114 Analysis of Plastics Solidworks Simulation

Analy An alyzin zingg Plast Pl astic ic Parts Part s in i n COSMOSWorks COSMOSWor ks

The COSMOS Companion Analyzing Plastic Parts in COSMOSWorks

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Volume 114

Unit #114

Image courtesy of National Optical Astronomy Observatory, operated by the Association of Universities for Research in Astronomy, under cooperative agreement with the National Science Foundation.

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Analy An alyzin zingg Plast Pl astic ic Parts Part s in i n COSMOSWorks COSMOSWor ks

What is the COSMOS Companion? 

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The COSMOS Companion is a series of short subjects to help design engineers build better products with SolidWorks Analysis Video presentations and accompanying exercises A tool for Continuous Learning on your your schedule Pre-recorded videos are accompanied by a more detailed webcast with Q & A – Download videos and review webcast schedule at: 

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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

© 2006 SolidWorks Corp. Confidential. Confidential.

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If you are new to the COSMOS Companion, 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 have been asking for clarification of common design analysis questions to enable them to make more representative analysis models and make better better decisions with the data. What’s more, users have asked for this material to be made available in a variety of formats so they can review review it how and when they 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 webcast enabling attendees attendees to ask questions and engage engage in additional discussion. We are trying to 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 COSMOS products is in a class led by your reseller’s 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 online help but to augment that information.

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Analyzing Plastic Parts in COSMOSWorks

Topics to be Covered… 

Solid modeling considerations for plastic parts & assemblies

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Loads, restraints, and contact

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Properties of plastic materials

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Other nonlinearities

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Interpreting results

© 2006 SolidWorks Corp. Confidential.

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In this edition, we’ll be reviewing tips and tricks for building your solid models so that the analysis can proceed more quickly. More thoughtful CAD can really facilitate conceptual analysis and product innovation. We’ll 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. We’ll 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, we’ll review other nonlinearities that must be considered when analyzing plastic parts and assemblies. We’ll 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, we’ll 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 – – – –

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Machine from bar stock Thermoforming Roto or Blow Molding Injection or Compression Molding

Machined parts tend to be blocky and no special solid modeling techniques apply. Solid elements are usually applicable. Remember that when complex parts are not tooled off of the solid model, significant variations between “plan” and “product” may exist


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You only need to consider alternate SolidWorks modeling techniques for plastic components when their geometries differ from your typical parts. The geometry of plastic components 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. I’ve been involved in a number of analyses involving plastic parts where adjusting the CAD model to better represent the as-manufactured, failed part, was a major part of the project.

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Using Outer or Offset Surfaces 

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Blow Molded, Rotomolded, and Thermoformed parts involve wrapping around or otherwise forming plastic onto a mold or die. These processes create geometries that lend themselves well to modeling with a continuous surface Shell elements are usually applicable Watch out for wall thickness variations at radii or transitions


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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 to be 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 don’t 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 

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Injection molded parts can be thin, thick, large, small They can be as simple as a flat washer and as complex as one’s imagination permits General modeling guidelines: – Attempt to determine if shells or solids are appropriat e up front – Shells 

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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  

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Consider starting with conceptual geometry Don’t automatically include small details until macro-level performance is validated Add in small features and fillets in stress risers to validate performance and optimize as req’d


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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 captured well 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 the part and use that to drive your shell models. If solids are the best, or most efficient option, again consider using conceptual geometry until you’ve 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 don’t 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 many ‘rigid’ restraints that are valid for steel – Solution validity breaks down at large changes in stiffness

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Contact behavior may not be as predictable as steel Flexibility of mating plastic components may require more of assy to be modeled in study In general, L/Rs must be thought out more carefully in the analysis of plastic components


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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 with stiffer 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 what’s going to happen at that interaction. If you can’t honestly say that, don’t jump head first into a restraint. For these reasons, it is important to consider assembly modeling with contact conditions at the 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 makes convergence critical – If you felt feature was important to include, it is important to get right – Adaptive techniques can help with this

