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FatigueTesting(1)
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1
Mechanical characterization
Fatigue testing
28/11/2013 1CompositesIves De Baere and Joris Degrieck – 2013‐2014
What is fatigue?
• A periodically changing load (or deformation) on a structure is a fatigue load
D t titi th t t• Due to numerous repetitions, the structure will fail at (significantly) lower stresses or deformations than would be the case under the (quasi‐)static loading
understanding of this behaviour is important
28/11/2013 2CompositesIves De Baere and Joris Degrieck – 2013‐2014
2
What is fatigue?
Realistic loads (see example) are quite random, although in most cases (rotating structures) one or some frequencies are more important than others
Normal stress in an airplane wing during ground‐air‐ground manouvre
28/11/2013 3CompositesIves De Baere and Joris Degrieck – 2013‐2014
Fatigue tests usually consist of imposing a sinusoidal load (or deformation) on the structure and observing its behaviour
Sinusoidal loading
• Realistic loading is often not possible with a servo‐hydraulic test machine
• Realistic loading is even more expensive and time consuming g p gthan sinusoidal ones
• Often, the realistic loading conditions are not know.
28/11/2013 4CompositesIves De Baere and Joris Degrieck – 2013‐2014
3
Sinusoidal loading
Some parameters
Mean stress
Stress amplitude
Stress ratio
max min
max min
min
max
2
2
mean
amp
R
28/11/2013 5CompositesIves De Baere and Joris Degrieck – 2013‐2014
Parameters for fatigue
• Sample– Material– Lay‐upy p– Dimensions and geometry
• Loading– Type of loading (uni‐axial, bi‐axial, bending, …)– Orientation of the loading with respect to the fibre lay‐up– Frequentie of the loading– Stresses and stress ratio
28/11/2013 6CompositesIves De Baere and Joris Degrieck – 2013‐2014
• Environment– Temperature– humidity
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Sinusoidal block loading – Miner’s rule
Block loading:To simulate realistic loading, tests are conducted, consisting of different periods each with its own mean stressperiods, each with its own mean stress and stress amplitude.
To predict damage, usually Miner’s rule is applied:During a constant amplitude fatigue loading, damage D increases linearly with the lifetime, going from 0 at start to 1 at
28/11/2013 7CompositesIves De Baere and Joris Degrieck – 2013‐2014
failure:
For different loading‐blocks:
f
ND
N
1 21 2
,1 ,2 ,
... ... ii
f f f i
NN ND D D D
N N N
Sinusoidal block loading – Miner’s rule
• The rule is commonly applied, but the predicted lifetime often exceeds the actual lifetime (unsafe!!!)
• Miner assumes linear damage growth which is (quite• Miner assumes linear damage growth, which is (quite often) not the case
• Miner does not take the order of the blocks into account, but for fibre reinforced composites, the fatigue life for low‐high amplitude significantly differs from high‐low amplitude
Mi i t ti b t d
28/11/2013 8CompositesIves De Baere and Joris Degrieck – 2013‐2014
• Miner assumes no interaction between damage corresponding to different load levels. Damage growth, however, is definitely influenced by the damage, already present
5
Representation – S‐N curves
• Typical representation of fatigue results
• For a given load case (e.g. maximum stress and R‐ratio), the b f l f il i l d
28/11/2013 9CompositesIves De Baere and Joris Degrieck – 2013‐2014
number of cycles to failure is plotted.
• Due to inherent scatter (which is even larger for composites compared to metals), a lot of experiments are necessary for 1 S‐N curve (meaning 1 load case)
S‐N curves: not suited for composites
• S‐N curve only valid for one specific combination of sample, loading case, environmental conditionsenvironmental conditions.– Results from literature are often not comparable
• Damage mechanics in composites:– Each layer has its own stress state, depending on its on mechanical properties (and orientation)
28/11/2013 10CompositesIves De Baere and Joris Degrieck – 2013‐2014
– Damage lower mechanical properties stress redistribution damaged layer is subjected to a lower loading and other layers carry the load damage will first grow in other layers …
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Why composites for fatigue?
28/11/2013 11CompositesIves De Baere and Joris Degrieck – 2013‐2014
Different damage behaviour in composites
• Stress concentrations present between fibre and matrix, resulting in micro‐cracks
• Other anomalies present (voids foreign particlesOther anomalies present (voids, foreign particles, thermal cracks, …)At the beginning of the fatigue, damage is
already present• However, micro cracks will lower the local stress concentrations, so damage will grow very gradually
28/11/2013 12CompositesIves De Baere and Joris Degrieck – 2013‐2014
gradually• Metals: most of the fatigue life is spent on the initiation of cracks. Crack growth till final failure is only a small part.
