Strain-controlled load

Conclusion In strain-controlled loading (booth A), increasing the stifTness increases the elastically stored energy, whereas decreasing the stiffness increases the elastically stored energy in stress-controlled loading (booth B), see figure 13.2. 

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. 

The second type of load is called a strain-controlled load. A thermal expansion load is a strain-controlled load. Using the brick stack example, assume that the brick is restrained in the vertical direction by a steel structure. Let us also assume, for the simplicity of the calculation, that the thermal expansion is totally restrained. The compressive thermal 5tc in th brick stack is calculated as... 

Constant strain for stress relaxation tests and constant load creep tests may be conducted in simple devices. Temperature control is critical since the results are usually applied as a spectral representation for structural analysis or research purposes. Figure 8 illustrates a multistation creep tester with automated data recorders. Strain and load endurance tests are conducted in similar devices, but the conditions existing at failure and time to failure are normally the only data required. The endurance tests are used frequently to supplement the constant displacement rate tests for routine evaluation. 

The kind of test that is appropriate in the two cases is different in the first case, load should be increased until failure while in the second, some sort of deformation (or strain) control is appropriate. (A practical case of the latter is the Marshall stability test for evaluating the performance of asphalt concrete). In reality some combination of stress and strain capacity is required. 

The strain-control test has the advantage that information on the strain capacity is obtained, as well as the maximum stress that can be sustained, the latter value being similar to that obtained in a conventional test with constant loading rate. In the following we shall discuss behaviour in compression in tests with a constant strain rate. 

A Procedurefor the Specification, Calibration and Testing of Strain Gage Load Cellsfor Industrial Process Weighing andForce Measurement, Institute of Measurement and Control, London. 

Cyclic loads were applied to the specimens using a hydraulic feedback load system which could be either load-, stroke-, or strain-controlled. Sinusoidal and linear-ramp waveforms were generally used. Fatigue frequencies were varied from 0.1 to 15 Hz. Unless otherwise stated, mechanical property data were obtained at a fatigue frequency of 10 Hz. 

A strain-controlled or fully confined recovery test was conducted. It started immediately after completion of the programming with the same loading device and environmental chamber. The chamber was heated at a ramp rate of 0.6°C/min. The heating was continued until the temperature starting the programming (71 °C or 79 °C) was reached. After that, the temperature was maintained constant for several hours and the stress was continuously recorded. This process was stopped when further stress recovery was negligible. 

In order to conduct 2-D programming, a special truss fixture was fabricated and cruciform specimens were used [57]. It has been realized that this may not be realistic for large panels or structures. Together with the requirement for high temperature, and loading rate and pre-strain control, it turns out to be a challenging task. Therefore, an alternative programming approach is... 

The secant modulus, E(t), is defined as the relationship between stress and strain at the loading time, t, for a material subjected to controlled strain rate loading that is ... 

The typical type of loading used in all stiffness modulus laboratory test determinations is either sinusoidal, pulse or controlled strain rate loading. The pulse or the sinusoidal type of loading is explained in Figure 7.1. 

Table 7.1 Strain to be applied during a controlled strain rate loading test ...

Strain gauge load cell design is a perfected science. In general, the load cell structure is cylindrical or ring shaped and is precision machined from a material such as I7-4ph stainless steel. The physical size of the structure controls the deflection that it experiences under load and, therefore, the capacity rating... 

---