Dynamic mechanical analysis of fiber reinforced composites

Dynamic mechanical and thermal properties were determined for unidirectional epoxy/glass composites at various fiber orientation angles. Resonant frequency and relative logarithmic decrement were measured as functions of temperature. In low angle and longitudinal specimens a transition was observed above the resin glass transition temperature which was manifested mechanically as an additional damping peak and thermally as a change in the coefficient of thermal expansion. The new transition was attributed to a heterogeneous resin matrix induced by the fiber. The temperature span of the glass-rubber relaxation was found to broaden with decreasing orientation angle, reflecting the growth of fiber contribution and exhibiting behavior similar to that of Young's modulus. The change in resonant frequency through the glass transition was greatest for samples of intermediate fiber angle, demonstrating behavior similar to that of the longitudinal shear modulus

Dynamic analysis of flexible mechanical systems using LATDYN

A 3-D, finite element based simulation tool for flexible multibody systems is presented. Hinge degrees-of-freedom is built into equations of motion to reduce geometric constraints. The approach avoids the difficulty in selecting deformation modes for flexible components by using assumed mode method. The tool is applied to simulate a practical space structure deployment problem. Results of examples demonstrate the capability of the code and approach

Dynamic mechanical properties of oral mucosa: comparison with polymeric soft denture liners.

The purpose of this work was to characterize the viscoelastic behaviour of oral mucosa and compare it with the dynamic mechanical properties of different soft liners. For this purpose, a sample of pig oral mucosa and six commercialized soft liner samples have been investigated. A comparison was also carried with the first suitable hard rubber for dental prosthetics: vulcanite. Creep recovery (CR) and dynamic mechanical analysis (DMA)have been used to determine the mechanical modulus of oral mucosa and soft liners respectively. The Poisson ratio is used to compare mucosa bulk modulus and soft liner shear modulus. The biomechanical concept of conventional complete dentures needs a good adjustment of dynamic mechanical impedance between the base and oral mucosa. The viscoelastic mechanical property of the oral mucosa as a referent biopolymer has been confirmed in vitro. The modulus value, adjusted for old patients in physiological conditions, is in the order of 3 MPa. This study underlines the plasticization effect of absorbed water on the mechanical properties of the underlying tissue. This study allows us to define some characteristics of the most adapted biomaterial according to the clinical exigency. The required biomaterial must display the following properties: compatibility and chemical resistance with biological environment perpetuated mechanical properties during physiological conditions and clinical use, good adjustment of dynamic mechanical impedance with supporting mucosa and easy sample processing

Probing microplasticity in small scale FCC crystals via Dynamic Mechanical Analysis

In small-scale metallic systems, collective dislocation activity has been correlated with size effects in strength and with a step-like plastic response under uniaxial compression and tension. Yielding and plastic flow in these samples is often accompanied by the emergence of multiple dislocation avalanches. Dislocations might be active pre-yield, but their activity typically cannot be discerned because of the inherent instrumental noise in detecting equipment. We apply Alternate Current (AC) load perturbations via Dynamic Mechanical Analysis (DMA) during quasi-static uniaxial compression experiments on single crystalline Cu nano-pillars with diameters of 500 nm, and compute dynamic moduli at frequencies 0.1, 0.3, 1, and 10 Hz under progressively higher static loads until yielding. By tracking the collective aspects of the oscillatory stress-strain-time series in multiple samples, we observe an evolving dissipative component of the dislocation network response that signifies the transition from elastic behavior to dislocation avalanches in the globally pre-yield regime. We postulate that microplasticity, which is associated with the combination of dislocation avalanches and slow viscoplastic relaxations, is the cause of the dependency of dynamic modulus on the driving rate and the quasi-static stress. We construct a continuum mesoscopic dislocation dynamics model to compute the frequency response of stress over strain and obtain a consistent agreement with experimental observations. The results of our experiments and simulations present a pathway to discern and quantify correlated dislocation activity in the pre-yield regime of deforming crystals.Comment: 5 pages, 3 figure

Investigating the thermal stability of 1-3 piezoelectric composite transducers by varying the thermal conductivity and glass transition temperature of the polymeric filler material

The thermal behaviour of a number of 1-3 piezoelectric composite transducers is discussed. In particular, devices manufactured from a polymer filler with a relatively high glass to rubber transition temperature (T-g), and from polymer systems with increased thermal conductivity, are evaluated. The mechanical properties of the various filler materials were obtained via ultrasonic measurements, with the thermal properties extracted using dynamic mechanical thermal analysis (dmta), differential scanning calorimetry (dsc) and laserflash studies. A range of ultrasonic transducers were then constructed and their thermal stability studied using a combination of impedance analysis and laser surface displacement measurement

Chopped basalt fibres: A new perspective in reinforcing poly(lactic acid) to produce injection moulded engineering composites from renewable and natural resources

This paper focuses on the reinforcing of Poly(lactic acid) with chopped basalt fibres by using silane treated and untreated basalt fibres. Composite materials with 5–10–15–20–30–40 wt% basalt fibre contents were prepared from silane sized basalt fibres using extrusion, and injection moulding, while composites with 5–10–15 wt% basalt fibre contents were also prepared by using untreated basalt fibres as control. The properties of the injection moulded composites were extensively examined by using quasi-static (tensile, three-point bending) and dynamic mechanical tests (notched and unnotched Charpy impact tests), dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC), heat deflection temperature (HDT) analysis, dimensional stability test, as well as melt flow index (MFI) analysis and scanning electron microscopic (SEM) observations. It was found that silane treated chopped basalt fibres are much more effective in reinforcing Poly(lactic acid) than natural fibres; although basalt fibres are not biodegradable but they are still considered as natural (can be found in nature in the form of volcanic rocks) and biologically inert. It is demonstrated in this paper that by using basalt fibre reinforcement, a renewable and natural resource based composite can be produced by injection moulding with excellent mechanical properties suitable even for engineering applications. Finally it was shown that by using adequate drying of the materials, composites with higher mechanical properties can be achieved compared to literature data

North Wind 4kW passive control system design

An overview of a mechanical rotor control design is presented. Operation at constant RPM and rapid response are obtained by using blade pitch moments for both sensing control need and blade pitch actuation. The basic concept, static or equilibrium design, and dynamic analysis are briefly presented

Macroscopic yielding in jammed solids is accompanied by a non-equilibrium first-order transition in particle trajectories

We use computer simulations to analyse the yielding transition during large-amplitude oscillatory shear of a simple model for soft jammed solids. Simultaneous analysis of global mechanical response and particle-scale motion demonstrates that macroscopic yielding, revealed by a smooth crossover in mechanical properties, is accompanied by a sudden change in the particle dynamics, which evolves from non-diffusive motion to irreversible diffusion as the amplitude of the shear is increased. We provide numerical evidence that this sharp change corresponds to a non-equilibrium first-order dynamic phase transition, thus establishing the existence of a well-defined microscopic dynamic signature of the yielding transition in amorphous materials in oscillatory shear.Comment: 7 pages, 4 figures, Phys. Rev. E (in press

Merging of the alpha and beta relaxations and aging via the Johari–Goldstein modes in rapidly quenched metallic glasses

This paper provides evidence that the physical aging of deeply and rapidly quenched metallic glasses is promoted by the Johari–Goldstein slow beta relaxation, resulting in a significant irreversible increase in the mechanical modulus on initial heating. Dynamic mechanical analysis has been used to characterize relaxation phenomena of a strong and a fragile metallic glass. In addition, we can extrapolate the temperature dependence of beta- and alpha-relaxation peaks to higher temperatures and calculate the merging temperature for both types of glasses