Issue 29

M. Scafè et alii, Frattura ed Integrità Strutturale, 29 (2014) 399-409; DOI: 10.3221/IGF-ESIS.29.35 402 In summary, the 0° lamina compressive strength is determined by equations: - (1) and (3), through the mechanical characterization of two UD laminates (0° and 90°) and a cross-ply laminate with a maximum of 50% of 0° plies [2]; - (1) and (4), through the mechanical characterization of two UD laminates (0° and 90°) and an angle-ply laminate with a maximum of 50% of 0° plies. E XPERIMENTAL ix separate stacking sequences were used in the present experimental test campaign, as shown in Tab. 1. Test specimens were produced according to ASTM [2], [7]. This procedure was simplified because untabbed specimens are permitted, as reported in ASTM D 6641/ D 6641 M: “The specimen may be untabbed (Procedure A) or tabbed (Procedure B), as required. (…) Untabbed specimens are usually suitable for use with materials of low orthotropy, for example (…) laminates with a maximum of 50% 0° plies” [2]. Lay Up ID Code Configuration A Cross-ply with 21.1 % of 0° plies B Cross-ply with 42.1 % of 0° plies C Angle-ply with 21.1 % of 0° plies D Angle-ply with 42.1 % of 0° plies E UD 0° F UD 90° Table 1 : Stacking sequences of test specimens. The experimental tests were conducted at ambient laboratory conditions, using an MTS electro-hydraulic universal testing machine, equipped with an MTS 100 kN load cell. The test procedure is in accordance with ASTM D 6641 / D6641 M [2] which is referred to “Combined Loading Compression test fixture”. All tests were performed at a constant displacement rate of 1.3 mm/min, while the data were acquired at a rate of 10 samples/s and processed in accordance with the same ASTM standard. A Wheatstone bridge system in half-bridge configuration, was used for strain measurements. This system was composed by an active strain gauge and a “dummy” for temperature compensation. The acquisition unit and the strain gauges used are HBM products. The specimens were instrumented with two strain gauges applied in two alternative back-to-back configuration in the gauge section, thus distinguished: 1) two unidirectional strain gauges; 2) an unidirectional strain gauge and a bidirectional strain gauge. R ESULTS AND D ISCUSSION verall 60 tests were carried out, as follows: 24 tests for cross-ply laminates (materials A and B with 10 instrumented and 14 not instrumented tests), 24 tests for angle-ply laminates (materials C and D with 12 instrumented and 12 not instrumented tests) and 12 tests for UD laminates (materials E and F, with 10 instrumented and 2 not instrumented tests). Normalized results are summarized, relative to an appropriate experimental value, in tables from 2 to 7, where the symbols are: σ max = compressive strength E SG1chord = axial compressive stiffness measured by strain gauge 1 1 E SG2chord = axial compressive stiffness measured by strain gauge 2 ΔE % = variation between ESG1chord and ESG2chord referred to their average value 1 Strain gauge 1 is identified with the bidirectional strain gauge (if present), or with the strain gauge whose data are recorded first. S O