In our last blog post of this series, I dive into how we can simulate cracked structures using Ansys simulation software, Ansys Mechanical. As before, if you’ve not read the previous two posts, go back and read ‘em!!! 

How Engineers Use Ansys Mechanical Software to Model Cracks 

Ansys Computer Aided Engineering (CAE) simulation software allows engineers to study cracks in structures via fracture mechanics, along with a host of other structural simulation needs. Ansys has a long history of simulation development since the 1970’s in creating tools for engineers to design and virtual prototype their products. As a quick note, Ansys is not limited to just structural physics either. Fluids, electromagnetics, systems and optics are some of the other fields Ansys offers in its portfolio of simulation capabilities. 

The options to create a crack in Ansys simulation software generally fall in two categories: either a) use a CAD surface that represents the crack, that overlaps with the structure or b) use the auto-generation tool in Ansys to add a crack at the mesh level. 

The former works well in all scenarios but is very useful when the crack is not a simple analytical shape, i.e., a penny-shaped crack. The latter is great for those simple, penny-shaped cracks, where the engineer can input two radii to define the shape, input where the crack is located, and they’re done. 

Here’s an example; take this simplified cast bearing/shaft support. The machinist finds a crack when machining the bearing support housing (outlined in the blue box). Perhaps this is caused by an incorrect casting process. 

Representative CAD Model, Crack Surface on Right 

When magnafluxed or cut open, the crack is not a simple shape. This is perhaps an extreme example, but it gets the point across. With a few inputs and clicks, Ansys overlaps the crack surface with the solid CAD, splits the mesh where these intersect, buffers the elements from the new crack mesh into the existing base mesh, and voila! The finite element model crack is ready to analyze.

Representative Finite Element Mesh of Structure with Crack Inserted: Back View on Left, Top View on Right (with red line indicating part boundary) 

What About Crack Growth? 

Ansys requires no special treatment of the crack to determine the relevant fracture parameters when evaluating a crack for simple comparison to material fracture toughness. The simplicity of the Ansys workflow mirrors the simplicity of what the engineer is after, i.e., a single value for Stress Intensity Factor. Using the methods described previously, engineers can model a crack and then mesh the structure with hexahedra, tetrahedra, or a mix of element shapes and get results for Stress Intensity Factor. 

For fatigue cracks, the requirements are greater. Engineers must provide the crack growth equation constants, i.e., the Paris constants C and m, then Ansys will do the rest. Ansys’ technology for general, 3D crack growth is quite extraordinary. This technology is referred to as SMART – Separating, Morphing, and Adaptive Remeshing Technology. To put it simply, automatic remeshing occurs as the crack grows in simulation. 

Representative Crack Growth Simulation Showcasing Automated Solution Remeshing 

For a nice overview of fracture mechanics in Ansys, you can watch an on-demand webinar on DRD’s website. In the webinar, I provide a brief overview of fracture mechanics and Ansys capabilities in fracture analysis, much like this paper. I also discuss damage tolerant design, material data acquisition, and Ansys CAE simulation of cracks in structures. 

Head over to DRD’s website for two on-demand webinars I conducted in October and November, ‘Simulating Crack Propagation Part 1 and 2.’ 

https://www.drd.com/resources-all/simulating-crack-propagation-part-1-webinar-recording/ 

https://www.drd.com/resources-all/simulating-crack-propagation-part-2-webinar-recording/ 

This concludes our 3-part series on fracture mechanics. We have a few other resources engineers can dig into on this topic, including the two on-demand webinars mentioned above. DRD has a fracture mechanics training course that I teach as demand requires, https://www.drd.com/project/ansys-mechanical-fracture-mechanics/. If you are interested in this course, please let us know at support@drd.com. 

Let’s continue our discussion on fracture mechanics with this second blog post, where I dive into the methods engineers have available to evaluate cracked structures. If you’ve missed part 1 of this blog series, go back and read it here. 

Stationary, Static and Fatigue Cracks 

When evaluating a structure with cracks, engineers have a few options with respect to the level of involvement in solving the problem. From least to most involved: 

• Stationary: review of status of crack, ignoring crack growth. 
• Static: review of status of crack under single, monotonically increasing load, crack growth is assumed. 
• Fatigue: review of status of crack under cyclic loading, crack growth is assumed. 

