Spoormaker Consultancy in Reliability & Liability of Plastic Products
Approaches to Failure analysis of polymeric materials
Jan Spoormaker Spoormaker Consultancy / Delft University of TechnologySpoo a e Co su a cy / e U e s y o ec o ogy
If you don't know the fords, don't step in the water!p
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Contents Spoormaker Consultancy in reliability and liability of plastic products
Co te ts• Polymers
• Typical failure mechanisms of plastics: failure might occur.Typical failure mechanisms of plastics: failure might occur.
• Failure causes of plastic products: failure has occurred .
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Polymerisation of ethylene C2H4 PESpoormaker Consultancy in Reliability & Liability of Plastic Products
y y 2 4
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Polyethelyne C2H4
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y y 2 4
length
unbranched
length
unbranched
chin
gbr
an
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branched
Verdeling MolecuulmassaSpoormaker Consultancy in Reliability & Liability of Plastic Products
Verdeling Molecuulmassa
Molecular mass
IO-2040-20091130Chain length
Molecular mass
Chain lengte of PE (CnH2n+2)Spoormaker Consultancy in Reliability & Liability of Plastic Products
g ( n 2n+2)
paraffin (500 g/mol ) glue container(105 g/mol) UHMW-PE (5.106 g/mol)
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Amorphous and semi-crystalline Spoormaker Consultancy in Reliability & Liability of Plastic Products
high binding energylow binding energy tie-moleculehigh binding energylow binding energy
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Bending strength and impact strength as a function of Mn
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Viscosity as a function of molecular massSpoormaker Consultancy in Reliability & Liability of Plastic Products
y
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Injection moulding of thermoplasticsSpoormaker Consultancy in Reliability & Liability of Plastic Products
j g p
All you need is speed !!!All you need is speed !!!
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Failure mechanisms and causesSpoormaker Consultancy in reliability and liability of plastic products
a u e ec a s s a d causesFailure Mechanisms
• UV degradation
Failure Causes
• faulty ribbing• UV-degradation• creep & stress relaxation • environmental stress cracking
• faulty ribbing • difference in stiffnesses • high stiffness of mating parts
• dimensional stability• static fatigue
• grade and polymer selection
• dynamic fatigue • stress concentrations• dynamic fatigue• wear
• stress concentrations
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Fatigue Spoormaker Consultancy in reliability and liability of plastic products
at gueFailure Mechanisms Failure Causes
• UV-degradation• creep & stress relaxation • environmental stress cracking
• faulty ribbing • difference in stiffnesses • high stiffness of mating partsenvironmental stress cracking
• dimensional stability• static fatigue
high stiffness of mating parts• grade and polymer selection
• dynamic fatigue• wear
• stress concentrations
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FatigueSpoormaker Consultancy in Reliability & Liability of Plastic Products
FatigueDisentanglement in polymers can occurpolymers can occur, resulting in stable crack extension.extension. This is also referred to as static fatigue. gDynamic fatigue is a similar mechanism.
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Dynamic crack growth Paris lawSpoormaker Consultancy in Reliability & Liability of Plastic Products
y g
Crack growth per cycleCrack growth per cycle
Constant for dynamic loads
St i t it diffStress intensity difference
exponent
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Crack extension crack initiation
Stable crack extension
unstablecrack extension
Threshhold
of initiationof initiation
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Static fatigue Spoormaker Consultancy in Reliability & Liability of Plastic Products
g
Crack growth rate
Crack growth rateC ac g o t ate
Constant
Stress intensityStress intensity
exponent
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Static fatigue crack growthSpoormaker Consultancy in Reliability & Liability of Plastic Products
g g
unstablecrack extension
Stable crackwrh
rate
Stable crack extension
rack
gro
ww
crack initiation
Cr
Polycarbonate spring Spoormaker Consultancy in reliability and liability of plastic products
o yca bo ate sp gFailure Mechanisms
• UV degradation
Failure Causes
• faulty ribbing• UV-degradation• creep & stress relaxation • environmental stress cracking
• faulty ribbing • difference in stiffnesses • high stiffness of mating parts
• dimensional stability• static fatigue
• grade and polymer selection
• dynamic fatigue • stress concentrations• dynamic fatigue• wear
• stress concentrations
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PCPC--spring in service positionspring in service positionSpoormaker Consultancy in Reliability & Liability of Plastic Products
PCPC spring in service positionspring in service position
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Photo of PC spring in service positionSpoormaker Consultancy in Reliability & Liability of Plastic Products
Photo of PC spring in service position
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Spring in unlocked positionSpring in unlocked positionSpoormaker Consultancy in Reliability & Liability of Plastic Products
Spring in unlocked positionSpring in unlocked position
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Photo of Spring in unlocked positionSpring in unlocked positionSpoormaker Consultancy in Reliability & Liability of Plastic Products
p g pp g p
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Crack extension
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Polycarbonate (PC) has relatively smooth ymolecular chains and has a low resistance to stable crack extension. It has, however, a l i t tlarge resistance to crack initiation.