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Many plastic parts lend themselves to shell modeling but effects at fillets drafted walls, and parting lines may be lost – Consider wall thickness vs. typical feature size

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Plan for assy modeling and contact

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Consider element distortion in large displacement problems


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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 the features 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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Material Properties of Plastics… 

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The most important difference between prediction and failure analysis of plastics vs. steel (classical engineering) is in material properties Most plastics have significant elastic and/or plastic nonlinear material behavior in normal operating strains Mat’l properties, both elastic/plastic & failure are very sensitive 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…


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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 the properties. 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 rates you expect for your system. Your testing lab can help you determine the appropriate tests for your application.

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Modulus of Elasticity 

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The Young’s Modulus for plastics is the slope in the first few data points reported by a tensil e test. It is a Tensile Modulu s. 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 equals tensile mod if the material is symmetric, or the compressive and tensile stiffnesses are the same. – Furthermore, Flex Mod is calculated using linear equations from measured Force-Displacement data. Once displacement approaches specimen thickness, this calculation becomes unreliable due to nonlinearity in the system.

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COSMOSWorks is looking for a tensile data for both linear and nonlinear analyses.


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One common point of confusion with designers who are trying to analyze plastic parts is the difference between Tensile, or Young’s Modulus and Flexural Modulus. It is important to remember that COSMOS is looking for the Young’s 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 Young’s 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 of the curve, is the Tangent Modulus. This is used in a COSMOSWorks bi-linear plasticity model. – Tangent Modulus of elasticity is the slope of the stress-strain diagram at any point. – Tangent and Young’s modulii are equal up to the Proportional Limit of Young’s Modulus a material. σengr

Tangent Modulus

Bi-linear Material Model Proportional Limit


εengr

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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 the plasticity range of the stress-strain curve. We’ll discuss where this is used in COSMOSWorks later in the unit.

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Material Properties of Plastics… Ductile vs. Brittle Behavior 

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

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

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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 to 4x increase in compressive vs. tensile strength

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Stiffness DOES NOT indicate ductility

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Ductility varies with temperature, strain rate, and material composition


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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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Material Properties of Plastics… Ductile vs. Brittle Behavior 9233G

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33% GF Nylon Relatively Brittle 13

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-S curve 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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Material Properties of Plastics… Ductile vs. Brittle Behavior

Room Temperature


Low Temperature

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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 the limits 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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Material Properties of Plastics… Orientation

Flow

Cross-Flow


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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 without any 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 crossflow direction indicate. Although the few examples shown in this unit might lead you to this conclusion, don’t 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 Curv e for Polyethy lene (PE)


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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 the bottom-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. We’ll use this data to explore the options for material model definition.

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Nonlinear Materials in COSMOSWorks 

Linear Elastic

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Nonlinear Elastic

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Plasticity-Von Mises Viscoelastic

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COSMOSWorks provides three more commonly used material models for plastic part analysis, Linear Elastic, Nonlinear Elastic, and Von Mises Plasticity. The Viscoelastic model 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, we’ll focus on the previous three.

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Nonlinear Materials in COSMOSWorks

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0,0 assumed No yielding will be calculated Yield Strength in table for Factor of Safety plots and reference only • • • • • © 2006 SolidWorks Corp. Confidential.

0,0 assumed Final point should be at strain far beyond expected strain No yielding will be calculated

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0,0 assumed First point must be Yield Strength Only post-yield nonlinearity allowed in S-S curve Young’s Mod and Yield Strength entered in table is ignored 18

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 of interest so a linear material model provides perfectly valid results. Unfortunately, you won’t 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 doesn’t need to be entered. You don’t need to enter a Young’s 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 req’d. 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 Young’s 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 can’t 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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Nonlinear Materials in COSMOSWorks Linear

Yield Strength

s s e r t S


Nonlinear Elastic

Ac tu al Plasticity

Strain

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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 elastic model, 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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Nonlinear Materials in COSMOSWorks 

Most plastic materials have a significant nonlinear elastic portion of their stress-strain curve – Do you need to consider plasticity?

