Torsional Mechanical Properties Research Articles

Chromatin Buffers Torsional Stress During Transcription.

Transcription through chromatin under torsion represents a fundamental problem in biology. Pol II must overcome nucleosome obstacles and, because of the DNA helical structure, must also rotate relative to the DNA, generating torsional stress. However, there is a limited understanding of how Pol II transcribes through nucleosomes while supercoiling DNA. In this work, we developed methods to visualize Pol II rotation of DNA during transcription and determine how torsion slows down the transcription rate. We found that Pol II stalls at ± 9 pN·nm torque, nearly sufficient to melt DNA. The stalling is due to extensive backtracking, and the presence of TFIIS increases the stall torque to + 13 pN·nm, making Pol II a powerful rotary motor. This increased torsional capacity greatly enhances Pol II's ability to transcribe through a nucleosome. Intriguingly, when Pol II encounters a nucleosome, nucleosome passage becomes more efficient on a chromatin substrate than on a single-nucleosome substrate, demonstrating that chromatin efficiently buffers torsional stress via its torsional mechanical properties. Furthermore, topoisomerase II relaxation of torsional stress significantly enhances transcription, allowing Pol II to elongate through multiple nucleosomes. Our results demonstrate that chromatin greatly reduces torsional stress on transcription, revealing a novel role of chromatin beyond the more conventional view of it being just a roadblock to transcription.

Torsional Mechanics of Circular DNA.

Circular DNA found in the cell is actively regulated to an underwound state, with their superhelical density close to σ ∼ - 0.06. While this underwound state is essential to life, how it impacts the torsional mechanical properties of DNA is not fully understood. In this work, we performed simulations to understand the torsional mechanics of circular DNA and validated our results with single-molecule measurements and analytical theory. We found that the torque generated at σ ∼ - 0.06 is near but slightly below that required to melt DNA, significantly decreasing the energy barrier for proteins that interact with melted DNA. Furthermore, supercoiled circular DNA experiences force (tension) and torque that are equally distributed through the DNA contour. We have also extended a previous analytical framework to show how the plectonemic twist persistence length depends on the intrinsic bending persistence length and twist persistence length. Our work establishes a framework for understanding DNA supercoiling and torsional dynamics of circular DNA.

K-wire is more damaging than standard or acrylic drill bits when evaluating torsional properties of rabbit (Oryctolagus cuniculi) femurs.

The objective of this study is to compare drilling variables and torsional mechanical properties of rabbit femora after bicortical drilling with a 1.5-mm standard surgical drill bit, acrylic drill bit, and K-wire. 24 pairs of rabbit femora. After drilling under controlled axial displacement rate, each bone was biaxially loaded in compression followed by rapid external torsion to failure. Maximum axial thrust force, maximum drill torque, integral of force and displacement, change in temperature, maximum power spectral density of the torque signal, torque vibration, and torque and angle at the yield and failure points were collected. Pre- and postyield stiffness, yield and failure energies, and postyield energy were calculated. The work required to drill through the cis- and transcortices (integral of force and displacement) was greater for the K-wire, followed by the acrylic and then standard drill bits, respectively. The K-wire demonstrated higher maximum torque than the drill bits at the ciscortex, and the force of drilling was significantly greater. The vibration data was greater with the acrylic and standard drill bits than the K-wire. There was no difference in torsional strength between drilling types. Mechanical differences exist between different drill bits and K-wire and demonstrate that the K-wire is overall more damaging than the surgical drill bit.

