CS stress at tang to blade corner, Cold Steel

F

fnc

Joined
Nov 13, 2005
Messages
328
Have read some postings that Cold Steel knives
fail at the junction of the tang and blade. Some
possible causes are 1) heat treatment 2) overly
abusive prying 3) blade does not taper to the
tang, but has a corner where stress would build.

I did a search on "SRK stress" and came up with
several threads and here is a long, long one.
Please read the first page or look at the picture.
http://www.bladeforums.com/forums/showthread.php?t=328256&highlight=stress+srk

I am intersted in the SRK specifically, but also other
Cold Steel knives. Has CS changed the corner
junction of blade and tang to a taper junction?

frank
 
While that isn't ideal from an durability standpoint, others have done similar work with no issues so likely it was a heat treatment issue in those that failed, thus a more redial design may have prevented the problem. As far as I know they have not changed the design. I severely doubt they will anytime soon considering the problems at Camillus.

-Cliff
 
What's interesting is that the carbon V is the same steel that is used in the Becker line of knives. I remember reading about a few of the Becker knives breaking as well. I own a BK-7 and have done some fairly heavy work with the blade and havn't been let down yet. I do carry an extra fixed blade (Mora) and a SAK with me, when I'm carrying the Becker, so I don't worry too much.
 
I have seen erratic performance with that steel, in both the Cold Steel and Becker blades. However it is hard to find fault with its composition. It is likely just a processing problem.

-Cliff
 
Is good engineering too much to expect?

I have only a casual knowledge concerning
strength of materials and materials failures;
but I know that things tend to break where
strength changes abruptly and where severe
angles (90 degrees) are involved.
In addition, it is very predictable that the
blade to handle area is likely to be a high
stress area, so why not pay special attention
to it?

I had tried a BK-7 and it was a big hunk of steel,
that cut well, though I did no formal testing.
It appeared to be massively strong,
even though it had a skeletonized handle.
It weighed more than the specification indicated.
Have you guys weighed yours?

If I knew I was going to be in a tough or survival
situation, I would carry a BK-7, gladly. But, as has
been discussed at length, some of us tend to leave
heavy stuff in the truck. We try to find lighter, yet
still effective tools.

So, I was looking for a well engineered stick tang
knife, as a compromise. Regarding the handle area
and its strength, a fiber reinforced handle material
could be much stronger than steel, by weight.
Check me on this last factoid; but in any case, the
handle area could be made light and very strong, if
tang and handle were well engineered.

I also see why some manufactures put so much
steel in their blades and handles: 1) it is easy to add
(keep) strength that way 2) some people will do
destructive tests 3) real world use may be very
demanding 4) if you make a knife really heavy, people
are less likely to carry it, and therefore less likely to
break it, preserving the knife's reputation.

Forgive my covering the map with many opinions,
I am a little frustrated. I am sad about Camillus.

Cold Steel will have to find a new maker, for several
of its knives; maybe this would be a good time to
re-spec the blade/handle area.

Frank
 
fnc said:
Have read some postings that Cold Steel knives
fail at the junction of the tang and blade. Some
possible causes are 1) heat treatment 2) overly
abusive prying 3) blade does not taper to the
tang, but has a corner where stress would build.

I did a search on "SRK stress" and came up with
several threads and here is a long, long one.
Please read the first page or look at the picture.
http://www.bladeforums.com/forums/showthread.php?t=328256&highlight=stress+srk

I am intersted in the SRK specifically, but also other
Cold Steel knives. Has CS changed the corner
junction of blade and tang to a taper junction?

frank
Click to expand...


Here is a picture of the SRK tang sans handle:



Not the greatest picture, but it shows there are radii at the tand/blade juncture. Sharp inside corners create stress concentrations and will weaken a blade under certain conditions. The Recon Scout and Trailmaster both have slide-on brass guards and have sharper corners to make fitting the guard easier. Many knives are built this way, pretty much all of them really with that type of guard and stick tang. I seem to remember not too long ago someone posted about a Benchmade Rant bowie that broke in a similar way under similar conditions as the Recon Scout did in the thread you referred to (and for similar reasons I'm sure). The SRK's Kraton handle doesn't require this and is stronger as a result.
 
