Environmental stress cracking of low-density polyethylene in normal alcohols

Fracture mechanics was used to investigate the environmental stress cracking of low-density polyethylene with 4.0 melt flow index. Annealed samples were prepared; a single edge notch was made and then the samples were fractured under constant load in four different liquid alcohols. The relation between the stress intensity factor K and the crack speed a˙ has been investigated. The log a˙ vs. logK curves are influenced not only by temperature but also by the alcohol used as the environment. This influence has been studied in detail. The conclusions are as follows: the crack speed at high K is determined by the diffusion mechanism, and this mechanism cannot be explained in terms of hydrodynamics but can be explained in terms of thermally activated molecular motion. On the other hand, the crack speed at low K is strongly related to the plasticization and the stress relaxation of the crack tip material.     

Electron-beam-induced fracture of polymers

When a notched polymeric material is stressed, the notch opens into a wide crack tip, exposing a region of high stress concentration. The consequences of electron bombardment of the tip of such a stressed material under vacuum are explored here for the first time. Evidence is presented for electron-induced crack growth at stress far below that needed for crack growth due to stress alone. The electron current densities used in these experiments are sufficiently small that thermal heating of the zone near the crack tip is minimal. To provide information on the phenomena involved, we present simultaneous measurements of electron current, gas pressure, and sample load in response to periodic bombardment of the sample. Experiments involving the bombardment of un-notched polymers under stress are also described. Fractography of the unique structures obtained by fracture due to the combination of electron bombardment and stress are presented and interpreted in terms of a crosslinking mechanism.     

A finite element method based comparative fracture assessment of carbon black and silica filled elastomers: Reinforcing efficacy of carbonaceous fillers in flexible composites

This study is focused on numerical investigation on fracture behaviors of carbon black (CB) and silica filled elastomeric composites. Finite element analysis (FEA) in compliance with multi-specimen method is used to calculate J-integral and geometry factor of the rubber composites up to a displacement of 20 mm for single edge notch in tension (SENT) and double edge notch in tension (DENT) specimens. An empirical relationship between crack tip opening displacement (CTOD) and crack advancement is established depending on notch to width ratio (NWR). The stress contours across the notches for SENT and DENT specimens is discussed briefly. It is found that fracture propagation resistance of CB filled elastomer is 125% more than that of silica filled elastomer. Although, Silica filled elastomer have good tensile strength and crosslink density but it fails to replace carbon black in terms of fracture properties. The critical J-integral for CB filled elastomer is 18.7% and 32.2% more than silica filled elastomer for SENT and DENT specimens respectively. The effect of specimen type on various fracture properties is also explored. The factor of safety is found to be significantly more in case of CB filled elastomers making them less vulnerable to crack propagation and catastrophic failure.     

Direct fracture toughness determination of a ductile epoxy polymer from digital image correlation measurements on a single edge notched bending sample

This work presents a combined experimental and numerical study on the fracture toughness behaviour of a ductile epoxy resin system. Quasi-static fracture tests using single edge notched bending (SENB) specimens were conducted under room temperature conditions. In addition, the digital image correlation technique was employed to experimentally map the full-field displacements and strains around the notch and crack tip, allowing direct calculation of the J-integral fracture toughness. The magnitude of fracture toughness was found to be 1.52 ± 0.03 kJ/m², showing good consistency with the results measured according to the standard analytical formulations. A numerical model of the single edge notch bending specimen was built to compute the local strain field around the crack tip, together with the fracture toughness parameter. Good agreement was confirmed for both the experimental J-integral fracture toughness and the local surface strains around the crack-tip from the digital image correlation based optical technique, compared to the results obtained by numerical simulation. The fracture surfaces of the samples were examined using an optical microscope to analyze the failed surface morphology and the corresponding failure mechanisms.     

The molecular structure of cis- and trans-bicyclo [4.2.0]-octane in the gas phase as studied by electron diffraction and molecular mechanics

The molecular structure of Cis- and trans-bicyclo[4.2.0]octane in the gas phase was studied. Molecular mechanics calculations applying Boyd's force Held were used for constraining differences between structural parameters during least squares analysis and for calculating vibrational amplitudes. The cyclohexane ring was found to have a distorted chair conformation, the ring in the cis isomer being flattened along the junction and more twisted in the other part. For the trans compound the reverse is true. The following structural parameters were obtained (r_a-structure):cis: r(C-C)_av. = 1.535 Å. Cyclohexane ring: average bond angle 112.9°; average torsional angle 48°. Cyclobutane ring: average bond angle 88.9°; puckering 157°. The dihedral angle between the bisecting planes of the C(2)-C(1)-C(6)-C(5) and C(8)-C(1)-C(6)-C(7) torsional angles, is 119° - the “connection angle” of the two rings.trans: r(C-C)_av.= 1.532 Å. Cyclohexane ring: average bond angle 110.4° ; average torsional angle 57°. Cyclobutane ring: average bond angle 87.3°; puckering 145°. The “connection angle” is 180° (C₂ symmetry).Comparison is made with structures of related compounds.     