Torsional mechanical properties and damage mechanism of glass fiber-ramie hybrid circular tube

Compared with other green natural fibers, ramie has higher mechanical properties and lower cost. In this study, ramie and glass fiber are made into composite circular tubes by two-dimensional braiding, and their torsional mechanical properties and damage mechanisms are investigated. The results show that under the unit wall thickness, compared with the pure glass fiber circular tube, the maximum torsion angle of the hybrid circular tube with ramie and glass fiber spindle ratio of 1:3 increases by 78%, the weight is reduced by 22%, the cost is reduced by 14%, the maximum torque is increased by 10% and the torsional stiffness is reduced by 18.75%. When the loading direction is the same as the implantation direction with more glass fiber content, the failure is caused by buckling. When the loading direction is the same as the implantation direction with more ramie content, the failure is caused by matrix fracture or fiber fracture. The numerical simulation results of the damage model are consistent with the experimental data and the damage morphology, which verifies the effectiveness of the finite element model. These conclusions provide a reference for the engineering application of composite circular tubes with environmental protection, lightweight and anti-torsion failure ability.

Stress torsional magneto-impedance effect and mechanical properties of Co-based metallic microfibers

GMI characteristics of Co-based metallic microfibers under different stresses are tested by stress torsional magneto-impedance comprehensive platform. The internal stress distribution of microfibers is simulated with ANSYS finite element software and the fracture mechanism is analyzed based on fracture morphology. The experimental results indicate that tensile stress can enhance the GMI effect of metallic microfibers, and the significant [ΔZ/Zmax]max of 293.63% at a frequency of 3 MHz is obtained under the tensile stress of 482 MPa. The simulation results demonstrate that the internal stress concentration occurs at clamping end under tensile stress, and which accumulating on the surface of microfibers after applying torsional strain, thus weakening GMI effect. The fracture morphology will change with external stress, some vein-shaped patterns and molten drops can be seen in the cross-section under tensile stress, and the helical vein-shaped patterns appear with the application of torsional strain. In addition, the circular domain wall energy will be increased due to magnetoelastic anisotropy has been affected by tensile stress, which promotes an increment of circular magnetic permeability, thus improving the GMI effect of microfibers. However, the appearance of helical anisotropy after torsional strain is added will weaken the circular domain wall energy.

Mechanical enhancement and weakening in Mo6S6 nanowire by twisting

The torsional, bending and tensile mechanical properties of Mo6S6 nanowire are examined by molecular dynamics (MD) simulations with a first-principles-based reactive force field (ReaxFF). It is found that Mo6S6 nanowire shows unique mechanical properties such as high torsional and bending flexibility, high Young’s modulus and strength, and negative Poisson’s ratio. The Mo6S6 nanowire can be strengthened or weakened via twisting, depending on the twist angle. The Mo6S6 nanowire with a slight twist angle shows brittle failure, whereas it with a large twist angle exhibits ductile failure and necking behavior. Twisted Mo6S6 nanowires show a crossover in the negative Poisson’s ratio at critical strains, that is, Poisson’s ratio first decreases but then increases, with a minimum value down to around −0.8 at the strain of 0.01 as the twist angle is 21.0 °/nm. The negative Poisson’s ratio and the crossover are explained by the bond transform that makes zero angles to the wire cross-section.

Improved Equations for the Torsional Strength of Reinforced Concrete Beams for Codes of Practice Based on the Space Truss Analogy.

Design codes provide the necessary tools to check the torsional strength of reinforced concrete (RC) members. However, some researchers have pointed out that code equations still need improvement. This study presents a review and a comparative analysis of the calculation procedures to predict the torsional strength of RC beams from some reference design codes, namely the Russian, American, European, and Canadian codes for RC structures. The reliability and accuracy of the normative torsional strengths are checked against experimental results from a broad database incorporating 202 RC rectangular beams tested under pure torsion and collected from the literature. The results show that both the readability and accuracy of the codes’ equations should be improved. Based on a correlation study between the experimental torsional strengths, and geometrical and mechanical properties of the beams, refined yet simple equations are proposed to predict torsional strength. It is demonstrated that the proposed formulation is characterized by a significant improvement over the reference design codes. The efficiency of the proposed formulae is also assessed against another equation earlier proposed in the literature, and an improvement is noted as well. From the results, it can be concluded that the proposed equations in this study can contribute to a more accurate and economical design for practice.

