Question
What is an interface?
Answer
Interface is a type of element that enables the user to simulate the relative slip behavior between two different material types or element types. Application of interfaces are prevalent in any type of model, provided there is a large stiffness difference between the two elements which will allow both to move independent of each other unless they are rigidly connected.
Interfaces are mostly used to simulate the scenarios like rock-joint, ground structure, retaining wall-soil interface, tunnel lining ground interactions etc.
Interface non linearity can be simulated using the Coulomb Friction Behavior as shown below;
Coulomb Friction Function
The main input parameters for creating an interface element are as shown below:
Normal Stiffness Modulus (Kn) - Is the Elasticity modulus for bonding and un-bonding behavior in the normal direction to the interface element. The general range is 10~100 times the smallest value of the Elasticity modulus on the oedometer of adjacent elements.
Shear Stiffness Modulus (Kt) - Is the Elasticity modulus for slip behavior in the normal direction to the interface element. The general range is 10~100 times the smallest value of the shear strength of adjacent elements.
Normal Stiffness in Tension Part - Input Normal Stiffness Modulus in tension part. Otherwise, It will be substituted with Normal Stiffness Modulus.
Mode-II Model - The Mode-II model expresses shear behavior and defines the tangential slip behavior or the interface. For the 2 models below, the failure envelopes are shown for when the ‘Constant Shear Retention’ function is considered suitable in terms of numerical analysis stability etc.
• Brittle model
The structure does not receive any loading if the vertical force is higher than the tensile strength.
• Constant Shear Retention
Apply the input value on the shear direction such that the structure can receive loading in that direction.
Multi-linear Hardening
If a function is entered in the multi-linear hardening, the cohesion and friction angle used in the Coulomb friction failure criterion changes with plastic displacement. Note that the cohesion and friction angle both need to increase as the plastic displacement increases. This behavioral characteristic must be defined by experimentation and is mainly used for research purposes than practical purposes.
Seepage
Conduction for Seepage flow : Sets allowable flow rate at the interface.
Thermal
Convection coefficient : Controls allowable heat exchange at the interface.
When the dilatancy angle and tensile strength is defined, a smaller or equal value needs to be defined for the interface element and the cohesion; friction angle can be multiplied with the strength reduction factor. For the interface dilatancy angle, the same angle can be applied as the ground when the ground is under rigid body motion without strength reduction (R=1). When considering strength reduction, entering '0(zero)' is the general definition for rigid body motion. When creating an interface element, the following Wizard can be used for automatically calculate, according to the element properties of the neighboring ground element, using the 2 parameters (tv, R).
The general Strength reduction factor for structural members and neighboring ground properties are as follows.
• Sand/Steel : R ≈ 0.6~0.7
• Clay/Steel : R ≈ 0.5
• Sand/Concrete : R ≈ 0.8~1.0
• Clay/Concrete : R ≈ 0.7~1.0
In case of multiple soil layers with the same structural component, the smaller value of R is recommended.