In-plane Material Properties Research Articles

Measurements of In-Plane Material Properties with Scanning Probe Microscopy

AbstractScanning probe microscopy (SPM) was originally conceived as a method for measuring atomic-scale surface topography. Over the last two decades, it has blossomed into an array of techniques that can be used to obtain a rich variety of information about nanoscale material properties. With the exception of friction measurements, these techniques have traditionally depended on tip—sample interactions directed normal to the sample's surface. Recently, researchers have explored several effects arising from interactions parallel to surfaces, usually by deliberately applying a modulated lateral displacement. In fact, some parallel motion is ubiquitous to cantilever-based SPM, due to the tilt of the cantilever. Recent studies, performed in contact, noncontact, and intermittent-contact modes, provide new insights into properties such as structural anisotropy, lateral interactions with surface features, nanoscale shear stress and contact mechanics, and in-plane energy dissipation. The understanding gained from interpreting this behavior has consequences for all cantilever-based scanning probe microscopies.

In-plane contributions to phase contrast in intermittent contact atomic force microscopy

Contrast in the phase response of intermittent-contact atomic force microscopy (IC-AFM) reveals in-plane structural and mechanical properties of polymer monolayers. This result is unexpected, as IC-AFM has previously only been considered as a probe of out-of-plane properties. Until now, AFM measurements of nanoscale in-plane properties have employed contact mode techniques. In-plane property measurements are possible with intermittent contact AFM because there is a small but significant component of tip motion parallel to the sample surface. This in-plane component of tip displacement is virtually universal in AFM, implying that oscillating-tip techniques generally are sensitive to in-plane material properties. We present a simple Hertzian model of intermittent-contact AFM that includes such an in-plane displacement.

Material anisotropy revealed by phase contrast in intermittent contact atomic force microscopy.

Phase contrast in intermittent-contact atomic force microscopy (AFM) reveals in-plane structural and mechanical properties of polymer monolayers. This is surprising, because measurements of nanoscale in-plane properties typically require contact mode microscopies. Our measurements are possible because the tip oscillates not just perpendicular but also parallel to the sample surface along the long axis of the cantilever. This lateral tip displacement is virtually universal in AFM, implying that any oscillating-tip AFM technique is sensitive to in-plane material properties.

An Approach for Obtaining the Young's Modulus in Woven Glass/Epoxy Reinforced Composites

A number of equations for predicting the in-plane material properties of woven reinforced composites have been suggested in the available literature using empirically evaluated correction factors. The use of such factors in the determination of dynamic properties has, however, led to signficant errors. It is therefore imperative that a method of obtaining high speed material properties of composite laminates must be sought since micro-mechanics equations have not been found suitable. Tensile tests were conducted on woven reinforced glass/epoxy laminates at increasing rates of strain and the fractured surfaces were viewed under a scanning electron microscope. In this work a semi-empirical relationship for the tensile modulus was proposed using micro-mechanics and data from tests conducted.

Optimal design of grid-stiffened panels and shells with variable curvature

A design strategy for optimal design of composite grid-stiffened structures with variable curvature subjected to global and local buckling constraints is developed using a discrete optimizer. An improved smeared stiffener theory is used for the global buckling analysis. Local buckling of skin segments is assessed using a Rayleigh–Ritz method that accounts for material anisotropy and transverse shear flexibility. The local buckling of stiffener segments is also assessed. Design variables are the axial and transverse stiffener spacing, stiffener height and thickness, skin laminate, and stiffening configuration. Stiffening configuration is herein defined as a design variable that indicates the combination of axial, transverse and diagonal stiffeners in the stiffened panel. The design optimization process is adapted to identify the lightest-weight stiffening configuration and stiffener spacing for grid-stiffened composite panels given the overall panel dimensions, in-plane design loads, material properties, and boundary conditions of the grid-stiffened panel or shell.

Development of a Semi-Empinical Method for Obtaining the Dynamic Young's Modulus in Random Continuous Reinforced Glass/Epoxy Composites

