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i was in one of my classes today (engineering materials), and the section my professor covered in class today was specifically about "Failure," and i was thinking about this (turbo manifold failure and AshsZ's "manifold project" during the class lecture. here are some thoughts about the subject. there are basically four main types of material "fractures": 1)failure - fracture due to too much force/ load, 2)impact - fracture due to too much energy transfer, 3)fatigue - fracture due to too much viration and 4) creep - fracture due to too high of temperatures. obviously the latter two (fatigue and creep) are the most relavant types of fractures when considering the reasons for turbo manifold cracking (or partial fracture); then *maybe* the failure-type comes into play. CREEP. probably the most relavant and obvious is creep failure. assuming a constant stress (let's just say the force or weight of the turbo(s) and relavant comonents on the manifolds/ headers), the higher the temperature, the less stress the material can withstand. therefore, a material more resistant to heat will lessen the amount of stress and therefore chance of cracking. (this was a duh, but i might as well have mentioned it). FATIGUE. according to my book, for those that care: Materials Science and Engineering: An Introduction (6th ed.). W.D. Callister (Jr.), "fatigue is a form of failure that occurs in structures subjected to dynamic and fluctuating stress". imagine all of that (i.e. turbo movement, engine movement, exhaust flow) in addition to CREEP failure, and other stresses placed on the turbo manifold. also, regardless of the type of stress applied (axial: tension-compression, flexural (bending), or torsional:twisting), fatigue will ultimately occur regardless of material's fatigue strength (stress level at which failure will occur for some specified number of CYCLES). therefore, a material more resistant to fatigue fracture will prevent it from happening, but only to a certain point; what that certain point is depends on 1) the material, 2) the stress (or force) applied to the material and 3)all of the other stresses acting on the material (i.e. creep fracture, failure). it's a little easier to understand, now, why cracks in turbo manifolds are inevitable: it's just a matter of time. case in point: greddy's turbo manifolds for the Z32 are renown for cracking, but also, the stock OEM manifolds have although much longer than most aftermarket manifolds have been known to crack. however, there's a section in my class book specifically on "Crack Initiation and Crack Propogation". there is a process of fatigue failure (for cracki propogation) characterized by three distinct steps: 1)crack initiation, wherein a small crack forms at some point of high stress concentration; 2)crack propagation, during which this crack advances incrementally with each stress cycle; 3) final failure, which occurs very rapidly once the advancing crack has reached a critical size. so the material (as well as design) that best suits a turbo manifold will 1)have higher reistance to FATIGUE, 2)have higher resistance to CREEP and 3) have higher resistance to FAILURE in general compared to other materials. just some thoughts from my class. hope you found it interesting.
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