Tensile modulus, longitudinal

The commercial composite materials being marketed today are optimized in order to make the interfacial properties acceptable in the sense that they will not fail at such low levels as to detract from the overall composite behavior. Considering a unidirectional specimen, where the fibers are all aligned parallel to each other, commercial systems can be described by a rule of mixtures661 relationship (Fig. 10). Properties of the matrix and fiber can be linearly combined based on the volume fraction of each constituent. For example, the longitudinal tensile modulus is the sum of the proportion of each component. The interface in these systems is considered ideal in that it efficiently transmits forces between fiber and matrix without failure. Using this model as a basis for micromechanical analysis and discussion, the magnitude of the forces present at the interface can be predicted. 

Earlier work (4,6) demonstrated that a high gradient in the volume strain-longitudinal strain curve is associated with a large drop in the modulus of the material. This is to be expected, as the crazes have much lower moduli than the material from which they were formed (7). As a result of the creep tests described above, the 100 sec tensile modulus of Luran S 757R at a strain of 0.5% fell from 1.99 to 1.30 GN/m2, and the modulus of Luran S 776S at a similar strain fell from 1.65 to 0.98 GN/m2. 

Volume Composite Constituents fraction Longitudinal tensile, MPa (ksi) Longitudinal tensile modulus, Shear, GPa(psixl0 ) MPa (ksi) Longitudinal expansion, fim/m-K Transverse expansion, fim/m-K... 

Kevlar fibers are transversely isotropic with the tensile modulus in the axial direction around 52 times the transverse compressive modulus of Kevlar 49 and 38 times for Kevlar 29. Additionally, the longitudinal tensile strength is about 42 times the transverse compressive strength of Kevlar 49 and 54 times for Kevlar 29 (Kawabata, 1990). 

Notes The first subscript 1 or t refers to directions longitudinal and transverse to the fibre. The second subscript, t, f or c, refers to tensile, flexural and compressive properties. For example, is the transverse tensile modulus. 

There is little to choose between the two epoxies. The longitudinal tensile modulus is what would be expected from the rule of mixtures, section 1.3, though the strength is below the expected figure. The extreme anisotropy of unidirectional composites is noticeable and in both cases compressive strength is less than tensile strength. 

Fiber Matrix Fiber Content (yol%) Density (g/cm ) Longitudinal Tensile Modulus (GPd) Longitudinal Tensile Strength (MPa)... 

One of the simplest deformations to study experimentally is simple extension which involves the longitudinal extension of a sample (whose length is usually much greater than its width or thickness). This type of deformation, however, induces both shear and volume deformations on the specimen. The tensile modulus E is obtained from... 

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