114 Analysis of Plastics Solidworks Simulation

8/10/2019 114 Analysis of Plastics Solidworks Simulation

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Analyzing Plastic Parts in COSMOSWor

Unit #114

Image courtesy of National Optical AstronomyObservatory, operated by the Association of Universitiesfor Research in Astronomy, under cooperativeagreement with the National Science Foundation.

The COSMOS Companion

Analyzing Plastic Parts in

COSMOSWorks

Volume 114

Sponsored by:


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Unit #114

2 2006 SolidWorks Corp. Confidential.

What is the COSMOS Companion?

The COSMOS Companion is a series of short subjects to

help design engineers build better products with SolidWorksAnalysis

Video presentations and accompanying exercises

A tool for Continuous Learning on your schedule

Pre-recorded videos are accompanied by a more detailedwebcast with Q & A Download videos and review webcast schedule at:

http://www.cosmosm.com/pages/news/COSMOS_Companion.html

It is not an alternative to instructor-led introductory training We highly recommend you take a course with your local reseller to

build a solid knowledge base

If you are new to the COSMOS Companion, a few comments on the program are warranted. The COSMOS

Companion series was developed in response to the request from many of our users for more detailed

information on specific and/or new functionality within the COSMOS products. Additionally, many users havebeen asking for clarification of common design analysis questions to enable them to make more

representative analysis models and make better decisions with the data. Whats more, users have asked for

this material to be made available in a variety of formats so they can review it how and when they wish. To

address this, each COSMOS Companion topic has been pre-recorded and made available thru the COSMOS

Companion homepage as a downloadable or streaming video with audio, as static PDF slides for printing, or

as a live webcast enabling attendees to ask questions and engage in additional discussion. We are trying to

provide continuous learning on your schedule so you can be as effective and efficient as possible when using

COSMOS for design analysis and validation.

It is important to note that this material is not developed as an alternative to instructor led training. We still

believe that the best introduction to any of the COSMOS products is in a class led by your resellers certified

instructor. In this program, we are hoping to build on the lessons learned in your initial training. In fact, we will

make the assumption that you have basic knowledge of the interface and workflow from intro training or

equivalent experience. We will try not to repeat what was taught in those classes or can be found in the on-

line help but to augment that information.


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Topics to be Covered

Solid modeling considerations for plastic parts & assemblies

Loads, restraints, and contact

Properties of plastic materials

Other nonlinearities

Interpreting results

In this edition, well be reviewing tips and tricks for building your solid models so that the

analysis can proceed more quickly. More thoughtful CAD can really facilitate conceptualanalysis and product innovation.

Well discuss some things to keep in mind when choosing loads and restraints for plastic

components. As we discussed in the previous session on loads and restraints, this must

also include considering modeling additional components with contact conditions to get a

more natural and reasonable final response.

Well spend a good portion of time talking about how the material properties for plastics

differ from engineering metals and then discuss the various material model options in

COSMOSWorks and the Pros & Cons of each.

Next, well review other nonlinearities that must be considered when analyzing plastic parts

and assemblies. Well reference, but not repeat, information discussed in the COSMOS

Companion unit on Nonlinearities in COSMOSWorks, #110, and introduce a few other

options based on the analysis of an actual part.

Finally, well talk about the variability that is unavoidable when analyzing plastic

components and how this affects your interpretation of the results or correlation to test.


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Solid Modeling of Plastic Parts

Primary manufacturing methods for plastic parts

Machine from bar stock

Thermoforming

Roto or Blow Molding

Injection or Compression Molding

Machined parts tend to be blocky and no special solidmodeling techniques apply. Solid elements are usuallyapplicable.

Remember that when complex parts are not tooled off of thesolid model, significant variations between plan andproduct may exist

You only need to consider alternate SolidWorks modeling techniques for plastic

components when their geometries differ from your typical parts. The geometry of plasticcomponents is very dependent upon the manufacturing method used. Parts machined

from stock are typically very similar to metal parts fabricated in the same manner and lend

themselves well to traditional solid modeling and analysis techniques. Thermoformed,

Rotomolded, Blow Molded, and Injection molded parts can take on much more complex

forms and may need to be handled differently.

Keep in mind for any of these scenarios that there always exists the possibility that the

manufactured part may not match up well to the CAD model you build. When you consider

shrinkage and tooling complexities, this is even more of a concern for plastic parts. Ive

been involved in a number of analyses involving plastic parts where adjusting the CADmodel to better represent the as-manufactured, failed part, was a major part of the project.


