Aerospace Aluminium Alloy
in Mechanically Fastened
Joints
Results Seminar
Student: Andrew Steele
Project Supervisor: Dr Reza Oskouei
10th October 2014
Background
• Aircraft components and structures are attached to the
airframe using bolted or riveted fastening methods.
• Investigations by aviation authorities have determined
material failure at fastened joints have caused many fatal
aircraft accidents.
• These failures are due to high stress concentrations at the
joint where fretting is initiated. This leads to cracks which
propagate if they remain undetected during inspections.
Bolted Joint
• Bolted joints are used to allow engineers to remove
components for periodic inspection and servicing.
• Bolts can be used in shear and tensile applications when a
suitable bolt material is selected.
Riveted Joint
• Components requiring tamper proof joints are fastened using
riveted mechanical fasteners.
• Rivets are designed for shear applications only, but do resist
minor tensile forces during aircraft operation.
Aloha Airlines Flight #243 (1988)
El Al Flight #1862 (1992)
South West Flight #812 (2011)
Failure Modes in Mechanical Joints
Aircraft components are designed with a limited service life due to the
significant consequences when critical component failure occurs.
In aviation it is essential to determine the precise nature of
component failure to prevent the event reoccurring.
Research into material failure has identified specific failure modes
which are classified as:
• Net section
• Shear out
• Bearing
Net Section
• Net section is observed when high load levels are applied to
fastened joints causing stress concentrations at the hole edge.
• It is at these locations where cracks initiate and propagate as
the material plastically deforms.
𝑃 =𝐴×𝜎𝑢
𝑃 = axial force (kN)
𝐴 = 𝑐𝑟𝑜𝑠𝑠 𝑠𝑒𝑐𝑡𝑖𝑜𝑛𝑎𝑙 𝑎𝑟𝑒𝑎
𝜎 𝑢 = ultimate strength
Shear out
• Shear out is caused when fasteners are positioned too close to
an end or edge of a component.
• Material above the hole diameter shears from the surrounding
material causing the joint to fail.
𝜏=
𝑃
𝑚𝑡𝐻
𝜏 = shear strength
P = axial force (kN)
m = number of shear areas
t = thickness of plate
H = distance from fastener to edge of plate
Bearing
• Bearing is the plastic deformation of material around fastening
holes when excessive loadings are applied.
• This deformation causes fretting on the material surfaces
while elongating the fastener hole.
𝜎𝑏 =
𝑃
𝑚 𝑑 𝑙𝑔
𝜎𝑏 = 𝑏𝑒𝑎𝑟𝑖𝑛𝑔 𝑦𝑖𝑒𝑙𝑑 𝑠𝑡𝑟𝑒𝑛𝑔𝑡ℎ
P = axial force (kN)
m = number of fasteners in joint
d = fastener diameter
𝑙𝑔 = thickness of plate
Project Objectives
• Compare bolted fasteners with riveted fasteners using:
identical failure modes
identical loading conditions
identical specimen dimensions
identical materials and testing method
• Examine loading and displacement data between each
fastening method.
• Investigate joint behaviour after increasing the total number
of fasteners in the joint.
• Analyse and document physical specimens after testing.
Method
• Joint design
• Risk Assessment & Safe Operation Procedure
• Specimen assembly (riveting / bolting)
• Equipment Training & Testing
Joint Design
• Researched aircraft joint construction in order to replicate
joints used in the manufacture of aircraft.
• Material selected for joint construction was 2mm 2024-T3
ALCLAD sheet
Net Tension & Bearing (Bolted)
Net Tension & Bearing (Riveted)
Shear Out (Bolted)
Shear Out (Riveted)
Risk Assessment & Safe Operating
Procedures
Before initiating fabrication of specimens:
• Risk Assessments were required for:
Heat treatment furnace
Solid rivet gun
Fabrication process
• Safe Operating Procedures were required for:
Heat treatment furnace
Solid rivet gun
Fabrication process
• These documents are now registered on Flinders University wiki for
future projects.
Specimen Assembly
• Bolted specimens were assembly using Class 8.8, 1/4”
diameter, 1.5” long bolts.
• These bolts were torqued to 15 Nm using a Matador digital
torque wrench to ensure consistency.
• Riveted specimens required the rivets to be heat treated at
500°C for 1 hour before quenching in 21 °C water. These were
then place on ice to stop the hardening process.
Training & Testing
• Before testing, training on the Instron 5969 was completed.
• A testing method was developed specifically for the project to
ensure each specimen was tested with identical settings.
• The loading rate 0.3 mm/min
• 12 bolted specimens were tested
• 12 riveted specimens were tested
Double Riveted Joint
Results
22%
55%
1.4%
7.5%
Results
3.2%
1.8%
29.2%
21%
Results
15 kN
12.2 kN
10.3 kN
Results
Results
Results
Results
Results
Results
Conclusion
• Evidence of increased rivet clamping force was found when
specimens were disassembled. Fretting had been initiated on
the mating material surfaces.
• Fail mode changed from Shear out to Net tension when a
second fastener was introduced. This is beneficial as Shear out
is undesired in mechanical fastened joints.
• After carefully disassembling the specimens reasons for
greater joint displacement became evident. Bolted fasteners
are made from material with a higher yielding strength than
2024-T3 sheet. This caused the material to plastically deform
and displace the joint further than riveted joints.
Further Work
• Finite Element Analysis is being developed to compare
computational results again physical testing.
• These results will be combined with physical testing to
produced a paper, which will be submitted to aeronautical
engineering journal for review.