Correlation between the fractal dimension of fracture surfaces and fracture toughness for ductile polymer materials

A microcrack-shear band chain model for the fracture of ductile materials is proposed. The fractal dimension (D) of the fracture surfaces is derived and correlated with the fracture toughness (K_Ic) of ductile materials. The fractal dimension of the fracture surface is predicted to have an inverse trend with the fracture toughness. The theoretical results are consistent with the experimental results of some polymers and metals. © 1994 John Wiley & Sons, Inc.     

多层螺旋CT三维重建在脊椎骨折诊断中的应用价值

为分析多层螺旋电子计算机断层扫描(computed tomography,CT)三维重建在脊椎骨折诊断中的应用价值,本研究选取2017年1月~2018年10月期间于我院就诊的70例脊椎骨折患者,在征得患者知情同意下分别实施X线检查及多层螺旋CT三维重建,以手术结果为参照,对比两种诊断方式所得结果的差异。结果发现,X线检查骨折的部位诊断符合率、类型诊断符合率、合并椎管狭窄诊断符合率均低于多层螺旋CT三维重建,差异有统计学意义(P<0.05);两种诊断方式的敏感度、特异度、准确度相比较,多层螺旋CT三维重建均高于X线检查,差异有统计学意义(P<0.05)。据此可知,对脊椎骨折进行临床诊断时,多层螺旋CT三维重建能够准确揭示骨折部位及骨折类型,且敏感度、特异度、准确度水平均较高,值得在今后临床工作中推广使用。     

国产聚芳醚酮（PEK—C）的断裂韧性

由于材料内部缺陷和裂纹的存在,导致材料在使用过程中发生破坏的现象,已引起了广泛的注意,并发展了一门专门的学科——断裂力学。断裂力学不仅研究材料中裂纹扩展的规律,还通过应力及能量分析,建立和测定材料断裂的判据,如材料的断裂韧性,为结构设计提供必需的材料参数。目前断裂力学的研究对象已从金属和陶瓷扩充到聚合物和复合材料。     

In Situ tensile tests to analyze the mechanical response,crack initiation,and crack propagation in single polyamide 66 fibers

Single fiber mechanical testing is challenging to perform, especially when the diameter is as small as tens of micrometers. For this reason, real‐time observations of crack propagation mechanisms have been rarely been investigated experimentally. This article presents experimental and numerical investigations of fracture of monofilamentary high performance polyamide 66 fibers. Their engineering stress–strain curves are compared. The mechanisms of failure starting from crack initiation until the final brittle fracture are studied by in situ tests in Scanning Electron and optical microscopes. Finite element modeling at the individual fiber scale has been performed in three‐dimensional (3D), as a reverse engineering method. The compliance method was used to determine the crack depth that triggers the final failure. The fracture toughness was numerically determined using the J‐integral concept, accounting for the geometry of the crack front (3D) together with plastic deformation. 3D meshes were designed especially from postmortem observations. The average value deduced was about 47 ± 7 kJ m^?2, which will be discussed with other estimates using linear elastic fracture mechanics. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 680–690     

Temperature measurements of the gaseous emission during the fracture of polystyrene: A determination of the fracture energy and fracture surface temperature

The emission of volatile species accompanying the fracture of polystyrene has been monitored as a function of time by two quadrupole mass spectrometers mounted at different distances from the sample. The resulting time-of-arrival signals were numerically modeled to yield an estimate of the energy per unit area dissipated as heat by fracture. Neutral emissions of ethyl benzene molecules (a common volatile impurity) and a noble gas (Ar? introduced by exposure) were monitored. Dissipation (fracture) energies determined ranged from 680 to 1480 J/cm², consistent with the range of energy release rates in the literature. The peak surface temperatures calculated from the measured heat dissipation ranged from 600 to 1300 K. © 1994 John Wiley & Sons, Inc.     

Slow crack growth in blends of HDPE and model copolymers

The resistance to slow crack growth (SCG) was measured in binary blends of high density polyethylene (HDPE) and 5–10% concentrations of model ethylene-butene random copolymers by measuring the time to failure (t_f) under a constant stress intensity. An increase of t_f with the addition of the copolymer if the copolymer could crystallize and the increase was greater the higher branch density. The copolymer with 117 branches/1000C could not crystallize and therefore its blend had a t_f that was less than that of the HDPE. The fracture energies of the blends as determined by their resistance to SCG were compared with the energy by rapid fracture, J_c, as previously measured by Rhee and Crist. It is concluded that SCG is more sensitive to variations in the microstructure than is rapid fracture and that the differences in SCG behavior can be qualitatively explained in terms of the differences in microstructure of the blends. ©1995 John Wiley & Sons, Inc.     