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Impact & notch sensitivity
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Failed springsSpoormaker Consultancy in Reliability & Liability of Plastic Products
p g
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SEM of fracture surface (static fatigue)Spoormaker Consultancy in Reliability & Liability of Plastic Products
( g )
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SEM of fracture surface (dynamic fatigue)Spoormaker Consultancy in Reliability & Liability of Plastic Products
( y g )
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Melt Flow Rate (MFR)Spoormaker Consultancy in Reliability & Liability of Plastic Products
Melt Flow Rate (MFR)
MFR - the mass of polymer, in grams, flowing in 10 minutes through a capillary of a specific diameter and length by a pressure applied via prescribed weights for prescribed temperatures.
ECEFA -IV
Molecular mass (Mn) -MFR
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Relationship between the Molecular Weight and theRelationship between the Molecular Weight and the MFR for PC (Calibre 300 of DOW Plastics)VeMet
R l ti MFR I t t th
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Relation MFR-Impact strength
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Kid-SitSpoormaker Consultancy in reliability and liability of plastic products
d S tFailure Mechanisms
• UV degradation
Failure Causes
• faulty ribbing• UV-degradation• creep & stress relaxation • environmental stress cracking
• faulty ribbing • difference in stiffnesses • high stiffness of mating parts
• dimensional stability• static fatigue
• grade and polymer selection
• dynamic fatigue • stress concentrations• dynamic fatigue• wear
• stress concentrations
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Kid-Sit Spoormaker Consultancy in Reliability & Liability of Plastic Products.
d S t
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Kid-Sit stiffness differences Spoormaker Consultancy in Reliability & Liability of Plastic Products
Extreme high stiffness differences between swivel plate (steel) and the board adjacent to the swivel plate (polymer)
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Kid-Sit - cracking Spoormaker Consultancy in Reliability & Liability of Plastic Products.
g
cracks
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Fatigue Spoormaker Consultancy in Reliability & Liability of Plastic Products
g
Semi-crystalline (final material selection PA6 with 30 % glass fibres
Amorphous p(initial material selection)
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S-N curve of PA66-30GF (flow direction) Spoormaker Consultancy in Reliability & Liability of Plastic Products
( )
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Kid-Sit
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Kid Sit
Extreme high stiffness gdifferences between swivel wheel plate (steel) and the board adjacent to the swivelboard adjacent to the swivel plate (polymer)
282 Nmsw s naD E IBending stiffnesss sw s na
swp22.9 Nmb p bD E IBending stiffness b
Dsw \Db = 31
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pg
Impact factor Spoormaker Consultancy in Reliability & Liability of Plastic Products
p21 1impact
tot
F h kF totstaticF mg
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Impact factor Spoormaker Consultancy in Reliability & Liability of Plastic Products
p21 1impact
tot
F h kF totstaticF mg
h 0 1h 0.1 mm 18 kgg 9.8 m/s2
k = 22000 N/m2
2 0.11 22000 1 6.118 9.8
ktot = 22000 N/m2
6
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Kid-Sit FEM calculation ISpoormaker Consultancy in Reliability & Liability of Plastic Products.
