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Assuming Linearity: Linear Static vs. Nonlinear

Background
SolidWorks has several different tools available for analyzing structures. Static analysis (discussed in my previous blog article) is available with the purchase of SolidWorks Simulation (included in SolidWorks Premium). Nonlinear analysis is available with the purchase of SolidWorks Simulation Premium (see Figure 1).

Figure 1 - SolidWorks Simulation study types

When a force is applied to a structure (a pencil, a building, a fixture, etc.) the internal pressure in the structure builds up. We call this internal pressure, stress. As stresses rise, the structure gets longer or shorter. We call this elongation, strain. When we run a Static analysis, we are assuming that the relationship between stress and strain is a simple one; we assume the relationship is linear. By making this assumption, we can potentially make an otherwise difficult or sometimes impossible problem very easy to solve.

However, this assumption isn’t all true. In reality, the relationship between stress and strain is never “perfectly” linear. But this assumption is close enough for many scenarios, and the amount of error it introduces may be negligible.

The Test
So what about if the error isn’t negligible? What about those scenarios where we should avoid from making the linear assumption? To understand this, I constructed a very simple example model and ran a linear and a nonlinear analysis on it. My linear analysis results looked like what is shown in Figure 2 below.

Figure 2 - Linear Static stress results

I applied a 100lb load to this small model. This load happened to be quite a lot for this structure. You see the amount of stress before the structure yields is about 90ksi (shown below the chart). This “yield strength” is associated to the material I used (i.e., Alloy Steel). From this result, I noted the maximum stress of about 240ksi.

Then I applied the same 100 lb load using nonlinear analysis and saw the results shown in Figure 3.

Figure 3 - Nonlinear stress results

I noted that my maximum stress is now much lower at about 125ksi. This is almost half of that reported for the linear analysis (125 divided by 240). Also, it was interesting for me to take a look at how different the way the stress appeared to be distributed. It was almost as if the stress had to figure out a way to redistribute itself along the cross section so it could somehow bear the load.

I then ran the test with smaller loads.  With only 10 lbs applied, I hardly noticed a difference in the results when I compared my nonlinear results to my linear results.  I did the same with other loads (15, 25, 35, etc.) as well and still didn’t notice much of a difference.  So when did this difference begin to emerge?  After running more tests, I summarized my results with the graph seen in Figure 4 below.

Figure 4 - Linear vs. Nonlinear graph

Conclusion
It appears that differences begin to emerge only once the stresses have exceeded the yield strength of the material. Not too far beyond the yield strength, it seems to me that we at least have to consider nonlinear affects if we want accurate stress results. Also, the graph shows that displacements appear to diverge as well, but they appear to take longer to do so in this particular example model.

Shaun BentleyShaun Bentley
DASI Solutions

SolidWorks Webcast Series – May 4, 2012


An in-depth look at designing complex products:
Workflow Opportunities with SolidWorks

In this series, you will see how SolidWorks solutions enhance your ability to design complex products. Please join SolidWorks technical experts for this 3-part complimentary webcast as they outline benefits of SolidWorks products for your business needs.

Friday, May 4th at  12 PM ET
Presented by: Matt Derov -
SolidWorks Simulation Technical Manager

This presentation will cover how SolidWorks users can use SolidWorks Plastics  to facilitate plastics part design all within the SolidWorks and Windows  interface. Highlights include:

  • Injection molding simulation
  • State-of-the art meshing
  • Optimize multi-cavity and family mold layouts and feed systems
  • Assist in selecting materials help to create “greener” products

Friday, May 4th at 1PM ET
Presented by: Jeff Osman -
SolidWorks Technical Manager

In this one-hour webinar, we will see how SolidWorks can not only save time by creating injection mold splits along with mold base components, but also achieve a higher fidelity solution. The knowledge gained from using SolidWorks can then be used throughout the design process in order to optimize the performance of your design. Highlights include:

  • Create cavity core splits
  • Assembly visualization
  • SolidWorks Costing to help predict overall project cost
  • Seamless integration with outside vendors for communication
Friday, May 4th at 2 PM ET
Presented by: Elton Smith -
SolidWorks Technical Manager

In this webinar, we’ll show examples of maintenance instructions used in a mold
tooling environment. We’ll also show the creation of user manual materials that would ship with a consumer product. Whether you’re involved in the initial design, or downstream manufacturing; 3DVIA Composer offers a unique and compelling way to create the much needed technical content.

  • Injection mold tooling maintenance instructions
  • User manuals for consumer products

Checking Out the Enterprise PDM Check In Window

The SolidWorks Enterprise PDM check in window tells us a lot about the files that we and our organization interact with on a daily basis. Often times, this window may be viewed as ‘just another click’ to get our files into the system in order collaborate with others or complete a job. This cannot be further from the truth! This window offers a snapshot of the history, locations, and integrity of the data we provide to others in our organization. Let’s dive a little deeper into it and see how we can get the most use out of this display.

