Indentation resistance

At the target composition, the following empirical properties should be determined (a) indentation (resistance to permanent deformation) and, when required, (b) resistance to abrasion by studded tyres, (c) resistance to fuel for application on airfields and (d) resistance to de-icing fluid for application on airfields. 

PU two-part sealants cure to a durable rubber consistency with high elasticity, abrasion/indentation resistance and bonding strength over a wide range of temperatures. 

The three-layer PP system offers a high degree of impact resistance, peel strength especially at high temperatures, indentation resistance and flexibility. The main properties of the coating are reported in Table 2. 

FIGURE 10.2 (a) Indentation resistance of auxetic materials compared with a conventional... 

Hardness is a measurement of material resistance to plastic deformation in most cases. It is a simple nondestructive technique to test material indentation resistance, scratch resistance, wear resistance, or machinability. Hardness testing can be conducted by various methods, and it has long been used in analyzing part mechanical properties. In reverse engineering, this test is also widely used to check the material heat treatment condition and strength, particularly for a noncritical part, to save costs. The hardness of a material is usually quantitatively represented by a hardness number in various scales. The most utilized scales are Brinell, Rockwell, and Vickers for bulk hardness measurements. Knoop, Vickers microhardness, and other microhardness scales are used for very small area hardness measurements. Rockwell superficial and Shore scleroscope tests are used for surface hardness measurements. Surface hardness can also be measured on a nanoscale today. 

The radiation and temperature dependent mechanical properties of viscoelastic materials (modulus and loss) are of great interest throughout the plastics, polymer, and rubber from initial design to routine production. There are a number of laboratory research instruments are available to determine these properties. All these hardness tests conducted on polymeric materials involve the penetration of the sample under consideration by loaded spheres or other geometric shapes [1]. Most of these tests are to some extent arbitrary because the penetration of an indenter into viscoelastic material increases with time. For example, standard durometer test (the "Shore A") is widely used to measure the static "hardness" or resistance to indentation. However, it does not measure basic material properties, and its results depend on the specimen geometry (it is difficult to make available the identity of the initial position of the devices on cylinder or spherical surfaces while measuring) and test conditions, and some arbitrary time must be selected to compare different materials. 

Knoop developed an accepted method of measuring abrasive hardness using a diamond indenter of pyramidal shape and forcing it into the material to be evaluated with a fixed, often 100-g, load. The depth of penetration is then determined from the length and width of the indentation produced. Unlike WoodeU s method, Knoop values are static and primarily measure resistance to plastic flow and surface deformation. Variables such as load, temperature, and environment, which affect determination of hardness by the Knoop procedure, have been examined in detail (9). 

Hardness. The resistance of a fabricated mbber article to indentation, ie, hardness, is influenced by the amount and shape of its fillers. High loadings increase hardness. Fillers in the form of platelets or flakes, such as clays or mica, impart greater hardness to elastomers than other particle shapes at equivalent loadings. 

Hardness. Glass hardness tests usually measure the resistance to abrasion by grinding or grit-blasting, resistance to scratching, or penetration by an indenter. The method to be used depends on expected service conditions. Knoop hardness (Table 4) is commonly used, because other methods usually fracture the glass. 

Hardness is a measure of a material s resistance to deformation. In this article hardness is taken to be the measure of a material s resistance to indentation by a tool or indenter harder than itself This seems a relatively simple concept until mathematical analysis is attempted the elastic, plastic, and elastic recovery properties of a material are involved, making the relationship quite complex. Further complications are introduced by variations in elastic modulus and frictional coefficients. 

A hardness indentation causes both elastic and plastic deformations which activate certain strengthening mechanisms in metals. Dislocations created by the deformation result in strain hardening of metals. Thus the indentation hardness test, which is a measure of resistance to deformation, is affected by the rate of strain hardening. 

Most instmments make use of a probe geometry which gives an increasing area of contact as penetration proceeds. In this way, at some depth of penetration, the resisting force can become sufficient to balance the appHed force on the indentor. Unfortunately, many geometries, eg, diamonds, pyramids, and cones, do not permit the calculation of basic viscoelastic quantities from the results. Penetrometers of this type include the Pfund, Rockwell, Tukon, and Buchholz testers, used to measure indentation hardness which is dependent on modulus. 

