Quantitative fractography of mixed mode fracture in glass and ceramics

Many ceramic structural applications involve a combination of tensile (mode I) and shear (mode II) loading conditions. Quantitative fractography was performed on monolithic and R-curve materials. Soda-lime-silica glass was selected as the monolithic material. A mica glass ceramic was selected to represent R-curve materials. Mixed-mode fracture surfaces in both materials were characterized by an absence of the mist region. For the mica glass ceramic, crack-to-mirror size ratios were found to be a function of the crack length and mode mixity. Hackle markings on mixed-mode fracture surfaces appear as lances and differ from those observed on surfaces failed in pure Mode I. Atomic force microscopy showed that the features in different regions on the mixed-mode fracture surfaces are similar and differ only by scale. The practical implications of these observations are that forensic analyses can be used without a priori knowledge of the loading conditions.     

Deformation and fracture behaviour of a rubber-toughened epoxy: 1. Microstructure and fracture studies

The microstructure and fracture behaviour of an unmodified and a rubber-modified epoxy have been studied. Values of the stress intensity factor, K_Ic, at the onset of crack growth, the type of crack growth, and the detailed nature of the associated fracture surfaces have been ascertained. Both materials exhibit essentially the same types of crack growth but the values of K_Ic for the rubber-modified material were usually significantly higher than those for the nmodified epoxy. The mechanisms for this increased toughness have been considered and a mechanism that accounts for all the observed characteristics has been proposed.     

环氧胶粘剂的韧性与增韧机理

本文通过测定三种典型环氧结构胶粘剂：环氧-聚矾，环氧-丁腈40环氧-CTBN的动态力学性能，剥离强度和断裂韧性，观察其断面的电镜照片，分析了三者不同的韧性特点和增韧机理。并发现了PSF改性胶固化后奇特的两相结构。     

Deformation and fracture behaviour of a rubber-toughened epoxy: 2. Failure criteria

In part 1 the microstructure and fracture characteristics of a rubber-modified epoxy, and for comparison that of the unmodified epoxy, were examined in detail. Based on this analysis a qualitative mechanism involving cavitation, shear yielding and plastic flow was proposed. As an extension of this work, the present paper considers the yield behaviour of the epoxy material and uses the data determined, together with the previously reported fracture results, to calculate values of the crack opening displacement. The rate/temperature dependence of the crack opening displacement and the correlations established between stress intensity factor, K_Ic, yield stress and type of crack growth suggest that the extent of crack tip blunting largely governs the relative toughness of the epoxy materials and induces transitions in the types of crack growth observed. A quantitative expression is then presented which successfully describes the fracture toughness values over a wide range of temperatures and rates. The two parameters in this expression are shown to be material constants and therefore provide a unique failure criterion. They can be viewed simply as curve-fitting parameters but they may also have some significance in terms of a critical stress that must act over a critical distance ahead of the crack tip to produce crack growth.     

Effect of Bondline Thickness on Mixed-Mode Fracture of Adhesively Bonded Joints

The effect of bondline thickness on the critical strain energy release rate (CSERR) was investigated using aluminum adherends and an epoxy adhesive. Complete mixed Mode I/II fracture envelopes for adhesive thicknesses ranging from 0.203 to 1.52 mm were developed using double cantilever beam (DCB), mixed-mode bending (MMB), and end notch flexure (ENF) tests methods. Bondline thickness strongly affects the CSERR for different amounts of Mode II component. Pure Mode II had the largest CSERR and showed monotonic increase with bondline thickness, whereas pure Mode I had the lowest CSERR and exhibited non monotonic relationship with bondline thickness. The shape and size of the plastic zone that develops prior to crack propagation was predicted by finite element analysis. Variation of CSERR as a function of adhesive layer thickness was shown to be related to the size of the plastic zone for Mode I and MMB 50% fracture. No correlation was observed for the MMB 75% and Mode II, however.     

Mixed-mode fracture behavior of glass fiber reinforced polymer concrete

Fracture toughness of chopped strand glass fiber reinforced particle-filled polymer composite beams was investigated in Mode I and Mode III loading conditions using three-point bend tests. Effects of crack angles on fracture behavior were also studied. The specimens, which have inclined crack at an angle θ to the axis of the specimens, were used to carry out the tests. The specimens were tested with inclination angles 30°, 45°, 60° and 75°. The results are compared with the values of K_IC obtained using conventional (θ=90° ) specimens. In addition, J integrals were also determined. J_IC increases continuously with increasing in crack angle from θ=30° to θ=90°. In contrast, J_IIIC decreases with the crack inclination angle θ from 30° to 90°.     

