Finding Ground Contact Times in SolidWorks Simulation Drop Tests

By Keith Frankie - Mechanical Engineer at TriAxial Design and Analysis, Inc.

When performing a drop test the ground contact time can be an important result. Longer contact times mean lower average acceleration and thus lower impact forces.

As with other parameters of the test, such as impact velocity, knowing these “intermediate” results can be very helpful in confirming that the test is proceeding in a fashion similar to what hand calculations predict.

As an example the specimen’s stiffness can be estimated with a simple linear study. The impact event can then be modeled as a simple ‘mass on a spring’.  From this the ground contact time can be calculated and then compared to the SW drop test impact time.

We’ll use a bouncing rubber ball as a reference while explaining several methods to retrieve ground contact time. (Here is the sldprt for the ball in a zip file ball)

The ball is a simple 1 inch rubber ball (generic material ‘rubber’ from the SW material library) dropped from 0.1m (centroid). Velocity on impact can be hand calculated as v=(2gh)^1/2 = 1.31 m/s.

By definition ground contact begins at the start of the drop test. Remember that the default drop test plot animation includes 10 “fake” frames of free fall before impact.

A first pass approximation of the contact time is usually made by observing the test animation and taking note at which result step the specimen last touches the ground. Having a visual picture of the part’s behavior during impact certainly helps provide a sanity check against the more numerical results we’ll see later.  Sometimes it can be hard to gauge exactly when contact ends.

A more robust method of checking contact times is to look at the .OUT file that SW writes while solving the simulation. Every time a plot is generated SW records which nodes are in contact with the ground.  Simply look for the last entry with contact and the first entry without contact.

If it’s clear which node will be the last to loose contact with the surface a sensor can be placed there.   A split feature is used here to create a vertex on the bottom, middle, and top of the ball.

A graph of vertical motion vs. time can be created at both the top and bottom point. A plot of velocity gives the clearest indication of the takeoff time.  In the plot below the blue line shows velocity at the top.  Initial speed can clearly be read as -1.31 m/s, which matches the calculated value.  Takeoff occurs when velocity at the contact point becomes non zero at 1524 µs.

Let’s do some hand calculations as a sanity check.  We’ll model the system as a mass on a spring.

According to the drop test the center of the ball will move down a maximum of 0.67mm during impact. Because of the ball’s geometry the effective stiffness of the ball varies throughout the impact event.  We’ll simplify to an “average” displacement of .010 in, a little less than half of the maximum displacement.

We’ll run a static test on half the ball to determine the approximate stiffness of the structure. On the cut “half face” we’ll apply a forced displacement of .010 in.  On the bottom we’ll use a virtual wall to simulate the ground.

This static study solves quickly. Querying reaction forces we see a load of 1.22 lbs is required to squish the ball.  This converts to 5.43 N per .254 mm, or 21.4 kN/m.

Now we can model the system as a simple oscillator consisting of a mass on a spring. The natural frequency can be calculated as f = 1/2 π *(k/m)^1/2 . Substituting in our values we get 353.7 Hz.  Half of a cycle is 1.41 ms.  Since our theoretical model doesn’t account for the changing stiffness of our ball we know it’s not totally accurate, but it indicates we haven’t made any egregious errors with our FEA study.

Since solve time can be lengthy when testing real parts it’s rare to run the model much past the initial impact. With this simple (4k DOF) model it’s fun to run through a few bounces.  Displacement results look as expected, with decreasing rebound height at each bounce.

Keith Frankie is a Mechanical Engineer at TriAxial Design and Analysis, Inc. with a BSME from UC Berkeley. He is a Certified Solidworks Expert (CSWE) with additional certifications in Simulation, Advanced Sheet Metal, Advanced Drawing Tools, and Advanced Weldments.  In his 10 year career as a mechanical engineer he has worked on a wide variety of projects in both the commercial and defense sectors.  He is active in the leadership of the San Diego chapter of the American Society of Mechanical Engineers (ASME). He can be reached at TriAxial Design and Analysis (619) 460-0216 or engineering@triaxialdesign.com

Bumps in the Road to Expert

By James Woodward - Mechanical Engineer at TriAxial Design and Analysis, Inc.

Certification has never been a particularly important thing in my mind, as I generally look at CAD software as a tool for design in the same way a lathe is a tool for fabrication.  Certification definitely sets you aside for consideration in the rigorous competition of the engineering world, but beyond that, what's the point?

Having coworkers with broader skillsets than mine, however, has changed my mind quite a lot.  I'm surrounded by people that have explored all of the various minutiae of the program itself, led not only by the necessities of problem solving and finding the simplest and most efficient ways to model complex geometry, but also by the more rounded aspects of a standardized testing protocol.

Progression from the CSWA to CSWP and onward to the CSWE, it turns out, is a very good way to flush out the aspects of SolidWorks that you may have overlooked in your daily duties.  

The special case of the CSWE, however, is an interesting one.  Previous tests all function in a general knowledge-base sense, with a cumulative score, but the CSWE requires a passing grade in 4 of 6 subtopics in order to allow taking the complete test; Sheet Metal, Weldments, Surfacing, Mold Tools, and Drawing Tools. 

My particular experience has thoroughly educated me in the ins-and-outs of several of these topics, to the point where a brief review of the topic at hand and a quick overview of the test prep materials available online was sufficient for the Drawings, Surfacing, and Sheet Metal modules.  There is a degree of comfort and competence with the topics relevant to each subtopic which I think is necessary to proceed effectively, and achieving this was a pretty simple matter for everything with which I was mostly familiar.

