Failure Analysis of Panel Coil in a Sterilization Process

The Failure Analysis of Panel Coil Components in a Large Industrial Sterilizer Revealed a Complex Interplay between Mechanical Vibration and Structural dynamics. After a specimen exhibited cracking during operation, a multidisciplinary investigation was conducted to determine the root cause and implement corrective measures.

Failure Analysis Methodology

• Macroscopic and Microscopic Examination: Initial visual inspection identified fracture surfaces characteristic of fatigue failure, including beach marks and crack propagation patterns. Scanning electron microscopy (SEM) likely revealed striations consistent with cyclic loading, confirming fatigue as the primary failure mechanism.
• Material Characterization: Chemical composition and hardness testing were performed to rule out material defects or degradation from the sterilizer’s operational environment (e.g., steam, thermal cycles).

Dynamic Load Assessment

Bounding Frequency Analysis: First-principles calculations estimated the panel coils’ natural frequencies between 8–100 Hz in air. However, immersion in water significantly altered the system’s dynamic response by adding hydrodynamic mass, effectively reducing stiffness and shifting natural frequencies downward.
Forcing Frequency Identification: The six-bladed agitator’s rotational speed generated a dominant excitation frequency of 15 Hz, which aligned with the panel’s modified natural frequency when submerged. This resonance condition amplified stresses, accelerating fatigue crack initiation and growth.

Finite Element Analysis (FEA) Validation

A refined FEA model incorporated fluid-structure interaction to simulate the panels’ behavior in water. Results showed:
– A wet natural frequency close to 15 Hz, corroborating resonance as the driving factor.
– Stress concentrations at unsupported regions of the panel coils, exacerbated by cyclic loading from agitator-induced vibrations.

Design Modifications and Mitigation

• Structural Reinforcement: Additional supports were installed at critical locations to eliminate low-order vibration modes, decoupling the system from the agitator’s forcing frequency
• Welding Process Optimization: O’Donnell engineers specified improved welding techniques to enhance joint integrity, including:
  – Post-weld heat treatment to relieve residual stresses.
  – Non-destructive testing (e.g., ultrasonic inspection) to ensure defect-free welds.

Outcome

Post-modification vibration spectra confirmed the elimination of resonant peaks near 15 Hz. The sterilizer has since operated without recurrence of fatigue-related failures, demonstrating the effectiveness of combining empirical failure analysis with computational dynamics.

This case underscores the importance of accounting for environmental factors (e.g., fluid immersion) in dynamic systems and highlights the value of cross-validating analytical and numerical methods in failure investigations.