UNIAXIAL COMPRESSION

Compared to uniform compression, uniaxial strain along one of the lattice vectors is more likely to lead to detonation. Dick has proposed [43] that detonation initiation in nitromethane is favored by shock-wave propagation in specific directions related to the orientation-dependent sterie hindrance to the shear flow. This proposal is based on a model according to which the sterically hindered shear process causes preferential excitation of optical phonons strongly coupled with vibrons. 

A closer look at the atomic configurations reveals structural transitions which are triggered preferentially by pressure anisotropy. The most frequent transition in the x compression involves of course molecule translations, rotations of the methyl groups (which occur even for V/Vo close to 1) and reorientations of the molecules, primarily via relative translations and rotations aroimd their C-N axes. Two such abrupt transitions are observed when V/Vo = 67-70% and the estimated pressure is 25-30 GPa, and V/Vo = 59-62% with an estimated pressure of 45-55 GPa. In the z compression the methyl groups seem to undergo smoother transitions as each molecule s center-of-mass is translated in z. In contrast, the y compression causes abrupt transitions when V/Vo - 76 81% and 63-66% and the pressure is estimated to lie in the ranges 2-5 and 13-16 GPa, respectively. 

Notably, the dramatic stretching of four C-H bonds, one in each nitromethane molecule of the unit cell, occurs under stress in y when V/Vo is between 59% and 62%, with an estimated pressure of 25-40 GPa more precisely, these bond lengths are stretched by more than 10-12% of their original values in this case. The increase of strain in y above 50% causes these bonds to be stretched further and leads to the abstraction of their protons. This indication of proton dissociation corresponds to the steep part of the curve in Fig. 2, and renders the y and x compressions qualitatively different. 

Mulliken charges changes of carbon and hydrogen of the nitromethane molecules 1 and 2 in the unit cell under uniaxial compression along b. 

The mould used for forming a column of powder has a 7.5 cm square outside cross-section and 17.5 cm high with a 5 cm diameter axial hole drilled along its length. To achieve the elimination of wall friction the column is formed in a number of equal increments (N). Powder was poured into one section, compacted and then another section added. To assure good bonding between the several layers of compacted powder a compaction disk, with a profiled surface, is used instead of one with the usual flat surface. Once a complete column is formed and the mould removed, weights are added at timed intervals (circa 10-15 s) until 

Since the L/D ratio of the mould designed by Williams et al. was 3.5, the strength of the compact was deemed to be independent of the L/D ratio. 

2 The Gerritsen method Gerritsen (1986) pointed out that the densification of each of the increments in the Williams et al. method was not consolidated for the same period of time. Thus, the measured unconfined yield stress may not be applicable for materials which exhibit a time consolidation dependence. 

3 Peschl unconfined yield strength tester A sample is vertically compacted in a rectangular mould, and after the top lid and normal stress are removed one of the sidewalls of the tester is driven horizontally into the sample up to the point of failure. The stress applied to fail the powder is an estimate of the unconfined yield strength. If a bulk solid shows anisotropic behaviour, it might be expected that the described tester would produce 

The L/D ratio of this device is not fixed since it is dependent upon the powder compressibility. The sleeve is 15.25 cm long and has an inner diameter of 6.35 cm the pinhole is located 6 cm below the top end of the solid cylinder, so that in case of an incompressible powder the L/D ratio will be roughly 1.5, and even less for compressible powders. 

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Thomas, D.A. Uniaxial compressive creep studies. Plastics and Polymers, Oct(1969) p. 485. 

Fig. 4.10. The conductivity of uniaxially compressed (111) and (100) high purity germanium crystals leads to a determination of the shear deformation potential for the designated valley minima in the energy band (after Davison and Graham [79D01]).

Figure 5-26 Buckling Loads for Anfisymmefric Cross-Ply Laminated Plates under Uniform Uniaxial Compression (After Jones [5-19])...

Foam Density lb./ft.3 Glass Microballoons Epoxy Macroballoons Uniaxial Compressive Yield Strength, psi Hydrostatic Compressive Strength, psi Method of Preparation Resin System... 

Darvell, B. W. (1990). Uniaxial compression tests and the validity of indirect tensile strength. Journal of Materials Science, 25, 757-80. 

The stress-strain response of ideal networks under uniaxial compression or extension is characterized as follows ... 

The networks studied were prepared from reactions carried out at different initial dilutions. Aliquots of reaction mixtures were transferred to moulds, which were maintained at the reaction temperature under anhydrous conditions, and were allowed to proceed to complete reaction(32). Sol fractions were removed and shear moduli were determined in the dry and equilibrium-swollen states at known temperatures using uniaxial compression or a torsion pendulum at 1Hz. The procedures used have been described in detail elsewhere(26,32). The shear moduli(G) obtained were interpreted according to Gaussian theory(33 34 35) to give values of Mc, the effective molar mass between junction points, consistent with the affine behaviour expected at the small strains used(34,35). 

The analysis in the last paragraph has shown that the incommensurate Xe layer on Pt(lll) at misfits of about 6% is a striped phase with fully relaxed domain walls, i.e. a uniaxially compressed layer. For only partially relaxed domain walls and depending on the extent of the wall relaxation and on the nature of the walls (light, heavy or superheavy) additional statellites in the (n, n) diffraction patterns should appear. Indeed, closer to the beginning of the C-I transition, i.e. in the case of a weakly incommensurate layer (misfits below 4%) we observe an additional on-axis peak at Qcimm + e/2 in the (2,2) diffraction pattern. In order to determine the nature of the domain walls we have calculated the structure factor for the different domain wall types as a function of the domain wall relaxation following the analysis of Stephens et al. The observed additional on-axis satellite is consistent with the occurrence of superheavy striped domain wails the observed peak intensities indicate a domain wall width of A=i3-5Xe inter-row distances. With... 

