Large deformation mechanical behavior of gelatin–maltodextrin composite gels

The large deformation failure behavior of gelatin–maltodextrin composite gels was assessed. All the studied compositions were selected to lie within the incompatibility domain of the gelatin–maltodextrin phase diagram at 60°C, which produced gelatin continuous (maltodextrin included) and maltodextrin continuous (gelatin included) composites. Composite microstructural evaluation was performed using confocal laser scanning microscopy (CLSM). The large deformation mechanical behavior was measured in tension and compression experiments. Crack–microstructure interactions were investigated by dynamic experiments on the CLSM. The gelatin continuous composites exhibited pseudo-yielding behavior during tension and compression testing, and there was a significant decrease in modulus that arose from interfacial debonding. Conversely, the maltodextrin continuous composites exhibited an essentially brittle failure behavior, and there was an approximately linear increase in stress with increasing strain until fracture (which occurred at significantly lower strains than for the gelatin continuous composites). The CLSM observation of the failure of the notched samples also demonstrated interfacial debonding in the crack path; however, this occurred at significantly smaller strains than for the gelatin continuous samples with minimal elastic–plastic deformation of the maltodextrin matrix. The Poisson ratio was estimated to be close to 0.5 for these composites for all examined compositions. Compositions corresponding to a tie line of the phase diagram were also investigated to assess the influence of the relative phase volume (for constant phase compositions) on the failure behavior. The majority of the parameters subsequently extracted from the stress–strain curves were apparently functions of the individual phase volumes. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 124–135, 2001     

Experimental studies on fracture of unidirectional glass-fibre-reinforced plastics composites with V-notch

Abstract

The digital speckle correlation method is used to study the deformation and fracture behaviour of glass-fibre-reinforced plastics (GFRP) composites with V-notch under tensile loading. The deformation images surrounding the notch tip at varying load levels were captured; both the horizontal and vertical displacement fields are acquired. At the same time, the strain evolution information at the notch tip during the whole damage and fracture process of the notched sample is recorded by a three-element strain gauge. The effects of notch angle and notch depth on deformation and fracture of notched GFRP are analyzed. Some microscopic fracture characteristics such as interfacial debonding, fibre fracture and matrix damage are shown.     

Large deformation and ultimate properties of biopolymer gels: 2. Mixed gel systems

Biphasic, mixed gels of agarose and gelatin were prepared, and their mechanical behaviour in tensile tests was determined, up to failure, utilising four decades of (constant) strain rate. The behaviour of pure agarose and pure gelatin in such tests has been determined previously. Suitable ‘blending-laws’ relating the small deformation shear modulus of the composite to the moduli of the component phases have also been discussed elsewhere. This report extends the latter treatment to the more aggressive large deformation regime, deriving bounds for modulus and break stress which closely model observed behaviour.     

Interface effect on the mechanical behaviour of rigid particle filled polymer

Glass beads, non‐modified and modified separately with two different coupling agents, were incorporated in high density polyethylene to prepare composite materials with different interfacial adhesion strengths. Tensile tests show that the mechanical behaviour of the materials is sensitive to the strain rates. The strong interfacial adhesion can delay the occurrence of damage and so increase the load‐bearing ability under both monotonic and cyclic loading. In situ tensile tests give damage mechanisms mainly induced by the interfacial debonding. The stronger the interfacial adhesion, the lower the number of glass beads debonded from the matrix under a given stress. The degree of microdamage defined as the percentage of debonded particles is obtained as a function of the applied load. © 2001 Society of Chemical Industry     

Orientation effects on the deformation and fracture properties of high‐density polyethylene/ethylene vinyl acetate (HDPE/EVA) blends

In this paper, the tensile deformation and fracture toughness of high‐density polyethylene (HDPE)/ethylene vinyl acetate (EVA) blends, obtained by dynamic packing injection moulding, have been comprehensively investigated in different directions of rectangle samples, including longitudinal, latitudinal and oblique directions relative to the flow direction. Two kinds of EVA were used with VA content 16 wt% (16EVA) and 33 wt% (33EVA) to control the interfacial interactions. The results indicate that molecular orientation and interfacial interaction play very important roles to determine the tensile behaviour and fracture toughness. Biaxial‐reinforcement of tensile strength was seen for HDPE/16EVA blends but only uniaxial‐reinforcement was observed for HDPE/33EVA blends. The difference is caused by the different interfacial interactions as highlighted by the peel test, scanning electron microscopy (SEM) observation as well as theoretical evaluation. Very high impact strength, decreasing with increasing EVA content, was observed when the fracture propagation is perpendicular to the shear flow direction, while a low impact strength, increasing slightly increasing with EVA content, was seen when the fracture propagation is parallel to the shear flow. The fracture of oblique samples is always along the flow direction instead of along the impact direction or tensile direction. The tensile behaviour and fracture toughness are discussed on the basis of the formation of transcrystalline zones, orientation of EVA particles and matrix toughness of HDPE in different directions. Copyright © 2004 Society of Chemical Industry     

