ABSTRACT
Mechanical properties and creep behavior of vehicle tires representing vulcanized rubber was studied. A cut piece of dimension 4 cm/0.04 m by 2.5cm/0.025m was used for the tensile test experiments. The cut piece had a tensile strength of 0.014Mpa, a modulus of 0.024Nm2, and an extension to break at 0.575 when tested at room temperature. Increasing the temperature by heating in water to about 500c produced a tensile strength of 0.0093Mpa, modulus of 0.0077Nm2 and an elongation to break at 1.2. The creep material used had initial length of 7.5cm/0.075m, the material had a creep of 0.0086Nm when tested with 500g/0.5kg weight at room temperature. The creep when heated in water at 500c was 0.00417Nm2. Temperature affected both the mechanical and the creep properties of vulcanized rubber. Unvulcanized rubber produced different result. It had a dimension of 6cm/0.06m by 4cm/0.04m and was used for the tensile test experiment. In this, tensile strength was 0.00417Mpa, a modulus of 0.10Nm2, and an extension to break at 4.16 when tested at room temperature. Increasing the temperature by heating in water to about 500c produced a tensile strength of 0.05Mpa, a modulus of 0.019Nm2 and an elongation of 2.52. The material had creep of 0.00089Nm at room temperature. The creep material 0f dimension 8cm/0.8m by 4cm/0.4m when heated in water at 500c was 0.00019Nm2.
CHAPTER ONE
INTRODUCTION
BACKGROUND OF PROBLEM
We observe that the use of polymers example plastics and rubbers has grown drastically since world II, when they assumed enhanced commercial importance. This explosive growth in polymer application derives from their competitive costs and versatile properties. Polymers vary from liquid and soft rubber to hard and rigid solids. The unique properties of polymer coupled with their light weight makes them preferable alternatives to metallic and ceramic materials in many applications. In selection of a polymer for specific use, careful consideration must be given to its mechanical properties. This consideration is important not only in those applications where the mechanical properties play a primary role but also in other applications where other characteristics of polymer such as electrical, thermal etc. are of crucial importance. In the latter case, mechanical stability and durability of polymer maybe required for the part to perform its functions satisfactorily.This combination produces a material that is further improved compared to their respective building blocks (Taranu et al., 2012).
The mechanical behavior of a polymer is a function of its microstructure and morphology. Polymer morphology itself depends on many structural and environmental factors. Compared with those of metals and ceramics. Polymers properties show a much stronger dependence on temperature and time. ( Yuan et al 2002 )
This strong and temperature sensitivity of polymer properties is a consequences of the viscoelastic nature of polymers. This implies that polymers exhibits combined viscous and elastic behavior. For example depending on the temperature and stress levels, a polymer may show linear elasticity behavior, yield phenomena, plastic deformation, cold drawing. ( Taranu et al 2009 ). An amorphous polymer with Tg below ambient temperature may display nonlinear but recoverable deformation or even exhibit viscous flow. Given the complexity of polymer response to applied stresses and strains, it is imperative that of their judicious use, those who work with polymers have an elementary knowledge of how polymer behavior is influenced by structural and environmental factors.
( Huang et al 2013 )
Polymers components like other materials may fail to perform their intende functions in specific applications as a result of ;
Excessive elastic deformation
Yielding or excessive plastic deformation
Fracture
Polymers show excessive plastic deformation particularly in structural load bearing applications due to inadequate rigidity or stiffness. For such failure, the controlling material mechanical properties is the elastic modulus.
Failures of polymers in certain applications to carry design load or occasional accidental overloads maybe due to excessive plastic deformation resulting from the inadequate strength properties of the polymer. ( Hartman et al 2006 ).
STATEMENT OF PROBLEM
Neither the Maxwell model nor Voight model accurately predicts the behavior of real polymeric materials therefore further research are carried on to better understand the behaviors better. (Ferry et al, 1980).
Further experiments were carried out by Maxwell and Voigt to better understand the mechanical and viscoelastic behavior of polymer.
MAXWELL MODEL
To overcome the poor description of real polymeric material by either the spring or the dashpot, Maxwell suggested a simple series combination of both element. In Maxwell model, E, the instantaneous tensile modulus, characterizes the response of the spring while the viscosity, η, defines viscous response. In the following description we make no distinction between the types of stress. ( Ward et al, 1971 ) Thus, we use the symbol E even in case where we actually referring to the shearing stress( Ward et al, 1971 ). This, of course does not detract from the validity of the argument.
In the Maxwell element, both the spring and the dashpot support the same stress. Therefore,
σ = σa = σd (01 )
where σa and σd are both stress on the spring and the dashpot, respectively. However, the overall strain and strain rates are the sum of the elemental strain and strain rates, respectively.(Ferry et al, 1980) That is,
εT = εs + εd(02)
εs = σ/E and εd = σ/ η (03)
where εT is the total strain rate, while εs and εd are the strain rates of the spring and dashpot respectively.
Maxwell element equation on substitution becomes;
εT = 1/E σ + 1/ησ (04)
Creep experiment
In creep, the sample is subjected to an instantaneous constant stress σ0 and the strain is monitored as a function of time.(Ferry et al, 1980) Since the stress is constant, dσ/dt is zero and therefore, the equation becomes;
εT = 1/ησ0 (05)
solving for the equation and noting that the initial strain is σ0/E, the equation for the Maxwell element for creep can be written as;
εT = σ0 1/E + t/ η
On removal of the applied stress, the material experience creep recovery and the creep recovery of the Maxwell element. It shows that the instantaneous application of a constant stress σ0 is initially followed by an instantaneous deformation due to the response of the spring by an amount σ0/E. With the sustained application of this stress, the dashpot flows to relieve the stress. ( Alklonis et al, 1972 ).
The dashpot deforms linearly with time as long as the strain is maintained. On the removal of the applied stress, the spring contracts instantaneously by an amount equal to its extension. However, the deformation due to the viscous flow of the dashpot is retained as permanent set. Thus, Maxwell element predicts that in creep and creep recovery experiment, the response includes elastic strain and strain recovery, creep and permanent set. . (Ferry et al, 1980)
