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Mechanical Properties of FRPs
Many FRPs have tensile strengths that exceed the strength of steel, but their stiffness is generally lower than that of steel. When loaded along the fiber direction they behave essentially linearly until fracture, and are therefore brittle by nature.
Stress Corrosion and Stress Rupture
The average ultimate tensile strength of freshly drawn glass fibers may exceed 3500 MPa. However, surface flaws tend to reduce the tensile strength to values in the range of 1750 to 2100 MPa. Strength degradation is increased as the surface flaws grow under cyclic loads. This is one of the major disadvantages of using glass fibers in applications where fatigue may be an issue. Sustained loads also cause surface flaws to grow, resulting in reduced tensile strength.
Figure 1. shows reduction of strength with time for E-glass fiber under different temperatures.
Stress corrosion refers to the characteristic property of FRPs in which the failure strength under long term sustained loads in a chemical environment is lower than its short term tensile strength. In air, this phenomenon is referred to as “stress rupture” (Sen et al. 1993). Stress rupture tests are usually performed by applying a constant tensile stress to a specimen until it fractures completely. The time at which fracture occurs is termed the “lifetime”. Creep, on the other hand, is defined as the increase in strain with time at a constant load level (Mallick 1993).
Glass, Aramid, and Boron fibers and their composites exhibit failure by stress rupture. Carbon fibers, on the other hand, are relatively less prone to stress rupture failure. In order to prevent stress rupture in glass FRP for a period of 10, 30, and 50 years, the sustained strains in the GFRP should be less than about 0.35 , 0.32 , 0.30 , respectively (Sen et al.1993, ACI 2000).
The relationship between the sustained stress (or strain) and the logarithm of time is approximately linear as shown in Figure 1. ACI Committee 440R recommends the use of a safety factor of 1.67 and hence a safe level of sustained strain to prevent stress rupture in glass is about 0.2 .