4.13. Gasket

The gasket model (TB,GASKET) enables simulating the gasket joints with the interface elements. The gasket material is usually under compression and is highly nonlinear. The material also exhibits quite complicated unloading behavior when compression is released.

You can define some general parameters including the initial gap, stable stiffness for numerical stabilization, and stress cap for a gasket in tension. You can also directly input data for the experimentally measured complex pressure closure curves for the gaskets.

Suboptions are also available to define gasket unloading behavior including linear and nonlinear unloading. Linear unloading simplifies the input by defining the starting closure at the compression curves and the slope. Nonlinear unloading option enables you to input unloading curves directly to more accurately model the gasket unloading behavior. When no unloading curves are defined, the material behavior follows the compression curve while it is unloaded.

Enter the general parameters and the pressure closure behavior data via the TBOPT option on the TB,GASKET command. Input the material data (TBDATA or TBPT) as shown in the following table:

Gasket Data Type	`TBOPT`	Constants	Meaning	Input Format
General parameters	PARA	C1	Initial gap. Default = 0 (no initial gap).	TB,GASKET,`MATID`,,3,PARA TBDATA,1,C1,C2,C3
		C2	Scaling factor to produce stable stiffness Ks = C2 * K0, where K0 = Y1 / X1 is the initial compressive loading stiffness. Default = 1E-7.^[a]
		C3	Maximum tension stress allowed when the gasket material is in tension. Default = 0 (no tension stress in the gasket material).
Compression load closure curve^[b]	COMP	Xi	Closure value.	TB,GASKET,`MATID`,,2,COMP TBPT,,X1,Y1 TBPT,,X2,Y2
Compression load closure curve^[b]	COMP	Yi	Pressure value.	TB,GASKET,`MATID`,,2,COMP TBPT,,X1,Y1 TBPT,,X2,Y2
Linear unloading data	LUNL	Xi	Closure value on compression curve where unloading started.	TB,GASKET,`MATID`,,2,LUNL TBPT,,X1,Y1 TBPT,,X2,Y2
Linear unloading data	LUNL	Yi	Unloading slope value.	TB,GASKET,`MATID`,,2,LUNL TBPT,,X1,Y1 TBPT,,X2,Y2
Nonlinear unloading data^[c]	NUNL	Xi	Closure value.	TB,GASKET,`MATID`,,2,NUNL TBPT,,X1,Y1 TBPT,,X2,Y2
Nonlinear unloading data^[c]	NUNL	Yi	Pressure value.	TB,GASKET,`MATID`,,2,NUNL TBPT,,X1,Y1 TBPT,,X2,Y2
Transverse shear stiffness and membrane stiffness per unit thickness (Unit = Force / Length³)	TSS	XY, XZ	Transverse shear stiffness. Default = Maximum of stable stiffness (C2) or 1E - 4 * K0 (where K0 = Y1 / X1 is the initial compressive loading stiffness).^[d] TSS XZ is set to TSS XY if not specified.	TB,GASKET,,,3,TSS TBDATA,1,TSSXY,TSSXZ,MSTIF
	TSS	MS	Membrane stiffness. TSS MS is set to TSS XY + TSS XZ if not specified.	TB,GASKET,,,3,TSS TBDATA,1,TSSXY,TSSXZ,MSTIF
Transverse shear stiffness and membrane stiffness (Unit = Force / Length²)^[e]	TSMS	GXY	Shear modulus XY.	TB,GASKET,`MATID`,,6,TSMS TBDATA,,GXY,GXZ,EY,EZ,GYZ,NUYZ
		GXZ	Shear modulus XZ.
		EYY	Elastic modulus YY.
		EZZ	Elastic modulus ZZ.
		GYZ	Shear modulus YZ.
		NUYZ	Minor Poisson’s ratio YZ.

^[a]Stable stiffness is used for numerical stabilization. For example, numerical instability can occur when the gasket is opened up, thus contributing no stiffness to the element nodes. A realistic value of C2 can be given by C2 = (E / h) / K0, where E is the Young's modulus characteristic of the gasket material, h is the initial gasket thickness and K0 is the initial gasket compression stiffness. As the stable stiffness is adopted solely for numerical stabilization, however, a proper value of C2 should be C2 = C * (E / h) / K0, where C is a small scaling factor (such as 1E-7). Default: C2 = 1E-7.

^[b]If the gasket deformation exceeds the maximum defined closure value, the last two defined closure-pressure points are used to calculate the slope (stiffness) needed for the gasket pressure (stress) calculation.

^[c]Multiple curves may be required to define the complex nonlinear unloading behavior of a gasket material.

When several nonlinear unloading curves are defined, the starting point of each unloading curve must be on the compression curve to ensure that the gasket unloading behavior is correctly simulated. For each successive unloading curve, the first unloading point (closure point on the compression curve) must be in increasing order.

The temperature-dependency of unloading data does not need to match the compression data; however, if a temperature is missing, the program uses linear interpolation to obtain the material data of the missing temperature, possibly resulting in a mismatch between the compression data and the unloading data. It is therefore good practice to have the same number of temperatures and temperature points for each unloading curve and compression curve.

^[d]With a known shear modulus G characteristic of the gasket material, the transverse shear stiffness values TSSxy and TSSxz can be calculated via TSSxy = TSSxz = G / h, where h is the initial gasket thickness (Unit = Force / Length³). If using transverse shear stiffness primarily for overcoming numerical instability, the values should be further scaled with a small number C, as shown in this example:

TSSxy = TSSxz = C * (G / h), where C = 1E-4.

^[e]If specified, TBOPT = TSMS takes priority over TBOPT = TSS.

GXY defaults to maximum of initial compression stiffness * factor (1E-4) or stable stiffness (C2) (defined via TB,GASKET,,,,PARA). Initial compression stiffness is defined as h * Y1/X1, where (X1,Y1) is the first data point provided in the gasket-loading curve.

If not specified, GXZ defaults to GXY, EYY defaults to GXY + GXZ, and EZZ defaults to EYY. (If EZZ is specified, GYZ and NUYZ are required.)

NUYZ defaults to 0.0.

Units of the moduli parameters are Force / Length². Poisson’s ratio parameter NUYZ is unitless.

You can enter temperature-dependent data (TBTEMP) for any of the gasket data types. For the first temperature curve, issue TB,GASKET,,,,TBOPT (where TBOPT is the gasket material option), then input the first temperature (TBTEMP), followed by the data (TBDATA or TBPT depending on the TBOPT, as shown in the table). The number of data points defined (TBPT commands issued) for the specified TBOPT must be the same across all temperatures.

The program automatically interpolates the temperature data to the material points using linear interpolation. When the temperature is out of the specified range, the closest temperature point is used.

For more information, see Gasket Material in the Mechanical APDL Theory Reference.

For a detailed description of the gasket joint simulation capability, see Gasket Joints Simulation in the Structural Analysis Guide.