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Unit #114


Using Outer or Offset Surfaces

Blow Molded, Rotomolded, and

Thermoformed parts involvewrapping around or otherwiseforming plastic onto a mold ordie.

These processes creategeometries that lend themselveswell to modeling with acontinuous surface

Shell elements are usuallyapplicable

Watch out for wall thicknessvariations at radii or transitions

Blow Molded, Rotational Molded, or Thermoformed parts tend to take on the geometries of

continuous surfaces, either wrapped around a form or pushed up into one. They tend tobe thin compared to their overall size although there may be features that are similar in

size to the wall thickness. These parts rarely have standoffs or ribs that form a t-like

intersection with the main surface. For these reasons, a shell mesh is often appropriate

and most efficient.

One thing to keep in mind when considering shells is that most parts manufactured in

these ways do not have a true uniform and constant wall thickness. Thickness tends to

vary at bend radii and in features where the material has been pulled to cover a larger

area. Rotationally and Blow Molded parts tend to be thicker at outside radii and thinner at

inside radii due to the fact that they are formed from the inside of the tool. The opposite is

true for thermoformed parts as they are wrapped around the outside of a pattern. If the

areas of concern dont fall directly on these radii, or if you are focusing solely on trend

data, it may suffice to assume a uniform wall or assign a different shell property to these

radii. If the response at these areas is important, you may need to consider solids.


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Injection Molded Parts

Injection molded parts can be thin, thick, large, small

They can be as simple as a flat washer and as complex asones imagination permits

General modeling guidelines:Attempt to determine if shells or solids are appropriate up front

Shells If geometry is similar to a blow molded part, consider the outer or offset

surface technique just described

Otherwise, modeling surfaces at midsurface may be best solution

Solids Consider starting with conceptual geometry

Dont automatically include small details until macro-level performance isvalidated

Add in small features and fillets in stress risers to validate performanceand optimize as reqd

Injection molded parts are the toughest to categorize because their geometry is only

limited by the imagination of the designer. Many are truly thin walled and can be capturedwell with shell elements. Many have such varying walls and feature size that solids are the

only option.

Regardless of geometry, it is in your best interests to make some determination up front as

to whether solids or shells are appropriate. Consider conceptual geometry as shells to

develop the macro level features and then a more detailed solid model for final validation

as an option. Once you know which way you are going, you can choose the best

SolidWorks methods for the project.

If you decide on shells and the part is otherwise similar to a rotomolded part, the offset or

outside surface options might work well. If there are a number of free-standing ribs or

bosses, you may want to consider building an initial surface model at the midsurface of thepart and use that to drive your shell models.

If solids are the best, or most efficient option, again consider using conceptual geometry

until youve validated the design direction. As you build complexity into the model, keep in

mind the trade off between CAD detail and mesh/solution baggage. If you dont think

details are likely to affect your results of interest, leave them out until the analysis results

tell you to put them back in. Otherwise, use the analysis data to drive your optimization.


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Loads & Restraints

Relative flexibility of most plastic materials invalidates manyrigid restraints that are valid for steel

Solution validity breaks down at large changes in stiffness

Contact behavior may not be as predictable as steel

Flexibility of mating plastic components may require more ofassy to be modeled in study

In general, L/Rs must be thought out more carefully in the

analysis of plastic components

Choosing loads and restraints for plastic parts and assemblies requires more care than

with typical engineering metals. This is not to say that these should be taken lightly withstiffer materials. However, you must keep in mind that when a finite element model

encounters gross changes in stiffness, there will be error introduced into the model. If

steel parts are restrained with a fixed restraint, the stiffness differential is not as great as

with plastic parts. Also, where plastic parts in an assembly interact with metal parts, a

stiffness difference is inevitable.

Another consideration is that the actual deflection of plastic parts under load is often hard

to predict. Choosing loads and restraints to represent interactions is your way of telling

COSMOS that you know whats going to happen at that interaction. If you cant honestly

say that, dont jump head first into a restraint.

For these reasons, it is important to consider assembly modeling with contact conditions atthe beginning of the study. This means you may need to model all or portions of the

interacting parts before starting the analysis.