Stationary cracks provide an instantaneous view of the state of a crack in the structure. The engineer can only know one thing from this type of analysis: will the crack grow or not. No insight is provided into the second and third of the common questions asked in the previous section. Simple closed-form solutions are available for engineers to estimate the integrity of a cracked structure, and these can be found in literature reviews and textbooks. Many closed-form solutions take the resulting stress field caused by loading, the current crack length, and an empirically determined factor to determine stress intensity. A few examples are shown here, for plate geometry of varying sizes. 

Image Source: Elementary Engineering Fracture Mechanics, 4th Ed., Broek, pg. 85 

Static cracks allow the engineer to determine if a crack will grow and fast fracture, or if the crack will arrest. Static cracks are subjected to a single, increasing load, from unloaded to fully loaded. In this case, we are not interested in a time frame for the crack to grow or arrest; ultimately, engineers simply determine if the structure will break with the presence of the crack. 

Fatigue cracks, or fatigue crack growth, is the most complex case, both for understanding and to consider when designing a product. Fatigue crack growth considers the structure under cyclic loading, where the structure is repeatedly loaded and unloaded. There are variations to this load pattern as well, which we will not go into here. 

When it comes to fatigue cracks, there are additional test procedures to determine a crack growth rate versus the applied stress intensity. Engineers will typically see this abbreviated as da/dN vs. dK, i.e., the change in crack extension (da) over cycles per extension (dN) vs. change in stress intensity (dK). Like critical fracture toughness, every material will have a different crack growth curve. Examples of some different material curves are shown here. 

Image Source: Effects of ductile laminate thickness, volume fraction, and orientation on fatigue-crack propagation in Ti-Al3Ti metal-intermetallic laminate composites, Adharapurapa et al, 2005. 

The unique aspect of fatigue crack growth that harkens back to what Griffith found is the stress levels in the structure can be much less than those that would normally cause plastic collapse. Cyclically loading the structure will continue to grow the crack, under no threat of plastic collapse, and when the maximum stress intensity factor is less than the critical fracture toughness; we call this subcritical crack growth. 

Most crack growth data focus on this subcritical crack region, however, two other regions exist. Let’s limit the data shown in the previous graph to one material’s data set and expand the representative data out; we get a graph that looks like this. 

Image Source: A review of fatigue crack propagation modelling techniques using FEM and XFEM, Rege et al, 2017. 

The material data mentioned fits into the area marked ‘Region II’; on a log-log plot of crack growth rate versus change in stress intensity, this is commonly referred to as the Paris regime, and it is generally a straight line on this plot. A simple equation is used to describe this region, which takes the form of: 

where C and m are material constants determined via the graphed data. The other two regions, I and III, refer to the threshold and fast fracture regions, respectively. The threshold region describes when the crack grows slowly, either by small stress intensity or small crack size. Conversely, the fast fracture region describes rapid crack growth, which may result in surprise failure of the structure. Engineers use this crack growth data in damage tolerance assessment. 

In both the first and second blog posts, I’ve not touched on Ansys simulation to solve fracture mechanics problems. In the next blog post, I will discuss Ansys’ capability to model cracks and solve crack growth problems. 

Before we jump into the topic at hand, I’d like to introduce myself. My name is Alex Austin, and I am the Structural Team Lead at DRD Technology, an Ansys Channel Partner. I studied Mechanical Engineering at the University of Tulsa, OK, from which I graduated with a BS and MS in Mechanical Engineering a little over a decade ago (woo… it’s already been that long!). My graduate work was in the fatigue and fracture space. My primary area of expertise is structural mechanics; as many engineers may know, this is quite a large field of physics when we look at Ansys simulation capabilities. Fracture mechanics is a small part of that overall field, is relatively new in the world of engineering, and is very complex. 

What is Fracture Mechanics? 

When engineers evaluate stress in a structure, the common, simple equation that comes to mind is stress = force/area. This equation carries several assumptions: static equilibrium, uniform cross-sectional area, uniaxial stress, to name a few. With the introduction of a crack to the structure, the state of stress at the crack tip is not uniaxial. Cracks are sharp corners, or notches. In the computer simulation (finite element) world, we call these singularities. In fact, singularities are locations where the theoretical stress is infinite. A strength of materials approach does not account for these singularities. When a crack exists, we need a method to analyze it. Fracture mechanics is that method. Fracture mechanics is the study of crack propagation in materials. 

