Effective Interfacial Shear Strength Research Articles

Microstructural evolution in graphene nanoplatelets reinforced magnesium matrix composites fabricated through thixomolding process

The distribution characteristic of reinforcements is a critical issue for the final performance of magnesium matrix composites (MMCs). Here, the microstructural evolution of graphene nanoplatelets (GNPs) reinforced MMCs fabricated by thixomolding process was investigated. The GNPs successfully penetrated through the grain boundaries and embedded in two adjacent grains. Those intragranular GNPs/Mg interfaces not only could provide efficient barriers to pin dislocations, but also contribute to the multiplying dislocations to accumulate rather than dissipate. In addition, the finer MgO particles and elongated Mg17Al12 were anchored around the GNPs, which could effectively improve the interfacial load transfer due to the increased load-bearing capability and the effective interfacial shear strength. These special distribution characteristic of GNPs was beneficial to the significant enhancement of mechanical properties for the composites. The concept of intragranular reinforcements anchored by finer particles in this work may bring positive and directive significance for the structural design of the composites.

Tailored glass fiber interphases via electrophoretic deposition of carbon nanotubes: Fiber and interphase characterization

Electrophoretic deposition (EPD) has been used to deposit carbon nanotubes (CNTs) onto glass fibers to strengthen the fiber/matrix interphase of glass/epoxy composites. CNTs were functionalized using an ultrasonicated-ozonolysis followed by polyethyleneimine (PEI) treatment, creating a stable, aqueous dispersion suitable for EPD. Single E-glass fiber filaments were successfully coated with the functionalized CNTs using EPD. Single fiber tensile and microdroplet debond tests were conducted to investigate the tensile properties of the CNT modified E-glass fibers and their interfacial structure and properties in an epoxy matrix, respectively. Weibull analysis of the fiber testing revealed no detrimental effects resulted from the CNT coating, with some evidence to suggest slightly improved strength. Microdroplet tests revealed changes in the fracture modes due to the application of the CNT coating. The interfacial shear-sliding shifted from the fiber/resin interface to the CNT/resin interphase for the CNT modified fibers. Higher effective interfacial shear strength corresponded to the results where fracture propagated deeper into the CNT-rich interphase layer, confirming the trends observed in previous model interphase studies.

Interfacial strength and debonding mechanism between aerogel-spun carbon nanotube yarn and polyphenylene sulfide

The interfacial bonding properties between the carbon nano-tubes yarn and polyphenylene sulfide are investigated using the micro-bond test. Carbon nano-tubes yarn fabricated by floating catalyst chemical vapor deposition with a high Poisson’s ratio of 3.5, and high-performance thermoplastic resin polyphenylene sulfide are used as matrix. In order to improve the tensile strength of the yarn so as to get sufficient data points for the micro-bond test for interfacial bonding strength, a pretreatment that combines drafting and dichloromethane shrinking processes is applied. The pretreated carbon nano-tubes yarn shows a 23% increase in tensile strength (from 117 to 144MPa) and a 260% increase in initial Young’s modulus (from 0.8 to 3.2GPa). The effective interfacial shear strength is calculated to be 13.1MPa and analyzed based on fracture mechanism of a mixed failure mode.

Stiffness, strength and adhesion characterization of electrochemically deposited conjugated polymer films

