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12/11/2020
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Ductile‐to‐Brittle Transitions in Plastics
Jeffrey JansenDecember 10, 2020
Ductile to Brittle Transitions
The Madison Group 608‐231‐1907
Goal
• Why do I experience sudden unexpected failure in my plastic components?
• Why do the properties of my plastic parts change?
• Why do I get brittle failures in my ductile material?
Ductile to Brittle Transitions The Madison Group 608‐231‐1907
To address questions that arise regarding plastic component failure:
Jeffrey A. [email protected]
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Agenda
• Cracking in Plastics• Viscoelasticity• Temperature• Strain Rate• Time Under Load• Chemical Contact – ESC• Dynamic Stress• Molecular Weight• Molecular Degradation• Molecular Fusion• Stress Concentration
Ductile to Brittle Transitions The Madison Group 608‐231‐1907
Jeffrey A. [email protected]
CRACKING IN PLASTICS
Ductile to Brittle Transitions The Madison Group 608‐231‐1907
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Characteristics of Plastics Cracking:
• Covalent polymer backbone bonds are not broken by mechanical forces
Ductile to Brittle Transitions The Madison Group 608‐231‐1907
Plastics Cracking
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Characteristics of Plastics Cracking:
• Disentanglement mechanism in which polymer chains slide past each other
Ductile to Brittle Transitions The Madison Group 608‐231‐1907
Plastics Cracking
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Characteristics of Plastics Cracking:
• Applied stresses – both internal and external ‐overcome inter‐molecular forces such as, Van derWaals forces, London dispersion forces, hydrogen bonding, and dipole interactions
Ductile to Brittle Transitions The Madison Group 608‐231‐1907
Plastics Cracking
Jeffrey A. [email protected]
Ductile to Brittle Transitions The Madison Group 608‐231‐1907
Characteristics of Plastics Cracking:
• Mechanism is the same for amorphous and semi‐crystalline polymers
Amorphous Semi‐crystalline
Plastics Cracking
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Cracking ‐ Overload
Ductile to Brittle Transitions
The Madison Group 608‐231‐1907
Cracking ‐ Overload
Ductile to Brittle Transitions
The Madison Group 608‐231‐1907
Linear elastic deformation
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Cracking ‐ Overload
Ductile to Brittle Transitions
The Madison Group 608‐231‐1907
Non-linearelastic
Linear elastic
Cracking ‐ Overload
Ductile to Brittle Transitions
The Madison Group 608‐231‐1907
Non-linearelastic
Yielding
Linear elastic
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Cracking ‐ Overload
Ductile to Brittle Transitions
The Madison Group 608‐231‐1907
Non-linearelastic
Yielding
Necking
Plastic deformation
Linear elastic
Cracking ‐ Overload
Ductile to Brittle Transitions
The Madison Group 608‐231‐1907
Non-linearelastic
Yielding
Necking
FailurePlastic deformation
Linear elastic
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• Cracking occurs as a stress relief
• Ductile fracture is a bulk molecular response through yielding (macro molecular rearrangement) followed by disentanglement
• Brittle fracture is a localized molecular response where disentanglement is favored over yielding
Cracking is Simply a Response to Stress
Plastics Cracking
Jeffrey A. [email protected]
Ductile to Brittle Transitions
The Madison Group 608‐231‐1907
Plastics Cracking
Ductile to Brittle Transitions
The Madison Group 608‐231‐1907
Brittle vs. Ductile Fracture• Ductile fracture ‐ extensive plastic deformation and energy absorption (“toughness”) before fracture•Brittle fracture ‐ little plastic deformation and low energy absorption before fracture
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Plastics Cracking
Ductile to Brittle Transitions The Madison Group 608‐231‐1907
Macro‐DuctilityNo Macro‐Ductility
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Plastics Cracking
Ductile to Brittle Transitions The Madison Group 608‐231‐1907
No Micro‐Ductility Micro‐Ductility
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Plastics Cracking
Ductile to Brittle Transitions The Madison Group 608‐231‐1907
Most Failures Represent Brittle Behavior of a
Normally Ductile Plastic
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Production• Low Molecular ‐Weight
Material Selection
• Poor Fusion / Molecular Entanglement
• Contamination
• Increased Filler Level
• Stress Concentration –Design or Defects
• Molecular Degradation
Service• Reduced Temperature
• Elevated Strain Rate
• Extended Time Under Load
• Dynamic Stress Loading
• Chemical Exposure
• Loss of Plasticizer
• Molecular DegradationDuctile to Brittle Transitions The Madison Group
608‐231‐1907
Plastic Ductile‐to‐Brittle Transitions
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Plastics Cracking
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VISCOELASTICITY
Ductile to Brittle Transitions The Madison Group 608‐231‐1907
Jeffrey A. [email protected]
Viscoelasticity
Because of the molecular structure of the molecules, thermoplastic materials have different properties compared to other materials, like metals.
