Transcript CHE 333 Class 20 - Chemical Engineering

CHE 333 Class 20
Fracture continued
Dynamic Fatigue
Fatigue is failure due to a stress being repeatedly applied
to a metal. After a number of cycles the component fails.
Examples include aircraft, axles, and shafts.
Stresses
1.
Cyclic – tension compression fully reversed
tension – zero – tension
tension – non zero – tension
sine wave, saw tooth,
trapezoidal with delay.
2.
Stress Rato
R= smax/smin
Stress V Log Nf Curves
Ferrous, that is BCC iron based have a “knee” in the stress, number of cycles to failure
data, while non-ferrous does not. Safety ranges can be determined for ferrous, while the
Stress range for 107 cycles is used for non-ferrous metals.
Micro Process
Stage I – Slip lines intersect with surface of material at 45o to stress axis. Single slip system operating.
Form “Persistent Slip Band” – can’t be removed by annealing processes.
Crack initiates in slip band and grows at 45o to stress – single slip system. 25 -50 % Nf
Stage II – At crack tip, as tensile stress increases, multiples slip systems, crack grows along them both
producing “ Striations”, identifying fatigue., 74 – 49% Nf
Stage III – Crack length such that it is approached K1c value, so overload failure as stress increases.
Fatigue Fracture Surface
.
Fracture
Surface
Shaft broken by fatigue. Initiation site is
at the top. Note the change in appearance
when going from stage II to stage III.
Exposed Surface
Tube fractured by fatigue. Note river lines
running from initiation sites on surface as well as
multiple stage I initiation sites on surface. River
lines connect cracks.
Stage II Crack Growth Striations
Coarse striations on surface
Fine striations on surface. River line indicates
F
crack
growth direction while striations indicate
i
crack
front.
n
Mixed Mode Crack Growth
Transition where some striations and some overload failure. Anisotropic effect?
Striations indicate fatigue process lead to failure
Fatigue Crack Growth Testing
Cyclic stress on a pre cracked sample and measure crack length “a” as a function of
number of cycles to produce a da/dN curve. Know “a” so can calculate DK, also
from da/dN curve, for given value of N slope of curve is da/dN, growth rate per cycle.
DK= YDs (pa)0.5
Crack Growth Rate Data
with corrosion
log (da/dn)
Stage 11 C rack
Propag ation Reg ion
DKth
log DK
Typical crack growth curve shown with three regions. The first is a threshold region
which indicates a safe crack size below which no crack growth occurs. Then a steady
state region of crack growth, and the third stage of rapid failure when plane strain
fracture toughness is met. Lifing of components can be done using stage II. If the crack
length and service stress is known, DK can be calculated and the crack growth rate
obtained from graphs. The number of cycles before the crack length becomes critical
can be determined and so the remaining life of the cracked component can be estimated
Factors Affecting Fatigue.
Microstructure – plate structure of Ti-6Al-4V better fatigue life than particle structure.
Compressive stresses in the surface stop cracks opening – shot peen
Surface roughness – rougher surface initiates cracks easier than smooth surface.
Corrosion decreases fatigue life by initiating cracks easily so modifying stage I.
Cyclic frequency – increase frequency decreases fatigue life.
Cyclic waveform – trapezoidal detrimental for some titanium alloys
Examination – ultrasonic inspection, penetration X ray, die penetrant,
Creep Mechanisms
Creep in metals is extension to failure under combined stress and temperature. The temperature is
usually above 0.4 the melting point in oKelvin. The process occurring during creep involves grain
boundaries at 45o to the stress axis sliding relative to each other. Diffusion of atoms is involved.
s
Grain Boundary
slides due to atoms
moving. Voids open
up and failure occurs
grain boundary controlled
Creep
Creep can be split into three sections – primary, decreasing creep rate, secondary with
a steady state creep rate and tertiary creep with an increasing creep rate.
Factors affecting creep are grain size, temperature,
Small grain size detrimental for creep – single crystal turbine blades used these days.
Don’t want blades elongating as they would rub against containment. Other area
include nuclear reactors.
Creep Polymers
Creep in polymers is measured by applying a load and measuring the extension after a set
period of time. The creep modulus is then the stress divided by the strain. The higher the
creep modulus the better. Note the low temperatures compared to metals and reinforcement effect.