Bodo Fiedler

Influence of surface treatment on mechanical behaviour of fumed silica/epoxy resin nanocomposites

The present paper shows the potential of fumed silica as nano-reinforcements in polymers, by considering the limitations and challenges one has to face dealing with nanoparticles in general. The dominating effect of the manufacturing route and surface properties of fumed silica influencing the resulting degree of dispersion and the interfacial adhesion were investigated by electron microscopy (TEM, SEM). The resulting (fracture-) mechanical properties of the fumed silica/epoxy composites were investigated for volume contents of 0.5 vol% and below. Independent of the surface modification, static and dynamic modulus decreased slightly by adding the fumed silica. Hence, the fracture toughness K Ic turned out to be significantly increased (54%) adding only 0.5 vol% of surface modified fumed silica.

The influence of thermal residual stresses on the transverse strength of CFRP using FEM

The failure of transversely loaded unidirectional CFRP has been investigated using mechanical and thermo-mechanical test methods as well as finite element analysis (FEA). The FEA analysis consists of two cases: a high interfacial strength between fiber and matrix, so that matrix failure governs the fracture process of the composite, as well as a weak interface, so that fiber matrix debonding is the dominating failure process of the composite. The failure dependence of the resin on the actual stress-state could be described. Furthermore, the influence of the thermal residual stresses on the initial matrix failure has been investigated, and the actual stiffness as well as the thermal expansion change of the epoxy resins and the composites as a function of temperature have been determined experimentally. The results of the mechanical and thermo-mechanical tests performed on the neat resin and on the composites were incorporated into FEA and compared with the transverse tensile properties of the composite laminates. In the FE-analysis, the local fiber volume fraction was varied over a wide range in order to investigate its influence on the thermal residual stresses and transverse composite strength. The results can explain the low strain to failure of transverse laminates under tensile loading. The calculated interfacial shear strength (ISS) and the interfacial normal strength (INS) are in good agreement with values found in the literature.

Photo-elastic analysis of fibre-reinforced model composite materials

The phenomena of matrix cracking and fibre/matrix load transfer in single-fibre model composites have been studied. A special device was developed which allows in situ observation of fibre fracture during loading. Load- and stroke-controlled stress/strain curves of the test samples were measured. The local load and strain distributions were determined by photo-elastic analyses. This allowed a study of the micromechanical interdependence between the local fibre/matrix load transfer and crack propagation in the matrix starting from the location of fibre failure.

Low powered, tunable and ultra-light aerographite sensor for climate relevant gas monitoring

Increasing atmospheric CO2 gas pollution and emergence of new types of green energy sources require continuous environmental monitoring. In this context, fast, efficient, light, robust, and reliable gas sensors that can work at room temperature are in high demand. We report on a low-powered type of ultra-light sensor, based on a 3-D-microtube network from a 2-D graphene/nanographite, called aerographite, and a method to tune the nanosensor’s selectivity by a simple variation of the applied bias voltage. Adequate selectivity to CO2 and ultra-fast sensing of H2 by applying 1 V and 5 V, respectively, is obtained. At ultra-low applied bias voltages (1–100 mV) only very low power consumption (≈3.6 nW for 1 mV) is needed. This is most important, as it can be run by energy harvesting methods. The presented results are of the highest interest in terms of low-cost production of ultra-light and ultra-low-power consumption gas sensors for environmental monitoring of greenhouse gases and their simplicity from the technological/engineering points of view.

Mode I and II delamination fatigue crack growth behavior of alumina fiber/epoxy laminates in liquid nitrogen

Mode I and II interlaminar fracture toughness and delamination fatigue crack growth behavior were investigated with unidirectional alumina fiber (ALF)/epoxy laminates at 77 K in liquid nitrogen. The mode I fracture toughness values at 77 K were higher than those at room temperature in laboratory air (RT). Although initial values of the fracture toughness under mode II loading was higher at 77 K than those at RT, the propagation values of the fracture toughness was insensitive to the test temperature. The fatigue crack growth threshold at 77 K under mode I loading was higher than that at RT. The stress ratio dependency under mode II loading at 77 K was completely different from that at RT. Then, the increase of the fatigue crack growth resistance at 77 K from that at RT was observed only under stress ratio, R=0.1. The difference of the fracture mechanism due to the test temperature and the loading mode was discussed on the bases of fracture mechanics and microscopic fracture mechanism consideration.

Nondimensional simulation of influence of toughness of interface on tensile stress-strain behavior of unidirectional microcomposite

Abstract For description of the stress–strain curve, variation of Youngs modulus with increasing applied strain and fracture morphology of composite, a nondimensional shear lag method combined with a Monte Carlo simulation procedure was proposed, in which the statistics of the occurrence of damages such as breakage of components (fiber and matrix) and debonding of interface, mechanical interactions among the damaged and undamaged portions and variation of the geometry of location of the damages with increasing applied strain, are incorporated. The proposed method was applied to microcomposite with different interfacial toughness, and variations of the order and location of occurrence of damages, stress–strain curve and fracture morphology with varying interfacial toughness were shown.

