Low temperature fracture properties of polymer-modified asphalts relationships with the morphology

A methodology for studying the relationships between fracture behavior and morphology of polymer-modified asphalts used as binders was developed by using the linear elastic fracture mechanics (LEFM) method and confocal laser scanning and environmental and cryo-scanning electron microscopies. Different types of polymers were used as modifiers: (i) copolymers from ethylene and methyl acrylate (EMA), butyl acrylate (EBA) or, vinyl acetate (EVA); (ii) diblock or star-shape triblock styrene-butadiene copolymers (SB or SBS^*). The 4 to 6 wt. % blends display an heterogeneous structure with a polymer-rich dispersed phase based on the initial polymer swollen by the aromatic fractions of the asphalt. The fracture toughness of the blends is higher than for the neat asphalt even if KIc of blends remains low compared to usual polymer blends due to the brittleness of the asphalt matrix. The fracture behavior which is strongly dependent on the nature of the polymer is discussed from the toughening mechanisms given for the filled polymers and the polymer blends. The EBA, SB, and SBS-based blends compared to the EMA and EVA-based ones display a higher KIc due to the elastomeric behavior of the polymer phase leading to a more efficient energy dissipation during crack propagation. The sample prepared with 4% crosslinked SB (Styrelf) and the corresponding physical blend (non-crosslinked) display the better fracture properties.     

Resistance Against the Intrinsic Rate of Fracture Mechanics Parameters for Polymeric Materials Under Moderate Impact Loading

This study contributes towards understanding crack toughness as resistance against the intrinsic rate of fracture mechanics parameters. Up to now only few investigations have been done under moderate impact loading conditions. Based on experimental investigations using the crack resistance (R) concept, it has been shown that the stop block method combined with the multiple-specimen technique is a unique method for polymers under impact loading conditions in comparison with different R-curve methods. Other methods for the determination of R curve such as the low-blow technique are normally not applicable for polymers due to their time-dependent mechanical properties. The crack-tip opening displacement (CTOD) rate is a measurement of the rate sensibility of stable fracture process depending on the type of deformation, which can provide deep insights into the micromechanics and activation mechanisms during the fracture processes. In the polymeric materials mostly investigated, one can understand the stable crack propagation with three-stage processes; crack-tip blunting/crack initiation, non-stationary stable crack growth and steady-state stable crack growth (an equilibrium state). In this stable crack propagation, the values of normalized CTOD rate converge rapidly to a ‘matrix’-specific threshold. The stop block method in the multiple-specimen technique assures the criteria of the time-independent strain field around the crack tip and constant crack speed therewith and the J-integral is a valid toughness parameter.     

Morphology, mechanical and thermal behavior of acrylate rubber/fluorocarbon elastomer/polyacrylate blends

Novel thermoplastic elastomers derived from binary and ternary blends of polyfunctional acrylates, acrylic rubber (ACM) and fluorocarbon rubber (FKM) were analyzed by using Transmission Electron Microscopy (TEM), Differential Scanning Calorimetry (DSC), Dynamic Mechanical Thermal Analysis (DMTA) and mechanical tests. TEM revealed the presence of a single-phase structure for both acrylate rubber/fluorocarbon elastomer (ACM/FKM) and ACM/polyacrylate binary blends. Increase of FKM concentration in the ACM/FKM/polyacrylate ternary blend resulted in phase separation of FKM from the ternary blend. The FKM formed a dispersed phase with polynodal particle distribution and irregular shape ranging from ellipsoidal to highly elongated form with inclusion of ACM. The FKM/polyacrylate binary blend showed complete phase separation. Ageing of the blend increased the domain size of the dispersed phase. Differential scanning calorimetric (DSC) and DMTA studies showed no major changes in the T _gs of individual polymers in the blend, although the peak tan values were affected on changing the composition of the blends. Vulcanization of the thermoplastic elastomer (TPE) changed the phase morphology with increase in particle size. There is a distinct difference in morphology of statically and dynamically vulcanized blends.     