Unfortunately, most hidden tang knives are not heavily radiused at the blade/tang juncture.

If you are looking for a full tang knife about the size of the SRK, you might want to take a look at the Ontario RAT-7, I haven’t used one myself, but it has gotten good reviews on the forums.

A smaller (4.5” blade) full tang knife, I would recommend is the J. Wayne Fears Survival Knife made by Foster Knives. The FSK is both sturdy and light weight and is offered in a choice of 3 handle colors (including hard to loose blaze orange G10) and comes with a custom Kydex sheath from Eric at On Scene Tactical.





- Frank
 
Brusletto has at least in one instance used an interesting configuration. One of their blades has a tang that starts out almost full width - just apx. 1/8" shoulders to fit the guard -- then tapers over apx. 1.25" to the eventual full-length stick tang width of about 3/16".
 
fnc said:
...a fiber reinforced handle material
could be much stronger than steel, by weight.
Check me on this last factoid...
Click to expand...

Which fiber reinforced materials are you talking about?

FRN doesn't come close to the tensile strength/density of hardened steel (although it is now being used to
reinforce concrete in some environments where corrosion is a problem). Carbon fiber composites is the
material with the highest such ratio (Ti alloys 2nd, then steel, magnesium alloys)

In terms of strength/cost, steel is hard to beat, and in terms of tensile strength/density steel is one amazing
material.

http://www.immnet.com/articles?article=2893

The above site cites 30% reinforced nylon to have a tensile strength of 30 kpsi, density of 1.37 g/cc
(ratio = 21.9). Steel with a hardness up the mid to upper 50's have a tensile strength of >300 kpsi, density
of around 8 g/cc (ratio > 37.5). Carbon fiber composites are cited to be 80-152 kpsi, density of 1.5 - 1.6 g/cc
(ratio of 53 - 95).

2 small picture files:
Tensile Strength vs Density and then Price. The line in each graph is of slope 1. (Upper left = best ratio,
lower right = worst ratio. The nylon here in the graph is not reinforced. Carbon fiber is off to the right in
the price graph. CFRP = Carbon fiber composite, GFRP = polyester-glass composite)

strengthvdensity.jpg


strengthvprice.jpg
 
kel_aa,

Thanks for the info.
I would have guessed that nylon to be stronger
than steel for the same mass (or weight) of material,
taint so.

Unfortunately the way engineers are measuring things
is more complex than Force required to break a mass.

Interpreting the graphs is not easy. I guess you look
at the distance from the diagonal line. Using that,
CFRP (Carbon Fiber) looks better than Titanium which
looks better than Steel. The Steel is quirky having a
wide dispersion, I guess they are summarizing a lot
of steels and a lot of heat treatments.
I did not see Kevlar, which is better than steel.
Here is a link for Kevlar, skip the text,
go to the bottom chart
http://www.chemsoc.org/chembytes/ezine/2001/gross_oct01.htm

The graph of the cost is not important because,
1)the cost of fabrication is far greater
2)the amount of these materials is very small.

The problem with adding (keeping) more steel,
in the tang.

Looking at the cross section of a stick tang,
it is a small "o". A full tang would look like a vertical
bar "|".
Neither is a very good structure to resist lateral stress.
An "H" shape would resist lateral stress just as an H-beam
in designed to do. The strength of the sides are multiplied.
Adding material to the sides of "H" is far more effective than
adding more to a "|' or a "o". The sides of the "H" are the
sides of your knife handle. You do not want metal there and
it is expensive to put there. Put fiber there.

