Products: ABAQUS/Standard ABAQUS/Explicit
This example illustrates two elementary nonlinear cases of one-dimensional, fully coupled, heat transfer and stress analysis. The problems are simple enough that exact solutions are obtained easily, thus providing verification of the numerical solutions obtained with ABAQUS.
The model is shown in Figure 1.6.31. A conductive rod of unit area is fixed at one end, A, and free at the other end, Between the free end and an adjacent fixed wall, C, there is a gap across which heat will be conducted or radiated.
In case 1 two forms of clearance-dependent heat transfer are considered: in the first, the conductivity for the gap drops linearly as the clearance increases; in the second, the gap radiation viewfactor drops linearly as the clearance increases. The fixed ends of the rod, A, and the wall, C, are both held at fixed temperatures, and Initially the gap is open, so the distance between B and C is (). The objective is to predict the steady-state displacement, , and temperature, , of the free end of the rod. We assume that the strains are small and that the behavior of the rod is linear elastic, with constant modulus and thermal expansion coefficient. In this case the gap never closes, so the rod is always stress-free.
In case 2 it is assumed that the conductivity across the closed gap increases linearly as the pressure transmitted through the gap, p, increases. The fixed end of the rod, A, and the wall, C (which is also fixed in position), are both held at fixed temperatures, and Since in this case the gap never opens, the axial stress in the rod will be nonzero. We solve for the pressure across the gap, p, and the temperature, , of the end of the rod, assuming that the strains are small and the behavior of the rod is linear elastic with constant modulus and thermal expansion coefficient.
In ABAQUS/Standard the bar is modeled with either two- or three-dimensional elements; the contact between the end of the bar and the wall is modeled in one of three ways: as a gap element (GAPUNIT) or as an element-based rigid surface made of T2D2T or S8RT elements. In ABAQUS/Explicit the bar is modeled with either two- or three-dimensional elements; the wall is modeled one of two ways: either as an analytical rigid surface or as an element-based rigid surface. Surface-based contact is employed between the bar and the wall; both kinematic and penalty mechanical contact are considered.
Mechanical equilibrium along the rod requires that
Thermal equilibrium requires that the heat flux along the rod, q, has no gradient:
Since we assume that the strains are small, the strain at any point in the rod is
The rod is assumed to be made of a linear elastic material, so the stress constitutive equation is
Heat conduction in the rod is assumed to be governed by Fourier's law, which states that the heat flux is determined by
The heat flux in the gap, , between the end of the rod, B, and the fixed point C is assumed to be proportional to the difference in temperature between B and C:
First we assume that the gap is open and that the gap thermal conductivity, , increases linearly as the gap reduces, so
The thermal boundary conditions are that the temperatures at A and C, and , are held constant. The mechanical boundary conditions are that points A and C are fixed. Since in this case the end of the rod never touches C, force equilibrium requires that
The equations given above define the problem. Their solution is readily developed as follows. Combining integrated force equilibrium with the linear elastic constitutive equation and the displacement relationship, , gives
Thermal equilibrium combined with Fourier's law and the gap heat flux equation then gives
Substituting for then gives a quadratic equation for :
The roots of this quadratic equation provide two solutions for The solutions for are available once is determined. Only one of the two solutions gives a value of for which 0; hence, this is the only physically acceptable solution.
As a numerical example the parameters are chosen in consistent units as 1.0; 105, 1.0; 100; 400°; and 200°.
These values give 285.4° or 4485°, so 3.427 × 103 or 2.043 × 102. The second solution must be rejected as it gives 0. The first solution is valid so long as
Next we assume that the gap is open and that the gap radiation viewfactor, , increases linearly as the gap reduces; so
The thermal and mechanical boundary conditions are the same as the gap conduction problem considered above. Force equilibrium requires that
Following a procedure similar to that used in the gap conduction problem and combining integrated force equilibrium with the linear elastic constitutive equation and the displacement relationship, , gives
As a numerical example the parameters are chosen in consistent units as 1.0; 105; 1.0; 50; 1.E8; 1.0; 400°; 200°; and absolute zero –460.
These values give 222.4°, so 3.112 × 103. This solution is valid so long as All other solutions must be rejected since they give or
In this case the rod is always in contact with C. Combining the integrated equilibrium equation with the mechanical constitutive model gives
In this case the conductivity of the closed gap is proportional to the contact pressure:
The roots of this equation provide two solutions for , and the corresponding values of p are then defined by Only one solution gives a positive value for p; the other must be rejected because it is inconsistent with the assumption that the gap is closed.
As a numerical example the parameters are chosen in consistent units as 10; 2; 0.2; 105; 105; 1.0; 200°; and 100°.
These values give 122.6° or 342.6°, so 161.3 or 71.3. The second solution must be rejected as it gives
In both cases ABAQUS/Standard uses a full Newton method and obtains the solution in one or two increments requiring two or three iterations per increment. The values for and in case 1 and p and in case 2 agree with the exact solutions obtained above.
The results obtained with ABAQUS/Explicit also agree with the analytical solutions.
T2D3T elements and GAPUNIT elements.
CPE4T elements for the rod and an element-based rigid surface made of T2D2T elements for the wall.
CPE4RT elements for the rod and an element-based rigid surface made of T2D2T elements for the wall.
CPE4RHT elements for the rod and an element-based rigid surface made of T2D2T elements for the wall.
CPEG4T elements for the rod and an element-based rigid surface made of T2D2T elements for the wall.
C3D8HT elements for the rod and an element-based rigid surface made of S8RT elements for the wall.
T2D3T elements and GAPUNIT elements.
CPE4T elements for the rod and an element-based rigid surface made of T2D2T elements for the wall.
CPEG4T elements for the rod and an element-based rigid surface made of T2D2T elements for the wall.
C3D8HT elements for the rod and an element-based rigid surface made of S8RT elements for the wall.
T2D3T elements and GAPUNIT elements.
CPS4T elements for the rod and an element-based rigid surface made of T2D2T elements for the wall.
CPEG4T elements for the rod and an element-based rigid surface made of T2D2T elements for the wall.
C3D8T elements for the rod and an element-based rigid surface made of S8RT elements for the wall.
CPE3T elements.
CPE4RT elements.
CPS3T elements.
CPS4RT elements.
C3D4T elements.
C3D6T elements.
C3D8RT elements.
CPE4RT elements.
C3D4T elements.
CPE3T elements.
CPE4RT elements.
CPS3T elements.
CPS4RT elements.
C3D4T elements.
C3D6T elements.
C3D8RT elements.
CPE4RT elements.
C3D4T elements.
CPE3T elements.
CPE4RT elements.
CPS3T elements.
CPS4RT elements.
C3D4T elements.
C3D6T elements.
C3D8RT elements.
CPS4RT elements.
C3D6T elements.
CPE3T elements.
CPE4RT elements.
CPS3T elements.
CPS4RT elements.
C3D4T elements.
C3D6T elements.
C3D8RT elements.
CPE3T elements.
C3D8RT elements.
CPE3T elements.
CPE4RT elements.
CPS3T elements.
CPS4RT elements.
C3D4T elements.
C3D6T elements.
C3D8RT elements.
CPE3T elements.
C3D8RT elements.
CPE3T elements.
CPE4RT elements.
CPS3T elements.
CPS4RT elements.
C3D4T elements.
C3D6T elements.
C3D8RT elements.
CPS3T elements.
C3D8RT elements.