Mechanical testing creep

Mechanical Properties Instron Mechanical Test Creep test Stress relaxation test... 

Rheometric Scientific markets several devices designed for characterizing viscoelastic fluids. These instmments measure the response of a Hquid to sinusoidal oscillatory motion to determine dynamic viscosity as well as storage and loss moduH. The Rheometric Scientific line includes a fluids spectrometer (RFS-II), a dynamic spectrometer (RDS-7700 series II), and a mechanical spectrometer (RMS-800). The fluids spectrometer is designed for fairly low viscosity materials. The dynamic spectrometer can be used to test soHds, melts, and Hquids at frequencies from 10 to 500 rad/s and as a function of strain ampHtude and temperature. It is a stripped down version of the extremely versatile mechanical spectrometer, which is both a dynamic viscometer and a dynamic mechanical testing device. The RMS-800 can carry out measurements under rotational shear, oscillatory shear, torsional motion, and tension compression, as well as normal stress measurements. Step strain, creep, and creep recovery modes are also available. It is used on a wide range of materials, including adhesives, pastes, mbber, and plastics. 

Oilfields in the North Sea provide some of the harshest environments for polymers, coupled with a requirement for reliability. Many environmental tests have therefore been performed to demonstrate the fitness-for-purpose of the materials and the products before they are put into service. Of recent examples [33-35], a complete test rig has been set up to test 250-300 mm diameter pipes, made of steel with a polypropylene jacket for thermal insulation and corrosion protection, with a design temperature of 140 °C, internal pressures of up to 50 MPa (500 bar) and a water depth of 350 m (external pressure 3.5 MPa or 35 bar). In the test rig the oil filled pipes are maintained at 140 °C in constantly renewed sea water at a pressure of 30 bar. Tests last for 3 years and after 2 years there have been no significant changes in melt flow index or mechanical properties. A separate programme was established for the selection of materials for the internal sheath of pipelines, whose purpose is to contain the oil and protect the main steel armour windings. Environmental ageing was performed first (immersion in oil, sea water and acid) and followed by mechanical tests as well as specialised tests (rapid gas decompression, methane permeability) related to the application. Creep was measured separately. 

Foamed blends of ethylene-styrene interpolymer and LDPE were subjected to a range of mechanical tests, including compressive impact testing, Instron compression and Poisson s ratio measurements, compressive creep measurements and compression set and recovery measurements. The data obtained were compared with those for EVA and the suitability of these foamed blends as replacements for EVA in the manufacture of soccer shin guards and midsoles for sports shoes was evaluated. 20 refs. 

Figure 3.1 The five most common mechanical tests (I) constant elongation for tensile strength determinations, (2) constant force for creep determinations, (3) fixed elongation for stress relaxation determinations, (4) cyclic strain for dynamic mechanical determinations, and (5) impact for impact determinations. (After J. Fried, Plastics Engineering, July 1982, with permission.)...

Stress relaxation and creep are measured with the aid of two distinct experimental methods, which are frequently used in the mechanical testing of solid polymers, especially over longer periods. 

Mechanical Characterization of Sulfur-Asphalt. The serviceable life of a pavement comes to an end when the distress it suffers from traffic and climatic stresses reduces significantly either the structural capacity or riding quality of the pavement below an acceptable minimum. Consequently, the material properties of most interest to pavement designers are those which permit the prediction of the various forms of distress—resilient modulus, fatigue, creep, time-temperature shift, rutting parameters, and thermal coefficient of expansion. These material properties are determined from resilient modulus tests, flexure fatigue tests, creep tests, permanent deformation tests, and thermal expansion tests. 

In Figure 21.8, a softening temperature for the polystyrene endblock is shown as a function of endblock molecular weight (ranging from about 6000 to 30000). The softening temperature is characterized as the onset of test specimen creep (in a small-strain dynamic mechanical test in tensile mode), the creep point occurring when strain extension becomes considerable in order to maintain the appropriate stress level to continue the test. This softening temperature lies below the measured Tg, and it is an indication of the... 

