Showing papers on "Indentation published in 1970"

The correlation of indentation experiments

Abstract: The theory of rigid perfectly-plastic solids predicts indentation pressures, using wedge-shaped or conical indenters, which depend only on the geometry of the indenter and the yield stress of the material. With blunt wedges or with materials having a low ratio of Young's modulus, E, to yield stress, Y, the material displaced by the indenter is accommodated by an approximately radial expansion of the surrounding material. The indentation pressure then falls below the rigid perfectly-plastic value. In these circumstances, measurements of indentation pressure for a variety of indenter geometries are shown to correlate with the single parameter (E/Y) tan β, where β is the angle of inclination of the indenter to the surface at the edge of the indentation. This parameter may be interpreted as the ratio of the strain imposed by the indenter to the yield strain of the material. A simplified theoretical model of this behaviour is obtained by extending R. Hill's theory of expanding a cylindrical or spherical cavity in an elastic-plastic material to ensure compatibility between the volume of material displaced by the indenter and that accommodated by elastic expansion.

The hardness of solids

Abstract: This review is concerned with the basic physical meaning of hardness. It is shown that indentation hardness of ductile materials is essentially a measure of their plastic properties. With brittle solids the high hydrostatic pressures around the deformed region are often sufficient to inhibit brittle fracture. Under these conditions both indentation and scratch hardness are essentially a measure of the plastic rather than the brittle properties of the solid. This provides a simple physical basis for the Mohs scratch hardness scale

Densification and flow phenomena of glass in indentation experiments

Abstract: Flow of glass may occur during indentations. Remarkable accordance with conclusions from the theory of plasticity was found on micrographs. Simultaneously densification beneath the indentation must be noticed to be a more general property of glasses, whereas flow at room temperature seems to require a minimum percentage of network modifiers.

The micro-hardness of metals at very low loads

Abstract: The micro-hardness of gold has been investigated over a range of indentation sizes from 0.5 mm to 0.2 μm using loads down to 1.0 mg. The indentations were made using a series of spherical indenters of varying radius. In this way it was possible to keep the shape of indentations geometrically similar over a wide range of indentation sizes and so obtain comparable results. The smallest indenters (0.5 μm radius) were prepared using techniques similar to those used for making field-ion microscope tips and the experiments at the small sizes were carried out inside a scanning electron microscope. The measurements were made on specimens in the annealed condition and in the heavily cold-worked condition; considerable care was taken to exclude any possible effect arising from the preparative treatment of the surface. The results show that the hardness is increased by a factor of between two and three at sizes down to 0.2 μm in both the annealed and cold-worked conditions. Possible reasons for this increas...

Plastic deformation and anisotropy in the hardness of diamond.

Abstract: A NUMBER of methods may be used to determine the hardness of solids. The most reproducible and most widely used for crystalline solids are those in which the size of an indentation is measured after the surface of the specimen has been subjected to deformation by an indenter under a known normal load. Ideally, the indenter material should be considerably harder than the specimen in order to avoid inaccuracies due to deformation of the indenter tip, but this implies that the hardness of diamond cannot be measured, for it is the hardest indenter material available. A limited amount of data concerning the hardness of diamond, using the static indentation technique, has nevertheless been published. More than thirty years ago the Knoop indenter was developed and used to measure the hardness of a wide range of ductile and brittle materials. This indenter produces an indentation in the form of a parallelogram with one diagonal at least seven times longer than the shorter diagonal. In that early work, the hardness of diamond was quoted as 8,000–8,500 kg mm−2 (ref. 1). No details were given concerning the normal load used, the type of deformation associated with the indentation or the crystallographic relationships between the indenter and the indented diamond surface. It was reported subsequently that the hardness of a (001) diamond surface, again using the Knoop indenter and a normal load of 500 g, was 6,000–6,500 kg mm−2. Apparently, anisotropy in the hardness could not be detected at that time2. Bernhardt3 gave a limiting value of 10,000 kg mm−2 on the (111) surface. He used a conventional square based pyramidal indenter and a normal load of 200 g. Kruschov and Berkovich4, using an indenter in the form of a three sided pyramid with an angle of 65° between its geometrical axis and the side faces, derived a hardness of 10,060 kg mm−2 from one indentation also on a (111) plane and with a load of 200 g. On the basis of a very limited number of indentations, therefore, it seems that the hardness of diamond lies between 6,000 and 10,400 kg mm−2 with no evidence of anisotropy. On the other hand, studies of abrasion on diamond surfaces have established marked anisotropy in wear resistance. These observations have been used to suggest that the directions of high and low wear resistance represent the hard and soft directions respectively on a given diamond surface. For example, it has been proposed5 that, on the (001) plane of diamond, the hard directions are 〈110〉 and the soft directions are 〈100〉. It is now clear that directions of high wear resistance, on a given crystal plane, are not necessarily the hard directions6.

The indentation of elastic layers

Abstract: The depth of indentation of a thick layer by an axially symmetric rigid punch depends on its shape, the elastic constants of the layer and the force. A brief discussion of these relations and their physical significance is given in more general form than previously. The corresponding theory for very thin layers is given in detail. Results are calculated in particular for spherical indenters. Available experimental results are reviewed, and it is shown that for thin layers they need supplementing, in order to cover satisfactorily the conditions in which the theory is applicable.