Comparison of torsional properties between a Fixateur Externe du Service de Santé des Armées and an acrylic tie-in external skeletal fixator in a red-tailed hawk (Buteo jamaicensis) synthetic tibiotarsal bone model.

To compare the torsional mechanical properties of 2 external skeletal fixators (ESFs) placed with 2 intramedullary pin (IP) and transfixation pin (TP) size combinations in a model of raptor tibiotarsal bone fracture. 24 ESF-synthetic tibiotarsal bone model (polyoxymethylene) constructs. Synthetic bone models were fabricated with an 8-mm (simulated fracture) gap. Four types of ESF-synthetic bone model constructs (6/group) were tested: a FESSA with a 1.6-mm IP and 1.6-mm TPs, a FESSA with a 2.0-mm IP and 1.1-mm TPs, an acrylic connecting bar with a 1.6-mm IP and 1.6-mm TPs, and an acrylic connecting bar with a 2.0-mm IP and 1.1-mm TPs. Models were rotated in torsion (5°/s) to failure or the machine angle limit (80°). Mechanical variables at yield and at failure were determined from load deformation curves. Effects of overall construct type, connecting bar type, and IP and TP size combination on mechanical properties were assessed with mixed-model ANOVAs. Both FESSA constructs had significantly greater median stiffness and median torque at yield than both acrylic bar constructs; FESSA constructs with a 1.6-mm IP and 1.6-mm TPs had greatest stiffness of all tested constructs and lowest gap strain at yield. No FESSA constructs failed during testing; 7 of 12 acrylic bar constructs failed by fracture of the connecting bar at the interface with a TP. Although acrylic bar ESFs have been successfully used in avian patients, the FESSA constructs in this study were mechanically superior to acrylic bar constructs, with greatest benefit resulting from use with the larger TP configuration.

A multi-scale method for predicting ABD stiffness matrix of single-ply weave-reinforced composite

Single-ply weave-reinforced composites (SWRC) with asymmetric large textile unit cells are increasingly used in thin-walled aerospace structures because of the high specific stiffness than the plain weave-reinforced composites. The microstructural heterogeneity makes SWRC possess different tensile, flexural, shear and torsional mechanical properties, enabling the ABD stiffness matrix in classical laminate theory (CLT) to become a suitable candidate for the description of homogenized properties. As CLT is limited for SWRC, a multi-scale method to predict the homogenized ABD matrix is concentrated in the paper, and the whole process is captured through the single-ply broken twill 1/3 weave-reinforced composite. The representative volume element is generated with realistic internal geometry, and the periodic boundary conditions are extended to include the rotational degrees of freedom. The asymmetric weave architecture makes the neutral plane shift from the midplane, if this is not considered, the inaccurate coupling effects arise and the calculation error of tensile modulus reaches 35.9%. The obtained ABD matrix is macroscopically similar to those from two-ply cross-ply [0/90] laminate with only two small entries B12 and B21 in the B coupling matrix. The predicted values agree well with experimental results, and the reasonable strain distributions under various deformations are addressed thoroughly.

Mechanical behavior of submarine cable under coupled tension, torsion and compressive loads

Developing an offshore wind farms, the submarine cables are required as power transmission media to connect offshore wind farms and onshore substations. Axial forces are one of the main causes of the damage to the submarine cables which consist of spiral armor, sheath, insulation and conductors. In addition to the axial elongation when the cable was subjected to tensile loads, the helical armor also produces torque and the coupled tensile and torsional mechanical properties of the submarine cable should be analyzed. In a deep water environment, the effect of water pressure on cable stiffness is important and seldom investigation was made in the past. This paper studied in detail the mechanical behavior of submarine cables under coupled tensile, torsional and compressive loads. When the cable is located 4000 m below sea level, the coupled tensile, torsional and compressive loads may reduce the tensile stiffness as a whole to 30%. The coupling effects of tensile, torsional and compressive loads, therefore, must be carefully considered in analysis and design of the submarine cables.

