STRAIN RATE DEPENDENCE OF FRACTURE IN A RUBBER-TOUGHENED EPOXY SYSTEM

The toughening mechanisms in rubber-modified epoxies appear to be viscoelastic in nature since their fracture behavior is dependent on loading rate. This behavior has been studied in detail and modeled for only one system, a model toughened epoxy often used in research work. The present study examines the loading rate effect for a new material based on acrylic rubber by measuring the fracture energy in constant cross-head speed tests conducted over a wide range of speeds. As expected, decreasing the loading rate produced an increase in toughness. Just as in the previous studies, the fracture energies could be modeled with a power law relationship when the loading rate was characterized by the time of failure. Moreover, the parameters involved in the model are quite consistent with the earlier results. For most rates, the behavior was approximately linear elastic with little or no r-curve behavior. Below a critical rate, however, there was a transition to ductile failure with a large r-curve and very high fracture energies. The transition is very sudden which may help explain why some previous studies have observed this effect while others have not.     

Time Dependent Crack Growth and Loading Rate Effects on Interfacial and Cohesive Fracture of Adhesive Joints

Double cantilever beam fracture specimens were used to investigate rate dependent failures of model epoxy/steel adhesively bonded systems. Quasi-static tests exhibited time dependent crack growth and the maximum fracture energies consistently decreased with debond length for constant crosshead rate loading. It was also possible to cause debonding to switch between interfacial and cohesive failure modes by simply altering the loading rate. These rate dependent observations were characterized using the concepts of fracture mechanics. The time rate of change of the strain energy release rate, dG/dt, is introduced to model and predict failure properties of different adhesive systems over a range of testing rates. An emphasis is placed on the interfacial failure process and how rate dependent interfacial properties can lead to cohesive failures in the same adhesive system. Specific applications of the resulting model are presented and found to be in good agreement when compared with the experimental data. Finally, a failure envelope is identified which may be useful in predicting whether failures will be interfacial or cohesive depending on the rate of testing for the model adhesive systems.     

Contents Lists and Abstracts from the Journal of the Adhesion Society of Japan

Double cantilever beam fracture specimens were used to investigate rate dependent failures of model epoxy/steel adhesively bonded systems. Quasi-static tests exhibited time dependent crack growth and the maximum fracture energies consistently decreased with debond length for constant crosshead rate loading. It was also possible to cause debonding to switch between interfacial and cohesive failure modes by simply altering the loading rate. These rate dependent observations were characterized using the concepts of fracture mechanics. The time rate of change of the strain energy release rate, dG/dt, is introduced to model and predict failure properties of different adhesive systems over a range of testing rates. An emphasis is placed on the interfacial failure process and how rate dependent interfacial properties can lead to cohesive failures in the same adhesive system. Specific applications of the resulting model are presented and found to be in good agreement when compared with the experimental data. Finally, a failure envelope is identified which may be useful in predicting whether failures will be interfacial or cohesive depending on the rate of testing for the model adhesive systems.     

Influence of triblock copolymer interfacial modifiers on fracture behavior of polystyrene/ethylene-propylene rubber blends

In a previous study, two triblock copolymers of styrene/ethylene-butylene/styrene (SEBS), of different molecular weights, were used to compatibilize a blend of 80 vol% polystyrene (PS) and 20% ethylene-propylene rubber (EPR). The emulsification curve, which relates the average minor phase particle diameter to the concentration of interfacial agent added, was used to quantify the effect of the interfacial agents on the blend morphology. Links between morphology, interface, and properties were established by combining the emulsification curve with a fracture mechanics approach. The aim of this work is to foster the understanding of the effects of these two triblock copolymers on the fracture behavior of the blend over various loading rates and temperatures. The focus is on the brittle-ductile transition in fracture behavior, which is a critical condition for the application of these materials. It has been found that adding an interfacial agent lowers the temperature at brittle-ductile transition. However, this effect is much more pronounced for the copolymer with a lower molecular weight. The time-temperature dependence of fracture performance of the blend is also affected by the interface and morphology. When loading rate increases, the shift of the temperature at brittle-ductile transition is less significant for the blend with an interfacial agent having a lower molecular weight. The effect of loading rate and temperature on the brittle-ductile transition in fracture performance of the blends is controlled by an energy activation process. Adding the interfacial agents results in a plasticizing effect of the polystyrene matrix and a reduction in the energy barrier controlling the fracture process. With the addition of interfacial agent, the yield stress slightly increases at low concentration, attains a maximum value, and then decreases. The increase in yield stress confirms the coupling role of the copolymer and is in agreement with the observed emulsification curves. The reduction of yield stress and increase in ultimate strain with the copolymer concentration demonstrate the plasticizing effect of the interfacial agent. The result of stress relaxation tests also confirms the above effects of the interfacial agent.     

