The soft constraint method seeks to maximize contact stiffness while also maintaining stable contact behavior. When several parts of dissimilar mesh sizes and/or dissimilar material properties are included into one global slave set for *AUTOMATIC_SINGLE_SURFACE, the soft constraint stiffness method ( SOFT =1) is recommended. Standard penalty-based or soft constraint stiffness method In future version of LS-DYNA, the *AUTOMATIC_GENERAL option will also include these improvements. One advantage of the *AUTOMATIC_SINGLE_SURFACE contact starting with LS-DYNA version 950d is in its more rigorous treatment of interior sharp corners within the finite element mesh and in the handling of triangular contact segments consequently, the *AUTOMATIC_SINGLE_SURFACE contact is usually superior for parts meshed from triangular and tetrahedron elements. *AUTOMATIC_GENERAL contact should be used sparingly and only where conditions dictate its use. To treat special contact conditions where shell edge-to-edge or beam-to-beam contact is anticipated, the additional use of the *AUTOMATIC_GENERAL contact in localized regions is recommended. The *AUTOMATIC_SINGLE_SURFACE contact option is recommended for global contact. Of the two single surface contact types listed in Table 7.1, *AUTOMATIC_GENERAL is computationally more expensive owing to its additional capabilities and its more frequent and thorough contact search. Table 7.1 Difference between *AUTOMATIC_SINGLE_SURFACE (13) and *AUTOMATIC_GENERAL (26) Table 7.1 highlights the important differences. Though both contact algorithms belong to the single surface contact type, several key parameters distinguish these two contact types. AUTOMATIC_SINGLE_SURFACE or AUTOMATIC_GENERAL In such instances the user is encouraged to define local contact interfaces with non-default parameters that would best suit the contact condition. Frequently, there exist certain areas of the vehicle that require special contact considerations where the global contact definition is observed to fail. This, however, does not mean that one should always avoid local contact definitions. We often casually refer to this single contact approach as a global contact approach.

From the standpoints of simplicity in preprocessing, numerical robustness, and computational efficiency, it is now usually advantageous to forsake the use of numerous contact definitions in favor of ONE single-surface-type contact that includes all parts which may interact during the crash event. The development and implementation of a robust single surface type of contact has changed the way engineers model the contact today. Historically, many individual contact definitions were used for the treatment of contact.

Some of the modeling practices based on experience are discussed below. Guidelines should be followed to achieve stability in contact as well as reasonable contact behavior. Another challenge is handling contact at corners or edges of geometrically complex parts. This is especially important when occupants are included in the model. One of the challenging aspects of contact modeling in crash analysis is the handling of interactions between structural metallic parts and non-structural components typically made from foam and plastic.

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