The Informative Check In Window

First, we should cover the general setup and information being displayed on the screen. The furthest left column, File Name, lets us know what files are being taken into consideration for a check-in. In the window above, we can see that it is a standard check in of a SolidWorks assembly. The second column, Result, lets us know whether Enterprise PDM had any issues when reviewing the data being submitted to the system. If the result does not permit, we will not be able to check in a document until we resolve our problem.

Next, we have three check boxes that control how the data will be transmitted to the Enterprise PDM Archive Server: Check In, Keep Checked Out, Remove Local Copy. When working with Enterprise PDM, it is important to keep in mind that all modifications made to your files are local to your machine until you check in your work to the server. Other users can only see files that have been checked in to the archive server. By selecting this check box, it tells the system to send any updated files you have generated to the server – in other words check in your work. The option to Keep Checked Out is often used in tandem with the check in box. By selecting this checkbox, we’re telling the system to send the latest version of work to the server for others to see, but keep the ownership in your name so that you may continue to modify the document. As you continue to work from project to project, you are pulling files local to your machine to gain the benefits of not having to work across the network. While over the course of a couple weeks, you may interact with 5, 10, or even 50 assemblies with hundreds of parts. Keep in mind these are all now local on your machine. The option to Remove Local Copy tells the check in process to wipe your local copies from your machine after you have checked in your work to the server. This will help to keep your local cache free of back logged files.

The next two columns show us information about the files we are checking in. The first column, Local Ver., displays information about the file we are checking in. Above, we see the figure 1/1 in grey. This indicates to us that the version we are checking in is identical to version 1 in the vault. If we were making subsequent changes, we’d expect to see something such as -/2 meaning the version we are checking in is not similar to the latest version in the vault which is version 2. The next column, New Ver., lets us know what the new version of the file will be when checked in. Again, this is displaying 1/1 and is grey as there will not be a new version checked into the vault in this screenshot.

The Checked Out By column displays the user who has modify rights to a particular referenced document. In this case, for this article, I am logged in as Admin. We will only be permitted to check in files that we have ownership of. If we are not currently the owner of the document, we will not be allowed to check the box for Check In.

The last two (displayed) columns, tell us information about the locations of the documents. The first of these two columns, Checked Out In, lets us know what machine the document was checked out on. In this case, I only see my machine, but in a collaborative environment or an environment where you might use a laptop and a desktop, this information is useful in letting us know where the most accurate data is located. The last displayed column, Found In, tells us what folder within the vault the file is referenced from. In situations where you might have the same file name for many files within your vault, this folder might be the only identifier as to the accuracy of the referenced file.

Right clicking anywhere with the file listing will produce the menu above. These options help with working with documents in the check in dialog.

There are several columns we can turn on or off under the Columns sub menu – in this case Referenced as and State. By turning these options on, we will add two more columns to our file listing. Referenced as will display the information from the SolidWorks file header as to where its parent is pointing to for the location. This is helpful for locating files that have been referenced outside of Enterprise. The State column will tell us what state of a workflow a particular file is in as well. These column settings are a per machine option so your settings may differ than what are displayed in these screen shots or what your neighbor might have turned on!

Select All, Select Files, Check In All Files, Keep All Files Checked Out, and Remove All Local Local Copies are all options to help expedite interacting with the checkboxes available to us. Select All and Select File will highlight either All files or files that meet a file naming convention. Using Select File from this menu or the first button in the upper right hand corner of the check in dialog, we can highlight all drawings (*.SLDDRW) or maybe parts that are named a particular way (SW-201*.SLDPRT). After the files are highlighted, you can check a box and that will be checked for all files that are highlighted if possible. Windows standard Ctrl+A, Shift+click, and Ctrl+click selection methods also work within this window. Check In All Files, Keep All Files Checked Out, and Remove All Local Local Copies will attempt to check the Check In, Keep Checked Out, and Remove Local Copy respectively for ALL files in the window where applicable – this operation will override any selections.

The last two buttons, Open File List and Save File List which also correspond to the 2nd and 3rd buttons in the upper right hand corner of the check in dialog, offer way to work with file listing in an outside program or for reporting. The Open File List will open the file list in Excel where each column and row displayed appear in a column and row in Excel. Save File List gives us a method to save this information to a text file without opening directly into Excel.

Lastly, at the bottom of the screen, is the Comment text box. By entering data into this box, you can document important information about the document(s) you are checking in. This can be to identify the less obvious (i.e. moved the main connector .1 mm along the y-axis for clearance) or to correspond to a particular change request (i.e. implemented items 1,2, and 4 of change notice 12345). This information can be helpful in searching, project review, or even as a friendly reminder as to where you left off prior to a vacation.

What if the Check In Window looks like this?

We have quite a few things being displayed here. Some good, some informative, and some frightening. When things look like this we want to refer to the following icons in the Result column. This will give us an idea as to what’s going on with our files.