Impact of characteristic length and loading rate upon dynamic constitutive behavior and fracture process in alumina ceramics

Understanding the impact performance of ceramic materials requires accurate corresponding relationship between mechanical response and fracture behavior. In this study, constitutive behaviors of alumina ceramics were successfully determined via split-Hopkinson pressure bar (SHPB) system coupled with high-speed camera to track the deformation and failure process. Failure strength of alumina demonstrated a strong dependency on strain rate beyond a critical value (namely transition strain rate). Inelastic deformation in the dynamic stress-strain curves implied that degradation of modulus does occur. The incorporating such degradation (damage evolution) in modulus enabled a more accurate evaluation of transition strain rate as a function of characteristic length of specimen. On-line observation revealed that longitudinal cracks dominated the failure process of alumina with negligible interfacial friction. However, interfacial friction became significant with the decreased characteristic length, thus the inclined cracks dominated fracture in alumina. It was found that the effect of interfacial friction can be minimized by lowering the impact velocity to maintain the uniaxial loading status in SHPB loads. Finally, it is suggested that an aspect ratio of 1.0 for the specimen should be suitable for alumina due to its insensitivity to interfacial friction within the achievable strain rate.     

Effect of molecular weight between crosslinks on the fracture behavior of rubber‐toughened epoxy adhesives

The effect of molecular weight between crosslinks, M_c, on the fracture behavior of rubber‐toughened epoxy adhesives was investigated and compared with the behavior of the bulk resins. In the liquid rubber‐toughened bulk system, fracture energy increased with increasing M_c. However, in the liquid rubber‐toughened adhesive system, with increasing M_c, the locus of joint fracture had a transition from cohesive failure, break in the bond layer, to interfacial failure, rupture of the bond layer from the surface of the substrate. Specimens fractured by cohesive failure exhibited larger fracture energies than those by interfacial failure. The occurrence of transition from cohesive to interfacial failure seemed to be caused by the increase in the ductility of matrix, the mismatch of elastic constant, and the agglomeration of rubber particles at the metal/epoxy interface. When core‐shell rubber, which did not agglomerate at the interface, was used as a toughening agent, fracture energy increased with M_c. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 38–48, 2001     

Fracture Energy of GlassAlumina Interfaces via the Bimaterial Bend Test

The fracture energies of glassalumina interfaces were measured using the bimaterial bend test. These experiments comprise one of the few studies in which the bimaterial bend test has been applied to an all-ceramic materials system. The experiment was used to evaluate the effect of materials purity, processing and environmental conditions on the interface toughness. Changes in interface fracture energy were measured as a function of glass content, interface roughness, phase angle, and testing atmosphere. Alumina glass content and testing atmosphere had the greatest effect on the interface toughness and the overall mechanical behavior of the composite. The effect of phase angle of loading on the interface fracture energy was assessed over several degrees by varying the relative heights of the two layers. The results from these interface fracture experiments offer insight into the fracture behavior of multiphase and composite ceramics where deflection of cracks at bimaterial interfaces is an important concern.     

An anomaly in the necking behavior of polyethylene,part 4

It is shown that the annealing of a high molecular weight, high density polyethylene at different temperatures ranging from 393.2 to 405.2 K influences the density of the material, the lamellar structure as studied by differential scanning calorimetry and transmission electron microscopy, and the necking and fracture behavior at constant uniaxial tensile loading in air at 313 K. In previous reports, a marked transition in the necking and fracture behavior of high density, high molecular weight polyethylene under constant uniaxial tensile loading has been reported. The nominal stress and the maximum strain rate of this transition show minima for polyethylenes annealed at temperatures of about 401 K. By combining these data with data for the lamellar structure a hypothesis that explains the necking/fracture behavior is set up. The heat treatment at temperatures from 393.2 to 403.2 K of the original non-equilibrium lamellar structure causes a molecular fractionation preferentially of low molecular weight and branched material. These segregated parts may then act as fracture initiators and thus lower the resistance towards fracture. Other structural effects such as those proposed by McCready and co-workers may also be of importance. The fracture curves at nominal stresses below transition of the materials annealed at 396.7 and 401.2 K for 24 h are shifted to shorter times in comparison with that of the non-annealed material and this can also be explained by molecular fractionation. The time to necking at 14 MPa nominal stress seems to be related to the lamellar thickness of the samples.     

