Structural Acrylic

Acrylic is a very unusual material. It serves not only optical but also structural purposes. Its optical properties are well understood and it finds use in diverse optical applications, for example, lenses, fiber optics, light pipes, display cases, and many others too numerous to mention here.

Because of its transparency, it is also utilized in applications where it must resist static and dynamic forces, for example, as windows or shells of pressure vessels under external or internal pressure. In such applications the mechanical properties of acrylic rather than the optical properties become of paramount importance. Properly applied, the acrylic structural members not only provide visibility but can safely contain static or dynamic loading of almost any magnitude.

The key item in the design of structural acrylic components for the ability to carry loads in the form of pressure, is the selection of the maximum working stress level that will allow the component to function properly without precipitating the initiation of fracture. For typical structural materials, such as steel, aluminum, or titanium, the maximum working stress levels are known, having been established over the years by extensive test programs and experience, and are certified in many design standards.

For these materials, safe maximum working stress levels have been established for any type of loading and in any type of environment that conceivably could be encountered by the designed structure in service. The engineer designing the structure only needs to review existing technical literature to retrieve the necessary data pertinent to his design on which to base the magnitude of working stress appropriate to the loads and environment encountered by the structure during its service life.

For acrylic materials, the case is quite different. First, the response of acrylic to applied loads is more complex than for metals. The response of an acrylic structure to stresses is nonlinear, a function of both the temperature and duration of stress application (i.e., it exhibits viscoelasticity). Because of its temperature and time-dependent viscoelasticity, the designer has to consider in the selection of safe working stress both temperature and duration of loading, as they both affect not only the magnitude of strain but also the yield point of the material.

Second, since acrylic is not widely used in the industry and is not a strategically important material, there is a scarcity of published technical data on which to base the selection of working stress magnitude. Because of this, the selection of a safe working stress level is not based on a precise analytical calculation, but on empirically derived procedures based on the experience of engineers engaged in the design and field observations of operators or users of vessels incorporating acrylic components.

Because the performance record of acrylic structures, designed on the basis of empirical rules summarizing published test data and past performance observations, has been found to be free of in-service failures, most of them have been incorporated in the ANSI/ASME Pressure Vessels for Human Occupancy Safety Standard. As more knowledge on structural performance of acrylic accumulates, some, if not all, empirical rules for design of acrylic structures found in this book will be replaced by design procedures employing finite element analysis and experimentally validated maximum working stress values.

To date, only the design of acrylic panels in aquaria is well enough understood where their selection can be based alone on FEA and an empirically-selected working stress level. This has been feasible because, in this case, the engineer only has to consider in his design a benign environment, a steady loading condition, and an absence of restraint on the weight of the structure allowing him to use a single low-working stress level for acrylic aquaria of any configuration. Perhaps someday all acrylic structures can be designed with FEA alone. However, that is not the case now and therefore this book presents a proven way to design with acrylic in almost any environment under a multitude of loading conditions and in unlimited configurations.

One feature of the acrylic material that must be considered by the engineer in the design of acrylic components for pressure vessels and subsequently monitored by the operator of the pressure vessel containing acrylic components is the degradation of their mechanical properties with exposure to service environment. Exposure to weathering, in general, and sunshine and organic solvents, in particular, will degrade acrylic's properties rapidly where replacement may be needed in 10 years or less to avoid catastrophic failure of the acrylic component under working pressure.

To prevent catastrophic failure of the acrylic components in service, they must be periodically inspected and their condition noted. With proper care and inspection the acrylic components may exceed their design life without fracturing or removal from service. In this respect their record is good. To date, the service life of acrylic windows and pressure hulls based on the working stress levels specified by ASMIE PVHO-1 Safety Standard and applied in this book to design of acrylic pressure-resistant components has exceeded their design life without failure. That does not mean that under some unusually severe service environment conditions they may degrade to such an extent that removal from service may become necessary prior to expiration of their design life. Frequent inspections insure that severely degraded acrylic windows and vessels are detected and removed from service in a timely manner. Under proper care, the acrylic components perform as reliably as the metallic vessel to which they are attached.

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