Stress creep test

Load sample (units H3.i H3.2), then select a stress (creep test) or strain (stress... 

Fig. 11.3. Schematic of strain as a function of time in a constant-stress creep test (adapted from Courtney (1990)).

An interesting three-parameter model (the Burger model has four parameters) was proposed by Hsueh [6] and is shown in Fig. 3b. He demonstrated that for a Hookean elastic element (Ei) in series with a Kelvin solid (E2,ry), the stress-strain rate relations for constant strain rate and constant stress creep tests are,... 

Merry and co-workers [5] used an axisymmetric tension test to perform constant stress tests and compare the creep response of new and old HDPE geomembranes. A 36-h constant stress creep test was performed on new HDPE using a temperature range of 2-53 °C and stresses ranging from 2-15 MPa. Excess material was then stored in a laboratory for an additional seven years, after which time tests on old geo-membranes were performed. Because the experimental results could not be compared directly, an adaption of the Singh-Mitchell creep model for soil, which is a rate process equation. 

Figure 1. Extensometer strains tor constant stress creep tests on OS-44.

Relaxation of the residual stresses induced by autofrettage at 720 MPa (104,400 psi) in reactor tubes k = 2.4), of AISI 4333 M6 at a uniform temperature of 300°C has been studied and it was concluded, on the basis of creep tests for 10,000 h, that after 5.7 years 60% of the original stress would remain (161). 

Mechanical properties of plastics can be determined by short, single-point quaUty control tests and longer, generally multipoint or multiple condition procedures that relate to fundamental polymer properties. Single-point tests iaclude tensile, compressive, flexural, shear, and impact properties of plastics creep, heat aging, creep mpture, and environmental stress-crackiag tests usually result ia multipoint curves or tables for comparison of the original response to post-exposure response. 

Creep, creep mpture, and stress relaxation tests are multiple-point tests requiring long periods of time (1000 h min) to generate useflil data these are standard tests for determining more fundamental polymer properties (202,203). Data for these tests are generated under several... 

Tensile Testing. The most widely used instmment for measuring the viscoelastic properties of soHds is the tensile tester or stress—strain instmment, which extends a sample at constant rate and records the stress. Creep and stress—relaxation can also be measured. Numerous commercial instmments of various sizes and capacities are available. They vary greatiy in terms of automation, from manually operated to completely computer controlled. Some have temperature chambers, which allow measurements over a range of temperatures. Manufacturers include Instron, MTS, Tinius Olsen, Apphed Test Systems, Thwing-Albert, Shimadzu, GRC Instmments, SATEC Systems, Inc., and Monsanto. 

An alloy tie bar in a chemical plant has been designed to withstand a stress, ct, of 25 MN m at 620°C. Creep tests carried out on specimens of the alloy under... 

Type of stress. A uniaxial tensile creep test would not be expected to give the required data if the designer was concerned with torsional or compressive creep. 

It may be felt that the initiation of a stress-corrosion test involves no more than bringing the environment into contact with the specimen in which a stress is generated, but the order in which these steps are carried out may influence the results obtained, as may certain other actions at the start of the test. Thus, in outdoor exposure tests the time of the year at which the test is initiated can have a marked effect upon the time to failure as can the orientation of the specimen, i.e. according to whether the tension surface in bend specimens is horizontal upwards or downwards or at some other angle. But even in laboratory tests, the time at which the stress is applied in relation to the time at which the specimen is exposed to the environment may influence results. Figure 8.100 shows the effects of exposure for 3 h at the applied stress before the solution was introduced to the cell, upon the failure of a magnesium alloy immersed in a chromate-chloride solution. Clearly such prior creep extends the lifetime of specimens and raises the threshold stress very considerably and since other metals are known to be strain-rate sensitive in their cracking response, it is likely that the type of result apparent in Fig. 8.100 is more widely applicable. 

