Time-dependent properties creep

The time dependent properties creep and stress relaxation can be considered as tests to monitor degradation or as degradation tests that add mechanical stress. 

Sintered sihcon carbide retains its strength at elevated temperatures and shows excellent time-dependent properties such as creep and slow crack growth resistance. Reaction-bonded SiC, because of the presence of free sihcon in its microstmcture, exhibits slightly inferior elevated temperature properties as compared to sintered sihcon carbide. Table 2 (11,43) and Table 3 (44) show selected mechanical properties of sihcon carbide at room and elevated temperatures. 

Researchers have examined the creep and creep recovery of textile fibers extensively (13-21). For example, Hunt and Darlington (16, 17) studied the effects of temperature, humidity, and previous thermal history on the creep properties of Nylon 6,6. They were able to explain the shift in creep curves with changes in temperature and humidity. Lead-erman (19) studied the time dependence of creep at different temperatures and humidities. Shifts in creep curves due to changes in temperature and humidity were explained with simple equations and convenient shift factors. Morton and Hearle (21) also examined the dependence of fiber creep on temperature and humidity. Meredith (20) studied many mechanical properties, including creep of several generic fiber types. Phenomenological theory of linear viscoelasticity of semicrystalline polymers has been tested with creep measurements performed on textile fibers (18). From these works one can readily appreciate that creep behavior is affected by many factors on both practical and theoretical levels. 

Strictly speaking, there are no static viscoelastic properties as viscoelastic properties are always time-dependent. However, creep and stress relaxation experiments can be considered quasi-static experiments from which the creep compliance and the modulus can be obtained (4). Such tests are commonly applied in uniaxial conditions for simphcity. The usual time range of quasi-static transient measurements is limited to times not less than 10 s. The reasons for this is that in actual experiments it takes a short period of time to apply the force or the deformation to the sample, and a transitory dynamic response overlaps the idealized creep or relaxation experiment. There is no limitation on the maximum time, but usually it is restricted to a maximum of 10" s. In fact, this range of times is complementary, in the corresponding frequency scale, to that of dynamic experiments. Accordingly, to compare these two complementary techniques, procedures of interconversion of data (time frequency or its inverse) are needed. Some of these procedures are discussed in Chapters 6 and 9. 

Stress relaxation, creep, and other time-dependent properties... 

Vegetable fibers also show time-dependent properties associated with other polymeric materials. There is, however, only limited data available, which analyzes creep, relaxation, or strain-rate behavior of vegetable fibers. 

Creep / relaxation test time dependent properties compliance... 

All polymer materials used in reinforced plastics display some viscoelastic or time-dependent properties. The origins of creep in composites stem from the behaviour of polymers under load together with local stress redistributions between fibre and matrix as a function of time. There is little creep at normal temperatures in the reinforcing fibres. The origin of the creep mechanisms is related to the nature and levels of internal bonding forces between the chains of the polymer, which are influenced by temperature and moisture. 

Viscoelastic characteristics of polymers may be measured by either static or dynamic mechanical tests. The most common static methods are by measurement of creep, the time-dependent deformation of a polymer sample under constant load, or stress relaxation, the time-dependent load required to maintain a polymer sample at a constant extent of deformation. The results of such tests are expressed as the time-dependent parameters, creep compliance J t) (instantaneous strain/stress) and stress relaxation modulus Git) (instantaneous stress/strain) respectively. The more important of these, from the point of view of adhesive joints, is creep compliance (see also Pressure-sensitive adhesives - adhesion properties). Typical curves of creep and creep recovery for an uncross-Unked rubber (approximated by a three-parameter model) and a cross-linked rubber (approximated by a Voigt element) are shown in Fig. 2. 

Stress rupture—Italicized values in Section II, Part D indicate that allowable stress values are governed by time-dependent properties, e.g. stress rupture and creep rate. 

Polymers have viscoelastic properties they contain both elastic and viscous components. When the solicitation submitted to polymeric materials is mechanical, their time-dependent properties can be examined by creep, stress relaxation, and DMA. In DMA, a sample is subjected to a sinusoidal load (stress or strain), with the... 

Thus, if the time dependent creep or relaxation properties of a material are known, the complex moduli and compliances can be calculated simply via Fourier transforms (Eqs. 6.49-6.50). Comparison back to the Laplace transforms (Eq. 6.44), we see that s or ioi times the Laplace or Fourier transform, respectively, of the time dependent properties provide the transformed properties which can be used in the correspondence principle forms Eqs. 6.43 and 6.46. 

Polymers are viscoelastic materials in other words, their mechanical properties have both elastic and viscous components. Their response to an applied stress depends on the time-scale of the experiment. The phenomenon of creep, which is the increase in strain with time at constant applied stress, and stress relaxation, which is the decrease in stress at constant strain, are examples of time-dependent properties. 

The relaxation of polymers is another time-dependent property. The standard relaxation test is opposite that of the creep test the strain is kept constant, and the relaxation in stress is measured as shown in Figure 2.5. The relaxation in stress is measured as shown in Figure 2.9. The relaxation test is a standard test to determine if a polymer can be classified as a fluid. Fluid behavior is characterized by the stress relaxing to zero, o —> 0 as t 0. 

A fully automated microscale indentor known as the Nano Indentor is available from Nano Instmments (257—259). Used with the Berkovich diamond indentor, this system has load and displacement resolutions of 0.3 N and 0.16 nm, respectively. Multiple indentations can be made on one specimen with spatial accuracy of better than 200 nm using a computer controlled sample manipulation table. This allows spatial mapping of mechanical properties. Hardness and elastic modulus are typically measured (259,260) but time-dependent phenomena such as creep and adhesive strength can also be monitored. 

The viscoelastic nature of the matrix in many fibre reinforced plastics causes their properties to be time and temperature dependent. Under a constant stress they exhibit creep which will be more pronounced as the temperature increases. However, since fibres exhibit negligible creep, the time dependence of the properties of fibre reinforced plastics is very much less than that for the unreinforced matrix. 

In this book no prior knowledge of plastics is assumed. Chapter 1 provides a brief introduction to the structure of plastics and it provides an insight to the way in which their unique structure affects their performance. There is a resume of the main types of plastics which are available. Chapter 2 deals with the mechanical properties of unreinforced and reinforced plastics under the general heading of deformation. The time dependent behaviour of the materials is introduced and simple design procedures are illustrated. Chapter 3 continues the discussion on properties but concentrates on fracture as caused by creep, fatigue and impact. The concepts of fracture mechanics are also introduced for reinforced and unreinforced plastics. 

Depending on construction and orientation of stress relative to reinforcement, it may not be necessary to provide extensive data on time-dependent stiffness properties since their effects may be small and can frequently be considered by rule of thumb using established practical design approaches. When time dependent strength properties are required, creep and other data are used most effectively. There are many RP products that have had super life spans of many decades. Included are products that have been subjected to different dynamic loads in many different environments from very low temperatures to very high corrosive conditions, etc. An example is aircraft primary structures (10,14,62). 

---