Engineering topics

Said, Magdi A.

Publications and source records attributed to Said, Magdi A..

6 records

NASA NTRS2000

Recent Progress in Materials Selection and Characterizations for Ultra Long Duration Balloon (ULDB) Missions

The development and characterization of materials suitable for ultra long duration balloon flights has recently been the focus of the materials R&D efforts for the NASA balloon program. Although basic materials selection criteria is similar to those used for conventional balloon missions, additional considerations related to balloon design, fabrication, durability, environmental effects, and cost must also be considered. Among these, the highest impact on material selection is, related to the design shape of the balloon. Work done by independent researchers indicate that for pumpkin type balloons, the load-carrying member is the tendon rather than the structural envelope. This in turn lowers the strength requirements on the envelope material to a large extent. Several materials and material combinations were explored for both design systems. This paper will present the progress made to date in the selection and characterization of these materials and the technical challenges remaining to be overcome,

Said, Magdi A.↗

NASA NTRS1999

Special Considerations in Selection of Fabric Film Laminates for Use in Inflatable Structures

Inflatable structures are gaining wide support in planetary scientific missions as well as commercial applications. For such applications a new class of fabric/film laminates is being considered for use as a structural gas envelope. The emerging composite materials are a result of recent advances in the manufacturing of lightweight, high strength fibers, fabrics and scrims. The lamination of these load-carrying members with the proper gas barriers results in a wide range of materials suitable for various loading and environmental conditions. Polyester-based woven fabrics laminated to thin homogenous film of polyester are an example of this class. This fabric/film laminate is being considered for the development of a material suitable for building large gas envelopes for use in the NASA ultra long duration balloon program (ULDB). Compared to commercial homogenous films, the material provides relatively high strength to weight ratio as well as better resistance to crack and tear propagation, The mechanical, creep and viscoelastic properties of these fabric film laminates have been studied to form a material model. Preliminary analysis indicates that the material is highly viscoelastic. The mechanical properties of this class of materials will be discussed in some details.

Said, Magdi A.↗

NASA NTRS1998

Mechanical and Tear Properties of Fabric/Film Laminates

Films reinforced with woven fabrics are being considered for the development of a material suitable for long duration scientific balloons under a program managed by the National Aeronautics and Space Administration (NASA). Recently developed woven fabrics provide a relatively high strength to weight ratio compared to standard homogenous films. Woven fabrics also have better crack propagation resistance and rip stop capabilities when compared to homogenous lightweight, high strength polymeric films such as polyester and nylon. If joining is required, such as in the case of scientific balloons, woven fabrics have the advantage over polymeric thin films to utilize traditional textile methods as well as other techniques including hot sealing, adhesion, and ultrasonic means. Woven fabrics, however, lack the barrier properties required for helium filled scientific balloons, therefore lamination with homogenous films is required to provide the gas barrier capabilities required in these applications.

Said, Magdi A.↗

NASA NTRS1994

Characterization of path effects on the properties of balloon films

The effect of stress and temperature path on the mechanical properties of a balloon film material (Astrofilm E2) have been studied using two techniques. The first technique is based on uniaxial tensile measurements of prestrained strips of the film as a function of temperature. Data are analyzed in terms of two temperature coefficients, defined here as a stiffness coefficient and a strength coefficient; and by comparing the areas under the stress-strain curves. The second technique investigated the effect of stress and temperature path on inflated cylinders of the same material. The changes in the mechanical properties due to preconditioning in the biaxial stress state were evaluated using the ball burst test. Preliminary findings indicate that the material is stress and temperature path dependent. Changes in stiffness coefficient, draw strength and toughness were measured and discussed. The above techniques may be suitable to document path dependent changes, and to discriminate among films of different manufacture.

Said, Magdi A.↗

NASA NTRS1994

Morphological characterization of selected balloon films and its effects on balloon performances

Morphological characterization of several polyethylene balloon films have been studied using various techniques. The objective is to determine, if any, differentiating structural or morphological features that can be related to the performance of these balloon film materials. The results of the study indicate that the films are composed of either linear low denstiy polyethylene (LLDPE) or low density polyethylene (LDPE). A selective examination of these data imply that films limited degree of branching and larger crystallites size (same % crystallinity) showed good mechanical properties that appear to correlate with their high level of success in balloon flights.

Said, Magdi A.↗

NASA NTRS1991

An assessment on the validity of toughness as a balloon film discriminator

Uniaxial and ball burst toughness for eight commercial polyethylene balloon films have been investigated and compared. Uniaxial toughness was measured in both machine and transverse directions using a rectangular sample 25.4 mm wide by 50.8 mm gage length with an average nominal thickness of 20.32 microns (0.8 mils). Ball burst toughness was measured using a ball burst test fixture with 127 mm hole diameter and 50.8 mm ball diameter resulting in a 0.4 ball to hole diameter ratio. Both the size and shape of the deformed area were observed and the ball burst toughness was calculated based on the deformed area of the film instead of the full area. Specimens were tested at 23 C and -80 C and cross head speed was maintained at 508 mm per minute. The uniaxial toughness were in general, higher than the ball burst toughness. The toughness of the films varied from 12 to 365 MPa when measured uniaxially and from 24 to 73 MPa when measured in the ball burst test. Further investigation showed an apparent correlation between film performance in flight as measured by its stress index and its toughness.

Said, Magdi A.↗