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Cold flakes, one of casting defects, were observed through acoustic microscopy in aluminum alloy die-cast plates (ADC12), and specimens having a cold flake of different sizes were prepared and subjected to the tensile testing. The tensile strength linearly decreased with the size of the cold flake, and the decreasing rate of the strength was greater for the specimen with the exposed cold flake than that with the embedded cold flake. The in-process ultrasonic measurement was carried out during tensile testing, and it was found that the oxide layer of the cold flake was detached from the matrix to form a crack before final failure. Then by using the linear fracture mechanics, the critical value K c à at failure was evaluated to be 8-10 MPaÁm 1=2 from the tensile strength, which value was slightly lower than the fracture toughness of the matrix of