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Meshing Considerations

Presence of many local stress concentrators makesconvergence critical

If you felt feature was important to include, it is important to get right

Adaptive techniques can help with this

Many plastic parts lend themselves to shell modeling buteffects at fillets drafted walls, and parting lines may be lost

Consider wall thickness vs. typical feature size

Plan for assy modeling and contact

Consider element distortion in large displacement problems

From a meshing standpoint, once the issue of shells vs. solids has been resolved, the

biggest concern is making sure that there is enough mesh and mesh control in all thefeatures you have included in the model to ensure the results on them are meaningful.

Using h or p-adaptive meshing techniques will help on problems that allow this technique.

Make sure you plan for assemblies and any contact that might occur in your meshing

decisions by placing split lines and mesh control where appropriate.

As a final thought, if large strain or excessive bending is seen or expected, solution failure

or local error might occur because good elements in the undeformed state of the model

become overly distorted as the model reaches equilibrium. A more refined mesh can

alleviate these problems as the distortion is shared by more elements.


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Unit #114


Material Properties of Plastics

The most important difference between prediction and failureanalysis of plastics vs. steel (classical engineering) is inmaterial properties

Most plastics have significant elastic and/or plastic nonlinearmaterial behavior in normal operating strains

Matl properties, both elastic/plastic & failure are verysensitive to temperature, strain rate, humidity, flow direction,skinning, regrind, fillers, notching, environment and glass

orientation & length

Test data more accurate than datasheet properties

What Test Data to use

A colleague of mine once said, The 3 most important things when analyzing plastic

components are (1) Know the properties, (2) Know the properties, and (3) Know theproperties. Not only do the properties for a given polymer vary from manufacturer to

manufacturer, they vary due changes in temperature, strain rate, orientation and a host of

other things. This applies to the input properties as well as the failure quantities.

Additionally, most plastics have a significant nonlinear elastic response before yield is

reached which may require a nonlinear analysis. Contrast this with steel which as linear as

it gets until the yield strength.

At a very minimum, you should try to get your hands on representative stress-strain curves

for the materials you design with. Even better, have a lab test material samples cut from

similar parts to the one you are designing or analyzing at the temperatures and strain ratesyou expect for your system. Your testing lab can help you determine the appropriate tests

for your application.


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Unit #114


Modulus of Elasticity

The Youngs Modulus for plastics is the slope in the first few data points

reported by a tensile test. It is a Tensile Modulus.

Flexural Modulus is often reported in datasheets for plastic materials

Flex Mod is calculated using a 3 point bending test defined in ASTM D790.

Since bending involves compression and tension, Flex Mod only equalstensile mod if the material is symmetric, or the compressive and tensilestiffnesses are the same.

Furthermore, Flex Mod is calculated using linear equations from measuredForce-Displacement data. Once displacement approaches specimenthickness, this calculation becomes unreliable due to nonlinearity in thesystem.

COSMOSWorks is looking for a tensile data for both linear and nonlinearanalyses.

One common point of confusion with designers who are trying to analyze plastic parts is

the difference between Tensile, or Youngs Modulus and Flexural Modulus. It is importantto remember that COSMOS is looking for the Youngs Modulus in the material input

tables. This is determined in an ASTM D638 tensile test. Flex Mod is calculated from the

force-displacement measurements in an ASTM D790 3 Point bending test. As you know,

when a beam is placed in bending, one side of the beam is in tension and one is in

compression. Consequently, the response measured will be impacted by both the tensile

and compressive properties of the sample, not just tensile. When the tensile and

compressive properties of a material are identical, the Flex Mod should match the Youngs

Mod for small strains. Beyond small strains, (& it is difficult to know always how small

small is,) the linear calculations used in D790 to estimate modulus fall apart as

geometric, or large displacement, nonlinearities enter into the flex response.

If you only have access to Flex Mod data, be sure you are focusing on comparative data

from one run to the next. Again, you should try to get your hands on a tensile stress-strain

curve for your material.


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Modulus of Elasticity

One other elastic modulus, sometimes referred to for plastic

materials due to the difficulty of isolating the linear portion ofthe curve, is the Tangent Modulus. This is used in aCOSMOSWorks bi-linear plasticity model.

Tangent Modulus of elasticity is the slope of the stress-strain diagramat any point.

Tangent and Youngs modulii are equal up to the Proportional Limit ofa material.

engr

engr

Tangent Modulus

Youngs Modulus

Proportional Limit

Bi-linear Material Model

The tangent modulus of a material is typically only important when estimating a bi-linear

plasticity material model. The tangent modulus is a representative stiffness in theplasticity range of the stress-strain curve. Well discuss where this is used in

COSMOSWorks later in the unit.