Energy-consuming micromechanisms in the fracture of glassy polymers. I. Chain scission in polystyrene

The number of chain scissions per unit area that occur during the fracture of partially annealed latex films from M_n ? 180,000 g/mol polystyrene particles of about 275 Å radius were measured and correlated to annealing times. A curve with four regimes was found. At short annealing times the curve is nearly flat, in what is called the chain pull-out regime. In the second regime, the number of chains broken per unit area increases with a 0.8 power of annealing time as entanglement of the diffusing polymer chains increases in neighboring host particles. This is in good agreement with Wool's theory which predicts a 0.75 power dependence. Then, after reaching a peak, the number of scissions decreases in the third regime, indicating a change in fracture mechanism. The number of chain scissions increases again in the fourth regime, as final healing of the film interface takes place. Fracture surface analysis reveals a rough surface for short annealing times and a smooth surface for longer annealing times. The number of polymer chain scissions per unit area of fracture surface showed no dependence on initial molecular weights for t ? τ_r where t and τ_r are annealing and relaxation times, respectively. The number of chain bridges crossing a unit area of interface was suggested as the basic molecular property. © 1992 John Wiley & Sons, Inc.     

Properties of fracture surfaces of glassy polymers: Chain scission versus chain pullout

Fresh fracture surfaces formed by tensile failure of craze in molded polystyrene (PS) bars have been compared with the molded surfaces of the same bars, using an atomic force microscope with a thermal probe and operated in local thermal analysis. The results indicate that molecular weight is much higher in the interior of the sample than at the surface. No evidence was found for degradation of the PS chains via chain scission during crazing. Alternative explanations for the low‐molecular weights at the molded surface are discussed. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2010     

Thermomechanical properties of liquid‐crystalline epoxy networks arranged by a magnetic field

During the curing process of a liquid‐crystalline epoxy resin, a relatively strong magnetic field was applied, and the thermomechanical properties of the cured resin were investigated. The network orientation and mechanical properties of the cured system were evaluated with wide‐angle X‐ray diffraction, dynamic mechanical analysis, and fracture toughness testing. The cured system was found to have an anisotropic network structure, which arranged along the applied field, and the anisotropy was reflected in the thermomechanical properties. In particular, the fracture toughness of the system dramatically increased when the network chains were arranged across the direction of the crack propagation. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 758–765, 2004     

Fracture mechanism of liquid‐crystalline epoxy resin systems with different phase structures

A liquid‐crystalline epoxy resin was cured at two different temperatures. The phases of the cured systems clearly showed isotropic and nematic polydomain structures, which depended on the curing temperature. The fracture toughness of the systems was measured, and the fracture mechanism was investigated with polarized IR measurements. The nematic polydomain structure system showed considerably higher fracture toughness than the isotropic structure. Moreover, both systems exhibited a reorientation of the network chains near the fracture surface during the fracture process, and the region of the network reorientation in the nematic polydomain structure system was larger than that in the isotropic structure system. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 4044–4052, 2004     

Influence of the loading rate on liquid‐crystalline epoxy resin systems

The fracture toughness of liquid‐crystalline epoxy systems, which had a nematic polydomain structure (domain size about 40 μm), with an increasing loading rate was evaluated. In this system, the fracture toughness dramatically decreased from 1.96 to 0.22 MN/m^3/2 with an increasing loading rate (0.1–5 mm/min). The network orientation near the fracture surface of different loading rate systems was investigated with polarized optical microscopy and polarized infrared spectroscopy. As a result, a large oriented region of mesogenic groups was observed near the fracture surface in the relatively low loading rate (0.1 and 0.5 mm/min) systems, but such a phenomenon was not observed in the high loading rate (2 and 5 mm/min) systems. These results showed that the high fracture toughness of the system at the low loading rate was due to the magnitude and region of the reorientation of the mesogenic groups in the fracture process and that high toughness could not be achieved at a high loading rate because the loading rate was too fast to allow orientation of the networks containing the mesogenic groups. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1296–1302, 2005     

Chain scission in polystyrene by impact fracture

The number of chain scissions n_s per unit fracture area by impact in high-molecular weight polystyrene is determined to be approximately 3.3 × 10¹⁴/cm² at room temperature. This is almost 20 times larger than would be expected if chain scissions took place only at, or very close to, fracture surfaces. This result was obtained by measuring the molecular weight decrease and the total fracture area of the impact fragments by using size exclusion chromatography and statistical particle size measurements, respectively. The large n_s strongly indicates that significant chain breakage occurs during crazing before the propagation of cracks. An average craze thickness before breakdown under impact is estimated from n_s to be around 2 μm. In a diluted polymer, n_s is found to be significantly lower than the extrapolated value, assuming a linear dilution of entangled chain crossings at the fracture surface. This low chain scission density, however, can be explained by taking into account the reduction of craze breakdown strain in the diluted polymers. Finally, the broken chain ends of polystyrene appear to be stable under ambient conditions. © 1992 John Wiley & Sons, Inc.     

