Loading programme

As an example, fig. 7.5 illustrates a two-step loading programme. The dashed curve shows the strain Acti J t — tj) that would have been present at time t > 2 if the second step in stress Act2 had not been applied at tj. The actual strain is this strain plus the strain Act2 J t — t ) produced by the second step in stress applied at ti-... 

It is easy to generalise to a multi-step loading programme ... 

Figure 4.6 illustrates the creep response to a multistep loading programme, in which incremental stresses Aoi, A02,. .., are added at times ti, t2,. .., respectively. The total creep at time t is then given by... 

The integral in Equation (4.3) is called a Duhamel integral, and it is a useful illustration of the consequences of the Boltzmann superposition principle to evaluate the response for a number of simple loading programmes. Recalling the development that leads to Equation (5.2) it can be seen that the Duhamel integral is most simply evaluated by treating it as the summation of a number of response terms. Consider two specific cases ... 

A Maxwell element eonsists of an elastic spring of modulus , = lO Pa and a dashpot of viscosity lO" Pa. Calculate the stress at time t= 100 s in the following loading programme ... 

The strain follows the pattern of the loading programme exactly and in exact proportionality to the magnitude of the loads applied. 

The effect of applying a similar loading programme to a linear viscoelastic solid has several similarities (Figure 5.2(b)). In the most general case, the total strain e is the sum of three separate parts e, and ez. e and are often termed the immediate elastic deformation and the delayed elastic deformation respectively, ez is the Newtonian flow, that is that part of the deformation, which is identical with the deformation of a viscous liquid obeying Newton s law of viscosity. 

Figure 11.4 (a) Loading programme, (b) deformation and (c) direct comparison of creep ec(t) and recovery edt) for a non-linear viscoelastic solid. 

In 1974, the Harmonized Monitoring Programme was set up by the Department of the Environment (DoE). The objective was to provide a network of sites at the lower end of catchments, where water quality data could be collected and analysed in a nationally consistent manner, allowing the loads of materials carried through river catchments into estuaries to be estimated and long-term trends in river quality to be assessed. The complete list of substances to be monitored is diverse and specifies about 115 substances. The pesticides aldrin, dieldrin, y-HCH, heptachlor, p,p -DDT and p,p -DDE are included. Figures 1 and 2 show the downward trend of y-HCH and dieldrin over the past 20 years at the Harmonized Monitoring Sites. This confirms that reductions in environmental concentrations have been achieved, particularly over the past 10 years. 

These latter curves are particularly important when they are obtained experimentally because they are less time consuming and require less specimen preparation than creep curves. Isochronous graphs at several time intervals can also be used to build up creep curves and indicate areas where the main experimental creep programme could be most profitably concentrated. They are also popular as evaluations of deformational behaviour because the data presentation is similar to the conventional tensile test data referred to in Section 2.3. It is interesting to note that the isochronous test method only differs from that of a conventional incremental loading tensile test in that (a) the presence of creep is recognised, and (b) the memory which the material has for its stress history is accounted for by the recovery periods. 

In DI-MS, solid or liquid samples are introduced into a small glass cup and then the cup is inserted into a spring-loaded holder (probe tip). This is heated by a coiled heater up to a maximum of 450 °C thus allowing the sample to be volatilised. The probe can be heated in two different ways ballistic mode or temperature-programme mode. In ballistic mode the probe heats at the maximum rate (150 °C min J) up to the final temperature desired. In the temperature-programme mode the probe heats at a specified rate (from 10 to 100 °C min 1) up to the final set up temperature. 

In one application, the same programmable controllor coordinating the actions of the machine is also providing control over an intrinsically safe robotic arm which loads and unloads the heavy projectiles being processed. In this way, the maximum protection is afforded not only to the operator, but also to the facility. 

LOAD COUNTERS The load and hold registers of the programmable counters I and 2 are loaded to produce variable duty cycle pulse trains. The values loaded to the counters are determined by Eqs. (9.103) and (9.104). 

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