Due to the inherent inertial effects associated with high speed testing, it may be beneficial if material properties at high strain rates could be obtained from the use of micro-mechanics. However, it has been demonstrated that using the rule of mixtures to predict material properties at high strain rates results in errors since the rate sensitivity of the Young's modulus and tensile strength cannot be explained solely in terms of the rate dependence of the resin modulus and strength (respectively) measure in isolation. A number of equations for predicting the in-plane material properties of random continuous reinforced polymer composites have been suggested in the available literature using empirically evaluated correction factors. The use of such factors in the determination of dynamic properties have, however, led to significant errors. It is therefore imperative that a method of obtaining high speed material properties of composite laminates must be sought since micro-mechanics equations have not been found suitable. This was set as the primary aim of this research work in obtaining accurate material data. In addition, the understanding of the mechanisms governing failure under high speed loadings remain largely unknown. This prompted the need to characterise the effect of loading rate on failure modes. Tensile tests were conducted on random continuous glass/epoxy composites at increasing rates of strain. The effect of loading rate on failure mechanisms was investigated by viewing fractured surfaces of the tensile specimens using a scanning electron microscope (SEM). This work postulated a semi-empirical relationship for the tensile modulus using micro-mechanics and data from tests conducted. The validity of this relationship however, needs to be investigated further in future work.

Optimal Design of Grid-Stiffened Composite Panels

A design strategy for optimal design of composite grid-stiffened panels subjected to global and local buckling constraints is developed using a discrete optimizer. An improved smeared stiffener theory is used for the global buckling analysis. Local buckling of skin segments is assessed using a Rayleigh-Ritz method that accounts for material anisotropy and transverse shear flexibility. The local buckling of stiffener segments is also assessed. Design variables are the axial and transverse stiffener spacing, stiffener height and thickness, skin laminate, and stiffening configuration, where the stiffening configuration is herein defined as a design variable that indicates the combination of axial, transverse, and diagonal stiffeners in the stiffened panel. The design optimization process is adapted to identify the lightest-weight stiffening configuration and stiffener spacing for grid-stiffened composite panels given the overall panel dimensions, in-plane design loads, material properties, and boundary conditions of the grid-stiffened panel.

Optimal design of general stiffened composite circular cylinders for global buckling with strength constraints

A design strategy for optimal design of composite grid-stiffened cylinders subjected to global and local buckling constraints and strength constraints was developed using a discrete optimizer based on a genetic algorithm. An improved smeared stiffener theory was used for the global analysis. Local buckling of skin segments were assessed using a Rayleigh-Ritz method that accounts for material anisotropy. The local buckling of stiffener segments were also assessed. Constraints on the axial membrane strain in the skin and stiffener segments were imposed to include strength criteria in the grid-stiffened cylinder design. Design variables used in this study were the axial and transverse stiffener spacings, stiffener height and thickness, skin laminate stacking sequence and stiffening configuration, where stiffening configuration is a design variable that indicates the combination of axial, transverse and diagonal stiffener in the grid-stiffened cylinder. The design optimization process was adapted to identify the best suited stiffening configurations and stiffener spacings for grid-stiffened composite cylinder with the length and radius of the cylinder, the design in-plane loads and material properties as inputs. The effect of having axial membrane strain constraints in the skin and stiffener segments in the optimization process is also studied for selected stiffening configurations.

Local buckling of pultruded beams — nonlinearity, anisotropy and inhomogeneity

Local buckling of pultruded fibre-reinforced plastic beams is discussed. The paper focuses on three issues related to the prediction of buckling loads both from experimental data and from analytical and numerical approaches, viz. nonlinearity, anisotropy and inhomogeneity. Experimental data obtained from full-scale buckling tests are reviewed and a method proposed for estimating the buckling stress in pultruded beams. Analytical studies based on classical orthotropic plate buckling theory are used to determine the edge restraint coefficient for pultruded beams and also to show the influence of the in-plane material properties on the buckling loads. Numerical studies using the finite element method in which inhomogeneous material properties in the beam cross-section are considered are used to give predictions of buckling loads of the beams. Although the paper focuses on local buckling of pultruded beams, it raises issues which are relevant to the analysis of pultruded material structures of all types.

Characterization of advanced flexible thermal protection material for space applications

This paper describes experimental and analytical characterization of a new flexible thermal protection material known as Tailorable Advanced Blanket Insulation (TABI). This material utilizes a three-dimensional ceramic fabric core structure and an insulation filler. TABI is the leading candidate for use in deployable aeroassisted vehicle designs. Such designs require extensive structural modeling, and the most significant in-plane material properties necessary for model development are measured and analytically verified in this study. First, nonlinear modulus curves are presented for the directions parallel and normal to the fill direction. Trends observed in the modulus profiles are explained in terms of filament behavior. Next, the dominant damping type and approximate magnitudes are determined using log decrement and hysteresis methods. Because of the clothlike nature of the material, unusual test methods are used for damping measurements. Mathematical models are developed for verification of the experimental modulus and damping data, and finally, transverse properties are described in terms of the in-plane properties through use of a finite difference model of a simple TABI configuration.