X线平片和CT扫描在外伤性脊椎骨折部位及解剖学改变诊断中的应用

分析X线平片和CT扫描对外伤性脊椎骨折部位的检出情况、骨折部位解剖学改变的显示情况,并对治疗效果进行评价。本研究选取2018年1月~2019年6月于我院进行手术治疗的外伤性脊椎骨折患者90例,所有患者均行X线平片和CT扫描检查,以手术结果为参考,观察X线平片和CT扫描对外伤性脊椎骨折部位(前柱、中柱、后柱)的检出情况、骨折部位解剖学改变(小关节滑脱、骨碎片、椎体移位、椎管狭窄)的显示情况,以及对术后椎弓根未完全到达椎体的检出情况。结果显示,CT扫描对中柱骨折、后柱骨折的诊断符合率均明显高于X线平片(P<0.05);对前柱骨折的诊断符合率与X线平片比较无差异(P>0.05)。CT扫描对小关节滑脱、骨碎片、椎体移位、椎管狭窄的诊断符合率均明显高于X线平片(P<0.05);对术后椎弓根未完全到达椎体检出率为88.89%,高于X线平片的77.78%,但差异无统计学意义(P>0.05)。CT扫描对外伤性脊椎骨折部位检出率及骨折部位解剖学改变的显示均优于X线平片,有利于临床治疗方案的制定,可作为外伤性脊椎骨折的首选诊疗检查方法。     

An experimental investigation of fracture by cavitation of model elastomeric networks

A new methodology to investigate the failure of elastomers in a confined geometry has been developed and applied to model end-linked polyurethane elastomers. The experimental in situ observations show that the elastomers fail by the growth of a single cavity nucleated in the region of maximum hydrostatic stress. Tests carried out at different temperatures for the same elastomer show that the critical stress at which this crack grows is not proportional to the Young's modulus E but depends mainly on the ratio between the mode I fracture energy G_IC and E. A reasonable fit of the data can be obtained with a model of cavity expansion by irreversible fracture calculating the energy release rate by finite elements with a strain hardening constitutive equation. Comparison between different elastomers shows that the material containing both entanglements and crosslinks is both tougher in mode I and more resistant to cavitation relative to its elastic modulus. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48:1409–1422, 2010     

Localized Yielding Around Crack Tips of Double‐Network Gels

Double‐network (DN) gels, a type of interpenetrating polymer network (IPN) consisting of rigid and flexible polymer components, exhibit two outstanding mechanical behaviors: yielding deformation of the entire specimen in tensile tests and quite high fracture energy in tearing tests. In this study, atomic force microscope (AFM) measurements were conducted on DN gels to determine the local Young's moduli immediately below the fracture surfaces E_f and below the usual molded surfaces E_m, and compare the local modulus with bulk Young's moduli measured before and after the yielding deformation, denoted as E_h and E_s, respectively. E_m and E_h are around 0.1 MPa; E_f and E_s, around 0.01 MPa, one order lower than the former two moduli. The order relation indicates that yielding deformation occurred locally around the crack tip of the DN gel during fracture. This supports the basic assumption of phenomenological models recently proposed to explain high fracture energy of DN gels. (H. R. Brown, Macromolecules 2007 , 40, 3815–3818; Y. Tanaka, Europhys. Lett. 2007 , 78, 56005).

     

经皮中空螺钉内固定与张力带内固定内踝骨折的临床疗效观察

研究探讨经皮置入中空螺钉内固定治疗闭合性内踝骨折(Ashurst和Bromer分类的内翻内收型骨折Ⅰ度)的临床实用性,收集60例闭合性内踝骨折的患者,随机分为两组各30例。A组予传统切开复位张力带内固定术治疗,B组予闭合复位经皮置入中空螺钉内固定治疗。术后采用Takakura踝关节评分系统来评价疗效,并对结果进行分析。     

The dependence of slow crack growth in a linear polyethylene on test temperature and morphology

The slow crack growth behavior of a linear polyethylene with different morphologies was studied by using three point bending with a single edge notched specimen at testing tem-peratures from 30 to 80°C. The morphology was varied by annealing the quenched material at temperatures from 86°C to 135°C. It was found that at test temperatures of 60°C or less, the changes in failure time with annealing temperature are very similar to the change in density with a maximum at 130°C. At testing temperatures above 60°C, the relationship of between failure time and annealing temperature is altered when the test is in the range of the α transition temperature. These results indicate that with respect to slow crack growth in the case of a homopolymer the strength of the crystals is relatively more important than the number of tie molecules. © 1993 John Wiley & Sons, Inc.     