Ductile to Brittle Transitions The Madison Group 608‐231‐1907
Jeffrey A. [email protected]
The polymer molecules consist of very long chains – high molecular weight.
The individual polymer chains are entangled in each other.
The polymer chains are mobile and can slide past each other because they do not share chemical bonds with the other chains around them.
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• Viscoelasticity is the property of materials that exhibit
both viscous and elastic characteristics when
undergoing deformation.
• Viscous materials, like honey, resist shear flow and
strain linearly with time when a stress is applied.
• Elastic materials, like a rubber band steel rod, strain
when stressed and quickly return to their original state
once the stress is removed.
• Viscoelastic materials have elements of both of these
properties and, as such, exhibit time‐dependent strain.
Ductile to Brittle Transitions The Madison Group 608‐231‐1907
Viscoelasticity
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• Viscoelastic materials have elements of both of these properties and, as such, exhibit unique behavior
Ductile to Brittle Transitions The Madison Group 608‐231‐1907
Viscoelasticity
Spring and dashpot model
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Viscoelasticity
• There are three main factors that will affect the viscoelasticity of a plastic part:
Temperature, Time, and Strain Rate
• As the temperature is increased, the polymer chains are further apart, there is more free volume and kinetic energy, and they can slide past one another and disentangle more easily.
• As the strain rate is increased, the polymer chains do not have enough time to undergo yielding and they will disentangle.
• Over time, there is mobility between the polymer chains.
Ductile to Brittle Transitions The Madison Group 608‐231‐1907
Jeffrey A. [email protected]
Viscoelasticity
Why is viscoelasticity important?• Thermoplastic materials have both long‐term and short term
properties – they flow due to the application of stress over time.
• Most mechanical testing of plastic materials is actually testing the material’s viscoelasticity – how the plastic flows when different stresses are applied.
• Plastics are time, temperature and strain rate sensitive.
• It is important to understand the viscoelastic nature of plastic materials so that that their behavior in the intended application can be understood.
Viscoelasticity is a fundamental concept of plastic behavior that needs to understand!
Ductile to Brittle Transitions The Madison Group 608‐231‐1907
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TEMPERATURE
Ductile to Brittle Transitions The Madison Group 608‐231‐1907
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Temperature
Temperature• Lower temperature → lower duc lity
• Kinetic energy
• Molecular response shifts to disentanglement/slippage away from yielding
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Temperature
Ductile to Brittle Transitions The Madison Group 608‐231‐1907
Strain
Str
ess
Soft and Tough
Hard and Tough
Hard and Strong
Hard and Brittle
ꞏ Source: “Ductile-To-Brittle Transition of Plastic Materials”, Advanced Materials and Processes, ASM International, February 2006, pg. 39-42.
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Temperature
Ductile to Brittle Transitions The Madison Group 608‐231‐1907
Noryl 731
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Temperature
Ductile to Brittle Transitions The Madison Group 608‐231‐1907
Jeffrey A. [email protected]
0
10
20
30
40
50
60
70
80
90
100
0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0
Stress (MPa)
Strain (%)
PET
23 °C55 °C
Temperature
Ductile to Brittle Transitions The Madison Group 608‐231‐1907
Temperature Polycarbonate ABS
73 °F 33 ft‐lbs/in.2 9 ft‐lbs/in.2
‐22 °F 6 ft‐lbs/in.2 3 ft‐lbs/in.2
Notched Izod Impact Strength
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Ductile to Brittle Transitions The Madison Group 608‐231‐1907
Liquid
Random movement of chains
Difficult movement of chains
Flexible
Glassy
SoftLimited local
chain movement
Tg
Tm
Temperature
Stif
fnes
sSource: “Understanding the Consequence of Ductile-to-Brittle Transitions in a Plastics Materials Failure”, Published and Presented at ANTEC, 2008
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Temperature – Semi‐crystalline
Temperature – Amorphous
Ductile to Brittle Transitions The Madison Group 608‐231‐1907
Viscous
Temperature
Source: “Understanding the Consequence of Ductile-to-Brittle Transitions in a Plastics Materials Failure”, Published and Presented at ANTEC, 2008
Limited local chain movement
Movement of chains under stress
Rubbery
Flexible
Rigid
Glassy
Tg
DBTT
15-17 ft-lbs/inch
2-3 ft-lbs/inch.