Effect of torsional pre-strain on low cycle fatigue performance of 304 stainless steel

Uniaxial tensile tests and low cycle fatigue tests were performed on 304 stainless steel treated with different torsional pre-strains. The relationship between torsional pre-strain and mechanical properties is established. Furthermore, the changes in the amount of martensite phase transitions are measured during low cycle fatigue tests and after specimen fracture are observed. This is mainly because martensitic transformation is a frequent phenomenon during plastic deformation, which is closely related to the mechanical properties. In this study, it is found that torsional pre-strain increases yield strength and reduces elongation. The ductility of 304 stainless steel decreases with the increase of torsional pre-strain, while its fatigue life does not. As the amplitude of torsional pre-strain increases, the fatigue life of the material changes in s-type. In particular, the fatigue life increases when the torsional pre-strain is between 5% and 8%. From the analysis of the microcosmic distribution of the martensite, it is found that the fatigue life is affected by the combined action of martensite transformation and ductility. The test results suggest that it should skip the relatively dangerous small strain area when improving mechanical properties of 304 stainless steel by way of changing shear pre-strain.

Effects of Hole Diameter on Torsional Mechanical Properties of the Rabbit Femur.

The aim of this study was to evaluate and compare the effect of three clinically applicable screw hole diameters on rabbit femoral torsional structural properties. Eighteen pairs of skeletally mature New Zealand White rabbit femora (36 bones). Femora with a bicortical hole at mid-diaphysis from one of the 3-drill bit sizes, 1.1 mm, 1.5 mm, 2.0 mm, and intact bones were studied. Each bone was bi-axially loaded in a servo-hydraulic load frame with the bone positioned so the neutral axis of torsion was aligned with the centre of the bone diaphysis. Axial compression to 35% body weight was applied to represent compression at stance, and rapid external torsion was applied to failure. Torque and angular deformation data were plotted for each test, with pre-yield and post-yield stiffnesses derived. Yield and failure torques and angles were determined, along with calculated yield, failure and post-yield energies. Failure torque was reduced compared with that of intact bone; weakened by 37% in 1.1-mm hole models, 53% in 1.5-mm hole models and 65% in 2.0-mm hole models. The torque angular deformation curves lacked plastic deformation. This study demonstrates the unique, brittle biomechanics of rabbit bone. Based on data from other species that strength loss of no more than 50% is acceptable when placing orthopaedic implants, no defect greater than 1.1 mm (15% bone diameter) is recommended in rabbit femora.

Effects of axial compression and rotation angle on torsional mechanical properties of bovine caudal discs.

The intervertebral disc is a complex joint that acts to support and transfer large multidirectional loads, including combinations of compression, tension, bending, and torsion. Direct comparison of disc torsion mechanics across studies has been difficult, due to differences in loading protocols. In particular, the lack of information on the combined effect of multiple parameters, including axial compressive preload and rotation angle, makes it difficult to discern whether disc torsion mechanics are sensitive to the variables used in the test protocol. Thus, the objective of this study was to evaluate compression-torsion mechanical behavior of healthy discs under a wide range of rotation angles. Bovine caudal discs were tested under a range of compressive preloads (150, 300, 600, and 900N) and rotation angles (± 1, 2, 3, 4, or 5°) applied at a rate of 0.5°/s. Torque-rotation data were used to characterize shape changes in the hysteresis loop and to calculate disc torsion mechanics. Torsional mechanical properties were described using multivariate regression models. The rate of change in torsional mechanical properties with compression depended on the maximum rotation angle applied, indicating a strong interaction between compressive stress and maximum rotation angle. The regression models reported here can be used to predict disc torsion mechanics under axial compression for a given disc geometry, compressive preload, and rotation angle.