Temperature and Loading Rate Effects on the Fracture Behaviour of Adhesively Bonded GFRP Nylon-6,6

The combined effect of varying test temperature and loading rate on the Mode II fracture toughness of plasma-treated GFRP Nylon-6,6 composites bonded using a silica-reinforced epoxy adhesive has been studied. End notch flexure tests have shown that the adhesive system used in this study offers a wide range of fracture energies that are extremely sensitive to changes in temperature and loading rate. Increasing the test temperature resulted in a substantial reduction in the Mode II fracture toughness of the adhesive, with the value of G_IIc at 60°C being approximately one-half of the room temperature value. In contrast, increasing the crosshead displacement rate at a given temperature has been shown to increase the value of G_IIc by up to 250%. Compression tests performed on bulk adhesive specimens revealed similar trends in the value of [sgrave]_y with temperature and loading rate. In addition, it was found that the plasma treatment employed in this study resulted in stable crack propagation through the adhesive layer under all testing conditions.

A more detailed understanding of the effect of varying temperature and loading rate on the failure mechanisms occurring at the crack tip was achieved using the double end notch flexure (DENF) geometry, which was considered in tandem with the fracture surface morphologies. Here, changes in the degree of matrix shear yielding and particle-matrix debonding were used to explain the trends in [sgrave]_y and G_IIc.     

The effect of temperature and loading rate on the mode II interlaminar fracture properties of a carbon fiber reinforced phenolic

The combined effect of varying loading rate and test temperature on the mode II Interlaminar fracture properties of a carbon fiber reinforced phenolic resin has been investigated. End notch flexure tests at room temperature have shown that this composite offers a relatively modest value of G_IIc^NL at non‐linearity and that its interlaminar fracture toughness decreases with increasing loading rate. As the test temperature is increased, the quasistatic value of G_IIc^NL increases steadily and the reduction in G_IIc^NL with loading rate becomes less dramatic. At temperatures approaching the glass transition temperature of the phenolic matrix, the interlaminar fracture toughness of the composite begins to increase sharply with crosshead displacement rate. A more detailed understanding of the effect of varying the test conditions on the failure mechanisms occurring at the crack tip of these interlaminar fracture specimens has been achieved using the double end notch flexure (DENF) geometry.     

PVA/PVAc复合黏结剂制备Li1.075Nb0.625Ti0.45O3水基流延膜

采用PVA/PVAc复合黏结剂制备Li_1.075Nb_0.625Ti_0.45O₃微波介质陶瓷基片。研究了PVAc用量和固相含量对浆料流变性能以及流延膜片力学性能的影响。结果表明,随着PVAc添加量的增加,浆料的黏度和屈服应力不断下降,膜片的断裂方式由韧性断裂转变为脆性断裂。随着浆料固相含量的增大,浆料的流变模型从Bingham型转变为Casson型。SEM观察表明,膜片的微观结构非常均匀。     

The Influence of Surfactants on the Peel Strength of Water-based Pressure Sensitive Adhesives

In order to study the effect of surfactants on the adhesive properties, peel measurements were performed with two series of model polymers of ethylhexylmethacrylate (PEHMA), the first prepared by emulsion polymerization with four anionic surfactants, and the second by post-adding the same surfactants to a surfactant-free latex. Cohesive fracture is observed at low peel rates; the peel strength depends on the bulk mechanical properties and is independent of the emulsifier. A transition to another type of separation occurs at higher peel rates, which seems to be an interfacial failure by visual inspection. Surface analytical studies, however, give evidence that this “interfacial” failure is, in fact, a mixed failure, leaving traces of the polymer on the substrate surface. The peel rate at this transition as well as the peel strength at mixed fracture are influenced by the surfactants. Large differences were observed between the four surfactants as well as between both series of polymers, leading to the conclusion that the surfactants have a different mobility within the film. This is also reflected by a different aging behaviour of the films.     

