材料期刊网

Serrated plastic flows in an Al sheet (AA5182-H19) were first characterized temporally and spatially under quasistatic uniaxial tension. Discrete types B and C serrations or Portevin Le-Chatelier (PLC) bands started to appear at a plastic strain of 2.54% with an average stress drop of 2.5 MPa and an average time interval of 7.75 s between serrations. The width of the individual PLC bands was about twice of the thickness of the sheet (0.235 mm) and the plastic strain inside the bands was as high as 2%. V-notched tear test coupons made of the same Al alloy were then stretched under both continuous monotonic and periodic interrupted loading conditions and their crack initiation and growth processes were monitored by direct digital imaging. The crack tip opening angle (CTOA) and the crack tip opening displacement (CTOD) were measured to be about 2.5～3℃ and 0.07～0.1 mm, respectively. While no major load serrations were observed before crack initiation at the notch tip, discontinuous or intermittent crack advances or crack jumps were unambiguously detected in the crack growth stage. The unstable crack propagation in the AA5182-H19 sheet seems to be related closely with its strong dynamic strain aging behavior exhibited under quasistatic loading conditions at the ambient temperature.