Strain-controlled experiment

In the formulas, e t), e(t), a t) is respectively strain rate, strain and stress. C is the elastic wave velocity in pressure bar, 5060 m/s. is the elastic modulus of bar material, 200 Gpa. A, L is respectively cross-sectional area and length of specimen. The impact velocity of experiment was controlled within 2-6 m/s. Because the impact velocity of bullet is provided by high pressure gas, the impact speed of coal and limestone samples will vary slightly. 

The light-induced shape-memory functionality of the polymers is quantified by cyclic, photomechanical experiments luider stress-controlled and/or strain-controlled conditions. These cyclic experiments have been designed in analogy to those characterizing a thermally-induced SME. 

Low-Cycle Fatigue Properties. Results of low-cycle fatigue experiments under strain control on as-worked W plate material at 815 °C are shown in Fig. 3.1-172. Low-cycle fatigue tests of pure W were performed in the temperature range between 1650 °C and 3300 C [1.184]. A relationship Afaiiure = exp(—aT) was found to be valid up to test temperatures of 2700 °C [1.185]. In all cases the failure mode was intercrystalline. Similar results were also obtained at a test temperature of 1232 °C [ 1.186]. The deformation behavior of Nb and Nb IZr under plastic-strain control at room temperature was investigated and cyclic stress-strain curves published [1.182]. 

An important issue is the influence of an electrochemical environment on the cyclic deformation behavior of metals [74,33-35]. As illustrated by the data in Fig. 1 for a carbon-manganese steel in high-temperature water, environment does not typically affect the relationship between stresses and strains derived from the maximum tensile (or compressive) points of steady-state (saturation) hysteresis loops [36]. Such loops should relate to elastic and plastic deformation prior to substantial CF microcracking. CF data of the sort shown in Fig, 1 are produced by either stress or total strain controlled uniaxial fatigue experiments, identical to the methods... 

Experiments to characterize low-cycle CF life according to the Coffin-Manson relationship (Eq 2) foUow from an ASTM standard for LCF of metals in air, and a classic ASTM manual on laboratory methods (see Ref 37 and ASTM E 606, Practice for Strain-Controlled Fatigue Testing). Low-cycle CF... 

C-VOR-200 Rheometer Bohlin Strain-controlled creep experiments... 

Fig. 10.6. Formation of slip bands in ALMg 3 in a strain-controlled fatigue experiment Rs = —1, a = 0.5%, grain size 50iim). Optical micrograph (after [148])...

If we look at an S-N curve (figure 10.18), we can see that the number of cycles to failure strongly depends on the stress in the LCF regime. Small scatter in the stress-strain properties of different specimens (due to scatter in the material properties, for example) would cause large changes in the number of cycles to failure measured in the experiment. The scatter band would thus be rather wide. In this regime, strain-controlled experiments are more useful since, with a prescribed strain amplitude, the scatter of the stress amplitude is small. Furthermore, stress-controlled experiments would also cause more rapid failure due to the reduction in the cross section of the specimen caused by crack propagation ]113]. 

Fig. 10.28. Cyclic stress-strain behaviour at the beginning of strain-controlled fatigue experiments (after [130]). The controlled variable is shown on the left, the material answer in the centre...

If a strain-controlled fatigue experiment is performed at a non-zero mean strain, cyclic relaxation may occur in addition to cyclic hardening or softening, with the mean stress decreasing over time (figure 10.31(a)). If, on the other hand, the experiment is stress-controlled at a non-zero mean stress, the hys-... 

Analyzers are available for both strain (displacement) and stress (force) control. On the one hand, under strain control, the probe is displaced and the resulting stress of the sample is measured by implementing a force balance transducer, which utilizes different shafts. The advantages of strain control include a better short-term response for materials of low viscosity, which allows experiments of stress relaxation to be performed with relative ease. On the other hand, under stress control a set force is applied while several other experimental conditions (temperature, frequency, or time) are varied. Although stress control analyses are... 

All the experiments shown here are performed in a strain-controlled manner with a strain rate of H0 s. ... 

The experiments are performed in a strain-controlled mode, which means that the X, Y and Z axes are the principal axes of strain. If the powder causes the differences in stress between XI and X3 or between Y2 and Y4, then the principal axes of strain are not equal to the principal axes of stress. 