If the loaded specimen is allowed to elongate for some lime and the stress is then removed, creep recovery will be observed. An uncross-linked amorphous polymer approximates a highly viscous Iluid in such a mechanical test. Hence the elongation-time curve of Fig. l-3c is fitted by an equation of the form... 

Similar curves can be constructed for creep or dynamic mechanical test data of amorphous polymers. Because of the equivalence of time and temperature. 

Nelson investigated the relationship between density and physical properties, e.g., flexural modulus, Gardner impact, heat distortion, ten-sile/flexural strength, coefficient of linear thermal expansion, dynamic mechanical testing, and creep testing. The specific gravity of the SRIM obtained was changed from about 0.3 to 1.2. 

See, for example, the product literature for Perkin Elmer s Diamond TMA and PYRIS software. The TMA and the DMA can both be used for running simple mechanical tests like stress-strain curves, creep-recovery, heat set and stress relaxation. Other vendors have similar packages. 

The microstructures of the consolidated and deformed samples were characterized by X-ray diffraction, optical and electron microscopy (SEM and TEM). The samples for mechanical testing have been prepared by spark erosion. The linear thermal expansion was determined by using a thermomechanical system (TMA). The temperature-dependent elastic moduli have been measured by the resonance frequency and the pulse-echo method. The bulk moduli were determined by synchrotron radiation diffraction using a high-pressure diamond-die cell at HASYLAB. The compression and creep tests were performed with computer-controlled tensile testing and creep machines. 

Mechanical Tests. Impact strength, tensile strength, elongation and hysteresis, creep and recovery, flexibility and foldability, mold shrinkage, heat distortion. 

Mechanical tests using bending, tensile and compressive load conditions including the determination of elastic constants of the orthotropic material were carried out under room and high temperature conditions. For creep tests in tension four testing devices were established and creep tests longer than 6,000 h were carried out with different CMC qualities. The WHIPOX CMCs show much better creep resistance compared to state-of-the-art metallic combustor materials. 

In vitro mechanical tests should also focus on the dynamic behavior of a given polymer scaffold, which is very crucial for applications like knee/hip joint repair and vascular grafts. Certain polymers like polypropylene show creep behavior, i.e., exhibit dimensional changes nnder continuous load and cannot be used to make vascular grafts [15]. 

J. R. Zuiker, A model for the creep response of oxide-oxide ceramic matrix composites, in Thermal and Mechanical Test Methods and Behavior of Continuous-Fiber Ceramic Composites, M. G. Jenkins, et al., Eds., American Society for Testing and Materials, West Conshohocken, PA, (1997). 

Entry data in SMAT are currently more than 12,000 data points on 11 different kinds of mechanical tests (including tensile, low cycle fatigue, creep) for 10 kinds of FBR structural steels. 

The transition from linear to non-hnear viscoelastic behaviour and the transition threshold has been investigated with time and temperature. Thermal and mechanical tests were applied followed by isothermal creep tests at temperature steps and with differing stress levels. Isochronal creep curves were constructed to reveal the non-linearity threshold at different times and temperatures. The study was important for design and performance of elastomeric components. ... 

Figure 17.62 The creep response of a composite specimen to an applied stress. Source Reprinted from Course on Mechanical Testing of Advanced Fibre Composites, University of London, Imperial College, Sep 1995.

Graph of stress as a function of strain constructed from data obtained in any mechanical test in which a load is apphed to a material and continuous measurements of stress and strain are made simultaneously. It is constructed for tensile, creep, or torsional loadings. 

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. 

The mechanical testing of MI SiC/SiC has been reported previously [9,10]. This work has been focused on the long-term durability response of the material under different loads and times creep and dwell fatigue. Most of the testing was done at 1204°C with limited testing at 815 C. 

---