Development of Design Data for FSSW Joints · • Minimum values calculated by direct or indirect statistical procedures – Calculated by direct statistical procedures • F tu =

Development of Design Data for FSSW Joints

Dwight Burford, PhD, PE

Joining Innovations, LLC September 17, 2014

Handbook Design Data

• Template for FSSW joints – MMPDS: Metallic Materials Properties

Development & Standardization • formerly MIL-HDBK-05J • formal procedures • proven methodology for structural joints • only source of publically available design data accepted

for metallic materials by the FAA

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Structural Joints

• Design Data Handbooks – Legacy: MIL-HDBK-5J

• Referenced in this presentation

– Current: MMPDS-08

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http://quicksearch.dla.mil/qsDocDetails.aspx?ident_number=53876

http://projects.battelle.org/mmpds/

SAE Paper 2003-01-2897

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SAE Paper 2003-01-2897


• Process Development via Design of Experiments (DOE) – Coarse DOE

• Identify possible failure modes • Identify process window with a single consistent failure

mode – Fine DOE

• Optimize process window (i.e. speeds, feeds, forces, etc.) to establish a “tight” specification

• Apply a fuse concept: consistent HAZ failures

Impact of Scatter in a Data Package

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Shift in minimum due to scatter in data

NOTE: Ultimately a reflection of the quality / capability of the standard.

Illustration of a Normal distribution

(NTS) Population A

Population B

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Impact of Scatter in a Data Package


Same allowable

value

NOTE: Ultimately a reflection of the quality / capability of the standard.

Illustration of a Normal distribution

(NTS) Population A

Population B


SAE Paper 2003-01-2897


Design Allowables

• Minimum values calculated by direct or indirect statistical procedures – Calculated by direct statistical procedures

• Ftu = Design tensile stress • Fty = Design tensile yield stress (0.2% offset)

– Calculated by indirect (derived/ratioed) statistical procedures

• Fsu = Design ultimate stress in pure shear (averaged)

• Fcy = Design compressive yield stress (0.2% offset)

• Fbru = Design ultimate bearing stress

• Fbry = Design bearing yield stress

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Data Basis

• Four basis types for mechanical property data (in increasing order of statistical confidence) – Typical basis – S-basis – B-basis – A-basis


FTU, FTY, etc. Illustration of an example Weibull distribution

*Note: T99 and T90 are statistically calculated local tolerance bounds.

A=T99

B=T90

S=Specification Minimum

Sample Population Distribution


S-Basis

• Specification minimum value – Specified by the governing industry specification

(e.g. SAE AMS specs, ASTM, etc.) or federal or military standards

– Statistical assurance is not known – May include

• Tensile ultimate strength (TUS) • Tensile yield strength (TYS) • Elongation • Reduction in areas


B-Basis

• Statistically calculated value where at least 90 percent of the population of values is expected to equal or exceed the B-basis mechanical property allowable with a confidence of 95 percent – Established from computed T90 values – Use of B-basis design properties is permitted in

design by USA government agencies, subject to certain limitations specified by the individual agency.


A-Basis

• The lower of either a statistically calculated number or the specification minimum (i.e. S-basis) – As a statistically calculated number, it represents a value

for which at least 99 percent of the population of values is expected to equal or exceed the computed mechanical design property with a confidence of 95 percent.

– Established from computed T99 values and depends upon the following two cases:

• CASE 1: A value that is higher than corresponding S-basis value is presented as a footnote in the property table and is NOT qualified for general use in design unless the specification requirement is increased to equal the calculated value.

• CASE 2: A value that is equal to or lower than a corresponding S-basis value replaces the S-basis values in the document.


Data Basis

• CASE 1: For T99 > S-basis, A-basis = S-basis – T99 is published in the footnote to the table.


FTU, FTY, etc.

(Illustration of an example Weibull distribution)

A=T99

Specification Minimum

1) Perhaps suggestive of an improved industry process.

2) A potential action could then be to update the specification minimums



MP159


Data Basis

• CASE 2: For T99 < S-basis, A-basis = T99 – T99 is published in table, replacing spec. min.


FTU, FTY, etc.

