# The Effect of Hardness upon Resistance to Penetration



## glen (Mar 24, 2008)

Effect of Hardness



> An increase in the hardness of a given thickness of homogeneous armour may result in an increase or in a decrease or in no change at all in resistance to penetration, depending upon the ratio of the thickness of the armour to the calibre of the projectile concerned. Where under-matching projectiles (those whose calibre is less than the thickness of the armour) are concerned resistance to penetration at normal impact increases as hardness increases; where overmatching projectiles (those whose calibre is greater than the thickness of the armour) are concerned, resistance to penetration at normal impact decreases as hardness increases; and where matching projectiles (those whose calibre equals the thickness of the armour) are concerned, little change in resistance to penetration at normal impact occurs over a considerable range in hardness. As the ratio of thickness to calibre approaches closer to 1:1, the effect of hardness upon resistance to penetration becomes less, and as that ratio either increases or decreases from 1:1, the effect becomes greater. The relationships are illustrated in Fig VII-4 .


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## wilbur1 (Mar 24, 2008)

Thats pretty cool glen thanks


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## merlin (Mar 24, 2008)

Is it me? Don't tell I was the only one, on reading the 'thread' title wondered 'what's this got to do with aircraft, surely this should be in Off-topc talk'!?

And besides when hardness occurs, who wants resistance!?


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## wilbur1 (Mar 24, 2008)

lol


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## Njaco (Mar 24, 2008)

Merlin I was thinking the EXACT same thing, just didn't want to be the first to bring it up!! The way the title is cut off....


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## glen (Mar 25, 2008)

LOL, There is amor on aircrafts, and 20mm cannon's penetration is important.


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## Soren (Mar 25, 2008)

You still don't understand the importance of energy concentration I see.


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## merlin (Mar 25, 2008)

Oh yes I do, my energy can be very concentrated!


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## delcyros (Mar 26, 2008)

Glen,

this chart is interesting but You have to be somehow careful in extrapolating the results from this to other conditions.

What we see here is basically an effect well known under the term SCALING - the reduction in the ballistic limit of an armor type when all metallurgical properties of the plate and projectile, the projectile shape, the impact obliquity, projectile damage, and so forth are kept constant, but the size of both the projectile and the plate are increased by a given amount. Face-hardened armors have a scaling effect that increases rapidly with a decrease in the percentage thickness of the plate's unhardened back layer when the back layer goes below about 65% of the total plate thickness (due to brittle fracture of the hard face layers being a surface phenomenon, increasing only with the square of the scale (much slower than increased projectile and armor plug weight increase), while ductile deformation and tearing of the soft back is a volume-related phenomenon which increases with the cube of the scale, in step with increased projectile weight). *The most ductile homogeneous armors only have a very tiny scale effect*, but this increases as they get less and less ductile (due to reduced ability to stretch sideways to get out of the projectile's path before the armor splits apart, which is an increasing problem for the plate material near the impact center as the projectile gets wider), as measured by a decrease in the "Percent Elongation" from the circa 25% of the best ductile homogeneous armors. For homogeneous armors, the optimum ductility increases with absolute scale-due to the sideways stretch problem just mentioned, largely caused by factors such as the metal's speed of sound and crystal size that do not change with scale. Relative scale effects due to a thicker or more oblique plate having a greater ballistic limit at any scale also require that the plate have increased toughness for a maximum BL, usually obtained by reduced hardness and increased Percent Elongation, on top of the absolute scale effects applicable to all plates. 

With other words, the six data points of the latest graph show a plate with very low % elongation, representing a low ductility and large associated scaling effects.

best regards,

edit#: partly exzerpted from Nathan Okuns article of metal properties 2002


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## glen (Mar 27, 2008)

delcyros said:


> Glen,
> 
> this chart is interesting but You have to be somehow careful in extrapolating the results from this to other conditions.
> 
> ...



That's OKUN's article, I understand scaling effects. Anyway, this issue is complex and I don't want to spend more energy on it. The better way is to exam the tanks' performance in battle field.


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## delcyros (Mar 27, 2008)

glen said:


> That's OKUN's article, I understand scaling effects. Anyway, this issue is complex and I don't want to spend more energy on it. The better way is to exam the tanks' performance in battle field.



absolutely correct, its part of an exzerpt You could find interesting. I choosed that to show that the relationship between t/d and hardness is dependent on another factor, which is not substantiated on the tests (...and of course I cannot explain as well as Okun can). Therefore we don´t have conflicting approaches here.

