Static Failure Mode Prediction Models Mechanically Fastened Joints - PowerPoint PPT Presentation

Static Failure Mode Prediction Models Mechanically Fastened Joints Joining & Assembly

Static Failure Modes Net Section Shear Fastener Bearing Fastener Shear Tension Tear Out Pull Out

Basic Assumptions for our Predictions Failure of ductile materials (both the fastener and the sheets) • Brittle Material Ductile Material Initial stress concentration at Initial stress concentration hole edge at hole edge Plasticity causes load Failure when stress redistribution concentration reaches strength

Basic Assumptions for our Predictions Failure of ductile materials (both the fastener and the sheets) • All fasteners carry the same load at static failure • 25mm 1mm 12 = F kN total 12 25mm kN 3.5mm 1 = = F kN rivet 12 15mm 100mm 12kN (120MPa)

Basic Assumptions for our Predictions Failure of ductile materials (both the fastener and the sheets) • All fasteners carry the same load at static failure • Be mindful of load path • F F Load F is transmitted from one plate to another by two rows of bolts in each case F F

Net Section Tension Failure Failure will occur when net section area reaches the ultimate strength of the sheet material Ultimate tensile = ⋅ σ strength of sheet F A material NST net tu ( ) = − ⋅ ⋅ σ t W n d tu Number of fasteners along net section F NST

Bearing Failure Failure will occur when the stress acting on the projected area of the sheet on the bolt exceeds the bearing stress allowable for the sheet material. Bearing strength of sheet material = ⋅ σ F A br br br = d t σ ⋅ ⋅ ( ) 1 d A br br This equation gives you the force acting t on the sheet by the bolt at failure, not the force applied to the entire joint! F ( ) 2 br A br

Shear Tear Out Failure Failure will occur when the shear stress acting on the area between the bolt and joint edge exceeds the shear strength of the sheet material. Ultimate shear strength of sheet material = ⋅ σ F A STO STO su 2 = ⋅ ⋅ ⋅ b t σ su This equation gives you the force acting on the sheet by the bolt at failure, not t the force applied to the entire joint! F STO b A STO

Bolt Shear Failure Failure will occur when the shear stress acting on the area of the bolt exceeds the shear strength of the bolt material. Ultimate shear strength of bolt material = ⋅ σ F A , BS BS s bolt n = 1 π   2 = σ n d   , s bolt 4   n = 2 This equation gives you the force acting on the bolt at failure, not the force applied to F the entire joint! n = 3 BS

Fastener Pull Out Failure Fastener pull out failure is more complex and highly dependent on bolt head geometry, so it is typically evaluated via test. So we will not consider it here.