Dynamic fracture testing of polymethyl-methacrylate (PMMA) single-edge notched beam

Dynamic fracture in single-edge notched polymethyl-methacrylate (PMMA) beams have been investigated by three-point-bending impact testing with a drop-weight machine. A high-speed camera combined with the digital image correlation (DIC) method is used to capture the impact-induced crack initiation and propagation, as well as the beam deformation fields and the open mode strain at the original notch tip. The crack propagation length is recorded and the instantaneous crack velocity is calculated. Furthermore, the dynamic fracture toughness K_Id is quantified from the loading-displacement relations at different impact velocities. The effects of the impact velocity and impact energy on dynamic fracture toughness, fracture initiation strain, as well as the corresponding influences on the fracture propagation velocity, are discussed.     

Temperature effects in the environmental stress cracking of polyethylene

An already existing model describing three types of behavior in the environmental stress cracking (ESC) of polyethylene (PE) subjected to uniaxial tensile loading at 25°C has been extended to cover the temperature range 25-50°C. Three effects have been observed and are found to be qualitatively consistent with the model. The transition between “pure” ESC (zone 1) and liquid-flow-controlled behavior (zone 2) occurs at shorter times for higher temperatures—an effect mainly due to reduced liquid viscosity. “Pure” ESC is less temperature dependent than failure occurring without the liquid playing a significant role (zone 3). This has been attributed to a compensation of reduced mechanical resistance of the polymer at higher temperatures, by reduced stress concentrations at the crack tip due to local absorption of the liquid. The transition time between zones 2 and 3 increases with temperature. Although it is surprising at first sight, this effect is shown to be compatible with the ESC model.     

Effect of crack tip sharpness on the strength of vulcanized rubbers

The effect of crack tip sharpness on crack propagation in vulcanized rubbers has been studied. For very sharp cracks, tearing is found to occur on a small scale at very low energies not far above the threshold required for the onset of mechanical crack growth. The “small-scale” tearing energies show relatively little variation for rubbers that differ widely in tear strength as normally measured. Thus the latter property appears to be strongly influenced by variations in the ability of rubbers to promote tip blunting. The small-scale tear behavior is of relevance to other fracture phenomena, including cutting by sharp objects and tensile failure. Natural variations in tip sharpness occur during cyclic or time-dependent mechanical crack growth and influence the form of the crack growth characteristics.     

Essential work of fracture,crack tip opening displacement,and J-integral relationship for a ductile polymer film

A unique set of double-edge notched tension specimens of a Polyethylene Terephthalate Glycol-modified film was tested in mode I, plane stress. The load was registered on a universal testing machine. The displacements, ligament lengths, and video frames were recorded by a Digital Image Correlation system. With these registered data, the essential work of fracture, J-integral, and crack tip opening displacement (CTOD) fracture concepts have been applied. The onset of crack initiation was through a complete yielded ligament. The analysis showed that the intrinsic specific work of fracture, w_e, is the specific energy just up to crack initiation, which is an initiation value. w_e has both a coincident value and the same conceptual meaning as J_o, the J-integral at the onset of crack initiation. The relationship between J_o and CTOD is also determined. The influence on the notch quality when the specimens were sharpened by two different procedures, femtosecond laser ablation and razor blade sliding, was analysed in detail.     

Photoelastic analysis of matrix crack-tilted fiber bundle interaction

Matrix crack-tilted fiber bundle interaction was explored using photoelasticity. First, the isochromatic fringe patterns near the matrix crack tip, either shielded by a tilted fiber bundle or crossed by a broken fiber bundle, were observed. Then, the stress intensity factors of cracks at varying distances from the tilted fiber bundle were extracted from the isochromatic fringe patterns. Finally, finite element simulation was conducted in ABAQUS software to verify the experimental results, and the difference between photoelasticity measurement and FEM simulation were discussed. The results show that the mode I stress intensity factor of the crack near a tilted fiber bundle increases with the increase of crack length and decreases with the increase of the Young's modulus of the fiber bundle. However, the mode II stress intensity factor, which clearly increases as crack length increased and, as opposed to mode I, increases as the Young's modulus of the fiber bundle increased.     