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Unit #114



Ductile vs. Brittle Behavior

Most plastics are ductile at room temperature They deform and yield significantly before failure

Some ductile materials never rupture in a tensile test Ductile fracture is rare in practice unless planned or part grossly under-

designed

Ductile materials generally behave similarly in tension and compression(Symmetric)

Brittle materials fail with little or no plastic deformation; Low elongation Tensile failure typically result of flaws or micro-cracks in test; in theory, cause

is at molecular level

Since compression tends to close cracks, brittle materials often show up to4x increase in compressive vs. tensile strength

Stiffness DOES NOT indicate ductility

Ductility varies with temperature, strain rate, and material composition

The difference between ductile and brittle material behavior was discussed in depth in the

COSMOS Companion units on Material Properties (#104) and Static Failure Analysis(#106). For plastic materials, it is important to also note that the ductility of a plastic is

sensitive to temperature, strain rate, and material composition as shown in the following

examples.


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Unit #114



BU50I

0

1000

2000

3000

4000

5000

6000

7000

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07

Strain

Stress

(psi)

9233G

0

5000

10000

15000

20000

25000

30000

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035

Strain

Stress

(psi)

Unfilled Nylon

Extremely Ductile

33% GF Nylon

Relatively Brittle


Unfilled Nylon is typically very ductile. It can be pulled & twisted in knots without any

fracture or rupture. The stress-strain curve for unfilled Nylon is shown on the left. The S-Scurve shows a response up to more than 6% strain although it is likely the sample could

have been pulled farther without failure. Contrast this with a 33% glass filled Nylon, shown

in the rightmost S-S curve. This material failed in a tensile test at 3% strain. However, it is

a much stronger, stiffer material so a designer has to decide between these trade offs

when choosing a material.


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Room Temperature Low Temperature


In this example, identical polyethylene (PE) samples were pulled at room temperature and

at a reduced temperature. At room temperature, the sample stretched and necked to thelimits of the tensile tester. The sample was manually cut to fit into the picture alongside an

untested dogbone. When the temperature was reduced, the sample fracture with no

discernible yielding at a very low strain level. This is indicative of brittle fracture. If the

sample was pulled quickly, or at a high strain rate, the results might have been the same.

If you have access to Silly Putty, you can reproduce this strain rate dependent ductility

experiment.


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Flow

Cross-Flow


Orientation

The material properties of a plastic part may also vary with orientation. The primary cause

of orientation in an injection molded part is flow direction and this can occur with or withoutany filler. In this PPO example from GE Plastics, you can see that the strength in the flow

direction is much greater although apparently less ductile than the properties in the cross-

flow direction indicate.

Although the few examples shown in this unit might lead you to this conclusion, dont

assume that stiffer stronger materials are less ductile by default. The two characteristics

are not necessarily related.


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Nonlinear Materials in COSMOSWorks

Stress-Strain Curve for Polyethy lene (PE)

00

27 MPa1

27 MPa0.075

26 MPa0.0625

25 MPa0.05

23 MPa0.0375

19 MPa0.025

12 MPa0.0125

StressStrain

The image above shows representative stress-strain curves for polyethylene (PE). The

different curves in the plot represent tensile tests at different strain rates. Using thebottom-most curve, generated by the slowest test, the data was reduced to a tabular

format which is how it needs to be entered in COSMOSWorks. Well use this data to

explore the options for material model definition.


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Linear Elastic

Nonlinear Elastic

Plasticity-VonMises

Viscoelastic

COSMOSWorks provides three more commonly used material models for plastic part

analysis, Linear Elastic, Nonlinear Elastic, and Von Mises Plasticity. The Viscoelasticmodel is noted as important for designers since these effects, such as creep and

relaxation, are important aspects of plastic product response. However, the material

properties to take advantage of this model in COSMOSWorks or any FEA package require

specialized testing and this is usually left to specialists. For this discussion, well focus on

the previous three.


