A method of testing thin webs in shear

This paper describes a method for measuring the shear strengths and moduli of foils, films, fabrics and papers as thin as a fraction of one mil. The technique involves the use of two steel loading plates, to which the edges of the specimen are adhesively bonded, and a stabilizing system which prevents the specimen from buckling before shear yield strengthis reached. The practicality of the method is demonstrated on aluminum foil, as thin as 0.9 mil, by duplicating handbook values. Repeatability of results is verified by scatter within 2 percent in strength and 4 percent in modulus. The applicability of this method to several boron and glass-fiber composites is then shown.     

Quasi-Static Four-Point Bend Testing of Macro-Fiber Composite Unimorphs

This research examines the application of Macro-Fiber Composites on substrate materials in the form of unimorph actuators. The intent is to characterize the behavior of the unimorphs and to gain an understanding of the load bearing capacity as a function of the substrate. The results indicate that thin substrates, on the order of 0.1 mm, with a high modulus, on the order of 200 GPa or more, provide the largest displacements and load bearing capacity. Both classical laminate plate theory and experimental results are used to support this conclusion. The experimental tests used a four point bend setup to load the unimorphs through their entire range of deflection and quantified their load bearing capacity throughout this range. These results were compared to Classical Laminate Plate Theory, in terms of loading and curvature, with good agreement. The primary application of this research is for use on small unmanned vehicles, but these results can be expanded to other Macro-Fiber Composite applications as well.     

The effect of a coupling agent on the viscoelastic properties and internal stresses in high density polyethylene filled with glass spheres

It is shown that covalent bonding between high density polyethylene (HDPE) and glass spheres can have a significant influence on the stress relaxation behaviour and the creep properties of the corresponding composites at room temperature. The bonding is obtained by reacting the glass spheres with an azide functional alkoxysilane which is capable of bonding to the HDPE-chain. The internal stress, evaluated from relaxation experiments, increased markedly as a result of this treatment, and it is suggested that the internal stress level reflects the properties of the interphase region between the filler and the bulk matrix and its effect on the viscoelastic properties.     

Behavior of surface flaws in reactor pressure vessels under thermal-shock loading conditions

A pressurized-water, nuclear-reactor pressure vessel can be subjected to a severe thermal shock in the event of a loss-of-coolant accident (LOCA). If at the time of the LOCA there is a crack-like defect on the inner surface of the vessel, the crack may propagate as a result of the thermal shock. This paper discusses the conditions necessary for crack propagation during a LOCA, the detailed behavior of the cracks under these specific conditions, and an experimental program designed to determine the validity of the method of analysis (linear-elastic fracture mechanics) used to predict the behavior of flaws under severe thermal-shock loading conditions. A detailed fracture-mechanics analysis of the LOCA thermal shock was performed to help establish the scope of the experimental program. The results of this analysis indicate that present-generation and future PWR vessels will not experience excessive crack propagation. This is also true of earlier PWR vessels, which contain rather high concentrations of copper and, thus, are more susceptible to radiation damage, provided a phenomenon referred to as warm prestressing is effective. Eventually, the experimental program will include investigations of all the major fracture-mechanics phenomena predicted to occur under adverse LOCA-ECC conditions. Two of the experiments conducted thus far were designed for the study of long axial flaws that would penetrate no more than 20 percent of the wall of thick-wall steel test cylinders (533-mm OD×146-mm wall×914-mm length). During one of these experiments, no fast fracture took place, as predicted using LEFM. In the other experiment crack initiation and arrest were expected and took place, with a total penetration of ～16 percent. The agreement between experimental results and the LEFM analysis was very good, indicating that the LEFM analysis is valid at least for shallow flaws in thick-wall steel cylinders subjected to severe thermal shock.     

Plasticity size effects in free-standing submicron polycrystalline FCC films subjected to pure tension

The membrane deflection experiment developed by Espinosa and co-workers was used to examine size effects on mechanical properties of free-standing polycrystalline FCC thin films. We present stress-strain curves obtained on films 0.2, 0.3, 0.5 and thick including specimen widths of 2.5, 5.0, 10.0 and for each thickness. Elastic modulus was consistently measured in the range of 53- for Au, 125- for Cu and 65- for Al. Several size effects were observed including yield stress variations with membrane width and film thickness in pure tension. The yield stress of the membranes was found to increase as membrane width and thickness decreased. It was also observed that thickness plays a major role in deformation behavior and fracture of polycrystalline FCC metals. A strengthening size scale of one over film thickness was identified. In the case of Au free-standing films, a major transition in the material inelastic response occurs when thickness is changed from 1 to . In this transition, the yield stress more than doubled when film thickness was decreased, with the thick specimen exhibiting a more brittle-like failure and the thick specimen exhibiting a strain softening behavior. Similar plasticity size effects were observed in Cu and Al. Scanning electron microscopy performed on Au films revealed that the number of grains through the thickness essentially halved, from approximately 5 to 2, as thickness decreased. It is postulated that this feature affects the number of dislocations sources, active slip systems, and dislocation motion paths leading to the observed strengthening. This statistical effect is corroborated by the stress-strain data in the sense that data scatter increases with increase in thickness, i.e., plasticity activity.The size effects here reported are the first of their kind in the sense that the measurements were performed on free-standing polycrystalline FCC thin films subjected to macroscopic homogeneous axial deformation, i.e., in the absence of deformation gradients, in contrast to nanoindentation, beam deflection, and torsion, where deformation gradients occur. To the best of our understanding, continuum plasticity models in their current form cannot capture the observed size scale effects.     

