Autoclave history

Perhaps a more severe comparison of model response is the time history of the centerline temperature. These values reflect the interaction of several phenomena the reaction itself, the heat liberated by the reaction, the heat storage capacity of the material, and the rate at which heat can be carried away from the centerline region by conduction. Figure 8 shows the temperatures predicted by the Chiao and finite element models, as well as the imposed autoclave temperature history. It also includes five thermocouple readings which were reported in Chiao s manuscript (8). 

Figure 8.7 Centerline and autoclave temperature histories during cure of a 25-mm (200 plypthick AS4/3501-6 laminate...

Once the elastic residual moment from Equation 8.30 or the viscoelastic residual moment history from Equation 8.39 have been determined, the final value may be applied elastically as platens open (hot pressing) or as the applied pressure is released (autoclave processing) to find the resulting cross-ply curvature. This curvature can then be correlated to experimental data. 

In traditional process control, models are often used to predict the deviation of the controlled variable from the desired state, the process error. This assumes that one knows the desired state. In complex batch processes, the desired state of the process is also dependent on history and changing dynamically. Further, most process models have to predict the outcome of an entire cycle to determine if the product will be good, so predictions are not available in real time, even for a slow process like the autoclave cure however, partial models have been used as virtual sensors to expand on the information available from sensors [38]. Saliba et al. used a kinetic model to predict the degree of cure as a function of time and temperature in a mold and used that predicted degree of cure to time pressure application and determine the completion of cure. Others [39] have used the predictions of models together with the measured progress of the process to predict future trends and even project process outcomes. 

The major problem for control based on material states, however, is the quality control culture that requires that parts be accepted based on adherence to a preset cycle within specified limits. Because state-based inferential control systems could theoretically come up with a new cure cycle every time, this sort of specification cannot be used with such systems. Specifications instead have to be in terms of the process plan used for the cure. The satisfactory completion of a certain cure history without alarm states would be assumed to produce an acceptable part. Once the culture was able to accept that difference for autoclave curing, production costs at the U.S. air force s McClellan AFB Logistic Center were substantially reduced [32], This type of specification could also give material review boards a head start on investigations because they would know that a part did not meet specification as well as what sorts of flaws might result from the deviation. The experience at McClellan is that there are fewer parts to review. It is even conceivable that, with improvements to sensors, much of the current postcure nondestructive evaluation used to verily the quality of parts could be incorporated into the process, building quality in rather than inspecting it in after the fact. 

Figure 1. Diphenamid and MBC degradation in mixtures composed of 1-9 percent of soil that has acquired enhanced degradation capacity (P) with non-history soil (NP) and, for MBC soil, also mixed with autoclaved soil and sterile sand. MBC data was adapted from Ref. 11.

While validation has a relatively short technical history, in practice it has largely been directed at aseptic processing (autoclaves and media fill vials, etc.). In recent years validation has been progressively expanded, firstly to cover all manufacturing, then computer systems, analytical methodology and more recently into packaging processes. Coincident with this, validation has itself evolved into qualification, with defined parts ... 

The second test apparatus mentioned is used to determine the maximum explosion pressure and the maximum pressure rate. It is a spherical stainless steel autoclave of 20 liters volume. Dust samples of up to 40 g per trial, covering a concentration range of up to 2 kg/m, are blown into the sphere through special ring nozzles. The dispersed dust is ignited with varying delay time. The pressxue/time-history is measured. The maximum pressure rate observed is transferred into the safety number Kst value with the help of the so-called cubic-root-law. 

Starting with the silicone elastomer hydrocephalus shunt in 1955, silicone elastomer has become widely used as a soft, flexible, elastomeric material of construction for artificial organs and implants for the human body. When prepared with controls to assure its duplication and freedom from contamination, specific formulations have excellent biocompatibility, biodurability, and a long history of clinical safety. Properties can be varied to meet the needs in many different implant applications. Silicone elastomer can be fabricated in a wide variety of forms and shapes by most all of the techniques used to fabricate thermosetting elastomers. Radiopacity can be increased by fillers such as barium sulfate or powdered metals. It can be sterilized by ethylene oxide, steam autoclave, dry heat, or radiation. Shelf-life at ambient conditions is indefinite. When implanted the host reaction is typically limited to encapsulation of... 

As the temperature increases, the cure kicks off, and the heat generated during cure increases the local temperature, which further accelerates the cure, and consequently the temperature history of a curing part rapidly diverges from that of an equivalent inert part. An example of this is shown in Fig. 13.10, where the part was subjected to the same autoclave cycle twice. ... 

Experience to date indicates that if there is adequate characterization of material properties (in particular the cure kinetics response) and the HTC boundary conditions, then the prediction of thermal history and consequent degree of cure evolution is sufficient for most practical purposes. Currently, the biggest impediments to further adoption of process simulation for thermal management are materials characterization costs (which are one-off) and cost-effective boundary condition characterization, particularly for the case of process variations and deviations such as random positioning of tools in autoclaves and similar uncontrolled practical issues. 

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