Because dents can reduce the fatigue life of airframes, they are a significant problem for aircraft maintenance and repair organizations. The size, shape, and type of dent determine the impact on fatigue life and whether or not the airframe requires repair. Absent the appropriate data, aircraft maintenance and repair organizations frequently make repairs that may not be needed and may overlook dents that require attention. This whitepaper provides and overview of how accurate data on airframe dents can be collected and analyzed.
Dents form in a variety of shapes. Some examples of dents are shown at left (please see the article THE EFFECT OF DENTS IN FUSELAGE STRUCTURES ON FATIGUE AND STATIC STABILITY by Guijt et. al. US Airforce Academy). Measuring the dent depth is commonly performed with a pit gage. A rule is generally used to measure the dent length. However, dents with very different shapes, or profiles, can have the same depth and length. Measuring the dent profile requires more sophisticated tools than are generally being used today. Accurate measurements of the dent profile can be made using 3D image tools.
Below is a 3D image of a section of an airframe that has multiple dents. The data was acquired using the 3DSL Rhino. Because the 3DSL Rhino operates at over 500fps, the data was collected by hand without the need for a tripod. In addition, the use of the MT Eagle optical tracking system allowed for collection of the data in a global coordinate system without the need to put markers or other reference objects on the airframe.
Generally, the as-built or as-designed CAD files for an airframe are not readily available to the maintenance and repair organization. Consequently, the depth of the dent must be calculated from the data alone. This requires the ability to calculate the undamaged shape of the airframe from the 3D data of the damaged airframe. Airframe geometries are complex. Below is the typical geometry of a wing (for more detail, see the section on Wing Geometries at the NASA website)
In order to accommodate all of the potential geometries on an airframe, the local area around an area of damage is modeled as a quadric surface. A quadric surface can be defined as a surface that fist the following equation:
The quadric surface is used as the reference to determine the dent depth and length. The parameters needed to determine the appropriate quadric surface are extracted from the local data that is in the vicinity of the dent. This ensures that the dent depth and length can be accurately determined directly from the data without the need to compare the data to a CAD model of the airframe.
The plots below show the profile of the dent through the deepest point along the horizontal axis. The position of the dent, as shown along the x-axis, is with respect to a user-selectable reference point.
The dent profile along the vertical axis is shown below at left. The x-axis for this profile is also user-selectable.
The detailed result for the four dents shown in the 3D image are tabulated below.
Using the 3DSL Rhino, the data for this whitepaper was acquired in less than five minutes. The analysis software provides produces Go/No results and generates a report within minutes of acquiring the data.
In addition, the 3D data, along with the quadric surface models, can be imported into finite element analysis (FEA) software for further assessment.