Mechanical fracture behavior of polyacetal and thermoplastic polyurethane elastomer toughened polyacetal

Polyacetal (POM) toughening with thermoplastic polyurethane (TPU) elastomer was investigated in terms of Theological, mechanical, and morphological properties. Polyacetal can be effectively toughened by the blending with TPU elastomer and the improvement on toughness is found most significant with TPU content from 20 to 30 percent. POM does fracture in ductile mode under extremely low deformation rate and the ductile-brittle transition rate is at 0.5 mm/min. The transition rate is increased with the increase of elastomer content. The precrack hysteresis energy is important in dictating the failure mode. The experimental results show the hysteresis energy (under constant load) increases with the increase of elastomer content and the decrease of deformation rate. Greater hysteresis energy results in larger precrack plastic zone size and thus tends to shift the fracture mode from brittle to ductile as the critical size of the plastic zone is reached. The adoption of the slow rate fracture method has the advantages of ranking toughness of very brittle polymeric materials vs. the conventional Izod or Charpy impact method by varying temperatures. FTIR shows significant interaction between POM and TPU which is probably responsible for the TPU elastomer being such an efficient toughening agent for POM. Delamination in the buffer zone between the plane-strain and the plane-stress is discovered and the possible mechanism is discussed.     

Craze dissemination during fatigue fracture in polystyrene

Results of studies on fatigue fracture in polystyrene are reported. Experiments were carried out on tension-tension single edge notched specimens, 0.25 mm thick. Macroscopic studies involved the propagation of the crack and its associated active zone (or the so-called process zone) evolution. Microscopic studies consisted of fracture surface examination and quantitative characterization of the crazing distribution along the trailing edge and within the active zone. The width and length of the active zone increased monotonically during the quasi-static phase of crack layer growth. The crack growth kinetics followed an S-shaped curve. Analysis of crazing distribution showed that; (i) the distribution of crazing along the trailing edge of the active zone is related by a scaling parameter, (ii) the average crazing density along the trailing edge as well as within the active zone is constant, (iii) the specific energy of crazing evaluated here compares well with previously reported data. The results of this work support the form of self similarity of damage evolution adopted by the crack layer model, and that the specific energy of damage is a material constant.     

Morphological evolution and property changes of acrylate rubber/polyacetal thermoplastic vulcanizates during dynamic vulcanization process

A new type of thermoplastic vulcanizates (TPV) that is resistant to hot oil, high temperature, and aging was prepared using acrylate elastomer (ACM) and polyoxymethylene (POM) as raw materials. The phase transition process and micromorphology development of the blends with the change of dynamic vulcanization (DV) time, and to illustrate the effects of the phase transition and the degree of elastomer phase crosslinking on the macroscopic properties of TPV were investigated. It was shown that ACM crosslinking occurs continuously during 0–8 min of DV, whereas the phase inversion process occurs predominantly during 0–3 min. In contrast, the cross-section of the blends after phase inversion is flat, The constant elongation stress under small deformation, hardness, and permanent deformation after break are reduced, and the elongation at break, nonrelaxable modulus, and aging resistance are improved. The increase in the degree of vulcanization of the ACM phase resulted in a significant increase in the modulus of the blends under large deformation, an increase in the nonrelaxable modulus and relaxable modulus, an increase in the aging resistance, and a decrease in the elongation at tear were observed.     

The effect of molecular weight on fatigue crack propagation in nylon 66 and polyacetal

The influence of molecular weight on fatigue and fracture behavior in nylon 66 (N66) and polyacetal (PA) is examined. Fatigue crack propagation (FCP) resistance and apparent fracture toughness (K_cf) in these two semicrystalline polymers increase with increasing molecular weight in a manner consistent with that reported for another semicrystalline polymer (HDPE) as well as for several amorphous polymers. The improved FCP resistance with increasing molecular weight is attributed to the development of a molecular entanglement network that more effectively resists cyclic-load-induced breakdown. A type of discontinous crack growth is identified in PA at 100Hz and in N66 (2.6% H₂O) at 50 Hz and compared with that observed in amorphous polymers.     