Stif
fnes
s
Easy movement of chains
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STRAIN RATE
Ductile to Brittle Transitions The Madison Group 608‐231‐1907
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Ductile to Brittle Transitions The Madison Group 608‐231‐1907
Characteristics of high strain rate cracking:• Elevated strain rate → lower duc lity
• Polymer chains slide past each other through disentanglement mechanism.
• At increasingly elevated strain rates, the polymer molecules making up the formed plastic component are precluded from having sufficient time to undergo plastic deformation and yielding.
Reduced impact properties
Reduced fracture toughness
Strain Rate
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Ductile to Brittle Transitions The Madison Group 608‐231‐1907
High Strain Rate Applications:
• Impact
• Snap‐fit Installation
• Water Hammer / Rapid Pressurization
Strain Rate
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Ductile to Brittle Transitions The Madison Group 608‐231‐1907
ꞏ Source: “Ductile-To-Brittle Transition of Plastic Materials II”, Advanced Materials and Processes, ASM International, February 2007, pg. 25-27
Strain
Str
ess
Ductile
Ductile-Brittle
Brittle-Ductile
Brittle
Strain Rate
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Strain Rate
Ductile to Brittle Transitions The Madison Group 608‐231‐1907
Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is
a trademark used herein under license.
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Strain Rate
Ductile to Brittle Transitions
The Madison Group 608‐231‐1907
Incr
easi
ng T
oug
hnes
s
Low density polyethyleneNylon – wet
PolycarbonateAcrylonitrile:butadiene:styrene resin
Polypropylene copolymers High density polyethylene
Nylon – dry Poly(vinyl chloride)
Polyacetal Poly(ethylene terephthalate)Polypropylene homopolymer
Poly(methyl methacrylate) Polystyrene
-20 0 20 40 60 Temperature °C
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Time of Displacement
Stiff material
Less stiff material
Energy required to Initiate cracking
Energy absorbed after crack commencement
First Crack
Strain Rate
Ductile to Brittle Transitions
The Madison Group 608‐231‐1907
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Ductile to Brittle Transitions The Madison Group 608‐231‐1907
Not much deflection, but almost instantaneous –Very High Strain Rate
Catch
Snap Fit Installation
Strain Rate
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TIME UNDER LOAD
Ductile to Brittle Transitions The Madison Group 608‐231‐1907
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Creep is…..
the tendency of a solid material to deform permanently under the influence of constant stress (tensile, compressive, shear, or flexural). It occurs as a function of time through extended exposure to levels of stress that are below the yield strength of the material.
Ductile to Brittle Transitions The Madison Group 608‐231‐1907
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Time Under Load
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Time Under Load
• Low to moderate forces exerted over an extended me → lower duc lity. Can result in bri le fracture
in normally ductile plastics
• Inherent viscoelastic nature of polymers leads to time dependency
• Prolonged static stresses lead to a decay in apparent modulus through localized molecular reorganization of polymer chains
• At stresses below the yield point molecular reorganization includes disentanglement as there is no opportunity for yielding
Ductile to Brittle Transitions The Madison Group 608‐231‐1907
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Ductile to Brittle Transitions
The Madison Group 608‐231‐1907
Initial Placement
Time Under Load
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Ductile to Brittle Transitions
The Madison Group 608‐231‐1907
Starting Placement
Initial Deformation
Day 1
Time Under Load
Ductile to Brittle Transitions
The Madison Group 608‐231‐1907
Starting Placement
Initial Deformation
Day 10
Time Under Load
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Ductile to Brittle Transitions
The Madison Group 608‐231‐1907
Starting Placement
Initial Deformation
Day 100
Time Under Load
0
500
1000
1500
2000
2500
0.001 0.01 0.1 1 10 100 1000 10000 100000
Apparen
t Modulus (M
Pa)
Time (hours)
Apparent Modulus v. TimePolyacetal
Creep at 25 °C
Time Under Load
Ductile to Brittle Transitions
The Madison Group 608‐231‐1907
1000 MPa
11,000 hours
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Ductile to Brittle Transitions
The Madison Group 608‐231‐1907
Starting Placement
Initial Deformation
Day 100
Time Under Load
Ductile to Brittle Transitions
The Madison Group 608‐231‐1907
Starting Placement
Initial Deformation
Day 1000?
