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ASME STP-PT-070-2014 — Design Guidelines for the Effects of Creep, Fatigue & Creep-Fatigue Interaction with Design-by-Analysis and Nondestructive Inspection Acceptance Criteria
⏳ ASME STP-PT-070-2014 is a high-temperature component design report sponsored by ASME ST-LLC and written jointly by Structural Integrity Associates (Ian Perrin, Dan Peters, Nat Cofie) and the Electric Power Research Institute (Jonathan Parker, John Shingledecker). Addressing the failure of high-temperature pressure-retaining components in power plants (main steam piping, headers, high-temperature manifolds) under creep, fatigue, and creep-fatigue interaction, the report first systematically reviews the leading international codes (ASME VIII-2, API 579/ASME FFS-1, Section III NH, EN 12952/13445, UK R5, BS7910, French RCC-MR), then develops a design-by-analysis methodology intended for ASME Boiler and Pressure Vessel Code Section I, demonstrates a complete worked example on Grade 91 (T/P91) steel, and proposes radiographic (RT) and ultrasonic (UT) flaw acceptance criteria. It is an important milestone toward an integrated “analysis-based design plus in-service nondestructive evaluation” methodology for high-temperature components.
💡 The fundamental difference between high-temperature components and ambient-temperature pressure vessels is “time-dependent damage”: above the creep regime, even stresses well below the ambient allowable cause time-dependent deformation and locally accumulated damage. High-temperature design therefore cannot stop at stress checks — it must include a design life assessment.
1. Overview: High-Temperature Failure Modes and the Design Starting Point 📌
Creep, fatigue, and their interaction are the three sources of damage in high-temperature components:
- Creep: time-dependent plastic deformation accumulated under constant load and high temperature, ultimately leading to creep rupture.
- Fatigue: crack initiation and growth driven by cyclic loads (startups, load changes) at stress concentrations (nozzle roots, branch connections).
- Creep-fatigue interaction: hold time at high temperature superimposes creep damage and fatigue damage at the same location; the interaction is far from linear and is the part of high-temperature design requiring the most careful treatment.
| Damage Mode | Driving Load | Key Parameters | Typical Locations |
|---|---|---|---|
| Creep | Constant load + high temperature | Temperature, stress, time | Main steam pipe straights, headers |
| Fatigue | Cyclic loads (startup/shutdown, load changes) | Cycle count, stress range, notches | Nozzle roots, branch connections |
| Creep-fatigue interaction | Cyclic loads + high-temperature hold | Hold time, linear vs. interactive damage summation | High-temperature headers, dissimilar-metal welds |
⚠️ Creep-fatigue interaction cannot be treated simply by “creep damage + fatigue damage ≤ 1” linear summation: depending on material and temperature, the interaction can amplify or shrink the damage tolerance. The report systematically compares how each code handles the interaction (damage envelopes, etc.); the method matched to the material data should be selected.
2. Review of International Codes and Key Differences 🔍
Chapter 3 systematically reviews the applicability and differences of the leading codes:
- ASME BPVC Section VIII, Division 2: provides a complete design-by-analysis framework (including elastic-plastic analysis and fatigue assessment), but high-temperature creep material data are limited.
- API 579-1 / ASME FFS-1: fitness-for-service methods for rating in-service components with flaws under creep and fatigue.
- ASME BPVC Section III, Subsection NH (now HBB): the most complete high-temperature nuclear design flow, but data and procedures target nuclear applications.
- EN 12952-3/4 and EN 13445-3: European boiler and pressure vessel creep design clauses; conservative and easy to apply.
- R5 (UK): an advanced procedure for high-temperature creep-fatigue and flaw assessment, with both elastic and inelastic routes.
- BS7910 and RCC-MR: flaw assessment and high-temperature design guidance, respectively.
The report also distills the similarities and differences among the codes in yield criterion, reference temperature, perfectly-plastic vs. strain-hardening behavior, cyclic hardening/softening, and the treatment of creep-fatigue interaction (Section 3.1).
3. Development of the Design-by-Analysis Methodology 🛠️
Chapter 4 proposes a design-by-analysis method for high-temperature boiler components under ASME Section I. The protective design checks progress layer by layer:
- Design specification and material data: define design life, load cycles, and temperature history; establish high-temperature strength properties (for Grade 91 as an example, including allowable stress, creep rupture data, and fatigue curves).