Threshold toughness of polymers in the ductile to brittle transition region by different approaches

The fracture behavior of polymers in the ductile-to-brittle region is neither completely brittle nor entirely ductile. Besides, scatter in toughness results impairs the situation. Consequently, conventional methods based exclusively either on linear elastic fracture mechanics theory (LEFM) or on non-linear elastic fracture mechanics theory (NLEFM) are not suitable. It was demonstrated previously, that Weibull statistical method could be successfully used to determine the toughness threshold of polymers displaying ductile-to-brittle behavior. The present study compares the threshold toughness value determined by the statistical approach with other critical values calculated following other different suitable approaches: Low temperature plane strain fracture toughness, Plastic zone corrected fracture toughness, Stable and unstable propagation combined model, J extrapolated at zero stable propagation value, and Quasi J-R curve. The analysis was carried out on data points taken from fracture tests performed on polypropylene homopolymer, PPH, and on a blend of PPH and an elastomeric polyolefin, PPH/POes. The results of this analysis indicate that statistical, stable and unstable propagation combined model, and the J extrapolated at zero stable propagation value methods yield to very similar toughness threshold values being practically equivalent. In this case, threshold value was slightly smaller than the minimum J displayed by the experimental replicas, suggesting that it is an actual representative material toughness. Among these methodologies, the Statistical Method is applicable even if stable crack growth is difficult to determine. On the other hand, the methodologies based on LEFM tended to underestimate the fracture toughness, being very conservative while Quasi J-R curve method based on NLEFM overestimated the PPH/POes toughness value.     

Mechanical behaviour of poly(methyl methacrylate)

A series of tensile and three-point bending studies was conducted at various temperatures and loading rates using a commercial poly(methyl methacrylate) (PMMA). Tensile properties and fracture toughness data were obtained for the various conditions. In general, both tensile strength and fracture toughness increase with increasing loading rate and decreasing temperatur E. However, when the temperature reaches the glass transition region, the relationships between fracture toughness, loading rate, and temperature become very complex. This behaviour is due to the simultaneous interaction of viscoelasticity and localized plastic deformation. In the glass transition region, the fracture mechanism changes from a brittle to a ductile mode of failure. A failure envelope constructed from tensile tests suggests that the maximum elongation that the glassy PMMA can withstand without failure is about 130%. The calculated apparent activation energies suggest that the failure process of thermoplastic polymers (at least PMMA) follows a viscoelastic process, either glass or transition. The former is the case if crack initiation is required.Deceased.     

Thermoplastic-toughened epoxy polymers

The microstructure and properties of two epoxy-resin systems which have been modified with varying amounts of a thermoplastic to improve the toughness of the thermosetting epoxy polymers, have been studied. The curing agent was 4,4 diaminodiphenylsulphone and the thermoplastic was a reactively terminated poly (ether sulphone) copolymer. Different microstructures were found to occur as the concentration of the thermoplastic component was steadily increased. In particular, the relationships between the microstructures and values of stress-intensity factor, K_Ic, and fracture energy, G_Ic, were explored.     

Effect of hybridization of liquid rubber and nanosilica particles on the morphology, mechanical properties, and fracture toughness of epoxy composites

A liquid carboxyl-terminated butadiene–acrylonitrile copolymer (CTBN) and SiO₂ particles in nanosize were used to modify epoxy, and binary CTBN/epoxy composites and ternary CTBN/SiO₂/epoxy composites were prepared using piperidine as curing agent. The morphologies of the composites were observed by scanning electron microscope (SEM) and transmission electron microscope (TEM), and it is indicated that the size of CTBN particles increases with CTBN content in the binary composites, however, the CTBN particle size decreases with the content of nanosilica in the ternary composites. The effects of CTBN and nanosilica particles on the mechanical and fracture toughness of the composites were also investigated, it is shown that the tensile mechanical properties of the binary CTBN-modified epoxy composites can be further improved by addition of nanosilica particles, moreover, obvious improvement in fracture toughness of epoxy can be achieved by hybridization of liquid CTBN rubber and nanosilica particles. The morphologies of the fractured surface of the composites in compact tension tests were explored attentively by field emission SEM (FE-SEM), it is found that different zones (pre-crack, stable crack propagation, and fast crack zones) on the fractured surface can be obviously discriminated, and the toughening mechanism is mainly related to the stable crack propagation zone. The cavitation of the rubber particles and subsequent void growth by matrix shear deformation are the main toughening mechanisms in both binary and ternary composites.     