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Plasticity models are the most resource intensive Visco-elastic effects (Creep / Relaxation) must be considered – especially if unloading is of interest. – Difficult to develop material models – Consider Apparent Modulus to estimate creep


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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 under operating loads is preferred. If significant plasticity is not a valid operating condition, you don’t 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 can’t be avoided but remember that these models are very resource intensive. In some cases, you’ll 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 very common 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 might come 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 of each…refer to COSMOS Companion Unit #110 – Introduction to Nonlinear An alys is i n COSMOSWo rks


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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 reviewed in more detail in the COSMOS Companion unit #110 – Intro to Nonlinear in COSMOSWorks so that info won’t 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…


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Some of the advanced nonlinear options available in Advanced Professional involve advanced solution control. Due to the unpredictable nature of plastics and their tendency to 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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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

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Residual stress from molding must be acct’d for Other effects such as weld lines, UV or chemical degradation, notch sensitivity, nonlinearity, and other operating conditions must be considered Due to near “perfectly plastic” behavior of most ductile plastics beyond yield, failure strain may be more indicative of problems Long Term Effects must be considered for many parts – Creep, Relaxation, Fatigue, Aging (Thermal, UV, Oxidizing, Chemical)


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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 didn’t have an opportunity to fully sort thru this problem as we weren’t able to repeat the brittle failure again but i t highlighted the unpredictability of plastic components. One thing I’ve learned in my years of supporting analysis is that you can’t 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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Why Test & FEA Don’t Match 

Wrong material properties entered

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Wrong material model used

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Plastic-specific issues not considered – Strain rate, Temperature, Viscoelastic effects, Moisture effects, Thickness effects, Orientation effects, Residual stress, Local property variation, (Weld lines, skinning), Processing effects

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Wrong failure quantities examined

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Loads & Restraints invalid

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Poorly constructed mesh

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One or more nonlinearities not considered

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Absolute data (vs. Trend) relied upon too heavily


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In general, the biggest reason I’ve seen as to why test results don’t match FEA results is improperly specified material properties. If testing at temperature, strain rate, orientation, and thickness wasn’t 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 won’t be affected and will remain out of your control. Therefore, an improvement is likely to be valid even in you can’t 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. Don’t 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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Presentation Summary… 

In this COSMOS Companion unit, we reviewed: – Tips for modeling your parts in SolidWorks to improve the efficiency and 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 in COSMOSWorks – Guidelines for comparing test results to analysis results


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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 are using 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 Young’s 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 isn’t the difficult part. Obtaining and understanding these properties is. Finally, we talked about interpreting your analysis results and why you’ll need to keep an open mind when comparing analysis results for plastic parts to test. If you don’t get good test correlation, it is more likely that there was some aspect of the problem you didn’t 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… 

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Contact your local reseller for more in-depth training or support on using COSMOSWorks to analyze plastic parts or assemblies or to discuss modeling techniques Review the on-line help for a more detailed description of the features discussed Attend, or better yet, present at a local COSMOS or SolidWorks user group. – See http://www.swugn.org/ for a user group near you

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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


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I’d like to thank you for taking the time to j oin 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 don’t 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 aren’t 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, I’ve listed a couple of references that have been helpful to me in my years of analyzing plastic components. The inclusion of the Bayer and GE websites doesn’t imply an endorsement of these suppliers by SolidWorks or any relationship between the companies. These are just websites I’ve 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 I’d suggest taking a few evenings and reading thru it. It is one of my most valuable references. With that, I’d like to thank you again for your time and interest and I look forward to seeing you next time on the COSMOS Companion.

Unit #114

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114 Analysis of Plastics Solidworks Simulation

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