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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Fig. 1. C-ELS specimen and test setup.

The C-ELS specimens and test apparatus are presented in Fig. 1. For all specimens, the total specimen length l , thickness 2 h , initial insert length a 0 measured between the load line and the PTFE tip, illustrated in Fig. 1, were mea sured to be approximately 200 mm, 5 mm and 50 mm. The free length L f measured between the load line and the test fixture, also shown in that figure, was set to be 4 a 0 / 3 for each test in order to promote delamination propagation stabil ity. The width B , which does not appear in the figure, was approximately 20 mm. The tests were performed for the NPC stage described in the ISO-15114 standard [4]. In the NPC stage, the specimen is loaded with a constant displacement rate of 0.5 mm / min until the delamination is observed to propagate a short distance, and then unloaded. The load was transferred to the specimen through a load block by means of an Instron loading machine (model number 8872; High Wycombe, England), with a 250 N load cell. The load block was pulled vertically through a pin. A clamping arrange ment creates free horizontal sliding of the specimen by means of bearings, but restricts vertical motion and rotation as may be observed in Fig. 1. A LaVision system composed of one monochrome CCD 5 MP Imager Pro SX camera of LaVision (Go¨ ttingen, Germany) with resolution of 2456 × 2058 pixels, and a Nikon Micro-Nikkor 105 mm f / 2.8 lens was employed during the test. Also, a programmable timing unit (PTU) controlled by DaVis [19] computer software was used. During each test, images of the specimen were acquired at a rate of 2 Hz as the displacement increased until fracture. 2.2. Methods for determining G ic Several LEFM data reduction methodologies may be used to obtain the initial critical interface energy release rate G ic [4, 15, 20]. The load at failure P , the actuator displacement d , and the corresponding delamination length a are measured. In the C-ELS standard [4], the initiation load P is determined in three ways: non-linear (NL), visual, and 5% o ff set or maximum load. It may be noted that since the delamination considered here is along an interface between two dissimilar plies in an MD laminate, where the two specimen arms have di ff erent thicknesses and mechanical properties, some of the methods presented in the ISO standard [4] are not applicable. As expected, the C-ELS test produces dominant in-plane shear deformation; however, as a result of the interface between dissimilar plies, some delamination opening occurs. Three methods for determining G ic are considered here. The first is the ECM described in [4] and recommended to be used for both initiation and propagation C-ELS tests. The specimen compliance C , determined from the load and actuator displacement, is assumed to behave as C = C 0 + ma 3 (1) where C 0 and m are fitting parameters. These may be found by plotting the measured compliance C versus a 3 . Since the standard has been developed for UD laminates, the energy release rate is G IIc . For an interface delamination with dominant in-plane shear deformation, it is assumed here that G ic ≈ G IIc , (2) so that,

3 P 2 a 2 m

F N

(3)

G ic =

2 B ·

.