Fatigue analysis is a critical aspect ofpressure vessel calculations, ensuring structural integrity under cyclic loading. Different standards provide various methodologies to assess fatigue, ranging from simplified calculations to advanced Finite Element Analysis (FEA). This article compares fatigue assessment methods in EN 13445, AD 2000, and PD 5500 (simplified and rigorous approaches) against ASME VIII Div. 2, which mandates FEA for fatigue analysis.

Simplified Fatigue Analysis: EN 13445, AD 2000, and PD 5500

Advantages of Simplified Methods

1. Ease of Use

   & Reduced Complexity

EN 13445, AD 2000, and PD 5500 provide closed-form equations and tabulated data, making calculations straightforward. Engineers can perform fatigue evaluations quickly without needing specialized FEA expertise or software.

1. Lower Cost & Time Efficiency

Unlike ASME VIII Div. 2, which requires detailed computational analysis, simplified methods save engineering hours and eliminate expensive FEA software licenses.

1. Conservatism &

   Reliability

These methods have been extensively validated through years of industry use. They incorporate safety margins that prevent unexpected failures while avoiding the excessive conservatism of ASME VIII Div.2

1. Regulatory Acceptance & Standardization

EN 13445 and AD 2000 are widely accepted across Europe, while PD 5500 is commonly used in the UK. Compliance with these standards often simplifies regulatory approvals.

1. Best for Repetitive or Standardized Designs

When dealing with well-documented stress cycles and standardized geometries, the simplified approach is more practical than FEA.

ASME VIII Div. 2: FEA-Based Fatigue Analysis

Advantages of ASME VIII Div. 2

1. More Accurate Stress Distribution Analysis

ASME VIII Div. 2 requires FEA, allowing for detailed stress analysis and capturing localized stress concentrations that simplified methods might miss.

1. Better for Complex or High-Risk Components

If a vessel has complex geometries, stress risers, or non-standard load cases, ASME VIII Div. 2’s FEA approach provides a more reliable fatigue life prediction.

1. Material & Cost

   Optimization

Although FEA is costly upfront, it enables optimized material use, reducing excess conservatism and allowing for lighter, more efficient designs.

1. Higher Allowable Design Stresses

ASME VIII Div. 2 permits higher design stresses, making it more efficient for cyclically loaded vessels where weight or material costs are critical.

Are EN 13445 (Rigorous) and AD 2000 (Rigorous) Equivalent to ASME VIII Div. 2?

1. Stress Evaluation Methods

EN 13445 (rigorous) and AD 2000 (rigorous) allow stress calculations without mandatory FEA. ASME VIII Div. 2 requires FEA, making it superior in evaluating localized stress concentrations.

1. Fatigue Curves & Safety Margins

ASME VIII Div. 2 fatigue curves incorporate higher safety factors (factor of 2 on stress or 20 on life).

EN 13445 and AD 2000 are typically less conservative, leading to potentially longer fatigue life predictions.

1. Suitability for Complex Geometries

ASME VIII Div. 2 excels in complex designs where local stress effects are significant. EN 13445 and AD 2000 can handle complexity but rely on simplified assumptions if FEA is not used.

Conclusion: Which Method is Better?

The choice between EN 13445, AD 2000, PD 5500, and ASME VIII Div. 2 depends on the application:

Use EN 13445 (simplified), AD 2000 (simplified), or PD 5500 if:

• You need a faster, cost-effective, and conservative approach.
• The design is standard, with well-documented stress cycles.
• Regulatory compliance favors European standards.

Use EN 13445 (rigorous) or AD 2000 (rigorous) if:

• You require more detailed stress evaluations but want to avoid mandatory FEA.
• The vessel operates in moderate fatigue conditions but does not have extreme stress concentrations.

Use ASME VIII Div. 2 (FEA) if:

• The vessel has complex geometries or high localized stresses.
• Material efficiency and precise fatigue life estimation are critical.
• You are designing for high-risk applications where FEA provides a competitive advantage.

Conclusion

For most pressure vessel applications, simplified fatigue methods in EN 13445, AD 2000, and PD 5500 are preferable due to their ease of use, lower cost, and proven reliability. These methods provide sufficiently conservative fatigue assessments while avoiding the complexity and expense of FEA-based analysis in ASME VIII Div. 2.

However, when dealing with complex geometries, localized stress risers, or highly cyclic loading, ASME VIII Div. 2 remains the superior choice due to its precision and ability to handle detailed stress distributions.

For ASME VIII Div. 1 designs, where fatigue analysis is often not explicitly required, simplified methods from EN 13445, AD 2000, or PD 5500 can be useful to provide an additional layer of safety without resorting to full FEA. If fatigue is a concern, screening criteria from ASME VIII Div. 2 can be applied first, followed by a simplified method if the component does not warrant a full FEA approach.

Ultimately, the best method depends on the complexity, cyclic loading conditions, and regulatory requirements of the static equipment in question.