Stress concentrations in multiple fibre model composites

Abstract The finite element method is used to examine the load transfer in the fibre–matrix interface and between broken and neighbouring unbroken fibres. Three different models for the stress–strain dependence of the matrix were chosen for modelling the stress transfer: linear elastic, linear elastic–ideal plastic and hypoelastic. The hypoelastic behaviour is similar to the time independent viscoelastic behaviour of the polymer matrix. The hypoelastic model of the stress–strain behaviour can describe the mechanical properties of different matrix materials taken from simple tensile tests. The values of the stress intensity factors were used to discuss the different material models. The hypoelastic matrix behaviour results in realistic stress concentration factors K caused by a single fibre break.

A shear-lag approach to the early stage of interfacial failure in the fiber direction in notched two-dimensional unidirectional composites

Abstract When the interface between fiber and matrix is not strong, interfacial failure (debonding) occurs in the fiber direction in notched unidirectional composites. To simulate the early stage of such longitudinal debonding, a shear-lag approach has been applied to a two-dimensional, double-edge-notched composite composed of elastic fiber and elastic matrix. The shear stress and energy release rate criteria are used in the analysis. These assume that debonding occurs when the shear stress exerted at the interface and the energy release rate exceed critical values. The main results are summarized as follows. (1) The shear stress and energy release rate to cause debonding depend on the number of cut elements (fiber and matrix) and on the species of the final cut element in the notch. Debonding tends to occur at a lower applied stress when the number of cuti elements is large and the final cut element is a fiber in both criteria. (2) For the growth of debonding, three different types of behaviour appear in both criteria. In type (a), debonding grows unstably upon initiation. This type tends to occur when the final cut element in the notch is matrix and the frictional shear stress acting at the interface after debonding is very low. In type (b), debonding grows to some extent upon initiation, but then stops and starts to grow again with increasing applied stress. This type tends to occur when the final cut element in the notch is matrix and the frictional shear stress is low. In type (c), debonding grows stably with increasing applied stress. This type tends to occur when the final cut element in the notch is matrix and the frictional shear stress is high and when the final cut element is a fiber for any frictional stress. (3) Both shear stress and energy release rate criteria give similar tendencies for initiation and growth of debonding.

Nanomechanics of individual aerographite tetrapods.

Carbon-based three-dimensional aerographite networks, built from interconnected hollow tubular tetrapods of multilayer graphene, are ultra-lightweight materials recently discovered and ideal for advanced multifunctional applications. In order to predict the bulk mechanical behaviour of networks it is very important to understand the mechanics of their individual building blocks. Here we characterize the mechanical response of single aerographite tetrapods via in situ scanning electron and atomic force microscopy measurements. To understand the acquired results, which show that the overall behaviour of the tetrapod is governed by the buckling of the central joint, a mechanical nonlinear model was developed, introducing the concept of the buckling hinge. Finite element method simulations elucidate the governing buckling phenomena. The results are then generalized for tetrapods of different size-scales and shapes. These basic findings will permit better understanding of the mechanical response of the related networks and the design of similar aerogels based on graphene and other two-dimensional materials.

Individual hollow and mesoporous aero-graphitic microtube based devices for gas sensing applications

In this work, individual hollow and mesoporous graphitic microtubes were integrated into electronic devices using a FIB/SEM system and were investigated as gas and vapor sensors by applying different bias voltages (in the range of 10 mV–1 V). By increasing the bias voltage, a slight current enhancement is observed, which is mainly attributed to the self-heating effect. A different behavior of ammonia NH3 vapor sensing by increasing the applied bias voltage for hollow and mesoporous microtubes with diameters down to 300 nm is reported. In the case of the hollow microtube, an increase in the response was observed, while a reverse effect has been noticed for the mesoporous microtube. It might be explained on the basis of the higher specific surface area (SSA) of the mesoporous microtube compared to the hollow one. Thus, at room temperature when the surface chemical reaction rate (k) prevails on the gas diffusion rate (DK) the structures with a larger SSA possess a higher response. By increasing the bias voltage, i.e., the overall temperature of the structure, DK becomes a limiting step in the gas response. Therefore, at higher bias voltages the larger pores will facilitate an enhanced gas diffusion, i.e., a higher gas response. The present study demonstrates the importance of the material porosity towards gas sensing applications.In this work, individual hollow and mesoporous graphitic microtubes were integrated into electronic devices using a FIB/SEM system and were investigated as gas and vapor sensors by applying different bias voltages (in the range of 10 mV–1 V). By increasing the bias voltage, a slight current enhancement is observed, which is mainly attributed to the self-heating effect. A different behavior of ammonia NH3 vapor sensing by increasing the applied bias voltage for hollow and mesoporous microtubes with diameters down to 300 nm is reported. In the case of the hollow microtube, an increase in the response was observed, while a reverse effect has been noticed for the mesoporous microtube. It might be explained on the basis of the higher specific surface area (SSA) of the mesoporous microtube compared to the hollow one. Thus, at room temperature when the surface chemical reaction rate (k) prevails on the gas diffusion rate (DK) the structures with a larger SSA possess a higher response. By increasing the bias volt...