MODELLING INTERFACIAL DEGRADATION USING INTERFACIAL RUPTURE ELEMENTS

Reliable predictive modelling of the environmental degradation of adhesively bonded structures is required for a more widespread use of this joining technique. Recent durability modelling has coupled moisture diffusion and stress analysis, where the joint response is controlled by continuum degradation of the adhesive. However, the joint response is more commonly controlled by degradation of the interface. Current research extends existing durability modelling to include interfacial degradation and failure. Experimental studies have been undertaken to provide the moisture uptake parameters and moisture-dependent properties, both for the constitutive behaviour of a bulk epoxy and for the fracture energy of an epoxy-steel interface that has been exposed to various uptake levels of moisture. The mixed mode flexure (MMF) test was used to determine the interfacial strength. It was found that the interface fracture energy reduces with increasing interfacial moisture concentration. Interfacial rupture elements were developed to model the complete progression of damage within a joint from a single FE analysis. These rupture elements were formulated for mixed mode conditions and followed a separation law that used the fracture energy and the tripping strain as the controlling parameters. The role of these parameters was investigated, and it was shown that as long as there is a continuous process zone these elements respond well. This can be achieved as long as the tripping strain remains below a (mesh-dependent) critical value. Moisture-dependent fracture energies and tripping strains were then determined by calibration using the initial crack length data from the MMF specimens. These parameters were subsequently used to predict the response with increasing crack length, and excellent predictions were obtained.     

Modelling interfacial degradation using interfacial rupture elements

Reliable predictive modelling of the environmental degradation of adhesively bonded structures is required for a more widespread use of this joining technique. Recent durability modelling has coupled moisture diffusion and stress analysis, where the joint response is controlled by continuum degradation of the adhesive. However, the joint response is more commonly controlled by degradation of the interface. Current research extends existing durability modelling to include interfacial degradation and failure. Experimental studies have been undertaken to provide the moisture uptake parameters and moisture-dependent properties, both for the constitutive behaviour of a bulk epoxy and for the fracture energy of an epoxy-steel interface that has been exposed to various uptake levels of moisture. The mixed mode flexure (MMF) test was used to determine the interfacial strength. It was found that the interface fracture energy reduces with increasing interfacial moisture concentration. Interfacial rupture elements were developed to model the complete progression of damage within a joint from a single FE analysis. These rupture elements were formulated for mixed mode conditions and followed a separation law that used the fracture energy and the tripping strain as the controlling parameters. The role of these parameters was investigated, and it was shown that as long as there is a continuous process zone these elements respond well. This can be achieved as long as the tripping strain remains below a (mesh-dependent) critical value. Moisture-dependent fracture energies and tripping strains were then determined by calibration using the initial crack length data from the MMF specimens. These parameters were subsequently used to predict the response with increasing crack length, and excellent predictions were obtained.     

Microstructure and fracture behavior of polypropylene/barium sulfate composites

The microstructure, mechanical properties, and fracture behavior of polypropylene (PP)/barium sulfate (BaSO₄) composites were studied. Four composite samples with different PP‐BaSO₄ interface were prepared by treating the filler with different modifiers. The fracture behavior of the composites under different strain rates was studied by means of Charpy impact tests and essential work of fracture (EWF) tests. It is shown that a moderate interfacial adhesion is favorable for toughening, which ensures that the particles transfer the stress and stabilizes the cracks at the primary stage of the deformation, and satisfies the stress conditions of plastic deformation for matrix ligaments subsequently via debonding. Very strong interfacial adhesion is not favorable for toughness, especially under high strain rate, because the debonding‐cavitation process may be delayed and the plastic deformation of matrix may be restrained. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 1207–1213, 2006     

Stress-strain behavior and tensile dilatometry of glass bead-filled polypropylene and polyamide 6

