February 28,2006 The Focus Issue 45

February 28, 2006 A Publication for ANSYS Users Issue 45 Modeling the “Plastic Knee” in Workbench By Doug Oatis best analyst would get models that would So you’ve mastered the basics of Work- not solve because the first slope was slightly bench and are ready to tackle a plastic off of the Young’s modulus. This approach analysis. If you look at your stress-strain used a TB,MISO material property to define curve and it looks like a bilinear model is multi-linear plasticity. not going to work, you need to be aware of Based upon this issue and several others, how the WB multilinear model works to the new multilinear plasticity model, TB, make sure you are capturing all of your PLAS was developed so that users only had plastic behavior, especially if your material to define the plastic portion of the stress- has a big “plastic knee”. To understand the strain curve. You do this by defining the why and how, we first need to look at the Total Strain - 0.2% (Yield Strain) for the way it used to be done in ANSYS. x-axis and the total stress for the y-axis. In olden days, you gave the elastic behavior Since this is simpler and produces a lot less as Young’s modulus (EX), a yield stress, user frustration, it was chosen for the multi- linear plasticity model in Workbench. Figure 1: Typical Stress Strain Curve and the whole stress-strain curve. If the havior, you should be using the elastic limit equivalent stress was under the yield stress, So for Workbench the following rules apply: or proportional limit (which are often very ANSYS calculated strain using the Young’s close), as your yield stress and use that point 1: The first data point must be at 0 for plastic modulus. If it was over, it used the curve. strain (X) and the Yield Stress for stress (Y) as zero for your strain. There were three rules that you had to meet: 2: The slopes of all the multi-linear segments If you do this, however, you run into two 1: The initial slope of the curve MUST equal the must have a slope less than or equal to the problems. The first is that you need to Young’s modulus Young’s modulus redefine the yield strength of your material 2: The slopes of all the multi-linear segments 3: No segment can have a slope less than zero in Workbench. This is because Workbench must have a slope less than or equal to the does an internal check to make sure all rules Young’s modulus So instead of calculating a total strain from are met with the plastic curve. If the pre- 3: No segment can have a slope of less than zero a curve when stress is above yield, the solver calculates a plastic strain and adds it defined yield stress doesn’t equal the initial The first rule proved to be especially trou- to the elastic strain. For many materials this y-value of the plasticity curve, the solve blesome. But if you didn’t have it, ANSYS is perfect and saves a lot of headache. But, won’t be executed. This problem gets even would predict a jump in strain when a stress if you can remember back to your Strength more complicated if you have a stress tool moved from below to above yield. Even the of Materials textbook (like “Introduction to calculating margin off of the yield strength, Mechanics of Solids”, since your yield strength is now actually Popov, Pg 111) a lot of your elastic limit. You will need to set the materials show behav- criteria to calculate margin from a user-de- ior like the curve shown fined value. in Figure 1. Pulling out The second problem this creates is that the a definitive yield point elastic limit is not a commonly listed mate- is tough so a 0.002 in/in rial property. Finding this point on an ac- offset of the linear por- tual curve obtained from MIL-HDBK can tion of the curve is used prove to be a long and tedious task. to set a yield stress. (Continued on Page 2) So, if your material Contents doesn’t have a large non-linear “knee” be- Plastic Material Model in fore the yield stress, Workbench ...... 1 you can move on to Substructuring Review...... 2 reading other articles in ANSYS Buys Fluent...... 3 this issue. But if your material shows this be- 2006 IAC...... 4 www.padtinc.com 1 1-800-293-PADT February 28,2006 The Focus Issue 45 (“Plastic Knee” cont.) The same geometry, element types, and x-axis and define the non-linear portion of loading were used for each model. The the stress strain curve. ANSYS Classic model used the tried-and- true method which requires the first slope equal the Young’s Modulus. Aluminum shows a significant “knee” between Fel and Fty so it was chosen for this study. Figure 2 shows the data used and is based on a MIL- HDBK-5 graph. The results between the models are shown below in Figures 3 and 4. Figure 2: Material Curve Used As you can see, all three models obtain Figure 3: Comparison of Strain So now you must be asking, “Well, is it similar strain results. However, when really worth the extra effort to use my start- equivalent stresses are viewed, the Work- ing point as the elastic limit instead of using bench model which used 0,Fty as it’s start- the easy-to-obtain 0.2% yield stress?” ing point is different from the other two, and In order to answer this, I created three mod- by a considerable amount. els: So, after all is said and done, if you want to Platform Plasticity Model Starting Point more closely match ANSYS Classic meth- Workbench PLASTIC,,,MISO 0,Fty odology, you need to define 0,Fel as your Workbench PLASTIC,,,MISO 0,Fel Classic MISO 0,0 starting point. Just ignore the values on the Figure 4: Comparison of Stress Reviewing Substructuring By Rod Scholl ces… a method to break a matrix into Substructuring, once a cornerstone of FEA say, quarters, and solve four (N/2 x N/2) system analysis, has continued to fade in matrices can represent significant sav- popularity as computer speed and available ings in solution