Use of Percent Elongation Numbers 9

[colar]

Does anybody have an engineering method for actually putting percent elongation numners to use? We all know how the numbers are arrived at, but how can the numbes be used in a mathematical formula to help an engineer in selecting a material?

This stems from a problem we are currently having at our plant. We have recieved some Ductile Iron castings which are unusually hard. They are supposed to be 65-45-12. Our accepted hardness range is 157 to 217 HB for these parts, but we are seing some much higher. The question put to me as the engineer in charge of the product they are used on is "how hard is to hard?" and "so what if their hard, does that mean they will fail?". I can easily draw a line in the sand and say "they are out of spec therfore...", but I would feel more confident if I could say " the corresponding elongation of these castings based on their hardness tells me that they will fail".

Does anyone have some insight on how someone can use percent elongation and ductility to analyse a material?

 

[vonlueke]

Some sources indicate that a material can be considered brittle if the strain at rupture (break) is less than or equal to 5%.

A few sources indicate if the strain at rupture dips below 10%, one should consider it a flag to start taking a closer, more careful look at the material properties and analyses.

Brittle materials generally have low toughness (high stiffness but low strain at rupture; therefore the total deformation energy they can absorb before breaking is low compared to more ductile metals). Thus, stress concentration factors become even more important in the analysis of brittle materials.

Juvinall, Fundamentals of Machine Component Design, 2 ed., Wiley, 1991, p. 224, suggests the factor of safety for analysis of brittle materials should be approximately doubled or tripled relative to what you would use for ductile materials in a similar situation.

If the stresses are cyclic, then the analysis should be performed using the fatigue strength of the material rather than the yield or ultimate strength.

Therefore, if you can perform the careful analyses required on a more brittle material, properly in all aspects, using the higher factors of safety required, and still get a positive safety margin, then the analysis would indicate the part is OK. Good luck.

 

[TVP]

Can you provide some additional details on what application the castings are used for? Also, why are the castings so much harder? Is the elongation substantially lower than 12% when the parts are higher than 217 HB? vonluecke provided you with some good information, but there is another level of analysis that you should be performing to fully understand your current problem.

ASTM A 536 is the standard industry specification for ductile iron castings, and it uses the nomenclature that you stated, so I will assume you are using this spec. Chemical composition is subordinate to mechanical properties in this spec, so it is possible that you are getting castings with a relatively high carbon content, which will increase hardness, but also decrease elongation and fracture toughness. Are you specifying any heat treatment for these castings? This is another area that can lead to increases in strength/hardness, but decreases in elongation and fracture toughness. Are you actually receiving documentation on the tensile test requirements (Section 4 of A 536)?

The data that I have indicates that anything higher than your upper limit of HB 217 will result in elongation lower than 12%. If your application requires good fracture toughness (impact loads, variable amplitude fatigue loading, etc.) then this could be a problem. Fracture toughness, which is usually measured by the Plane Strain Fracture Toughness, K1c, of a material, usually correlates with elongation-- high toughness materials have high elongation, and vice versa. This is definitely true within a specific material category such as ductile iron-- increasing elongation indicates higher toughness, since microstructural effects that improve one, improve the other.

 

[CoryPad]

colar,

Your post discusses two materials properties, ductility and hardness, which are not interchangeable. vonlueke gave you an adequate method for deciding initially when you need significant ductility, but I will try to go further.

Ductility and elongation are other names for plastic strain. Plastic strain is useful for deforming materials into a desired final shape or as a damage accomodation mechanism. Since you are using a cast material, you are interested only in the latter. One way to incorporate plastic strain into your design is to estimate deformation and energy/work during overload events. These events induce plastic strain, and this is when you want damage tolerance. The work per unit volume during plastic straining is:

w = integral (stress * strain)

Thus, you can use mechanics (software or hand calculations) to decide how much strain is needed to allow gradual failure + energy dissipation during overload.

The above was for single event loading. For cyclic loading, plastic strain accumulated during a lifetime can be used as a failure criterion. The Coffin-Manson relationship shows:

epsilon_p = epsilon_f * (2 * N_f)^c

where

epsilon_p is the plastic strain amplitude
epsilon_f is failure strain
N_f is the number of cycles to failure
c is a fitting parameter

Thus, you can estimate how much plastic strain is needed to provide the required lifetime.

Lastly, concerning hardness and "how hard is hard" and "does that mean they will fail?". This is best answered by considering fracture mechanics - fracture resistance scales with the inverse square of yield strength. So, if your yield strength is directly proportional to hardness, then your material has severely degraded fracture resistance if it is allowed to be stronger/harder than specified. For a flaw sensitive material like cast iron (even the ductile variety), I would not allow material to be used that exceeds the maximum hardness.

Good luck.

Cory
cpadfield@omnimetalslab.com

 

[CoryPad]

After reviewing my earlier post, I thought I should clarify a point. I stated that fracture resistance scales with the inverse square of yield strength. This assumes two pieces of a material with everything else essentially constant but yield strength. Obviously you can get ultra-high strength steel with higher toughness than a low strength steel, cast iron, Al alloy, etc. The relevance of my point is that if you are receiving different batches of the same material and one is much harder/stronger than another, then you will have much reduced fracture resistance.

