Thin-walled Specimens Research Articles

Evaluation of yield function including effects of third stress invariant and initial anisotropy

By means of combining Drucker's yield function with Hill's quadratic yield function, the anisotropic yield function of the sixth degree is proposed. It is able to include the effects of the third deviatoric stress invariant and initial anisotropy. The experimental evaluation is made on thin-walled cylindrical specimens of mild steel (in the fully annealed condition and the stress-relief annealed condition after a tensile pre-strain) and 2024 aluminum alloy (in the -0 and -T6 tempered conditions). By applying proportional combined loadings of axial load, internal pressure, and torsion to the specimens, a change of yield stress with a rotation of the principal stress axes and a difference between the directions of the principal stress and principal strain increment are examined Under tension–internal pressure and tension–torsion, the yield surfaces and strain behaviour are determined. The fully annealed steel is almost isotropic for yielding although it reveals the effect of the third deviatoric stress invariant. The stress-relief annealed steel and the 2024–0 and -T6 aluminum alloys exhibit axi-symmetric anisotropy. All of the yield surfaces can be expressed precisely by the proposed yield function. In addition, the normality rule is obeyed in strain behaviour.

High-temperature creep tests on specimens of single-crystal molybdenum with a complex stressed state

High-temperature (1550°C) creep tests are performed on tubular thin-walled specimens of single-crystal molybdenum loaded by internal pressure in a 1849P-1/2500 unit with a number of modifications. Strains are measured periodically in the circumferential and axial directions. Test results are processed statistically by two methods.

Hollow Cylinder Tests on Natural Rock Salt

Abstract Constitutive equations for the thermomechanical behavior of salt must accurately model material behavior when all three principal stresses are independent. Conventional triaxial compression and extension tests measure material response for stress states in which two of the principal stresses are equal. A computer-controlled, servohydraulic system is described for testing hollow cylinder specimens at stress states in which the principal stresses are controlled independently. Results of validation tests are given to show that the test system provides accurate measurements of the behavior of salt.

A Simple, High Precision Biaxial Extensometer for High Temperature Creep Study

Abstract A biaxial extensometer for high temperature creep study is described. The axial displacement is measured by a linear variable differential transformer, the angular displacement by a rotary variable differential transformer. The possible interference between the axial and angular displacement measurements is minimized by using instrument linear motion ball bushing bearings and case hardened shafts. Examples of applications are given for a constant shear strain rate test and a creep test at 200°C under abrupt changes of stress states performed on a thin-walled tube specimen made of 2618 aluminum alloy.

Two-Dimensional Strain Cycling in Plasticity

Detailed calculations are presented for strain cycling in a homogeneous deformation that can be sustained by a biaxial state of stress in thin-walled specimens of OFHC copper. These calculations are made with a set of relatively simple constitutive equations within the framework of the strain-space formulation of plasticity. The predicted theoretical calculations, carried out in the context of small deformation, are in good agreement with corresponding available experimental results for saturation hardening and erasure of memory in two-dimensional strain cycling. Also, with the use of the calculated results, a scalar quantity that characterizes strain-hardening is plotted as a function of plastic strains. Such plots are likely to be useful for computational purposes.

Acritarch evidence for the Cambrian age of phyllites in the agordo area (south-alpine basement of eastern Alps, Italy)

Abundant but poorly preserved acritarchs have been found in black metapelites. The assemblage consists mainly of small (5–15 μm), smooth, thin-walled specimens, without processes. Eight subgroups are represented by 19 genera. Stratigraphically diagnostic acritarchs are: Ovulum saccatum, O. lanceolatum, Volkovia flagellata, Aranidium cf. confusum, Veryhachium minutum, Acanthodiacrodium sp. (div. sp.), Cymatiogalea sp., Impluviculus sp.. The assemblage suggests an age in the range of late Early Cambrian to Early Ordovician (Tremadoc), the Late Cambrian being the more probable. The acritarchs described in this paper are the first Cambrian fossils discovered in the Eastern Alps, where until now no fossils older than the Late Ordovician have been found, and only one other record of Cambrian fossils is known from the whole of the Alps.

A comparison of palynological extraction techniques using samples from the Silurian Bainbridge formation, Missouri, U.S.A.

Well-preserved assemblages of organic-walled microphytoplankton (prasinophytes and “acritarchs”), chitinozoans, and spores of land-plant type were recovered from the Upper Silurian (Pridolian) Bainbridge Formation. The Bainbridge samples were used to test the effects of altering oxidation and debris removal techniques during extraction on the resulting numerical composition of assemblages. Oxidation in cold, concentrated HNO 3 for 25 min. or less caused no selective destruction of thin-walled specimens when compared to unoxidized controls. Controlled centrifugation proved both more reliable and more time-consuming than debris removal by passive settling in a Calgon solution or screening through 10 or 20 μm meshes. Calculated diversity values ( H) are higher in the biased samples screened through a 20 μm mesh than values for replicates prepared by controlled centrifugation. The effect of extraction bias on calculated diversity cannot be predicted without reference to unbiased assemblages. The importance of documenting extraction procedures where relative abundances are considered important is emphasized, as is the importance of running replicate samples.

Cyclic Torsion of a Circular Cylinder and Its Residual Stress Distribution

The endochronic theory of plasticity is applied to discuss the cyclic full-reversed torsional loading of a solid bar with circular cross section. Numerical techniques are employed to obtain the solution. The parameters of the constitutive equations are determined from the test data of thin-walled specimens. These parameters are then used without alteration to compute stress distributions within the solid specimen. Special attention is given to the residual stress distribution. It is shown that reasonable results are obtained. The relation of torque versus strain at the outermost fiber of the solid specimen provides an ultimate check of the theory as applied to this case.