Conjugated polymers such as poly(3,4-ethylenedioxythiphene) (PEDOT) are of interest for a variety of applications including interfaces between electronic biomedical devices and living tissue. The mechanical properties, strength, and adhesion of these materials to solid substrates are all vital for long-term applications. We have been developing methods to quantify the mechanical properties of conjugated polymer thin films. In this paper, the stiffness, strength and the interfacial shear strength (adhesion) of electrochemically deposited PEDOT and PEDOT-co-1,3,5-tri[2-(3,4-ethylene dioxythienyl)]-benzene (EPh) were studied. The estimated Young’s modulus of the PEDOT films was 2.6±1.4GPa, and the strain to failure was around 2%. The tensile strength was measured to be 56±27MPa. The effective interfacial shear strength was estimated with a shear-lag model by measuring the crack spacing as a function of film thickness. For PEDOT on gold/palladium-coated hydrocarbon film substrates an interfacial shear strength of 0.7±0.3MPa was determined. The addition of 5mole% of a tri-functional EDOT crosslinker (EPh) increased the tensile strength of the films to 283±67MPa, while the strain to failure remained about the same (2%). The effective interfacial shear strength was increased to 2.4±0.6MPa. Statement of significanceThis paper describes methods for estimating the ultimate mechanical properties of electrochemically deposited conjugated polymer (here PEDOT and PEDOT copolymers) films. Of particular interest and novelty is our implementation of a cracking test to quantify the shear strength of the PEDOT thin films on these solid substrates. There is considerable interest in these materials as interfaces between biomedical devices and living tissue, however potential mechanisms and modes of failure are areas of continuing concern, and establishing methods to quantify the strengths of these interfaces are therefore of particular current interest. We are confident that these results will be useful to the broader biological materials community and are worthy of broader dissemination.

The effective interfacial shear strength of carbon nanotube fibers in an epoxy matrix characterized by a microdroplet test

The tensile properties of continuous carbon nanotube (CNT) fibers spun from a CNT carpet consisting of mainly double- and triple-walled tubes, and their interfacial properties in an epoxy matrix, are investigated by single fiber tensile tests and microdroplet tests, respectively. The average CNT fiber strength, modulus and strain to failure are 1.2 ± 0.3 GPa, 43.3 ± 7.4 GPa and 2.7 ± 0.5%, respectively. A detailed study of strength distribution of CNT fiber has been carried out. Statistical analysis shows that the CNT fiber strength is less scattered than those of MWCNTs as well as commercial carbon and glass fibers without surface treatment. The effective CNT fiber/epoxy interfacial shear strength is 14.4 MPa. Unlike traditional fiber-reinforced composites, the interfacial shear sliding occurs along the interface between regions with and without resin infiltration in the CNT fiber. Guidelines for microdroplet experiments are established through probability analysis of variables basic to specimen design.

Evaluation of interfacial shear strength by tensile tests of impregnated flax fiber yarns

Adhesion of flax fibers and polymer matrix as well as mutual bonding of elementary fibers in a technical fiber are among the principal factors governing the mechanical response of flax fiber-reinforced polymer–matrix composites. A method for evaluation of adhesion is proposed based on tension tests of impregnated fiber yarns, with subsequent characterization by optical microscopy of length distribution of fibers pulled out of the yarn fracture surfaces. An elementary probabilistic model is derived relating aspect ratio distribution of the pulled-out fibers to the fiber tensile strength distribution and the effective interfacial shear strength (IFSS). The method was applied to flax fiber/vinylester resin yarns and an estimate of IFSS at 17 MPa was obtained.

Energy-Based Debonding Model for Steel-Concrete Composite Structures

This paper present a energy-based modelling approches for interfacial debonding between steel and concrete. Steel-concrete composite structural member is considered as a generalized elastic body with both the applied load and the interfacial shear stress acting as boundary stresses, and the debonding is modeled as crack propagation along the interface. The energy relationship is discussed in the process of debonding and an energy-based criterion for steel-concrete composite structure is proposed. Following, the debonding process is analyzed through energy-based criterion. The analysis is first performed for special case with constant shear stress along debonded interface, and then for the general case with shear stress softening in the debonded zone. A direct correspondence between energy-based and strength-based analysis can be established for arbitrary softening behavior along the interface. Specifically, through the proper definition of effective interfacial shear strength, the conventional strength-based approach can be employed to give the same results as the much more complicated energy-based analysis.

Energy-Based Modeling Approach for Debonding of FRP Plate from Concrete Substrate

For concrete beams and slabs strengthened with bonded fiber reinforced plastic (FRP) plates, plate debonding from the concrete substrate is a common failure mode. In this paper, the debonding process is modeled as the propagation of a crack along the concrete/adhesive interface, with frictional shear stress acting behind the crack tip. Crack propagation is taken to occur when the net energy release of the system equals the interfacial fracture energy. The analysis is first performed for the special case with constant shear stress along the debonded interface, and then for the general case with slip softening in the debonded zone. From the results, a direct correspondence between energy-based and strength-based analyses can be established for arbitrary softening behavior along the interface. Specifically, through the proper definition of an effective interfacial shear strength, the conventional strength-based approach can be employed to give the same results as the much more complicated energy-based analysis. Also, based on the relation between the effective shear strength and other material parameters, it is possible to explain the very high interfacial shear stresses observed in experimental measurements. As an application example, distribution of plate stress and interfacial shear stress for the linear softening case is derived. The model results are found to be in good agreement with experimental measurements, showing that the simple linear softening model can describe the debonding process in real material systems.

