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Dealing with Time and Temperature Dependence Design With Viscolelastic Materials

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Design with Viscoelastic Materials How are Properties Defined? Introduction to Viscoelasticity Strain Rate and Temperature Effects Simple Material Models Empirical Methods 2

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Learning Objectives Upon completion of this session, participants will be able to: 1. Describe how temperature and loading rate affect mechanical properties. 2. Define creep and stress relaxation and describe design situations for each. 3. Apply manufacturer’s data to design for applications in both short term and long term loading. 4. Relate data from creep curves and isochronous stress strain curves. 5. Apply snap fit design guidelines. 3

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Review Basic definitions: thermoplastic, thermoset, elastomer. Let’s talk about the kind of mechanical behavior seen in polymers. 1. Strength 2. Stiffness 3. Ductility Factors which can determine the strength of a polymer. 4

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Tensile Properties for Polymers Polymer Yield Strength is defined by the first peak on the stress strain diagram, not the 0.2% offset used for metals. 5

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Strength is: A measure of stress (load per unit area with units of ksi or MPa) Yield Strength (1 st peak in uniaxial tension test) Ultimate Tensile Strength (Highest stress in uniaxial tension test) 6

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Stiffness is: Young’s Modulus (Elastic Modulus), E with units of ksi or MPa The slope of the straight line part of the stress- strain curve The ratio of stress to strain (where strain is the change in length with respect to the original length, ΔL/L 0 ) E 7

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Ductility is: % Elongation (with units of in/in or mm/mm) The permanent percentage change on length after fracture (from a uniaxial tension test) ΔL/L 8

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Mechanical Properties Stress-strain behavior of polymers brittle polymer plastic elastomer FS of polymer ca. 10% that of metals Strains – deformations > 1000% possible (for metals, maximum strain ca. 10% or less) elastic modulus – less than metal Adapted from Fig. 15.1, Callister 7e. 9

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Question 10

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Introduction to Viscoelasticity 1.Mechanical properties depend on Temperature 2.Mechanical properties depend on Strain Rate 3.Creep (progressive change in strain at constant stress) 4.Stress Relaxation (progressive change in strain at constant strain) 5.Hysteresis (significant difference in load and unload stress-strain curves) 11

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Effect of Temperature on Strength “Celanese Nylon 6/6 Processing and Troubleshooting Guide” by Ticona As Temperature Increases Strength Decreases Stiffness Decreases Ductility Increases 12

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Time Temp for Delrin (Strain Rate) cs/en_US/assets/downloads/desi gn/230323c.pdf 13

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Effects of Strain Rate and Temperature stress strain Increasing temp Increasing strain rate 14

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Time Temp for Delrin (Strain Rate and Temp) cs/en_US/assets/downloads/desi gn/230323c.pdf 15

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Time Temp Dependence Plastic deformation of polymers involves chain uncoiling and chain sliding Increasing temperature increases relative space between chains and makes uncoiling easier. Slowing the strain rate means there is more time for chain reconfiguration. 16

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Questions 17

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Creep Take a tension specimen made from a polymer and apply a constant stress. We observe Creep: Progressive strain (deformation) over time at constant stress (load), usually at high temperatures 18

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Creep Test We instantly load with constant stress for a certain time, and instantly unload. Note that both linear elastic and viscous fluid behaviors are present. Note that there seems to be some residual strain at the end, i.e. the material does not completely recover. There is both elasticity and plasticity. 19

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Load-Unload Cycle in Nylon “Zytel/Minlon Design Guide” DuPont 20

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Creep of PEEK “PEEK Properties Guide” Victrex 21

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Write down two examples of parts that see constant tensile or bending load. 22

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Stress Relaxation Think of a polymer specimen loaded with a constant strain. Note that both linear elastic and viscous fluid behaviors are present. Note that there seems to be some residual stress at the end, i.e. the material does not completely recover. There is both elasticity and plasticity. Stress Relaxation: Progressive loss of stress (load) over time under constant strain (deformation), usually at high temperatures 23

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Stress Relaxation of Delrin cs/en_US/assets/downloads/desi gn/230323c.pdf 24

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Write down two examples of parts that see constant strain. 25

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Questions 26

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Time Dependent Deformation Stress relaxation test: -- strain to and hold. -- observe decrease in stress with time. Relaxation modulus: Sample T g ( C) values: PE (low density) PE (high density) PVC PS PC Selected values from Table 15.2, Callister 7e. time strain tensile test oo (t)(t) Data: Large drop in E r for T > T g. (amorphous polystyrene) Adapted from Fig. 15.7, Callister 7e. (Fig is from A.V. Tobolsky, Properties and Structures of Polymers, John Wiley and Sons, Inc., 1960.) rigid solid (small relax) transition region T(°C) TgTg E r (10s) in MPa viscous liquid (large relax) 27

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Or why does Garden Hose behave the way it does? Vinyl Garden hose can go from flexible to rigid as the seasons change. Effect of Temperature-Glass Transition 28

