Viscoelastic material selection

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. 

There are several ways in which the impact properties of plastics can be improved if the material selected does not have sufficient impact strength. One method is by altering the composition of the material so that it is no longer a glassy plastic at the operating temperature of the product (Chapter 6). In the case of PVC this is done by the addition of an impact modifier which can be a compatible plastic such as an acrylic or a nitrile rubber. The addition of such a material lowers the glass transition temperature and the material becomes a rubbery viscoelastic plastic with much improved impact properties. This is one of the methods in which PVC materials are made to exhibit superior impact properties. 

The three polymers that were chosen for study, e.g. PMMA (2), BPDN (3), and Hytrel (4), were selected because they represent a wide range of viscoelastic materials. These materials were processed into plaques. The plaques were annealed at 12S°C between highly polished chrome plated flat plates and cooled slowly to minimize the effects of residual stresses. Viscoelastic measurements were made under conditions cited above on two test specimens that were cut from... 

An approach suggested by Williams et al. (117) uses an energy balance equation for initiation of flaw growth in a linearly viscoelastic material. A spherical flaw geometry was selected for simplicity since the expressions for the critical values of applied stress to cause fracture were similar for several flaw geometries. The critical conditions are based on a power (energy rate) balance... 

Morrow, C.T. and Mohsensin, N.N. 1966. Consideration of selected agricultural products as viscoelastic materials. J. Food Sci. 31 686-698. 

From a more fundamental point of view, the selection of different inden-ter geometries and loading conditions offer the possibility of exploring the viscoelastic/viscoplastic response and brittle failure mechanisms over a wide range of strain and strain rates. The relationship between imposed contact strain and indenter geometry has been quite well established for normal indentation. In the case of a conical or pyramidal indenter, the mean contact strain is usually considered to depend on the contact slope, 0 (Fig. 2a). For metals, Tabor [32] has established that the mean strain is about 0.2 tanG, i.e. independent of the indentation depth. A similar relationship seems to hold for polymers although there is some indication that the proportionality could be lower than 0.2 for viscoelastic materials [33,34], In the case of a sphere, an... 

Dynamic properties tests on viscoelastic materials fall into the general categories of resonance tests and non-resonance tests (1). They can be further subdivided into tests using base motion excitation (2), (3) and direct force excitation (4), (5). Still another classification may be made according to whether the specimen is stressed in extension, shear, or dilatation. An important issue in the selection of test methods is whether it will... 

In this section we want to indicate the broad range of problems in which damping can be useful, and to present a few specific examples. Further, we list some of the principal properties and characteristics of viscoelastic materials that must be considered in selecting appropriate candidate materials for applications. 

Some of what we see as the principal properties and characteristics of import in the development and selection of viscoelastic materials for structural damping are listed below. Table IV notes "passive" properties and characteristics, that is, those that pertain to the material itself and to its performance in the specified operating conditions. Somewhat in contrast. Table V lists "Active or Interactive" properties and characteristics that concern the interaction of the viscoelastic material with other materials and with the environment of the treatment. These tabulations are surely incomplete, but are intended to suggest the breadth of considerations that may be involved in material selection. 

Viscoelastic layers convert strain energy into heat, suppressing the harmful effects viscoelastic damping systems reduce overall vibration response and remove resonant peaks. Co-curing viscoelastic materials (adhesives, etc.) with RPs reduces the space required by damping suppression systems and cuts manufacturing costs. FEA coupled with materials databases enables selection of materials with optimum damping properties. 

The Autovibron system is designed to measure the temperature dependence of the complex modulus (E ), dynamic storage modulus (E ), dynamic loss modulus (E") and dynamic loss tangent (tan 6) of viscoelastic materials at specific selected frequencies (0.01 to 1 Hz, 3.5, 11, 35, 110 Hz) of strain input. During measurement, a sinusoidal tensile strain is imposed on one end of the sample, and a sinusoidal tensile stress is measured at the other end. The phase angle 6 between strain and stress in the sample is measured. The instrument uses two transducers for detection of the complex dynamic modulus (ratio of maximum stress amplitude to maximum strain amplitude) and the phase angle 6 between stress and strain. From these two quantities, the real part (E ) and the imaginary part (E ) of the complex dynamic modulus (E ) can be calculated. 

Treny and Duperray [32] have pointed out that issues of human comfort relating to noise and vibration are one of the major priorities for materials structural research and development in the field of transportation. Dynamic mechanical analysis (DMA) testing provides a way to characterise in an accurate manner the viscoelastic properties of all the material used in vehicle interiors. Using a unique database software allowed easy material selection according to their viscoelastic properties. An approach for the optimisation of materials through the combination of selective database software and specific numerical calculation methods, to predict the final acoustic behaviour during the materials selection and systems development period are presented. 

Impact Behavior. The Fracture (qv) behavior of polymers under large-scale deformations is of considerable interest for engineering applications of polymers. It is important for material selection to have some practical criteria for decision making. Such criteria are discussed by Menges and Boden (99), who indicate how linear viscoelastic DMA data can be of benefit as a decision-making aid. 

In coping with vibration problems, there are various approaches. Depending on the application and object, selection of the modulus of the material and loss tangent will be important. A viscoelastic material such as soft silicone gel is suitable for recent lightweight, precision devices. 

Mechanical properties of the knee components are derived from selected referred articles (See Table 1). Bone s density and its mechanical properties varied along its length, but in this study tibia and femur simplified as a non-linear isotropic material. Meniscus is composed of fibrocartilage, an anisotropic nonlinear viscoelastic material and its viscoelastic behavior make constant after 1500s [2]. Because an instance load applied in the dynamic analysis, so the properties variation is neglected. Articular cartilage and tibia plateau cartilage are assumed as isotropic elastic material. 

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