Measuring effective interfacial shear strength in carbon fiber bundle polymeric composites

Adhesion in composite materials is often quantified using the single fiber fragmentation (SFF) test. While this method is believed to provide accurate values for the fiber–matrix interfacial shear strength (IFSS), these may not accurately reflect the macroscopic mechanical properties of specimens consisting of tows of thousands of tightly spaced fibers embedded in a resin matrix. In these types of specimens, adhesion may be mitigated by fiber twisting and misalignment, differences in the resin structure in the confined spaces between the fibers and, most importantly, by any incompleteness of the fiber wetting by the resin. The present work implements fiber band fragmentation (FBF) testing to obtain effective interfacial shear strengths, whose values reflect the importance of these factors. The fiber fragmentation in these specimens is tracked through the counting and sorting of acoustic emission (AE) events occurring during the tensile testing of the specimen and yields the average critical fiber fragment length. AE results, in conjunction with stress-strain data, show that fiber breakage events occur at acoustic wavelet amplitudes substantially greater than those generated by fiber/matrix debonding. Kelly–Tyson analysis is applied, using the measured critical fiber fragment length together with known values for the fiber diameter and tensile strength to yield the effective IFSS. FBF tests are performed on carbon fiber/poly(vinyl butyral) (PVB) dog-bone fiber-bundle systems, and effective IFSS values substantially lower than those typically reported for the single fiber fragmentation testing of similar systems are obtained, suggesting the importance of multi-fiber effects and incomplete fiber wetting.

Elastica bend testing of the effective interfacial shear strength and critical deformation strains of brittle coatings on ductile substrates

An elastica bend test is described that provides a rapid, convenient, and reproducible means for examining the interfacial delamination of brittle coatings on ductile substrates. By bending a coated strip into a half-loop and examining the microscopic cracking that is observed on the sample surface as the loop size decreases, information is obtained about the characteristic initial, mean, and final strains ε i, ε m and ε f that describe the distribution in strength of the coating. By measuring the distance between the cracks at high strains, we obtain an estimate of the interfacial shear strength τ.

Scanning force microscope study of detachment of nanometer adhering particulates

We employ salt particles deposited on soda lime glass substrates as a model system for particle detachment in chemically active environments. The chemical activity is provided by water vapor, and detachment is performed with the tip of a scanning force microscope. The lateral force required to detach nanometer-scale salt particles is a strong function of particle size and relative humidity. The peak lateral force at detachment divided by the nominal particle area yields an effective interfacial shear strength. The variation of shear strength with particle size and humidity is described in terms of chemically assisted crack growth along the salt/glass interface.

A scanning force microscope study of detachment of nanometer‐sized particles from glass surfaces

The nominal shear stress required to detach nanometer‐scale, single‐crystal salt particles from a soda lime glass substrate is a strong function of particle size and relative humidity. We use the tip of an atomic force microscope to detach these particles from a glass substrate under controlled atmospheres of known humidity. The peak lateral force at detachment was divided by the nominal particle area to yield an effective interfacial shear strength. We describe the variation of shear strength with particle area and humidity in terms of detachment by chemically assisted crack growth along the salt–glass interface.

Scanning Force Microscope Studies of Detachment of Nanometer Adhering Particulates

We employ salt particles deposited on soda lime glass substrates as a model system for particle detachment in chemically active environments. The chemical activity is provided by water vapor, and detachment is performed with the tip of a scanning force microscope. The later force required to detach nanometer-scale salt particles is a strong function of particle size and relative humidity. The peak lateral force at detachment divided by the nominal particle area yields an effective interfacial shear strength. The variation of shear strength with particle size and humidity is described in terms of chemically assisted crack growth along the salt-glass interface.