time and memory re- memory have increased dramatically. It still quirements. Of course these matrices has its place in structural analysis, but it is have to be relatively independent of one definitely diminished. And when this tech- another as we will see. Also, for some nique is applicable, the savings can be huge nonlinear problems, the predominately and makes a nice implement on the Bat Belt linear portion can be scuttled away as a of the FEA analyst. substructure to be solved essentially once, leaving only the nonlinear portion What it isn’t to be iterated upon. Afterward the whole First let’s note that performing a substructred solution is reassembled. Of course not modal anlaysis (prerequisite for many types all problems lend themselves to substruc- of dynamic analysis) requires Component turing… (if not, why would one ever Mode Synthesis (CMS) and although a type mess with the huge NxN global stiffness of substructuring, is not covered in this arti- matrix?!) So expect that only a limited cle, but was covered in the a previous issue class of problems are benefited by the Again, read the manual for more details, but Further, the term “submodeling” is often con- substructuring method. substructuring analysis can be broken into fused with the term “substructuring”. Sub- these steps: modelling refers to the method of taking a How It Works coarsely-meshed solution, remodeling a por- The help manual is decent in this re- Get the macros at: tion of it , meshing with greater refinement, spect… but with all the files and naming ftp.padtinc.com/public/downloads to keep track of, an extra example is and applying the results of the previous run as 1) TSUB1.mac – Generation Pass useful. (attach macros 1,2,3). These boundary conditions (CBDOF) to the new Define how the model will be sliced up into three macros can be run sequentially to model and resolving. chunks, and form a Matrix50 Super-Element model two blocks pushed into contact. (couldn’t resist making the graphic here, get What it Is The majority of the block is sub-struc- it, “Super Element”) of each chunk. What Substructuring is about breaking a model up tured, leaving only the contact surfaces exactly is a super-element? Well, you know into a set of smaller models, or to be solved during the contact iterations. that feeling when you lean back in a chair… “superelements” and then solving them si- The only real trick to substructuring is and you lean back too far, and you thrash multaneously. Since we know solution time getting the file naming correct. There are is roughly related to N^3 for (NxN) matri- a bunch of ways to do this. (Continued on Page 3) www.padtinc.com 2 1-800-293-PADT February 28,2006 The Focus Issue 45 quickly and barely recover? That was a lit- system level results at the superelement tle help from the often invisible, but ever- boundaries and applies them to the superelement present Super-Element… fighting evil in our and solves. Some people still call this an “Unwind” daily lives. Okay, okay, it’s an element with pass. varying number of DOF’s at the boundary it Another Example: encloses which has an equivalent stiffness These 3 macros are very similar to TSUB1-3… for each DOF. What makes the savings, is except they do not have the contact elements, rather that only the stiffness at the boundary the blocks are connected via couples, and only one DOF’s need to be carried… or more accu- of them is placed inside the superelement. rately, at the Master DOF’s defined at the boundaries. The Big Caveat The size of the boundary between substructured You don’t actually need each node at the regions is the FIRST thing to consider when deter- There is a maximum wavefront limitation of 46340 boundary to be a Master DOF node… but mining if substructuring is right for you. This often at the superelement boundary. But on many ma- realize force/deflections only transfer at the overlooked step (guilty) has sent many folks down chines RAM limitations will run out first. On my Master DOF’s, so I can’t think of a case the substructuring road only to find longer solves 2GB Ram, WINXP machine the maximum size of where a coarse mesh with every node defined times, or complete failure of runs due to explosively the boundary that I could solve was about 3,500 as a Master isn’t superior to a fine mesh with increasing RAM requirements. As we mentioned, nodes (3 Master DOF’s at each node). There only some boundary nodes defined as Mas- if breaking a simple cube into 8 smaller cubes actu- wasn’t much improvement going to a 6GB 32-bit ters. ally saved time and memory… then that would Linux machine. 2) TSUB2.mac – Use Pass already be in the ANSYS solver! The cases we are On the 6GB Ram 64-bit Linux machine, I’m not Solve the model of superelements and any looking for, is where regions of the model are rela- sure the maximum size of the boundary but hit a elements that were not put into superele- tively independent and share only a few nodes with 8GB file size failure at about 10,000 nodes (30,000 ments. one another, but yet the regions are needed in their MDOF’s). fully geometric detail and complexity for proper 3) TSUB3.mac – Expansion Pass system behavior -- or, also cases where the non- So on my machine, for example, we’re talking a If you need to know stresses and deflections linearity is localized, such as resolving plasticity in a maximum interface of about 30x30 mid-side nod- for the regions inside the superelement, you small fillet, or small regions of contact between ded elements. Results will vary depending on run an “expansion pass” which takes the many relatively large rigid bodies, etc. shape of the region, and of course your hardware. Commentary: ANSYS Inc’s Purchase of Fluent By Eric Miller lion from software and services. This is a huge ness. What we have seen in the past is that the They