Deformation indentation test

The present review shows how the microhardness technique can be used to elucidate the dependence of a variety of local deformational processes upon polymer texture and morphology. Microhardness is a rather elusive quantity, that is really a combination of other mechanical properties. It is most suitably defined in terms of the pyramid indentation test. Hardness is primarily taken as a measure of the irreversible deformation mechanisms which characterize a polymeric material, though it also involves elastic and time dependent effects which depend on microstructural details. In isotropic lamellar polymers a hardness depression from ideal values, due to the finite crystal thickness, occurs. The interlamellar non-crystalline layer introduces an additional weak component which contributes further to a lowering of the hardness value. Annealing effects and chemical etching are shown to produce, on the contrary, a significant hardening of the material. The prevalent mechanisms for plastic deformation are proposed. Anisotropy behaviour for several oriented materials is critically discussed. 

The plastic properties of a material are often determined by an indentation test [28]. Both static and dynamic test methods are available, but all generally determine the pressure necessary to cause permanent and nonrecoverable deformation. 

Hardness is measured by the Rockwell A-scale diamond cone indentation test (HRA) or by the Vickers diamond pyramid indentation test (HV). Although the Rockwell scale has been used for decades in the carbide industry as a measure of hardness, a true indication of the resistance of the tool to deformation in metal-cutting operations can be obtained only by measuring hardness at elevated temperatures. The hardness of cemented carbides decreases monotonically with increasing temperatures. 

Considering that a standard procedure in estimation of the mechanical properties of minerals is followed in different laboratories, Yushin proposed a scheme, given in Table 6.5.2, where, next to an ordinary hardness test, he proposes calculation of the elastic strain. This is the difference of hardness calculated from the diagonal of undeformed indentation (f/pUsl) and the diagonal of deformed indentation (jffpiast+eiasi)- ... 

Surface destructive alterations around the action site of the indenter are used to determine the degree of brittleness of a material (Section 6.3). They are the outcome of a destructive deformation of the crystal induced by multidirectional shear forces released in the crystal structure. The shear stresses liberated in static indenter tests are significantly in excess of those registered under other methods, being of the order of 50 MPa (Yushkin,... 

Scratch tests have been used for microhardness measurements of polymeric materials (Bierbaum Scratch Hardness Test (ASTM D 1526)). These tests are related to cuts and scratches, and, to some extent, to the wear resistance of materials. Scratch tests are not always related to the resistance to local deformation and they are now being replaced by the preferred indentation test. 

Let us consider one final example the application of atomic force microscopy (AFM) relating to nanoscale scratch and indentation tests on short carbon-fibre-reinforced PEEK/polytetrafluoroethylene (PTFE) composite blends (Han et al, 1999). In the scratch test, the tip was moved across the surface at constant velocity and fixed applied force to produce grooves with nanometre scale dimensions on the PEEK matrix surfaces. The grooves consisted of a central trough with pile-ups on each side. These grooves provide information about the deformation mechanisms and scratch resistance of the individual phases. In the nanoscale, indentation and... 

In particular, phase transformations under contact loading need a more detailed investigation. Both static and dynamic interactions between hard surfaces may result in phase transformations. Hydrostatic and deviatoric stresses must be taken into account and phase transformations in contact loading can be described as deformation-induced transformations. At the same time, the transformation pressures for silicon obtained in indentation tests are in good agreement with the results from high-pressure cell experiments, which utilize hydrostatic loading. 

The resistance of a material to the formation of a permanent surface impression by an indenter is termed hardness. The deformation process must be inelastic and, hence, it is inherently related to the resistance of a material to such a deformation (indentation). Hardness impressions can be formed even in brittle materials, though at higher loads this is usually accompanied by localized cracking. For more ductile materials, however, one would expect hardness to be related to the yield stress of a material. In order to create the surface impression, various geometries are used for the indenter (Fig. 6.30). In most tests, hardness is defined as the applied load divided by the actual or projected area of the impression and, thus, the units are the same as stress (Pa). 

The resistance to permanent deformation of MA is determined by the indentation test conducted in accordance to CEN EN 12697-20 (using cube or Marshall specimens), CEN EN 12697-21, Test W or Test B (using plate specimens) or CEN EN 12697-25 (2005) (cyclic... 

