PSI - Issue 14

K. Chawla et al. / Procedia Structural Integrity 14 (2019) 571–576 K.Chawla et al./ Structural Integrity Procedia 00 (2018) 000 – 000

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2.2. Testing, measurements and observations DCB samples of dimension, 150 mm x 25 mm x 3.5 mm, are machined from the laminates with pre-cracks of lengths, varying from 30 mm to 50 mm. The standard test procedure provided in ASTM D5528-13 (2013) is followed while performing the experiments. The tests are carried out at ambient temperature in INSTRON 3345 universal tensile testing machine with a load cell of capacity 5 KN. A pre-crack is induced in specimens to develop a natural crack prior to loading. First, the specimens are loaded at a crosshead speed of 1.2 mm/min until the delamination length increased by ~ 5 mm. This is followed by unloading the samples. Next, the specimens are reloaded while recording the synchronize load vs displacement data through optical imaging. The representative load vs. displacement data obtained from the experiments are shown in Fig. 1(a). Initial linear load vs. crosshead displacement curve indicates double cantilever beam loading prior to crack growth. After a critical load is attained the curve shows a gradually decreasing trend. During crack growth a constant bridging zone is formed ahead of the crack tip which develops the stable crack growth. In case of the reinforced specimens, the stable crack growth region is elongated due to additional stiffening provided by the short fibres. The resultant increase in the laminate’s resistance to crack growth can be inferred from the higher load values correspond to a given crosshead displacement. The strain energy release rate ( G I ) from modified beam theory method is given by,

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

2 ( b a G P I 

,

(1)

)

 

where ‘ P’ is the applied load; ‘ δ’ is the cross-head displacement; ‘ a ’ and ‘ b ’ are the delamination length and the width of the specimen, respectively. The ‘Δ’ is the applied compliance correction which is defined as the negative intercept on x-axis of the least square plot of the cube root of compliance ( C 1/3 ) with delamination length ( a ) as shown in Fig. 1(b). The compliance values used in the plot is calculated from the load vs. displacement data ( δ / P ).

(b)

(a)

Fig. 1. (a) Representative load vs. crosshead displacement plot obtained from DCB tests; (b) Compliance correction Δ determination.