Surfacing, in general, is a thing most users of parametric modeling have a hazy grasp on, I believe.  It's certainly a different sort of approach - instead of forming extruded and swept and revolved prismatic bodies and sculpting them with increasingly more complex additive and subtractive geometries, it's starting with a desired shape (either in your mind or based on what's in front of you) and manipulating the edges and surfaces that compose it in order to redefine where the material boundaries are.  Working from the outward in, you could say. 

Most often, this really only manifests in the manipulation of existing bodies or to design around an envelope, in a sort of smooth or organic fashion.  The most important element, I would say, is certainly the ability to correctly identify the portions of a surface model which need to be removed, and to remove them cleanly.  Making a working edge from a damaged edge allows the eventual filling of open edges and recreation of a solid body, which comes up regularly.  Besides this, the ability to correctly locate a sketch picture and use it for design guidelines is critical.

Drawings were a curious thing to study - through many a year manipulating and creating drawings, the basic elements are certainly already in place.  Creating a view, adding or altering a dimension, reconnecting a leader line to redefine a dimension, and creating a client-specific standard for BOM or Revision Block were pretty well clear in my mind, as these are things that one does on a day-to-day basis.  But there are definitely other, deeper facets that came as something more of a learning experience than anticipated.

I had limited use, for instance, for the various options available for placing drawing views relative to model features.  It's one of those things that will occasionally come up, and each time it had I'd looked it up for the immediate necessity, but never quite understood fully in terms of operations and potential.  Very much in the same vein, the wide range of potential in any given Bill of Materials is staggering.

If one had to study for anything in the Drawings, section, I'd recommend that.  Manipulating the fields shown in the BOM and understanding how everything is tied to everything else, changing custom properties and configurations, and editing what and how things are displayed via the model properties was the lion's share of the Drawings side of things in my view.

The Mold Tools was also reasonably straightforward, with one very important caveat - as both of my parents used to say (repeatedly and with evident joy) - RTFQ.  Read The...  Question.   This piece of wisdom has cropped up over and over again throughout the years, whenever I'd waste time on an exam trying to solve for a variable that wasn't the one in question, or missing the relevant line in a paragraphs-long essay topic and taking too large or small a bite of the subject.  Over and over, the sound of my parents hammering this one home.  R, T, F, Q.

It seems that, due to a slight peculiarity in the intricacies of the program itself, the correct answer to one aspect of the Mold Tools test can ONLY be correctly answered by selecting the Top Plane as the pull direction as the question outlined.  The topmost surface, despite being parallel to the Top Plane, yielded an answer that was very slightly different from the correct one.  Slight, yes, but definitely enough to be incorrect.  Hours of repeating the operations on the sample I had available and sometimes yielding the correct answer and sometimes not, changing the order of the features in the tree, selecting different options for each one all led me to the inevitable conclusion that when the test called out the Top Plane as the correct pull direction, it meant it.  R, T, F, Q.

And finally, Sheet Metal.  Once again, a perfect application of the 'RTFQ' idea.  If you understand what how the model is supposed to end up, and you understand the basics of K-factors and break styles (and most importantly, where to alter those values within the feature tree) you should largely be prepared.  Assigning materials is quite straightforward, but important.  Mitered edges and sketch bends are commonly encountered (and fortunately for me, had been encountered many a time prior to the test), and knowing the intricacies of how the model is affected by the values used in the bend features will smooth the test considerably.

San Diego SolidWorks User Group

A Year of Success - Thank you from TriAxial Design and Analysis, Inc.

What's New

TriAxial Design and Analysis, Inc. has experienced exciting growth in the last year, and we now have more capacity than ever. This means larger projects and more bandwidth to handle more projects simultaneously.

We have recently completed numerous successful projects encompassing skills such as advanced surfacing, detailed mechanism design, project management, and a variety of rapid prototyping techniques. Our diverse customer base includes industries developing handheld scanners, ablation catheters, DNA sequencing equipment, and commercial energy generation.

As an example of our software expertise, one of the members of Team TriAxial, Satakal Khalsa, finished first among all users and resellers in the Model Mania contest held at the recent SolidWorks® World Conference in Orlando. This is a timed modeling and analysis challenge that requires quick thinking, a cool head, and lots of SolidWorks® skills and experience to perform well.

We are continuously improving in our in-house standards and documentation in areas such as project definition, design and drafting standards, design validation, and even time tracking and invoicing.

• We are confident that we can assist you quickly and effectively by filling in the gaps when your workloads increase.  The threshold where a new staff member cannot be justified but a project must be completed.
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Through all this forward progress, our vision remains the same; TriAxial Design and Analysis provides totally reliable design consulting services using SolidWorks® 3-D Solutions. We have unmatched excellence in mechanical engineering: design, analysis, industrial design, documentation, and training.  We are committed to researching and applying the most effective methods and technically advanced techniques.  We strive to be courteous and responsive to each and every customer.

Our designs have been widely innovative, manufacturable, embraced by our customers, their industries, and after developed have seen little change over life of the products. We also provide complete design packages including drawings, assemblies, technical manuals, BOM's, component specifications, and validation test data. These are produced to your standards and requirements.

I want to offer our assistance to meet your current technical resource shortages. I would like to introduce our capabilities to your extended team. Together we can determine how good of a fit we are with your company and discuss assisting in a collaborative development effort.

Thank you,

Phil Sluder | Mechanical Engineer

TriAxial Design and Analysis, Inc.

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