overshot Valentine’s Day a bit, but AN- increase in market for the new company. first benefit to users is the sharing of brain SYS, Inc. just announced their intent to pur- Looking at ANSYS, Inc.’s 2005 revenue of power across companies. Published comments chase the Fluent Software business from $153.3 million you can see that this predict that the Fluent “development plans will AAvid Thermal Technologies. Since the deal purchase is going to most likely be augmented with additional is not completed there is not a lot we can say greatly expand the features and capabilities that will come about it. But we can comment on some of the number of customers from access to ANSYS technology.” under the ANSYS um- facts that are included in various press releases. This will take some time to do brella. The first comment is that this deal is big. Look- correctly but a few years ing at the numbers, Fluent has the largest share So what does this mean down the road there of the CFD market. The Fluent web site quotes to the users? For a while, should be some awe- Daratech as giving them “over one-third of the nothing will change. The deal some things happen- estimated global CFD market, and is twice as will not be consummated for a few ing. In the end, ANSYS, big as its nearest competitor.” That nearest months and until then everything is busi- Inc. users will have a compat- competitor is probably ANSYS/CFX or CD- ness as usual. After that, past history and ible and robust CAE solution that Adapco, depending on what you count as rev- official comment points toward taking their will be able to tackle almost any simulation enue. According to published numbers, time on integrating different parts of the busi- problem. Fluent’s 2005 revenue was around $121 mil- Learn more at : media.corporate-ir.net/media_files/IROL/11/118715/021606.pdf

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A new book, “The Finite Element Method and Applications in Engineering Using ANSYS” is Resources now available from two University of Arizona Professors. Learn more at Amazon.Com www.padtinc.com 3 1-800-293-PADT February 28,2006 The Focus Issue 45 An Engineer’s Travelogue: Anticipating the 2006 International ANSYS Conference leap across the surging rivers linking the wild center of the city with the outside world. The Top 10 Reasons And what a center it is. Buildings of metal, glass Why You Should Attend and stone surge upwards to great a leaden and IAC’06 cloudy sky. If you are lucky, a slight rain will be falling, just enough to impart a sense of scale and 10: Meet developers face to face and proportion to the amazing tableau before you. discuss your needs and concerns In and amongst this urban jungle sits a marvel of 9: Learn about how other companies modern engineering: the convention center. are using ANSYS products Featuring cantilevered terraces and soaring 8: Be able to finish a story with "he By: Egbertius Fog, MBE, PE cables that anchor the roof, this building is a meshed his sheet metal parts with Those of you have been faithful readers of my showplace of “green” design that manages to Tets!" and get a laugh every time travel stories know that I love a good jaunt to an merge function and beauty into one. But what excites me as I sit down to prepare with my 7: Attend presentations that focus on exotic local that features fascinating technology. In real world usage of ANSYS. your letters you have expressed your appreciation trusted outfitters is not the wild landscape it is to for tales of my close encounter at the Balanese be held in, and not the beauty of the structure that 6: Network with potential future Robotics Plantations and my four weeks at the will be my home for five days. No, what excites bosses... not that your intent is to South Pole studying the bridge building habits of me is the chance to meet the members of the find new employment, but you never Poindexter Penguins. It is therefore with a delicious various ANSYS, Inc. tribes. know... anticipation that I tell you about my next adventure: Sir Fog's pre-journey discussion will be 5: Meet John Swanson a journey to the wilds of Pittsburgh to partake in continued in the next issue of The Focus under that biennial congress of adventurers and risk the title: 4: Take training classes from experts takers: the 2006 International ANSYS Conference. “Interacting with the Natives and 3: You can have dinner with people This congress of capable calculators supplies that Understanding what they Worship” who not only know what a Newton- unique combination of thrill and knowledge that Raphson is, but they actually want causes my adrenaline to surge just thinking about it. to talk about it at dinner The roller coaster ride begins when we leave the 2: Voice your opinions to people who relative civility of the airport and journey towards can actually make a change. the hidden splendor of the city itself. Perched at the confluence of two great and wild rivers, Pittsburgh 1: It's in Pittsburgh, your spouse won't is shielded from the outside world by high bluffs get suspicious that are magnificently pierced by long, tile-lined tunnels that are vivid reminders of adolescent locker room nightmares. Bursting from the tunnels Visit: into the light of day you are greeted by one of the www.ansys.com/conf2006 finest collections of steel bridges in the world. to Register In a fascinating array of shapes and colors, they

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The Focus is a periodic publication of Phoenix Analysis & Design Technologies (PADT). Its goal is to educate and entertain the worldwide ANSYS user community. More informa- tion on this publication can be found at: http://www.padtinc.com/epubs/focus/about www.padtinc.com 4 1-800-293-PADT February 28,2006 The Focus Issue 45

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