Literature Review - Finite Element Analysis for Satellite Structures - page 6

and unique load path. The flexural shear loads are transferred from the matrix to
axial loads on the high-strength fibers, creating a stiffer and lighter structure. These
fibers may either be discontinuous or continuous entities. Discontinuous reinforced
composites comprise ceramic or fiber particles that are randomly distributed
throughout the matrix. Aluminum reinforced with silicon carbide particles is the
most widely used discontinuous composite.
The majority of continuous fiber composites are generally called laminate
composites. Laminate composites are manufactured from several layers of woven
fibers called laminae. The laminae are composed of several parallel fibers arranged
in sheets. The sheets themselves are anisotropic and have few structural applica-
tions. However, stacking several of the laminae with fibers aligned at different
angles, called lamina angles, creates a more stable laminate composite structure.
The laminate may be customized for individual applications by varying the fiber
type and the lay up. For example, some graphite/epoxy laminates are modified to
have a nearly zero coefficient of thermal expansion, and others may be laid up to
exhibit extraordinary specific stiffness properties.
Polymer-matrix composites (PMCs) are the most widely used continuous fiber
composites in spacecraft. The matrices consist of two polymers: thermoplastics, and
thermosets. Thermoplastics may be melted and solidified multiple times, whereas
thermosets are not reusable after curing. These properties enable a multitude of
bonding techniques and lay up procedures, many of which are currently proprietary.
The downside of fiber composite structures is the large development cost required
for reliable manufacturing. The large cost is due to the sensitivity of adhesive
bonding to process variables. This sensitivity makes each part a unique entity which
must be tested to verify strength. Extensive testing is required for fiber composite
flight hardware to verify its structural integrity. The requirements typically create
the need to fabricate a protoflight structure dedicated to qualification testing fol-
lowed by acceptance testing of the light article. This qualification procedure pres-
ently prohibits the use of fiber composite structures on small-budget programs.
1.3.3 Methods of Attachment
To attach structural modules, mechanical fastening hardware (such as bolts and
rivets), welding, and adhesive bonding are used. In selecting from these options and
designing the attachments, the following considerations should be considered [ 1 ]:
• Type of structure and application
• Materials of the joining parts
• Joint strength
• Joint stiffness
• Fatigue life
• Alignment
• Access