Ball Indentation Test Research Articles

Recovered elastic and plastic strains at residual micro-indentations in an MgO crystal

Abstract Elastic and plastic unloading effects are described for aligned diamond pyramid indentations imposed on a (001) MgO crystal surface. Substantial elastic recovery effects are explained in terms of the theoretical Hertzian behaviour for a ball indentation test. Limited reversal of the plastic deformation field occurs, involving dislocation processes which first produce an extensive amount of slip and cracking during the loading portion of the indentation cycle.

Effect of heating on the hardness of human bone.

Specimens of dry human bone wee subjected to gradual or sudden heating up to different temperatures. After heating they were cooled suddenly or gradually to room temperature. Their hardness was studied by the Vickers and ball indentation tests. The deformation produced was studied by interferometric and phase-contrast techniques. Heating caused the bone to become brittle without any considerable changes in the surface features. Gradual cooling induced surface hardening. Heating was not found to affect indentation-induced surface distortion.

Young's modulus of lignin from a continuous indentation test

The deformation of lignin in a continuous ball indentation test was almost entirely elastic up to a stress of 2.2 × 108 Pa (22 kg mm−2). The load versus depth of indentation curve of the lignin followed closely the classical Hertz equation thus enabling the Young's modulus of lignin to be calculated. From these results a stress-strain curve for lignin was drawn.

Ball indentation tests and the grain structure of lead

An investigation of the connexion between the appearance of “hilling” around ball indentations in cast lead and the relationship between the ball indentation pressure and the uniaxial stress behavior of lead reveals a strong influence of grain orientation. Both static and dynamic tests were conducted on lead specimens, and a careful correlation was made between Tabor's indentation constant C and the microstructure of the metal. Experiments conducted by other investigators had established the value of Tabor's constant at approximately 2·8. Such tests, however, were confined to essentially isotropic and homogeneous metal specimens. Present results indicate substantially higher values of the indentation constant for surfaces of polycrystalline lead of either columnar or oriented grain structure, but significantly lower values for less-oriented grain structures. Furthermore, tests of single crystals of lead gave values of Tabor's constant ranging from 2·3 to 5·5, depending on the orientation, with the smallest value resulting from indentations of a (111) or principal slip surface. However, static tests of randomly oriented polycrystalline lead gave a value of 2·9, very near the values obtained by other investigators and predicted on a theoretical basis for isotropic homogeneous materials.

The dependence of yield stress on strain rate as determined from ball-indentation tests

A ball-indentation experiment was conducted to explore the possibility of using this test for the investigation of strain-rate dependence of yield properties of materials. The initial velocities of indentation used were between 0.0002 and 3 ips. Force and depth of penetration were measured continuously during a test. Using these and the results given in a previous report, the dependence of yield pressure of the indentation test on penetration velocity is shown for a range of velocities from 0.0002 to 300 ips. Results are presented for annealed C1018 steel, annealed 1100 F and annealed 6061-T6 aluminums. For all materials tested, the dependence of yield stress on strain rate seems to be derivable from the ball-indentation results by using Tabor's empirical formulas and an equation relating strain rate and velocity of penetration. The increase of yield pressure over the range of indentation velocities is 100 percent for steel, 30 percent for 1100 aluminum and 20 percent for 6061 aluminum.

Meyer Analysis of Metals

Messrs. Finniston, Jones and Madsen discuss in a recent communication1 the variation of ultimate hardness number Pu with the Meyer index n, in the ball indentation test. There are several good reasons for associating a decrease in the n value with a decrease in the work-hardening capacity of a metal, especially since n and Pu have hitherto shown a negative correlation coefficient. Finniston et al., however, suggest that this generalization should be reconsidered, and they give results showing a positive correlation coefficient between Pu and n for metals of non-cubic structure and of increasing anisotropy as judged by results for thermal expansion. Their proposal is interesting because certain anomalies exist regarding the n value. Thus, during the progressive cold rolling of copper it may drop from 2.34 to 2.0 at an early stage and then remain more or less constant, while the Pu value will continue to increase and thus indicate further work-hardening2. This peculiarity has not been explained, and it should be remarked that the progressively rolled copper increasingly develops anisotropic properties (preferred orientation) as shown by X-ray diffraction and etching tests. This effect does not appear to be inconsistent with their proposal.

