CHAPTER8. Failure Analysis and Prevention

Transcription

1 CHAPTER8 Failure Analysis and Prevention The repetitive loading of engineering materials opens up additional opportunities for structural failure. Shown here is a mechanical testing machine, introduced in Chapter 6, modified to provide rapid cycling of a given level of mechanical stress. The resulting fatigue failure is a major concern for design engineers. ( Courtesy of Instron Corporation)

2 Scale Pointer Starting position Hammer End of swing Specimen h h Anvil Figure 8-1 Charpy test of impact energy. (From H. W. Hayden, W. G. Moffatt, and J. Wulff, The Structure and Properties of Materials, Vol. 3: Mechanical Behavior, John Wiley & Sons, Inc., New York, 1965.)

3 100 Testing temperature, ºF Impact energy, J Mg: AM100A Cu: Testing temperature, ºC Impact energy, ft lb Figure 8-2 Impact energy for a ductile fcc alloy (copper C , red brass ) is generally high over a wide temperature range. Conversely, the impact energy for a brittle hcp alloy (magnesium AM100A) is generally low over the same range. (From Metals Handbook, 9th Ed., Vol. 2, American Society for Metals, Metals Park, Ohio, 1979.)

4 Temperature, F % C 150 Impact energy, J % C % C 0.60% C 0.41% C 0.49% C % C % C Temperature, C (a) Impact energy, ft lb Temperature, F Impact energy, J % Mn 1% Mn % Mn 150 0% Mn Impact energy, ft lb Temperature, C (b) Figure 8-3 Variation in ductile-to-brittle transition temperature with alloy composition. (a) Charpy V-notch impact energy with temperature for plain-carbon steels with various carbon levels (in weight percent). (b) Charpy V-notch impact energy with temperature for Fe Mn 0.05C alloys with various manganese levels (in weight percent). (From Metals Handbook, 9th Ed., Vol. 1, American Society for Metals, Metals Park, Ohio, 1978.)

5 (a) Figure 8-4 (a) Typical cup and cone ductile fracture surface. Fracture originates near the center and spreads outward with a dimpled texture. Near the surface, the stress state changes from tension to shear with fracture continuing at approximately 45. (From Metals Handbook, 9th Ed., Vol. 12, ASM International, Metals Park, Ohio, 1987.) (b) Typical cleavage texture of brittle fracture surface. (From Metals Handbook, 9th Ed., Vol. 11, American Society Metals, Metals Park, Ohio, 1986.) (b)

6 a Figure 8-5 Fracture toughness test.

7 1 a (flaw-induced fracture) a critical Log flaw size (log a) Figure 8-6 A design plot of stress versus flaw size for a pressure vessel material in which general yielding occurs for flaw sizes less than a critical size, a critical, but catastrophic fast fracture occurs for flaws larger than a critical.

8 Figure 8-7 Two mechanisms for improving fracture toughness of ceramics by crack arrest. (a) Transformation toughening of partially stabilized zirconia involves the stress-induced transformation of tetragonal grains to the monoclinic structure which has a larger specific volume. The result is a local volume expansion at the crack tip, squeezing the crack shut and producing a residual compressive stress. (b) Microcracks produced during fabrication of the ceramic can blunt the advancing crack tip. Tetragonal ZrO 2 grain Monoclinic ZrO 2 grain Cubic ZrO 2 matrix (a) Microcrack (b)

9 Stress Tensile strength (T.S.) Fracture Time Figure 8-8 Fatigue corresponds to the brittle fracture of an alloy after a total of N cycles to a stress below the tensile strength.

10 Bearing housing Specimen Flexible coupling Counter Fulcrum P/2 P/2 Fulcrum Bearing housing High-speed motor Figure 8-9 Fatigue test. (From C. A. Keyser, Materials Science in Engineering, 4th Ed., Charles E. Merrill Publishing Company, Columbus, Ohio, 1986.)

11 1000 Applied stress, S (MPa) 500 T.S. Fatigue strength or endurance limit Number of cycles to failure, N Figure 8-10 Typical fatigue curve. (Note that a log scale is required for the horizontal axis.)

12 Specimen surface Localized slip (a) (b) Intrusion (c) (d) Figure 8-11 An illustration of how repeated stress applications can generate localized plastic deformation at the alloy surface leading eventually to sharp discontinuities.