Image Source: Wikipedia 

Motivation for Fracture Mechanics 

Often, cracks naturally form during the manufacturing process, either through casting or machining methods. Cracks can exist in a product and never cause issues with the working of the structure. In fact, cracks may be invisible to the naked eye. However, when this is not the case, what happens? 

Let’s say we’ve designed and manufactured structure that is currently out in the field, and our customer notices a crack… is this a problem? Let’s say a customer reported cracks popping up on some rotating machinery housings and they simply asked, ‘Is this a problem?’ though, the more likely case is no questions and, ‘Please fix this!!!’. What is the engineer typically tasked with determining? The common questions we ask are: 

1. Will the crack grow? If so, 

1. How quickly will the crack grow? And then, 

1. Will the structure fail catastrophically? 

As stated, the customer absolutely thinks the presence of a crack is a problem. This is commonly the case when the customer is not an engineer, and even if they’re an engineer, they had no insight into the design and manufacture of the product. Answering the above questions will directly determine if the crack is or is not a problem. 

What about the case where the engineer designs a structure and during the design process must consider a structure that has cracks? This is a common practice in regulatory bodies, namely, the FAA (Federal Aviation Administration). In this case, the engineer assumes a crack or cracks exist in the designed components and must design for this potential failure mode. This is referred to as Fatigue and Damage Tolerance. The engineer establishes inspection intervals for components based on this analysis. The maintenance crew knows how many hours the component can be used before it needs to be checked for integrity and possibly replaced. 

In the next blog post, we will discuss the methods to evaluate cracked structures. 

If you’ve paid attention to new and emerging technologies in the world of simulation, you may be wondering, “What’s all this hype about Ansys Discovery?” In this blog, we will help answer this question by discussing some of the key features in Discovery, as well as what the future holds for current SpaceClaim users.

Is Discovery Modeling replacing SpaceClaim?

The older SpaceClaim interface has joined DesignModeler as a maintenance mode product in 2023R2. This means the SpaceClaim GUI will continue to be available, and Ansys will continue to perform major bug fixes. However, all new modeling capabilities are being developed within the Ansys Discovery Modeling interface.

How does the new license structure work?

The Discovery Modeling license give you access to all 3 of Ansys’ modeling applications:

• Discovery Modeling (Simulation comes with Discovery Simulation license)
• SpaceClaim
• DesignModeler

Discovery Modeling and SpaceClaim can also be accessed through Enterprise or PrepPost bundles

SpaceClaim and DesignModeler standalone licenses have been discontinued

Benefits of Switching to Discovery Modeling

I already use SpaceClaim, why should I consider learning Discovery Modeling?

If you are a SpaceClaim user, you may be wondering if it is worth your while to switch to something new. Before we dive into new features of the Discovery Modeling package, let’s talk about what this transition actually looks like:

• With Discovery Modeling license, you can simply open your existing SpaceClaim geometry in Discovery from the workbench page and pick up your projects right where you left off.

If you’re starting a new model from scratch, Discovery continues to support direct import of major external CAD formats.

• Discovery gives you access to the same features you love from SpaceClaim with a new and improved UI. Ie, Familiar tools with new and improved functionality.

Still intimidated by the new look? Discovery has a plethora of online training materials, as well as tutorials and documentation baked into the application so you can spend less time learning a new interface and more time getting work done. You can also check out DRD’s Discovery learning page for more information.

Ok, Ok, But is Discovery Modeling actually better than SpaceClaim?

So far we’ve covered that Discovery Modeling is highly accessible to existing SpaceClaim users, but is it actually better? We could spend all day discussing the various reasons people are switching to Discovery Modeling, but here are a few of the highlights:

If you have questions about transitioning from SpaceClaim to Discovery Modeling, one of our experienced staff would love to chat. Contact DRD today to see if Discovery is right for your team!