Time Under Load
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Why the difference?
Viscoelastic nature of polymers
Ductile to Brittle Transitions The Madison Group 608‐231‐1907
Metals: Ionic Bondingvs.
Polymers: Covalent & Intermolecular Bonding
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Time Under Load
Brittle Fracture
Normalized Creep Rate
Adapted from NIST modelhttp://www.metallurgy.nist.gov/
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Time Under Load
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Ductile to Brittle Transitions The Madison Group 608‐231‐1907
Failure Through Craze Formation
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Time Under Load
Ductile to Brittle Transitions The Madison Group 608‐231‐1907
Higher Applied Stress
Lower Applied Stress
Stress Level – Ductile vs. Brittle
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Time Under Load
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Time Under Load
Ductile to Brittle Transitions The Madison Group 608‐231‐1907
Time
Stress
1,000 psi
5,000 psi
7,500 psi
2,500 psi
9,000 psi
10,000 psiDuctile Failure
Brittle Failure
Yield Point
Source: “Understanding the Consequence of Ductile-to-Brittle Transitions in a Plastics Materials Failure”, Published and Presented at ANTEC, 2008
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CHEMICAL CONTACT ENVIRONMENTAL STRESS CRACKING
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Environmental Stress Cracking is…
the premature embrittlement and subsequent cracking of a plastic due to the simultaneous and synergistic action of stress and contact with a chemical agent
Ductile to Brittle Transitions The Madison Group 608‐231‐1907
Chemical Contact ‐ ESC
Jeffrey A. [email protected]
Ductile to Brittle Transitions The Madison Group 608‐231‐1907
Chemical
Plastic
Stress
Chemical Contact ‐ ESC
Environmental Stress Cracking
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• No chemical reaction between the polymer and the chemical agent
• No molecular degradation• The plastic would undergo stress cracking in
air given sufficient time• The chemical accelerates the stress cracking
Ductile to Brittle Transitions The Madison Group 608‐231‐1907
Chemical Contact ‐ ESC
Environmental Stress Cracking
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• Chemical agent permeates into molecular structure → lower duc lity
• Mechanism includes interference with inter‐molecular forces bonding polymer chains
• Reduces the energy required for disentanglement/slippage to occur producing a shift in the preferred mechanism from yielding
Ductile to Brittle Transitions The Madison Group 608‐231‐1907
Chemical Contact ‐ ESC
Environmental Stress Cracking
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Chemical Contact ‐ ESC
The Madison Group 608‐231‐1907
0 0
Chemical Agent
Ductile to Brittle TransitionsJeffrey A. Jansen
Chemical Contact ‐ ESC
The Madison Group 608‐231‐1907
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Day 1
45
Ductile to Brittle TransitionsJeffrey A. Jansen
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Chemical Contact ‐ ESC
The Madison Group 608‐231‐1907
45
Day 10
45
Ductile to Brittle TransitionsJeffrey A. Jansen
Chemical Contact ‐ ESC
The Madison Group 608‐231‐1907
4545
Day 100Ductile to Brittle Transitions
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Diagram from Smithers RAPRAhttp://www.rapra.net
Ductile to Brittle Transitions The Madison Group 608‐231‐1907
Chemical Contact ‐ ESC
Jeffrey A. [email protected]
Chemical Contact ‐ ESC
• Bent Strip Test
• ASTM D 543 Practice B –Mechanical Stress / Reagent Exposure
• Chemical Contract While Under Stress
• Inspection for Crack Formation
The Madison Group 608‐231‐1907
Ductile to Brittle TransitionsJeffrey A. Jansen
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Ductile to Brittle Transitions
The Madison Group 608‐231‐1907
Ductile to Brittle Transitions
The Madison Group 608‐231‐1907
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Ductile to Brittle Transitions
The Madison Group 608‐231‐1907
Ductile to Brittle Transitions
The Madison Group 608‐231‐1907
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Ductile to Brittle Transitions
The Madison Group 608‐231‐1907
Ductile to Brittle Transitions The Madison Group 608‐231‐1907
Bend Fixture
Plastic Sample
Chemical Agent
Chemical Contact ‐ ESC
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Ductile to Brittle Transitions The Madison Group 608‐231‐1907
Elevated stress zone at defect, scratch, crack Typically these defects range in stress concentration factor from 1 to 50.