Evaluation of the weld-line strength of thermoplastics by compact tension test

In order to understand the relationship between processing conditions and the properties of weld-lines on a molecular level, it is necessary to evaluate the true strength of the weld-line that is not affected by the V-shape notch on the surface of the weld-line zone. In this experiment, the weld-line strength of several brittle, ductile, or phase-separated polymers was evaluated using the compact tension test by measuring the critical stress intensity factor, K_IC, or the critical J-value, J_IC, and the results were compared with those obtained by tensile testing. For brittle polymers such as poly(methyl methacrylate) (PMMA) or styrene acrylonitrile copolymer (SAN), the value of the weld-line factor, i.e., the strength ratio between the welded and the non-welded specimen, is higher than that measured by tensile testing, because of the notch sensitivity of brittle thermoplastics and the notch dependence of tensile strength. On the other hand, in the case of ductile polymers such as polycarbonate (PC), the weld-line factor is similar for both the tensile and compact tension tests. However, the dependency of the weld-line factor on melt temperature is more obvious in the compact tension test. From these results, it seems that the compact tension test is more appropriate for measuring the interfacial adhesion strength across the weld-line, which excludes the notch effect.     

A discrete crack approach to normal/shear cracking of concrete

This paper presents a numerical procedure for mixed mode fracture of quasi-brittle materials. The numerical procedure is based on the cohesive crack approach and extends it to mixed mode fracture. The crack path is obtained, and the mixed mode fracture model is incorporated into the crack path. The crack model is based on the formulation of the classical plasticity. The model is incorporated into a commercial finite element code by an user subroutine and is contrasted with experimental results. The numerical results agree quite well with two experimental sets of mixed mode fracture of concrete beams; one from Arrea and Ingraffea, the other from a nonproportional loading by the authors. Two other sets of experimental fracture results were modeled based on double-edge notched testing. The numerical procedure, mainly based on standard properties of the material measured by standard methods, predicts the experimental records of the load versus displacement at several control points of the specimens for three homothetic sizes of specimen.     

Strength of epoxy-resin-based insulation systems in transverse tension and shear using two novel test pieces

A novel test specimen type has been developed to generate engineering data on bonded aluminium surfaces. The test is designed to simulate conditions in a superconducting coil, operating at 4.2 K, in a region where a glass/epoxy composite laminate is loaded primarily in through-thickness tension and in shear. We include the effects of thermal contraction from the cure temperature to the operating temperature of 4.2 K. The test specimen avoids the large stresses associated with the edges of the composite in other through-thickness tension test pieces and results calculated from the measured failure loads of specimens are the peak stresses in the centre of the specimen.     

Cure behavior of epoxy/MWCNT nanocomposites: The effect of nanotube surface modification

The effect of carboxyl and fluorine modified multi-wall carbon nanotubes (MWCNTs) on the curing behavior of diglycidyl ether of bisphenol A (DGEBA) epoxy resin was studied using differential scanning calorimetry (DSC), rheology and infrared spectroscopy (IR). Activation energy (E_a) and rate constants (k) obtained from isothermal DSC were the same for the neat resin and fluorinated MWCNT system (47.7 and 47.5 kJ/mol, respectively) whereas samples containing carboxylated MWCNTs exhibited a higher activation energy (61.7 kJ/mol) and lower rate constant. Comparison of the activation energies, rate constants, gelation behavior and vitrification times for all of the samples suggests that the cure mechanisms of the neat resin and fluorinated sample are similar but different from the carboxylated sample. This can be explained by the difference in how the fluorinated nanotubes react with the epoxy resin compared to the carboxylated nanotubes. Although the two systems have different reaction mechanisms, both systems have similar degrees of conversion as calculated from the infrared spectroscopic data, glass transition temperature (T_g), and predictions based on DSC data. This difference in reaction mechanism may be attributed to differences in nanotube dispersion; the fluorinated MWCNT system is more uniformly dispersed in the matrix whereas the more heterogeneously dispersed carboxylated MWCNTs can hinder mobility of the reactive species and disrupt the reaction stoichiometry on the local scale.     

Moisture sorption-desorption-resorption characteristics and its effect on the mechanical behavior of the epoxy system

Thermosetting epoxy resins are attractive materials for many engineering applications, as they are low in density, with excellent mechanical properties and easily fabricated by processes such as injection molding, extrusion and vacuum forming. However, the hostile hygrothermal environment can degrade the epoxy system. In this study, moisture sorption-desorption-resorption characteristics of the DGEBA/DDA epoxy system have been investigated by the hygrothermal aging and molecular dynamic (MD) simulation. Also, the effects of moisture on the mechanical behavior of the epoxy system have been studied by the uniaxial tensile test and a scanning electron microscopy (SEM), for the unaged, moisture saturated, completely desorbed and moisture re-saturated specimens, respectively. Results show that the moisture diffusion in epoxy system is not only dependent on the hygrothermal conditions, but also on the specimen thickness and hygrothermal history. Due to the effect of the hygrothermal aging, both the tensile elastic module and tensile strength of the studied epoxy system have been reduced, that is, the absorbed moisture has deleterious effects on the physical properties of epoxies and can, therefore, greatly compromise the performance of an epoxy-based component.     