The torsional mechanical properties of copper nanowires supported by carbon nanotubes

The torsional mechanical properties of hollow Cu nanowires supported by carbon nanotubes (CNTs) are studied by all atoms molecular dynamic simulation. It is found that the critical angles of Cu nanowires almost do not decrease when the temperature increases to a limit value, and this invariant feature also has been found as the torsional loading rate is lower than 4.5×1012°s−1. Due to the support of CNTs, Cu nanowires can bear larger torsional angle at low torsional rates and high temperatures compared with those without CNTs, which means the CNTs will increase the torsion-tolerance of Cu nanowires.

Nonlinear Torsional Mechanical Properties of ZnO Nanowire Based on Completely Nonlocal Theory

Nonlinear Torsional Mechanical Properties of ZnO Nanowire Based on Completely Nonlocal Theory

Self-deploying shape memory polymer scaffolds for grafting and stabilizing complex bone defects: A mouse femoral segmental defect study

Treatment of complex bone defects places a significant burden on the US health care system. Current strategies for treatment include grafting and stabilization using internal metal plates/screws, intramedullary rods, or external fixators. Here, we introduce the use of shape memory polymer (SMP) materials for grafting and adjunct stabilization of segmental defects. Self-deploying SMP grafts and SMP sleeves capable of expanding and contracting, respectively, under intraoperative conditions were developed and evaluated in a mouse segmental defect model in vivo. Integration between grafts/sleeves and native bone was assessed using x-ray radiography, microcomputed tomography, and torsional mechanical testing. We found that SMP grafts were able to integrate with the native bone after 12 weeks, maintain defect stability, and provide torsional mechanical properties comparable to an allograft alone treatment; however no gross de novo bone formation was observed. SMP sleeves did not inhibit bony bridging at the margins, and limbs treated with a sleeve/allograft combination had torsional mechanical properties comparable to limbs treated with an allograft alone. In vitro torsional and bending tests suggest sleeves may provide additional torsional stability to defects. Incorporation of shape memory into synthetic bone graft substitutes and adjunct stabilization devices is anticipated to enhance functionality of synthetic materials employed in both applications.

Torsional properties of hexagonal boron nitride nanotubes, carbon nanotubes and their hybrid structures: A molecular dynamics study

The torsional mechanical properties of hexagonal single-walled boron nitride nanotubes (SWBNNTs), single-walled carbon nanotubes (SWCNTs), and their hybrid structures (SWBN-CNTs) are investigated using molecular dynamics (MD) simulation. Two approaches - force approach and energy approach, are adopted to calculate the shear moduli of SWBNNTs and SWCNTs, the discrepancy between two approaches is analyzed. The results show that the shear moduli of single-walled nanotubes (SWNTs), including SWBNNTs and SWCNTs are dependent on the diameter, especially for armchair SWNTs. The armchair SWNTs show the better ability of resistance the twisting comparable to the zigzag SWNTs. The effects of diameter and length on the critical values of torque of SWNTs are obtained by comparing the torsional behaviors of SWNTs with different diameters and different lengths. It is observed that the MD results of the effect of diameter and length on the critical values of torque agrees well with the prediction of continuum shell model. The shear modulus of SWBN-CNT has a significant dependence on the percentages of SWCNT and the hybrid style has also an influence on shear modulus. The critical values of torque of SWBN-CNTs increase with the increase of the percentages of SWCNT. This phenomenon can be interpreted by the function relationship between the torque of different bonds (B-N-X, C-C-X, C-B-X, C-N-X) and the angles of bonds.

