---
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]]"
source_refs:
  - source: "[[Midas-FEA-Analysis-Manual|Midas FEA Analysis Manual]]"
    raw_path: ".raw/MidasFEAAnalysisManual/"
    raw_files:
      - "MidasFEAAnalysisManual_002.md"
      - "MidasFEAAnalysisManual_029.md"
      - "MidasFEAAnalysisManual_047.md"
    md_indices:
      - 2
      - 29
      - 47
    match: "heuristic-heading-keyword"
    confidence: high
---

# 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.