Fracture behavior of carbon/epoxy laminated composite reinforced by iron powder

The fracture behaviors of a newly developed iron-powder reinforced carbon/epoxy laminated composite are investigated in this paper. Three kinds of DCB (double cantilever beam) specimens (without iron powder, with iron powder and with iron powder in a magnetic field) were prepared by the ASTM D 5528-94a. For the third DCB specimen, the unidirectional laminas were stacked with iron powder spread evenly on each lamina’s surface. This process was performed in a magnetic field to keep the iron powder standing along the out-plane direction. From the test data of Instron 5567, the fracture toughness, G _I , was calculated by using the compliance calibration method for each of the three kinds of specimens. The calculated fracture toughness shows that the iron powder effectively disturbs the progress of fiber branching between the laminates and provides a good stitching to the in-plane laminates during the fracture.     

Creep response of thixotropic ambient temperature cure adhesives measured by DMTA in static tension and shear

The technology for bonding in rods into timber structures for repair, reinforcement and forming primary connections is now well established. An ambient temperature cure adhesive is required for bonding on site and for overhead application thixotropic (shear thinning) characteristics are essential. At the same time bonded-in components may experience service temperatures of 50 °C or more, especially in roof spaces. It is commonly supposed that an adhesive with a glass transition temperature (Tg) below the in-service temperature will suffer from potential creep unless it is tightly cross-linked. In this paper the creep properties of three epoxy-based, thixotropic adhesives are investigated, which are less heavily cross-linked and possess Tg values between 30 and 60 °C. The adhesives are subjected to a creep load in tension within a Dynamic Mechanical Thermal Analyser used in static mode with a step-wise increase in temperature and a range of stress levels. A unique laminated shear specimen has been developed comprising an adhesive layer sandwiched by two thin wood veneers so that the adhesive layer can be stressed in shear. The results demonstrate that in the temperature range between Tg and Tg+15 °C the thixotropic adhesives creep to a limit, behaving as classic viscoelastic polymers and above Tg+15 °C they behave like rubbers with no creep. At high stresses and temperatures the adhesives eventually fail by rupture of the adhesive bonds. In conclusion, thixotropic adhesives are seen to possess a unique combination of physical and chemical properties, which enable them to function above Tg under creep load.     

The influence of clay and elastomer concentration on the morphology and fracture energy of preformed acrylic rubber dispersed clay filled epoxy nanocomposites

The influence of toughener and clay concentration on the morphology and mechanical properties of three-phase, rubber-modified epoxy nanocomposites was studied. Nanocomposite samples were prepared by adding octadecyl ammonium ion exchanged clay to a dispersion of pre-formed acrylic rubber particles in liquid epoxy, so as to minimize alteration to the rubber morphology in the final cured specimen. The state of clay platelet exfoliation and rubber dispersion in the cured nanocomposites was studied using transmission electron microscopy. The amounts of clay platelet separation and dispersion of clay aggregates in the epoxy matrix were found to be sensitive to clay and toughener concentration, and clay platelets preferentially adsorb to the rubber particles. Tensile modulus and strength increase and ductility decreases with increasing organoclay content, while rubber has the opposite effects on the properties of epoxy resin. When both additives are present in epoxy resin, a favorable combination is produced: ductility is enhanced without compromising modulus and strength. Modulus and strength are improved by nano and micro dispersion of nanoclay in the epoxy matrix, whereas elongation and toughness are improved by clay adsorption to the rubber particle surface, which promotes cavitation. The glass transition temperature of epoxy resin remains relatively unchanged with clay addition.     

The relationship between dielectric and rheological behavior of epoxy resin during cure

Measurement of the frequency-dependent, vector voltage (V_c) provided an in-situ and non-destructive technique to measure continuously the rheological change of a resin due to polymerization, and can be used as the basis of real-time control. The vector voltage depends on the degree of polarization of the dipolar molecules and on the change of viscosity during cure; both result from the modified structure of the epoxy resin during cure, The initial stage of curing, represented by the former portion Of the Vc curve (divided at the minimum of the Vc curve), was caused mainly by the effects of temperature and viscosity. During the latter stage of the cure reaction, Vc alters because of the effect of the lightened matrix structure that inhibits alignment of dipoles. The duration of reaction. temperature of curing and degree of conversion all have the same effects on both vector voltage and viscosity, The minimum value of vector voltage is correlated to the minimum viscosity, and there is a nearly quantitative relationship between them, One can determine the viscosity of the epoxy resin during cure from reading of the vector voltage. Various reaction mechanisms may be explained based on the graphs of vector voltage of various types.     

