Rebound hardness

Rebound flexible foam Rebound hardness test Receptor... 

The so-called Shore hardnesses are measured differently for metals and plastics. With hard materials (metals), a scleroscope is used to measure the rebound of a small steel ball. This Shore hardness is thus measured by a dynamic method, which yields the rebound hardness (the impact elasticity of the rubber industry). Soft plastics, on the other hand, are tested with a Shore durometer. This measures the resistance to the penetration of the point of a cone through the contraction of a calibrated spring. The durometer thus works according to a static method, and yields the true Shore hardness as understood by the rubber industry. Like the Rockwell hardness, the Shore hardness is given in scale divisions. 

Hardness Hardness refers to the properties of a material resistant to various kinds of shape changes when force is applied. It is fundamental for many applications and is an important mechanical parameter of materials. There are three principal types of hardness scratch hardness (resistance to fracture or plastic deformation due to friction from a sharp object), indentation hardness (resistance to plastic deformation due to impact from a sharp object), and rebound hardness (height of the bounce of an object dropped on the material). There are several different definitions of hardness, including Brinell hardness, Knoop hardness, Vickers hardness. Shore hardness, etc. 

Many types of hardness tests have been devised. The most common in use are the static indentation tests, eg, Brinell, Rockwell, and Vickers. Dynamic hardness tests involve the elastic response or rebound of a dropped indenter, eg, Scleroscope (Table 1). The approximate relationships among the various hardness tests are given in Table 2. 

Mohs hardness values may vary Shore hardness number is derived from rebound height of standard steel ball when dropped on material from standard height. 

Filler loading Volume, parts Mooney viscosity Optimum cure (at 141°C), min Modulus (at 300%), MPa Tensde strength, MPa Elongation, % Hardness, Shore A NBS abrasion (ASTMD1630) Rebound, %... 

The material known as bouncing putty is also a silicone polymer with the occasional Si—O—B group in the chain, in this case with 1 boron atom to about every 3-100 silicon atoms. The material flows on storage, and on slow extension shows viscous flow. However, small pieces dropped onto a hard surface show a high elastic rebound, whilst on sudden striking they may shatter. The material had some use in electrical equipment, as a children s novelty and as a useful teaching aid, but is now difficult to obtain. 

Vickers and Knoop indentors, Barcol hardness, and Shore durometers (2) (b) to measure the resistance of a material to scratching by another material or by a sharp point, such as the Bierbaum hardness or scratch-resistance test and the Moh one for hardness and (c) to measure rebound efficiency or resilience, such as the various Rockwell hardness tests. The various tests provide different behavior characteristics for plastics, as described by different ASTM standards such as D 785. The ASTM and other sources provide different degrees of comparison for some of these tests. 

Indentation hardness determinations were performed in dynamic mode ( 1500 mm/sec impact speed) using a pendulum impact device and in quasistatic mode ( 0.008 mm/sec impact speed) with a custom-built indentation tester. The spherical indenters were of 2.54 cm diameter and 65.6 g mass, and the pendulum length was 92.3 cm with a release angle of 30°. Quasistatic indentation forces were selected to produce indentations of a similar size to the dynamic indentation test (1.5 to 2.0 mm radius). The compact indentations were measured using a white light interferometer (Zygo Corporation, Middlefield, Connecticut, U.S.A.) and the dent depth, dent diameter, apparent radius of curvature, and pendulum initial and rebound heights were used to calculate the indentation hardness of the compacts. 

The most straightforward way to measure the effect of low temperatures on recovery is by means of a compression set or tension set test. Tests in compression are favoured and a method has been standardised internationally. The procedure is essentially the same as set measurements at normal or elevated temperatures and has been discussed in Chapter 10, Section 3.1. As the recovery of the rubber becomes more sluggish with reduction of temperature the dynamic loss tangent becomes larger and the resilience lower (see Chapter 9), and these parameters are sensitive measures of the effects of low temperatures. Procedures have not been standardized, but rebound resilience tests are inherently simple and quite commonly carried out as a function of temperature. It is found that resilience becomes a minimum when the rubber is in its most leathery state and rises again as the rubber becomes hard and brittle. 

A separate group of methods of limited usefulness in regard to brittle materials, making use of impact action with an indenter or steel ball, with registration of elastic rebound of a hammer or beater from the sample, consists of measurements with the Shore scleroscope, the duroscope and several other testers based on this principle, where the results come close to those obtained by the Brinell method. Working on the same principle is also the impact hardness tester, in which the depth of the indent or scratch made by a diamond indenter is measured. 

Interesting are attempts to design dynamic hardness testers with a higher level of automation. Among these, we should mention the Equotip hardness tester manufactured by Proceq of Switzerland and the Sonodur A probe manufactured by Branson-Krautkramer. In the former, the sampler in the shape of a ball of sintered carbide is mounted in a beater weighing 5.5 g, which is also fitted out with a permanent magnet. This enables measurement of the speed of impulse and rebound by proportional current induction in the measuring coil. The quotient of rebound and impulse speed multiplied by 1000 is defined as hardness L (after the name of the method s author—Leeb). The hardness measurement result L can be converted to HKC, Hb or Hv with the aid of tables. 

SCLEROMETER. An apparatus for determining the hardness of a material by measuring the pressure on a standard point that is required to scratch the material. A scleroscope is a similar apparatus, which measures hardness by determining the rebound of a standard ball dropped on the subject material from a fixed height. 

The following symbols are used in Table 1.3 ot> is strength limit in extension e is defoimation at break E is modulus of elasticity Er is residual deformation (after elastic recoil) Hb is Shore hardness Eel is rebound elasticity T, is glass transition temperature... 

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