:a and B:b knob-hole interactions under varying solution conditions using molecular dynamics simulations of the structural models of fibrin(ogen) fragment D complexed with synthetic peptides GPRP (knob 'A' mimetic) and GHRP (knob 'B' mimetic). The strength of A:a and B:b knob-hole complexes was roughly equal, decreasing with pulling force; however, the dissociation kinetics were sensitive to variations in acidity (pH 5-7) and temperature (T = 25-37 °C). There were similar structural changes in holes 'a' and 'b' during forced dissociation of the knob-hole complexes: elongation of loop I, stretching of the interior region, and translocation of the moveable flap. The disruption of the knob-hole interactions was not an "all-or-none" transition as it occurred through distinct two-step or single step pathways with or without intermediate states. The knob-hole bonds were stronger, tighter, and more brittle at pH 7 than at pH 5. The B:b knob-hole bonds were weaker, looser, and more compliant than the A:a knob-hole bonds at pH 7 but stronger, tighter, and less compliant at pH 5. Surprisingly, the knob-hole bonds were stronger, not weaker, at elevated temperature (T = 37 °C) compared with T = 25 °C due to the helix-to-coil transition in loop I that helps stabilize the bonds. These results provide detailed qualitative and quantitative characteristics underlying the most significant non-covalent interactions involved in fibrin polymerization.

Constant force elongational flow of polymer melts: Experiment and modelling

Characterizing elongational behavior of polymer melts in constant elongation rate or constant tensile stress experiments is hampered by the onset of “necking” instabilities according to the Considère criterion: For an elastic filament, homogeneous uniaxial extension is not guaranteed for strains larger than the strain at which a maximum occurs in the force versus extension curve. Although simulations and experiments seem to indicate that in viscoelastic fluids viscosity effects delay the onset of necking to higher strains, integral measurements of elongational viscosities in Meissner- or Münstedt-type elongational rheometers are often affected by inhomogeneous deformations. A simple means of avoiding the consequences of the Considère criterion consists in operating at constant tensile force, but little attention has been paid so far to constant force elongational rheometry since the pioneering work of Raible et al. [J. Non-Newtonian Fluid Mech. 11, 239 (1982)]. Constant force elongation is also of great industrial relevance, since it is the correct analog of steady fiber spinning, as, e.g., exemplified by the so-called Rheotens test. We present experimental data for two long-chain branched polyethylene melts obtained in constant elongation rate and constant tensile force modes by use of a Sentmanat extensional rheometer in combination with an Anton Paar MCR301 rotational rheometer. The accessible experimental window and experimental limitations are discussed. Constant force elongation can be subdivided into three distinct deformation regimes: At small deformations (regime 1), constant force elongation is equivalent to creep at constant tensile stress. This is followed by regime 2, which is characterized by a steep increase in tensile stress at roughly constant strain rate, while regime 3 corresponds to elongation of a viscous power-law fluid. All three regimes can be modelled quantitatively by a single integral constitutive equation, and a constitutive analysis reveals substantial underestimation of the effective strain-hardening effect observed in constant strain-rate elongation in comparison to constant tensile force extension, which may be indicative of the effect of necking under constant elongation-rate conditions.

Retraction: Unraveling Solvent-Mediated Reaction Pathways Leading to Regiospecific Mechanochemical Cleavage of Disulfide Bonds in Peptides

Stressing disulfide bonds! Nucleophilic thiol-disulfide exchange reactions within the I27 domain of titin were previously investigated with force clamp AFM. Here, all possible pathways associated with disulfide bond scission at constant tensile force are revealed in terms of end-to-end distances by using force clamp molecular dynamics. The simulations, together with experimental data unravel the competition between mechanochemical bond activation and solvent-mediated regiospecificity exhibited during SS bond cleavage due to the nucleophilic substitution mechanism within a stretched peptide.

