ASE324: Aerospace Materials Laboratory

Transcription

1 ASE324: Aerospace Materials Laboratory Instructor: Rui Huang Dept of Aerospace Engineering and Engineering Mechanics The University of Texas at Austin Fall 2003

2 Lecture 20 November 11, 2003

3 Fracture toughness tests Quantitative measurements of fracture toughness Room temperature Quasi-static loading Uniaxial tensile stress However, it cannot predict failure under some special circumstances.

4 Charpy impact test The impact energy absorbed by the specimen is computed from the difference between the maximum heights of the pendulum before and after it breaks the specimen. Qualitative measurement of fracture energy High strain rate Triaxial stress at the notch Low and high temperatures

5 Ductile-to-brittle transition The measured impact energy decreases with decreasing temperature. For steels, the impact energy drops remarkably over a narrow temperature range, indicating a ductileto-brittle transition phenomenon.

6 Fracture surface character Shear character (fibrous and dull) for ductile fracture. Cleavage character (shiny granular texture) for brittle fracture. The percentage of shear character indicates the ductile-tobrittle fracture transition.

7 Determination of transition temperature Temperature at specific impact energy (e.g., 15 ft-lb or 20 J). Temperature corresponding to some given fracture surface character (e.g., 50% shear fracture). No unified criterion!

8 Effects of microstructure Decreasing the grain size results in lower transition temperature. Increasing the carbon content in steels raises the transition tempertaure.

9 Fracture transition behavior FCC alloys (including aluminum- and copper-based alloys) remain ductile at extremely low temperature. BCC and HCP alloys experience ductile-to-brittle transition. Ceramics and polymers also experience ductile-tobrittle transitions.

10 Specimen thickness effects Transition temperature Sample thickness The transition temperature increases with increasing Charpy bar thickness. Plane-stress/plane-strain transition. Laboratory results may not be directly used for design components. To overcome this difficulty, use dynamic tear (DT) test and drop-weight tear test (DWTT).

11 Crack-length effects Stress σ Y (T 2 ) σ Y (T 1 ) σ = c K c πa Crack length a 1 : brittle at both temperatures; Crack length a 2 : ductile at T 1, but brittle at T 2 ; Crack length a 3 : ductile at both temperatures. a 3 a 2 a 1 Crack length, a Transition temperature values determined in the lab require careful interpretation when used for full-scale components.

12 Energy-toughness correlation It is found that, for some materials, the fracture toughness (G c ) and the impacted energy absorbed by unit cross-sectional area of a pre-cracked Charpy specimen (W/A) are correlated. Restricted to materials that exhibit little or no strain-rate sensitivity. No good for brittle materials. W/A G c

13 Instrumented Charpy test To provide more information about the load-time history of the sample during the test, from which the general yield load, maximum and fracture loads, and time to fracture can be determined. load time

14 Fracture energy Fracture energy can be calculated from the load-time curve assuming the pendulum velocity is constant. Correction for changing velocity. Separation of initiation and propagation fracture energy. E1 = V0 Pdt E t = E1(1 α) α = E1 4E 0 E = E + t I E P

15 Summary Charpy impact test Ductile-brittle transition Thickness and crack length effects Instrumented Charpy test