Actually it just occurred to me that a very strong design
would be:
small “o” or dot in the middle representing stick tang,
"t" (cross) at the “o”, representing fiber inner-structure,
Big "O" surrounding fiber, representing the outside-handle.

This has the strength of an “H” and an “I” at the same time.
That is, added strength in both directions.

IMO, most variations on this would be far stronger per unit weight than
adding steel to the center.

frank
 
Fnc said:
I guess you look at the distance from the diagonal line. Using that,
CFRP (Carbon Fiber) looks better than Titanium which looks better than Steel. The Steel is quirky having a wide dispersion, I guess they are summarizing a lot of steels and a lot of heat treatments.
Click to expand...

That is all correct. I stated the same order in my description above. If you raise the line it'll eventually exclude everything until steel (stainless), Ti alloys, and finally carbon fiber composites. I kept the line lower however to show more materials in colour.

The site you posted does show a high tensile strength for kevlar. What can be misleading is however that the stated value is for 1 fiber. Whereas with steel you can get 100% density for any given shape, you cannot get make 100% dense kevlar with the same properties as stated. When woven into sheets and composited it'll probably be along the line of fiberglass and carbon fiber.

As for the handle, my view is this: The weakest part is not going to be along the length of the tang, it's going to be junction between the blade and the tang (for stick tangs). As far as most slab constructions go, I don't see them forming an adquate bond with the metal to actually contribute much to prevent failure at the junction. How many use rivet or screw construction as opposed to a strong epoxy? I'd think a tapered (width and thickness) is a good idea.
 
There is a Master Hunter in 'San Mai' available now if that interests you. Anything in Carbon V is going to be tough to source I think considering the Camillus situation.

Unsolicited opinion here, get a Fallkniven S1 and then you dont have to worry about durability or cutting or anything else.
 
kel_aa

I read your last post quickly and I agree at
first reading. The junction area is the most
critical area for failures due to prying action.
The prying action is not always intentional,
so even if non-prybar fans should take note.

I could see an argument that this area of
the knife should be the thickest and widest
and taper thickness and width in both
directions. A blade cross section in this area
could be shaped like a "t" and it would be
very strong.

Slightly disagree:
Around the house, I use many kinds of Scandi blades
with no handles at all. One, in particular, has a very
small stick tang that has bent (not broken) in my hand.
I think a wood or plastic handle would have prevented
this. BTW, the knife blade/tang was tapered, it bent
about in the middle of the handle. I do not see this as
bad design or workmanship; it was designed to have a
handle for support.
No other Scandi's have bent.

I must go now. I am late for my Knife Destruction 101
class at the Community College.

frank
 
kel_aa,
Besides Scandi blades that remain with no handle,
I have a few with a handle. I have paid very little
attention to how they are handled.

I will not be able to lay my hands on the Scandi
that bent until Saturday. It looks roughly like the
one in the link. From memory, I think mine was a
Swedish Frosts, very cheap=$3, 1995, 3 1/2 inch blade.

Thanks for that Scandi handle link; I will study and
think about it.

Back to your larger point that the handle must
mate to the tang or it provides almost no added
strength. I agree, and I had not given that a
sufficient amount of thought.
If the tang had some irregularities that the handle
could mate to, with 3 or 4 screws, and the handle
was also epoxied; then the handle would add strength.

To the extent that the tang has wiggle-room, the handle
adds No strength. Your point, in different words.

There is an analogy with ropes. Two ropes of the same
design and materials double the strength of one. Two
ropes that are dissimilar may be no stronger than the
strongest and quite likely much weaker than the strengths
of both added together; it depends.

On another note, I am glad that some of you guys are
testing knives to the breaking point; intentional
and otherwise.
Thanks for all the feedback on my theories, also.

frank
 
fnc said:
It weighed more than the specification indicated.

Frank
Click to expand...

Yep, I have weighed the Becker 7. 13.7oz out of sheath and 1# 7oz. in sheath with kit. Not invisible on the belt, but it carries well.
 
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