Structural changes in polyester fibers during fatigue

Structural changes occurring during the fatigue failure of polyester fibers have been identified, and a comparison has been made with untested fibers and fibers which were subjected to cyclic loading conditions which did not produce fatigue. Fatigue failure was seen to result in a distinctive fracture morphology. Infrared spectrometry and X-ray diffraction revealed a lowering of crystallinity under fatigue conditions but not under other loading conditions. Transmission electron microscopy and electron diffraction revealed the creation of amorphous zones which are supposed as coalescing to form an amorphous band seen along and ahead of the fatigue crack. The zone just ahead of the fatigue crack tip is shown to contain voids. Crack propagation involves, therefore, the joining up of these voids and development along the amorphous band.     

Micromechanisms of interlaminar fracture and fatigue

The interlaminar fracture and fatigue properties of AS/3501-;6 graphite/epoxy are discussed from a mechanistic point of view. Particular emphasis is placed on the interaction between the loading mode and the local geometry of the interlaminar zone and on how this affects the stresses close to the crack tip and the resulting failure path. Delamination growth under Mode I loading is shown to depend on the likelihood of fiber bridging occurring and on how effective these bridged fibers are at diverting strain energy away from the crack tip. The Mode II behavior is controlled by both the work required to shear the fibers from the matrix and the ease with which tensile failure of the matrix between the fibers can occur.     

Proton-NMR studies of polyacetal copolymers

Sequence length of ethylene oxide and the amount of end groups in polyacetal copolymers were determined using proton NMR. Peaks from ethylene oxide usually appear in the region from 3.5 to 3.8 ppm, when tetramethylsilane was used as the internal standard. These peaks were divided into three triad sequences, i.e., MEM, MEE, EEE (E = ethylene oxide, M = methylene oxide) by referring to several polymers of similar structure. The sequence length in a copolymer prepared from ethylene oxide was 1.37 and that from trioxocan was 2.00. Small peaks from end groups usually appear in the region higher than 3.5 ppm. The peaks from methoxy and ethoxy groups were easily detected and determined quantitatively. The peak from hydroxy groups was detected after acetylation of a copolymer, because the original sample did not give a distinct peak corresponding to that group. The number-average molecular weight of the copolymer prepared from trioxocane was estimated from the content of methoxy, ethoxy, and hydroxy groups. The value thus obtained was consistent with that obtained by gel permeation chromatography.     

Determination of changes in mass and volume of linseed oil during drying

Plates of stainless steel were coated with a thin film of seven different types of oxidative drying oils in order to follow the development charge in mass and volume during 42 days of curing. The plates were weighed in air and in water, and the volumes determined by applying Archimedes’ principle. All of the oils gained considerably in mass, with a maximum of 12% within seven days. The volume, however, decreased with large individual differences from one oil to another. A cold-pressed raw linseed oil decreased in volume by almost 15% whereas the volume of an alkali-refined linseed oil only decreased by a few percent. It is well known that linseed oil-based coatings are prone to crocodiling (surface crack formation), it is suggested that the severity of this phenomenon may be related to the volumetric reduction of the binder during curing. The described method is suggested as a simple means of screening linseed oil binders for formulation and development purposes.     

Mechanism for compensating slip volume changes during hot casting of ceramic

It is shown that during hot casting of ceramic the slip volume changes in the thermoplastic state, which comprise up to 85% of the total shrinkage, can be compensated by plastic deformation of the casting. The pressure gradient produced in the casting as it solidifies will determine the completeness of the compensation of the volume changes in the slip. __________ Translated from Steklo i Keramika, No. 7, pp. 10–12, July, 2007.     

Mechanisms of fatigue damage and fracture in semi-crystalline polymers

Fatigue crack profiles and fracture surfaces of poly(vinylidene fluoride) (PVDF), nylon-6,6 (N66), and poly(acetal) (PA) were studied to ascertain the mechanisms of cyclic damage and fatigue crack propagation in semicrystalline polymers. Crack tip damage is believed to begin as small trans-spherulitic and inter-spherulitic tensile crazes. However, compressive yielding within the reverse plastic zone at the crack tip crushes and elongates the spherulites in the direction of crack growth. Consequently, the microstructure of the polymer in advance of the crack front is different from the original morphology of the spherulitic bulk material as evidenced by the resulting fracture surface appearance. When the test temperature is below the glass transition temperature, however, plastic deformation is limited, and fatigue fracture occurs before significant disruption of the spherulitic structure. In this case, the fracture surface morphology reflects the original microstructure of the bulk polymer.     