Contents Lists and Abstracts from the China Journal “Chemistry and Adhesion”

In order to study the effect of surfactants on the adhesive properties, peel measurements were performed with two series of model polymers of ethylhexylmethacrylate (PEHMA), the first prepared by emulsion polymerization with four anionic surfactants, and the second by post-adding the same surfactants to a surfactant-free latex. Cohesive fracture is observed at low peel rates; the peel strength depends on the bulk mechanical properties and is independent of the emulsifier. A transition to another type of separation occurs at higher peel rates, which seems to be an interfacial failure by visual inspection. Surface analytical studies, however, give evidence that this “interfacial” failure is, in fact, a mixed failure, leaving traces of the polymer on the substrate surface. The peel rate at this transition as well as the peel strength at mixed fracture are influenced by the surfactants. Large differences were observed between the four surfactants as well as between both series of polymers, leading to the conclusion that the surfactants have a different mobility within the film. This is also reflected by a different aging behaviour of the films.     

Quasi-static and dynamic compressive fracture behavior of carbon/carbon composites

To understand the dynamic compressive fracture behavior of carbon/carbon composites, their compressive behavior was investigated at a strain rate of 500/s using a modified split Hopkinson pressure bar. Quasi-static compressive tests were conducted on a universal test machine and compared with those at high strain rate. Scanning electron microscopy was used to observe the compressive fracture surfaces. The results show that the compressive strength and stiffness are increased at high strain rate. Fiber failure under quasi-static compressive loading is characterized by fiber bundle debonding, breakage and pull-out, while the fiber failure under dynamic compressive loading is characterized by multiple splitting without extensive debonding.     

Crack Path Selection in Adhesively-Bonded Joints: The Role of Material Properties

This paper investigates the role of material properties on crack path selection in adhesively bonded joints. First, a parametric study of directionally unstable crack propagation in adhesively-bonded double cantilever beam specimens (DCB) is presented. The results indicate that the characteristic length of directionally unstable cracks varies with the Dundurs' parameters characterizing the material mismatch. Second, the effect of interface properties on crack path selection is investigated. DCB specimens with substrates treated using various surface preparation methods are tested under mixed mode fracture loading to determine the effect of interface properties on the locus of failure. As indicated by the post-failure analyses, debonding tends to be more interfacial as the mode II fracture component in the loading increases. On the other hand, failures in specimens prepared with more advanced surface preparation techniques appear more cohesive for given loading conditions. Using a high-speed camera to monitor the fracture sequence, DCB specimens are tested quasi-statically and the XPS analyses conducted on the failure surfaces indicate that the effect of crack propagation rate on the locus of failure is less significant when more advanced surface preparation techniques are used. The effect of asymmetric interface property on the behavior of directionally unstable crack propagation in adhesive bonds is also investigated. Geometrically-symmetric DCB specimens with asymmetric surface pretreatments are prepared and tested under low-speed impact. As indicated by Auger depth profile results, the centerline of the crack trajectory shifts slightly toward the interface with poor adhesion due to the asymmetric interface properties. Third, through varying the rubber content in the adhesive, DCB specimens with various fracture toughnesses are prepared and tested. An examination of the failure surfaces reveals that directionally unstable crack propagation is more unlikely to occur as the toughness of the adhesive increases, which is consistent with the analytical predictions that were discussed using an energy balance model.     

Crack Path Selection in Adhesively-Bonded Joints: The Role of Material Properties

This paper investigates the role of material properties on crack path selection in adhesively bonded joints. First, a parametric study of directionally unstable crack propagation in adhesively-bonded double cantilever beam specimens (DCB) is presented. The results indicate that the characteristic length of directionally unstable cracks varies with the Dundurs' parameters characterizing the material mismatch. Second, the effect of interface properties on crack path selection is investigated. DCB specimens with substrates treated using various surface preparation methods are tested under mixed mode fracture loading to determine the effect of interface properties on the locus of failure. As indicated by the post-failure analyses, debonding tends to be more interfacial as the mode II fracture component in the loading increases. On the other hand, failures in specimens prepared with more advanced surface preparation techniques appear more cohesive for given loading conditions. Using a high-speed camera to monitor the fracture sequence, DCB specimens are tested quasi-statically and the XPS analyses conducted on the failure surfaces indicate that the effect of crack propagation rate on the locus of failure is less significant when more advanced surface preparation techniques are used. The effect of asymmetric interface property on the behavior of directionally unstable crack propagation in adhesive bonds is also investigated. Geometrically-symmetric DCB specimens with asymmetric surface pretreatments are prepared and tested under low-speed impact. As indicated by Auger depth profile results, the centerline of the crack trajectory shifts slightly toward the interface with poor adhesion due to the asymmetric interface properties. Third, through varying the rubber content in the adhesive, DCB specimens with various fracture toughnesses are prepared and tested. An examination of the failure surfaces reveals that directionally unstable crack propagation is more unlikely to occur as the toughness of the adhesive increases, which is consistent with the analytical predictions that were discussed using an energy balance model.     