Very high secondary stresses in the
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y g yregion near the stiffness difference
FEM calculation (filliting)
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ca cu at o ( t g)
Still very high secondary stresses in the region near
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Still very high secondary stresses in the region near the stiffness difference. Filleting does not help
UV-degradationSpoormaker Consultancy in reliability and liability of plastic products
gFailure Mechanisms
• UV degradation
Failure Causes
• faulty ribbing• UV-degradation• creep & stress relaxation • environmental stress cracking
• faulty ribbing • difference in stiffnesses • high stiffness of mating parts
• dimensional stability• static fatigue
• grade and polymer selection
• dynamic fatigue • stress concentrations• dynamic fatigue• wear
• stress concentrations
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WeatheringSpoormaker Consultancy in Reliability & Liability of Plastic Products
Weathering
ECEFA -IV
WeatheringSpoormaker Consultancy in Reliability & Liability of Plastic Products
Weathering
1.Definition1.Definition2.Sunlight and spectral sensitivity3.Effect of UV on polymers4.Attack of bonds5.Change in mechanical properties
ECEFA -IV
Definition of weatheringSpoormaker Consultancy in Reliability & Liability of Plastic Products
g
Weathering is the adverse response of a material or d t t li t ft i t d dproduct to climate, often causing unwanted and
premature product failures.
Primary Factors of Weathering:
• solar radiation (light energy)• temperature• water (moisture/dew)
ECEFA -IV
WeatheringSpoormaker Consultancy in Reliability & Liability of Plastic Products
Weathering
1.Definition1.Definition2.Sunlight and spectral sensitivity3.Effect of UV on polymers4.Attack of bonds5.Change in mechanical properties
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Solar radiation spectrum Spoormaker Consultancy in Reliability & Liability of Plastic Products
p
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WeatheringSpoormaker Consultancy in Reliability & Liability of Plastic Products
Weathering
1.Definition1.Definition2.Sunlight and spectral sensitivity3.Effect of UV on polymers4.Attack of bonds5.Change in mechanical properties
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UV- induced reactionsSpoormaker Consultancy in Reliability & Liability of Plastic Products
UV induced reactions
Loss of H-atomLoss of H atom
Reaction with O2
Reaction with a molecule
Reaction with O2
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Hydrogen peroxide & hydroperoxideSpoormaker Consultancy in Reliability & Liability of Plastic Products
yd oge pe o de & yd ope o de
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Degradation reactionsSpoormaker Consultancy in Reliability & Liability of Plastic Products
g
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WeatheringSpoormaker Consultancy in Reliability & Liability of Plastic Products
Weathering
1.Definition1.Definition2.Sunlight and spectral sensitivity3.Effect of UV on polymers4.Attack of bonds5.Change in mechanical properties
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Attack on bondsSpoormaker Consultancy in Reliability & Liability of Plastic Products
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PESpoormaker Consultancy in Reliability & Liability of Plastic Products
Chain scissionChain scission
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branched/side group
Pattack on bondsSpoormaker Consultancy in Reliability & Liability of Plastic Products
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Transferring of energySpoormaker Consultancy in Reliability & Liability of Plastic Products
g gy
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WeatheringSpoormaker Consultancy in Reliability & Liability of Plastic Products
Weathering
1.Definition1.Definition2.Sunlight and spectral sensitivity3.Effect of UV on polymers4.Attack of bonds5.Change in mechanical properties
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Scission and cross-linkingSpoormaker Consultancy in Reliability & Liability of Plastic Products
Scission and cross linking
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Chain scission and cross linkingSpoormaker Consultancy in Reliability & Liability of Plastic Products
g
.
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UV - embrittlement
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60
Polypropene (PP)Spoormaker Consultancy in Reliability & Liability of Plastic Products.