 

[colar]

Thanks for your posts. Some more info; These castings are to be used on agricultural equipment. Bearings are pressed in to machined cavities in these parts. In some of the hard ones, we have had cracks form from this process. They are subject to all manner of loads, from impact to fatigue. However the magnitudes of thse loads are fairly small. We do not specify any heat treating. Also, we do not recieve mechanical property results from our supplier, only hardness readings (now). We have always done our own in-house hardness tests.

I get the feeling from reading the posts, that a number like elongation may only serve as qualitative number. The number itself does not seem to have any quantitative use. It is a little bit frustrating as an engineer to not be able to use a quantified characteristic like this.

 

[TVP]

colar,

If you are considering a press-fit operation, then elongation certainly CAN be used in a quantitative manner. You want the amount of deformation that takes place during the press-fitting to be below some nominal value, which at the most would be equal to the maximum elongation the material can undergo before fracture (cracking). Usually, one would specify the deformation to be a fraction of the fracture strain/elongation of the material.

Calculating the change in diameter of a hole after press-fitting will give you a certain % elongation that the assembly requires. You need the castings to have more elongation than this minimum requirement, which needs to account for variations in casting dimensions, bearing tolerances, etc. This is precisely why you should be specifying on your casting drawings a standard specification like ASTM A 536, grade 65-45-12, which outlines the minimum mechanical property requirements, including elongation. You should also place the hardness restriction of 217 HB max, and consider placing additional restrictions for quality and soundness (inclusions, porosity, etc.).

It sounds like your casting vendor is giving you inferior castings, with the yield and tensile strength, and thus hardness, being in excess of your specification, with the resulting elongation being WAY TOO LOW!!!! I would start by rejecting anything in excess of your hardness specification. Next, I would perform a correlation study that takes into account the following items:

1. Mechanical properties of separately cast test specimens (see ASTM A 536)
2. Mechanical properties of specimens machined from actual castings
3. Microstructural evaluation of both separately-cast test specimens, and machined-from casting specimens--grain size, graphite shape appearance (degree of nodularity), inclusion content, porosity (both microporosity due to solidification shrinkage and macro due to entrapped gas), etc.
4. Bearing assembly/press-fit-- do you see a definite transition from acceptable performance to unacceptable performance as the strength/hardness increase, and elongation decreases? This could even be performed on the separately cast test specimens if you didn't want to waste a lot of castings.
5. Some type of component or assembly test that simulates actual use conditions-- impact, fatigue, etc. Do you see a noticeable difference in performance that can be correlated with the mechanical property information? I assume that cracks that appear during press-fit are not subsequently used, but are there marginal parts (say from 220-250 HB) that may not produce visible cracks (perhaps die penetrant testing or magnetic particle testing (Magnaflux) should be considered for testing purposes) but do not pass an accelerated life test, due to insufficient fracture toughness and/or fatigue strength?

If you consult very many references on this topic, I think you will see that elongation is usually a decent predictor of field performance when it comes to impact and fatigue loading, especially for a material like cast iron. You also may want to consider adding a simple impact test like a Charpy pendulum impact test (ASTM E 23), which would highlight immediate differences in toughness. Also, you may want to consider placing restrictions on the chemical composition that is used, especially for impurity elements like sulfur and phosphorus, and possibly on carbon.

 

[Viktor]

Dear Colar

Please do not be frustrated.
The elongation is a very valuable measure of material ductility.
In general, measurements of ductility are of interest in three ways: (1)To indicate the extent to which a metal can be deformed without fracture in metalworking operations. (2) To indicate to the designer, in a general way, the ability of the metal to flow plastically before fracture. A high ductility indicated that the metal is ‘forgiving’ and likely to deform locally without fracture should the designer err in the stress calculations or the prediction of severe loads. (3) to serve as an indicator of changes in impurity level or processing conditions. Ductility measurements may be specified to assess material ‘quality’ even though no direct relationship exists between the ductility measurement and performance in service.

The conventional measures of ductility that are obtained from the tension test and the engineering strain at fracture (usually called the elongation) and the reduction of area at fracture. Both of these properties are obtained after fracture and both elongation and reduction of area usually expressed as a percentage.

Now back to your problem. There two types of ductile iron 65-45-12 – compression and torsion. You should know which one you’ve got. Please note that the chemical composition for such a material is as follows

Ductile iron grade 65-45-12
Component Wt. %


C 3.6 - 3.8
Ce 0.005 - 0.2
Cr 0.03 - 0.07
Cu 0.15 - 1
Component Wt. %


Fe 90.738 - 94.175
Mg 0.03 - 0.06
Mn 0.15 - 1
Mo 0.01 - 0.1
Component Wt. %


Ni 0.05 - 0.2
P Max 0.03
S Max 0.002
Si 1.8 - 2.8



Carbon represents the total carbon in the above composition. Cerium is an optional constituent in ductile iron. Most ductile irons are specified based on mechanical properties and have loosely defined compositions. For example, 65-45-12 ductile iron is specified to have a minimum tensile strength of 65 ksi (448 MPa), a yield strength of 45 ksi (310 MPa) and an elongation of 12%. Determined for a single temperature of ductile iron, heat treated to approximate various standard grades. Properties were obtained using test bars machined from 25 mm keel blocks.

The standard does not specify the hardness of this material. Moreover, broad ranges of C and Si may result in different hardness (this can be also affected by endless number of metallurgical characteristics of the material). Therefore, to understand if your material corresponds to the specification you should measure the tensile strength, yield strength and elongation and compare these characteristics with the spec.


Viktor P. Astakhov

http://viktorastakhov.tripod.com