Polymer fatigue fracture diagrams: BPA polycarbonate

A fatigue fracture diagram for BPA polycarbonate has been created from fatigue lifetime data obtained from knit line notched samples. This fatigue fracture diagram maps out stress-temperature zones where fatigue fracture is dominated by crack growth through leading crazes and zones where fatigue fracture occurs through shear fracture at 45 degrees to the load direction. Both craze and shear planes coexist in the fatigue crack tip plastic zone, and both compete to determine the ultimate crack growth behavior. The shear planes preferentially develop (and fracture) at higher temperatures and stresses, but this fracture process is quite slow. Consequently, an inversion in the fatigue lifetime curve is observed, with longer lifetimes at higher stresses. This inversion is easily understood as a transition between a craze branch and a shear branch on the fatigue lifetime plot. When the fatigue lifetime curve is plotted for data at different temperatures, with the stresses normalized to the yield stress at the respective test temperatures, the craze branch data from different temperatures overlap. This overlap can be explained by the N = 2 power law dependence of crack growth in the discontinuous crack growth regime.     

高性能纤维扭转疲劳断裂研究

在自制的扭转疲劳装置上,进行对位芳纶和超高相对分子质量聚乙烯(UHMWPE)纤维扭转疲劳实验,分析扭转机理及其扭转疲劳断口形貌特征。结果显示,相同条件下,UHMWPE纤维的耐扭转疲劳性能优于对位芳纶。对位芳纶扭转疲劳断口呈现原纤分裂,UHMWPE纤维皮层呈蠕变特征,芯层有脆性断裂特征。     

Properties of copolymer-type polyacetal/polyurethane blends

Mechanical and physical properties of the blends of copolymer-type polyacetals (POM) with polyurethane (PU) were investigated. The properties relationships of POM/PU blends are established by studying their morphology and compatibility. For the blends rich in POM, the morphology of the blends observed with a scanning electron microscopy (SEM) indicates that the blends containing lower than 50 wt % (46 vol %) PU are almost completely filled with spherical particles of the dispersed PU. As the concentration of PU increases, the spherulites of the POM observed by SEM become less perfect with coarse fine structure. Furthermore, when the concentration of PU was increased up to 50 wt %, the spherulties of POM in the blends are smaller than those of unblended POM. X-ray diffraction studies reveal that the degree of crystallinity of POM decreases with increasing PU content, which is nonmonotonic. This conclusion agrees with the observations made by differential scanning calorimetry (DSC) and density measurements. For the blends rich in POM, mechanical properties show that the impact strength of POM/PU blends increased with decreasing spherical size of the dispersed PU.     

Effect of short chain branching on the viscoelastic behavior during fatigue fracture of medium density ethylene copolymers

A method to determine viscoelastic changes in medium density polyethylene (MDPE) pipe specimens associated with the crack tip during fatigue crack initiation (FCI) and propagation (FCP) experiments is described. The load-displacement curves are analyzed to obtain the phase angle, δ. Changes in δ are related to the number of cycles of crack initiation of three different MDPE copolymers: hexene (H), butene (B), and methyl pentene (MP) copolymers. These changes are related to craze formation and growth at the notch tip, leading to crack initiation and to the irreversible work, W_i, expended on them. Within a given material, step wise increments in δ distinguish the onset of crack initiation and the brittle-to-ductile transition in crack growth. The magnitudes of tan δ and W_i are noted to be in quantitative agreement with the resistance of the three copolymers to FCI and brittle propagation that rank in the order: isobutyl (MP) > ethyl (B) > butyl (H). Similar crystallinity of the three copolymers insinuates a hypothesis that variance in the nature of chain entanglements associated with the respective branch type might be accountable for the observed differences in viscoelastic character. The final stage of failure by ductile tearing is dominated by large scale plastic flow that seemingly overshadows the material differences governing time dependent brittle fracture.     