Polypropene (PP)
degraded specimens
notched specimens
ECEFA -IV
Ultra violet stabilisation (Tinuvin)
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62
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KeysaverSpoormaker Consultancy in reliability and liability of plastic products
eysa eFailure Mechanisms
• UV degradation
Failure Causes
• faulty ribbing• UV-degradation• creep & stress relaxation • environmental stress cracking
• faulty ribbing • difference in stiffnesses • high stiffness of mating parts
• dimensional stability• static fatigue
• grade and polymer selection
• dynamic fatigue • stress concentrations• dynamic fatigue• wear
• stress concentrations• improper mould design
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Construction of KEYSAVERConstruction of KEYSAVER
bobber with spool
Detail of click boss Keysaver in
t t ABSbobber with spool transparant ABS
bossClick disk Parts of Keysaver with
keys to be saved
sponge
y
ROSMOULD 2006
Floating deviceSpoormaker Consultancy in Reliability & Liability of Plastic Products
Floating device
ANTEC2008
Functioning and non functioning KEYSAVERSSpoormaker Consultancy in Reliability & Liability of Plastic Products
Functioning and non functioning KEYSAVERS
Critical pointsSpoormaker Consultancy in Reliability & Liability of Plastic Products
p
Dimensions
• Diameter of snap-fit disk
• Height of bosses
Material selection
• Snap-fit disk (PA6)
• Inner housing ABS
Processing and gatingProcessing and gating
Lay-out of KEYSAVER
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Lay out of KEYSAVER
Critical dimensions
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Critical dimensions
Dd – diameter of disk
Db – diameter of boss circle
Force to unclick is depends on:
Dd - Db
Macro photo of boss
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Macro photo of boss
Equilibrium of water absorption in airSpoormaker Consultancy in Reliability & Liability of Plastic Products
q p
3 % water uptake!!
Deformation of inner house (ABS) – (bw)Spoormaker Consultancy in Reliability & Liability of Plastic Products
( ) ( )
Deformation of ABS test barsSpoormaker Consultancy in Reliability & Liability of Plastic Products
Critical dimensions Spoormaker Consultancy in Reliability & Liability of Plastic Products
Dd – diameter of disk
Db – diameter of boss circle
Force to unclick is depends on:
Dd - Db
StiffnessSpoormaker Consultancy in Reliability & Liability of Plastic Products
Effect of stiffness on scatter
15
10
Forc
e (N
)
5
0.0 0.5 1.0 1.5 2.00
displacement (mm)
Failure mechanisms and causesSpoormaker Consultancy in reliability and liability of plastic products
a u e ec a s s a d causesFailure Mechanisms
• UV degradation
Failure Causes
• faulty ribbing• UV-degradation• creep & stress relaxation • environmental stress cracking
• faulty ribbing • difference in stiffnesses • high stiffness of mating parts
• dimensional stability• static fatigue
• grade and polymer selection
• dynamic fatigue (temperature) • stress concentrations• dynamic fatigue (temperature)• wear (heat, hardness)
• stress concentrations• improper mould design
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Adhesive wearSpoormaker Consultancy in Reliability & Liability of Plastic Products
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Abrasive wearSpoormaker Consultancy in Reliability & Liability of Plastic Products
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Pitting as a result of fatigueSpoormaker Consultancy in Reliability & Liability of Plastic Products
g g
Fatigue cracks Pitting
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Wear Spoormaker Consultancy in Reliability & Liability of Plastic Products.
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Wear (NX-UHMWPE) Spoormaker Consultancy in Reliability & Liability of Plastic Products.
( )
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Conclusions Spoormaker Consultancy in Reliability & Liability of Plastic Products.
1. Basic understanding of structure related properties. 2. Typical failure mechanisms and causes.2. Typical failure mechanisms and causes. 3. Material and grade selection.4. Do not rely on injection moulders 5 More education in designing in plastics is necessary5. More education in designing in plastics is necessary
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Literature list Polymers
• McCrum, N.G. et al., Principles of Polymer Engineering, Oxford University Press. • Oberbach, K, Kunststoff Kernwerte für Konstrukteure, Carl Hanser Verlag, München
1980. • Spoormaker, J.L., Ten Essential Pictures for Understanding the Mechanical Behavior
of Plastics, Proceedings of ANTEC 2002. • Peter C. Powell & A.J. Ingen Housz Engineering with Polymers
Failure of Plastics
• M. Ezrin, Plastics Failure Guide – Cause and Prevention, Hanser, Cincinnati, OH (1996).
• M. Ezrin et al., The Role Of Analytical and Physical Methods in Plastics Failure Analysis, SPE-ANTEC Tech. Papers, 53, 2762 (2007).
• P.R. Lewis, Reynolds K., Gagg Forensic Materials Engineering, CRC Press, London 2004.
• Bohme, E., Failure Analysis using microscopic techniques. DuPont. http://plastics.dupont.com/plastics/pdflit/europe/ design/failureanalysise.pdf
• Brostow, W. and R.D. Cornelissen, Failure of Plastics, Hanser Publishers, New York (1986).