Uniaxial tension creep

By studying the sample dilatation versus strain in uniaxial tension creep tests. Buck-nail is able to determine the operative niechanism in each system. Fracture mechanics is used to evaluate the toughness parameters of the various systems. 

Fig. 2-3. Uniaxial tension creep and short-time stress-strain diagram for polymethylmethacrylate at 73.4 2 F. (From Engineering Design for Plastics, Eric Baer, Reinhold, 1964)...

Although nearly all creep and stress-relaxation tests are made in uniaxial tension, it is possible to make biaxial tests in which two stresses are applied at 90° to one another, as discussed in Section VI. In a uniaxial test there is a contraction in the transverse direction, but in a biaxial test the transverse contraction is reduced or even prevented. As a result, biaxial creep is less than uniaxial creep--in cquihiaxial loading it is roughly hall as much for equivalent loading conditions. In the linear region the biaxial strain 2 in each direction is (255.256)... 

Creep tests in uniaxial tension were made at 11 temperatures between —70° and 75°C on strips which were cut 1.27-cm wide, 0.12-cm thick, and 20.0-cm long from a sheet cast from benzene solution. The distance between the grips was 19.0 cm. Bench marks were placed 15.0 cm apart. Displacements were read from the bench marks using a power driven cathetometer of 0.0002-cm sensitivity. The creep tests covered a minimum range of about four decades of time beginning at 10 seconds. The measurements were repeated at 15°, 30°, 40°, and 50°C on a second sheet, also cast from benzene solution. Measurements derived... 

The Poisson ratio, like the bulk, tensile, and shear creep compliance, is an increasing function of time because the lateral contraction cannot develop instantaneously in uniaxial tension but takes an infinite time to reach its ultimate value. In response to a sinusoidal uniaxial stretch, the complete Poison ratio is obtained ... 

Glen Jones (1967) have examined the creep of ice crystals under uniaxial tension at very much lower temperatures, — 50 to — 70 °C, and have found results rather different from those discussed above. After times as long as 50 h the strain rate under constant stress had not become constant but continued to increase approximately as The strain itself was, however, still only a few per cent. The strain rate at fixed temperature was found to behave as in (8.37) but the exponent had the much larger value m = 4+1. The reasons for these differences are not clear but may be due to differences in temperature, deformation mode or crystal perfection in the specimens used. 

Experimental creep data for ceramics have been obtained using mainly flexural or uniaxial compression loading modes. Both approaches can present some important difficulties in the interpretation of the data. For example, in uniaxial compression it is very difficult to perform a test without the presence of friction between the sample and the loading rams. This effect causes specimens to barrel and leads to the presence of a non-uniform stress field. As mentioned in Section 4.3, the bend test is statically indeterminate. Thus, the actual stress distribution depends on the (unknown) deformation behavior of the material. Some experimental approaches have been suggested for dealing with this problem. Unfortunately, the situation can become even more intractable if asymmetric creep occurs. This effect will lead to a shift in the neutral axis during deformation. It is now recommended that creep data be obtained in uniaxial tension and more workers are taking this approach. 

The most common technique employed to date has been that of creep in uniaxial tension. It was shown above that with the inclusion of lateral strain measurements this is a powerful technique giving access to up to 6 independent creep compliance functions. This is more than for any other known method. It further has the overwhelming advantage over many methods, such as say torsional or flexural creep, that the stress is sensibly uniform over the working volume of the specimen. This advantage is paramount in studies of materials displaying non-linear behaviour in creep since analysis of the non-uniform stress situation in non-linear systems is not well developed. Attempts to overcome the non-uniform stress situation in torsion, by recourse to, say, torsion of thin walled tubes, lead to severe difSculties in specimen preparation in oriented materials, when anisotropy of behaviour is to be studied. 

In the original paper [20], the authors reported work on the uniaxial tension of plasticized poly(vinyl chloride), sulphur vulcanizates of butyl rubber and polyisobutylene. Very successful predictions were made at extension ratios of up to 5. Zapas and Craft [21] applied their formulation to multistep stress relaxation and creep and recovery of both plasticized poly(vinyl chloride) and pol3dsobuty-lene. McKenna and Zapas applied a modified form of the model to the torsional deformation of poly(methyl methacrylate) [22]. McKenna and Zapas [23] have used the model in analysis of the tensile behaviour of carbon-black-fllled butyl rubbers. 

As for the uniaxial tension case, while the elastic solution for angular displacement is constant in time for a constant torque input, the viscoelastic bar exhibits increasing displacement from creep over time. Note again that the expression Eq. 8.31 is quite simple in the step input case and analogous in form to the elastic solution Eq. 8.26. For time varying loading, the integration of Eq. 8.29 is nontrivial and results in a more complex form. 

Consistent results and ultimate failure in the middle third of the neck of the specimens indicates that the apparatus is effective at inducing uniaxial tension. Three specimens are mounted in series for creep testing enabling spread of values between specimens to be measured for identical load and environmental conditions. Five creep lines are operated in ambient conditions while a further five are mounted in an incubator cabinet - see Figure 6. 

In the determination of the reference stress, JNC accounts for the creep strain intensity due to heterogeneous stress distribution in the ligament for small scale yielding condition. However, since the reeommended values of pi and p2 in eqn. (13) are based on creep finite element analyses for plates eontaining a through wall notch subjected to uniaxial tension, the applicability to the semi-elliptical surface crack has not been verified yet. 

Thin film or fiber Film/fiber tension probe Significant load" penetration) Linear displacement (uniaxial Time Temperature temperatures creep and cure behavior T T - g) - m... 

Comparison of uniaxial ex-tensional viscosities from rod pulling (open symbols), entrance flow using Cogswell s analysis (solid symbols), and tensile creep (solid symbols with ticks) measurements. From Laun and Schuch (1989). 

There appears to be no information on the uniaxial creep of polymers used as structural adhesives such as is available referring to the creep of adhesive joints in lap-shear or torsion. The latter is reserved for Chapter 7 where the few data that are available are given. An apparent exception is the careful study of a nylon-epoxy adhesive (FM 1000) by Shen and Rutherford (1972). These authors achieved something approaching uniaxial stress using a cylindrical butt joint in direct tension. However, what they called creep was simply the delayed elastic response. They likened the adhesive to a metal in its behaviour and used the classical but inappropriate concept of separating the creep behaviour into three stages as was done many years ago for metals. 

---