Crack kinking from an initially closed interface crack

One studies crack kinking from an interface crack when this crack is closed prior to kinking but open after it. Contact is assumed to occur without friction. This problem was considered very recently by the authors in the special case of some homogeneous material (ordinary crack). One establishes the relation between the stress intensity factors just before and just after the kink. From there, and using Goldstein and Salganik's famous principle of local symmetry, one derives the value of the kink angle. This value is somewhat surprisingly found to depend only very weakly on the mismatch of elastic properties between the materials.     

Crack-craze opening profiles near a crack tip in a polytetrafluoroethylene

The crack opening and craze profiles near a crack tip in a polytetrafluoroethylene have been experimentally investigated. A double-edge-crack plate specimen under uniaxial tensile load was used in the experiment and the experimental procedure was performed using the Digital Image Correlation method, which is a well-established optical-numerical method for estimating full-field displacement. A theoretical model of the stress intensity factor based on linear elastic fracture mechanics combined with a classical saturated expression was proposed. The proposed model is in good agreement with experimental data and predictions of the model may be used to verify the non-linear behavior from crack and craze (cohesive) zones.     

Environmental stress cracking of polyethylene: Analysis of the three zones of behavior and determination of crack-front dimensions

Previous work has shown that in the environmental stress cracking (ESC) of polyethylene, there are three major zones of behavior depending on the applied stress and the nature of the liquid environment. These three zones correspond to “pure” ESC (zone 1), ESC controlled by the speed of penetration of the liquid within a growing crack (zone 2), and behavior as in the absence of liquid (zone 3). Analysis of the transitions between zones has shown that, in the present case, a given liquid will either be capable of giving rise to all three types of behavior depending on the stress applied, or will be totally inactive. A related analysis has enabled the order of magnitude of the dimensions of the crack tip to be estimated and this has been found to be in the range of a micron.     

尼龙-6/热塑性聚酰胺系弹性体合金的力学性能和破坏机理

尼龙 ６与在尼龙 ６中分别添加了两种热塑性聚酰胺系弹性体的尼龙 ６合金相比，前者的耐冲击强度低，后者的耐冲击强度高；通过偏光显微镜和扫描电镜的观察，尼龙 ６在Ｕ型缺口前部区域内的塑性约束下，呈脆性破坏；尼龙 ６的合金因塑性区域内的空洞化和空洞的变形及空洞之间区域的颈缩和纤维化，有效的吸收了冲击能量，使塑性约束得到缓解，促进了平面应变状态向平面应力状态转变，最终呈半韧性或韧性破坏．     

Notched Ring Test for Measuring Slow Cracking Resistance in Plastics Pipes and Fittings

It is known, that the lifetime of polyethylene pipes is essentially limited by slow crack growth (SCG). For state of the art PE materials common SCG testing methods have reached their limits with respect to extension of testing times. A comparatively new method is the Notched Ring Test (NRT) as developed by Choi et al.^[1] Pipe rings notched at the inner wall are used. The test is carried out in 80 °C water under constant bending load. The arrangement of the notch at the inner wall reduces testing times using the residual stress of extruded pipes. A disadvantage of this method is that there is no clearly defined failure time because SCG takes place between two phases of creeping. The output of this test is an “on-set slow cracking time” (crack initiation), obtained by analysis of the displacement curve. In this work it has been shown that the NRT method yields to brittle fracture within acceptable time frames.^[2] Methods for data analysis are presented. This test could be very useful applied in research and development for resin evaluation and as a tool in quality control in pipe production for evaluating the process conditions.     

Customization of the CEFM for predicting stress corrosion cracking in lightly sensitized Al–Mg alloys in marine applications

The Coupled Environment Fracture Model (CEFM) has been modified and calibrated to predict crack growth rate (CGR) in aluminum marine alloys. The customized CEFM provided quantitative predictions of the effects of O₂, electrochemical potential, stress intensity factor, and conductivity on CGR in lightly sensitized AA5083-H321 in 3.5 wt.% NaCl solution, as well as explaining the development of a semi-elliptical surface cracks. The importance of the properties of the external environment, such as conductivity, oxidant/reductant concentration, and the kinetics of the cathodic reactions on the surfaces external to the crack has been confirmed. Crack growth is attributed to a sequence of microfracture events at the crack front, the frequency of which is determined by the mechanical conditions that exist at the crack tip, as governed by the stress intensity while the microfracture dimension is determined by hydrogen-induced fracture, with the CGR being the product of these two quantities. The success in explaining the intergranular stress corrosion cracking (IGSCC) of aluminum alloys, argues that the basic concept of the CEFM, that the internal and external environments are strongly coupled, is sound and that the CEFM, which was originally developed to describe IGSCC in sensitized stainless steels is equally applicable for describing IGSCC in lightly sensitized aluminum alloys.     