In order to evaluate the actual bonding behaviour between fiber coating and matrix material as well as the load transfer limit of embedded sensing elements, the micro indentation test method can be used [57,58]. A thin slice of material containing the embedded sensing element (dimensions, coating, and position) is deformed this method delivers the shear stress behaviour at the interface when the material is deformed. The recorded force-deformation curve allows the estimation of the limit of reliable sensor operation. 

Hardness is defined as the resistance of a material to deformation, particularly permanent deformation, indentation or scratching. Hardness is purely a relative term and should not be confused with wear and abrasion resistance of plastics. For example, polystyrene has a high hardness but a poor abrasion resistance. Many tests have been devised to measure hardness. However, Rockwell and Durometer hardness tests are commonly used. The Rockwell hardness test measures the net increase in depth impression as the load on an indentor is increased from a fixed minor load to a major load and then returned to a minor load. The hardness numbers derived are just numbers without units. Rockwell hardness numbers in increasing order of hardness are R, L, M, E and K scales. The higher the number in each scale, the harder is the material. The Durometer hardness test is based on the penetration of a specified indentor forced into... 

The second model improves on the first one in the way that static indentation hardness test theory has advanced by introducing plastic deformation into this dynamic system as a parameter. Plastic deformation of the contact area between the particles and the target surface causes radial cracks to propagate outwards from the contact zone and subsurface lateral cracks to move outwards on planes nearly parallel to the surface. Here the parallel with overloaded Vickers static indent tests, which are discussed in detail in... 

LDPE is easily marked by a thumbnail, HOPE is scratched in this way with difficulty, but PP is marked little, if at all. Hardness is defined as the resistance of a material to deformation, particularly permanent deformation, indentation, or scratching. Hardness is a relative term and should not be confused with wear and abrasion resistance. Many tests have been devised to measure hardness. Rockwell and Durometer hardness test are commonly used. 

However, at higher velocities, and cases for which actual dummy head and the panel were us in the tests, the correlation was poor. This is due to the complications arised from modelling of the dummy head exterior (Butyl rubber) and also the plywood panel onto which a one-inch thick Nomax open all honeycomb with fiberglass laminate on both sides were fixed. From the experimental data, data fitting techniques were used to come-up with an alternative non-linear viscoelastic contact force model such that the contact force f is evaluated from the deformation (indentation) 6 and deformation rate 5 according to... 

The simplicity of this plastic model is that all the parameter that is required isp , which can be determined through routine static indentation testing, as also demonstrated for various ductile projectiles (steels and brass) impacted on silicon nitride targets [10]. However, pertinent experimental techniques should be sought to determine more accurately the related dynamic patameters such as impact force, stresses, deformation, duration of impact, coefficient of restitution, and stress wave propagations, etc. Frictional constraint by property mismatch between projectiles and targets of dissimilar materials needs to be taken into account in some cases. 

Depth-sensing Indentation Deformation Wear Tests and Their Effects on... 

The literatme on indentation testing of UHMWPE has covered a variety of aspects of the behavior of UHMWPE. These include effects of processing, oxidation, prior deformation, extent of crystallinity, frequency of loading, and several other issues related to UHMWPE. Indentation tests have spanned a range of sizes from nanoindentation up to indentation tests in the scale of tens of microns. The advantages of these nano- and microscale tests include the ability to sample very small regions of the material, an ability to profile mechanical properties in a single sample at small intervals (hundreds of nanometers and tens of microns, respectively), and an ability to correlate the indentation response with other measurements of material structure and properties. 

There are relatively few papers that have explored the viscoelastic response of UHMWPE to indentation. Typically, indentation tests of viscoelastic materials have included a pause in the load-unload curves at the maximum load to allow creeplike deformation to occur [21]. Researchers have typically inserted a hold time of between 10 and 100... 

This combination of single asperity wear testing, surface deformation mapping of the resultant deformation field, and the subsequent indentation test measurements of the changes in mechanical properties due to the wear process will provide detailed and significant information of the interrelationships between wear, structure, and property evolutions. These observations may also help discern the fundamental deformation mechanisms that result in wear particle formation and how different UHMWPE starting materials evolve with wear deformation. 

Fischer-Cripps AC (2000) A review of analysis methods for suh-micron indentation testing. Vacuum 58 569 Freund LB, Floro JA, Chason E (1999) Extensions of the Stoney formula for substrate curvature to configurations with thin substrates or large deformations. Appl Phys Lett 74(14) 1987... 

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