- Protection against collapse (net section rupture): ensure overall load-carrying capacity at both plastic collapse and net-section rupture levels.
- Protection against shakedown and ratcheting: verify elastic or low-cycle shakedown of the cyclic loading superposed on steady primary loads to prevent cycle-by-cycle deformation accumulation.
- Protection against fatigue: accumulate fatigue damage using the ASME fatigue curve for carbon and low-alloy steels.
- Protection against local creep damage and creep-fatigue: assess creep and creep-fatigue damage using time-fraction and damage/strain-fraction criteria, concluding with a design life statement.
The report completes a full worked example on a Grade 91 component (Section 4.3), from specification, material properties, and FE modeling to each check, demonstrating the operability of the method, and identifies the design resources required for implementation (Section 4.4).
🚨 The credibility of design-by-analysis depends on material data quality: if the creep rupture extrapolation, cyclic fatigue curves, and hold-time correction factors used in the creep-fatigue assessment have unclear origins or overly long temperature extrapolation, even the finest finite element analysis will produce non-conservative conclusions. Material data and the analysis method must be matched and traceable.
4. Nondestructive Inspection (NDT) Flaw Acceptance Criteria 📏
Because design-by-analysis permits demonstrating the safety of flawed components analytically, supporting flaw acceptance criteria are essential:
- Radiographic (RT) criteria: Section 5.1 gives recommended RT acceptance judgments based on crack-like flaw size and location, interfacing with the creep and fatigue assessments.
- Ultrasonic (UT) criteria: Section 5.2 gives recommendations for flaw sizing, location, and acceptance judgments in UT, emphasizing quantitative characterization of flaws rather than a simple “record or repair” decision.
- The acceptance criteria are formulated around the closed loop of “flaw size — remaining life — inspection interval”, so that NDT results feed directly into FFS and remaining-life assessment.
💡 Engineering practice note: the NDT acceptance criteria established at the design stage should be “back-propagated” to fabrication inspection and in-service inspection, ensuring that any flaw reaching the design-assumed initial size can be reliably detected; otherwise the “initial flaw assumption” of the design-by-analysis loses its foundation.
5. Frequently Asked Questions ❓
❓ Q: What is creep-fatigue interaction and why cannot it be a linear sum?
A: During high-temperature hold time, creep damage at the crack tip competes and cooperates with the fatigue damage of each cycle; the interaction damage envelope is often nonlinear. Linear summation may underestimate damage in some conditions and overestimate it in others, so the interaction model in the code must be matched to the material data.
A: During high-temperature hold time, creep damage at the crack tip competes and cooperates with the fatigue damage of each cycle; the interaction damage envelope is often nonlinear. Linear summation may underestimate damage in some conditions and overestimate it in others, so the interaction model in the code must be matched to the material data.
❓ Q: How does this report relate to ASME VIII-2 design-by-analysis?
A: VIII-2 is a mature framework for ambient/lower-temperature design-by-analysis but has limited high-temperature creep data; this report builds on its philosophy to add material data requirements, design check flows, and NDT acceptance criteria for creep, fatigue, and creep-fatigue assessment of boiler components.
A: VIII-2 is a mature framework for ambient/lower-temperature design-by-analysis but has limited high-temperature creep data; this report builds on its philosophy to add material data requirements, design check flows, and NDT acceptance criteria for creep, fatigue, and creep-fatigue assessment of boiler components.
❓ Q: What does the Grade 91 example tell us?
A: It demonstrates the complete flow from design specification, material properties, and FE modeling through “anti-collapse → anti-ratcheting → anti-fatigue → anti-creep → anti-creep-fatigue” checks, proving that the method can be applied to design-life assessment of real high-temperature components.
A: It demonstrates the complete flow from design specification, material properties, and FE modeling through “anti-collapse → anti-ratcheting → anti-fatigue → anti-creep → anti-creep-fatigue” checks, proving that the method can be applied to design-life assessment of real high-temperature components.
🔍 Q: What role do NDT acceptance criteria play in design-by-analysis?
A: The NDT acceptance criteria define the “acceptable initial flaw size”, which together with the creep/fatigue damage assessment forms a closed loop: design-assumed flaw → nondestructive detection → remaining life from damage assessment → inspection interval determination.
A: The NDT acceptance criteria define the “acceptable initial flaw size”, which together with the creep/fatigue damage assessment forms a closed loop: design-assumed flaw → nondestructive detection → remaining life from damage assessment → inspection interval determination.
