Engr 54 Failure (Fracture And Fatigue)

Front 1

fracture

Back 1

-separation of body into two or more pieces in response to applied stress
-involves two steps: crack formation and crack propagation

Front 2

ductile fracture

Back 2

-results in extensive plastic deformation in vicinity of propagating crack and cup and cone surfaces
-involves a stable crack that grows with increasing stress

Front 3

brittle fracture

Back 3

-results in very little plastic deformation around the crack and smooth fracture surfaces
-involves an unstable crack that continues to propagate more spontaneously without increased stress

Front 4

fracture strength

Back 4

-function of the forces that hold atoms together
-its observed value is much lower than its theoretical calculations because there are always cracks in material that lead to stress concentration

Front 5

stress concentration

Back 5

stress around flaw such as void or crack is amplified

Front 6

griffith theory

Back 6

-used with brittle materials
-involves crack propagation of an elliptical crack with a very sharp crack tip radius
-it is based on energy balance of the surface energy required by creation of the fracture surface and strain energy released by moving the crack

Front 7

fracture toughness k_c

Back 7

-comes from stress intensity factor k (different from k_t in that it considers geometrical factors
-is a measure of a material's ability to resist crack growth

Front 8

impact test

Back 8

-subjects a material to a high velocity impact blow
-a pendulum hammer is dropped from a fixed height and strikes the sample which rises to a slightly lower height
-qualitative test for making comparisons

Front 9

types of impact test

Back 9

charpy and izod

Front 10

yes mostly in BCC, HCP, ceramics and polymers not so much in FCC

Back 10

can ductile temperature become brittle when cooled below a certain temperature

Front 11

impact test at different temperatures

Back 11

How is ductile to brittle transition temperature calculated?

Front 12

fatigue

Back 12

-failure do to repeated stress or stain cycling
-its process involves initiation and propagation of cracks
-its the largest cause of failures in metals
-can occur unexpectedly and without warning

Front 13

reverse cycle loading

Back 13

type of fatigue cycling that uses a mean stress level around zero

Front 14

repeated stress cycle

Back 14

type of fatigue cycling that uses a non zero mean stress level.

Front 15

fatigue test

Back 15

-simulates fatigue by subjecting material to specific loading
-cycles around specified stress and counts the number of cycles to failure (repeat for different stresses)
-plotted as stress amplitude verse cycles to failure

Front 16

fatigue strength

Back 16

-stress level at which failure occurs for given number of cycles

Front 17

fatigue life

Back 17

-number of cycles to failure for given stress level

Front 18

fatigue (endurance) limit

Back 18

-this limit determines the amount of stress required for failure to be possible

Front 19

factors affecting fatigue life

Back 19

-mean stress (longest life is when mean stress is 0)
-any flaws on the surface
-design factors like geometrical discontinuities (holes, grooves, corners) increase stress concentration