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0,0 assumed

Final point should be at strain

far beyond expected strain

No yielding will be calculated

0,0 assumed

First point must be Yield

Strength

Only post-yield nonlinearity

allowed in S-S curve

Youngs Mod and Yield

Strength entered in table is

ignored

0,0 assumed

No yielding will be calculated

Yield Strength in table for

Factor of Safety plots and

reference only

The Linear Elastic material model should be familiar to most design engineers. It assumes that the stiffness of the

material is constant for all possible stresses and strains. No yielding will be calculated and it is as nearly straightforward

as a linear spring equation. In many cases, the stress-strain curve is linear or nearly linear for the response range ofinterest so a linear material model provides perfectly valid results. Unfortunately, you wont know if this is the case

without reviewing the stress-strain curve itself.

The Nonlinear elastic model in COSMOSWorks requires tabular stress-strain input. COSMOSWorks assumes a zero

initial stress state so the 0,0 point doesnt need to be entered. You dont need to enter a Youngs Modulus since the

material stiffness is extrapolated from the S-S curve for all strain levels. If the strain exceeds the data in the table, the

final stiffness, defined by the slope between the last 2 points it the curve, is extrapolated to infinity. Remember that no

yielding will ever be calculated by a nonlinear elastic material model.

Finally, the Von Mises Plasticity model is most appropriate when the plastic response, or the accumulation of permanent

strain, is required. Von Mises is the only plasticity model currently available for shell models. Solid models allow a

Tresca and a Drucker-Prager plasticity models. Users are encouraged to read thru the on-line help discussion of these

models to determine their applicability. Focusing on the more general Von Mises model, a zero strain state is again

assumed. You have the option of choosing a bi-linear material model by entering a Tangent Modulus into the table on

the Properties tab. If you choose this route, no stress-strain curve is reqd.If you instead choose to use a stress-strain curve, the first point in the data table must be the elastic limit of the material,

or the point where the plasticity begins to accumulate. The model assumes a linear elastic response until plasticity

occurs and then will follow the curve as input if an element goes plastic. The linear Youngs Modulus is calculated from

the first data point, the elastic limit, so any value entered in the table on the Properties tab will be ignored. In most

plastics, the S-S curve is pretty simple post-yield and since you cant account for the nonlinear elastic response that is

likely to occur, a bi-linear model with a carefully chosen Tangent Modulus may be your simplest solution.


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Unit #114



Strain

Stres

s

Yield

Strength

Linear

Actual

Nonlinear

Elastic

Plasticity

In this plot, the different material models are overlaid on each other. The red curve is the

actual stress-strain curve from the D638 tensile test. The blue curve, the linear elasticmodel, will clearly over-predict stress for moderate strain levels. The nonlinear elastic

curve clearly captures the bulk of the material response until plasticity occurs but will not

ever calculate any plasticity. Conversely, the plasticity model essentially trivializes the

elastic response and provides only valid data in the post-yield response range.

The most logical conclusion, as with many things in FEA, there are trade offs to all the

methods and you are responsible for understanding these trade-offs when choosing a

material model.


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Most plastic materials have a significant nonlinear elasticportion of their stress-strain curve

Do you need to consider plasticity?

Plasticity models are the most resource intensive

Visco-elastic effects (Creep / Relaxation) must beconsidered especially if unloading is of interest.

Difficult to develop material models

Consider Apparent Modulus to estimate creep

To sum up this portion of the discussion, most plastics do have a significant nonlinear

elastic response. In most designs, intended to actually be used, elastic behavior underoperating loads is preferred. If significant plasticity is not a valid operating condition, you

dont really need to know how plastic a problem got, just that it crossed the line. For these

cases, a linear or nonlinear elastic model is probably the best choice. When you truly need

to understand how much plastic strain has been accumulated, a plasticity model cant be

avoided but remember that these models are very resource intensive. In some cases,

youll have no option but to solve the problem using a nonlinear elastic and plastic model

to truly understand the entire response.

While it was mentioned only briefly, you do need to consider the visco-elastic effects in

your system. Creep, which time-dependent deflection under constant load, is verycommon in plastic parts with structural requirements. While analyzing this in any FEA

code is very difficult, most major resin manufacturers publish an Apparent Modulus table

based on initial stress values. By running your model with this reduced modulus, based on

the stress values calculated in an initial study, you can get a feel for how much deflection

to expect as the part or assembly creeps.