Epon 828 epoxy: A new photoelastic-model material

The photoelastic properties of Epon 828 are discussed. The epoxy is evaluated experimentally for its time-edge effect, optical creep, stress-optic relations, and mechanical properties at room and elevated temperatures. Epon 828 is extremely clear, has good transparency, and can be use for photoelastic stress analysis at room and elevated temperatures. It has a low-fringe constant at room temperature as well as at elevated temperatures coupled with a high critical modulus of elasticity. Epon 828 can easily be cast stress-free and has relatively small amounts of optical and mechanical creep. The machining characteristics of Epon 828 are excellent and it cements easily with itself and other epoxy materials.     

Machining unreinforced polymers with high-velocity water jets

Sapphire nozzles of 0.076-mm to 0.178-mm diameter have been used with a water intensifier, operating at pressures up to 4.2 kbar, to produce continuous jets with velocities around 6.6×10⁴ cm/s. These jets have been used to machine (slit) sheet specimens of polymethylmethacrylate (PMMA), polycarbonate and acetal at feed rates up to 4 cm/s. A previous paper describes the jet cutting system and analyzes data for PMMA and a phenolic laminate. This paper presents and analyzes data for polycarbonate and acetal; it also reanalyzes the PMMA data. A cutting theory described previously is modified and extended so that depth of cut can be directly related to nozzle pressure, jet flow rate, specimen feed rate and nozzle diameter. In its original form, this energy theory cannot account for the effect of changes in nozzle diameter. It is shown that there is excellent agreement between the predictions of the modified theory and the experimental data. The theory is also used to explain the surface appearance of a typical cut.     

Multi-scale Measurement of (Amorphous) Polymer Deformation: Simultaneous X-ray Scattering,Digital Image Correlation and In-situ Loading

This paper presents a method to investigate the behaviour of polymers on different scales during deformation using simultaneously collected synchrotron X-ray scattering, digital image correlation (DIC) and tensile loading. The method is demonstrated through experiments made on specimens of amorphous polycarbonate. Deformation is measured in-situ, simultaneously across different scales from the macroscopic deformation, measured using sensors on the tensile machine, to the full-field mesoscopic deformation, measured using DIC, down to the deformation of the nano-scale structure, studied using small and wide angle X-ray scattering (SAXS/WAXS). The DIC reveals highly inhomogeneous deformations that render conventional techniques for measuring deformation, such as extensiometers, virtually useless. The X-ray scattering is measured in several spatial points during continuous loading giving the evolution of the microstructure with respect to both spatial location and load level. The spatial mapping of the scattering reveals characters that would not be observed when only measuring at the centre point or measuring on a large area of the specimen, e.g. wide beam SAXS/WAXS or small angle neutron scattering (SANS). With these data, the macroscopic and the mesoscopic deformation can be correlated to the behaviour of the microstructure providing relevant information when developing micro-mechanical based constitutive models. The experimental results shown here indicate a direct correlation between the major principal strain direction and the maximum anisotropy direction of the SAXS patterns. The current approach can be extended to any kind of polymeric materials or polymer-based nano-composites.     

A LES study of the flow interference between tandem square cylinder pairs

The results of an investigation on the interference effects of the tandem square cylinders exposed to a uniform flow are presented in this paper. Time-dependent and three-dimensional flow simulations are carried out using large eddy simulation with a one-equation subgrid model. An incompressible three-dimensional finite volume code with a collocated grid arrangement is used for solving filtered Navier–Stokes equations. These equations are solved with an implicit fractional two-step method. Simulations are conducted with different Reynolds numbers between 10³ and 10⁵. The longitudinal spacing between the cylinders is selected 4D for the chosen Reynolds numbers, where D is the side of the cylinders. Also the effect of the spacing between cylinders, ranging from 1D to 12D, is studied for the selected Reynolds numbers. The instantaneous flow field is studied by analyzing the vortices, pressure, streamlines and Q-criterion to assist understanding of the various flow patterns, vortical structures and Kelvin–Helmholtz vortices in the separating shear layers. The hysteresis is observed in a certain range of the gap spacing, which this range depends on the selected Reynolds number. The global results are also computed and compared with available experimental results. The results indicate that there is a satisfactory agreement between the predictions and available experimental data considering the fine grid adopted.     