Btw, Glen, You seem to be fond of armour topics. Do You have M79APCLC? You can download it on Mr. Okuns page. It calculates armour penetration on the base of the 3" M79 AP-projectile (uncapped and w/o filler) with a very high degree of reliability. The only problem You will be conflicted with is to determine the exact quality of armour, which appears to be generally above 1.1 for tank armour.


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## glen (Mar 27, 2008)

delcyros said:


> absolutely correct, its part of an exzerpt You could find interesting. I choosed that to show that the relationship between t/d and hardness is dependent on another factor, which is not substantiated on the tests (...and of course I cannot explain as well as Okun can). Therefore we don´t have conflicting approaches here.
> 
> Btw, Glen, You seem to be fond of armour topics. Do You have M79APCLC? You can download it on Mr. Okuns page. It calculates armour penetration on the base of the 3" M79 AP-projectile (uncapped and w/o filler) with a very high degree of reliability. The only problem You will be conflicted with is to determine the exact quality of armour, which appears to be generally above 1.1 for tank armour.



delcyros, my apprehending of scaling effect of homogeneous armors is below:

Hardness and toughness conflict each other under human's amor technics(perhaps not for Martian,lol), if you increase amor's hardness, the toughness will drop, vice versa. But these two factors are both helpful for resistance ability, the issue is how we trade off.

There are two scaling effects: absolute scale and relative scale.

For a certain amor,such as classB(the most ductile homogeneous armors ), the absolute scale factors are constant because its speed of sound and crystal size are fixed. So if we compare classB with low Elongation plates, anything else(amor thickness, projectile caliber.....) being equal, the scaling effect of classB is smaller and classB outperforms others.

But there are some other factors such as thickness, oblique degree which are also important for amor's sideways stretch ability. For example, anything else (projectile caliber....)being equal, when thickness increases, the thick amor will do better in sideways stretching than previous thin amor. When oblique increases, amor will get better sideways stretching too. Thickness and oblique are easliy to change, so we call "relative scale" factors. 

Toughness beneifits from high Elongation(absolute scale ), big thickness( relative scale), high oblique( relative scale) and so on.

As OKUN said, " the size of both the projectile and the plate are increased by a given amount", high Elongation(ductile) amor(>=25%) only have a very tiny scale effect, that's right,however, IMO, if only projectile caliber increases, thickness still unchanged, the high Elongation(ductile) amor will have obvious scale effect because amor have nothing to deal with the increase of projectile caliber, if amor thickness or oblique increases, amor will overcome the greater carliber.

This link proves my opinion. 
Effect of Hardness

The thickness of amor remains 1½"(37mm) while projectile caliber increases from 20mm to 37mm and then 57mm.
Facing 20mm projectile, the amor's toughness(sideways stretch ability) is good due to it's enough thickness, therefore, if you increases the hardness, the resistance will be better. Of course in this period the toughness will decrease, but it's still above "base line".

Facing 37mm projectile, the amor's toughness(sideways stretch ability) is average or aound "base line", the decrease of toughness counteracts most of improvement of hardness.

Facing 57mm projectile, the amor's toughness is not good, below "base line", when you increases hardness, the toughness will be much worse, and then the improvement of hardness is not only completely counteracted but also "over counteracted".



As OKUN said, 25% Elongation amor has little scaling effect(both caliber and thickness increase), that's right, because the greater thickness counteracts the greater caliber. But for low elongation amor, the increase of thickness is not enough,then scaling effect comes out.

It seems that OKUN also said that below 8" barliber(203mm) which is above all tank gun's, there is a fuction:NBL = {1 - [1 - (%EL/25)0.5](D - 8)/8}(NBLSTS)(AVERAGE ARMOR QUALITY).....

It dosen't mean smaller caliber/thin amor has nothing to do with scaling effect. I think the navy amor is quite ductiler and thicker than tank amors, so navy thick amor's toughness is enough in front of smaller caliber(<203mm), so we can increase the hardness safely just like 37mm amor hit by 20mm carliber. However, for infriorer and thiner tank amor, the toughness is not enough when hit by 100mm,122mm,152mm... So the scaling effect is still with tank amor/gun issue.

BTW, as you can see the link is of "normal impact", therefore Panther's(80-85mm) and T34's(45-47mm) slope amors(55-60 degree) has much less scaling effect than vertical 80/45mm amor. All in All, sloped thin amor design is excellent, it can use high hardness low toughness plate without obvious scaling effect due to the high obliquity especially facing small caliber projectile.


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## Soundbreaker Welch? (Apr 6, 2008)

Yeah Merlin, the title sounded odd to me too, lol.


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