W.Tong
,
N.Zhang
材料科学技术（英文）
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.
关键词:
Crack growth
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Gaifei PENG Sammy Yin Nin CHAN
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Jianhua YAN
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Yefeng LIU
,
Quanxin YI
材料科学技术（英文）
An experimental investigation was conducted to identify the characteristics of crack growth in high performance concrete (HPC) subjected to fire, including two parts of work, i.e. crack growth resistance determinations and cracking observations, using concrete of three strength grades 40 MPa, 70 MPa, and 110 MPa. The crack growth resistance curves (R-curves ) of HPC subjected to high temperatures were determined using notched three-point bend beam specimens of 100 mm×100 mm×300 mm. The R-curve (crack growth resistance curve) flattening shows that the crack growth resistance has been significantly reduced by elevated temperature. Concrete with a higher strength grade has a steeper R -curve, with a higher fracture toughness but a shorter critical crack growth. The shorter critical crack growth means that concrete of a higher strength grade has a more brittle behavior. The concrete cracking observations reveal that the consequences of rapid heating are quite different from those of slow heating. For slow heating at a rate of 0.5℃/min, HPC suffered no obvious cracking below 600℃ even if it had a high moisture content. Explosive spalling is an extreme case of the internal cracking driven mainly by vapor pressure. All these results confirmed the vapor pressure mechanism for spalling behavior which should be more significant for denser concrete. The crack growth ranges obtained from the R -curve determination results are in good agreement with those measured in the concrete cracking observations.
关键词:
High performance concrete
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