For erosive wear. Rockwell or Brinell hardness is likely to show an inverse relation with carbon and low alloy steels. If they contain over about 0.55 percent carbon, they can be hardened to a high level. However, at the same or even at lower hardness, certain martensitic cast irons (HC 250 and Ni-Hard) can out perform carbon and low alloy steel considerably. For simplification, each of these alloys can be considered a mixture of hard carbide and hardened steel. The usual hardness tests tend to reflect chiefly the steel portion, indicating perhaps from 500 to 650 BHN. Even the Rockwell diamond cone indenter is too large to measure the hardness of the carbides a sharp diamond point with a light load must be used. The Vickers diamond pyramid indenter provides this, giving values around 1,100 for the iron carbide in Ni-Hard and 1,700 for the chromium carbide in HC 250. (These numbers have the same mathematical basis as the more common Brinell hardness numbers.) The microscopically revealed differences in carbide hardness accounts for the superior erosion resistance of these cast irons versus the hardened steels. 

The hardness and abrasion resistance of anodic coatings have never been easy properties to measure, but the development of a British Standard on hard anodising has made this essential. Film hardness is best measured by making microhardness indents on a cross-section of a film , but a minimum film thickness of 25 tm is required. For abrasion resistance measurements, a test based on a loaded abrasive wheel , which moves backwards and forwards over the film surface, has improved the sensitivity of such measurements. 

Hardness Coating hardness is related to the method of measurement. Results reflect the resistance to scratching as well as to indentation. 

Hardness basically is the resistance to indentation as measured under specific conditions such as depth of indentation, load applied, and time period. Different tests relate to different hardness behaviors of plastics. They include Barcol, Brinell, durom-eter, Knoop, Mohs, Rockwell, Shore, and Vicat (2). 

Hardness is closely related to strength, stiffness, scratch resistance, wear resistance, and brittleness. The opposite characteristic, softness, is associated with ductility. There are different kinds of hardness that measure a number of different properties (Fig. 5-5). The usual hardness tests are listed in three categories (a) to measure the resistance of a material to indentation by an indentor some measure indentation with the load applied, some the residual indentation after it is removed, such as tests using Brinell hardness,... 

D urometer hardness An arbitrary numerical value that measures the resistance to intention of a blunt indenter point of the durometer. The higher the number, the greater indention hardness. 

Although hardness is a somewhat nebulous term, it can be defined in terms of the tensile modulus of elasticity. From a more practical side, it is usually characterized by a combination of three measurable parameters (1) scratch resistance (2) abrasion or mar resistance and (3) indentation under load. To measure scratch resistance or hardness, an approach is where a specimen is moved laterally under a loaded diamond point. The hardness value is expressed as the load divided by the width of the scratch. In other tests, especially in the paint industry, the surface is scratched with lead pencils of different hardnesses. The hardness of the surface is defined by the pencil hardness that first causes a visible scratch. Other tests include a sand-blast spray evaluation. 

The drawback with the use of the Gillmore needle is that it is not a test of any well-defined rheological property, but of resistance to indentation. 

Hardness signifies the resistance to deformation shown by materials undergoing abrasion, cutting and indentation. As strength is also a measure of resistance to deformation, the two properties are, to some extent, related but not all hard metals are strong, for hardness takes... 

Hardness is determined by hardness tests which involve the measurement of a material s resistance to surface penetration by an indentor with a force applied to it The indentation process occurs by plastic deformation of metals and alloys. Hardness is therefore inherently related to plastic flow resistance of these materials. Brittle materials, such as glass and ceramics at room temperature, can also be subjected to hardness testing by indentation. This implies that these materials are capable of plastic flow, at least at the microscopic level. However, hardness testing of brittle materials is frequently accompanied by unicrack formation, and this fact makes the relationship between hardness and flow strength less direct than it is for metals. 

Durometer hardness is defined as the measure of resistance to indentation using either a macro- or microhardness tester. To the pharmaceutical drug manufacturer, hardness is important because of its relationship to ultimate mechanical properties— particularly modulus. In general, softer compounds of the same elastomer base have better coring and reseal properties, whereas harder compounds tend to process better on high-speed filling lines. 

The indentation process is driven by the applied load, and resisted by two principal factors the resistance of the specimen to plastic deformation (and elastic deformation) plus the frictional resistance at the indenter/specimen interface. The ratio of these resistances changes with the size of the indentation because the plastic resistance is proportional to the volume of the indentation, while the frictional resistance is proportional to the surface area of the indentation. Therefore, the ratio varies as the reciprocal indentation size. This interpretation has been tested and found to be valid by Bystrzycki and Varin (1993). 

Hardness also depends on which face of a non-cubic crystal is being indented. The difference may be large. For a crystal with tetragonal symmetry the face that is normal to the c-axis can be expected to be different from those that are normal to the a-axes. Similarly the basal faces of hexagonal crystals are different from the prism faces. One extreme case is graphite where the resistance to indentation on the basal plane is very different than the resistance on the prism planes. 

---