PSI - Issue 28

Mor Mega et al. / Procedia Structural Integrity 28 (2020) 917–924 M. Mega and L. Banks-Sills / Structural Integrity Procedia 00 (2019) 000–000

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structural design, reliable tests are necessary. Currently, several specimens and test configurations have been proposed in the literature including five existing ASTM and ISO standards [2–6] for determination of interlaminar fracture toughness for mode I, mode II and mixed mode I / II conditions. All five standards are limited to UD composites. However, most applications involve MD laminates where a delamination between plies of di ff erent orientations tends to occur [7]. Various types of test configurations exist for determining the mode II initiation fracture toughness G IIc and propa gation resistance G IIR [4, 5, 8, 9] in UD laminates. In [10, 11], it was shown that lower initial critical G IIc values are expected from a precrack than from an insert. Yet, if a precrack is generated in mode II, it is recommended in the C-ELS standard [4] to measure both the critical initiation value G IIc from the insert, as well as from the precrack. Note that the C-ELS configuration is known to be the most stable mode II test which results in less scatter in the steady state energy release rate G iss [10–14]. For each test geometry, several linear elastic fracture mechanics (LEFM) data reduction methodologies to obtain G c or G R , based on BT or ECM are presented in the standards, as well as in the literature. Some of the methods described in the standards make use of delamination length measurements to define a relation between the compliance C and the delamination length a . It may be noted that the latter is known to be di ffi cult to measure for mode II testing [8, 9, 14]. It is important to note that the equations derived by means of the BT method and used in [4, 5] are based on the assumption that the two specimen arms are of the same geometrical measurements and sti ff ness. Since this is not the case for a delamination along an interface between two dissimilar plies in an MD material, modification of these equations is required. In [15], based on the specimen curvature and moments, using BT, mode separation was achieved for the case of a specimen with two arms of di ff erent thicknesses. It may be noted that this derivation does not take into account the dissimilar mechanical properties of the two arms. In this study, six C-ELS quasi-static tests were performed following the ISO-15114 [4] standard for a UD compos ite. The delamination is between a UD fabric ply with fibers oriented mainly in the 0 ◦ - direction and a plain balanced woven ply with tows oriented in the + 45 ◦ / − 45 ◦ - directions. In Section 2, the CFRP material tested, as well as the test procedure will be described. The analysis methods used to calculate the mixed mode critical initiation interface energy release rate or fracture toughness G ic , from a film insert, for ∆ a = 0, will also be introduced in this section. In Section 3, test and analysis results will be discussed, including the calculated phase angle ˆ ψ , as well as the G ic values obtained by means of the various methods. In [16], the same material and interface were studied using BD specimens under various mixed mode conditions to obtain G ic . Failure criteria were proposed in [17]. In Section 4, the G ic values obtained from the C-ELS tests will be compared to results found from the criteria in [17]. Also, in that section, conclusions from this study will be presented. In this section, details of the specimen, test procedure and analyses methods will be presented. In Section 2.1, the investigated CFRP composite material and interface, specimen details and the C-ELS test procedure will be discussed. The various methods used for the calculation of G ic will be introduced in Section 2.2. 2.1. Specimen details and experimental procedure In this study, six C-ELS specimens were fabricated by means of a water-jet process from an MD CFRP laminate composed of 19 plies. The laminate was manufactured by means of a wet-layup process, and composed of the follow ing layup [( + 45 ◦ / − 45 ◦ ) , (0 ◦ / 90 ◦ )] 4 , ( + 45 ◦ / − 45 ◦ , 0 ◦ // ( + 45 ◦ / − 45 ◦ ) , [( + 45 ◦ / − 45 ◦ ) , (0 ◦ / 90 ◦ )] 4 . Note that the double slash indicates the initial delamination location which was introduced using a 13 µ m thick polytetrafluoroethylene (PTFE) film. The delamination is between a UD ply composed of approximately 97% T300 carbon fibers oriented in the x 1 - direction and approximately 3% glass fibers oriented in the transverse direction. Below the PTFE film is a plain, balanced woven ply also containing T300 carbon fibers; its tows are oriented in the + 45 ◦ / − 45 ◦ - directions. The fibers in both plies are embedded in an EPR-L20 / EPH960 epoxy matrix. The 0 ◦ / 90 ◦ woven ply used in the layup is the same as the + 45 ◦ / − 45 ◦ ply only placed in the laminate with the tows in the 0 ◦ / 90 ◦ - direction. Note that the specimens used here were cut from the same plate in [18]. The fiber volume fraction and mechanical properties of each ply may be found in [18]. 2. Materials and methods