On the effect of pulsed laser ablation on shear strength and mode I fracture toughness of Al/epoxy adhesive joints

The present work describes an experimental study about the shear strength and the mode I fracture toughness of adhesive joints with substrates pre-treated by pulsed laser ablation. An ytterbium-doped pulsed fiber laser was employed to perform laser irradiation on AA6082-T4 alloy. Morphological and chemical modifications were evaluated by means of surface profilometry, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Thick adherend shear tests were carried out in order to assess the shear strength while the mode I fracture toughness was determined using the double cantilever beam. For comparison, control samples were prepared using classical surface degreasing. The results indicated that laser ablation has a favorable effect on the mechanical behavior of epoxy bonded joints; however, while a + 20% increase was recorded for shear strength, a remarkable threefold enhancement of fracture toughness was observed with respect to control samples. XPS analyses of treated substrates and SEM observations of the fracture surfaces indicated that laser pre-treatment promoted chemical and morphological modifications able to sustain energy dissipation through mechanical interlocking. As a result cohesive failure within the adhesive bond-line was enabled under predominant peel loading.     

Influence of substrate on the cathodic electrodeposition behavior of waterborne epoxy resins

A modified epoxy–amine adduct was prepared and was then emulsified in water, and was subsequently deposited on a cathode substrate at constant voltage (200 V) or constant current (1 mA/cm²) by the aid of a DC power supply. The cathode was made of different substrates such as bare steel, phosphated steel and aluminum. The results show that the film conductance was greatest on the aluminum substrate which was followed by bare steel and phosphated steel.     

Experimental and numerical investigations of crack face adhesive bonding effect on the mixed-mode fracture strength of PMMA

Experimental and numerical investigations have been conducted to evaluate the effect of adhesive bonding of crack surfaces on the mixed-mode (I and II) fracture strength and effective stress intensity geometry/loading factor of a plate with an edge crack. The experimental tests were carried out on five batches of simple edge crack and specimens in which adhesive bonding was used on crack faces at different distances from the crack tip. The cracked specimens made from poly methyl-methacrylate rectangular plates. The specimens’ fracture strength was obtained by employing a tensile testing machine at different loading angles using a modified Arcan fixture. In the numerical part, finite element simulations were used to model the test specimens and thereby establishing their stress intensity geometry/loading factors. The results show that the adhesive bonding of the crack surfaces has a significant effect on reducing the equivalent mixed-mode stress intensity factor for all loading angles. The bonded specimens show considerable fracture force enhancement compared to the simple edge crack specimens.     

Analysis of the fracture behavior of epoxy resins under impact conditions

In this work an analysis of the fracture behavior under impact of four epoxy resins was performed. The morphology of the fracture surfaces was analyzed by scanning electron microscopy and the topographic marks observed could be related to the thermal behavior of each epoxy system. The relevant properties that determine the thermal behavior were the thermal diffusivity, which was measured by using the open photoacoustic cell technique, and the glass transition temperature. As the thermal diffusivity of these materials is very low, and therefore also is their heat dissipation capacity, the impact test occurs under adiabatic conditions and a temperature increase occurs at the tip of the running cracks. Therefore, thermal blunting may occur at the crack tip and the energy absorption capacity of the resins is increased. The topographic marks observed at the fracture surface help to identify how efficient this mechanism is for each of the epoxy systems analyzed. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 2486–2492, 2000     

Fracture behavior of silicon nitride ceramics under combined compression-torsion stresses analyzed by multiaxial fracture statistics

Combined compression-torsion tests were performed on the thermal-treated and as-machined silicon nitride ceramics to investigate their fracture behavior under multiaxial stress states. The thermal-treated samples showed considerable high strength and low anisotropy to the grinding direction in flexure tests compared to the as-machined samples. Under combined compression and torsion stress states, the thermal-treated samples showed considerably higher tensile strength than that of as-machined samples at low compressive stress states and weakening with increasing compression stress. The as-machined samples showed little decrease in tensile strength with increasing compression stress and comparable tensile strength with the thermal-treated samples under a highly compressive stress state. The behavior of thermal-treated samples were well described by the statistical theory of multiaxial fracture for volume-distributed flaws combined with a mixed-mode fracture criterion with the shear sensitivity constant of 1.75 and 1.65 for Shetty’s criterion and the ellipsoidal criterion, respectively.