The mechanical behavior of multiphase materials is closely related to the interfacial adhesion between their various components. There is considerable interest in the development of simple experimental techniques for characterization of interfacial debonding during mechanical loading. Probably the best known method is tensile dilatometry, in which the onset and progression of debonding are related to the volume of microvoids generated in the material as it undergoes mechanical loading. Several authors have suggested that equivalent information can also be extracted from stress-strain data generated during a simple constant strain rate test. In practice, however, the transition between the initially well-bonded and the debonded state is obscured by the strain-induced softening of the matrix, which is usually observed in the same strain range as the debonding. In this work the filler/matrix debonding in polypropylene and polyamide 6 filled with up to 50 vol % of glass beads is examined using both tensile dilatometry and an analysis of tensile stress-strain curves. It was found that, depending on the level of adhesion, either a complete or partial debonding occurs in the strain range studied (0–8%). It appears that the volume change due to debonding is a small part of the total volume strain recorded. Therefore, the accuracy of the tensile dilatometry is not sufficient to detect the onset of debonding. The loss of stiffness of the composite, particularly when compared to the loss of stiffness of the matrix offers a more promising way to follow the debonding process. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 653–665, 1997     

The effect of physical ageing on the properties of poly(ethylene terephthalate)

Physical ageing rates of poly(ethylene terephthalate) have been measured, and ageing is interpreted to be associated with the conventional glass formation process, which occurs at a more rapid rate at higher temperatures. Ageing is accompanied by a marked change in mechanical properties, increased tensile yield stress and drawing stress, more localized yielding of the polymer and a marked decrease in impact strength. The fracture results have been attributed to the increased yield stress and a change in contribution of plane stress and plane strain conditions in the samples. Fracture surfaces show evidence of mixed modes of fracture.     

Uniaxial tensile and creep behaviour of an alumina fibre-reinforced ceramic matrix composite: I. Experimental study

The uniaxial tensile and creep behaviour of an alumina fibre-reinforced silicon carbide composite is studied. The damage mechanisms during tensile loading are identified on the basis of the elastic response and in-situ morphological analysis. Tensile tests show that the composite presents a pseudoductile behaviour due to matrix microcracking and fibre-matrix debonding. Temperature induces changes in the tensile behaviour because of variations in load transfer conditions and in the axial residual stress borne by the fibres and the matrix. The creep curves at 1100°C under vacuum present an extended tertiary part, especially at low creep stress. The unloading-reloading loops periodically performed during creep show a progressive decrease in longitudinal stiffness. Progressive interface debonding during creep is invoked to explain: (i) the strain rate increase during tertiary creep, (ii) the decrease of the elastic modulus and (iii) the large fibre pull-out observed on the creep fracture surface. The different creep rupture modes at low and high stresses are related to the capability of the remaining intact fibres to support the overload failure of the first fibres.     

Use of a Crack-Bridging Single-Fiber Pullout Test to Study Steel Fiber/Cementitious Matrix Composites

The fracture process of steel fiber/cementitious matrix composites has been studied using a single-fiber pullout test that permits detailed measurements of the load-crack opening displacement relationship during fiber debonding and unloading. Using a suitable analytical model, the interfacial fracture energy and interfacial sliding friction have been calculated for composites incorporating steel fibers with cement paste or mortar matrices. Comparison of theoretical debonding curves with the experimental data show that the model accurately represents the fiber debonding process, except for a decrease in interfacial sliding friction due to wear of matrix asperities at the interface. Differences between the calculated interfacial properties of several specimens are associated with changes in the interfacial microstructure.     

Kinetics of ageing and re-embrittlement of mechanically rejuvenated polystyrene

Pre-deforming polystyrene by rolling results in elimination of strain softening and induces ductile deformation behaviour in a subsequent tensile test. However, both yield stress and strain softening recover in time as a result of ageing, resulting in renewed brittle failure behaviour. The kinetics of this process is addressed in this paper. Although the process of recovery of yield stress and strain softening shows no molecular weight dependence, the time-scale of renewed brittle fracture after rejuvenation does. Any localisation of strain can only be stabilised if the molecular network can transfer sufficient load. For relatively low molecular-weight polystyrene, the load bearing capacity is already exceeded at short ageing times, whereas for higher molecular-weight grades this takes longer. Since the creep compliance and shift-rate of mechanically rejuvenated polystyrene shows a pronounced increase as compared to thermally rejuvenated polystyrene, the segmental mobility in the mechanically rejuvenated samples has increased, despite a lower free volume. This indicates that a new explanation for ageing should be postulated, which is discussed.     

