Material Behavior Modeling



Material behavior and its incorporation into Finite Element (FE) constitutive models is often the most important aspect of any analysis, especially nonlinear predictive/failure analysis. One of the common threads that link our diverse business areas is material modeling. Useful analysis requires accurate predictions, which require advance material models which accurately model the stress-strain behavior of complex materials.

Most FE programs include a large library of material models, many of which are advanced nonlinear models. For example, elastic-plastic material behavior with combined isotropic and kinematic hardening. While classic plasticity theory provides an excellent approximation to the behavior of metals that are monotonically loaded at room temperature, these same models offer poor predictions of the cyclic behavior of both material and structural models. Also, many of material models available in general purpose FE programs do not allow the treatment of strain softening. This deficiency is a major reason why most general purpose FE programs fail to provide accurate or reasonable predictions of the observed response of concrete structures to cyclic loading.

Most general purpose FE programs allow the user access to a "user-provided constitutive model" where the user typically provides a FORTRAN coded subroutine that defines the stress-strain behavior of the material. We have developed and coded many user material models. This modeling requires advanced training and considerable skill in both mechanics and computer programming. Few FE program users have the skill or experience to develop such software. Most users are content to limit their consideration to those few material models generally available. Unfortunately, this limits the accuracy and value of their analytical results. We took an empirical approach with the "Form Follows Function" philosophy in order that these models provide "engineering," that is, useful solutions as opposed to simply programming an "established" theory.

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We have been at the forefront of concrete material modeling for over thirty years and is internationally recognized as one of the leaders in simulating the complex behavior of reinforced and prestressed concrete. The core of our concrete material modeling capabilities have been collected into a software package called ANACAP.

ANACAP is a structural analysis program developed for 2D, axisymmetric or 3D, static or dynamic response and failure analysis of plain (mass), reinforced, or prestressed concrete and steel structures. ANACAP is cast in finite element methodology especially formulated for the highly nonlinear material response associated with concrete structures due to cracking, creep, aging, crushing, reinforcement yielding, and bond failure. This program is also well equipped to handle metal plasticity for steel structural elements. ANACAP can be applied over the entire range of analysis requirements including design acceptance verification, construction optimization, ultimate load prediction and failure determination. The software provides users with capabilities which have evolved from over thirty years of research and experimental verification by ANATECH, a Structural Integrity Associates company. The software is currently being applied in many areas of national concern and has gained the acceptance and approval of peer review committees, in particular for the seismic retrofitting of concrete and steel highway bridges in California.