High Interfacial Shear Stress Research Articles

Mechanics of Muscle Injury Induced by Lengthening Contraction

Skeletal muscle is composed of two primary structural components, contractile myofibrils and extracellular matrix (ECM). The myofibrils adhere to the surrounding endomysium through the basal lamina, sarcolemma and dystrophin, and dystrophin associated glycoprotein (DAG). In this study, a novel shear lag type model is developed to investigate the mechanics of injury to the single muscle fiber due to lengthening contractions. A single muscle fiber is considered as a composite system with reinforced by the contractile myofibrils. The lateral linkages between myofibril and endomysium is modeled as a zero thickness coating layer, that could be injured under high interfacial shear stress. The results shows that the degree of the muscle injury is correlated to the magnitude of the passive stretch during the contraction. Dystrophic muscles are more susceptible to contraction induced injury due to lack of DAG complex in lateral linkage.

Study on Interfacial Shear Stress in RC Beams Strengthened with Prestressed FRP Laminates

Reinforced concrete (RC) beams strengthened with prestressed fiber-reinforced polymer (FRP) laminates has been proved to be a rather effective strengthening technique in the field of bridge engineering. However, debonding failure usually occurs at the end of FRP in the strengthened beams on releasing the prestress due to the high interfacial shear stress. Analytical method to calculate the interfacial stress is developed in this paper. Through the establishment of mathematical model for the interfacial shear stress, the distribution of the interfacial shear stress and the longitudinal stress of FRP are presented explicitly in an analytical way. Moreover, the maximum prestress level is estimated to prevent debonding failure on releasing the prestress. Finally, experimental results of eight strengthened beams validate the analytical solution for the FRP longitudinal stress.

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.

SiC/SiC Composites Prepared with a BN Interphase Processed by LP-CVD from Molecular Precursor

A BN interphase has been deposited by infiltration of a Hi-Nicalon fibre tow in a Low Pressure Chemical Vapour Deposition reactor using a boron nitride molecular precursor: the tris(dimethylamino)borane (TDMAB), an halogen-free precursor. This precursor prevents fibre and CVD apparatus from chemical damage. Then SiC/BN/SiC 1D mini-composites were produced by classical CVD of SiC from hydrogen and methyltrichlorosilane mixture. The interphase composition was characterised using FT-IR and XPS analyses and the presence of carbon in the BN has be related to surface pollution and not from carbon bounded to the coating. The tensile properties of the mini-composites were tested with unload-reload cycles and have shown very good mechanical properties corresponding to a high interfacial shear stress. The observations of the interphase using TEM reveal that it is made of an anisotropic turbostratic BN. The fibre/matrix debounding, which occurs during mechanical loading, was shown to be located within the BN interphase.

Physical interactions at carbon nanotube-polymer interface

Mechanical properties of carbon nanotube (CNT) reinforced polystyrene rod and CNT reinforced epoxy thin film were studied and the CNT-polymer interface in these composites was examined. Transmission and scanning electron microscopy examinations of CNT/polystyrene (PS) and CNT/epoxy composite showed that these polymers adhered well to CNT at the nanometer scale. Molecular mechanics simulations and elasticity calculations were used to quantify some of the important interfacial characteristics that critically control the performance of a composite material. In the absence of chemical bonding between CNT and the matrix, it is found that the non-bond interactions, consist of electrostatic and van der Waals forces, result in CNT-polymer interfacial shear stress (at 0 K) of about 138 and 186 MPa, respectively, for CNT/epoxy and CNT/PS. The high interfacial shear stress calculated, about an order of magnitude higher than micro fiber reinforced composites, is believed attributed to intimate contact between the two solid phases at the molecular scale. Simulations and calculations also showed that local non-uniformity of CNT and mismatch of the coefficients of thermal expansions between CNT and polymer matrix also promote the stress transfer ability between the two.

An experimental method for quantifying tool–part shear interaction during composites processing

The success with which dimensional control during processing of composite structures can be modelled depends on the level of understanding of the underlying mechanisms that drive the accumulation of residual stresses in the part. Tool–part shear interaction during processing can cause substantial warpage in initially flat laminates, yet this phenomenon remains poorly understood. This paper presents an experimental technique in which a thin tool, instrumented with strain gauges, is used for characterizing the interfacial shear stresses that arise between the tool and part during processing. The results show that a sliding interface condition occurs during the majority of the cure cycle, although, at times the tool and part adhere together resulting in high interfacial shear stresses. This tool–part interaction occurs despite the use of a release agent, though the use of a fluoroethylenepropylene (FEP) release film at the tool–part interface reduces the effect.

Mechanical properties and interfacial characteristics of carbon-nanotube-reinforced epoxy thin films

Multiwalled carbon nanotubes (MWNT) reinforced epoxy composite thin films were prepared by a microfabrication process and their elastic modulus was determined using a shaft-loaded blister test and linear and nonlinear elasticity models. Compared to net resin thin films, a 20% increase in elastic modulus was seen when 0.1 wt % MWNTs were added, suggesting MWNT alignment by spin coating. Electron microscopic observations of the fracture surfaces suggested high interfacial shear stress between MWNTs and the epoxy matrix, a result supported by both molecular mechanics simulation and micromechanics calculations.

