The Design Engineering Technology Group of Mobay Corporation's Plastics and Rubber Division, in Pittsburgh, Pennsylvania, is responsible for offering assistance to customers in their design of components using the division's polymer materials. According to Brian Dowler, Design Engineer for Mobay, this job has been simplified through the use of a new "computerized engineering handbook" called The Desktop Engineer (Desktop Engineering/Computer Aided Analytical Solutions for Engineers), marketed by Desktop Engineering, Inc., Woodcliff Lake, New Jersey. Dowler says the software package helps to provide possible solutions for many calculations which aren't complex enough to warrant time-consuming finite element modeling yet are extremely tedious to solve by hand. He added that the program produces a wide range of graphic output which provides ideal illustrations for reports to the company's customers upon completion of the project.

The first example cited by Dowler was a set of two medical Bannisters molded of Mobay's Makrolon polyearbonate resin. Dowler was asked to analyze the Bannisters, which basically consisted of 1500 cc and 2500 cc cylinders, because they were failing a vacuum implosion test. The test consisted of generating an initial vacuum of 17 inches of mercury inside the cylinders which is equivalent to 8.35 psi, then pressurizing the outside until the walls of the cylinder failed. The larger cylinder was to withstand a total pressure of 22.35 psi and the smaller cylinder was to withstand 21.35 psi. Both cylinders, which were originally molded with a wall thickness of .080 inch, were failing very close to the specified pressure value. Dowler was asked to determine the minimum wall thickness that would provide adequate wall strength with a 2096 margin of safety.

Dowler pointed out that the conventional equations for solving cylinder buckling problems are quite lengthy, particularly when considering that they have to be solved for different waves in each load case. He estimated that writing, solving and checking the equations by hand would have taken about a day. Instead, using the '"cylinder buckling" module of The Desktop Engineer, he solved the complete problem in about 15 minutes. The procedure simply involved entering the height, diameter and thickness of the cylinders and the strength of the polymer. Based on these values, the program predicted a buckling pressure of 22.9 psi in the smaller cylinder which was remarkably close to the test results. Dowler then entered a new thickness value of .091 inch and ran the analysis again. The new pressure value was 30.5 psi which, although it exceeded the specification, did not provide an adequate margin of safety. Further increasing the thickness to .100 inch yielded a pressure value of 38 psi which did provide the necessary safety margin and ended up being the final recommended design value. An analogous procedure was used for the larger cylinder.

Another example cited by Dowler involved analysis of a blood oxygenator for a major manufacturer of medical equipment. The product is used in an operating room to add oxygen to the blood of a patient that has stopped breathing due to a surgical procedure. The initial design was a 7-inch-high and 3-inch-diameter cylinder with 1-inch high elliptical end caps on both ends, and a wall thickness of .100 inch. The primary performance requirement for the vessel is that the wall deflection cannot exceed 0.015 inch under a pressure drop ranging from 8.7 to 12.7 psi which may be cyclic from one side of the unit to the other at a frequency of 60 Hz. The equipment manufacturer asked Dowler's group to determine whether the shape of the oxygenator could be changed from cylindrical to rectangular while still meeting performance specifications. Because the endcap was elliptical and not spherical, conventional pressure vessel equations would not be able to predict the stress or deflection in the endcap region.

Dowler pointed out that this is a classic pressure vessel problem which could easily be solved with finite element analysis. However, he said that creating a finite element model and running the analysis would normally take about two hours. He solved the problem in about five minutes using the "thin-walled pressure vessel" module of The Desktop Engineer. His first step was to analyze the cylindrical tank using a modulus of elasticity of 3~0,000 psi and Poisson's ratio of 0.34. The indicated deflection was under .001 inch and the maximum hoop stress was only 193 psi which matched up very well with test results. Dowler then went to the "rectangular plate" module of The Desktop Engineer and constructed a model 7 inches long, 4-1/2 inches deep and wide and .100 inch thick. A maximum plate deflection value of .327 inches was determined by the analysis which was 27 times the allowable value. The maximum stress figure of 10,410 psi also exceeded the safe range of the material. As a result, Dowler concluded that a rectangular vessel without ribbing would not be practical and the customer maintained the original design.

The Desktop Engineer is basically a computerized handbook of solutions to over 5000 structural/mechanical engineering equations found in over 100 reference books. It includes 37 modules grouped into the following categories: geometric and material properties; beams and columns; rings, cables, arches and frames; plates, shells, and pressure vessels; natural frequencies; miscellaneous; user defined modules; and utilities. Modules are self-prompting to help the user find or calculate things like material properties, inertial, etc.

Once a problem is defined, the program will perform the necessary calculations to provide the necessary solutions and associated output. In most cases it will do the equation substitutions, integration, boundary condition analysis, exactly like one would do if the problem was done by hand. In some of the more complicated analyses, the program uses finite elements to get a solution, even though the input and output format remains similar to the other modules and the user is not required to know finite element techniques. Once a problem is accurately set up and modeled, it can be quickly changed and, e-run to allow the designer to look at various design options or modifications.

Several utility modules are included to assist the designer with complex problems. A superposition module allows the designer to look at the effects of several concurrent loads on the same structure. A module is included to produce Mohr's circle analysis ,or multi-axial states of stress and strain. Modules are also included to assist the user with calculating section properties, The program also has a convenient material database and several modules that look at complex geometries and dynamic conditions. Although the program is self-explanatory, the user manual is well-written and includes a theoretical section which provides the equations and references for the entire system.

The Desktop Engineer operates on most computers using Windows and supports color high resolution graphics for the IBM PC, AT, PS/o and compatibles.