Bulk Material handling can frequently involve fluid flow that impacts the behavior of the particulate. Luckily, Rocky couples with Ansys Fluent to enable representation of the fluid flow and the particulate behavior. This coupling can be either one or two-way. One-way coupling solves the fluid flow first and exports the resulting flow field into Rocky. This means that particles within Rocky are affected by the flow, but the flow is not altered by the particles. In addition to a constant flow field, Rocky also supports transient one-way coupling where a time varying flow field can be imported. This can include a periodic repeating transient flow. Two-way coupling, sometimes called co-simulation, is also possible. In this configuration, Fluent and Rocky exchange information back and forth as the solution moves forward in time.   

One common application of coupling Rocky with CFD is to perform density-based separation. One application of density separation is to separate lightweight plastics from a stream of compost. In the example below, an air knife and vacuum system is used to remove the plastic from the denser wood particles.  

The first step in a one-way coupled model like this is to setup and solve the CFD model. This particular application has a low particulate loading, so one-way coupling is appropriate. The setup of this particular model included specification of the air knife and suction outlet flowrates, while specifying pressure boundaries for the clean outlet and compost inlet. Once solved, the developed flow field will be exported to Rocky using the Rocky Export tab at the top of the Fluent interface. Note that the Rocky Export tab is only available after installation of Rocky along with the Rocky coupling module. To perform a one-way export, first select Rocky Export, then Export one way data, and finally export current data to Rocky.

Once this is complete, the next step is to setup the Rocky model as normal, that is leaving the coupling setup to later. This includes importing the geometry, setting up the particle inlets and particles, etc. Finally, proceed to the CFD Coupling entry in the Data panel. In the Data Editors panel, select the appropriate coupling type. For this model, 1-Way > Fluent (Fluid -> Particle) was selected.

Following this selection, Rocky will prompt the user to select the Fluent to Rocky (.f2r) file. This is the header file of the rocky export that was performed from the Fluent interface. Note that the export performed earlier resulted in several files. An .f2r file, two .dat files and several .stl files representing each boundary in the Fluent model. You will always point to the .f2r file when referencing the export in Rocky.

Once the Fluent to Rocky file has been read in, a new entry in the Data panel will appear under the CFD coupling item. Clicking on this item will reveal settings for the one-way coupling.

Common options are to change the drag law applied to different particle shapes. The Rocky CFD Coupling Technical Manual has good advice on appropriate drag models to use. The Coloring tab of the Data Editors panel allows you to visualize the CFD data you have imported as shown below. Be sure to turn off this visualization when you are done as it can impact performance.

Finally, your model is ready to solve like normal. All of the typical postprocessing results you can expect from Rocky are still available to you, only now particles can be affected by the imported airflow. The workflow demonstrated here works well for transient or transient periodic one-way coupling as well. Two-way coupling does not rely on the Rocky Export tab in Fluent. Instead, Rocky will launch Fluent itself once provided the appropriate Case and Data files to start from.

I hope you found this short article useful. Check out our website for other Rocky and Ansys content.

As mentioned in Part 1 of this blog, you reduce complex 3D flows down to cross-sectional averaged values for plotting against the distance along the flow path in either of Ansys’s dedicated post-processing tools: CFD-Post and EnSight.  Part 2 of this blog will focus on the method available in EnSight.

Method 3: Query

Application: Ansys EnSight

Pro: Utilizes EnSight, which can be used for much larger models than is practical with CFD-Post

Con: Defining cross-section location can be very difficult for complex geometry

EnSight’s Query tool has a built-in feature for cycling a location over a range of values, performing a calculation as it progresses, and making a plot of that data.

The first step to use this feature is to create a location that defines the cross-section of your flow path.  For simple geometry this will just be a Clip along a particular direction, but for more complicated flow paths this could involve defining a spline path for a clip to follow.

Next, you will need to create a Variable that calculates the cross-sectional average of the quantity of interest on the clip.  For transported quantities, this should be a mass-flow-weighted average.  Unfortunately, EnSight does not have a direction-independent mass-flow-weighted average function, but one can be built in a few steps.  First, a new variable for mass flux needs to be created.

Then, the weighted average can be calculated on the clip using the SpaMeanWeighted predefined function and the MassFlux variable that was created in the previous step.

To create the plot of the averaged value as the clip progresses along the flow path, create a new Query using Query > Over time/distance, set the Sample to By constant on part sweep, select the variable that was just created, Start, set the range and number of samples, and then Create query.

Note that the plot data can be exported to a file by right-clicking on the query and choosing Data > Save CSV file.