Differential transport and dispersion of colloids relative to solutes in single fractures

This work employed numerical experiments simulating colloid and solute transport in single parallel-plate fractures, using the random walk particle tracking method, to demonstrate that (1) there exists an aspect ratio of the colloid radius to half the fracture aperture, δ_o, where the average velocities of colloids and solutes are similar. When δ > δ_o, the velocity distribution assumption is satisfied, and the fact that the ratio of the colloid transport velocity to the solute transport velocity, τ_p, decreases as δ increases is well documented in the literature. However, when δ < δ_o, the velocity distribution assumption is violated, and τ_p increases as δ increases and (2) the Taylor dispersion coefficient and its extension by James and Chrysikopoulos [S.C. James, C. V. Chrysikopoulos, J. Colloid Interface Sci. 263 (2003) 288] will overestimate the colloid dispersion coefficient significantly. Additionally, numerical experiments simulating colloid and solute transport in variable-aperture fractures demonstrated that τ_p and D_L,coll/D_L,solute decrease with increasing CoV, and the anisotropy ratio only plays a minor role compared to the CoV. These observations have important implications towards the interpretation of colloid transport in both porous and fractured media.     

Characterizing fracture energy of proton exchange membranes using a knife slit test

Pinhole formation in proton exchange membranes (PEM) may be caused by a process of flaw formation and crack propagation within membranes exposed to cyclic hygrothermal loading. Fracture mechanics can be used to characterize the propagation process, which is thought to occur in a slow, time‐dependent manner under cyclic loading conditions, and believed to be associated with limited plasticity. The intrinsic fracture energy has been used to characterize the fracture resistance of polymeric material with limited viscoelastic and plastic dissipation, and has been found to be associated with long‐term durability of polymeric materials. Insight into this limiting value of fracture energy may be useful in characterizing the durability of proton exchange membranes, including the formation of pinhole defects. In an effort to collect fracture data with limited plasticity, a knife slit test was adapted to measure fracture energies of PEMs, resulting in fracture energies that were two orders of magnitude smaller than those obtained with other fracture test methods. The presence of a sharp knife blade reduces crack tip plasticity, providing fracture energies that may be more representative of the intrinsic fracture energies of the thin membranes. Three commercial PEMs were tested to evaluate their fracture energies (G_c) at temperatures ranging from 40 to 90 °C and humidity levels varying from dry to 90% relative humidity (RH). Experiments were also conducted with membrane specimens immersed in water at various temperatures. The time temperature moisture superposition principle was applied to generate fracture energy master curves plotted as a function of reduced cutting rate based on the humidity and temperature conditions of the tests. The shift with respect to temperature and humidity suggests that the slitting process is viscoelastic in nature. Also such shifts were found to be consistent with those obtained from constitutive tests such as stress relaxation. The fracture energy is more sensitive to temperature than on humidity. The master curves converge at the lowest reduced cutting rates, suggesting similar intrinsic fracture energies; but diverge at higher reduced cutting rates to significantly different fracture energies. Although the relationship between G_c and ultimate mechanical durability has not been established, the test method may hold promise for investigating and comparing membrane resistance to failure in fuel cell environments. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 333–343, 2010     

Deformation and fracture behavior of high‐energy electron irradiated poly(vinylidene fluoride‐trifluorethylene) ferroelectric copolymer films under uniaxial tension

The deformation and fracture behavior under uniaxial tension were characterized for high‐energy irradiated poly(vinylidene fluoride‐trifluorethylene) (P(VDF‐TrFE)) 68/32 mol % copolymer films. The results show that the stress–strain behavior of the irradiated copolymer films exhibits ductile polymeric behavior, with its fracture strain being more than five times of that of the nonirradiated ones but of much lower maximum strength. X‐ray diffraction (XRD) analysis and scanning electron microscope (SEM) observation are carried out to examine the microstructure and morphology changes caused by the uniaxial tension. It is demonstrated that the tensile mechanical field reintroduces the polar β‐phase that was previously lost through irradiation. It is suggested that the conformational change from the nonpolar phase to the polar β‐phase during the uniaxial tension, as well as the low crystallinity and loosely packed molecular chain structure, mainly contribute to the observed stress–strain behavior for the irradiated copolymer films. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2563–2567, 2007