Incremental-step test

To reduce the experimental efforts in measuring cyclic stress-strain curves, the incremental-step test can be used. In this test, the strain amphtude is varied block-wise between zero and a maximal value as sketched in figure 10.30. After the block has been repeated several times, the material behaviour does not change an5rmore and a stationary state is arrived at. If the stress is measured at each of the strain maxima, the... 

Fig. 1.25 Incremental step test. Blocks of increasing and decreasing strain amplitude fully...

Fig. 1.26 Cyclic stress-strain curve obtained on Cr-Mo-V steel of 660 MPa yielding strength and 820 MPa ultimate with the incremental step test [32]...

Testing and verification of the ISS have involved a number of unique requirements and constraints. The major contributing factors include the on-orbit assembly process, the series of vehicle stages defining the incremental steps in the assembly process (each with unique functional performance requirements), and the requirement to operate and support human activity under the extreme environmental conditions of space. Add to this that the system is designed, built, and deployed by a consortium of 16 nations, and its parts delivered to their final assembly location by the most complex space vehicle ever built (the Space Shuttle), and the result is arguably the most difficult test and verification program ever attempted. 

Ibid., Vol. 15.03, F 1624, Standard Test Method for Measurement of Hydrogen Embrittlement Threshold in Steel by the Incremental Step Load Technique. ... 

At each velocity a load-stepping test is conducted. The friction torque and bearing temperature, which are plotted continuously, are allowed to reach equilibrium at each loading (see Fig. 3-106) [323]. The equilibrium condition is maintained for about 30 min., at which point the load is increased. At an advance load increment the friction torque and temperature will not stabilize. The slope of the curve will increase sharply in the friction torque or temperature plot. An increase in temperature or torque will eventually result in bearing failure. The pressure limits at several velocities provide a curve showing the limiting PV capability of the bearing material [323]. 

How Many Samples. A first step in deciding how many samples to collect is to divide what constitutes an overexposure by how much or how often an exposure can go over the exposure criteria limit before it is considered important. Given this quantification of importance it is then possible to calculate, using an assumed variabihty, how many samples are required to demonstrate just the significance of an important difference if one exists (5). This is the minimum number of samples required for each hypothesis test, but more samples are usually collected. In the usual tolerance limit type of testing where the criteria is not more than some fraction of predicted exceedances at some confidence level, increasing the number of samples does not increase confidence as much as in tests of means. Thus it works out that the incremental benefit above about seven samples is small. 

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. 

The study of isotopes makes it necessary to introduce a further refinement in the general method of solution. I have been using a test of the relative increment to adjust the time step. The relative increment is the change in a dependent variable divided by the value of that variable. This is not a useful test, however, when the value of the variable approaches zero, because the test requires progressively smaller time steps. None of the variables I considered in previous chapters has approached zero, and so there has been no problem with this test. But carbon isotope ratios of seawater have delta values near zero, and a problem may occur when calculating these values. I have modified subroutine CHECKSTEP to permit a flexible response to this situation. 

The calculation is performed in terms of degrees Celsius, including values both above and below zero. It is not convenient, therefore, to use the relative increment of temperature as a test for step size in subroutine CHECKSTEP. I use absolute increments instead. At the end of subroutine SPECS, I set incind equal to 3 for all equations, limiting the absolute increment in temperature to 3° per time step. Zonally averaged heat capacity as a function of latitude is calculated in subroutine CLIMINP in terms of land fraction and the heat capacity parameters specified in SPECS. It is returned in the array heap. 

The fiber modulus and matrix shear modulus are also required for the analysis. The fiber s coordinates are recorded directly from the stage controllers to the computer. The operator begins the test from the keyboard. The x and y stages move the fiber end to a position directly under the debonder tip the z stage then moves the sample surface to within 4 yum of the tip. The z-stage approach is slowed down to 0.04 jan/step at a rate of 6 steps/s. The balance readout is monitored, at a load of 2 g the loading is stopped, and the fiber end returned to the field of view of the camera. The location of the indent is noted and corrections are made, if necessary, to center the point of contact. Loading is then continued from 4 g in approximately 1 g increments. Debond is determined to have occurred when an interfacial crack is visible for 90-120° on the fiber perimeter. The load at which this occurs is used to calculate the interfacial shear stress at debond. 

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