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Other Nonlinearities

COSMOSWorks supports other nonlinearities that mightcome in handy for the analysis of plastic parts

Large Displacement effects (Designer & up)

Sliding contact with friction (Designer & up)

Advanced load control (Advanced Professional)

Advanced solution control options (Advanced Professional)

Nonlinear response animations and plots (Advanced Professional)

For a more complete description ofeachrefer to COSMOS Companion

Unit #110 Introduction to NonlinearAnalys is in COSMOSWorks

There are many other nonlinearities that come into play when analyzing plastic parts in

COSMOSWorks which is why this area of study is so challenging. These were reviewedin more detail in the COSMOS Companion unit #110 Intro to Nonlinear in

COSMOSWorks so that info wont be repeated here. However, you should understand

each of these effects and their impact on your model behavior. Some of these require the

Advanced Professional package so if they are important to gaining a full picture of your

product response, you may want to talk to your reseller about upgrading.


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Other Nonlinearities

Some of the advanced nonlinear options available in Advanced Professional involve

advanced solution control. Due to the unpredictable nature of plastics and their tendencyto deform greatly, many times intentionally, the default Force Control solution algorithm

might not be sufficient. If you are running a static Large Displacement solution and it

keeps failing at about the same load factor, there might be buckling or other abrupt

stiffness change happening in the model. In these cases, switching to a Displacement or

Arc Length control might do the trick. The application of these features is discussed in

detail in the on-line help and in the COSMOSWorks Nonlinear course material.


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Unit #114


Results Interpretation

Even nonlinear test data of S-S curve is representative of

small sample and single condition Many other factors enter into plastic component failure

Residual stress from molding must be acctd for

Other effects such as weld lines, UV or chemicaldegradation, notch sensitivity, nonlinearity, and otheroperating conditions must be considered

Due to near perfectly plastic behavior of most ductile

plastics beyond yield, failure strain may be more indicative ofproblems

Long Term Effects must be considered for many parts Creep, Relaxation, Fatigue, Aging (Thermal, UV, Oxidizing,

Chemical)

A couple of thoughts on plastic part analysis results interpretation

Remember that no matter how diligent you are in researching properties or building CAD models,your analysis represents, at best, a snapshot of a single possible condition. Since material

properties, operating environments, processing parameters, and many other things will alter the

performance of a plastic part, catching every combination of these variables in COSMOSWorks is

nearly impossible. Therefore, some variation can be expected. In a recent project, a client ran

some tests on a PVC piping component and documented a brittle failure at 75% of the desired

load. It was a dramatic failure with a loud bang and parts flying into the air! When I came in to

observe the test the following week, a sample of supposedly the same parts behaved in a ductile

manner and withstood the full operating force. The engineer was sure that while the parts came

from different manufacturing dates, they were all unused and relatively new. After speaking with

people in the test lab, we ascertained two significant differences. The first batch of parts that failed

had been stored outside on a pallet and exposed to sunlight whereas the second group had never

left the building. Additionally, when the first test was completed, it was warm enough outside that

the shipping door to the lab was closed and the air conditioning had cooled the place off nicely. In

the test I observed, the door was open & the lab was warm and humid. We didnt have an

opportunity to fully sort thru this problem as we werent able to repeat the brittle failure again but it

highlighted the unpredictability of plastic components. One thing Ive learned in my years of

supporting analysis is that you cant expect analysis results to match test results any more closely

than the variability in response from test to test. Keep this in mind when reviewing your results.


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Unit #114


Why Test & FEA Dont Match

Wrong material properties entered

Wrong material model used

Plastic-specific issues not considered Strain rate, Temperature, Viscoelastic effects, Moisture effects,

Thickness effects, Orientation effects, Residual stress, Local propertyvariation, (Weld lines, skinning), Processing effects

Wrong failure quantities examined

Loads & Restraints invalid

Poorly constructed mesh

One or more nonlinearities not considered

Absolute data (vs. Trend) relied upon too heavily

In general, the biggest reason Ive seen as to why test results dont match FEA results is

improperly specified material properties. If testing at temperature, strain rate, orientation,and thickness wasnt performed, you should treat your analysis results as ball park and

focus on trend analysis. Many of the other reasons in this list were addressed previously

but the value of using trend analysis for plastic part design needs to be emphasized.

Since it is so hard to nail down exactly what conditions a plastic part will encounter in

actual use, it is safer to compare results from one iteration to another and assume that the

variability is constant, meaning that regardless of the features in your design, the issues

that are out of your control wont be affected and will remain out of your control.

Therefore, an improvement is likely to be valid even in you cant hang your hat on the

absolute value.

One common technique that I recommend is to test a part or system similar to the one you

are concerned about gradually to failure so a mean load to failure can be identified.