(Illustration of an example Weibull distribution)

A=T99

Specification Minimum


9.7.1 Mechanically Fastened Joints

• 9.7.1.2 Yield Load Determination • 9.7.1.3 Shear Strength of Fastener • 9.7.1.4 Sheet Critical and Transition Critical

Strengths – “…a plot of the average Pu/D2 and Py/D2 values for

each t/D tested is expected to yield a compact band of data points through which single ultimate and yield load curves can be determined…”

– P/D2 = A0 + A1 * (t/D) + A2 * ln (t/D)


Example Regression Analysis: B-Basis (T90)


Spot Welds

• 8.2.2.3 Spot and Seam Welding — “Permission to use spot and seam welding on structural parts is governed by the requirements of the procuring or certifying agency…” – 8.2.2.3.2.1 Effects of Spot Welds on Parent Metal Strength

of Aluminum Alloys — “In applications of spot welding other than splices, where ribs, intercostals, or doublers are attached to sheet, the allowable ultimate strength of the spot-welded sheet may be determined by multiplying the ultimate tensile strength of the sheet (A or S-values) by the appropriate efficiency factor shown on Figure 8.2.2.3.2.1. Efficiencies for gages under 0.020 shall be determined by test.”


RSW Joint Efficiency in Aluminums



• FSSW joints are Integral Fasteners – Discrete in nature – Similar to installed mechanical fasteners

• Installed through mechanical means • Tested with same/similar procedures to conventional

mechanical fasteners

– Suitable for handbook design data for FSSW joints


Example: ASTM Paper JAI101568, 2008


System to measure crack growth as a function of cyclic loading

D. A. Burford, B. M. Tweedy and C. A. Widener, "Fatigue Crack Growth in Integrally Stiffened Panels Joined Using Friction Stir Welding and Swept Friction Stir Spot Welding," Journal of ASTM International, vol. 5, no. 4, p. Online Publication, Paper ID JAI101568, 2008.




Riveted Panel Swept FSSW Panel







Typical Joint

Knife-Edge Joint

Integral Fasteners vs. Installed Fasteners and Spot Welds

• Consider factors that potentially influence the respective failure modes: – Mechanical notch (associated with flash, tool gouging, etc.) – Metallurgical notch (e.g. HAZ) – Transition location in spot path (entrance onto & exit from

periphery tool travel path) – Residual stress gradients (e.g. tilt / precession angle, etc.)

• Calculate stress on “integral fasteners” like a bearing stress (analogous to bearing stress of rivets ) based on: – Observations (preliminary) of location of fatigue crack initiation – Behavior of S-N curves plotted for different loading conditions


0.04 inch AA2024-T3 – Tensile Loading: 1.5 kN Transverse (X) Displacement


0.04 inch AA2024-T3 – Tensile Loading: 1.5 kN Transverse (X) Stress


0.04 inch AA2024-T3 – Tensile Loading: 1.5 kN Longitudinal (Y) Stress


0.04 inch AA2024-T3 – Tensile Loading: 1.5 kN von Mises Stress


0.04 inch AA2024-T3 – Tensile Loading: 6.0 kN von Mises Stress


Simulated Unguided Lap Shear Test 0.04” Al7075-T6 / 0.04” Al2024-T3

6.0 kN Tensile Load


Rivet Failure Mode vs. Swept Spot Integral Fastener Failure Mode


Fatigue crack initiation site in hole inhibited by presence of compressive contact stress field that is introduced by interaction with the rivet

Fatigue crack initiation site may be augmented by presence of metallurgical and mechanical notches and its alignment with the uniaxial loading path.

Riveted Joint

Integral Fastener

Joint

Hole elongation under applied uniaxial tension



• The stress distribution around the integral joints is uniquely different than for installed fasteners. – This leads to potential differences in fatigue

behavior between the two types of fastener systems (installed vs. integral fasteners) as well as between joints produced by the different FSSW variants.

– Joint aspect ratio and the orientation of joints affects the stress distribution.


Summary

• Developing allowables is an important step in overcoming barriers to implementation of FSW, FSP, FSSW, etc. in the aviation and aerospace sectors.

• Allowables for FSW are underway – Example Reference:

• Presentation by Jana Rubadue of Battelle • Technology Exchange at Penn State on 11/5/2013

– (See slide 22 for FSW reference)


http://www.cimp-3d.org/events/past/2013/nov/5/downloads/MMPDS_Handbook.pdf

Documents

Development of Design Data for FSSW Joints · • Minimum values calculated by direct or indirect statistical procedures – Calculated by direct statistical procedures • F tu =