However, the software of advanced stress controlled instruments allows for running an experiment at variable strain amplitudes. In this operation mode, several iterative cycles have to be measured before the actual measurement. In these iterations, the applied torque is adjusted to produce the desired strain amplitude [27]. In contrast to the classical way of amplitude adjustment, new operating modes of stress controlled rheometers (termed Direct Strain Oscillation or Continuous Oscillation) use a feedback control to compare the current strain signal y(t) at time t to the desired pure sinusoidal signal yd t) = y o sin(control loop then adjusts the torque accordingly in order to minimize the difference Yd t + At) — y (t -I- At)I for the next step at t -I- At. This deformation control enables a stress controlled rheometer to mimic a strain controlled experiment [27]. This holds true even beyond the linear regime where nonlinear contributions to the strain wave are compensated for and are then transferred into the stress wave, as the control loop tries to make the appropriate adjustments to the torque within minimum time. 

For the strain controlled LAOS experiment (LAOStrain), a pure sinusoidal strain input is needed, which is why in the past it was necessary to use a SMT-rheometer. With the new developments in the deformation control of CMT-rheometers that enable them to perform strain controlled experiments [27], the question arises whether these instruments can be used in the same way as SMT-rheometers for LAOStrain experiments. It is of great interest to know if they can deliver quantitatively identical results or if the deformation control loop of the CMT-Rheometers influences the measured nonlinearities in the stress wave. Since CMT-rheometers are much more common due to the lower price and simpler design, it would be a great advantage if they could also be used for LAOStrain experiments, therefore... 

LAOS measurements for two samples, a polyisoprene melt (abbreviated PI-84k, Mw = 84,000 g/mol, PDI = 1.04) and a 10 wt% solution of poly isobutylene (abbreviated PIB, Af = 1.1 xlO g/mol) in oligoisobutylene, were conducted on four different rheometers. The first two were separated motor transducer(SMT)-rheometers, namely the ARES-G2 (TA Instruments) and the ARES-LS (TA Instruments) with a IKFRTNl transducer. The DHR-3 (TA Instruments) and the MCR501 (Anton Paar) are in principle stress controlled instruments, but can be used for strain controlled experiments when using the deformation control feedback option (called continuous oscillation for DHR-3 and direct strain oscillation for MCR501). 

The stress-strain behavior of a material provides important information relevant to its range of applicability. Load bearing applications may require certain stiffness or strength properties, the latter of which has been addressed in (see Chap. 19). For strain-controlled loading modes experienced by sealants, the modulus must be sufflciently low and the strain capabilities sufficiently high to provide adequate flexibility to meet the mechanical or thermally driven deformations. Due in part to the popularity of screw-driven test frames, most stress-strain characterization experiments have traditionally been carried out at a constant crosshead displacement rate, effectively straining the specimen at the desired rate. Results obtained are often quite rate and temperature dependent, so care is needed in reporting these details. 

Both controlled-angular displacement (strain-controlled) and controlled-torque (stress-controlled) rotational rheometers are used, with the former giving superior performance at high frequencies and the latter better precision at low frequencies. There have also recently appeared on the market instruments said to be capable of operating in both modes. Controlled torque instruments can also be used to make creep and creep recovery tests, which are described in the next section. In order to obtain a linear viscoelastic characterization that includes the terminal zone, it is sometimes useful to combine data from oscillatory flow with those from a creep experiment, and this is also discussed in the following section. Rheometrical methods are described in some detail in several books [3,8,9]... 

Concentrations of hydrostatic stresses ahead of and equivalent plastic strains at the local carbide crack tips arise in both mode I and mode II loadings as shown in [5] and [11] - [13]. They give rise to the assumption of localized void nucleation and growth in both failure modes (Fig. 12). These processes are known from experiment to control the failure process unaer mode I loading [1]. The results presented suggest a crack propagation by void nucleation, growth and coalescence as shown in Fig. 13. 

The Institute has many-year experience of investigations and developments in the field of NDT. These are, mainly, developments which allowed creation of a series of eddy current flaw detectors for various applications. The Institute has traditionally studied the physico-mechanical properties of materials, their stressed-strained state, fracture mechanics and developed on this basis the procedures and instruments which measure the properties and predict the behaviour of materials. Quite important are also developments of technologies and equipment for control of thickness and adhesion of thin protective coatings on various bases, corrosion control of underground pipelines by indirect method, acoustic emission control of hydrogen and corrosion cracking in structural materials, etc. 

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