On the effect of the different notching techniques on the fracture toughness of PETG

The fracture toughness of poly (ethylene terephthalate) modified glycol (PETG) has been evaluated using notch sharpening techniques which could be grouped into contact and non-contact procedures. Razor blade tapping, razor blade sliding, razor pressing and broaching are part of the first group, while the femtolaser technique belongs to the second one. Not all the contact procedures generated valid sharp cracks for fracture parameter assessment; indeed none of the samples sharpened via razor sliding generated acceptable sharp cracks. The results revealed that the non-contact femtolaser technique produced the sharpest cracks in this polymer, with crack tip radii of only 0.5 μm, leading to the lowest fracture toughness values. On the contrary, the traditional notch sharpening technique based on razor tapping, recommended in ISO, ESIS and ASTM protocols and standards, generated larger crack tip radii than those introduced via femtolaser and, consequently, resulted in higher fracture toughness values. Both broaching and pressing methods created specimens with smaller crack tip radii compared to those obtained by razor tapping, and hence resulting in intermediate fracture toughness values.     

EPR study on tomatoes before and after gamma-irradiation

The results from the EPR studies on fresh, air-dried and lyophilized tomato samples before and after gamma-irradiation are reported. Before irradiation fresh and air-dried tomatoes exhibit one singlet EPR line characterized with common g-factor of 2.0048±0.0005, whereas freeze-dried tomato does not show any EPR spectrum. After irradiation, a typical “cellulose-like” triplet EPR spectrum appears in all samples, attributed to cellulose free radicals, generated by gamma-irradiation. It consists of intense central line with g=2.0048±0.0005 and two weak satellite lines separated ca. 3 mT left and right of it. In air-dried and lyophilized tomatoes the “cellulose-like” EPR spectrum is superimposed by an additional partly resolved carbohydrate spectrum. Fading measurements of the radiation-induced EPR signals indicate that the intensity of the EPR spectra of air-dried and freeze-dried tomato are reduced to about 50% after 50 days, whereas those of fresh irradiated tomatoes kept at 4 °C fade completely in 15 days. The reported results unambiguously show that the presence of two satellite lines in the EPR “cellulose-like” spectra of tomato samples can be used for identification of radiation processing.     

Effects of humidity on environmental stress cracking behavior in poly(methyl methacrylate)

Environmental stress cracking (ESC) in poly(methyl methacrylate) under different humidity conditions has been investigated. Constant stress‐intensity factor (K) ring‐type specimens were prepared, and all specimens were equilibrated at five different humidity conditions for about two years. ESC tests were carried out under the same humidity as specimens had been stored. Acoustic emission (AE) signals during ESC tests were also measured to examine the crack‐growth behavior. The threshold K value (K_th) tended to increase with increasing humidity. At a relative humidity (RH) of 11%, crack growth occurred gradually until 40 ks under a K value of 0.70 MPam^1/2, and then the crack‐growth rate began to increase and AE events were observed. A laser microscopic observation indicated that the crack extended by the coalescence between a main crack and a microcrack ahead of the main crack tip. AE signals generated are considered to be associated with the coalescence. At 98% RH, an incubation period where no crack growth was observed existed under a K value of 0.94 MPam^1/2, but the crack began to grow suddenly after that incubation period. This suggests that the craze at the crack tip may become weaker with increasing loading time under high humidity. Although the crack‐growth rate at 98% RH was higher than that at 11% RH, no AE events were observed. This suggests that the crack extended stably in the craze at a crack tip, and sorbed water may make the craze growth easy. All the results suggest that two different ESC mechanisms are activated depending on sorbed water that are varied by humidity. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 40: 1–9, 2002     

Mechanism of fracture in glassy polymers. III. Direct observation of the craze ahead of the propagating crack in poly(methyl methacrylate) and polystyrene

Observation of optical interference fringes at the tip of a crack in a glassy polymer allows the construction of the configurations of the crack tip and the craze that precedes it. The craze extends 25 μ beyond the crack tip in poly(methyl methacrylate) and 550 μ beyond the tip in the polystyrene studied. The craze at the crack tip in PMMA may be seen to deform elastically as much as 100% under stress before crack propagation recommences. Such deformation is estimated to account for as much as 40% of the nominal Griffith energy of crack propagation.