Chapter 1 Dynamic behavior of materials and structures

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Summary

The primary ingredient for successful simulations of dynamic events is

  • understanding the physical problem for which a solution is needed;

  • having or obtaining a keen knowledge of the computational tools;

  • gaining experience in the use of that computational tool, preferably under the tutelage of a senior engineer or scientist.

Such codes are in no way “black boxes” which can be taken off the shelf and used by anyone. Each has its own characteristics and eccentricities and it takes time—six months to two years—to become thoroughly familiar with a given code.

The best approach to acquiring these skills is to:

  • (a)

    Study the basic theory of finite elements, finite differences and other computational techniques used for continuum mechanics, preferably in formal college courses.

  • (b)

    Study, either in formal courses or on your own, the basic principles of continuum dynamics, which should include structural dynamics, thermodynamics, material behavior at high strain rates, wave propagation, shock wave physics and mathematical physics.

  • (c)

    Acquire fundamental modeling skills under the guidance of experienced colleagues or supervisors.

  • (d)

    Become familiar with the constitutive descriptions, numerical models, computational approaches and idiosyncrasies of the particular computer programs you will be using.

The above implies that it would be foolhardy to assign a junior engineer to a structural dynamics or wave propagation code and expect him or her to generate, in short order, the types of results seen in the literature. Commercial codes required 15–25 man-years of development. Their effective use requires a minimum commitment of six months to two years just to set up the code on an in-house computer and learn to use it with a reasonable degree of competency. This is after the educational requirements listed in (a) and (b) above have been met. More often than not, considerable on-the-job learning in physics goes on while the code is being mastered. During this period, adherence to a “one person-one code” philosophy is necessary. Equally necessary is contact with other experienced code users or code developers during the learning period. Mandatory is the acquisition of material data for the constitutive equations employed in the code determined from wave propagation experiments at strain rates appropriate for the problems being addressed.

Assuming all this has been done—it almost never is—there remains the problem of rendering the results of code computations in usable graphical form. While most current wave codes are readily transportable, post-processing routines are still highly device-dependent. The color slides and movies produced readily at one installation may require a major investment in graphics hardware at another.

All this is not meant to discourage you from the use of structural dynamics or wave propagation codes. In a later chapter, you will see some outstanding results obtained with present-day codes on technologically difficult problems. It is intended to remind you that the use of codes is a non-trivial exercise. Successful implementation of a computational capability in any organization implies a commitment to manpower, training and investments in hardware and software, which may not be obvious at first glance. Successful use of the codes inevitably entails tradeoffs between accuracy and economy. The rest of the book strives to give you the background necessary to understand the nature of the computer programs used to solve problems in dynamics and to evaluate and use those results intelligently.

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