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Different damage behaviour in composites
• Due to the local cracks, the mechanical properties deteriorate
• Usually, this results in a decrease in stiffness properties and an increase in permanent deformationincrease in permanent deformation
28/11/2013 13CompositesIves De Baere and Joris Degrieck – 2013‐2014
Different damage behaviour in composites
28/11/2013 14CompositesIves De Baere and Joris Degrieck – 2013‐2014
• As such, fatigue life for composites will be a function of the desired application for example:– maintaining stiffness for a composite spring– maintaining strength– Limitation in permanent deformation (geometrical stability)– Avoiding surface cracks (corrosion related)
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Influence of some important parameters‐specimen
• Material– Most fibres have a good
(glass) to very good (carbon,(glass) to very good (carbon, aramid) fatigue resistance.
– UD carbon or UD aramid have an extra increase in fatigue resistance in tension compared to glass, since they carry more load due to their higher stiffness
28/11/2013 15CompositesIves De Baere and Joris Degrieck – 2013‐2014
– The fibre volume fraction
Influence of some important parameters‐specimen
• Material– Matrices with higher failure strain
(typically thermoplastics) tend to have fewer crack initiationhave fewer crack initiation, resulting in longer fatigue life
– Crack density= number of cracks per surface area
For PEEK (thermoplastic)
For EPOXY
Cycles x 105
28/11/2013 16CompositesIves De Baere and Joris Degrieck – 2013‐2014
For EPOXY
(usually counted by hand under a microscope quite time‐consuming and inherent human error/scatter
Cycles x 104
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Influence of some important parameters‐specimen
• Geometry– With increasing dimensions, the
change of having area’s withchange of having area s with defects increases, so a lower fatigue life is usually the result
– Too small width, however, results in the fact that cracks grow to fast from one side to the other
– Dumbbell shape for CETEX: only
28/11/2013 17CompositesIves De Baere and Joris Degrieck – 2013‐2014
p ya small area (middle of the curvature) sees the highest stress, so less chances of defects, while width is still sufficient
Influence of some important parameters‐environment
Temperature
similar to the static properties, also the f i b h i ifatigue behaviour is influenced
Humidity
28/11/2013 18CompositesIves De Baere and Joris Degrieck – 2013‐2014
HumidityMoisture absorption usually results in softening and swelling of the matrix and a degradation of the fibre‐matrix interface
10
Influence of some important parameters‐loading
• Loading direction– Direction of loading
compared to the orthotropiccompared to the orthotropic directions determines whether the behaviour is fibre‐dominated or matrix‐dominated.
– Small deviations from the (intended) fibre directions result in significant reduction
28/11/2013 19CompositesIves De Baere and Joris Degrieck – 2013‐2014
of properties
– Chopped fibre composites mainly show a matrix dominated behaviour
Influence of some important parameters‐loading
• Loading type:tension versus compression
Types
min
max
R
– Types
Tension‐tension:
Tension‐compression:
Compression‐compression:
– Due to lower tensile strength of the matrix (compared to
its compressive strength), fatigue life with tension loading
max
max
max
0 and 0 1
0 and R 0
0 and 1
R
R
28/11/2013 20CompositesIves De Baere and Joris Degrieck – 2013‐2014
is mostly lower than when only compression is applied
– Due to tension, quite often ‘debris’ is formed within the matrix cracks, causing increase in permanent deformation, increase in temperature and increase in degradation
11
Influence of some important parameters‐loading
• Loading type:tension versus compression– Be careful when interpreting fatigue life: tension‐tension
fatigue may yield high fatigue life, even when the specimen is completely damaged. Residual compressive strength, however, will be negligible.
– Most real life components are loaded both in tension and compression
28/11/2013 21CompositesIves De Baere and Joris Degrieck – 2013‐2014
Influence of some important parameters‐loading
• Type of loading– Due to the orthotropic
nature, the type of loadingnature, the type of loading (uni‐axial, bending, shear, …) will have an important influence.
– E.g. a bending test will result in a combination of normal stresses and shear stresses (both inter‐ and intralaminar).
28/11/2013 22CompositesIves De Baere and Joris Degrieck – 2013‐2014
Therefore, there is no such thing as ‘bending fatigue life’
12
Influence of some important parameters‐loading
• Loading frequency– General preference: keep the loading frequency as high as possible,
to shorten the duration of the testto shorten the duration of the test.