A microindentation method for estimating interfacial shear strength and its use in studying the influence of titanium transition layers on the interface strength of epitaxial copper films on sapphire

An approximate elastic model is derived that converts measurements of indentation depth as a function of the applied load in a microindentation experiment, to an estimate of the interfacial yield strength of a metal film on a ceramic substrate. The analysis is done in two stages. In the first stage the indenter lies entirely within the overgrowth. In the second, the indenter pierces through the interface and into the substrate. The load is predicted to increase as the second power of the displacement in Stage I. In Stage II the net load supported in the film is predicted to increase linearly with the displacement. The coefficients for the parabolic and linear functions are related to the effective interfacial shear strength. Experiments conducted on epitaxial films of (111) copper on (0001) sapphire, show excellent fits with the Stage I and Stage II models, yielding self consistent values for the interfacial shear strength. The technique was used to measure the influence of titanium transitional layers, varying in thickness from 0.7 nm (∼ 3 monolayers) to 110 nm. Just 0.7 nm thick titanium produced a 40% increase in the interfacial strength of the copper/sapphire interface, with thicker titanium layers producing only a marginal further improvement in the interfacial strength. Possible reasons why the elastic analysis provides a good description of the data, even through the deformation underneath the indenter is plastic, are discussed.

Effect of lateral stresses on fiber debonding/pull-out

The performance of fiber reinforced composites is strongly dependent on the behavior of the fiber/matrix interface. The fiber pull-out test has been widely used to determine interfacial properties, from which fiber debonding/pull-out behavior in the composite can be deduced. Pull-out test results reported in the literature are almost always obtained under the condition of zero far-field lateral stresses. However, in many practical applications, crack bridging fibers can be under significant lateral compression (e.g. splitting cracks, shear cracks) or tension (cracks at the bottom of plates under biaxial bending). In this investigation, a novel experimental set-up is developed to study the effect of lateral stresses on fiber debonding and pull-out. Steel fiber reinforced mortar specimens were tested to provide an example. With lateral compression, both the initial interfacial friction and the effective interfacial shear strength are found to increase. A higher lateral compression, however, also results in a more rapid decrease in the interfacial friction during fiber pull-out. Therefore, while lateral compression can significantly increase the peak pull-out load, the energy absorption capacity (denoted by the area under the pull-out curve) does not increase to the same degree. Qualitatively, lateral tension imposes opposite effects to lateral compression. Quantitatively, a small lateral tension can result in changes in interfacial properties comparable in magnitude to those caused by a much higher lateral compression. Therefore, although the lateral tension that can act on a fiber is limited by the low tensile strength of the matrix, it may still impose a noticeable effect on the fiber debonding/pull-out behavior.

Effect of surfactant treatments on interfacial adhesion in single graphite/epoxy composites

Interfacial adhesion between graphite fibers and epoxy matrices is one of the important parameters in controlling composite performance. Experiments were performed to verify the idea that polymerizable and nonpolymerizable nonionic surfactants can promote wetting and increase interfacial shear strength. Using a single-fiber-composite (SFC) technique with a Poisson/Weibull model of fiber strength and flaws, the effective interfacial shear strength (ISS) was determined. The effect of surfactant treatments on interfacial adhesion in single graphite/epoxy composites was also compared with the effects of treatment with deionized water, absolute alcohol, and acetone. The experimental results showed that the fibers treated with the nonpolymerizable surfactant gained 37% in interfacial shear strength relative to those untreated or those treated with deionized water, absolute alcohol, or acetone. The polymerizable surfactants showed a 10% increase in interfacial shear strength. The lognormal distribution provided the best fit for critical aspect ratios obtained from the SFC tests.

