Issue 31

R.D.S.G. Campilho et alii, Frattura ed Integrità Strutturale, 31 (2015) 1-12; DOI: 10.3221/IGF-ESIS.31.01 4 30%. The plates were fabricated by hand lay-up and cured at room temperature in a vacuum bag. For the three joint configurations, for a uniform value of t A , calibrated spacers were inserted between the adherends. These spacers were inserted at both bonding edges between the adherends to control the value of t A . For the calibrated spacer at the crack tip, 3 plies were stacked and glued together, composed of a 0.1 mm thick razor blade between steel spacers to achieve the desired value of thickness, to create a pre-crack. For all specimens, stainless steel piano hinges were glued to both faces of the specimens at the cracked edge with a ductile adhesive, to provide a loading means in the testing machine grips. Also, a metric scale was glued with cyanoacrylate in both adherends to allow measurement of the crack length ( a ) and of the input data for the extraction of the J -integral. Six specimens of each configuration were tested at room temperature (≈20ºC), relative humidity of ≈40% and 2 mm/min in an electro-mechanical testing machine (Shimadzu AG-X 100) with a load cell of 100 kN. Data recording was carried out at 5 Hz for the load ( P ) and testing machine grips displacement (  ), registered during the test as a function of the time elapsed since its initiation. Pictures were recorded during the specimens testing with 5 s intervals using a 15 MPixel digital camera with no zoom and fixed focal distance to approximately 100 mm. J- INTEGRAL TECHNIQUE TO MEASURE G n c n the proposed method, the CZM law is measured by the direct method. Under this scope, the path-independence of the J -integral can be used to extract relations between the specimen loads and the cohesive law of the crack path [21]. Based on the fundamental expression for J defined by Rice [22], it is possible to derive an expression for the value of G n applied to the DCB specimen from the concept of energetic force and also the beam theory for this particular geometry, as follows [23]:   u n u o n u p 2 3 12 or P a G P G P Eh      (1) where P u represents the applied load per unit width at the adherends edges,  o the relative rotation of the adherends at the crack tip and   p the relative rotation of the adherends at the loading line (Fig. 2). Figure 2 : DCB specimen under loading, with description of the analysis parameters. In this work, the first expression of (1) is considered, using  o instead of  p , due to a simpler extraction of the parameter by the optical method. The J -integral can be calculated along an arbitrary path encircling the start of the adhesive layer, giving [21]:   nc n n n n 0 d G t      (2) where  nc is the end-opening at failure of the cohesive law (measured at the initial crack tip) and t n is the current normal traction. G n c can be considered the value of G n at the beginning of crack growth. Thus, G n c is given by the steady-state value of G n , at a  n value of  nc [13]. The t n (  n ) curve can be easily obtained by differentiation of Eq. (1) with respect to  n   n n n n G t      (3) I