A cumulative damage theory for fatigue crack initiation and propagation

A method for evaluating the cumulative damage resulting from the application of cyclic stress (or strain) sequences of varying amplitude is presented. Both the crack initiation and propagation stages of the fatigue failure process are included. The development is based on the concept of plastic strain energy dissipation as a function of cyclic life. The damage accumulated at any stage is evaluated from a knowledge of the fatigue limit in the initiation phase and an ‘apparent’ limit obtained through fracture mechanics for the propagation phase. The proposed damage theory is compared with two-level strain cycle test data of thin-walled specimens, and is found to be in fairly good agreement.

The effect of hydrogen on the multiaxial stress-strain behavior of titanium tubing

The influence of internal hydrogen on the multiaxial stress-strain behavior of commercially pure titanium has been studied. Thin-walled tubing specimens containing either 20 or 1070 ppm hydrogen have been tested at constant stress ratios in combined tension and internal pressure. The addition of hydrogen lowers the yield strength for all loading paths but has no significant effect on the strain hardening behavior at strains e ≥ 0.02. Thus, the hydrogen embrittlement of titanium under plain strain or equibiaxial loading is not a consequence of changes of flow behavior. The yielding behavior of this anisotropic material is described well by Hill’s quadratic yield criterion. As measured mechanically and by pole figure analysis, the plastic anisotropy changes with deformation in a manner which depends on stress state. Hill’s criterion and the associated flow rule do not describe the multiaxial flow behavior well because of their inability to account for changes of texture which depend on multiaxial stress path. Hence, a strain dependent, texture-induced strengthening effect in equibiaxial tension is observed, this effect having the form of an enhanced strain hardening rate.

Combined-load testing of the space-shuttle insulation at 1100°C

The Space Shuttle's insulation coating has been mechanically characterized at temperatures to 1100°C. A creep/combined-load test machine was fabricated, and a capacitive sensor system was devised to assist in the alignment and installation of fragile, thin-walled cylindrical specimens in the test frame. A biaxial strain sensor is described which features low cross-talk, excellent long-term stability and high resolution. Ceramic fixtures and silicate-coated refractory metals are used where temperatures exceed 600°C, and specimen heating is simplified by attaching the transducer to the inside of test cylinders.

Techniques for making thin-walled silica glass specimen containers

Two techniques are described for making lightweight silica glass buckets for weight-gain type kinetic experiments, in which it is necessary to contain spalled reaction products. The buckets are 25 mm diameter by 12 mm deep. They weigh 1-1.5 g, with wall thicknesses of 150-225 mu m. Methods of handling fragile silica glass during the various glass blowing operations are discussed.

Single crystal lamellar texture in a thin-walled cylindrical specimen of polyethylene

Single crystal lamellar texture in a thin-walled cylindrical specimen of polyethylene

Filament-wound rings tested by the application of concentrated loads

The stiffness and strength of rings and ring segments of fiber-reinforced materials have been investigated theoretically and experimentally in short-time tests with concentrated loads applied in the plane of the specimen. It is shown that the low shear resistance and the geometric nonlinearity must be taken into account in testing thick- and thin-walled specimens, respectively. The equations and graphs presented make it possible to allow for these effects in analyzing the test results or in design calculations. Experiments on unidirectional glass-reinforced plastics indicate good agreement with the proposed theoretical equations.

Biaxial fatigue tests with zero mean stresses using tubular specimens

Multiaxial fatigue stressing with zero mean stresses gives a materials fundamental fatigue strength. To obtain such stress states with both positive and negative ratios between the principal stress amplitudes the present work used large, thin-walled specimens made of low-carbon steel ( C = 0·35 per cent). The fatigue loadings consisted of a pulsating oil pressure inside the specimens, a constant oil pressure outside the specimens, a pulsating axial force and a constant axial force, the direction of which was opposite to the pulsating force. The tests divide into six loading series with different ratios between axial stress amplitude and circumferential stress amplitude: ∞, 0, 0·76, 1·24, −0·80, −1·32. Statistical analysis of test results showed agreement with two criteria proposed by Stanfield and Crossland respectively. The criterion due to Crossland agreed also with some biaxial tests with superimposed mean stresses.

A thin-walled specimen container for neutron scattering studies of liquids at high temperatures and pressures

The container described uses a grid formed of thin-walled stainless-steel tubing to retain liquid specimens at elevated temperature and pressure. The sample is pressurized by means of a standard compressed air cylinder and uniform heating of the specimen is achieved by passing electric current along the tube wall. Thermal expansion of the specimen is accommodated whilst retaining a completely liquid-filled container.

On Obtaining the Shear Stress-Strain Relationship from a Hollow Specimen in Torsion

A method is given whereby the shear stress-strain relationship of a material can be obtained from observations made during a torsion test on a hollow circular specimen. An examination is then made of the corrections necessary when using thin-walled specimens, and some advantageous definitions of the mean diameter of a tube are suggested.The use of torsion tests to obtain shear stress-strain relationships is now well established and takes one of two forms. A thin circular tube can be used, it being assumed that the stress distribution is uniform across the wall thickness, or a solid circular bar can be used, the results being analysed by a method ascribed to Nadai. Swift has shown that these two methods give comparable results for moderate strains.