FEA : beam-configuration fillet-weld tensile test

The "number 1" question...

The test has been called the "beam-configuration fillet-weld tensile test" - but is it truly tensile upon the test fillet weld ???
Anything else is subsidiary to that question. As, if the load on the test fillet weld isn't "pure" tension, the test has much lesser value.



The real physical BCFWTT being modelled by FEA

The physical test "in real life" - see:
Tensile-test rig for beam-configuration fillet-weld samples
plus with more pictures - first BCFWTT (sample identical),
Fillet welds tensile tested in beam test


Firstly - is the load on the test fillet weld pure tension brought by the top surface of the sample beam?
That being so...
For the test fillet weld at break - are the measured force needed and the observed fracture mode predicted by FEA?


Two simulations, sequentially

Beam only - all beam - origin and destination of forces

Preview - yes find the BCFWTT is good

You may want to jump ahead to the detailed BCFWTT weld-region simulation , accepting that consideration of the overall beam configuration test indicates it works as it should.

This FEA investigation

To the right-hand-side edge of the model as constructed is a plane-of-symmetry. So the representation is the left-hand-side half of the complete beam sample.

This model is qualitative only !
The "beam" is a solid cuboid.
The size is not literal.
The force applied is arbitrary.

Its sole purpose is to assist in a qualitative visualisation of the origin and destination of forces, and the flow of stresses.

What one would desire is a state of pure tension applied to the weld (located at top right-hand-side of represented beam).

Beam deflection is shown magnified 100000X times.


I am challenging viewers to consider whether I am right in evaluating these forces and stresses, and make informed comment.

Quick digression - in this unusual case "von Mises" [Wikipedia] isn't helpful - almost misleading, as

"Principal Stress 1" seems most useful.

It seems to me that it is possible to separately visualise - intuitively; from the analytical derivation see section "Stresses in weld analysed" ; and only partly from the FEA model :

Note : I've "bent reality" putting the "no vertical movement" restraint on the end face of the beam (see model) - to avoid creating a lurid local stress concentration and unreal long-range moment ("twist") from a non-rotating "no vertical movement" zone on the top of the beam end. Given the real "end-hoops" can rotate a bit, bedding onto the current plane of the beam top surface. Then these two impressions can be combined, for their combined effect.
At the lower right-hand-side, there is both vertical force from the piston and reacted horizontal force in that one local volume.
What happens at the left-hand-side doesn't matter to the weld test - but there is nothing going on there anyway.
Then crucial observation;
what is happening in the top right-hand-side region where the test weld is... This region is clear of the flow of force from the lower RHS to the upper LHS - so the tension there is in a "pure" state - nothing else mixed in. As we want.

BCFWTT - weld area only

This model is the left-hand-side half, given a plane-of-symmetry to the right-hand-side of the model.

Conditions - dimensions; materials; loads; constraints - are believed to be completely representative of the BCFWTT at the point of failure by abrupt fracture in test.

In particular - 14Tonnes-force (137340N) across the weld of 6mm leg-length and 40mm weld length.

SI units used, so

Shown with deflection magnified 4X ("4 times").

FEA "Open cracks mode" reveals the horizontal "plate", representing the top surface of the Rectangular Hollow Section, coming in from the left-hand-side and terminating against the vertical "middle plate".


The highest stresses are predicted to be along exactly the region where the real test weld is seen to fracture .
So the FEA model and the real test do correlate.


around the plane of the top surface of the "Rectangular Hollow Section", along the longitudinal "leg" where the weld metal meets the parent metal.

That is unequivocal.

I am soliciting mentoring on interpretation about forces, stresses, their interpretation and their effects on the following...

"von Mises" is showing a band of regional stress well above the known breaking stress seen of around 560MPa, along the longitudinal fillet "leg" (edge of the "triangle" of weld metal meeting the parent plate) - which "isn't good news" for survival of that region.

Other measures of stress are concurring.

The local point of extremely high stress at the "root" / "fillet corner" should not be taken too literally, as this is infinitely sharp in the model - but has some microscopic but finite radius in real life.
Also... As seen from the magnified deflection representation of this single-sided fillet weld - there is some rotation of the parent plate under load - which will intensify the stress at the fillet corner further. Where many fillet welds are double-sided, which makes rotation under stress tend to zero, and in reality random-and-negligible.

That said - the real fracture surface has its inner side from around the fillet corner - while the outer side "hits" the weld surface just above the weld toe. So the "von Mises" stress concentration at the weld toe cannot be fully compelling the fracture to go there.

Intermission - horizontal plate deflecting to much?

Attention went to whether the horizontal "8mm thickness" plate is rotating under load and intensifying the stresses in the weld-root / fillet-corner ?
Considering the magnified deflection, and the "Displacement in Y" output.

That being due to a Moment ("twist") caused the beam's cross-section and the fillet weld not being in-line for the forces flow through them. That Moment causing the horizontal plate to rotate / "twist".

I find that's not the case according to the FEA. That when I model with a 15mm thickness plate, while the rotation under load is much reduced, it makes a very minor difference to the stress in the weld-root / fillet-corner - and elsewhere.

I've provided a comparison page between the "actual" 8mm thickness and a hypothetical 15mm insert thickness.

That said - in the "real world" of the physical test, as any experimenter would do,

Resuming the narrative...

There is inexactness about finer details of the interpretation of the FEA in relation to the real outcome. However, the dominating overview is of theory as implemented in the FEA model predicting the nature of the fracture on overload of the BCFWTT.
Plus concurring that the force to cause fracture in the real weld looks like a breaking condition in the FEA model.

To put it in perspective - the established analytical / mathematical model for fillet weld strength has fracture through the fillet throat (its shortest thickness "on the 45degree" from the fillet corner to the weld face). Which is not correct. As discussed elsewhere .

Good outcome here.

(R. Smith, 05Feb2021, 06Feb2021 (dy, 15t), 07Feb2021 (anames, edits), 08Feb2021 (tensile q))