The NASA Glenn Research Center in partnership with the Ohio Aerospace Institute (OAI) collaborated with the NASA Kennedy Space Center and Cornell University to lead an experimental investigation to extract crucial mechanical characteristics from the Space Shuttle Columbia’s remaining composite overwrapped pressure vessels (COPVs). The work was focused on (1) determining manufacturing residual strains due to the overwrap/liner interference fit and (2) measuring the mechanical properties of Kevlar (DuPont)/epoxy strands extracted from Columbia’s composite overwrap.
Saw cut through overwrap showing expanded gap.
The investigation began with the development and approval of plans to section two of Columbia’s Main Propulsion System (MPS) COPVs: a 40-in.- and a 26-in.-diameter vessel. Glenn researchers were onsite at Kennedy to participate in and document the tank dissections. Researchers measured residual strains by placing several fiduciary markers on a tank’s surface and measuring the arc length between them. As the dissection proceeded, the manufacturing residual strains were relaxed, producing significant gaps in the progressing saw cuts. This photograph shows a finished meridian cut with the fiduciary markers. After the complete section was removed, significant residual strains were recovered. The bar chart compares the strains measured to those available from manufacturer’s data sheets. These data were the first available experimental verifications of the residual strains and were of significant value to the Orbiter COPV Team.
Summary of zero-pressure overwrap residual strains. Columbia MPS 40-in. overwrap linear fiduciary marker measurements and individual volumetric changes from data packs. SR, ratio of operating fiber stress to burst fiber stress; SAR, original manufacturer’s stress analysis report (1978 Stress Analysis Report); SN, vessel serial number; asterisk (*), original thin-shell analysis of 68-percent load share, with no adjustments for creep, rate effects, or through-the-thickness gradients.
Sections of the composite were sent to Glenn, where techniques were developed to extract individual fiber strands from the solid composite sections. These strands were then mounted on specially designed cardboard picture frames to provide support and alignment for mechanical testing. Strand specimen testing is often used only to provide strength data since it is difficult to apply extensometry without disturbing the specimen. This was overcome by employing Glenn’s unique digital imaging correlation (DIC) technology to obtain strains in the specimen gauge length without contact. The following photograph shows the typical test setup for the specimen along with the resolved DIC strain distribution and the digital wave acoustic emission sensors. The resolved strains show a shear concentration on one side of the specimen because of the preferential failure of a portion of the filaments. Together with the acoustic emission data, these results demonstrate that failure in these strands was progressive. The following graph summarizes the strength-versus-failure strain for the distinct vessels.
Tensile property testing of Columbia strands.
Columbia strand test results. Vendor minimum strength, 490 ksi; stresses were based on 0.000542 in.2 per roving. OMS, orbital maneuvering system.
These experiments have been extremely valuable in understanding the mechanical characteristics of COPVs. The data are being used in modeling efforts to assess the performance of COPVs on the remaining shuttle fleet.
Ohio Aerospace Institute (OAI) contact:Last updated: December 15, 2007
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