TBS Precision and Comparison between the Hologic and GE-Lunar DXA Scanners

Peripheral quantitative computed tomography (pQCT) is a popular tool for noninvasively estimating bone mechanical properties. Previous studies have demonstrated that pQCT provides precise estimates that are good predictors of actual bone mechanical properties at popular distal imaging sites (tibia and radius). The predictive ability and precision of pQCT at more proximal sites remain unknown. The aim of the present study was to explore the predictive ability and short-term precision of pQCT estimates of mechanical properties of the midshaft humerus, a site gaining popularity for exploring the skeletal benefits of exercise. Predictive ability was determined ex vivo by assessing the ability of pQCT-derived estimates of torsional mechanical properties in cadaver humeri (density-weighted polar moment of inertia [IP] and polar strength-strain index [SSIP]) to predict actual torsional properties. Short-term precision was assessed in vivo by performing 6 repeat pQCT scans at the level of the midshaft humerus in 30 young, healthy individuals (degrees of freedom = 150), with repeat scans performed by the same and different testers and on the same and different days to explore the influences of different testers and time between repeat scans on precision errors. IP and SSIP both independently predicted at least 90% of the variance in ex vivo midshaft humerus mechanical properties in cadaveric bones. Overall values for relative precision error (root mean squared coefficients of variation) for in vivo measures of IP and SSIP at the midshaft humerus were <1.5% and were not influenced by pQCT assessments being performed by different testers or on different days. These data indicate that pQCT provides very good prediction of midshaft humerus mechanical properties with good short-term precision, with measures being robust against the influences of different testers and time between repeat scans.

Torsional mechanical properties of (n, n)-(2n, 0) carbon nanotubes heterojunction

A coaxial cylindrical heterojunction of carbon tubes, which consists of alternant bands of 5- and 7-membered rings, can be formed by one armchair (n, n) carbon nanotube and one zigzag (2n, 0) carbon nanotube. The torsional mechanical properties of this kind of (n, n)-(2n, 0) heterojunction constructed by the same length of armchair and zigzag nanotubes are studied by using molecular dynamics method. In order to make a comparison, the relations of the torque and axial stress to torsional angle of (n, n) and (2n, 0) carbon tubes are also systemically calculated. Moreover, the transfer process of torsional stress in the (n, n)-(2n, 0) heterojunction is analyzed. Some important conclusions are obtained. Firstly, the torsional angle corresponding to the buckling point of carbon nanotubes is closely related to their torsional stiffness. The buckling angle decreases monotonically with torsional stiffness. Secondly, as the torsion develops, the torsional stress appears from the joint position due to the fact that the junction part in the (n, n)-(2n, 0) heterojunction has the smallest torsional stiffness and then transfers from the joint position to both ends. The propagation velocity of the torsional stress in (n, n) nanotube which has smaller stiffness is faster than that in (2n, 0) nanotube with bigger stiffness. Finally, for the process of torsion within the elastic limit, no axial stress is produced in (n, n)-(2n, 0) heterojunction during the torsion. This effect is of great significance for designing the carbon nanotube-based nano-oscillator devices.

Effect from surface treatment of nickel-titanium rotary files on the fracture resistance.

This study was aimed to compare the cyclic fatigue resistance and torsional resistance of rotary instruments with and without surface treatment. G6 A2 (Group A2) with and G6 A2 without surface treatment after machining (Group AN) were compared in this study. ProTaper F2 (Group F2) which has similar dimension and shape was also used for comparison. To evaluate the torsional resistance, ultimate torsional strength and distortion angle until fracture were recorded, and the toughness was calculated. The cyclic fatigue resistance was compared by evaluating the number of cycles to failure in a simulated canal. Statistical analysis was performed by one-way analysis of variance and Tukey post hoc test (p = 0.05). After torsional and cyclic fatigue tests, all fracture fragments were observed under a scanning electron microscope. Group A2 showed higher cyclic fatigue resistance than the groups AN and F2 (p < 0.05). Although group A2 demonstrated lower ultimate torsional strength than the others, there were no significant differences in toughness among the groups. While obvious machining grooves were seen in groups AN and F2, group A2 showed smooth surface resulting from the surface treatment. The specimens of fracture fragments showed typical features of cyclic failure such as micro-cracks, overloaded fast fracture zone, and torsional fracture such as unwinding helix, circular abrasion marks and dimples. Under the conditions of this study, the surface treated instruments may improve cyclic fatigue resistance while maintaining the torsional resistances and mechanical properties.