Chemical Contact ‐ ESC
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Ductile to Brittle Transitions The Madison Group 608‐231‐1907
Chemical Contact ‐ ESC
Chemical agent permeates into plastic surface –preferentially at elevated stress field. Localized plasticization via stress enhanced fluid absorption at stress.
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Ductile to Brittle Transitions The Madison Group 608‐231‐1907
Chemical Contact ‐ ESC
Jeffrey A. [email protected]
In response to stress and facilitated by the plasticizing effect of the chemical, the individual segments of the chains rotate, and become aligned parallel to the direction of the maximum strain. Crazes are formed as planar arrays of fine voids normal to the tensile stress. The voids are separated by ligaments of highly aligned polymer chains.
Ductile to Brittle Transitions The Madison Group 608‐231‐1907
Chemical Contact ‐ ESC
Crazes form and grow within chemically‐affected zone
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Ductile to Brittle Transitions The Madison Group 608‐231‐1907
Chemical Contact ‐ ESC
Crazes form and grow within chemically‐affected zone
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Ductile to Brittle Transitions The Madison Group 608‐231‐1907
Chemical Contact ‐ ESC
Crazes rupture to form a crack
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Ductile to Brittle Transitions The Madison Group 608‐231‐1907
Chemical Contact ‐ ESC
Crazes form and grow within chemically‐affected zone in front of crack
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Ductile to Brittle Transitions The Madison Group 608‐231‐1907
Chemical Contact ‐ ESC
Crazes rupture to extend the crack
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Ductile to Brittle Transitions The Madison Group 608‐231‐1907
Chemical Contact ‐ ESC
Crazes form and grow within chemically‐affected zone in front of crack
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Ductile to Brittle Transitions The Madison Group 608‐231‐1907
Chemical Contact ‐ ESC
Crazes rupture to extend the crack
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DYNAMIC STRESS
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Ductile to Brittle Transitions The Madison Group 608‐231‐1907
Fatigue is….
“the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. The nominal maximum stress values are less than the yield strength and tensile strength of the material”
Like creep, fatigue produces an apparent decay in strength.
Dynamic Stress
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Dynamic Stress
Ductile to Brittle Transitions
The Madison Group 608‐231‐1907
Fatigue:
The cyclic deformation of a plastic resulting from dynamic loading through stress or strain control
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Dynamic Stress
Ductile to Brittle Transitions
The Madison Group 608‐231‐1907
• Low to moderate forces exerted intermittently over an extended me → lower duc lity. Can result in brittle fracture in normally ductile plastics
• Cyclic stress application leads to a decay in apparent modulus through localized molecular reorganization of polymer chains
• At stresses below the yield point molecular reorganization includes disentanglement as there is no opportunity for yielding
Fatigue
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Dynamic Stress
Premise of fatigue is basic:
If a component is subjected to stress repeatedly, the material undergoes a decay in strength.
Ductile to Brittle Transitions The Madison Group 608‐231‐1907
Jeffrey A. [email protected]
• Lexan 101 Polycarbonate
• Tensile Strength: 69 MPa
• Fatigue Strength: 59 MPa at 70,000 cycles
45 MPa at 100,000 cycles
31 MPa at 750,000 cycles
7 MPa at 2,500,000 cycles
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Dynamic Stress
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Ductile to Brittle Transitions The Madison Group 608‐231‐1907
Under conditions of cyclic loading, the decay in strength occurs more rapidly than under conditions of static stress.
