Fracture Mechanics & Fatigue - Lunch & Learn
Introduction
Good afternoon, everyone. Thank you for attending our seminar/webinar on fracture mechanics. Today, we'll be discussing fracture mechanics and ANSYS in version 16. ANSYS released three versions back-to-back: 16.0, 16.1, and 16.2, each with new fracture mechanics capabilities. These enhancements are available in both ANSYS Classic and ANSYS Workbench.
What is Fracture Mechanics?
Fracture mechanics is the study of flaws and cracks in materials, focusing on crack growth and lifetime estimation. The total fatigue life of a component is the sum of the number of cycles for crack initiation and crack propagation.
Key Concepts:
- Crack initiation and propagation are critical aspects of fatigue life.
- In some components, crack initiation takes up 90% of the total time, while in others, like BGA solder joints, crack propagation takes 90% of the time.
- Strength of materials approaches do not account for the presence of a crack, leading to infinite stress at the crack tip.
Stress Intensity Factors
When dealing with cracks, we use measures like K1C or J1C, which are material properties similar to Young's modulus. ANSYS calculates the stress intensity factor at the crack tip, which indicates how much stress is concentrated there.
Important Points:
- The stress intensity factor reaches a critical value, leading to immediate fracture or crack propagation.
- Cracks can cause significant financial impact, as seen in industries like aviation and defense.
Historical Cases
Several historical cases highlight the importance of understanding fracture mechanics:
- Comet Airplanes (1950s): The first pressurized cabin airplanes had square windows, leading to stress singularities and crashes.
- Liberty Ships (WWII): Some ships cracked due to stress intensity factors changing with temperature.
- ICE Accident (Germany): Cracks in train wheels led to reduced speeds for safety.
Fracture Mechanics Analysis
In fracture mechanics analyses, we record crack length on the y-axis and the number of cycles on the x-axis. The slope of the curve, dA/dN, is plotted against the stress intensity factor range.
Analysis Steps:
- Perform a stress analysis to determine the state of stress and stress intensity factors.
- Use a failure criterion to assess if a crack will propagate or remain stable.
ANSYS Capabilities
ANSYS has made significant improvements in fracture mechanics capabilities, including:
- Unstructured mesh methods, allowing for more flexible crack modeling.
- New measures like T-stress and C-integral for specific scenarios such as creep cracking.
- XFEM (Extended Finite Element Method) for meshless crack modeling.
Conclusion
In conclusion, ANSYS Workbench provides powerful tools for setting up fracture mechanics problems. We offer a two-day class on fracture mechanics and fatigue for those interested in learning more. Visit our website at www.ozeninc.com for more information and resources.
Questions?
If you have any questions, feel free to ask. Thank you for attending, and we hope to see you in our future webinars.
Fracture Mechanics & Fatigue - Lunch & Learn 9 17 2015 Alright, well good afternoon everyone. Thanks for coming to our seminar/webinar on fracture mechanics.
Today, I'll be discussing fracture mechanics and ANSYS in version 16. This year, ANSYS released three versions back-to-back: 16.0, 16.1, and 16. 2. Starting with 16.0, they put new fracture mechanics capabilities in both ANSYS Classic and ANSYS Workbench.
Fracture mechanics is the study of cracks and flows in materials, primarily dealing with crack growth and lifetime estimation. Typically, the total fatigue life of a component is the number of cycles for crack initiation plus the number of cycles for crack propagation.
In ANSYS, we can look at crack initiation and propagation. The fatigue part ties in with crack propagation. Initiation plus propagation is the total fatigue life. In some components, it may take 90% of the total time to initiate the crack and only 10% for crack propagation.
However, in other applications like BGA solder joint reliability, it may be the opposite. When a crack is present, the strength of materials approach doesn't anticipate the infinite stress at the crack tip.
Instead, we use different measures like K1C or J1C, which are material properties similar to Young's modulus or the ultimate strength of a material. ANSYS calculates the stress intensity factor at the crack tip, showing how much it hurts at the crack tip.
When this stress intensity factor reaches a critical value, a crack will start propagating or there will be immediate fracture.
The reason we use ANSYS for fracture mechanics is to determine how much it hurts at the crack tip, allowing us to make a judgment call on whether the crack will propagate or not. Cracks can occur in various ways, such as from imperfections, existing cracks, or damaged areas.
As engineers, we must first determine the state of stress and stress intensity factors. Then, we need a failure criterion to determine if there will be immediate fracture or if the crack will start propagating or stay as is.
In the 1920s, Alan Griffith, known as the father of fracture mechanics, wrote a paper stating that stress times the square root of crack length is a constant. This was a major finding, and it wasn't until the 1950s and 1960s that people started using it.
When dealing with cracks, we also need to consider mode one, mode two, and mode three. Usually, cracks propagate in mode one, where the stress is always orthogonal to the plane of the crack.
ANSYS has made significant improvements in fracture mechanics capabilities, especially with the new technology in version 16. 0. The unstructured mesh option allows for more flexibility in meshing the crack tip. In 16.0, a new feature called T-stress was introduced.
T-stress is the stress acting parallel to the crack basis and can have both negative and positive values. Negative T-stress values promote fracture, while negative T-stress values result in larger plastic zones.
The material force method provides parameters for global material and dissipation forces that yield near-critical driving force, applicable to elastic, viscoelastic, and plastic materials. C-integral, introduced in 16.0, can be used to evaluate the crack tip fields under creep deformation.
The unstructured mesh method is a numerical tool used for evaluating fracture mechanics parameters on unstructured hex or tet element meshes. These new features in 16.0 help with crack initiation determination and crack growth simulation.
The vcct-based interface element method, cohesive zone element method, and xfam are used for crack initiation determination and answering questions about which way the crack will run and how it will run.
Xfam is a new method introduced in 16.0 that allows for meshless crack analysis, even when there isn't a crack right between the two elements. The crack may be running inside the elements. In ANSYS Classic, specific element types are used for different methods.
Solid 186 and solid 187 elements are used for C-integral, for example. Now, let's see how to put a crack in ANSYS Workbench. First, create a coordinate system at the desired location. Then, make the x-axis normal to the surface using the principal x-axis option and hit point normal.
Next, insert a fracture and create a crack using the local coordinate system. Specify the major radius, largest contour radius, and crack orientation. ANSYS will automatically align the x-axis normal to the surface and perpendicular to the plane of the crack. Now, generate the crack mesh.
ANSYS will automatically create a mesh around the crack. Finally, solve the problem and analyze the results, including stress intensity factors, J-integral, and vector principal stresses. These results can help determine if the crack will propagate or not.
Thank you for attending the seminar/webinar. If you have any questions, please feel free to contact us. We also have PDF files and recorded webinars available on our website.
