Contact Analysis

Very rarely is analysis of contact between two surfaces in finite element modeling an easy process. Due to the inherent difficulty in modeling this problem analytically, finite element approximations are also appropriately complex and rather arduous to model accurately. Analytical models have been developed for special ideal cases—

contact between two spheres, two parallel cylinders, cylinders on a flat plate, gear teeth, and some bearing applications such as roller bearings. Contact analysis in finite element modeling typically requires a much greater computational resource, and has many assumptions that go into generating a correct solution. These problems are also prone to convergence problems, often converging very slowly or not at all. Many modern upgrades in finite element software have eliminated various problems through advanced solving techniques and options that assist the calculations to allow for easier convergence.

Both AP1 and AP3 involve the use of contact elements to solve for the stresses due to the shrink fitting process. Additionally, the contact problem in AP3 requires an addition to the previous work done in AP1 in that contact between components does not occur initially. As per the second step of AP3, as the girder cools to steady state temperature, interference stress develops in the HG assembly slowly due to the contraction of the girder around the hub. Therefore, no contact between components exists initially. This differs from the contact analysis performed in AP1 where the entire TH assembly was dipped into liquid nitrogen. Interference between these components had already been made in the previous step, and so modeling the contact problem was solved with respect to this condition.

Solving contact problems in ANSYS requires the use of contact and target elements. These elements are specifically designed to model contact between components due to an applied load of some kind—change in temperature, force, stress, etc. Many different types of contact can be modeled in ANSYS—node, node-to-line, node-to-surface, and surface-to-surface contact. The specific behavior of the

“contact pair” created between the contact and target elements can also be specified—

rigid-to-flexible and flexible-to-flexible boundary conditions. The contact between the TH assembly and the HG assembly occurs over a relatively large area with respect to the element sizes used and so surface-to-surface contact analysis was performed. These components were also expected to undergo deformation, so flexible-to-flexible contact was chosen as the appropriate boundary condition.

From the onset of any contact problem, it is necessary to establish which surface will be the contact and target surfaces respectively. Many guides are available to help

distinguish these surfaces such as the ANSYS Structural Analysis Guide, Release 10.0 which states that “If one surface is markedly larger than the other surface, such as in the instance where one surface surrounds the other surface, the larger surface should be the target surface.” [21] Table 2 below shows the list of contact and target elements used in this thesis as well as surfaces that each of these elements were assigned to. Note that only the first stage of AP2 is studied—when the hub is immersed in liquid nitrogen. As such, no contact elements were required for AP2.

Each “contact pair” that is generated between a contact element and its associated target element shares a set of real constants which describe various aspects of the behavior of the elements at the contact region. It is important to note that each contact pair will share one set of real constants which apply to both elements.

Table 2 List of contact and target element usage in present work.

Assembly Procedure

Contact Surface Contact Element Used

Target Surface Target Element Used

diameter area CONTA174 Girder hole

area TARGE170

These elements were chosen as they are surface-to-surface contact elements, and can handle flexible-to-flexible contact conditions. A brief summary of the capabilities of each element will follow as well as its specific use in this thesis.

1. CONTA174: This is a three-dimensional, four-node, surface-to-surface contact that it is compatible with higher order elements with mid-side nodes. This element is considered a “deformable” element surface and takes on the geometric

element is used in AP1 and AP3 to study the interference stresses developed in various stages of these assembly procedures. It was chosen for use due to its compatibility with SOLID45, the element used to model the TH components.

2. TARGE170: This is a three-dimensional, four-node, surface-to-surface target element used to designate a “target surface” for many associated contact elements (CONTA173, CONTA174, CONTA175, CONTA176, and CONTA177). This element may or may not be initially in contact with its associated contact element, and contact can be made incrementally via various keyopts available to the user.

This target element can easily model complex target surface shapes. For flexible target areas, like the ones assumed in this work, the target elements generated will overlay the solid, shell, or line elements which define the boundary between the contact pair [21]. This element is used in both AP1 and AP3 as the target element in the contact pair. In AP1, the target surface is the hub inner diameter surface area, and in AP3 the target surface is the surface area of the hole in the girder.