Determine Limiting Crack Sizes and Remaining Life to Improve Your Next Equipment Inspection

Why It Matters

Knowing the limiting crack size and remaining life enables operators to prioritize inspections, avoid unexpected failures, and reduce costly unplanned downtime.

Key Takeaways

• Limiting crack size defines inspection detection tolerance.
• FAD combines toughness ratio (Kr) and load ratio (Lr).
• Points below FAD curve indicate stable, repair‑able cracks.
• API 579 methodology quantifies remaining fatigue life.
• Early crack sizing reduces unplanned downtime and costs.

Pulse Analysis

Equipment owners increasingly rely on proactive crack sizing to turn inspections from reactive check‑lists into strategic maintenance tools. By establishing a limiting crack size, teams can set detection thresholds that match the capabilities of ultrasonic, radiographic, or advanced phased‑array techniques, ensuring that inspections focus on defects that truly threaten integrity. This pre‑inspection insight also feeds directly into fatigue crack growth models, allowing engineers to forecast remaining service life and schedule repairs during planned outages rather than emergency shutdowns.

The Failure Assessment Diagram (FAD) is the analytical backbone of this approach. Developed under API 579‑1 and ASME FFS‑1, the diagram plots the toughness ratio Kr against the load ratio Lr, capturing the interaction of brittle fracture and ductile collapse mechanisms. A point plotted below the FAD curve signals a stable crack that can be monitored, while points above indicate imminent instability, prompting immediate repair or replacement. The methodology incorporates primary loads, residual stresses, and material properties, delivering a quantitative basis for fitness‑for‑service decisions.

Practical adoption is accelerated by tools such as Quest Integrity’s FEACrack software, which automates 3‑D crack mesh generation and integrates API 579 Part 9 calculations. These solutions reduce manual effort, improve accuracy, and enable rapid scenario analysis across multiple load cases. For the broader industry, embracing limiting crack size assessments translates into lower inspection costs, extended equipment life, and enhanced safety compliance, positioning firms to meet tighter regulatory expectations while maintaining operational efficiency.

Determine Limiting Crack Sizes and Remaining Life to Improve Your Next Equipment Inspection

By [Greg Thorwald](https://inspectioneering.com/author/783), Principal Engineer at Quest Integrity · Appears in the November/December 2025 issue of Inspectioneering Journal

Crack Analysis Introduction

When planning the next equipment inspection, operators may not expect to discover cracks in essential equipment, such as vessels, tanks, or pipes. Knowing the limiting crack sizes in these components before the inspection begins can improve efficiency and inform decisions about additional analysis or repairs. A remaining‑life fatigue crack growth analysis can help determine whether the equipment can operate until a more convenient future repair during a planned outage. This article describes various methods for crack analysis.

Knowing limiting crack sizes helps inspectors understand the size of cracks to search for and select the inspection method. If the limiting crack sizes are small, the inspection may require tighter detection tolerances and more advanced methods; for larger cracks, broader detection tolerances can accelerate the inspection process. If cracks are found, comparison to the limiting crack sizes can enable quick decisions about repair, replacement, or continuing to operate and monitor the equipment with additional analyses and inspections.

Failure Assessment Diagram

The API 579‑1/ASME FFS‑1 Fitness‑for‑Service (FFS) standard uses the Failure Assessment Diagram (FAD) method to evaluate cracks for both brittle fracture and plastic collapse. For example, Figure 1 shows a typical FAD plot for a pipe with three pressure load cases. The FAD’s y‑axis, Kr, is the toughness ratio of a crack’s stress intensity to the material’s toughness. Kr can include primary loads and secondary loads, such as weld residual stress. The FAD’s x‑axis, Lr, is the load ratio of the crack’s reference stress to the material’s yield strength. Conceptually, think of the reference stress as a type of limit load indicating how the remaining material carrying the load compares to a possible plastic collapse (i.e., net‑section collapse failure).

The FAD limiting curve starts at the upper left at a Kr value of 1.0 for the brittle fracture limit. If a crack’s stress intensity exceeds the toughness, the Kr ratio is greater than 1.0. The curving shape of the FAD limit curve from the upper left to the lower right accounts for the interaction between brittle and ductile failure behavior. The lower‑right end of the FAD limit curve has a maximum cutoff that can be specified by yield and tensile strength or by an alloy‑specific value, as described in API 579.

A crack’s FAD assessment point has an Lr, Kr coordinate, and the point is determined using the API 579 Part 9 methodology and crack solutions for stress intensity and reference stress, given in the Part 9 annexes. A point inside or below the FAD curve indicates a stable crack; a point outside or above the FAD curve indicates an unstable crack that could cause a failure; and a point on the FAD curve is at the critical limit, which is useful to determine limiting crack sizes and limiting load. From a technical perspective, the curve serves as a boundary—the critical transition between stable flaw tolerance and the onset of unstable crack propagation.

In Figure 1, the pipe example image shows a cutaway view of the surface crack (red profile), and the FAD plot shows results for three pressure load cases.

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About the Author

Greg Thorwald is a principal engineer at Quest Integrity in Boulder, Colorado, USA. He uses finite element analysis for structural, fracture, and fatigue crack growth assessments in a variety of fitness‑for‑service consulting projects. He is the lead developer of the commercial FEACrack software, an automated parametric 3‑D crack mesh generation program used to compute crack‑front stress… Read more »