Effects of mode mix upon fracture behavior of a solder joint

Results of fracture experiments of brass/solder/brass sandwich CTS (Compact Tension-Shear) specimens are presented together with observations of the crack propagation behavior and the fractographs. The fracture behaviors of the interface crack are analyzed by the finite element method with a modified boundary layer formulation. Several fracture mechanisms and the corresponding criteria are examined. And the crack growth behavior and fracture toughness are predicted. As the results various crack growth procedures such as the crack jump to another interface on the opposite side, the nucleation of a new crack far from the initial crack front, and the asymmetric relation of fracture toughness versus mode mix J _c– can be successfully explained. The fractographs, the crack growth behaviors, and stress-strain distribution along the interface are inter-related.     

On the environmental fracture of polymethylmethacrylate

Crack propagation of PMMA in some liquid environments is described for various testing conditions, such as fixed load, fixed displacement and monotonically increasing displacement. Fracture mechanics concepts have been used successfully in analysing the results. When continuous stable cracking is achieved, values of fracture toughness (R) for PMMA under these loading conditions are obtained as a function of crack velocity () using the method of Gurney and Hunt [11]. For crack velocities greater than 10^–2 mm sec^–1, the fracture toughness values in the environments are increased when compared with the corresponding air results. Unique relationships betweenR and have been shown to exist for cracking in ethanol and carbon tetrachloride.     

Toughening of epoxies through thermoplastic crack bridging

The fracture toughness and toughening mechanism of two epoxy matrices containing varying concentrations of pre-formed polyamide-12 particles was investigated. The pre-formed thermoplastic modifier was used to keep the physical and morphological characteristics of the second phase constant while varying the matrix intrinsic toughness to simplify the interpretation of toughening results. We observed that these particles toughened the epoxies through a crack bridging mechanism involving large plastic deformation of the second phase.This mechanism was found to be effective independent of the potential of the matrix for plastic deformation since the increasing fracture toughness was accomplished without significant amounts of plastic deformation in the epoxy matrix. A quantitative model was adapted to account for the increase in toughness due to the crack bridging mechanism. From this model, it was possible to determine the factors which are most important when attempting to toughen a material through thermoplastic crack bridging. A better understanding of the specific factors which influence the efficiency of the crack bridging mechanism enables the fracture properties of brittle materials to be further improved with thermoplastic addition. This was shown to be very important when attempting to enhance the toughness of materials which are believed to be un-toughenable by conventional rubber modification, or materials whose other mechanical properties suffer from the addition of elastomeric materials.     

Toughness and fatigue of encapsulation-processed silicon carbide-polymer matrix particulate composites

A new process for composite fabrication was developed which improves distribution of the particulate reinforcing phase by polymer encapsulation of the particulate prior to consolidation. The effect of such processing on the fatigue-crack propagation and fracture toughness behaviour of particulate thermoplastic composites was investigated. Composites of several particulate size ranges were fabricated into disc-shaped, compact tension specimens and tested under cyclic and monotonie loading conditions. For comparison, a composite was also fabricated using a standard casting technique. The observed fatigue-crack growth rates spanned three orders of magnitude (10^–11 to 10^–9 m per cycle) over an applied stress intensity range, K, of 0.3 to 1.1 MPa m^1/2. The measured fracture toughness values ranged from 0.69 to 2.95 MPa m^1/2. Comparison of the two processing techniques indicated that encapsulation processing increased the fracture toughness of the composite by approximately 33%; however, the fatigue-crack growth behaviour was unaffected. In addition, a trend of increasing crack growth resistance (toughness) with increasing reinforcement particle size was observed. These results are discussed in the light of crack shielding and bridging models for composite toughening.     

Fracture toughness of tooth acrylics

The hardness, fracture toughness, toughness, flexural strength and Youngs moduli of three acrylic tooth polymers were investigated. The first polymer was based on a conventional homopolymer poly(methylmethacrylate). The second was based on cross-linked poly(methylmethacrylate) with an uncross-linked poly(methylmethacrylate) coating. The third material was based on an interpenetrating polymer network (IPN) of a cross-linked and uncross-linked poly(methylmethacrylate). All three polymers had similar hardness values. The cross-linked and IPN polymers had higher fracture toughness (K_IC) and toughness (G_IC) values than the conventional homopolymer poly(methylmethacrylate) polymer and lower flexural strengths (_f). The toughness of the cross-linked and IPN polymers was higher due to crack deflection around the polymer bead structure and the polymer beads acting as crack pinning sites.     