Fracture behavior of sisal fiber–reinforced starch‐based composites

The fracture behavior of biodegradable fiber–reinforced composites as a function of fiber content under different loading conditions was investigated. Composites with different fiber content, ranging from 5 to 20 wt%, were prepared using commercial starch‐based polymer and short sisal fibers. Quasistatic fracture studies as well as instrumented falling weight impact tests were performed on the composites and the plain matrix. Results showed a significant increase in the crack initiation resistance under quasistatic loading. This was caused by the incorporation of sisal fibers to the matrix and the development of failure mechanisms induced by the presence of the fibers. On the other hand, a modest increasing trend of the resistance to crack initiation with fiber loading was detected. An improved fracture behavior was also observed when the impact loading was parallel to the thickness direction. Under these experimental conditions, the composites exhibited higher values of ductility index, energy at initiation and total fracture energy than the plain matrix. Furthermore, an increasing trend of these parameters with fiber content was detected in the biocomposites. Overall, the addition of sisal fibers to the biodegradable matrix appears to be an efficient mean of improving fracture behavior under both quasistatic and impact loading conditions. POLYM. COMPOS. 26:316–323, 2005. © 2005 Society of Plastics Engineers     

Acoustic emission behavior of epoxies during tensile loading

The acoustic emission behavior during tensile loading of two common epoxy systems of different ductility was investigated at different loading rates. At low threshold voltage, it was possible to register acoustic emissions before the final failure. Only very few emissions were recorded compared with the amount commonly recorded for metals and composite materials. The acoustic emissions detected were of burst-type, revealing a brittle damage accumulation process. They originated from the initiation and incremental growth of microcracks of stochastic nature. The events occurred before gross yielding and during the final “brittle” failure process. Basically no events were detected between gross yielding and the final failure during which large scale yielding, necking, and stable crack growth took place. The occurrence of events at the different loading rates was strongly influenced by the yielding behavior and fracture toughness, characterized by the yield stress σ_y and the plane-strain fracture toughness K_Ic respectively. K_Ic was inversely dependent on the total number of events up to gross yielding. The event distribution normalized with respect to the conditions at gross yielding was hardly affected by the loading rate.     

Notch Strength and Fracture Behavior of 2-D Carbon-Carbon Composites

This study examined Mode I in-plane fracture in two-directional carbon-carbon composites. Linear elastic fracture mechanics (LEFM) provided good predictions for the failure loads of the compact tension specimens, and consistent values of K _IC were calculated. However, values for the strain energy release rate G _IC were inconsistent. Analysis of loading/unloading cycles applied to the specimens showed that significant inelastic behavior occurred prior to fracture. Hence the J -integral method is not thought to be appropriate for fracture prediction. It was also found that the real crack tip stresses in carbon-carbon composites are much lower than theoretical predictions. An interesting discovery is that cracks in carbon-carbon composites appear to be naturally blunt. The lower limit on the crack tip diameter corresponds with the spacing of the woven fiber bundles. A significant conclusion from this study is that the material behavior can be modeled as the sum of two contributions: (i) a linear elastic solid, which fails according to LEFM, and (ii) an independent damage mechanism, which absorbs energy but does not alter the ultimate fracture load. Possible candidates for the damage mode are fiber pullout and inelastic deformations due to shear stresses.     

Temperature and loading rate effects in the mode II interlaminar fracture behavior of carbon fiber reinforced PEEK

The combined effect of varying loading rate and test temperature on the mode II interlaminar fracture properties of AS4/carbon fiber reinforced PEEK has been investigated. End notch flexure tests have shown that this thermoplastic‐based composite system offers a very high value of interlaminar fracture toughness at room temperature. Increasing the test temperature leads to a reduction in the mode II interlaminar fracture toughness of the composite, with the value at 150°C being approximately one half of the room temperature value. In contrast, increasing the crosshead displacement rate has been shown to increase the value of G_IIc by up to 25%. A more detailed understanding of the effect of varying temperature and loading rate on the failure mechanisms occurring at the crack tip of these interlaminar fracture specimens has been achieved using the double end notch flexure (DENF) geometry. Here, extensive plastic flow within the crack tip region was observed in all specimens. It is believed that the rate sensitivity of G_IIc reflects the rate‐dependent characteristics of the thermoplastic resin.