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Glass Transition Temperature Many amorphous materials show a change in behavior as the material changes from viscous to rigid. For polymers, the rigid behavior below Tg results from the inability of the chains to move easily (chains have insufficient free volume to coil and uncoil). 29

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Melting Temperature For polymers, T melt usually refers to the transition from semicrystalline to fully amorphous rather than a solid to liquid transformation. Thus, a melting temperature may not be reported for an amorphous polymer, and some polymers may be both liquid and crystalline. (Some companies report a crystalline temperature and a melting temperature.) 30

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Melting vs. Glass Transition Temp. What factors affect T m and T g ? Both T m and T g increase with increasing chain stiffness Chain stiffness increased by 1. Bulky sidegroups 2. Polar groups or sidegroups 3. Double bonds or aromatic chain groups Regularity – effects T m only Adapted from Fig , Callister 7e. 31

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T g and T m 32

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Questions 33

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Hysteresis Polymers often don’t load and unload on the same line on the stress-strain curve. The difference in areas under those curves represents energy loss (often to heat). This means that polymers can have inherent energy damping. This means plastic springs may not be as good an idea as plastic dampers. 34

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Sinusoidal Response Tests We have a polymer specimen experiencing a sinusoidal loading. Note that there is a phase shift, and that there is also hysteresis indicating that energy is being dissipated cyclically. This all suggests some simple material models. 35

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Hysteresis in Delrin “Delrin Design Guide” DuPont 36

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Time Dependent Response can be Modeled Maxwell Model Kelvin-Voight Model 4 Element Model 37

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Maxwell Model Here is an alternative to the simple spring model of linear elasticity. Add a damper. This gives what is called as the Maxwell model. strain time stress time Creep not too good! Stress relaxation is not bad In the limit, it’s a fluid! 38

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Kelvin-Voigt Model Try putting the spring and damper in series This gives the Kelvin-Voigt model. In the limit, it’s a solid! strainstress time Doesn’t really show stress relaxation! 39

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4 Element Model Standard Linear Solid Further improvement is possible. stress time Shows both creep and stress relaxation! strain 40

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Stress Strain Relationships We can get stress from strain history and strain form stress history through the following heriditary relationships. K is creep modulus, and F is the relaxation modulus. 41

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Examples of These Time Dependent Moduli MaterialCreep ModulusRelaxation Modulus Maxwell Kelvin Voigt Standard Linear Solid H(t) is the unit step function. (t) is the Dirac delta function 42

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More on the material models Testing needs to be done to fit the parameters of the model to the behavior of an actual material. Note the fact that the history of the material must be recorded to be able to complete the calculations. Some additional complexity. The parameters in the creep modulus and relaxation modulus are 1.Temperature Dependent 2.Strain Rate Dependent 43

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Summary Polymers exhibit: Great sensitivity to temperature. Great sensitivity to strain rate. Very complex behavior Model parameters are difficult to determine – Therefore we will use an empirical approach. 44

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Without Effective Math Models we will rely on Manufacturers Data to make Design Decisions What are the limitations of Material Data Sheets? What do the polymer companies recommend? How do companies report time and temperature dependent properties? Designing using Creep information. 45

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Empirical approach Use published information on behavior Suppliers data sheets Suppliers creep curves 46

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What can we learn from Supplier Data Sheets Polymer parts in service will generally have lower material property values Strength, Stiffness, and Impact Energy than the ones listed in the Supplier Data Sheet. 47

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What are the problems for Strength and Stiffness values? Tested at a single temperature (usually room temp) Tested at a single strain rate. Polymer flow is in the direction of loading (advantage of molecular alignment) Effects of colorants and other additives 48

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Impact data may thickness sensitive. Polycarbonate Resin – Product Brochure, Sabic 49

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Impact Data may Depend on Notch Radius Delrin Design Guide, DuPont 50

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Polymer Flow Affects Properties 51

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From Lavengood and Silver in “Interpreting supplier Data Sheets”, ASM Engineered Material Handbook, Polymers: Tensile and Flexural moduli “may be used directly in design calculations for items that do not carry sustained loads and are not exposed to elevated temperatures or adverse environmental factors.” In other words, we can use the supplier data for a clean, dry, indoor application of primarily decorative function. What does the Designer Do? 52

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Company Recommendations General Design Principles for DuPont Engineering Polymers 53

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Company Recommendations – Preliminary Design Designing with Plastics, The Fundamentals - Ticona What safety factors do these numbers represent? 54

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What do Plastics Companies Recommend? Checklists Use of Creep Curves Confirm by Testing 55

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Checklist from Bayer 56

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From the DuPont Checklist 57

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DuPont Checklist-Mechanical 58

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DuPont Checklist-Environment 59