Kinetic and structural interpretations of post-yield crack resistance behavior of polyamide-6/polyolefin-g-maleic anhydride blends

Binary blends of polyamide-6 (PA-6)/polypropylene-grafted-maleic anhydride (PP-g-MA) and PA-6/low density polyethylene-grafted-maleic anhydride (LDPE-g-MA) were prepared with varied concentration (0–30 wt%) of maleic anhydride-grafted polyolefinic (PP) moiety as the impact modifier. The microstructural attributes and thermal properties of the blends were characterized by WAXD, FTIR, SEM, DSC, and TGA. The WAXD/DSC studies have revealed that the crystallinity of the blends decreased with the increase in the PP modifier whereas the onset of degradation temperature remained nearly unaffected. Comparative assessment of the crack toughness behavior of the blends has been carried out following the essential work of fracture (EWF) approach based on post-yield fracture mechanics (PYFM) concept. The kinetics of crack propagation of the blends has been discussed in the realms of structural and compositional parameters. An enhancement in the toughness (resistance to stable crack propagation) as indicated by a maximum in the non-EWF (βw _p) values have been observed at 10 and 20 wt% followed by a sharp and consistent drop in the composition regime of 10–20 and 20–30 wt% of PP-g-MA and LDPE-g-MA, respectively; conceptually implying possible ductile-to-semiductile transitions in the blend systems. The equivalence of PYFM–EPFM fracture parameters have been discussed following inequality criterion. Fractured surface morphology investigations revealed that the failure mode of the blends undergo a systematic transition from matrix-controlled homogeneous flow/deformation in the PP/polyamide phase to blend composition-dependent changes in the modes and extent of fibrillation via cavitation and shear-banding mechanisms.     

Probabilistic fracture mechanics of 2D carbon-carbon composites

The fracture toughness of two types of carbon fabric reinforced carbon composite (KKARB^®, Types A and C) is evaluated, the mechanisms of crack propagation resistance are identified and both are related to microstructural differences.The two composites have the same constituents, i.e. fibers, yarns, fabric weaving and matrix precursor. However, different processing cycles result in apparent differences in microstructure (e.g. different number and length distribution of microcracks, crimp angle) and toughness.The crack diffusion model (CDM) is invoked to parameterize the fluctuating strength field of the composite in terms of an average fracture energy , a minimum fracture energy 179-1 and a shape parameter . The values of 179-1 are in direct correlation with the average size of microcracks in each composite, and is found to correlate with the scatter in fracture toughness.     

Fracture toughness behavior of weldments with mis-matched properties at elevated temperature

High temperature fracture toughness tests were performed on welded specimens of 1Cr-1Mo-14 V steel with different levels of mismatch between the base metal and the weld metal and the cracks lying along the fusion line. A wide range of fracture toughness values were obtained for weldments, as opposed to a unique value of J_IC and a unique J-R curve typically obtained for homogeneous materials. Detailed observations of the crack path within the weldments were made to understand the wide scatter in the fracture toughness behavior. The yield strength mismatch between the base metal and the weld metal was found to directly influence the stable crack path, and hence the fracture toughness behavior. The denomination of ‘apparent fracture toughness’ was used to describe the variability of the fracture toughness in the weld region due to microstructure and mechanical property gradients. The apparent fracture toughness exhibited a minima at a fixed distance from the fusion line for a specific weld. The relative position of the fatigue precrack with respect to the fusion line and the region of low fracture toughness was also shown to influence the measured fracture toughness behavior of the specimen. A frame-work is provided for representing the weld fracture toughness behavior and the associated variability due to microstructural gradients. This revised version was published online in August 2006 with corrections to the Cover Date.     

聚乙烯／橡胶共混物的断裂韧性

用单边缺口法研究了聚乙烯(PE)与顺丁橡胶(BR)共混物在裂纹亚稳临界扩展时的断裂韧性(G_(?))。结果表明,加入BR可以明显改善高密度聚乙烯(HDPE)的抗裂纹扩展能力,但对低密度聚乙烯(LDPE)的抗裂纹扩展能力有所降低。     

Interlaminar fracture (model II) of commingled yarn-based GF/PP composites

A 5050 wt % mixture of commingled glass/polypropylene fibre system was selected to study the correlations between the morphological details, mode II interlaminar fracture toughness and corresponding failure mechanisms. Mode II interlaminar fracture tests were performed by using the end-notched flexure test procedure. Compared to conventional composite laminates, mode II interlaminar crack extension in these commingled yarn-based composites was very stable, and extensive fibre nesting occurred along the main crack plane. Crack jumping and non-broken matrix links were observed.R-curve behaviour for these materials was identified and the toughness for initiation was much lower than that for propagation. Compared to mode I interlaminar fracture toughness, similar trends in effects of cooling rates and isothermal crystallizations on mode II interlaminar fracture toughness were observed. However, the effects were not as significant as those found for mode I interlaminar fracture toughness.Alexander von Humboldt Fellow.     