Adaptive control of MEMS gyroscope using global fast terminal sliding mode control and fuzzy-neural-network

An adaptive control of MEMS gyroscope using global fast terminal sliding mode control (GTSMC) and fuzzy-neural-network (FNN) is presented for micro-electro-mechanical systems (MEMS) vibratory gyroscopes in this paper. This approach gives a new global fast terminal sliding surface, which will guarantee that the designed control system can reach the sliding surface and converge to equilibrium point in a shorter finite time from any initial state. In addition, the proposed adaptive global fast terminal sliding mode controller can real-time estimate the angular velocity and the damping and stiffness coefficients. Moreover, the main feature of this scheme is that an adaptive fuzzy-neural-network is employed to learn the upper bound of model uncertainties and external disturbances, so the prior knowledge of the upper bound of the system uncertainties is not required. All adaptive laws in the control system are derived in the same Lyapunov framework, which can guarantee the globally asymptotical stability of the closed-loop system. Numerical simulations for a MEMS gyroscope are investigated to demonstrate the validity of the proposed control approaches.     

Strain-gage-stability measurements for years at 75°C in air

The authors have evaluated the performance of three types of metal-foil strain gages for two and a half years at a constant temperature of 75°C in air. The studies included polyimide-backed and phenolic-glass-backed gages with copper-nickel foil (constantan) and glass-fiber-rein-forced epoxy-phenolic-resin-backed gages with nickel-chromium foil (modified Karma). All gages were tested in a three-wire-lead, single-active-gage Wheatstone bridge system at nominal mechanical-strain levels of 0, +1000 and ?1000 μm/m. The polyimide-backed constantan gages used in Installation No. 1 typically zero-shifted more than +200 μm/m after 30 days' testing at 75°C. The phenolic-glass-backed constantan gages used in Installation No. 2 zero-shifted ?40 to +20 μm/m after 30 days and +270 to +350 μm/m after 900 days. The glass-fiber-reinforced, epoxyphenolic-resin-backed Karma gages used in Installation No. 3 zero-shifted about +5 μm/m after 30 days, and +20 to +45 μm/m after 900 days. The Karma gages used in Installation No. 3 performed better than the other two types of gages. The measured results for the Karma gages appeared to be independent of the adhesives, test-part material, and mechanical-strain levels that we tested. The high stability and low zero drifts of this gage at 75°C allowed us to reliably correct the measured static-strain data on actual test parts.     

The rheological behaviour of HDPE/LDPE blends

The rheological behaviour of three types of HDPE/LDPE blends at several compositions (various weight percentages of LDPE) has been studied with the aid of a capillary rheometer and three different capillaries. In particular, entrance effects and shear viscosities have been determined over a wide shear rate region and at different temperatures. Thus activation energies could also be evaluated. Synergistic effects are evidenced when the relevant properties of the homopolymers parents are not too different from one another.     

Application of faraday's effect in static and dynamic holographic photoelasticity

The basic principle of applying Faraday's effect to achieve the separation of fringes in static and dynamic holographic photoelasticity, and a study and application of Faraday's light rotator are described in this paper. It is proposed that Faraday's light rotator be used for automating photoelastic instrumentation for measuring isoclinics and the decimal orders of isochromatic fringes.     

Calculation of the rotational transition probabilities of diatomic molecules when they collide with heavy particles

The collisional rotational transition probabilities for molecule-molecule and molecule-atom interactions in three-dimensional space are calculated. The quasiclassical approach developed in [1] is used. Expressions are obtained that are suitable for practical calculations of single-quantum and double-quantum rotational transitions in diatomic molecules. The collisional rotational transition probabilities are averaged over the Maxwell velocity distribution and their dependence on the gas temperature is obtained. To illustrate the method the results of a calculation of the probabilities for HCl-HCl, HCl-He, CO-CO interaction are presented.     

A composite three-dimensional photoelastic method

Care must be taken in preparation and testing of three-dimensional composite photoelastic models. Some problems encountered in modeling the prototype and during model testing are: model-material failure, loss of fringe pattern in slicing, inherent shrinkage response in freezing, inadequate bonding between materials, and modular ratio difficulties. The selection of the correct plastic can eliminate the first four problems, but the correct modular ratio between the matrix and the insert has to be obtained. This investigation illustrated the behavior of commercially manufactured plastics as inserts, with a matrix material of Epon 828 epoxy. The effective moduli of elasticity of these plastics are reported for pure tension and for flexure. Since the manufactured plastics produced varying results, the use of Epon 828 epoxy as an insert was investigated. The inserts were cast in tygon tubing and their curing cycle was altered from that used for the matrix material to produce a different effective modulus of elasticity. The Epon 828 inserts gave excellent results in the beams. The use of the same material for matrix and insert eliminates many of the problems associated with composite three-dimensional photoelasticity.     