Dynamic Stress
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Ductile to Brittle Transitions The Madison Group 608‐231‐1907
Hysteresis vs. Brittle Fracture
Taken From “Fatigue of Engineering Plastics” by Hertzberg and Manson
Dynamic Stress
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REDUCED MOLECULAR WEIGHT MATERIAL SELECTION
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Molecular Weight
• Lower molecular weight → lower duc lity
• Level of entanglement is different
• Reduces the energy required for disentanglement/slippage to occur – shifts preferred mechanism from yielding
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Molecular Weight
Multiple entangled chains made up of repeating units
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Molecular Weight
16+1+1=18Sodium Chloride (Salt)
23+35=58
6.02 X1023 molecules / mole (molecular weight mass in grams)
Water
Molecular Weight: The sum of the atomic weights of the atoms in a molecule.
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Molecular Weight
Polyethylene
(12x2 + 1x4) x n
Polycarbonate
(12x16 + 1x14 + 16x3) x n
6.02 X1023 molecules / mole (molecular weight mass in grams)
Molecular Weight: The sum of the atomic weights of the atoms in a molecule.
Ductile to Brittle Transitions The Madison Group 608‐231‐1907
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Sodium Chloride
Polyethylene
Polycarbonate
1 mole = 18 grams
1 mole = 58 grams
1 mole = ~500 lbs.
1 mole = ~50 lbs.
Most commercial polymers have an average molecular weight between 10,000 and 500,000
Molecular Weight
Ductile to Brittle Transitions
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Water
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Ref: Ezrin, Plastics Failure Guide
Change of slope
around 20K
Condensation polymers: like PC & NylonPC Standard Grades @ 20K-35KPA Standard Grades @10K-40K
Addition polymers like PS and PMMA
Molecular Weight
Ductile to Brittle Transitions
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Molecular Weight
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Tensile Strength Decreases
Elongation at Break
Yield Strength
Impact Resistance
Toughness
Hardness
Abrasion Resistance
Thermomechanical Stability
Chemical Resistance
Creep and Fatigue Resistance
Molecular Weight
Selecting a Lower Molecular Weight Resin
Ductile to Brittle Transitions The Madison Group 608‐231‐1907
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Increasing MW
Molecular Weight
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Molecular Weight
MFR (g/10min) Notched Izod (ft‐lb/in) Unotched Izod (ft‐lb/in) Strain @ Break (%)
2.5 1.5 25.0 75
9.0 1.3 20.0 60
27.0 1.0 17.0 40
Effects of Molecular Weight on the Impact Properties of Acetal Copolymer
Ref: Mike Sepe
Longer molecular chains produce a higher level of entanglement
Ductile to Brittle Transitions The Madison Group 608‐231‐1907
Jeffrey A. [email protected]
Molecular Weight
MFR (g/10min) Fatigue Endurance Limit @ 107 Cycles (psi)
2.5 4000
9.0 3300
27.0 3000
Effects of Molecular Weight on the Fatigue Properties of Acetal Copolymer
Ref: Mike Sepe
Longer molecular chains produce a higher level of entanglement
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Molecular Weight
Ductile to Brittle Transitions
The Madison Group 608‐231‐1907
DBTT - Molecular Weight RealtionshipGeneral Purpose Polycarbonate
0
2
4
6
8
10
12
14
16
18
20
-40 -20 0 20 40 60 80 100 120
Temperature (deg F)
No
tch
ed
Izo
d (
ft-l
b/in
.)
4 MFR
6 MFR
10 MFR
15 MFR
22 MFR
Source: Dow Plastics
DBTT vs. Molecular Weight
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MOLECULAR DEGRADATION
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Molecular Degradation Mechanisms
Ductile to Brittle Transitions
The Madison Group 608‐231‐1907
• Oxidation
• Hydrolysis
• Chain Scission
• Side Chain Alteration
• Destructive Crosslinking
Molecular Weight Changes Permanently Through Chemical Reactions
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Molecular Degradation
• Reduc on in molecular weight → lower duc lity
• Loss of entanglement associated with shortening of polymer chains
• Reduces the energy required for disentanglement/slippage to occur and shifts the preferred mechanism from yielding
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Molecular Degradation
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OxidationThermal oxidation is the chemical reaction of a polymeric material with oxygen from an oxidizing material, including air. The rate of the degradation reaction increases with increasing temperatures.
The oxygen in the air is the reactant and ambient heat is the energy source which drives the reaction.
