The Columbia Accident Investigation Board’s conclusion that the tragic loss of the Space Shuttle Columbia was caused principally by the liberation of insulating foam from the bipod ramp of the external tank (ref. 1) launched an extensive series of studies to identify the cause of and the factors influencing foam shedding. Through various laboratory experiments, it became apparent that defects in the foam resulting from the spraying process have a significant influence on foam-shedding phenomena (ref. 2).
Although more stringent foam spraying and processing specifications have been established to limit both the size and number of defects, it is improbable that these defects will be completely eliminated. Vehicle safety, therefore, relies on developing an ability to assess the potential for a foam-shedding event with critical significance. To accomplish this, researchers need to accurately quantify the foam stress states during ascent and to understand how defects interact with the surrounding stresses to produce a fracture event.
Thermal expansion behavior of BX-265 foam normal-to-rise.
The stress analysis of foam is challenging by itself. For the shuttle application, it is further complicated by severe temperature gradients across the foam layer during shuttle ascent. Because the foam cell walls are a polymer, the glass transition, the release of volatiles, and the thermal decomposition of the material contribute to the complex thermal expansion and thermomechanical behavior (see the preceding graph). The foam cell structure shown in the following figure also plays a significant role because it introduces anisotropy in the mechanical and physical properties.
Factors influencing the complex thermomechanical behavior of polymer foams. Left: Accumulation of volatiles inside cells. Right: Foam microstructure.
Researchers in the Materials and Structures Division at the NASA Glenn Research Center are conducting a series of research studies to develop improved analysis methods for simulating the complex thermostructural behavior of the rigid closed-cell polymeric foams used to insulate the shuttle’s external tank. The goal is to develop accurate analysis tools to identify the mechanisms responsible for foam failure and debris liberation (see the final figure). These tools will also help to assess the criticality of specific foam defects on the basis of measurable defect characteristics, such as size, shape, depth, and location.
Illustration of the divot test setup. (a) Foam divot test setup. (b) Formation of a divot. (c) Finite-element model of divot test. (d) Stress contours in foam.
Both macromechanics and micromechanics models are being developed to account for the physical and chemical changes that occur in the material as well as the influence of the foam microstructure on the thermostructural behavior. Experimental studies are being conducted to (1) determine critical material property values for input to analysis models, (2) study the fracture behavior of rigid, closed-cell polymeric foams, and (3) study the significant physical and chemical processes that occur in the material to help guide the development of the physics-based models. So far, a formulation has been derived to account for the increase in the cell gas pressure due to thermal decomposition of the polymer (see the left side of the bottom figure on the preceding page). Also a finite element model has been established to describe the divot tests (see the figure on this page).
Find out more about the research of Glenn’s Life Prediction Branch: http://www.grc.nasa.gov/WWW/LPB/
Glenn contacts:
Dr. Roy M. Sullivan, 216-433-3249, Roy.M.Sullivan@nasa.gov; and Dr. Bradley A. Lerch, 216-433-5522, Bradley.A.Lerch@nasa.gov
Author:
Dr. Roy M. Sullivan
Headquarters program office:
NESC
Programs/Projects:
Space Shuttle
Last updated: October 16, 2006
Responsible NASA Official:
Gynelle.C.Steele@nasa.gov
216-433-8258
Point of contact for NASA Glenn's Research & Technology reports:
Cynthia.L.Dreibelbis@nasa.gov
216-433-2912
SGT, Inc.
Web page curator:
Nancy.L.Obryan@nasa.gov
216-433-5793
Wyle Information Systems, LLC