Kwade, A., Schulze, D., and Schwedes, ]., Determination of the Stress Ratio in Uniaxial Compression Tests - Part 2, Powder Handl. Process., 6, 2, 199 (1994)... 

Fig. 43. Load displacement curves under uniaxial compression for the neat epoxy line a) and macroporous epoxy networks prepared via CIPS with 20 wt % cyclohexane line b)...

When a closed cell foam is uniaxially compressed, it can be assumed that the compressive stress is a sum of the stresses taken by the polymer structure and that taken by the cell gas. For a foam with zero lateral expansion when uniaxially compressed, and isothermal gas compression, the latter contribution (Tg is given by (295) ... 

Mills and Gilchrist (270) analysed the heat transfer that occurs when closed cell foams are subjected to impact, to predict the effect on the uniaxial compression stress-strain curve. Transient heat conduction from the hot compressed gas to the cell walls occurs on the 10 ms... 

PP bead foams were subjected to oblique impacts (167), in which the material was compressed and sheared. This strain combination could occur when a cycle helmet hit a road surface. The results were compared with simple shear tests at low strain rates and to uniaxial compressive tests at impact strain rates. The observed shear hardening was greatest when there was no imposed density increase and practically zero when the angle of impact was less than 15 degrees. The shear hardening appeared to be a unique function of the main tensile extension ratio and was a polymer contribution, whereas the volumetric hardening was due to the isothermal compression of the cell gas. Eoam material models for FEA needed to be reformulated to consider the physics of the hardening mechanisms, so their... 

The effect of gas compression on the uniaxial compression stress-strain curve of closed-cell polymer foams was analysed. The elastic contribution of cell faces to the compressive stress-strain curve is predicted quantitatively, and the effect on the initial Young s modulus is said to be large. The polymer contribution was analysed using a tetrakaidecahedral cell model. It is demonstrated that the cell faces contribute linearly to the Young s modulus, but compressive yielding involves non-linear viscoelastic deformation. 3 refs. 

Gas compression in closed-cell polymer foams was analysed, and the effect on the uniaxial compression stress-strain curve predicted. Results were compared with experimental data for a foams with a range of cell sizes, and the heat transfer conditions inferred from the best fit with the simulations. The lateral expansion of the foam must be considered in the simulation, so in subsidiary experiments Poisson s ratio was measured at high compressive strains. 13 refs. 

Ito H, Ishihara T, Niwa M, Suzuki T, Onari S, Tanaka Y, Yamada J, Yamochi H, Saito G (2010) Superconductivity of -type salts under uniaxial compression. Physica B 405 S262-S264... 

ISO. TS 17996. IDF. RM205 2006(E). Cheese-determination of rheological properties by uniaxial compression at constant displacement rate. 

Texture has a number of component attributes, and some of them can be assessed by mechanical means. The texture or firmness of cooked potatoes is evaluated by subjecting each sample to a compression test using a universal testing machine equipped with a load cell. Cooked potato cylinders are compressed in a single-cycle compression-decompression test. Uniaxial compression is measured with an Instron machine with a lOON load cell. Measurements are performed on hot potato cylinders (depth 12 mm, height 10 mm) from 15 potatoes immediately after cooking, at a deformation rate of 20 mm/min. Stress and strain at fracture are calculated by the Instron series IX version 7.40 software and means of 15 repetitions are calculated. 

Thybo, A. K., Bechmann, I. E., Martens, M., Engelsen, S. B. (2000). Prediction of sensory texture of cooked potatoes using uniaxial compression, near infrared spectroscopy and low field H NMR spectroscopy. Lebensmittel-Wissenschaft und-Technologie, 33, 103-111. 

In the same way that close-packed directions in a crystal have larger refractive indices, so too can the application of a tensile stress to an isotropic glass increase the index of refraction normal to the direction of the applied stress. Uniaxial compression has the reverse effect. The resulting variation in refractive index with direction is called birefringence, which can be used as a method of measuring stress. 

In a recent attempt to bring an engineering approach to multiaxial failure in solid propellants, Siron and Duerr (92) tested two composite double-base formulations under nine distinct states of stress. The tests included triaxial poker chip, biaxial strip, uniaxial extension, shear, diametral compression, uniaxial compression, and pressurized uniaxial extension at several temperatures and strain rates. The data were reduced in terms of an empirically defined constraint parameter which ranged from —1.0 (hydrostatic compression) to +1.0 (hydrostatic tension). The parameter () is defined in terms of principal stresses and indicates the tensile or compressive nature of the stress field at any point in a structure —i.e.,... 

Very interestingly, a similar surface instability was predicted by Grinfeld on uniaxially compressed solids in contact with their melt [94]. The theory has been revived by Nozieres in a general context [95] and has been applied to quantum solids [96]. The theories are limited to linear stability analysis at present. 

In order to find the influence of compression on collapse of the polymer networks, the experiments on the swelling of the deformed gels of AA-SMA in water-methanol and water-dioxane mixtures were performed [29]. It was shown that uniaxial compression of the gel really affects the swelling curves and that, in a good agreement with the theory, the region of stability of the collapsed state increases and the sharpness of collapse decreases under compression. 

Deformational measurements were carried out in a uniaxial compression of cylindrical samples and the equilibrium shear modulus G was determined from [11]... 

A new ultrasonic technique for studying dewetting and cumulative internal damage in solid propints has been reported (Refs 17 20). This technique yields volume-dilatation data on proplnt in tension, and on damage in uniaxial compression and shear strain fields. Estimates of vacuole size and number density arising from dewetting can be made, as well as can the time dependent void growth at constant strain be observed... 

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