PSI - Issue 28

Anurag Singh et al. / Procedia Structural Integrity 28 (2020) 2206–2217 Anurag Singh/ Structural Integrity Procedia 00 (2019) 000 – 000

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where,  R: rate of crosshead motion, displacement rate, mm/min  L: support span, mm  d: depth of specimen, mm  b: width of the specimen, mm  Z: rate of straining, equivalent to 0.01  D: mid-span deflection, mm  r: strain, mm/mm  P: load at a point in a force vs displacement curve, N  σ f : flexural stress, MPa  ε f : Flexural strain, mm/mm  E f : Modulus of elasticity in bending, MPa  m: the slope of force vs displacement curve, N/mm 2.5. DCB Mode-I fracture test

Mode I fracture toughness tests were carried out for composites according to the test standard ASTM D5528 using a double cantilever beam (DCB) method (ASTM D5528-01 2014). Dimensions of the specimens were 25x180x3 mm; a PTFE film of 25 μm thickness and 50 mm in length was inserted in the mid -plane to create an initial crack. Experimental protocol suggests using five different specimens. Figure 3 a) shows the prepared specimen with end blocks bonded to one end of the specimen. Figure 3 b) shows the initial configuration of the DCB test setup; figure 3 c) shows the specimen at the end of the test, a usual open jaw configuration. A travelling optical microscope was positioned on the backside of the specimen to observe the delamination front as it extends along one edge during the test. Specimens were loaded at a constant rate of 5 mm/min, while continuously recording the load and displacement values. From the first record of visual onset of delamination, speed of 25 mm/min employed for unloading, after observing the crack jump. The experimental protocol uses the speed of 5 mm/min for reloading the specimens, on the continuation of the loading, the load and displacement values were recorded at delamination lengths increments of every 1 mm, in the first 5 mm. Subsequently, recording the load and displacement data every 5 mm until the crack delamination has propagated 45 mm from the tip of the pre-crack, at last for the final 5 mm, 1 mm increment of crack growth is recorded up to the total delamination length of 50 mm beyond the tip of the pre-crack. On reaching the final delamination length unloading of the specimens is done at a rate of 25 mm/min. The mode I interlaminar fracture toughness was calculated

Figure 2: Flexural test (a) Initial position of the specimen (b) Final position of the specimen before failure