Theory of Hardness and Measurements Applicable to Contact Problems

The theory of the hardness is reviewed and measurements are made with respect to the application of hardness data to contact problems, particularly for the determination of the real contact area. The ball indentation method is recommended, with hardness H defined as the ratio P/Sm between the contact load P and the mouth area Sm of the indentation. This hardness is not a single valued material constant but is a function of the specific depth D of the indentation, where D is the ratio between the actual depth and the radius of curvature of the indentation. The variation of H with geometric and metallurgical conditions is discussed together with some empirical formulae to describe the experimental results. The relation between the hardness and the yield point is discussed. A comparison with Brinell and Rockwell hardness numbers is also made. The indentation which two crossed cylindrical rods produce in each other is found to give nearly the same hardness as the ball indentation tests if D exceeds a certain value.

Hardness Testing of Vulcanized Rubber. V. Investigation of the Ball Indentation Test. Part 3. Condition of the Rubber Surface

Hardness Testing of Vulcanized Rubber. V. Investigation of the Ball Indentation Test. Part 3. Condition of the Rubber Surface

The Significance of Tensile and Other Mechanical Test Properties of Metals

Critical consideration of the conventional quantities obtained from the tensile test reveals their limitations for design purposes, especially where notch-fatigue effects predominate. Against such notch-fatigue the importance of high work-hardening capacity in a metal is emphasized, and its relation to tensile elongation behaviour outlined. It is suggested that even conventional tensile records may yield some rough measure of work-hardening capacity prior to cracking by means of quantities which have here been called “plasticity ratio” and “plasticity value”.Attention is drawn to certain refined methods of plotting tensile diagrams. A review of published results indicates a basic generalization for plastic deformation. If a graph of true stress P against one of the several forms of true strain ∈ be obtained, then the basic deformation diagram is in general the same for tension, compression, torsion, and indentation. It consists first of a curved portion extending up to the point of critical plasticity which corresponds with the onset of “necking” in tension. The curve afterwards flattens out and continues upwards almost as a straight line. The complete diagram may be considered very approximately as of the logarithmic form P = κεm, and its gradient (or the value of m) will be high in materials with very high strain-hardening capacity, and low in those which have been cold-worked.Since work-hardening capacity has hitherto been largely determined by ball indentation tests, a correlation has been made between these and tensile tests. This has revealed some shortcomings of the Meyer n value for indentation. Heat treatment and crystal grain size influence the plastic properties of a metal and the effects of both have been studied. For specimens of various crystal sizes there is similarity in form between certain tensile and indentation diagrams. From these an appropriate ultimate true stress value may be obtained which is practically independent of crystal grain size and therefore enables comparisons of materials to be made. Similarly there is a fundamental limiting “pressure of fluidity” for metals which is independent of their initial content of cold work. Values for this pressure have been determined. The pressure of fluidity obtained by linear extrapolation of a Stead true-stress tensile diagram is lower than that determined by heavy cold-rolling experiments.The deformation and strain-hardening which take place up to the point of “critical plasticity” (i.e. “necking” in the tensile test) appear to differ in internal crystallographic mechanism from that developing beyond this point. It is found that several physical properties reach limiting values at the “necking” stress.Investigation shows that heat-treated steels with sorbitic microstructures may have a relatively low capacity for strain-hardening. Their employment under conditions of notch-fatigue is therefore not attractive. On the other hand their resistance to wear may be relatively good. Wear remains a complex problem, but examples are given where relatively good wear resistance has corresponded with relatively high “plasticity values”.

Hardness Testing of Vulcanized Rubber. III. Investigation of the Ball Indentation Test. Part 1

Hardness Testing of Vulcanized Rubber. III. Investigation of the Ball Indentation Test. Part 1

Relation of the Hardness Number to the Applied Load in the Static Ball Indentation Test of Wood

Relation of the Hardness Number to the Applied Load in the Static Ball Indentation Test of Wood

On the depth of the impression in the ball indentation test of wood

On the depth of the impression in the ball indentation test of wood