Analyze this part using that mean load to failure and your best estimate of properties, load

distribution & restraints, and other model parameters and call that solution your baseline.

Dont worry too much about correlating FEA stress to test stress, just note the stress

levels in the analysis. Subsequent studies at the desired operating load should focus on

getting stresses at or below the baseline. This will provide valuable insight using

COSMOSWorks without the risk associated with a single data point solution.


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Unit #114


Presentation Summary

In this COSMOS Companion unit, we reviewed:

Tips for modeling your parts in SolidWorks to improve the efficiencyand accuracy of the analysis

Guidelines for selecting loads & restraints

Guidelines for modeling assemblies involving plastic parts

Important material property concerns for plastics

Nonlinear material models in COSMOSWorks

Other nonlinearities that might impact a plastic part analysis inCOSMOSWorks

Guidelines for comparing test results to analysis results

This wraps up out discussion of plastic part analysis in COSMOSWorks. This is a deep subject and you are

encouraged to research the topics introduced here in more detail. We spent some time talking about ways to

prep your analysis by building more efficient models in SolidWorks. This is especially important if you areusing conceptual elements such as shells for your plastic parts.

We discussed the importance of well thought out loads and restraints when using plastic parts due to the

danger of creating fictitious stiffness transitions at restraints. Assembly analysis using contact is often an

important task to get a more natural, reasonable response in your plastic parts.

We discussed the material properties of plastics including the difference between tensile, or Youngs

Modulus, and Flex Modulus. Remember that COSMOSWorks is looking for tensile properties and using flex

properties could invalidate your results. We also reviewed the nonlinear material models that you might need

for plastic part analysis and compared the relative pros & cons of each. Using nonlinear material properties

in COSMOSWorks isnt the difficult part. Obtaining and understanding these properties is.

Finally, we talked about interpreting your analysis results and why youll need to keep an open mind when

comparing analysis results for plastic parts to test. If you dont get good test correlation, it is more likely thatthere was some aspect of the problem you didnt fully understand than that the analysis was wrong.

Remember COSMOSWorks only answers your questions and the answer can only be as complete as the

information provided. With plastic components, finding all the information you need to fully characterize a

response can be very difficult and thus, using analysis for trend studies vs. attempting to ascertain an exact

response is probably more effective.


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Conclusion

For more information

Contact your local reseller for more in-depth training or support on usingCOSMOSWorks to analyze plastic parts or assemblies or to discussmodeling techniques

Review the on-line help for a more detailed description of the featuresdiscussed

Attend, or better yet, present at a local COSMOS or SolidWorks usergroup.

See http://www.swugn.org/ for a user group near you

Good resources for plastic part design and analysis:

Structural Analysis of Thermoplastic Components; Trantina & Nimmer;McGraw Hill

Bayer Plastics http://plastics.bayer.com

GE Plastics http://www.geplastics.com

Id like to thank you for taking the time to join in this edition of the COSMOS Companion. I hope you will

approach analyses involving plastic parts from a different perspective. There is so much more to know

about this topic and I welcome your questions and feedback.

I encourage you to talk thru your problem, model setup and material model options with the support team at

your local reseller and take advantage of their experience in using COSMOSWorks. If you have time, you

should also read thru the on-line help topics on the nonlinear material models available to you in

COSMOSWorks Advanced Pro, even if you dont have access to these features, so you might have a better

understanding of what you can do or why your large displacement solution is having problems.

I encourage you to get involved in a local COSMOS user group. This is one of the best vehicles for sharing

and learning from the experience of others who face the same challenges as you. You can locate a local

COSMOS group on the SolidWorks User Group network website shown. If there arent any COSMOS

groups near you, get involved in your local SolidWorks groups and introduce some COSMOS related topics

to foster some discussion on design analysis and validation.

Finally, Ive listed a couple of references that have been helpful to me in my years of analyzing plasticcomponents. The inclusion of the Bayer and GE websites doesnt imply an endorsement of these suppliers

by SolidWorks or any relationship between the companies. These are just websites Ive found useful

information on, both for material properties and design tips. The book by Trantina and Nimmer, however, is

a must for any designer hoping to use FEA to better understand plastic part response. You can order this

on Amazon and Id suggest taking a few evenings and reading thru it. It is one of my most valuable

references.

With that, Id like to thank you again for your time and interest and I look forward to seeing you next time on

the COSMOS Companion.

Documents

114 Analysis of Plastics Solidworks Simulation