The PLATELET INTEGRIN αIIbβ3 CHANGES FROM A LOWER- to A Higher-AFFINITY STATE DURING INTERACTION with FIBRINOGEN

Abstract 1130The integrin αIIbβ3 (GPIIb-IIIa) plays an essential role in platelet adhesion and cohesion by binding to a number of adhesive ligands including fibrinogen, von Willebrand factor, fibronectin, and vitronectin. Following ligand binding, αIIbβ3-mediated intracellular signaling is thought to stabilize platelet adhesive interactions, cause platelet spreading, and initiate clot retraction. Here we investigated the time course of interactions between purified αIIbβ3 and its major ligand fibrinogen when both were firmly attached to apposed surfaces, mimicking αIIbβ3-mediated platelet adhesive interactions. To measure the mechanical resistance of individual αIIbβ3-fibrinogen complexes under a constant tensile force, we used a previously described optical trap force clamp system (Litvinov et al., Biophys. J., 2011, 100, 165). Briefly, we trapped a fibrinogen-coated microscopic bead in a focused laser beam and intermittently brought it into contact with an αIIbβ3-coated silica pedestal. Repeated touching and separation of the bead and pedestal occurred with a compressive force of 20 pN and a constant pulling force of 50 pN. When the duration of contact between interacting molecules on the bead and pedestal was varied from 0.1s to 2s, the probability of αIIbβ3-fibrinogen interaction was time-dependent. This enabled us to extract two-dimensional kinetic parameters, i.e., how fast the αIIbβ3-fibrinogen complex forms and dissociates at the interface without the application of an external tensile force (zero-force kinetics). To convert the kinetic parameters to absolute values, we determined the density of αIIbβ3 molecules on the pedestal surface capable of binding 125I-fibrinogen to be ≈3,000 molecules per μm2. We found that the reactive αIIbβ3 exists in at least two states that differ significantly in their on-rates (kon1 =8×10−5 and kon2 =5.6×10−4 μm2/s), off-rates (koff1 =1.56 and koff2 =1.70 1/s) and affinities (Kd1 =2×104 and Kd2 =3×103 1/μm2) for fibrinogen. In the absence of αIIbβ3 activators, the interacting molecules reached a stable equilibrium in which 85% of the complexes were in the lower affinity form. In the presence of the activator Mn2+, the proportion of lower affinity forms decreased to 60%. Mn2+ also changed the on-rates (kon1 =3.38×10−5 and kon2 =9.9×10−4 μm2/s), off-rates (koff1 =2.99 and koff2 =4.82 1/s) and affinities (Kd1 =9×104 and Kd2 =5×103 1/μm2). We then tested whether the ratio between the two integrin activation states and the mechanical stability of the αIIbβ3-fibrinogen complex depends on the duration of contact between the interacting surfaces. As the contact duration was prolonged from 0.1s to 2s, the αIIbβ3-fibrinogen bond lifetime increased from shorter lifetimes (<2s) corresponding to the low-affinity state to longer lifetimes (up to 50s) corresponding to the high-affinity state. Moreover, the rate of transition from low- to high-affinity increased ∼3-fold, whereas the reverse rate decreased 1.5–2-fold, resulting in an overall 6-fold increase in the equilibrium rate constant. This means that reversible conformational transitions of αIIbβ3 from low- to high affinity states occur during its interaction with fibrinogen with the equilibrium gradually shifting toward the higher affinity state as contact duration increases. Thus, the strength of αIIbβ3-fibrinogen interactions is time-dependent due to a progressive increase in the affinity of the αIIbβ3-fibrinogen complex during the course of the interaction. These results provide kinetic and thermodynamic characteristics of the adhesive interaction between αIIbβ3 and fibrinogen at the single-molecule level and provide evidence for a direct fibrinogen-induced change in αIIbβ3 conformation as a potential mechanism for the initiation of αIIbβ3-mediated outside-in signaling in platelets. Disclosures:No relevant conflicts of interest to declare.

Molecular dynamics study of the role of friction on the thermal rupture of linear alternate copolymers

We investigated, using extensive molecular dynamics simulations, the role of friction on the thermal rupture of a linear alternating copolymer tethered at one end and pulled by a constant tensile force at the other. Rupture has been described as escape from a metastable minimum of effective bond potential. We studied the effect of friction on the average first passage time and the average rate of chain scission. The role of friction on healing/recombination was also studied.

Evaluation of the efficacy of several phytopolymers in terms of their duration of mucoadhesion

In recent years worldwide interest has grown towards the development of mucoadhesive drug delivery systems. These systems are capable of adhering to body's mucosal surfaces for an extended period of time, releasing their drug content in a retarded manner. The use of biopolymers, and in particular phytopolymers, could provide a natural basis for the development of such systems. The aim of this study was to evaluate the rank order of adhesion of several phytopolymers in terms of their invitro duration of mucoadhesion. Several phytopolymers including carrageenan, sodium alginate, tragacanth, locust bean gum, pectin, acacia and guar gum were directly compressed into 100mg flat-faced solid discs with a diameter of 9mm. The duration of mucoadhesion (maximum time interval disc remained in contact with the mucosal surface) of prepared discs to rat intestinal mucosa (model biological membrane), present within a pH 6.8 phosphate buffer at 37 C and under a constant tensile force of 15g, was evaluated. Each study was performed six times and the mean±standard deviation was calculated. Among the polymers investigated, carrageenan (4.6±0.8 h) and guar gum (4.1±0.5 h) containing discs resulted in the longest duration of mucoadhesion, which were significantly (P

