STRAIN-GAUGE SELECTION CRITERIA FOR TEXTILE COMPOSITE-MATERIALS

Composite materials reinforced with braided, woven, knit, and stitched fibrous preforms are emerging as potential alternatives to unidirecti onal prepreg tape. A variety of these material forms are currently bei ng evaluated by major airframe manufacturers. These new forms of compo site materials bring with them potential testing problems. The applica bility of existing test methods to the 2-D and 3-D braided, woven, sti tched, and knit materials being evaluated bears investigation. The ove rriding concern is that the values measured are accurate representatio ns of the true material response, This paper will provide a review of efforts to establish a set of strain gage selection guidelines for tex tile reinforced composite materials. A variety of strain gags were eva luated in the study to determine the sensitivity of strain measurement s to the size of the strain gage, The strain gages were chosen to prov ide a range of gage lengths and widths. The gage aspect ratio (the len gth-to-width ratio) was also varied. The gages were tested on a divers e collection of textile composite laminates. Test specimens featured e leven different textile architectures: four 2-D triaxial braids, six 3 -D weaves, and one stitched uniweave architecture. All specimens were loaded in uniaxial tension. The materials' moduli were measured in bot h the longitudinal (parallel to the 0 degrees yarns) and the transvers e (perpendicular to the 0 degrees yarns directions. The results of the se measurements were analyzed to establish the strain gage guidelines.