– However, the fatigue behaviour of (especially, but not limited to) matrix‐dominated composites is highly frequency dependent, due to the visco‐elasticity of the matrix.
– Higher frequencies will result in heating of the specimen (especially with shear loads)
– Lower frequencies give existing cracks more time to grow within one cycle reducing fatigue life
28/11/2013 23CompositesIves De Baere and Joris Degrieck – 2013‐2014
cycle, reducing fatigue life
Influence of some important parameters‐loadingHeating of the composite due to shear load: three rail shear test on CETEX
T of PPSTG of PPS is 90°C!
28/11/2013 24CompositesIves De Baere and Joris Degrieck – 2013‐2014
13
Influence of some important parameters‐loading
Heating of the composite due to shear load, [±45°]4s (CETEX)
• Temperature exceeds TG allowing the fibresTG, allowing the fibres to align themselves more along the loading‐direction.
• Thus, a new loading regime with less shear is present.
• Temperature drops
28/11/2013 25CompositesIves De Baere and Joris Degrieck – 2013‐2014
Temperature drops and fatigue continues under different conditions
Ultra high cycle fatigue
• Low cycle fatigue – up to 106 cycles– 1Hz 11.6 days
• High cycle fatigue – 106 tot 108 cycles• High cycle fatigue – 10 tot 10 cycles– 1Hz 1157 days !!!
• Very (or ultra) high cycle fatigue – more than 108
cycles– Application in aircraft, automobile, railway…– Gas turbine disk : 1010 cycles
28/11/2013 26CompositesIves De Baere and Joris Degrieck – 2013‐2014
– High speed trains: 109 cycles
• No longer achievable with standard tensile machines
14
Ultra high cycle fatigue
• Usually done ultrasonic (f=20kHz) to limit test time
Number of cycles Ulstrasonic (20kHz) Conventional (100Hz)
107 9 minutes 1 day
109 14 hours 4 months
1010 6 days 3 years
28/11/2013 27CompositesIves De Baere and Joris Degrieck – 2013‐2014
Ultra high cycle fatigue
• For composites: specimen is usually larger (a number of unit cells necessary to have an accurate representation
• Usually done with a shaker, in bending
Main problems
• Highest load occur at the clamping clampingfailure due to stressconcentration
• Mechanical properties (E
28/11/2013 28CompositesIves De Baere and Joris Degrieck – 2013‐2014
• Mechanical properties (Eii, Gij, …) change significantly compared to static values
• Interpretation/modelling/prediction is difficult
15
Thermal fatigue
• Instead of applying cyclic loading (forces or deformationg), the temperature has a cyclictemperature has a cyclic evolution.
• Very time‐consuming• Mostly done for Ceramic
matrix composites and metal‐matrix composites
• Is also important for fibre reinforced
28/11/2013 29CompositesIves De Baere and Joris Degrieck – 2013‐2014
fibre reinforced polymers, due to the degradation of the polymer.
Lifetime expectancy for real structures
28/11/2013 30CompositesIves De Baere and Joris Degrieck – 2013‐2014
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Full scale testing: Windturbine blade
• Before a new design of Wind turbine blade can be sold, it needs to be certified prove that it will survive
• Data on certification tests: case dependentBesides static failure loads, a fatigue life of 108
cycles is desired (lifetime about 20 years).• To shorten test time, stresses are usually higher, but given the low test frequency, certification may take up at least 1 year
28/11/2013 31CompositesIves De Baere and Joris Degrieck – 2013‐2014
may take up at least 1 year• Two test methods available
– Using hydraulic actuators– Using the eigenfrequency of the blade
Full scale testing: Windturbine blade
Using hydraulic actuators
28/11/2013 32CompositesIves De Baere and Joris Degrieck – 2013‐2014
http://www.youtube.com/watch?v=e8ePrX1GKck
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Full scale testing: Windturbine blade
Using the eigenfrequency of the blade
28/11/2013 33CompositesIves De Baere and Joris Degrieck – 2013‐2014
http://www.youtube.com/watch?v=XdsC73Q‐E2s after 1min
Full scale fatigue test on Boeing 747
28/11/2013 34CompositesIves De Baere and Joris Degrieck – 2013‐2014
more details: http://www.youtube.com/watch?v=TH9k9fWaFrs
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Full scale testing on bicycle framesFatigue test simulating pedal forces: go or no‐go test according to the standard
28/11/2013 35CompositesIves De Baere and Joris Degrieck – 2013‐2014