Determination of the rigidity modulus of thin soft coatings by indentation measurements

Abstract: The effect of layer thickness on the elastic response of thin layers loaded with a relatively large ball indentor was determined with a Wallace Indentation Tester. Thin coatings of gelatin gel on a rigid support were used and the identation results compared with absolute determinations of rigidity modulus. The empirical equation obtained relates rigidity modulus to the load, indentation, layer thickness and ball radius. The constant in the equation obtained for gelatin gels of rigidity modulus up to 1?5?105 dyn cm?2 was found to apply also to rubber of rigidity modulus 107 dyn cm?2.

The deformation behaviour of crossed diamond wedges in the scanning electron microscope

Abstract: Experiments have been carried out in which two sharp diamond wedges were pressed together orthogonally under a known load inside a scanning electron microscope. The object of the experiment was to ascertain whether plastic deformation is possible in diamond at room temperature when it is subjected to shear stresses approaching the theoretical strength of a perfect diamond crystal. Calculation shows that it should be possible to achieve shear stresses of this magnitude using a crossed-wedge geometry in which the stresses are predominantly compressive. The wedges were made from type IIb diamond by cleavage. The results showed that there was a threshold load below which no damage of any sort could be detected on the edges of the diamonds. Above this value, indentation occurred suddenly. Subsequent examination of the indentation showed it to have a very jagged appearance as if it had been formed by a fracture process. It was concluded that plastic deformation is not possible in diamond at room temperature except possibly in very small amounts not resolvable in the scanning electron microscope.

A method of forming a stack of laminations

Abstract: 1,200,279 Laminating metal sheets; jointing; shearing H LEUENBERGER 16 Dec, 1968 [29 Dec, 1967], No 59640/68 Headings B3A, B3J, B3V and B3W A method of laminating sheets comprises laying a plain sheet on an underlying sheet 2 having at least one indentation 3 formed therein and using the indentation as a die to form a further indentation in the plane sheet, which further indentation mates with the indentation in the underlying sheet to connect the sheets together The laminations are of iron and their surfaces are electrically insulated, the stacks of laminations forming magnetic cores for choke coils or transformers The bottom sheet is punched out initially and the indentations are stamped by a male and female die The second lamination is punched out and placed on the bottom lamination, which acts as a female die for the second lamination, the original male die being used again The process is repeated until the stack comprises the required number of laminations Alternatively the punching and stamping operations may be performed simultaneously The indentations are alternatively in the shape of a symmetrical trapezium with an open base, Fig 5 (not shown), and they may be formed in the sheet edges The stacks of laminations may also be used for stators and rotors of dynamo-electric machines

Easy-open tear strip reinforcing means

Abstract: D R A W I N G A COINED INDENTATION AT THE BENDING LINE OF AN EASYOPEN CAN END TEAR STRIP. THE INDENTATION HAS NEARLY VERTICAL SIDE WALLS OF INCREASED THICKNESS AND THE METAL BETWEEN THE COINED AREA AND THE TEAR-STRIP SEVERING SCORE IS IN COMPRESSION. THIS PREVENTS THE FORMATION OF CRACKS IN THE SCORE AND INCREASES THE STRENGTH OF THE STRIP TO PREVENT ITS BREAKING WHEN A CREASE IS FORMED WHEN THE CAN IS OPENED IMPROPERLY.

Hardness measuring apparatus

Abstract: An apparatus for measuring hardness of a specimen comprises indenting means for forming an indentation in the upper surface of the specimen, loading means for applying a predetermined load onto said indenting means and an optical system for measuring said formed indentation which is a measure of the hardness of the tested specimen. The loading means is mechanically disconnected from the indenting means and adapted to be engaged with the latter, only when the load is applied thereto upon the measurement. By such arrangement, the rigidity, mass as well as bulk of the supporting structure for the indenting means can be considerably reduced, and hence a dynamical load due to such factors which may constitute a source of error in the measurement can be effectively eliminated.

Indentation Of A Functionally Graded Elastic Solid:Application Of An Adhesively Bonded Plate Model

Abstract: This paper examines the influence of adhesive bonding on the flexural interaction between a thin plate and an isotropic elastic halfspace. The modelling is developed for the possible application of the results to the study of functionally stiffness graded elastic media where the grading is restricted to the near surface region.

On The Indentation Of Brittle Solids WhichExperience Fragmentation

Abstract: The paper examines the problem of the interaction between a rigid indentor and a brittle solid which can experience fragmentation during the indentation process. The analysis is achieved through the modification of an existing distinct element code, which incorporates viscoplastic effects in the medium and relatively simple criteria for fragmentation. The procedures are applied to investigate the scale effects in such indentation processes.

The Vickers Indentation Technique Applied ToThe Evaluation Of Thermal Transient Stresses DueTo Quenching Of Brittle Materials

Abstract: Use is made of the domain of stable extension of the Vickers indentation crack system in order to investigate both the thermal shock resistance materials and the thermal transient stresses which appear during quenching of brittle materials. The equivalency of thermally and mechanically induced stresses is assumed such as to describe the relative increase in length of the radial crack during quenching. This approach is checked in using two different glass ceramics, various quenching conditions and sample sizes. Finally is the increase in length of Vickers indentation cracks measured or samples loaded in bending and a maximum of the thermal transient stress evaluated. This offers the mean to approach an effective heat transfer coefficient.