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Other DuPont Checklists Writing Meaningful Specifications A specification is intended to satisfy functional, aesthetic and economic requirements by controlling variations in the final product. The part must meet the complete set of requirements as prescribed in the specifications. The designers’ specifications should include: Material brand name and grade, and generic name (e.g., Zytel ® 101, 66 nylon) Surface finish Parting line location desired Flash limitations Permissible gating and weld line areas (away from critical stress points) Locations where voids are intolerable Allowable warpage Tolerances Color Decorating considerations Performance considerations General Design Principles for DuPont Engineering Polymers 60

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DuPont Example General Design Principles for DuPont Engineering Polymers 61

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Long Term Properties for Example General Design Principles for DuPont Engineering Polymers 62

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General Design Principles for DuPont Engineering Polymers 63

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General Design Principles for DuPont Engineering Polymers 64

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General Design Principles for DuPont Engineering Polymers 65

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How Do Companies Report Time and Temperature Dependent Properties? Creep Curves (Strain vs. Time) Isochronous Stress Strain Curves Creep Modulus (Modulus vs. Time) Stress Relaxation Curves (Stress vs. Time) All curves must contain information on: Stress, Strain, Time, Temperature 66

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Designing with Plastics, The Fundamentals - Ticona Data is usually taken in a creep test and replotted for the other graphs. 67

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Creep Curve Creep curves show the data as it was most likely measured, as strain vs. time for constant stress. “PEEK Properties Guide” Victrex 68

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Isochronous Stress-Strain Curves Creep data is plotted as constant time (isochronous) stress vs strain curves at a given temperature 69

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Creep Modulus vs. Time Creep Stress is divided by strain at a given time to determine a “Creep” modulus. 70

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Exercise Confirm that the Creep Modulus Curve is a replotting of the Isochronous Stress-strain Curve. Use the data shown on the Isochronous curve for Apec 1745 polycarbonate to create the 15 Mpa line on the Creep Modulus plot for Apec

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Stress Relaxation Curve Stress Relaxation should be tested under constant strain, but most reported results and replotted creep curves. 72

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Designing with Celcon - Ticona Where do these ratios come from? Talk to the supplier. 73

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Designing with Celcon - Ticona 74

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A BASF Approach to Design A method described in BASF’s document, “Review of mathematical design methods for thermoplastic machine parts”, uses multiplied “efficiency factors” to account for the effects of long-term loading, temperature, or strain rate. These factors multiply together in a manner you may have seen before in fatigue design. 75

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A BASF Approach to Design The design stress is the published strength, K, divided by the safety factor and the product of all the “efficiency factors”. 76

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Suggested Safety factors 77

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Efficiency factors As can be seen below, the efficiency factors can add 50 to 300% each to the safety factor, 78

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Cumulative Effect of Factors If we start with a factor of safety of 3 on bending and buckling and have efficiency factors of 1.5 for sustained loading, 1.5 for dynamic loads, and 1.25 for temperature, we would have an effective Safety Factor of about 8. This is consistent with the preliminary design guidelines we saw earlier. 79

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Snap Fits Types of Snap Fits Snap Fit Issues Snap Fit Calculators 80

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Types of Snap fit -Annular Snap Fit 81

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Types of Snap Fit - Cantilever Snap Fit 82

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Snap Fit Issues Integral Fastener Can be designed for disassembly Snap fits represent an undercut that complicates molding Snap fits that remain under load will stress relax 83

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Eliminating Snap Fit Undercut 84

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Dealing with Snap Fit Undercut 85

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Snap Fit Calculators Manufacturers have design guidelines Type “Snap Fit Calculator” in to Google will get plenty of hits Most are a modification of cantilever beam analysis 86

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BASF Definition of Terms Snap-Fit Design Manual, BASF Plastics 88

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Classical Beam Theory Snap-Fit Design Manual, BASF Plastics 89

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“Improved” Method Snap-Fit Design Manual, BASF Plastics 90

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BASF’s “Improved Canitlever Snap-Fit Design”. The Q factor accounts for the flexibility of the part. Note that Example 1 has the least flexibility, so the Q factor is close to a value of one. From: Snap-Fit Design Manual, BASF Plastics 91

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Snap Fit - BASF Snap-Fit Design Manual, BASF Plastics 92

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Snap-Fit Design Manual, BASF Plastics 93

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snap_fit_tapered.htm Snap fit calculator from Engineer’s Edge Snap fit Design excerpt from Paul E. Tres’ book Annular Snap Fit article in Machine Design snap-fit-joints-0106 Additional Sources 94

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Press Fit Issues Initial stress could cause boss to crack Boss may have weakening weld line Continuous deflection and resultant stress could cause Cracking Stress Relaxation and reduced pull-out force 95

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Boss with weld line support General Design Principles for DuPont Engineering Polymers 96

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Loss of force in press fit When subjected to constant strain, the resulting stress in the plastics diminishes over time. Imagine a metal insert pressed into a boss. As time goes on, the plastic boss material grips the insert with less and less force. Eventually the insert may become too loose, resulting in a failed joint. 97

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Questions 98

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