Measurement of the fracture toughness of polymer-non-polymer interfaces

One of the primary factors limiting the development of a better understanding of polymer-non-polymer adhesion is the lack of a good testing method for the measurement of the strength of the interface. In this paper polymer-non-polymer adhesion is evaluated in terms of the fracture toughness of the interface using an asymmetric double cantilever beam testing geometry. The test is applied to the measurement of polystyrene (PS)-glass and PS-silicon (native oxide) interfaces modified by PS-poly(2 vinylpyridine) (PVP) and PS-poly methyl methacrylate (PMMA) diblock copolymers. The importance of mixed mode crack propagation is demonstrated and it is shown that through an appropriate choice of sample geometry, the crack-tip trajectory can be controlled so that the crack is forced to propagate along the interface. The PS-glass test, in particular, is shown to overcome many of the traditional problems of adhesion measurements, such as failure, away from the interface and effects of far-field deformation in the polymer. The interfacial fracture toughness of the PS-glass and PS-silicon interfaces without copolymer modification are approximately the same and weak with values of 1 J m^–2. The addition of the block copolymers results in significant (>40-fold) increases in the interfacial fracture energies. The increase in fracture toughness is dependent on the quantity and degree of organization of the block copolymer at the interface.     

The fracture of acrylic polymers in water

This paper describes the application of linear elastic fracture mechanics analysis to the fracture of acrylic polymers in water. Three denture base acrylics were studied in addition to Perspex. The effects of strain rate and temperature were investigated using double torsion specimens and three-point bend specimens. It was found for most materials that the fracture toughness was dramatically increased on testing in water compared with testing in air. Crack propagation at fast strain rates was unstable in water and the fracture toughness and flaw size were strain-rate dependent, increasing with decreasing test rate, whilst the un-notched fracture strength decreased with decreasing strain rate. At low strain rates, stable crack propagation was achieved and fracture toughness then decreased with decreasing strain rate. The results are discussed in terms of Williams' model for environmental fracture and the effect water has on the crazing process taking place at the crack tip.     

Poly(ether ether ketone) with pendent methyl groups as a toughening agent for amine cured DGEBA epoxy resin

A diglycidyl ether of bisphenol-A (DGEBA) epoxy resin was modified with poly(ether ether ketone) with pendent methyl groups (PEEKM). PEEKM was synthesised from methyl hydroquinone and 4,4′-difluorobenzophenone and characterised. Blends of epoxy resin and PEEKM were prepared by melt blending. The blends were transparent in the uncured state and gave single composition dependent T _g. The T _g-composition behaviour of the uncured blends has been studied using Gordon–Taylor, Kelley–Bueche and Fox equations. The scanning electron micrographs of extracted fracture surfaces revealed that reaction induced phase separation occurred in the blends. Cocontinuous morphology was obtained in blends containing 15 phr PEEKM. Two glass transition peaks corresponding to epoxy rich and thermoplastic rich phases were observed in the dynamic mechanical spectrum of the blends. The crosslink density of the blends calculated from dynamic mechanical analysis was less than that of unmodified epoxy resin. The tensile strength, flexural strength and modulus were comparable to that of the unmodified epoxy resin. It was found from fracture toughness measurements that PEEKM is an effective toughener for DDS cured epoxy resin. Fifteen phr PEEKM having cocontinuous morphology exhibited maximum increase in fracture toughness. The increase in fracture toughness was due to crack path deflection, crack pinning, crack bridging by dispersed PEEKM and local plastic deformation of the matrix. The exceptional increase in fracture toughness of 15 phr blend was attributed to the cocontinuous morphology of the blend. Finally it was observed that the thermal stability of epoxy resin was not affected by the addition of PEEKM.