Forms of glass fiber reinforcements
Glass fiber-reinforced composites contain fibers having lengths far greater than their cross sectional dimensions (aspect ratios > 10:1).
The largest commercially produced glass fiber diameter is a “T” fiber filament having a nominal diameter of 22.86 to 24.12 microns. A number of fiber forms are available.
Rovings—This is the basic form of commercial continuous fiber.
Rovings are a grouping of a number of strands, or in the case of so-called “direct pull” rovings, the
entire roving is formed at one time.
This results in a more uniform product and eliminates catenary associated with roving groups of strands under unequal tension.
Woven roving—The same roving product mentioned above is also used as input to woven roving reinforcement.
The product is defined by weave type, which can be at 0 and 90 deg; at 0 deg, +45 deg, -45 deg, and other orientations depending on the manufacturing process.
Mats—These are two-dimensional random arrays of chopped strands.
The fiber strands are deposited onto a continuous conveyor and pass through a region where thermosetting resin is dusted on them.
This resin is heat set and holds the mat together.
The binder resin dissolves in the polyester or vinyl ester matrix thereby allowing the mat to conform to the shape of the mold.
Combined products—It is also possible to combine a woven roving with a chopped strand mat.
There are several techniques for accomplishing this.
One-technique bonds the two reinforcements together with a thermosetting resin similar to that in the chopped strand approach.
Another approach starts with the woven roving but has the chopped strand fibers deposited onto the surface of the woven roving, which is followed immediately by a stitching process to secure the chopped fibers.
There are several variations on this theme.
Cloth—Cloth reinforcement is made in several weights as measured in ounces-persquare-yard. It is made from continuous strand filaments that are twisted and plied and then woven in conventional textile processes.
All composite-reinforcing fibers, including glass, will be anisotropic with respect to their length.
There are fiber placement techniques and textile-type operations that can further arrange fibers to approach a significant degree of quasi-iso-tropic composite performance.
Glass fibers and virtually all other composite fibers are also available in a range of fabric.
1.5.2 Behavior of glass fibers under load Glass fibers are elastic until failure and exhibit negligible creep under controlled dry conditions. Generally, it is agreed that the modulus of elasticity of mono-filament E-glass is approximately 73 GPa.
The ultimate fracture strain is in the range of 2.5 to 3.5 percent.
The stress-strain characteristics of strands have been thoroughly investigated. The fracture of the actual strand is a cumulative process in which the weakest fiber fails first and the load is then transferred to the remaining stronger fibers, which fail in succession.
Glass fibers are much stronger than a comparable glass formulation in bulk form such as window glass, or bottle glass.
The strength of glass fibers is well retained if the fibers are protected from moisture and air-borne or contact contamination.
When glass fibers are held under a constant load at stresses below the instantaneous static strength, they will fail at some point as long as the stress is maintained above a minimum value.
This is called “creep rupture.” Atmospheric conditions play a role, with water vapor being most deleterious. It has been theorized that the surface of glass contains submicroscopic voids that act as stress concentrations.
Moist air can contain weakly acidic carbon dioxide. The corrosive effect of such exposure can affect the stress in the void regions for glass fiber filaments until failure occurs.
In addition, exposure to high pH environments may cause aging or a rupture associated with time. These potential problems were recognized in the early years of glass fiber manufacture and have been the object of continuing development of protective treatments. Such treatments are universally applied at the fiber-forming stage of manufacture.
A number of special organo-silane functional treatments have been developed for this purpose.
Both multifunctional and environmental-specific chemistries have been developed for the classes of matrix materials in current use.