Temperature Aging, Compression Recovery, Creep, and Weathering of a Foam Silicone Sealant for Bridge Expansion Joints

Silicone foam was investigated as a sealant for small movement expansion joints in bridge decks. This paper presents results from the laboratory assessment of a model foam sealant subjected to thermal aging (exposure to high and low temperatures and temperature cycling), fatigue conditions, and outdoor weathering. Parallel tests were performed on a commercial solid-silicone bridge-joint sealant. Test results showed that the solid sealant recovered faster than the foam sealant after being subjected to prolonged compression at elevated temperature. When subjected to a constant tensile force, both the foam and solid sealants exhibited high initial creep rates (rate of elongation), but appeared to reach an equilibrium level of elongation at longer times. The foam sealant creeps at a slower rate and takes more time to get to the equilibrium elongation, whereas the solid sealant creeps much faster initially and reaches equilibrium faster. Thermal aging was found to have significant effects on the sealant modulus (increases with temperature); however, the effects on ultimate stress and strain were not apparent for both types of sealants. Temperature cycling between 24 and -29°C was observed to diminish the ultimate stress and strain of both the sealants by roughly 25%; however, no significant changes in modulus were found. Results from the tests on a limited number of outdoor-weathered sealant specimens showed that weathering appeared to produce an increase in sealant tensile and shear moduli (i.e., hardening effects because of weathering) and a decrease in ultimate strain. The weathered samples show tension loading-unloading behavior similar to the unaged samples. The tensile stress relaxation rate of the outdoor-weathered sealants could not be distinguished from those laboratory-cured (unweathered) counterparts.

Effects of Load and Contact Time on the Stability of Bimolecular Integrin-Fibrinogen Bonds Under a Constant Tensile Force

The regulated ability of integrin alphaIIbbeta3 to bind fibrinogen plays a crucial role in platelet function, thrombosis, and hemostasis. Previously, we found that the rupture force distribution of individual integrin alphaIIbbeta3-fibrinogen interactions under ramp loads display at least two components that differ in kinetics, loading rate dependence, and susceptibility to activation/inhibition of the integrin, which suggests that specific binding and unbinding of the integrin alphaIIbbeta3 with fibrinogen is a complex multi-step process (Litvinov et al., Biophys J. 2005; 89:2824). Employing a new laser tweezers-based electronic force clamp (Litvinov et al., Biophys J. 2008, 94, Suppl., Abstract, 1724-Pos.), we now study interactions of purified integrin alphaIIbbeta3 and fibrinogen under constant tensile force, mimicking the effect of hydrodynamic blood flow on an adherent platelet. Under a constant tensile force of 50 pN, the distribution of durations of the alphaIIbbeta3 and fibrinogen bonds is bimodal, with specific integrin-fibrinogen interactions mostly lasting more than 2 sec. At the same constant unbinding force, the bond lifetimes increase as the duration of contact between alphaIIbbeta3- and fibrinogen-coated interacting surfaces is increased from 0.1s to 2s, again suggesting that the initial interaction of fibrinogen with alphaIIbbeta3 is followed by reorganization of the binding interface, enhancing the strength and stability of binding. In further experiments, the average bond lifetimes exponentially decrease as tensile force is increased from 0 to 30 pN, suggesting that in this force range the alphaIIbbeta3-fibrinogen interactions represent classical “slip bonds”. Taken together, these data provide important quantitative characteristics of the alphaIIbbeta3-fibrinogen binding and unbinding, which underlie the dynamics of platelet adhesion and aggregation in the blood flow.

Measurement of the Lifetime of Bonds Between αIIbβ3 and Fibrinogen Using Constant Unbinding Forces Generated by Optical Tweezers

We have shown that the distribution of rupture forces between individual αIIbβ3 and fibrinogen molecules displays at least two components that differ in kinetics, loading rate dependence, and susceptibility to activation and inhibition of the integrin. This suggests that the binding and unbinding of αIIbβ3 and fibrinogen is a complex multistep process that depends on the conformational state of both αIIbβ3 and fibrinogen, the duration of their interaction, and environmental factors such as externally-applied shear force. To directly test these possibilities, we quantified the lifetime of bonds stabilizing individual αIIbβ3-fibrinogen complexes with a novel nanoscale laser tweezers-based system that uses an optical trap to apply a constant unbinding force to single-molecule protein-protein interactions. When a ligand-coated bead is brought into repeated contact with a receptor-coated silica pedestal using an optical trap, the signal parameters that are measured correspond to both compressive and rupture forces. To measure bond lifetimes, the amplitude of the generated tensile force signal must remain constant throughout the lifetime of the bond. This can be accomplished by incorporating an analog feedback circuit within the optical system. The system also enables us to control the time of contact between interacting surfaces, the magnitude of compressive force during contact, the magnitude of the tensile force, and the time of protein-protein separation when binding occurs. To quantify the forced unbinding of fibrinogen molecules covalently bound to latex beads and αIIbβ3 molecules covalently attached to silica microspheres, we measured the distribution of bond lifetimes obtained under constant tensile force, mimicking the effect of hydrodynamic blood flow on an adherent platelet. We found that the separation times of the αIIbβ3- and fibrinogen-coated surfaces varied, indicating that the interactions occurring at the interface had heterogeneous kinetic and thermodynamic properties. Discrimination of specific αIIbβ3-fibrinogen binding events versus non-specific interactions was based on comparison of bond lifetime distributions in the absence and presence of abciximab or eptifibatide, specific inhibitors of fibrinogen binding to activated αIIbβ3. We found that the separation times of the αIIbβ3- and fibrinogen-coated surfaces were bimodal, with specific integrin-fibrinogen interactions lasting more than 2s under a constant tensile force of 50 pN. Varying the time of contact between αIIbβ3 and fibrinogen from 0.1s to 2.0s at the same unbinding force revealed that the bond lifetimes increased as the duration of contact between that interacting surfaces increased, suggesting that stability of αIIbβ3-fibrinogen interactions is time-dependent. Because these measurements mimic the binding/unbinding parameters and the time course of the αIIbβ3-fibrinogen interactions under conditions of shear, they are relevant to physiological processes of fibrinogen-mediated platelet adhesion and platelet aggregation. Taken together, our data suggest a model for fibrinogen binding to αIIbβ3 in which the initial interaction is followed by reorganization of the binding interface, thereby enhancing the strength and stability of fibrinogen binding.

Corrected Formulas for Natural Frequencies of Cantilever Beams Under Uniform Axial Tension

N ATURAL frequencies of beams under axial tensile force are of interest in many applications such as rotating fan blades, turbine blades, etc. There have been extensive studies on beam dynamics considering the effects of rotation. Rosales and Filipich [1], for example, studied the dynamic stability of a spinning beam with an axial dead load.Naguleswaran [2] presented a study of lateral vibration of a centrifugally tensioned uniform Euler–Bernoulli beam. He developed the database of nondimensional parameters of speed and natural frequencies for various offset ratios of a beam. Lee [3] studied the dynamic response of a rotating Timoshenko beam subjected to axial forces and a moving load. Closed-form solutions for the natural frequencies of rotating pinned–pinned beams are available (for example, Timoshenko et al. [4]).However, other boundary conditions are not readily amenable to such a treatment. Gorman [5] presented the variation of the frequency parameter with axial tension parameter for the six modes of clamped–pinned and clamped–clamped beams. Bokaian [6] presented a comprehensive study of the effect of a constant axial tensile force on the natural frequencies andmode shapes of a uniform single-span beam, with different combinations of the end conditions. Although simple relations could capture the behavior of beams with various boundary conditions, clamped–free beams showed deviant behavior. Thus, the present study focuses on the natural frequencies of a clamped–free beam under uniform axial tension. The relations available for finding the fundamental frequency of a cantilever beam under uniform axial tension are summarized next: The Bokaian [6] characteristic equation is

Probability of Receptor Unbinding During Ligand Assisted Tether Elongation from the Red Cell Membrane

When a system having a series of non-covalently bonded multiple components is pulled to opposite directions from its two ends, sequential and/or simultaneous bond breakages take place. The probability of breakage of individual bonds depends on their intrinsic mechanical properties and on the experimental conditions. We are particularly interested in the pulling mechanics observed when a lectin functional AFM probe was used to pull out the membrane protein, glycophorin A of the red blood cell, together with lipid tether formation. Here our concern is which of the non-covalent interactions, i.e., lectin-glycophorin A or lipid-lipid interactions in the tether is disrupted when the probe is detached from the biomembrane surface. In this paper we conclude that, during the process of lipid tether elongation with a constant tensile force of ∼ 70-80 pN, unbinding of the lectin from glycophorin A takes place with a finite probability in addition to tether breakages that includes complete or incomplete delipidation of glycophorin A. [DOI: 10.1380/ejssnt.2006.273]

