Creep experiments, uniaxial

Uniaxial creep experiments were carried out on two types of commercial polyethylene In the presence of various environments. One polymer, which will be designated hereafter as sample D, was a linear polyethylene having a weight-average molecular weight (My) of 99,000 and a nominal density of 0.966 g/cm. ... 

Structural adhesives such as epoxy resins can be treated as any rigid polymer and samples can be machined from cast sheets to produce test-pieces. These can then be used to measure typical tensile properties such as failure stress and strain. Using accurate exten-sometry, it is possible to characterize completely the uniaxial properties of an adhesive. The Creep of adhesive joints is especially important for structural adhesives maintained at high temperature. It is possible to determine the creep resistance of such materials by applying suitable loads at an appropriate temperature to samples of the adhesive, and to record the deformation with time. From such data, it will soon be evident if the adhesive is suitable for use or if it will cause a joint to deform with time. It is important to remember that humidity is likely to affect the properties of the adhesive, and in a long-term creep experiment, the humidity could cause premature failure. 

More recently, in a series of papers Zapas and Crissman [5-7] reported the results of uniaxial creep experiments for different polyethylenes under varied... 

From equation (1), It can be seen that If the bar at time x=0 is subjected to a single step in strain, p(t), then the stress necessary to keep the bar stretched at time t is equal to H(p(t), t), where H(l,t) 0. From data obtained from single step stress-relaxation experiments carried out at different levels of strain. It is evident that one can determine the stress response for any other strain history in uniaxial extension. However, since equation (1) is nonlinear, one cannot determine the strain as a function of the stress, as for example In a creep experiment. Equation (1) applies to the type of experiment where, knowing the strain history, one can determine the stress response and the calculated values can then be compared with experimentally determined quantities. 

A direct simple method to study the viscoelastic properties of a given sample is the creep experiment It is carried out by instantaneously applying a constant force, which is then followed by a measurement of the resulting deformation as a function of time. Figure 5.1 indicates schematically a possible result, referring to the case where an uniaxial tensile load is applied, which then leads to an elongation AL. In general, it will be found that the creep curve represents a superposition of three contributions... 

This polymer in its commercial form exhibits sufficient photoconductivity to be used in imaging processes in spite of its stereochemical impurity and conformational disorder. Simple uniaxial creep experiments on thin films above the glass transition temperature were shown to produce only small orientations, i.e. order parameters < 2) when defined in the... 

The exact laws, based on continuum analysis of the fibers and the matrix, would be very complicated. The analysis would involve equilibrium of stresses around, and in, the fibers and compatibility of matrix deformation with the fiber strains. Furthermore, end and edge effects near the free surfaces of the composite material would introduce complications. However, a simplified model can be developed for the interior of the composite material based on the notion that the fibers and the matrix interact only by having to experience the same longitudinal strain. Otherwise, the phases behave as two uniaxially stressed materials. McLean5 introduced such a model for materials with elastic fibers and he notes that McDanels et al.6 developed the model for the case where both the fibrous phase and the matrix phase are creeping. In both cases, the longitudinal parameters are the same, namely... 

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. 

Based on the theory presented here, eq(9) is applicable not only to creep failure, but also to the other deformations where 0 varies with time. If we substitute n and c which are obtained from creep failure experiments into eq(9), the P(tg) for the constant rate extension process must be described by this equation. To examine this, the constant rate extension experiment has been carried out. In constant rate uniaxial extension, stretch ratio X is given by a function of time t as following. 

It has been shown that the life time in the creep process of rubbery polymers scatters largeley but obeys the specified statistical distribution which are introduced theoretically based on some assumptions. Two assumptions are made here that "one crack leads the body to failure" and "the v th crack leads the body to failure". The former assumption leads the exponential distribution of tg, and the latter the unimodal distribution when v>2. It has been explained from experiments that the distribution of tg for pure rubbers of vulcanized SBR and NR are the exponential, type and for filled systems the unimodal type. Theory introduced here can be applied not only to the creep failiire but also to the failure process varing stress level such as uniaxial extension with constant strain rate. It has been demonstrated that the distribution of Xg, the stretch ratio at breedc in the constant rate of extension, is well estimated by the theory substituting the parameters n and c which are obtained from creep failure experiment to eq(l9). 

The technique of loading dilatometry, in which a small, controlled uniaxial stress Pj is applied to a powda- compact during sintering has been used to investigate the simultaneous occurrence of densification and creep, as well as their interaction (57,58). Parameters such as the stress intensity factor < > and the sintering stress 2 can be determined from the data. In the experiments, simultaneous measurement of the time-dependent axial and radial strains allows the determination of the... 

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