Ductile to Brittle Transitions The Madison Group 608‐231‐1907
Jeffrey A. [email protected]
Molecular Degradation
Oxidation• Most polymers undergo thermal oxidation.
• Oxidation takes places via free radical formation.
• Chemical reaction – incorporation of oxygen into the backbone structure, creates carbonyl structural groups.
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Molecular Degradation
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Effects of Oxidation• Loss of Molecular Weight
– Embrittlement– Loss of Mechanical Integrity– Cracking– Catastrophic Failure
• Conjugation– Discoloration
– Loss of Gloss
– Loss of Transparency
Ductile to Brittle Transitions The Madison Group 608‐231‐1907
• Evolution of Volatiles– Foul Odor Generation
• Carbonyl Formation– Loss of Dielectric Properties
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Molecular Degradation
MOLECULAR FUSION
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Molecular Fusion
• Less molecular entanglement → lower ductility
• Less intermolecular force to overcome disentanglement
• Knit lines and molecular orientation
• Molecular response shifts to disentanglement/slippage away from yielding because it requires less energy and with little entanglement opportunities for yielding are diminished
Ductile to Brittle Transitions The Madison Group 608‐231‐1907
Jeffrey A. [email protected]
Molecular Fusion
Knit Lines• Knit lines are areas in the molded part where two or more flow
fronts converge.
• This area generally has lower strength than the other areas of the part.
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Molecular Fusion
Ductile to Brittle Transitions The Madison Group 608‐231‐1907
Jeffrey A. [email protected]
Knit Lines in Nylon 6 ‐ Conditioned
Molecular Fusion
Knit Lines• Flow analysis programs can be used to anticipate knit lines, and
direct them away from anticipated high stress areas of the part.
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Molecular Fusion
Knit Lines• Knit lines generally form on the opposite side of obstacles which
are in the way of the normal flow path, such as pins that form holes in the part or bosses designed to accept inserts.
Ductile to Brittle Transitions The Madison Group 608‐231‐1907
Jeffrey A. [email protected]
Molecular Fusion
Additive Manufacturing
Ductile to Brittle Transitions The Madison Group 608‐231‐1907
Jeffrey A. [email protected]
0
10
20
30
40
50
60
70
80
90
100
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
Stress (MPa)
Strain (%)
PET
Injection Molded
Additive Manufactured
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STRESS CONCENTRATION
Ductile to Brittle Transitions The Madison Group 608‐231‐1907
Jeffrey A. [email protected]
Stress Concentration
• Higher levels of stress concentra on → lower duc lity
• Irregularities in a structure subjected to loading may produce high localized stress (stress concentration).
• These irregularities or stress risers include holes, sharp corners, notches, abrupt changes in wall thickness, or other geometric discontinuities.
• Notches or sharp corners introduce stress concentrations and can induce premature failures, especially during impact.
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Stress Concentration
• The majority of plastics are notch sensitive and the increased stress at the notch is called the “Notch Effect”.
• Sharp corners results in stress concentra on → lower ductility.
Ductile to Brittle Transitions The Madison Group 608‐231‐1907
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Poor design
Internal sharp corner
Failure
Failure Better design
R
Ductile to Brittle Transitions The Madison Group 608‐231‐1907
Stress Concentration
Notch Radii
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Stress Concentration
Ductile to Brittle Transitions The Madison Group 608‐231‐1907
Stress concentration factor
1
1.5
2
2.5
3
0 0.25 0.5 0.75 1 1.25Internal radius/part wall thickness (R/T)
R
Force
T
Internal radius stress concentration factor graph for cantilever beam
Stress Concentration
Sharp Rounded
Sharp corner
Recommended
0.125
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Molecular WeightMolecular Weight Distribution
TemperatureRate of Loading
CrystallinityFiller ContentPart Radius
Wall Thickness TransitionsInternal StressesChemical ContactPlasticizer Content
Fusion
BRITTLElower
broaderlower
higherhigherhigher
smallerabrupthigher
yes/varies lessless
DUCTILEhighernarrowerhigherslowerlowerlowerlargergraduallowerno/variesmoremore
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Ductile‐to‐ Brittle Transitions
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Questions?
Ductile to Brittle Transitions
The Madison Group 608‐231‐1907
Jeffrey A. [email protected]
Thank You
Jeffrey A. JansenThe Madison Group608‐231‐[email protected]
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