•  Phenolic—very good fire-resistant properties and can  have very low density, but relatively low mechanical  properties. •  Polyurethane—used for producing the fuselage, wing  tips, and other curved parts of small aircraft; relatively  inexpensive, fuel resistant, and compatible with most  adhesives; do not use a hot wire to cut polyurethane  foam; easily contoured with a large knife and sanding  equipment. •  Polypropylene—used to make airfoil shapes; can be  cut with a hot wire; compatible with most adhesives  and epoxy resins; not for use with polyester resins,  dissolves in fuels and solvents. •  Polyvinyl chloride (PVC) (Divinycell, Klegecell,  and Airex)—a closed cell medium- to high-density  foam with high compression strength, durability, and  excellent fire resistance; can be vacuum formed to  compound shapes and be bent using heat; compatible  with polyester, vinyl ester, and epoxy resins. •  Polymethacrylimide (Rohacell)—a closed-cell foam  used for lightweight sandwich construction; excellent  mechanical properties, high-dimensional stability  under heat, good solvent resistance, and outstanding  creep compression resistance; more expensive than  the other types of foams, but has greater mechanical  properties. • Delamination •  Resin starved areas •  Resin rich areas •  Blisters, air bubbles • Wrinkles • Voids •  Thermal decomposition •  Improper cure or processing •  Improper machining • Mishandling •  Improper drilling •  Tool drops • Contamination •  Improper sanding •  Substandard material •  Inadequate tooling •  Mislocation of holes or details Balsa Wood $^{ }$Manufacturing and In-Service Damage Manufacturing Defects Fiber Breakage Matrix Imperfections  7-13 Matrix imperfections usually occur on the matrix-fiber  interface or in the matrix parallel to the fibers. These  imperfections can slightly reduce some of the material  properties but are seldom critical to the structure, unless the  matrix degradation is widespread. Accumulation of matrix  Manufacturing damage includes anomalies, such as porosity,  microcracking, and delaminations resulting from processing  discrepancies. It also includes such items as inadvertent  Fiber breakage can be critical because structures are typically  designed to be fiber dominant (i.e., fibers carry most of the  loads). Fortunately, fiber failure is typically limited to a zone  near the point of impact and is constrained by the impact  object size and energy. Only a few of the service-related  events listed in the previous section could lead to large areas  of fiber damage. Manufacturing defects include: Balsa is a natural wood product with elongated closed cells;  it is available in a variety of grades that correlate to the  structural, cosmetic, and physical characteristics. The density  of balsa is less than one-half of the density of conventional  wood products. However, balsa has a considerably higher  density than the other types of structural cores.  Damage can occur at several scales within the composite  material and structural configuration. This ranges from  damage in the matrix and fiber to broken elements and failure  of bonded or bolted attachments. The extent of damage  controls repeated load life and residual strength and is critical  to damage tolerance. edge cuts, surface gouges and scratches, damaged fastener  holes, and impact damage. Examples of flaws occurring  in manufacturing include a contaminated bondline surface  or inclusions, such as prepreg backing paper or separation  film, that is inadvertently left between plies during layup.  Inadvertent (nonprocess) damage can occur in detail parts or  components during assembly or transport or during operation.  A part is resin rich if too much resin is used, for nonstructural  applications this is not necessarily bad, but it adds weight. A  part is called resin starved if too much resin is bled off during  the curing process or if not enough resin is applied during  the wet layup process. Resin-starved areas are indicated by  fibers that show to the surface. The ratio of 60:40 fiber to  resin ratio is considered optimum. Sources of manufacturing  defects include: