Wedge Tensile Testing of Fasteners

[mshimko]

I've a couple of fastener suppliers that are claiming it's technically acceptable to perform wedge tensile testing (IAW ASTM F606) on a fastener that had been previously yield tested (but not to failure, obviously).

My position: yield testing of a fastener imparts at least limited work hardening on the piece. It is not acceptable to yield test the fastener, then remove the fastener from the test apparatus and re-install with the wedge, and then perform the wedge tensile test.

Any feedback or opinions are appreciated.

 

[TVP]

Your suppliers are testing in accordance with ASTM F606. Here is an excerpt from the relevant section:

3.5.1 Wedge Tension Testing of Fasteners— Determine the ultimate load of the fastener as described in 3.4 except place a
wedge under the fastener head. When both wedge and proof load testing are required by the product specification use the
proof load-tested fastener for wedge testing.


The purpose of the wedge tensile test is to obtain the tensile strength and demonstrate the “head quality” and ductility
of the product. Keep in mind that the proof load is 90% of the nominal yield strength, and therefore should not cause permanent plastic deformation of the fastener.

 

[mshimko]

For TVP: You're confusing PROOF load testing with YIELD testing (which, BTW, is the same arguement this supplier made).

My question pertains to wedge tensile testing AFTER a speciman had been YIELD tested.

 

[Screwman1]

Are we correct to assume then that this is a "non-standard" acceptance test? ISO 898-1 also does not require both yield and tensile except on machined specimens and necked down parts. I don't understand why you would need yield and wedge tensile instead of proof and wedge tensile.

 

[dbooker630]

I agree with TVP as far as interpreting ASTM F606-11.

 

[TVP]

mshimko,

I am not confused, but thank you for clarifying that you did indeed mean yield testing (permanent plastic deformation). The latest 2012 revision of ASTM F606M clarifies what is meant by proof load testing in the abstract:

For externally threaded fasteners, the mechanical tests describe the procedures for determining the following properties: product hardness; proof load by length measurement (Method 1), yield strength (Method 2), yield strength of austenitic stainless steel and nonferrous materials (Method 2A), and uniform hardness (Method 3); axial tension of full size products such as fasteners and studs; wedge tension of full size products such as fasteners and studs; tension of machined test specimens including yield point (by drop of the beam or halt of the pointer, autographic diagram, and total extension under load methods), yield strength (by offset, and extension under load methods), tensile strength, elongation, and reduction of area; and total extension at fracture.

As you can see, proof load testing can be performed using the conventional "proof load" method of length measurement or the yield strength method (Method 2), among others. ASTM, and the rest of the international fastener industry, deems that the purpose of the wedge tensile test is to ensure that the fastener does not fracture at the head to shank juntion, even at loads equivalent to the ultimate tensile strength. So imagine that an M10 PC10.9 externally threaded fastener is tested to yield (~ 48 kN), then beyond yield according to the wedge method in 2 kN increments up to the ultimate load of ~ 60 kN. It should look like this:

48 kN yield
50 kN more plastic deformation
52 kN more plastic deformation
54 kN more plastic deformation
56 kN more plastic deformation
58 kN more plastic deformation
60 kN fracture in the body or threaded portion

No matter what the load is, the stress in the head to shank junction is not sufficient to cause fracture. It doesn't matter if the yielded portion in the body or threads has strain hardened, the load keeps increasing but fracture occurs outside of the head area.