Joe Galliera

Terms used in analysis - Set 3 - #19-29

Blog Post created by Joe Galliera Employee on Feb 15, 2011

I thought it might be a good idea to provide some basic definitions of terms that are used in analysis that are good to know to be able to communicate intelligently about Simulation.  These are not meant to be definitive technical definitions but more fundamental knowledge of these terms (i.e. they are mostly coming off the top of my head as I'm typing).  If you believe that I am off on any of these, please let me know.

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Now that we learned about different stresses in the previous 2 sets, I'm going to explain some ways that they can be used to determine failure.  There are many types of mechanical failure, such as: yielding, fatigue, buckling, thermal stress, impact and creep; although there are also some other types of mechanical failure that cannot be predicted well by computational methods, such as corrosion and wear, so these will not be discussed.  In addition to something fracturing or breaking, a failure could also have said to occur if the body deforms or displaces more than its tolerance.

 

19. fatigue - occurs when an object is subjected to repeated loading and unloading that cause microscopic cracks to form initially on the surface and after spending about 95% of its time as a very small crack it will reach a critical size in which it will suddenly and violently fracture with the ability to cut through very thick structures (this is because the stress concentration factor of the growing crack goes exponential).  Fatigue can also sometimes occur due to internal defects in the material crystalline structure.  Tell-tale signs of a fatigue fracture are both dark and light areas showing slow crack growth areas followed by areas of granular sudden fracture; other visual features are from the crack growth striations called "beach marks" that look like sand washing up onto a beach.

 

20. high-cycle fatigue - greater than 1000 loading cycles and upwards of 10 million.  Max stresses are typically less than yield.  Tensile and shear stresses are important for high-cycle fatigue so focus is placed on keeping them low, and the Fatigue module in SW Simulation Professional can help determine the amount of damage a part undergoes by comparing the alternating stresses to an S-N material curve.  Cumulative damage of greater than or equal to 1 or 100% predicts fatigue failure.

 

21. S-N curve - characteristic material curve for fatigue (S for stress, and N for number of cycles) which is obtained by cyclically loading a sample material coupon and counting the number of cycles to failure; many samples are needed to generate a good curve because cycles to failure is not consistent.  Surfaces of coupons are typically well polished to reduce the surface microcracks, so this should be taken into consideration when comparing to actual design surface.  It's frequently hard to obtain a S-N curve for a given material (because of the large number of samples needed and expensive time for testing), except for well studied materials such as metals used in the aerospace field that has a published handbook of material curves formerly called MIL-HDBK-5 and is now called MMPDS handbook (Metallic Material Properties Development and Standardization).  ASM International publishes an Atlas of Fatigue Curves that SW Simulation uses for its material library and are denoted by (SN) at the suffix of the material name.  A common practice is to find a similar material and scale the values based on elastic moduli.

 

22. low-cycle fatigue - cycles less than 1000 and typically stresses go beyond a material's yield strength into plasticity.  Strain-based methods are a used for low-cycle fatigue prediction, and SW Simulation does not have a good solution to handle this type of fatigue well.

 

23. yielding - typically for ductile materials where the stress goes beyond the yield strength of the material.  You can also look at where strain goes beyond 0.2% or 0.002.

 

24. buckling - typically a slender body under a compressive stress (look at 3rd principal stress) can lead to buckling, where the body's transverse direction stiffness goes to zero causing a sudden and dramatic fracture.  The causes of buckling can be compounded by off-axis loading, geometry or material non-uniformity, and/or elevated temperatures.  Linear buckling (based on characteristic mode shapes from resonances) is handled by SW Simulation Professional where the value calculated is the BLF, buckling load factor, which is a ratio of critical buckling load to the applied load; linear buckling is a very non-conservative method meaning that you may get BLF values much higher than what happens in reality.  A better test of buckling is to run a nonlinear analysis where stress softening and geometry deformation is taken into consideration.

 

25. thermal stress - stresses imparted on a body due to the expansion (heating up) or contraction (cooling down) of a material, especially where bodies of different material (and thus differing thermal expansion coefficients, symbolized by Greek alpha a) are in contact with one another.

 

26. impact - a collision of 2 or more bodies causes a high force or shock over a very short period of time.  To analyze the contact of bodies together, one should perform a Nonlinear Dynamic analysis; if one can determine the force over time imparted and the area of contact, then it is much more computationally efficient (i.e a lot less time) to solve this as a Time History type of Linear Dynamic analysis.

 

27. static analysis - How to know if it is a static or dynamic analysis?  Static analysis assumes that the load is constant (or applied over a very long period of time) and the force vector (both magnitude and direction) do not change.  If the first natural frequency of the load has a period of more than 3 times the first natural frequency of the body in question, then the analysis could be run as a static problem.  (Since the period, t, is one over the frequency, f, that is t=1/f, another way to figure out if it's static is to say if the frequency of the load is less than 1/3 of the first natural frequency of the body.)

 

28. creep - refers to a permanent deformation that body takes on under constant loading typically at an elevated temperature.  If the body gets to half of its material's melting temperature, one should definitely check for creep.  I have a personal story where a friend had a TV sitting on some plastic shelves that were near a window; over time because of the sunlight heating the plastic you could visibly see the deformation of the shelf down under the weight of the TV that it didn't originally have when he put it on the shelf.

 

29. deformation - when a body displaces more than an allowed tolerance it could said to have failed even if the stresses did not cause the failure.  The point to make here is that it is also very important to look at and understand the displacements in addition to the stresses.


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