Effect of 60Co gamma radiation on the mechanical properties of epoxy blends and epoxy‐graphite fiber interface

AbstractThe effect of 60Co gamma radiation of up to 100 Mrads on an IM6‐G graphite fiber‐epoxy interface was studied using the single‐fiber‐composite (SFC) technique. Flexible epoxy blends were formulated using diglycidylether of bisphenol‐A (DGEBA) based and polyglycol diepoxide epoxies which were cured with aliphatic and aromatic curing agents. Bulk epoxy specimens and graphite fibers were tension tested to obtain their tensile properties. The fragment length distribution from SFC tests, single fiber strength data, and a Monte Carlo simulation of Poisson/Weibull model for fiber strength and flaws were used to obtain the effective interfacial shear strength values. The results indicate that while graphite fiber strength is not affected by radiation, the tensile properties of the epoxies used are adversely affected by the radiation. The interfacial shear strength, however, increases significantly with the radiation dose. The study also supports the earlier results of many workers that the interfacial shear strength for flexible epoxies is much higher than the shear yield strength of the epoxies.

Study of interfaces of high‐performance glass fibers and DGEBA‐based epoxy resins using single‐fiber‐composite test

AbstractThe single‐fiber‐composite (SFC) technique was used to study the interfacial behavior between two flexible blends of diglycidylether of bisphenol A (DGEBA)‐based epoxy and polyglycol epoxide and three glass fibers. Dog‐bone‐shaped SFC specimens were made and strained to obtain a distribution of fragment lengths. The fibers were tension‐tested at two different gauge lengths. The fragment length distributions, the fiber strength data, and a Monte Carlo simulation of a Poisson/Weibull model for fiber strength and flaws were used to obtain the effective interfacial shear strength values. The results show that the interface does not fail. Instead, penny‐shaped transverse cracks appear at every fiber break and grow as the specimen is strained. The interfacial shear strength values are many times higher than the yield shear strength values of bulk epoxy obtained from the tension test.

Interfacial shear strength studies using the single‐filament‐composite test. I: Experiments on graphite fibers in epoxy

AbstractThe failure of the interface in a carbon fiber‐epoxy system was studied for six different epoxy blends using the single‐filament‐composite technique. The blends were formulated to yield a wide range of stiffnesses, and their effect on interfacial failure was examined. Specimens were made from Hercules IM6‐G carbon fiber and the different blends of epoxy, and then strained to obtain a distribution of fiber fragment lengths. Birefringence patterns near the fiber breaks were observed and recorded. Some of the specimens were strained until they failed and the resulting fracture surfaces were observed under a scanning electron microscope to determine fracture patterns and the existence of debonding. The fragment length distributions were interpreted using a Monte‐Carlo simulation of a Poisson/Weibull model for fiber strength and flaw occurrence. The results were used to calculate an effective interfacial shear strength. From this analysis we conclude that one cannot accurately predict the interfacial properties of a composite based solely upon conventional single fiber and bulk matrix properties. Local matrix properties and fiber/matrix interactions, on a microscale, play a key role in composite strength.

Interfacial shear strength and failure modes of interphase‐modified graphite‐epoxy composites

AbstractElectrocopolymerization of acrylonitrile/methyl acrylate/acrylic acid, acrylonitrile/methylacrylate, and glycidyl‐acrylate/acrylonitrile interlayers onto Hercules AS4 graphite fibers was used to improve simultaneously the effective interfacial shear strength, τe, and the fracture toughness of graphite‐epoxy composite materials. With a single‐fiber fracture test, τe for these coated fibers (embedded in a diglycidyl ether of bisphenol A ‐ 4,4′‐methylenedianiline matrix) was determined at various temperatures and under various hygrothermal treatments. At room temperature, the coated samples showed slightly improved shear strength over the uncoated sample. At elevated temperatures, a plot of τe vs. temperature for the uncoated sample showed two distinctive regions: an interface‐controlled plateau region at low temperatures, and a matrix‐controlled region at high (>80°C) temperatures. Only one region, which was controlled by the matrix and the interlayer, was observed for the GA/AN coated sample. The τe values determined were slightly higher than the shear strength of the bulk matrix, possibly because of stronger matrix properties at the interface. Optical micrographs of the coated sample tested at temperatures less than 100°C showed no matrix crack perpendicular to the fiber axis, indicating that the interlayer has effectively blunted the crack tip and restricted its propagation. The effect of moisture in the sample was to reduce τe for the coated sample. Upon dehydration, the strength was partially recovered. The treatments did not affect the uncoated sample, however. The fragmentation length data were fitted well by both Gaussian and Weibull distributions.