Page 327 - Engineered Interfaces in Fiber Reinforced Composites

308 Engineered interfaces in jiber reinforced composites

remarkable 500% using multi-layers of perforated Mylar films for the best optimum
condition (Jea and Felbeck, 1980).
In a similar study using several other type of films, including aluminum foils,
bleached papers, polyester textile fabrics and polyimide Mylar, as delamination
promoters, it was concluded that the fracture mode and the subsequent energy
absorption mechanisms depended strongly on the loading directions relative to the
laminar plane and other testing parameters such as loading speed and the span-to-
depth-ratio in bending (Jang et al., 1987). The effectiveness of the intermittent
interlaminar bonding concept has been further proven for cement mortars with
embedded perforated papers as delamination promoters (Mai et al., 1982b). It was
noted that the large improvement of 200-800% in transverse fracture toughness was
accompanied by a huge 30-50% reduction in flexural strength. In summary,
although the delamination promoter concept is quite useful for enhancing the
transverse fracture toughness of laminate composites, extreme care must be exercised
in its application because delamination and other associated damage are some of the
most critical life-limiting failure modes in most engineering structures made from
composite materials. Excessive delamination growth may cause unacceptable
reduction in the composite stiffness and strength, which, in turn, impair the overall
mechanical performance and structural integrity of the composite structure.

7.5. Residual Stresses

7.5.1. Origin of residual stresses

Residual stresses are inherent in almost all fiber composites whether they are
based on polymer, metal or ceramic matrices, but they are often ignored or
underestimated in both design and analytic modeling. This oversight can lead to
incorrect interpretations of material characteristics and mechanical behavior. The
primary origins of residual stresses in fiber composites are twofold: thermal and
mechanical. The thermal origin is the most prevalent and arises from the different
CTE of the composite constituents. Composites in general achieve their structural
integrity by being cured or processed at elevated temperatures under pressure and/or
in vacuum. This process invariably induces residual stresses to build up in the
composite when it is at a temperature different from its process temperature. The
resulting residual stresses of thermal origin can be either micro or macroresidual
stresses, depending on the geometry and scale of the composites concerned (Chamis,
1971). Favre (1988) has given a review of these residual stresses, the experimental
techniques devised to measure them, and of ways to reduce them.

7.5.1 -1. Micro-residual thermal stresses
The micro-residual stresses arise from the differential CTE of the fiber and matrix,
and the temperature difference. Table 7.4 gives the linear CTE values for various
types of reinforcing fibers and matrix materials used widely for composite
fabrication. The CTEs of most fibers and ceramic matrices are relatively lower