Analysis and design of FRP externally-reinforced concrete beams against debonding-type failures

Epoxy-bonding of FRP plates to the tensile face of RC beams has been shown to be an effective repair and strengthening technique. However, local failure by debonding or ripping of concrete cover has been reported in experiments to be a likely mode of failure due to high interfacial shear and normal stress concentrations. Predictive models for finding the interfacial shear stress have been reviewed and evaluated using experimental data reported in the literature. The most critical parameters governing the interfacial shear strength and stress as determined by the models were also examined. Through understanding of the conditions that result in debonding failure, a better approach towards designing FRP-plated RC beams against this mode of failure might be achieved.

Delamination Failure in Concrete Beams Retrofitted with a Bonded Plate

Concrete beams are often retrofitted with bonded steel or composite plates. The failure of the retrofitted member is usually due to delamination of the plate from the beam. Delamination can occur either at the end of the reinforcing plate or at the location of a crack in the concrete beam. This paper will focus on the analysis of delamination initiated at the mouth of a flexural crack. When loading is applied, the flexural crack tends to open and hence induces high interfacial shear stress that may result in delamination. The relation between interfacial shear stress and applied moment is affected by many material and geometric parameters. In this study, the bridging stress in the bonded plate and the maximum interfacial shear stress τmax are first related to the crack mouth opening. A fracture mechanics based analysis is then carried out to obtain the crack mouth opening and τmax for a given applied moment. Through a systematic parametric analysis, the effect of various parameters on τmax is deduced. A combination of parameters that will increase τmax, and hence increase the likelihood of delamination, can hence be identified and avoided in design.

Effect of fiber coating on lifetime of Nicalon fiber-silicon carbide composites in air

Studies of the lifetime of Nicalon fiber-reinforced SiC matrix composites subjected to flexural stresses were conducted at temperatures of 600 and 950°C in air. The objective of this study was to evaluate the effectiveness of selected fiber coatings; i.e., carbon, boron-doped carbon (B-doped C), and boron nitride (BN), on the time-to-failure of Nicalon-SiC composites. Results at 600 and 950°C showed that the composite having a B-doped C fiber coating exhibited lifetimes that depend upon stress and temperature as found earlier for a composite with a carbon fiber coating. However, the replacement of the undoped carbon with a B-doped C fiber coating increased the lifetime by 10–100-fold and the apparent fatigue limit by a factor of two (to ≈150 MPa). On the other hand, the lifetime of composites at 950°C with a BN fiber coating was insensitive to the applied stress for levels up to ≈200 MPa (fatigue limit level) and was up to a thousand-fold longer than those composites having either B-doped C or undoped carbon fiber coatings. However, at 600°C, the composite with a BN fiber coating did exhibit a stress-dependent lifetime with an apparent fatigue limit of ≈150 MPa. The lifetime-limiting process in both of these composites is due to the progressive oxidation of the interface region and formation of boron-containing silicate glass, which forms a strong interfacial bond between fiber and matrix, (thus, high debond stress and interfacial shear stress), resulting in embrittlement of the composites.

Water damage in glass fibre/resin composites

Thin films of polyester resin containing glass fibres of several different compositions have been exposed to water at three different temperatures (20, 60, 100°C) and examined by means of optical and scanning electron microscopy. The incidence and development of fibre debonding has been studied by using the optical anisotropy arising from resin shrinkage on to the fibres during cure. To aid interpretation of these experiments, measurements have been made of resin dimensional changes produced by water immersions. At each temperature, the first response to diffused water is resin swelling and, in hot water, this is superseded by shrinkage, the magnitude of which becomes considerable after prolonged immersion, e. g. 8% linear shrinkage after 2000 h in boiling water. Evidence is reported suggesting that this shrinkage is mainly due to leaching of low molecular weight material from the resin. The interfacial bond between resin and clean glass fibres is rapidly destroyed by diffused water at all three temperatures. However, the use of a coupling agent produces vast improvements in bond life. In fact, debonding in the presence of a coupling agent has been observed only for hot water immersions; even then, the bond withstands the interfacial tension present during early resin swelling and is destroyed very much later when the interfacial stress is compressive owing to resin shrinkage. The mechanism by which this debonding is eventually initiated depends on glass composition. With ‘E’ and ‘C’ glass fibres, bond fracture is due to osmotic pressure generated at the interface by water soluble constituents leached from the fibre, and is often accompanied by the growth of cracks into the resin from the fibre surface. With fused silica fibres, which contain negligible amounts of impurities, small regions of debonded interface appear at fibre ends after much longer immersion times and are attributed to high interfacial shear stresses caused by resin shrinkage. Debonding facilitates relative longitudinal movement between fibre and resin, enabling each fibre end to act as a rigid indentor pushing into the adjacent resin. In hot water, resin indentation cracks result and subsequent resin shrinkage and gross plastic deformation lead to their displacement along the fibres, followed by the successive nucleation and displacement of further indentation cracks.