Fracture behavior of glass bead filled epoxies: Cleaning process of glass beads

Inorganic particles are commonly cleaned with solvents such as alcohols before being incorporated into thermoset polymers as fillers or tougheners, but the role of the cleaning process has never been examined. In this study, the effect of the cleaning process on the fracture behavior of particulate composites is investigated using glass bead filled epoxies as model systems. The cleaning process is shown to be a simple method to strengthen the interface between the glass beads and the epoxy matrix. Although the chemistry of the glass bead surface is unlikely to be altered by the cleaning process, submicron particles that exist on the glass bead surfaces are removed by cleaning with distilled water or ultrasonic vibration. The removal of submicron particles increases the interfacial strength between the glass beads and the matrix and changes the tensile strength of the composites. However, the modulus and fracture toughness of the composites is not significantly dependent on the cleaning process. Thus, it may be the case that debonding of the glass beads is not one of the major energy dissipating mechanisms in the fracture of glass bead filled thermoset systems. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 1371–1383, 2001     

Tensile properties and damage behaviors of glass‐bead‐filled modified polyphenylene oxide under large strain

Based on Continuum Damage Mechanics (CDM), a damage model for glass‐bead‐filled modified polyphenylene oxide (GB/PPO) has been proposed to describe its damage behavior at various levels of tensile strain by considering the reduction of effective loading area. Hence, an equation for prediction of effective elastic modulus of the damaged GB/PPO composites in terms of the three principal true strains was derived. The tensile properties and damage behaviors of the GB/PPO composites with different volume percentages of glass beads were investigated using standard tensile tests and load‐unload tests, respectively. The addition of glass beads increases Young's modulus of PPO but has a weakening effect on its tensile strength. A maximum value of tensile work to break and tensile strain at break was found when 5 vol% of glass beads with a mean diameter of 11 μm was blended with PPO. These results were justified through microscopic examination of the fracture surfaces of the tensile specimens by using a scanning electron microscope (SEM). In‐situ observations of the strain damage processes were made through the SEM equipped with a tensile stage to determine the strain at fully debonding of glass beads. The volumetric strain of GB/PPO composites increases because of microcavitation during strain damage. In general, the prediction for the effective elastic modulus of the damaged GB/PPO composites at different true strains is slightly higher than the experimental results. The damage evolution rates after fully debonding of glass beads from the matrix are close to those predicted by the proposed damage model.     

Effect of Carbon and Silicon Carbide/Carbon Interlayers on the Mechanical Behavior of Tyranno-SA-Fiber-Reinforced Silicon Carbide-Matrix Composites

Several CVI-SiC/SiC composites were fabricated and the mechanical properties were investigated using unloading–reloading tensile tests. The composites were reinforced with a new Tyranno-SA fiber (2-D, plain-woven). Various carbon and SiC/C layers were deposited as fiber/matrix interlayers by the isothermal CVI process. The Tyranno-SA/SiC composites exhibited high proportional limit stress (∼120 MPa) and relatively small strain-to-failure. The tensile stress/strain curves exhibited features corresponding to strong interfacial shear and sliding resistance, and indicated failures of all the composites before matrix-cracking saturation was achieved. Fiber/matrix debonding and relatively short fiber pullouts were observed on the fracture surfaces. The ultimate tensile strength displayed an increasing trend with increasing carbon layer thickness up to 100 nm. Further improvement of the mechanical properties of Tyranno-SA/SiC composites is expected with more suitable interlayer structures.     

Fracture behaviour of high impact polystyrene Influence of particle morphology

Abstract

The mechanical behaviour of high impact polystyrene has been examined with special attention to the influence of particle size and morphology. Video extensometry was used to follow the volume strain kinetics during tensile drawing and also to make direct observations of crack initiation in compact tension experiments at moderate rates. Materials incorporating composite particles with either small capsule (0.3 µm) or large salami (>1 µm) structures have been considered, as well as blends combining both morphologies. In the particular case of blends, a marked synergistic behaviour was achieved under impact conditions. Plasticity and fracture results, as well as morphological observations, suggest that the ability of the salami particles to stabilise the crazes initiated by the small capsules plays a key role, even at small volume fractions of salami particles.     

E‐glass/DGEBA/m‐PDA single fiber composites: Interface debonding during fiber fracture

In this paper, we examine the regions of debonding between the fibers and the matrix surrounding fiber breaks formed during single fiber fragmentation tests. The fiber breaks are accompanied by areas of debonding between the matrix and the surface of the fiber. With increasing applied strain, the lengths of these debonded regions generally increase. At the end of the test, the matrix tensile strain adjacent to the debond regions is an order of magnitude higher than the applied strain (40% vs. 4%). Although the debond edges typically remain attached at the same locations on the fiber fragments, debond propagation along fiber fragments under increasing strain has been observed in some cases. The phenomenon is termed secondary debond growth, and two mechanisms that trigger secondary debond region growth have been proposed. As expected, tests with bare fibers and with fibers coated to alter interface adhesion indicate that the average size of debonded regions at the end of the test increases as the calculated interfacial shear strength decreases. However, a decrease in the “apparent” interfacial shear strength resulting from an increase in testing rate results in a decrease in the size of the average debond region. This result suggests an increase in the amount of energy stored in the matrix from the fiber fracture process. POLYM. COMPOS. 28:561–574, 2007. © 2007 Society of Plastics Engineers     

Essential work of fracture of poly(ϵ‐caprolactone)/boehmite alumina nanocomposites: Effect of surface coating

The essential work of fracture (EWF) approach was adopted to reveal the effect of nanofillers on the toughness of poly (?‐caprolactone) (PCL)/boehmite alumina (BA) nanocomposites. Synthetic BA particles with different surface treatments were dispersed into the PCL matrix by extrusion melt compounding. The morphology of the composites was studied by scanning electron microscopy. Differential scanning calorimetry and wide‐angle X‐ray scattering were used to detect changes in the crystalline structure of PCL. Also, mode I type EWF tests, dynamic mechanical analysis, and quasi‐static tensile tests were applied to study the effect of the BA nanofillers on the mechanical properties of the nanocomposites. BA was homogeneously dispersed and acted as heterogeneous crystallization nucleant and a nonreinforcing filler in PCL. The tensile modulus and yield strength slightly increased and the yield strain decreased with increasing BA content (up to 10 wt %). The effect of the BA surface treatment with octylsilane was negligible by contrast to that with alkylbenzene sulfonic acid (OS2). Like the tensile mechanical data, the essential and nonessential work of fracture parameters did not change significantly either. The improved PCL/BA adhesion in case of OS2 treatment excluded the usual EWF treatise. This was circumvented by energy partitioning between yielding and necking. The yielding‐related EWF decreased, whereas the nonessential EWF increased with BA content and with better interfacial adhesion. This was attributed to the effect of matrix/filler debonding. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Study on interfacial debonding stress and damage mechanisms of C/SiC composites using acoustic emission

Among ceramic matrix composites (CMCs), carbon fiber-reinforced silicon carbide matrix (C/SiC) composites are widely used in numerous high-temperature structural applications because of their superior properties. The fiber–matrix (FM) interface is a decisive constituent to ensure material integrity and efficient crack deflection. Therefore, there is a critical need to study the mechanical properties of the FM interface in applications of C/SiC composites. In this study, tensile tests were conducted to evaluate the interfacial debonding stress on unidirectional C/SiC composites with fibers oriented perpendicularly to the loading direction in order to perfectly open the interfaces. The characteristics of the material damage behaviors in the tensile tests were successfully detected and distinguished using the acoustic emission (AE) technique. The relationships between the damage behaviors and features of AE signals were investigated. The results showed that there were obviously three damage stages, including the initiation and growth of cracks, FM interfacial debonding, and large-scale development and bridging of cracks, which finally resulted in material failure in the transverse tensile tests of unidirectional C/SiC composites. The frequency components distributed around 92.5 kHz were dominated by matrix damage and failure, and the high-frequency components distributed around 175.5 kHz were dominated by FM interfacial debonding. Based on the stress and strain versus time curves, the average interfacial debonding stress of the unidirectional C/SiC composites was approximately 1.91 MPa. Furthermore, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDXS) were used to observe the morphologies and analyze the chemical compositions of the fractured surfaces. The results confirmed that the fiber was completely debonded from a matrix on the fractured surface. The damage behaviors of the C/SiC composites were mainly the syntheses of matrix cracking, fiber breakage, and FM interfacial debonding.