Quantitative estimation of volume changes of granular materials during silo flow using X-ray tomography

The aim of the paper is to show the potential of the X-ray tomography to quantitatively measure volume changes in granular materials during silo flow in a rectangular model bin. The experiments were carried out with initially dense sand. The bin walls were smooth or very rough. In the first step, continuous X-ray radiographs were conducted. The results of volume changes were presented in form of 1D cross-sectional plots and 2D images. They were directly compared with corresponding experimental results obtained with a Particle Image Velocimetry (PIV) method allowing for measuring displacements on the solid surface. Measurement errors in both methods were discussed.     

Impact modification of polyacetal by thermoplastic elastomer polyurethane

The main goal of this study was impact modification of polyacetal [polyoxymethylene (POM)] with thermoplastic elastomer polyurethane (TPU). We modified the impact strength of POM 10‐fold. The mechanical properties, thermal behavior, and morphology of POM/TPU blends consisting of 5 to 50% of TPU were studied. It was found that the best impact modification of the blends was at 15% concentration of TPU and the maximum elongation at break was at 30% concentration of TPU. The impact strength of POM/TPU blends can be improved by using diphenylmethane diisocyanate (MDI) as compatibilizer. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 2573–2582, 2002     

The fracture toughness and fatigue crack propagation behaviour of annealed PET

The fracture toughness of poly(ethylene terephthalate) (PET) is correlated with various morphologies produced upon annealing. Annealing PET at a Hoffman regime III crystallization temperature (120°C) or inducing solid-state polymerization result in materials of high fracture toughness (K_c = 8.7 MPa m^1/2 and 9.5 MPa m^1/2, respectively); these high toughness values are related to multiple-crazing mechanisms produced by high tie-molecule densities. Annealing fully dried PET at 200°C transforms the material to a Hoffman regime I/II structure, and the fracture toughness decreases to 6.5 MPa m^1/2. Hydrolysis also reduces the fracture toughness (K_c < 3 MPa m^1/2). In these low toughness materials, multiple crazing is not observed. In addition, the fatigue crack propagation response of PET is found to be superior in samples annealed to produce high fracture toughness as compared with samples annealed to produce low fracture toughness values.     

Interlaminar fracture toughness and fatigue fracture of continuous fiber-reinforced polymer composites with carbon-based nanoreinforcements: a review

ABSTRACT

This review critically examines the recent developments in the use of carbon-based nanofillers as additional reinforcement to enhance the interlaminar properties of FRP composites. The low interlaminar strength of FRP composites results in delamination failure. The various nanoreinforcement strategies and their effect on fracture toughness, interlaminar shear strength (ILSS) and interlaminar fatigue are discussed in detail to prevent this delamination failure. Important findings on various factors that influence the interlaminar properties of multi-scale composites are presented by discussing various intrinsic and extrinsic toughening processes. Moreover, an overview of simulation techniques is provided to predict the delamination onset and propagation.     

Crack tip damage analysis in fatigue fracture of epoxy resin

The application of the crack layer theory to fatigue crack propagation (FCP) in epoxy is discussed. A crack tip damage evolution coefficient μ is introduced to assess the extent of damage as a fraction of the damage associated with critical crack propagation. The results can be expressed in the form where dl/dN is the rate of FCP, G₁ is the energy release rate whose critical value is G_1c, and β is a phenomenological constant. Although no damage was detected from microscopic analyses, μ increases fivefold during stable crack propagation. Fractal analysis of fracture surface profiles provides a quantitative measure of the roughness associated with crack advance. The fractural measure d is found to evolve in a similar fashion as μ, suggesting the applicability of d to quantify crack tip damage evolution.     

The tensile fatigue behavior of para-oriented aramid fibers and their fracture morphology

Samples of Fiber B and PRD 49 which were the forerunners of current Kevlar aramid fibers were subject to a limited number of tensile tests and tensile fatigue tests in order to determine their fracture morphology. The fibers were examined by optical and scanning electron microscopy. Both tensile and fatigue failure occurs by axial splitting, with the fatigue splits being much longer. Compressive effects in snap-back cause kink bands to form. The fatigue strength is only marginally less than the tensile strength.