There are several other comparable rheological experimental methods involving linear viscoelastic behavior. Among them are creep tests (constant stress), dynamic mechanical fatigue tests (forced periodic oscillation), and torsion pendulum tests (free oscillation). Viscoelastic data obtained from any of these techniques must be consistent data from the others. 

TTie strain readings of a creep test can be more accessible to a designer if they are presented as a creep modulus. In a viscoelastic material, namely plastic, the strain continues to increase with time while the stress level remains constant. Since the creep modulus equals stress divided by strain, we thus have the appearance of a changing modulus. 

Creep-test specimens may be loaded in tension or flexure (to a lesser degree in compression) in a constant temperature environment. With the load kept constant, deflection or strain is recorded at regular intervals of hours, days, weeks, months, or years. Generally, results are obtained at three or more stress levels. 

The tests are performed under carefully controlled stress (load), temperature, time, and creep (elongation) conditions. To save time, tests for different constant loads are performed simultaneously on different specimens of the same material. Creep tests may be rather extensively conducted, as for example when developing creep data prior to the design and fabrication of the first all-plastic airplane (41). The usual procedure is to plot the creep versus time curve, but other combinations are possible. 

Stress relaxation. In a stress-relaxation test a plastic is deformed by a fixed amount and the stress required to maintain this deformation is measured over a period of time (Fig. 2-33) where (a) recovery after creep, (b) strain increment caused by a stress step function, and (c) strain with stress applied (1) continuously and (2) intermittently. The maximum stress occurs as soon as the deformation takes place and decreases gradually with time from this value. From a practical standpoint, creep measurements are generally considered more important than stress-relaxation tests and are also easier to conduct. 

A creep test can be carried out with an imposed stress, then after a time have its stress suddenly changed to a new value and have the test continued. This type of change in loading allows the creep curve to be predicted. The simple law referred to earlier as the Boltzmann superposition principle, hold for most materials, so that their creep curves can thus be predicted. 

Third, creep data application is generally limited to the identical material, temperature use, stress level, atmospheric conditions, and type of test (that is tensile, flexural, or compressive) with a tolerance of 10%. Only rarely do product requirement conditions coincide with those of a test or, for that matter, are creep data available for all the grades of materials that may be selected by a designer. In such cases a creep test of relatively short duration, say 1,000 hours, can be instigated, and the information be extrapolated to long-... 

Creep test data when plotted on log-log paper usually form a straight line and tend themselves to extrapolation. Tlie slope of the straight line, which indicates a decreasing modulus, depends on the nature of the material (principally its rigidity and temperature of heat deflection), the temperature of the environment in which the product is used, and the amount of stress in relation to tensile strength. 

If data are not available on the effects of time, temperature, and strain rate on modulus, creep tests can be performed at various stress levels as a function of temperature over a reasonable period of time. In this regard, reasonable is a relative term. For applications like rockets and missiles, data obtained over a time period of 4-5 sec to an hour provide the essential information. For structural applications, such as pipelines, data over a period of years are required. 

In conclusion regarding creep testing, it can be stated that creep data and a stress-strain diagram indicate whether plain plastic properties can lead to practical product dimensions or whether a RP has to be substituted to keep the design within the desired proportions. For long-term product use under continuous load, plastic materials have to consider creep with much greater care than would be the case with metals. 

There are two further related sets of tests that can be used to give information on the mechanical properties of viscoelastic polymers, namely creep and stress relaxation. In a creep test, a constant load is applied to the specimen and the elongation is measured as a function of time. In a stress relaxation test, the specimen is strained quickly to a fixed amount and the stress needed to maintain this strain is also measured as a function of time. 

In creep tests, the parameter of interest is the creep compliance, J, defined as the ratio of the creep strain to the applied stress, i.e. 

Stress relaxation tests are alternative ways of measuring the same basic phenomenon in viscoelastic polymers as creep tests, Le. the time-dependent nature of their response to an applied stress. As such, they have also been of value in understanding the behaviour of these materials. The essence of stress relaxation tests is that strain increases with time for a given stress, so that if stress is decreased with time in a controlled manner ( relaxed ), a state... 

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