Unidirectional Tests Research Articles

Surface chemistry of PAN carbon fibres in relation to ILSS in epoxy-resin composites

In the manufacture of high performance carbon fibre re-inforced plastics (CFRP) good adhesion between fibre and matrix is essential in order to ensure optimum stress transfer across the resulting interfaces. The overall interfacial interaction is due to specific (chemical) and non-specific Van de Waals interactions between fibre surface atoms and matrix materials. Fibre surface topography, roughness and porosity, are also likely to contribute to interracial bonding by the mechanical keying together of the two components. In order to enhance the surface reactivity of carbon fibres the concentration of oxygen functionality is often increased by an oxidation step during manufacture. The process may also remove loosely bound, low molecular weight, carbon species from the fibl-e surfaces leaving a mechanically more suitable surface for adhesion. Detailed studies of the factors which determine interracial interactions are essential to the understanding of mechanical properties and modes of failure of composiite materials. We are engaged in a long-term investigation of how the surface chemistry of carbon fibre, and other fibre and particulate reinforcing materials, influences interface formation and the resulting mechanical properties of composites. In this communication we discuss the changes in surface chemistry, brought about by commercial anodic oxidation treatment, of PAN fibres and their effect on apparent interlaminar shear strength (1LSS) data for an epoxy composite. Batches of Akzo HTA polyacrylonitrile (PAN)based carbon fibres have been studied. Surface chemistry and contact angle data are compared for 0% and 100% (HTA 500X and 5001, Lots 11019 and 11011 respectively). Unoxidized fibres have quoted tensile strengths of 4012 (+1.5)MPa and tensile moduli of 239 (_+1.5) GPa. Oxidized materials have tensile strength of 3900 (+124)GPa and tensile moduli of 245.7 (+4.0)GPa. Unidirectional fibre test pieces have been produced from both fibre types after application of a bis-phenyl A size (HTA 5130 and 5131 same strength and modulus as above). A standard toughened epoxy (Ciba-Geigy F914) has been used as matrix. Apparent ILSS data have been calculated using the three-point bend test, short beam, method according to ASTM D2344-84. X-ray photoelectron spectroscopy (XPS) measurements have been carried out on a VG MK 1 Escalab using A1 K1 X-rays (energy 1486.6 eV) at a power of 200W in a residual vacuum of 1.333 × 10 -6 Pa. Binding energy resolution has been increased by removal of the X-ray line shape using in-house software on an IBM-AT computer, for functional group identification. Full experimental details have been published elsewhere [1]. Oxidation leads to a factor of 2 increase in measured surface oxygen (Table I). High energy resolution carbon ls spectra (not shown) show this to be distributed as hydroxyl/ether (C-O), carbonyl (C~---O) and carboxylic acid/ester(COOH/COOR). As expected both fibre types have very similar surface compositions after application of size (Table I). However, the oxygen levels recorded are slightly higher than those expected from a stoichiometric bisphenyl A layer (-11%). The 0% oxidized fibres generally have surface nitrogen levels of 2-3 at % which, after this method of oxidation, does not change significantly. In previous studies we have found that anodic oxidation in ammonium bicarbonate solution gives a similar increase in surface

The Evolution of the Pricing Mechanism in the Polish Vegetable Markets: An Empirical Test

Changes in price interaction between farmers' markets and retail stores were traced for Poland's vegetable markets. The vegetable market has been functioning within a competitive framework since the late 1950's. Using monthly price observations for cabbage, onions, potatoes, and carrots, the Granger causality test was applied to test the lead/lag relationship between the two types of outlets. Unidirectional causality test results indicate that a gradual price interaction occurred among the vegetable prices starting in the period of 1974 to 1979 and increased noticeably during the 1980's. This slowly unfolding process was a contributing factor in the forcing of economic liberalization.

Liquefaction resistance of cemented sands under multidirectional cyclic loading

This paper presents a methodology to predict liquefaction of uncemented and cemented sands under multidirectional cyclic loading conditions using solely conventional unidirectional cyclic test data. The method is found to be reasonably accurate based on the reported multidirectional cyclic test results using a true triaxial apparatus. Key words : sand, cementation, triaxial, experimental, theoretical, liquefaction.

