approaches to failure analysis of polymeric materials jan

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

VeMet

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 .

VeMet

Polymerisation of ethylene C2H4 PESpoormaker Consultancy in Reliability & Liability of Plastic Products

y y 2 4

VeMet

Polyethelyne C2H4


y y 2 4

length

unbranched

length

unbranched

chin

gbr

an

VeMet

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)

VeMet

Amorphous and semi-crystalline Spoormaker Consultancy in Reliability & Liability of Plastic Products

high binding energylow binding energy tie-moleculehigh binding energylow binding energy

VeMet


Bending strength and impact strength as a function of Mn

VeMet

Viscosity as a function of molecular massSpoormaker Consultancy in Reliability & Liability of Plastic Products

y

VeMet

Injection moulding of thermoplasticsSpoormaker Consultancy in Reliability & Liability of Plastic Products

j g p

All you need is speed !!!All you need is speed !!!

VeMet

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

VeMet

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


high stiffness of mating parts• grade and polymer selection

• dynamic fatigue• wear


VeMet

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.

VeMet

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

VeMet


Crack extension crack initiation

Stable crack extension

unstablecrack extension

Threshhold

of initiationof initiation

VeMet

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

VeMet

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







VeMet

PCPC--spring in service positionspring in service positionSpoormaker Consultancy in Reliability & Liability of Plastic Products

PCPC spring in service positionspring in service position

VeMet

Photo of PC spring in service positionSpoormaker Consultancy in Reliability & Liability of Plastic Products

Photo of PC spring in service position

VeMet

Spring in unlocked positionSpring in unlocked positionSpoormaker Consultancy in Reliability & Liability of Plastic Products

Spring in unlocked positionSpring in unlocked position

VeMet

Photo of Spring in unlocked positionSpring in unlocked positionSpoormaker Consultancy in Reliability & Liability of Plastic Products

p g pp g p

VeMet

Crack extension


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.

VeMet


Impact & notch sensitivity

ECEFA -IV

Failed springsSpoormaker Consultancy in Reliability & Liability of Plastic Products

p g

VeMet

SEM of fracture surface (static fatigue)Spoormaker Consultancy in Reliability & Liability of Plastic Products

( g )

VeMet

SEM of fracture surface (dynamic fatigue)Spoormaker Consultancy in Reliability & Liability of Plastic Products

( y g )

VeMet

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


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


Relation MFR-Impact strength

VeMet

Kid-SitSpoormaker Consultancy in reliability and liability of plastic products

d S tFailure Mechanisms

• UV degradation

Failure Causes







VeMet

Kid-Sit Spoormaker Consultancy in Reliability & Liability of Plastic Products.

d S t

VeMet

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)

VeMet

Kid-Sit - cracking Spoormaker Consultancy in Reliability & Liability of Plastic Products.

g

cracks

VeMet

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)

VeMet

S-N curve of PA66-30GF (flow direction) Spoormaker Consultancy in Reliability & Liability of Plastic Products

( )

VeMet

Kid-Sit

Spoormaker Consultancy in Reliability & Liability of Plastic Products.

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

VeMet

pg

Impact factor Spoormaker Consultancy in Reliability & Liability of Plastic Products

p21 1impact

tot

F h kF totstaticF mg

ECEFA -IV

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

VeMet

Kid-Sit FEM calculation ISpoormaker Consultancy in Reliability & Liability of Plastic Products.

Very high secondary stresses in the

VeMet

y g yregion near the stiffness difference

FEM calculation (filliting)

Spoormaker Consultancy in Reliability & Liability of Plastic Products.

ca cu at o ( t g)

Still very high secondary stresses in the region near

ECEFA -IV

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







VeMet

WeatheringSpoormaker Consultancy in Reliability & Liability of Plastic Products

Weathering

ECEFA -IV


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


Weathering


ECEFA -IV

Solar radiation spectrum Spoormaker Consultancy in Reliability & Liability of Plastic Products

p

ECEFA -IV


Weathering


ECEFA -IV

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

ECEFA -IV

Hydrogen peroxide & hydroperoxideSpoormaker Consultancy in Reliability & Liability of Plastic Products

yd oge pe o de & yd ope o de

ECEFA -IV

Degradation reactionsSpoormaker Consultancy in Reliability & Liability of Plastic Products

g

ECEFA -IV


Weathering


ECEFA -IV

Attack on bondsSpoormaker Consultancy in Reliability & Liability of Plastic Products

ECEFA -IV

PESpoormaker Consultancy in Reliability & Liability of Plastic Products

Chain scissionChain scission

ECEFA -IV

branched/side group

Pattack on bondsSpoormaker Consultancy in Reliability & Liability of Plastic Products

ECEFA -IV

Transferring of energySpoormaker Consultancy in Reliability & Liability of Plastic Products

g gy

ECEFA -IV


Weathering


ECEFA -IV

Scission and cross-linkingSpoormaker Consultancy in Reliability & Liability of Plastic Products

Scission and cross linking

ECEFA -IV

Chain scission and cross linkingSpoormaker Consultancy in Reliability & Liability of Plastic Products

g

.

ECEFA -IV

UV - embrittlement


60

Polypropene (PP)Spoormaker Consultancy in Reliability & Liability of Plastic Products.

Polypropene (PP)

degraded specimens

notched specimens

ECEFA -IV

Ultra violet stabilisation (Tinuvin)


62

ECEFA -IV

KeysaverSpoormaker Consultancy in reliability and liability of plastic products

eysa eFailure Mechanisms

• UV degradation

Failure Causes






• stress concentrations• improper mould design

VeMet


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


Lay out of KEYSAVER

Critical dimensions


Critical dimensions

Dd – diameter of disk

Db – diameter of boss circle

Force to unclick is depends on:

Dd - Db

Macro photo of boss


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





• dynamic fatigue (temperature) • stress concentrations• dynamic fatigue (temperature)• wear (heat, hardness)

• stress concentrations• improper mould design

VeMet

Adhesive wearSpoormaker Consultancy in Reliability & Liability of Plastic Products

ECEFA -IV

Abrasive wearSpoormaker Consultancy in Reliability & Liability of Plastic Products

ECEFA -IV

Pitting as a result of fatigueSpoormaker Consultancy in Reliability & Liability of Plastic Products

g g

Fatigue cracks Pitting

ECEFA -IV

Wear Spoormaker Consultancy in Reliability & Liability of Plastic Products.

ECEFA -IV

Wear (NX-UHMWPE) Spoormaker Consultancy in Reliability & Liability of Plastic Products.

( )

VeMet

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

VeMet

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).

approaches to failure analysis of polymeric materials jan

Documents