Total energy to catastrophic failure 11

[Manifolddesigner]

Hello all,
I’m wondering if there is information on common metals for “total energy” to catastrophic failure. E.g. The area under the stress strain curve.
It seems to me this information would be really useful. Back when I was in school when I chose a material I’d just look at the bottom of the chart, the one with the highest yield strength number. That material always had some ridiculous hardness and high strength numbers. When I’d try to use it they’d say “you can’t. it’s too brittle, you need to use something softer”. But they could never tell me what was too soft or too hard.
Example of two materials w/ same yield str.
Steering spindle.
1060 steel Q&T @1000F. Yield str 97ksi ultimate 140 17% elongation, 277 brinell
4140 steel Q&T @ 1200F. Yield str 95 ksi, ultimate 110. 22% elongation, 230 HB

Let’s assume they cost the same and the machinist will hate me with either material and ignore all other “I wouldn’t use this one becaue…”.

A bent spindle is bad, and would require a refund of the customer’s money. But a broken spindle is VERY bad (call the coroner). Even though the 1060 has a higher ultimate str, I wonder, in an impact situation, which one *really* has a higher factor of safety against death? There must be many more examples of materials w/ similar numbers, where one is considered “brittle” and another not.

I recently tried to cut a piece of carbide. The abrasive chopsaw wasn’t having it, but a chisel and a hammer owned it. A high speed steel piece would certainly have bent and deformed, but not broken. I have to assume carbide is far superior in yield and ultimate strength numbers; however, its “total energy to catastrophic failure” is much less than an equivalent piece of steel.

Is their data for Total Energy to catastrophic failure written anywhere?

Jason

btw, I thought I just posted something like this last night, but it doesn't show up?

 

[SJones]

Fracture toughness and fracture mechanics are the realms that you are entering. Try searching on the following:

CTOD
J-integral
R curve
Failure assessment diagram

You should get a first step onto the long and winding road!

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[CoryPad]

For large-scale plasticity, area under the stress-strain curve is used in some industries. Integration of the actual data, or simplifications (triangle for elastic region, rectangle for plastic region) are used.

For brittle fractures, fracture toughness is the appropriate measure. Fracture toughness is the resistance to crack propagation, and is related to strain energy release rate. This is a complex topic, and SJones has given you some good keywords.

Mixed in there is impact energy, usually determined by the Charpy impact test. This is a simple energy to fracture a notched (but not cracked) bar, and is commonly used to quantify the ductile to brittle transition temperature for steels.

Not an easy subject. Good luck with your education.

 

[imcjoek]

Another good search term here would be "fatigue strength"; particularly as it relates to terms like notch sensitivity, surface finish, hardness, etc.

 

[gariartola]

I'm 100% with what our eng-tips colleagues sugested, and just wanted to add one point on the change of the stress-strain curve with deformation speed: the material's curve will change under impact conditions. A related article to show what I talk about:

http://www.posco.com/homepage/docs/...el Sheets at the Intermediate Strain Rate.pdf

 

[unclesyd]

Here are two websites that you can check out for the fatigue aspect. The sites will guide you into the other aspects of your OP.

http://www.fatiguecalculator.com/

https://efatigue.com/

 

[Wrenchbender]

The short, simple answer to the question posed would be to look at Charpy impact energy (ASTM E23) as a qualitative comparison among candidate materials. But life is never simple. A more complete picture would be to also look at the material property called fracture toughness, which is covered by several ASTMs and the key words given in a previous post.

(The Charpy V-notch test has been a useful engineering tool for a hundred years, the databases are huge, and the test is easily performed. It has units of energy as does the area under the curve in a uniaxial tensile test. However CVN does have some limitations so for any quantitative predictions on fatigue life or catastrophic failure at less than yield stress, the science of fracture mechanics provides the answers. Any data you can find on ‘plain strain fracture toughness’ or KIc for the steel alloys and the carbides you mentioned would provide insight. For any design work, a few other factors to consider are service temperature, applied strain rate, chemical purity of the material, and sizes of the preexisting flaws on the part.)

 

[Manifolddesigner]

Bestwrench, you mention that the "databases are huge".
Any suggestions where to find said huge databases?

JM

 

[Maui]

Manifolddesigner, as others have correctly stated the question that you have posed can best be addressed by the discipline of fracture mechanics. There are two main approaches that are typically used to analyze the ability of a material to resist the formation and propagation of cracks: Linear Elastic Fracture Mechanics (LEFM) and Elastic-Plastic Fracture Mechanics (EPFM). Fracture mechanics can be used to determine how much energy a material is capable of absorbing before a crack or flaw of a given location, size, orientation, and geometry is able to propagate in an unstable manner, resulting in the catastrophic failure of the component. In Irwin's notation, there are three fundamental or standard modes of failure: modes I, II, and III. Mode I denotes a symmetric opening with the relative displacement of material normal to the fracture surface. Modes II and III denote antisymmetric separation through tangential relative displacements, normal and parallel to the crack front respectively. In the same manner that the points on a curve in three-dimensional space can be represented as a linear combination of the Cartesian position coordinates x, y, and z, the manner of crack propagation in a material body can be thought of as a similar linear combination of these modes. Both LEFM and EPFM principles have been successfully applied in analyzing the failure mechanisms under Mode I loading conditions. This success can be attributed to the ability of K and J-integral techniques to unambiguously describe the crack driving forces resulting from the crack tip stress fields produced during loading of elastic and elastic-plastic materials respectively. In order to accurately apply LEFM principles the physical size of the plastic zone surrounding the crack tip significantly smaller than the remaining unbroken ligament which holds the specimen together. This is often referred to as the small scale yielding criterion and usually results in the requirement that the estimated radius of the plastic zone, a calculation based on LEFM considerations, be much smaller than (usually 1/12 to 1/15) the size of the uncracked ligament. The application of J-integral techniques requires that two conditions be met:

1) the deformation theory of plasticity must adequately describe the small strain monotonic loading of a real material and

2) For the material under consideration the regions in which finite strain effects are important and in which the relevant microscopic processes occur must each be contained well within the region of the small strain solution dominated by the singularity fields.

The first requirement will be satisfied if proportional loading exists everywhere ( i.e. stress components changing in fixed proportion to one another) while the second, analogous to the small scale yielding criterion in LEFM, requires a complete understanding of the conditions required for j-dominance for the given geometry and loading conditions. A good presentation of the J-integral technique can be accessed here:

http://ocw.mit.edu/NR/rdonlyres/Mat...0A926-6353-44EE-8950-B862361D70A4/0/nlefm.pdf

Maui

 

[Wrenchbender]

WRT Charpy V-notch (CVN) data requested 2 posts up, there are tabulations in various handbooks and textbooks, e.g.:
1. ASM Handbook, Vol 1, Properties & Selection…, pp. 431 – 447, and elsewhere therein.
2. Hertzberg – Deformation and Fracture mechanics … has a good collection
3. journal papers
4. web searches on [Charpy + alloy number]

As noted previously, CVN tests are not the end-all and be-all of quantifying toughness, but are an easy lab test which accounts for the prevalence of data as opposed to the more desireable fracture toughness values K(or J)-Ic.

 

[mfgenggear]

Manifold Designer

Super good question, deserved a star

very very impresive explanations from all

 

[Manifolddesigner]

Thanx guys, I ordered set of ASM handbooks. Book 1 and book 20 appear very interesting regarding...uhhh..."material selection".