Indentor pressing

It can be assumed that epoxy polymers structure loosely packed matrix sustains the main load in indentor pressing process as the least stiff its component. Its dimension f can be calculated according to the mixtures rule [9] ... 

We will consider the problem of a series of rigid indentors pressed into a viscoelastic half-space (y>0) and moving across it. If plane strain conditions are to hold, the indentors must be infinitely long in one direction, taken to be the z direction, and of uniform cross-section. Also the loading distribution must be uniform along each punch. We consider a typical cross-section of this configuration. All subsequent discussion refers to this cross-section of which the material occupies the half-plane y>0. 

Consider the problem of a rigid indentor, pressed into a viscoelastic half-space, under a varying load. Friction between the indentor and the half-space is neglected here Graham (1980) considers this problem, in the case of complete and partial adhesion, and Graham and Sneddon (1981) examine the problem with rotational friction. As noted above, our approach will be based on (5.1.2). The strategy will be to reduce it to a form analogous to the elastic equation, and then use the familiar elastic solutions to determine the viscoelastic quantities. 

Penetration—Indentation. Penetration and indentation tests have long been used to characterize viscoelastic materials such as asphalt, mbber, plastics, and coatings. The basic test consists of pressing an indentor of prescribed geometry against the test surface. Most instmments have an indenting tip, eg, cone, needle, or hemisphere, attached to a short rod that is held vertically. The load is controlled at some constant value, and the time of indentation is specified the size or depth of the indentation is measured. Instmments have been built which allow loads as low as 10 N with penetration depths less than mm. The entire experiment is carried out in the vacuum chamber of a scanning electron microscope with which the penetration is monitored (248). 

Often, Hertz s work [27] is presented in a very simple form as the solution to the problem of a compliant spherical indentor against a rigid planar substrate. The assumption of the modeling make it clear that this solution is the same as the model of a rigid sphere pressed against a compliant planar substrate. In these cases, the contact radius a is related to the radius of the indentor R, the modulus E, and the Poisson s ratio v of the non-rigid material, and the compressive load P by... 

Some ductile plastics, such as PC and ABS, can be fabricated like metals with punching and cold-forming techniques. These processing techniques are analogous to the hardness tests in that a rigid indentor is pressed into a sheet of a less-rigid plastic. 

Hardness is measured by a specially shaped indentor that protrudes from a flat base. The other end is held against a spring. When held against a sample, the indentor is pressed into the case. The movement of the spring is measured either by analogue dial or by a digital display. The harder the sample, the more the indentor moves into the case. In all scales, 0 is very soft and 100 very hard. The two scales used in polyurethanes are the A and the D Durometer scales. Figure 7.5 shows the relationship between the Shore A and D scales. 

In addition to those standardized tests, two other test methods, monotonic creep and microhardness, have been developed by Hough and Wright [48]. In the monotonic creep test, the strain response to a constant stress rate is monitored. The deviation of the stress-strain characteristics in air and in the fluid of interest is taken to be the initiation of ESC. This method is shown to differentiate to a high resolution between polymers, and in the short term, the ESCR of polymer/fluid pairs that exhibit mild/weak interactions can be distinguished. The microhardness method, in which a pyramidal diamond indentor is pressed into the surface of the polymer component at a known load and for a known time, has the potential for mass screening of plastic/fluid compatibility, including extraction as well as absorption, and should be of interest to polymer suppliers. 

Conventional tensile and tear tests are not particularly suitable tools for a detailed investigation of strength-related properties of. small finished components or over the thickness of a bulky product, unless the item can be sectioned to size. An alternative approach, particularly suitable for diagnostic work, involves the use of a puncture test [17]. An indentor of specified geometry is pressed into the component, and the force to cause rupture is measured at a specified penetration or over a range of depths. One suitable application is the study of aging across the thickness of a bulky product [18]. 

A test more frequently found in Continental Europe than in the U.K. or the U.S.A. is the ball indentation hardness test, which is standardized in ISO 2039. Part 1 [5], also dual numbered as BS 2782. Method 365D [6]. In this a 5 mm diameter hardened steel ball is pressed into the test surface under a specified load so that the indentation is between 0.07 and 0.10 mm for method A. or between 0.15 and 0.35 mm for method B. The time of application of the load is 30 seconds, and a minimum test piece thickness of 4mm is recommended. Unlike the Shore or IRHD scales, vvherc hardness is directly related to the penetration of the indentor. the ball indentation hardness is given by ... 

A durometer is an instrument for measuring hardness by pressing a needle-like indentor into the specimen. Operationally, a durometer resembles the Barcol tester in that the instrument is pressed onto the sample surface until it reaches a stop ring. This forces the indentor into the material, and a system of levers transforms the depth of penetration into a pointer movement on an indicating dial, which is... 

Hardness. Hardness is probably the least difficult and most often measured property of vulcanized elastomers. It is in almost every fist of rubber specifications. As measured, hardness is the relative resistance of the rubber surface to deformation by an indentor. The hardness of rubber is measured by using a small spring-loaded gauge known as a durome-ter (ASTM D 2240). One measures the hardness by pressing the indentor against the rub-... 

Rockwell hardness (ASTM D785) is directly related to the indentation hardness of a plastic material. The indentor, a polished steel ball, is pressed into the surface... 

From a five-senses point of view, a ball indentor measures about the same properties as detected by pressing on a solid sample with one s thumb however, quantitative engineering values are obtained with proper instrumentation. 

Hardness - Hardness, as applied to rubber products, is the relative resistance of the surface to indentation under specified conditions. Hardness of rubber is usually measured with a small spring-loaded hardness gauge known as a durometer (ASTM D-2240). The durometer may be handheld or mounted on a stand. The measurement is made by pressing the indentor against the sample and reading the scale, which is calibrated in arbitrary units ranging from 0 (soft) to 100 (hard). A Type A durometer is used for most soft rubber products there is also a Type D durometer for hard rubber and plastic-like materials. On the A scale, a gum rubber band would measure around 40, a tire tread 60, and a shoe sole 80. 

The hardness range of this class of materials is from 60 Shore A to 60 Shore D. Hardness, modulus and tensile strength all increase as the PP content increases. (The hardness of rubber compounds is typically measured by the Shore Durometer test an indentor is pressed into the material, under a specified load, and the amount of penetration determines the hardness. A circular tip is used for the A scale and a pointed tip is used for the D scale. D means harder materids.)... 

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