Effect of Artificial Salting on Freezing Behavior of Silt Soil

The effect of artificial salting on the freezing behavior of a silt soil was investigated through open-system unidirectional freezing tests. Specimens were prepared in 5-cm-by-15-cm segmented columns. Initial soil dry densities ranged from 1.5 to 2.0 gm/c³. Artificial salting was applied by wetting the soil with salt solutions before compaction. Sodium chloride solutions at concentrations of 0.1 N, 0.25 N, and 0.5 N were used. Specimens were subjected to a top and a bottom temperature of -10°C and +4°C, respectively. The results show that artificial salting is an effective treatment in reducing rate and magnitude of heave, moisture transfer to the frozen zone, and depth of frost penetration. The freezing behavior of the soil tested was also influenced by the degree of compaction, suggesting that a critical density exists at which maximum frost heave is reached. Increasing or decreasing soil density from this value resulted in reduction of frost heave. An explanation of the soil behavior was attempted.

A fracture modes research of boron aluminum composite materials by acoustic emission (AE)

The acoustic Emission Characteristics are investigated for various failure modes (such as fibre breakage, matrix crazing and delamination) of Boron reinforced Aluminum Composites materials. Tensile Tests are conducted for specimens of four kinds of type (unidirectional, 0°/90° ply, boron fibre and shear lag test pieces). The results show that A. E. Characteristics (count rate, amplitude distribution and location) are different with failure modes. A. E. amplitude is above 90 dB for the fibre breakage of unidirectional ply test piece but below 90 dB for the transverse one of cross ply, which fractured at low level load, while the shear lag test pieces produce acoustic emission amplitude in the range of 40∼60 dB under delamination fracture mode.

Use of the flexure test for determining environmental behavior of fibrous composites

Use of the unidirectional flexure test for assessing environmental effects on organix matrix composites is investigated. Results show that moisture and temperature induce a change in failure mode from filament dominated to matrix dominated. Thus, the flexure test provides a simple means for determining environmental conditions which induce significant changes in mechanical behavior related to flexure-stress conditions. Environmental test procedures are also emphasized.

Comment on “A Unidirectional Test of Special Relativity?”

Comment on “A Unidirectional Test of Special Relativity?”

A Unidirectional Test of Special Relativity?

First Page

Tension-compression stress-strain curves from bending tests

A formula—Nadai's bending formula—is derived which enables the tension (or compression) stress-strain curve for a material to be obtained from the curve relating bending moment to curvature for a beam of solid rectangular section. The method is extended to give a formula which covers deformations in which reversals of plastic strain occur. The results obtained from a unidirectional bending test made on annealed copper are compared with those obtained from a tensile test made on the same material and the accuracy of the stress-strain values obtained from the bending test is discussed. The results obtained from a reversed bending test are also compared with those obtained from a tension-compression test in which a specimen was first stretched and then compressed to its original length. The limitations imposed by this method of obtaining the stress-strain curve for a material are examined and the advantages its presents in the study of the behaviour of materials under uniaxial stress are outlined.

Formation and propagation of cracks during cyclic deformation

Starting from observation of the geometric features of formation and propagation of cracks in cycled single crystals of copper a model was developed for fatigue cracking. The only and experimentally well established assumption of the model is that the slip during cyclic deformation tends to be coarse. By the cooperation of stress concentrations at the slip steps and the hardening of slip planes activated locally two slip systems (with different slip planes and Burgers vectors) are activated alternatively so that a crack develops from the slip step. It propagates without monotonically increasing the hardening at the crack tip. The coarse slip produces sharp slip steps at the surface for crack formation and prohibits crack blunting during propagation. In contrast to other models the one described can show how the irreversible process of crack formation and propagation can take place despite completely symmetrical push-pull stresses. The whole crack is formed merely by the motion of dislocations present in the material so that but comparatively small stresses are needed. As, moreover, no thermally activated processes are necessary, fatigue at 4·2°K can be explained too. The strong dependence of fatigue on the state of the surface can also be accounted for since the cracks form at the surface steps. Materials which tend to coarse slip even in unidirectional tests are expected to fatigue easily. This is corroborated experimentally. Finally, many details of crack geometry can be explained in terms of the model.