13 Crack length, a a 1 da dn da dn a 0 Cycles, N Figure 8-12 Illustration of crack growth with number of stress cycles, N, at two different stress levels. Note that, at a given stress level, the crack growth rate, da/dn, increases with increasing crack length, and, for a given crack length such as a 1, the rate of crack growth is significantly increased with increasing magnitude of stress.

14 da Crack growth rate, (log scale) dn da dn = A( K) m I II III Stress intensity factor range, K (log scale) Figure 8-13 Illustration of logarithmic relationship between crack growth rate, da/dn, and the stress intensity factor range, K. Region I corresponds to nonpropagating fatigue cracks. Region II corresponds to a linear relationship between log da/dn and log K. Region III represents unstable crack growth prior to catastrophic failure.

15 (a) (b) Figure 8-14 Characteristic fatigue fracture surface. (a) Photograph of an aircraft throttle-control spring (1 1 ) that broke in fatigue after 274 h of service. The alloy is 17 7PH stainless steel. (b) Optical micrograph (10 ) of the fracture ori- 2 gin (arrow) and the adjacent smooth region containing a concentric line pattern as a record of cyclic crack growth (an extension of the surface discontinuity shown in Figure 8 11). The granular region identifies the rapid crack propagation at the time of failure. (c) Scanning electron micrograph (60 ), showing a closeup of the fracture origin (arrow) and adjacent clamshell pattern. (From Metals Handbook, 8th Ed., Vol. 9: Fractography and Atlas of Fractographs, American Society for Metals, Metals Park, Ohio, 1974.) (c)

16 Pearlitic ( as cast) unnotched Fatigue strength, MPa Fatigue strength, ksi Fatigue life, cycles (a) Fatigue strength, MPa C (70 F) 65 C (150 F) 100 C (212 F) Stress cycles Fatigue strength, ksi (b) Figure 8-15 Comparison of fatigue curves for (a) ferrous and (b) nonferrous alloys. The ferrous alloy is a ductile iron. The nonferrous alloy is C11000 copper wire. The nonferrous data do not show a distinct endurance limit, but the failure stress at N = 10 8 cycles is a comparable parameter. (After Metals Handbook, 9th Ed., Vols. 1 and 2, American Society for Metals, Metals Park, Ohio, 1978, 1979.)

17 500 1 F.S. = T.S. 2 Fatigue strength, F.S. (MPa) 1 F.S. = T.S Tensile strength, T.S. (MPa) 1000 Figure 8-16 Plot of data from Table 8.4 showing how fatigue strength is generally one-fourth to one-half of the tensile strength.

18 Increased cold work or surface smoothness S Log N Figure 8-17 Fatigue strength is increased by prior mechanical deformation or reduction of structural discontinuities.

19 25,000 Breaking stress, psi 20,000 15,000 10,000 5,000 Fused silica glass (wet) Soda-lime silica glass (wet) Stress duration, sec. Figure 8-18 The drop in strength of glasses with duration of load (and without cyclic load applications) is termed static fatigue. (From W. D. Kingery, Introduction to Ceramics, John Wiley & Sons, Inc., New York, 1960.)

20 H Si O + O H Si Si OH HO Si Figure 8-19 The role of H 2 Oin static fatigue depends on its reaction with the silicate network. One H 2 O molecule and one Si O Si segment generate two Si OH units. This is equivalent to a break in the network.

21 Crack growth by local shearing mechanism (a) H 2 O H 2 O Crack growth by chemical breaking of oxide network (b) Figure 8-20 Comparison of (a) cyclic fatigue in metals and (b) static fatigue in ceramics.

22 cycles per minute 8 Stress (MPa) C(73 F) 21 C(73 F) 66 C(150 F) 100 C(212 F) Stress (10 3 psi) Cycles to failure Tensile stress only 2. Completely reversed tensile and compressive stress 3. Completely reversed tensile and compressive stress 4. Completely reversed tensile and compressive stress Figure 8-21 Fatigue behavior for an acetal polymer at various temperatures. (From Design Handbook for Du Pont Engineering Plastics, used by permission.)

23 x-ray source Test piece (containing high-density inclusion and low-density pore) Figure 8-22 A schematic of x-radiography. Film (showing corresponding lesser and greater film darkening)

24 Initial pulse Front surface Oscilloscope display Intensity Flaw Back surface Time Ultrasonic transducer Water bath Flaw Sample Figure 8-23 A schematic of a pulse echo ultrasonic test.

25 (Courtesy of Paramount Pictures and Twentieth Century Fox)