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+---
+type: concept
+title: "Midas FEA Interface Elements and Nonlinearities"
+created: 2026-06-02
+updated: 2026-06-02
+address: c-000148
+aliases:
+  - MIDAS FEA interface elements
+  - midas FEA interface nonlinearities
+tags:
+  - concept
+  - finite-element-method
+  - interface
+  - contact
+  - nonlinear-analysis
+  - midas-fea
+status: current
+related:
+  - "[[Midas-FEA-Analysis-Manual|Midas FEA Analysis Manual]]"
+  - "[[Midas FEA Element Library]]"
+  - "[[Midas FEA Static Contact Analysis]]"
+  - "[[Finite Element Contact Formulation]]"
+  - "[[Abaqus Cohesive and Gasket Elements]]"
+  - "[[Abaqus Contact Property Models]]"
+sources:
+  - "[[Midas-FEA-Analysis-Manual|Midas FEA Analysis Manual]]"
+---
+
+# Midas FEA Interface Elements and Nonlinearities
+
+## Definition
+
+Midas FEA interface elements and nonlinearities model relative displacement and traction across points, lines, or surfaces, including cracks, bond-slip, friction, and masonry-like joint behavior.
+
+## How It Works
+
+The element-library side describes point, line, and surface interface elements and their finite element formulation in terms of relative displacement. The material-library side then assigns nonlinear laws such as discrete crack behavior, crack dilatancy, bond-slip, Coulomb friction, and combined cracking-shearing-crushing.
+
+The manual emphasizes that normal and tangential interface behavior can be coupled. Crack dilatancy and frictional flow can introduce off-diagonal stiffness terms or nonsymmetric tangent behavior, which matters for solver selection and convergence.
+
+## Solver Development Notes
+
+- Store interface orientation and relative displacement components carefully.
+- Treat interface elements separately from general contact: an interface element has predefined connectivity, while contact often needs search and active-set updates.
+- Expect non-associated friction or dilatancy to produce nonsymmetric stiffness and slower convergence.
+- Include element-level tests for normal opening, tangential slip, coupled dilation, and unloading/reloading.
+
+## Connections
+
+- [[Finite Element Contact Formulation]] gives the broader contact and interface context.
+- [[Midas FEA Static Contact Analysis]] describes search-based contact in the same product.
+- [[Abaqus Cohesive and Gasket Elements]] and [[Abaqus Contact Property Models]] give parallel production concepts.
+