Effect of polymeric coatings on the static fatigue of double-coated optical fibers

The effect of polymeric coatings on the static fatigue of double-coated optical fibers is investigated. A closed form solution of the tensile stress in the glass fiber under the action of a constant tensile force in the fiber’s axial direction is obtained using viscoelastic theory. The tensile force is applied to the glass fiber and polymeric coatings. The applied tensile fore is gradually transferred from the polymeric coating to the glass fiber due to the viscoelastic behavior of the former, and the tensile stress in the glass fiber increases with time. This increase accelerates the failure of the glass fiber. The thickness and Young’s modulus of the secondary coating should be increased to ensure the long-term reliability of optical fibers, and the relaxation time of the secondary coating should be significantly larger than the expected lifetime of the optical fiber. However, the ratio of the proof-test load to the allowable external load should be increased if the secondary coating of the optical fiber has a high axial rigidity and low relaxation time.

Fictitious Crack Propagation in Fiber-Reinforced Concrete Beams

A nonlinear cracked hinge model is developed, aimed at the analysis of the bending fracture of fiber-reinforced concrete beams. The model is based on the fracture mechanics concepts of the fictitious crack model with a bilinear stress-crack opening relationship. Closed-form solutions are presented for the moment-rotation relationship of the hinge as a crack propagates, and the case of a nonzero normal force is covered. Special shapes of the stress-crack opening relationship are treated separately. These shapes are the so-called drop-linear and the drop-horizontal for which simplified expressions are obtained. The applicability of the hinge model is demonstrated through analysis of the bending fracture process in the case of a three-point bending beam and an infinitely long beam on a Winkler foundation, the latter analysis comprising the effect of a constant tensile normal force.

Stretch bending of aluminium extrusions for car bumpers

Stretch bending of two generic aluminium bumper sections are investigated experimentally and numerically. Six different stretch bending procedures, herein denoted tensile sequences, are studied. Laboratory tests are performed for one of these sequences, finding that the numerical model represents the observed behaviour satisfactorily. A sequence with constant tensile force during bending seems to be favourable with respect of cross-section deformations. These deformations increase with additional tensile force, whilst the springback during unloading shows a decreasing tendency. Post-stretching operations or membrane-induced stretch bending are also investigated, finding that these sequences result in a disadvantageous cross-section distortion.

Stretch Bending of Aluminum Extrusions: Effect of Geometry and Alloy

An experimental stretch bending study is presented in this paper. A total of 24 tests on 2-m-long rectangular hollow section aluminum specimens are performed in a specially designed stretch bending rig. All tests are carried out with a constant tensile force corresponding to the material's yield stress. Factorial analysis is systematically used in the planning of the experimental program as well as in the result interpretation. Special attention is paid to the influence of the alloy, die radius, and specimen wall thickness on certain response parameters. As stretch bending is an important industrial forming process, die force, sagging, and springback are considered to be the most relevant responses. The tests show that the alloy has the largest influence on the die force and springback, while the sagging behavior is primarily controlled by the geometric parameters (i.e., die radius and thickness).

A note on the melt strength of liquid crystalline polymer

Results of Rheotens experiments on a commercial liquid crystalline polymer melt are reported. In a nonisothermal extension of extruded filaments under the action of constant tensile force, we find a maximum melt tension at break or melt fracture stress of about σB=7 MPa, rather independent of extrusion speed or melt temperature.

Rheotens‐mastercurves and drawability of polymer melts

AbstractIn a Rheotens experiment, the tensile force needed for elongation of an extruded filament is measured as a function of the draw ratio. For thermo‐rheologically simple polymer melts, the existence of Rheotens‐mastercurves is proven. Rheotens‐mastercurves are invariant with respect to changes in melt temperature. Also, for polymer melts with different average molar masses, but similar molar mass distribution and branching structure, Rheotens‐mastercurves are invariant to changes in the average molar mass. It is shown, by testing several polyethylenes with different molar mass distribution and different long‐chain branching, that Rheotens‐mastercurves allow a direct and quantitative assessment of the drawability of polymer melts under actual processing conditions, i.e. under the action of a constant tensile force and including the effects of the rheological prehistory in the extrusion die.