Bolted joints

M11 introduced 3D contact between meshed bodies. M12 opens the third optional specialization: connecting two SOLID185 plates through pilot nodes, rigid regions with CERIG, and a comparable 1D connector (MPC184 or BEAM188). You will extract local forces to CSV for checking the bolt outside MAPDL under the applicable standard.

Objectives

After completing M12, you will be able to:

• Select nodes captured by the head and thread using auditable components.
• Apply CERIG,pilot,ALL to captured regions of three-dimensional solids.
• Build MPC184 (Lagrange KEYOPT) and BEAM188 (CSOLID) connectors.
• Avoid accidental bonding with a documented interface_gap.
• Extract connector SMISC results and interpret the local 1D coordinate system.
• Export forces to CSV for code verification outside the model.

Prerequisites and downloads

• M03: BEAM188, sections, and beam degrees of freedom.
• M06: global equilibrium and reaction summation.
• M02: selection with NSEL, CM, and SELTOL.
• M11 (recommended): gap criteria and diagnosis before trusting stresses.

How to use this lesson

Session map

Prediction — What happens before SOLVE?

Two 80×40×12 mm plates with a 0.1 mm gap and Fx=4000 N, Fy=−3000 N applied at head_pilot.

Prediction: if you omit the gap, the plates share nodes at y=plate_t and some of the load bypasses the bolt. With the gap, the local forces must balance the applied Fx and Fy.

Mental model — Head, thread, and connector

• Head: head_pilot + head_dep at y=2·plate_t+gap.
• Thread: thread_pilot + thread_dep at y=plate_t.
• CERIG: with ALL on SOLID185, only active translations (UX, UY, UZ) are coupled.
• Standards: MAPDL provides forces; code-based sizing is performed externally.

Step 1 — Geometry and mesh

BLOCK,0,plate_x,0,plate_t,0,plate_z
BLOCK,0,plate_x,plate_t+interface_gap,2*plate_t+interface_gap,0,plate_z
VMESH,ALL

Document interface_gap in the README and CSV. Confirm that there is no unintended continuity between the plates.

Step 2 — Captured regions and CERIG

NSEL,S,LOC,Y,2*plate_t+interface_gap
NSEL,R,LOC,X,bolt_x-head_absorb_r,bolt_x+head_absorb_r
NSEL,R,LOC,Z,bolt_z-head_absorb_r,bolt_z+head_absorb_r
CM,head_dep,NODE
*GET,head_pilot,NODE,0,NUM,MIN
CMSEL,S,head_dep
CERIG,head_pilot,ALL

Repeat at y=plate_t for the thread. With SOLID185, ALL is valid: MAPDL uses only the solid's active DOFs. Verify counts of 25+25 before creating the connector.

Step 3 — MPC184 and BEAM188 connectors

ET,2,MPC184
KEYOPT,2,1,1
KEYOPT,2,2,1
E,head_pilot,thread_pilot

ET,2,BEAM188
SECTYPE,1,BEAM,CSOLID
SECDATA,bolt_d
E,head_pilot,thread_pilot

MPC184 case: rigid beam with Lagrange multipliers to recover SMISC. BEAM188 case: finite axial bolt stiffness. Load: F,head_pilot,FX and FY.

Step 4 — Postprocessing and CSV

ETABLE,N_local,SMISC,1
ETABLE,V1_local,SMISC,6
*GET,N_local,ELEM,conn_elem,ETAB,N_local

Write m12_bolt_forces.csv with forces in the connector's local coordinate system. Measured reference: N_local=−3000 N, V1_local=−4000 N; study_passes=1. The file includes the note export_for_external_code_check.

Bug hunt

Five scenarios documented in 12_bug_hunt.mac:

1. Pilot incorrectly included only as a dependent node without a valid mesh.
2. Pilot included in the CERIG dependent set.
3. MPC184 with KEYOPT(2)=0 and empty SMISC output.
4. BEAM188 without SECTYPE,BEAM,CSOLID.
5. Trusting bolt forces without global support equilibrium.

Verifiable challenge — Stiffness or length

After validating the base deliverable, choose Challenge A (increase bolt_d) or Challenge B (increase plate_t and connector length). Predict the change in uy_head_beam before running.

Self-assessment

Learning evidence

• m12_connector_audit.csv — two rows with passes=1.
• m12_bolt_forces.csv — forces for external code checking.
• m12_summary.csv — study_passes=1.
• Brief note explaining the connector's local versus global coordinate system.

Exit checklist

• 0.1 mm gap documented; no numerical bonding.
• n_head_dep=n_thread_dep=25.
• CERIG,pilot,ALL at the head and thread (active solid DOFs).
• MPC184 and BEAM188 with local N/V consistent with the load.
• Global equilibrium <0.5% in both cases.
• study_passes=1 in a clean run.

Technical traceability

Commands: CERIG, MPC184, BEAM188, SECTYPE, SECDATA, ETABLE, SMISC, *GET. Reference: Structural Analysis Guide; Element Reference. Inherited from: M03 (beam), M11 (gap). Next: M13 modal analysis.