USE OF LEAST-SQUARES METHOD TO OBTAIN RELAXATION FUNCTION OF AS4 CARBON-PEEK 3D BRAIDED COMPOSITES FROM DMA DATA

Viscoelastic behavior is inherent to polymer matrix composites such as an AS4 Carbon - PEEK 3D composite system. Two common quantities of ch aracterizing viscoelastic behavior are the relaxation ''function'' G(t ), and the complex ''modulus'' G(omega). These two measures are relat ed: for linear viscoelastic materials, the complex modulus is the Four ier transform of the relaxation function. Thus, theoretically one only needs to perform either of the two experiments and mathematically rec over the other Practical considerations in converting G(omega) to G(t ) are discussed in this paper A method based upon the feast square tec hnique is used. The method is applied to a AS4-PEEK 3D braided composi te system. Complex shear modulii of the composite system are used to o btain the corresponding time domain shear relaxation modulii. The conv ersion is useful to generate time domain data from frequency domain da ta when such data is unavailable from a conventional creep or stress r elaxation test.