Browsing by Author "Johnson, David Page"

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    The effect of specimen size on the mechanical response of laminated composite coupons loaded in tension and flexure
    Johnson, David Page (Virginia Tech, 1994-04-05)
    The effect of specimen size on the uniaxial tensile stress/strain response of sublaminatelevel scaled composite specimens, and the four point flexure load/deflection response of ply- and sublaminate-level scaled composite specimens was investigated. Three laminates were studied in the tensile program, namely [+30/-30/90₂]ns, [+45/-45/0/90}ns and [90/0/90/0|ns, where n = 1, 2, 3, 4. Two material systems were used, namely AS4/3502 graphite/epoxy and APC-2 graphite/PEEK, to investigate the relative effect of resin toughness. Three laminates were also studied in the flexure program, The baseline lay-ups were (±45/0/90}2ns, [0/90/0/90J2ns and [±45/±45J2ns, where n = 1, 2, 4. Ply- and sublaminate-level scaling were used to increase specimen thickness. All flexure specimens were of AS4/3502 graphite/epoxy. Enhanced X-ray radiography and edge photomicroscopy were used to examine damage development in specimens loaded to various fractions of their ultimate stress. This nondestructive examination was coupled with observations of critical events in the stress/strain response to try to correlate scaling effects with the damage development in the specimens. Analytical and numerical methods were employed in order to understand the stresses driving certain damage modes observed. 2-D and 3-D finite element models were used to find delamination stresses in an undamaged laminate, and an approximate clasticity approach was used to find stresses duc to cracks in the 90° plies. It was found that the tensile strength of the [+30/-30/90₂]ns and [+45/-45/0/90}ns laminates gencrally increased as n increased. This effect was more pronounced for the matrixdominated [+30/-30/90₂]ns. Both the [+30/-30/90₂]ns and the quasi-isotropic [+45/-45/0/90}ns laminates seemed to be approaching a maximum strength, beyond which the strength scaling either stops, or is reversed. As # increased from 1 to 4, these two laminates exhibited a delay in the onset of certain damage mechanisms, such as delamination and transverse matrix cracking. The [90/0/90/0|ns laminates showed no tensile strcss/strain response scaling effects, although the stress at which first ply failure occurred was found to increase as 7 increased. (±45/0/90}2ns and [±45/±45J2ns flexure specimens showed no strength scaling cffects when sublaminate-level scaling was uscd, but significant decreases in s{rength were found when specimen size was increased using ply-level scaling. [0/90/0/90J2ns specimens showed no global load/deflection scaling effects.
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    A study of tension, compression, and shear test methods for advanced composites
    Johnson, David Page (Virginia Tech, 1991-05-05)
    A study of the literature pertaining to test methods for advanced composite materials has been carried out. Several test methods were discussed and compared for each of three areas of interest. These areas were uniaxial tension, uniaxial compression and in-plane shear. Test methods were selected for tension, compression and shear and guidelines set for the entry of material property data into a comprehensive mechanical property database being undertaken by Virginia Tech's Center for Composite Materials and Structures (CCMS). According to the findings, recommendations for future work were made.