Strain bursts during cyclic deformation and coarse slip

Single crystals of 99·98% copper were cyclically deformed in push-pull at 50 cps by an electrodynamical vibrator designed to make the stress amplitude independent of changes in plastic compliance of the specimen. Over a period ofN cycles (280 5000 the plastic strain amplitude did not rise monotonically but showed about 20 regularly spaced, strong maxima. These slip bursts lasted typically about 50 cycles and originated from slip distributed throughout the specimen. The next burst occurred when the stress amplitude had increased by about 12%. Observations of the influence of the rate of stress amplitude increase, unidirectional prestrain, and orientation are reported. If one stops increasing the stress amplitude between two strain bursts at a stress amplitude of only about 1·5 kp/mm2 and then pulls the specimen unidirectionally, a yield point with a lower yield stress is observed just at the stress where the next strain burst would have occurred had the increase of cycling stress been continued. During the subsequent yield elongation in a unidirectional test there form very strong (0·3 μm) and widely spaced (5 μm) slip lines. This coarse slip cannot be explained in terms of point defect clusters at the very beginning of a fatigue test of a pure metal. All of these observations can, however, be interpreted in terms of the formation and dissociation of dislocation dipoles formed by mutual trapping of dislocations of opposite sign on different slip planes. It seems reasonable that the main part of cyclic hardening is due to this mutual trapping even in tests performed so that no bursts occur. This is in agreement with the ample evidence supplied by electron microscopy that almost nothing but dipoles are observed after cyclic deformation.

The bidirectional in vitro mixed leukocyte reaction: a mutual inhibitory phenomenon?

SUMMARY The in vitro histocompatibility reaction consisting of a culture of lymphocytes from two unrelated persons was studied in bidirectional and unidirectional systems. The unidirectional test contained one population of viable, reactive cells incubated with a population of cells that had been treated with mitomycin C. This antibiotic served to inhibit cellular DNA activity as measured by incorporation of labeled thymidine but did not abolish cell-stimulating capability. In 23 of 30 experiments, the reciprocal unidirectional reactions either equaled or exceeded their bidirectional (both cell populations viable and reactive) counterpart, even though the latter test system had twice the number of cells capable of responding. This difference in reactivity was apparent only if the cell cultures were harvested for several consecutive days to obtain the peak reaction for each system. The data suggest that, in a unidirectional system, inhibition of one cell line allows the other uninhibited population to react maximally to the stimulus while, in the bidirectional test, a mutual inhibitory (as well as stimulatory) effect decreases the immunological capabilities of both cell populations.

Non-linear modes of vibration of a rigid rotor in short journal bearings : N. J. Huggins, Second Convention on Lubrication and Wear, 1964, Paper 18. To be published in the Proc. Inst. Mech. Engrs., London, 1964/1965; 8 figs., 12 refs.

Use of liquid lubricants in dry and dusty environments can lead to malfunction or jamming of small arms weapons. As such, dry or “lubricious” coatings are desired for use on weapons action components to eliminate the need for liquid lubricants. The tribological performance of a variety of coatings was assessed under unlubricated sliding conditions spanning the range of those expected to be found in the action components of small arms. The coating types included in the study covered a broad range of hardness, composition, and deposition methods. Coating types investigated included: hard vapor-deposited PVD TiCN coatings, relatively soft organic PTFE-filled resin-bonded coatings, modified anodized coatings (for aluminum), electroless nickel–cobalt alloy coatings, PTFE-filled electroless nickel coatings, phosphate pre-treatment combined with dry-film burnished MoS2, and a ferritic nitro-carburizing treatment. Substrates included medium carbon steel, D2 tool steel and 6061-T6 aluminum. Tests were run using a cylinder-on-flat geometry, with self-mated coatings in both reciprocating and unidirectional tests under sliding velocities up to 3.6 m/s, at room temperature and 150 °C. These screening tests ranked the tribological performance of the coatings using both the coefficient of friction and the wear coefficient. The best performing samples were tested up to total sliding distances of 3.6 km.

The dependence on temperature and strain rate of the flow stress of cyclically hardened copper single crystals

Abstract Copper single crystals were strained cyclically with a plastic tensile strain amplitude of 0.5%. The variation of flow stress with strain rate and temperature was measured. Comparison measurements were made on specimens strained unidirectionally in the same apparatus. The variation of flow stress with temperature was considerably larger for the cyclically strained specimens than for the unidirectionally strained ones. The variation with strain rate was slightly larger. It is concluded that the frequency factor of the thermally activated rate-controlling process is considerably higher in the cyclically strained specimens. Considerable long-range internal stresses were present. They appeared to be a larger fraction of the applied stress in the unidirectional tests.

The effects of extinction and “central tendency” on stimulus generalization in pigeons

Following VI training to respond to a monochromatic stimulus, pigeons were tested for generalization in extinction with stimuli of shorter wavelength than the CS (uni-directional test) or with both shorter and longer wavelengths (bi-directional test). Regardless of test condition the gradients became reliably steeper during extinction; when gradients were equated for amount of extinction during testing there was no significant difference in steepness attributable to test condition.