The green checkmark followed by OK is good news! These files are going to be processed without issue. In fact, in the image above, we can see files that are -/2 being checked in as 3/3. This looks to be pretty normal and as expected. But what about the Warning Icon and the Stop Sign Icon? These are less good and possibly very bad news. If you see the warning icon, this means you can check the file in still, but there ‘might’ be some downstream bad effects. If you see the Stop Sign Icon, this is a big error and the check in process will be halted. We get a report in the bottom left hand of the dialog that lets us know how many warnings we received and how many are preventing the check-in out right. Please note that most of these warnings and stop signs can be set on the administrative side of your environment. Your company may decide that a particular warning should stop a check in operation until resolved.


Let’s review some of the more common warnings and review their effects on your company:


(Not shown above) This one you’ll need to resolve locally, but it’s a fairly mundane warning. This typically only happens with non-SolidWorks files. It just means you have the file open in another program such as Microsoft Word and you need to save your work and close it before checking it in.


This warning indicates that you (or someone else) have updated a part without updating and rebuilding the drawing. This means that the drawing’s information may not be accurate and you really ought to open the drawing so that any eDrawings previews are as accurate as possible. Please note that this may show up commonly for assemblies that incorporate ‘standard parts’. Often times, these ‘standard parts’ haven’t been updated for years and might be in a workflow state that does not allow updating. What happens is when you open the drawing, it pulls in the file and updates it to the latest version of SolidWorks, however, when you save and close the drawing, it’s still referencing that ‘standard file’ from 2007! At this point, you can ignore the warning (not recommended) or have your systems administrator update the files to the current version of SolidWorks.


In a collaborative environment, you may see this quite often. If someone has checked out a part or subassembly to something you are working on, they may have updated their local copy of the file and your file may be out of date. In this screen shot, our sub-assembly is referencing version 2, but OtherUser has the file checked out, which we should interpret as a version 3 is on its way. Depending on why you are performing this check in action might determine whether to continue or to stop and contact OtherUser. If you are just trying to update your team with the latest files or checking in for backup purposes, I would continue with the check-in action. However, if you are finishing your portion on a project and submitting this for review, I would strongly reconsider. You are willingly submitting data to the system when you know a subcomponent is changing. This means that at some time prior to release you or someone else MUST reopen that assembly with the correct versions of files to ensure the integrity of the project.


This is another warning that should cause some concern. This is letting us know that the assembly being checked in is pointing to a file that is outside the vault. This is troublesome for a few reasons. While the file should work within SolidWorks in most situations, the user experience in Enterprise is greatly reduced. Enterprise PDM does not track files that are outside of the vault. This means BOM reports, the contains and where used information will all be missing this file. In addition to this, when files within Enterprise are renamed and moved, the references are maintained and the assembly will open correctly at a later date. When files are outside of the vault, the old convention is true where this will disassociate the references. This most often occurs with legacy data and users brining in files from outside of the vault. I strongly urge you and your coworkers to resolve this issue prior to checking the files in.


SolidWorks has checks and balances in place to not run into this one, but it can still happen. What this means is there is a reference in Assembly A to Assembly B which had a reference to Assembly A which has a reference to Assembly B which… well, you get the idea. This should be avoided at all costs as it is problematic to the functioning of SolidWorks, but this is also one of the only built in stop signs in Enterprise – you can’t override this value and you must correct the infraction. How does this happen? Typically, it is due to a designer brining in a parent assembly into a child assembly for lining up mates or brining showing geometric relationships (such as in a drawing). The only way to resolve this error is to completely remove (not suppress or hide) the infracting assembly which is usually the parent. This often will cause one to have to rethink how you are trying to communicate what it was.

That should be a fairly detailed overview of items on the check in screen. I think it is extremely important to not become frustrated with this dialog and move quickly through it. Take a moment to make sure the actions you expected to occur on your files are taking place. This can greatly reduce issues down the line.

Royce Cole

Royce Cole
DASI Solutions

Sub Contractors Needed

Sub contractors needed to create 2D drawings, at or near ASME Y14-5 standard
from SolidWorks 2011 models. If interested, please contact Bryon Morell at bryon@dasi-solutions.com or 734-634-2357.

Stratasys and Objet Agree to Combine to Create a Leader in 3D Printing and Direct Digital Manufacturing

April 16, 2012 press release from Stratasys and Objet can be found at
http://www.dasisolutions.com/index.php/news-events/press-releases/300-apr-16-2012-stratasys-and-objet-agree-to-combine.

David Darbyshire, owner and co-founder of DASI Solutions, responded by saying, “This is great news for DASI and Objet Corporation….

The bottom line is that our customers have always purchased Objet equipment based on the unique ability to create 3D models with digital materials. In many cases, our Objet customers already owned Stratasys equipment within their rapid prototyping lab and it was DASI’s goal to layer on capabilities for the Designing Engineer. The main area of being competitive lied in Objet and Stratasys competing for capital budget within a prospective customer.

Next week, DASI and Objet will be exhibiting at the SAE World Congress in Cobo Hall in Detroit, Michigan. Moving forward, DASI will remain an Objet reseller in the manner that we have done for the past few years.”