Fracture in Microscale SU-8 Polymer Thin Films

In this work, the fracture of spin coated SU-8 epoxy thin films was investigated under mode I loading using in situ optical experiments on specimens with double edge notched tensile geometry. A method was developed to fabricate 3 μm thick SU-8 films with tapered Chevron type notches using a combination of electron beam and ultra-violet lithography techniques. Subsequently, through speckle patterning under tensile loading, the local deformation fields around the crack tip were extracted using digital image correlation. Since the notches were tapered through the thickness, a crack nucleated from them and grew stably until it spanned the entire thickness before propagating unstably leading to catastrophic failure. As SU-8 underwent brittle fracture with no evidence of a large process zone, the critical energy release rate, J _{I C} was computed from deformation fields, and was found to be 106.6 ± 12.03 J /m ². As the film thickness was small compared to lateral dimensions, assuming plane stress conditions, the critical stress intensity factor was calculated as 0.57 ± 0.03 MPa\(\sqrt {m}\). Furthermore, to assess the validity of the experimental method, a finite element simulation on the exact specimen geometry was conducted with experimentally evaluated far field displacement boundary conditions. The strain fields and J-integral value obtained from the simulation were in good agreement with the experimental results, implying the validity of the in situ experimental method proposed given the challenges of small scale specimens. Furthermore, using fractography and optical imaging it was confirmed that the unstable crack propagation started once the crack front reached full thickness, thereby providing sharp crack at the time of failure, which is necessary for brittle materials for valid fracture toughness experiment. It is expected that the proposed methods of specimen preparation and fracture experiments on microscale polymer thin films can be used on other materials.     

Experimental investigation of jets from rectangular six-lobed and round orifices at very low Reynolds number

This article presents an experimental study conducted on a six-lobed rectangular jet at a very low Reynolds number of 800. The near-exit flow dynamics is compared to the reference counterpart circular jet with same initial conditions. Flow dynamics is analyzed using time-resolved flow-visualizations, hot-wire anemometry and laser Doppler velocimetry. In the round jet, flow motion is dominated by large primary Kelvin–Helmholtz (K–H) structures. In the six-lobed rectangular jet, the K–H vortices are very thin compared to the large secondary vortices generated by the high shear at the lobed nozzle lip. The inspection of mean-velocity profiles and streamwise evolutions of the spreading rates in the major and the minor planes of the lobed jet confirm the absence of the switching-over phenomenon not observed on flow images. The streamwise structures that develop in orifice troughs render the volumetric flow rate significantly higher than that of the reference circular jet. Comparison of the obtained results to available data of the literature of similar rectangular six-lobed jets investigated at very high Reynolds numbers reinforces the notion that the three-dimensional flowfields at very low and very high Reynolds numbers are similar if the geometry of the lobed nozzle is conserved. However, important variations in flow dynamics might occur if one or several geometric parameters of the lobed nozzle are modified.     

Performance of metal-foil strain gages during large cyclic strains

Constantan-alloy gages with polyimide backing were bonded to axial-fatigue specimens which were tested under cyclic loading. Constant-strain ranges between 0.0075 and 0.04 were applied to each specimen by means of a clip on extensometer, and the strain-gage signals were monitored for gage accuracy and life. Gage life varied upwards from about four cycles at the highest strains investigated. Although significant zero shift occurred, strain ranges were generally measured within five percent over most of the life of the gage. The gage-performance information obtained will aid in later study of local strain and low-cycle fatigue in notched members.     

An analytical approach to determine conventional S−N curves from accelerated-fatigue data

This paper describes an analytical method of obtaining conventional S?N curves from the accelerated-fatigue tests, namely the generalized Prot accelerated-fatigue-testing technique in which the stress amplitude increases linearly with respect to cycle. Miner's cumulative-damage theory was applied and an expression for the sum of a series of natural numbers raised to a certain nonintegral power was developed to achieve this. The agreement between analytical prediction and experimental verification is quite reasonable.     

Photoelastometric recording of stress waves

A very sensitive, fast-responding photomultiplier tube and laser light source were used to record stress-optic data associated with a moving stress wave. By using a “gray-field” (which is halfway between a dark and light field) polariscope, optical-electrical recordings were obtained which were linearly proportional to strain. A discriminatory record results which exhibits a higher signal-to-noise ratio than semiconductor strain gages. Gage length can be varied from 0.005 in. upwards.