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ASME PTB-6-2013 — Guidelines for Strain Gaging of Pressure Vessels Subjected to External Pressure Loading in the PVHO-1 Standard
📐 ASME PTB-6-2013 is a technical guideline sponsored by the ASME Pressure Technology Codes and Standards (PTCS) organization through the ASME Standards Technology, LLC (ASME ST-LLC), prepared by Lawrence J. Goland of Southwest Research Institute and issued on June 21, 2013. Its subject is strain gaging of pressure hulls — the pressure vessels of human-occupancy systems such as submersibles, diving bells, and atmospheric diving suits built to ASME PVHO-1, the Safety Standard for Pressure Vessels for Human Occupancy — when the hull is subjected to external hydrostatic test pressure loading. The guideline explains how to select and place strain gages so that the test monitors structural behavior and the onset of collapse, allowing the hydrostatic test to be halted before significant damage occurs.
💡 For a hull under external pressure, the governing limit state is buckling/collapse rather than internal-pressure leakage. The external hydrostatic test is essentially the proof test implied by the PVHO-1 design rules; strain gaging converts the invisible precursor of buckling into a measurable deviation in the strain-versus-pressure curve.
1. Nature and Scope of the Guideline ⚠️
The guideline states at the outset that strain gaging of a pressure hull under external hydrostatic test pressure serves two purposes. First, and foremost, it monitors the structural behavior and response of the vessel under external pressure so that the resulting strains and stresses can be compared with those obtained from the design analyses. Second, properly placed gages can indicate the onset of collapse of the hull during the external hydrostatic pressure test, so that the test can be halted prior to causing significant damage and/or collapse. The document explicitly declares that the two examples it presents are illustrative only: they establish no actual strain gaging requirements of any code, design rules, or jurisdictional body, and they do not establish mandatory gage locations, gage types, or numbers of gages. For each hull, the actual strain gaging plan is a function of many factors — the chamber’s configuration, the number and size of openings, attachments, actual as-built geometry, weld details, and whether the goal is merely validating an analysis and/or monitoring hull behavior to preclude collapse.
⚠️ This guideline is not a mandatory appendix of PVHO-1 and imposes no minimum gaging requirements. The example hulls (ring-stiffened cylinders, hemispherical heads, etc.) are typical geometries; the actual geometric deviations of each real hull — out-of-circularity (OOC), out-of-fairness (OOF), out-of-sphericity (OOS) — can be the sites of stress concentration and local buckling initiation and must be evaluated individually.
2. Strain Gage and Rosette Selection 🔧
Under external pressure, a hull develops compressive stress in both the circumferential and longitudinal (or radial) directions, so a single uniaxial gage cannot capture the biaxial stress state. The guideline systematically compares the strain gage and rosette types and their recommended uses:
| Gage Type | Stress State Measured | Typical Use and Limitations |
|---|---|---|
| Uniaxial strain gage | Linear strain in one direction | Locations where the principal direction is known (e.g., circumferential on the mid-shell of a cylinder); economical, but cannot resolve an unknown biaxial state |
| Biaxial (Tee) strain gage rosette | Linear strains in two perpendicular directions | Where the principal directions are roughly known but the transverse component must be confirmed, e.g., head-to-shell transition zones |
| Triaxial strain gage rosette | Three in-plane directions; principal strains and their directions can be resolved | Regions with unknown stress directions or complex stress fields (near openings, supports, attachments); most complete information |
💡 Selection rule of thumb: when the goal is to validate the axial-plus-hoop analysis on a geometrically regular hull, uniaxial or biaxial gages suffice; when the goal is to monitor complex regions that might initiate buckling, triaxial rosettes are required because the buckling initiation point is often accompanied by combined stresses of uncertain direction.
3. Placement Principles and Hull Nomenclature 📍
The guideline provides a nomenclature for pressure hulls and general placement principles, emphasizing that gages should cover three classes of locations:
- General locations: the smooth mid-shell regions away from openings and attachments, used to compare overall strain levels with the design analysis;
- Regions of concern: local positions where as-built geometry (out-of-circularity, out-of-fairness, out-of-sphericity), openings, attachments, or weld details could make the hull a starting point for high stress or instability;
- Complex layouts: systematic overlays of gages at the end bay regions of ring-stiffened cylindrical hulls (the most instability-prone zone), hemispherical heads, and other complex geometries, serving both “validate analysis” and “monitor behavior” objectives.
| Geometric Deviation | Symbol | Effect on the Hull |
|---|---|---|
| Out-of-Circularity | OOC | Cross-section departs from a true circle, lowering the external-pressure buckling critical pressure |
| Out-of-Fairness | OOF | Local concavity/convexity of the shell surface, causing local stress concentration |
| Out-of-Sphericity | OOS | Spherical/hemispherical head departs from an ideal sphere, reducing stability margin |
The guideline presents two worked examples, from a basic gaging plan (minimal number of gages at general locations, useful for validating strain and stress analyses) to a complex gaging plan (the largest number of gages, at both general locations and regions of concern due to as-built geometries, useful for monitoring hull behavior during the test), with thirteen illustrative figures (Figures 6.1–6.13) showing exact placement on ring-stiffened cylinder end bays and hemispherical heads.
🚨 As the external hydrostatic test approaches the critical pressure, the strain-versus-pressure curve departs from linearity (stiffness softening). The test team must stop pumping and vent immediately when obvious non-linearity or abnormal local strain growth is detected — never continue pressurizing merely to “watch the collapse.”
4. Engineering Practice Points 🛠️
- 🔍 Define the objective before placing gages: if only validating the FE/analytical solution, a basic plan (few gages at general locations) suffices; if also monitoring for buckling precursors, upgrade to a complex plan covering both general locations and regions of concern.
- 📊 Record strain-versus-pressure in real time: during the test, collect each gage’s strain as a function of external pressure and overlay it on the design linear/non-linear curve; any deviation must be evaluated immediately.
- 🧭 Use the actual as-built geometry, not the nominal: the measured OOC/OOF/OOS after fabrication usually differs from nominal design values, and these deviations are precisely what reduces the buckling critical pressure — place gages at the largest measured deviations.
- 🔧 Attachments and openings are weak links: manways, viewports, penetrations, and supports concentrate stress and destabilize locally; rosette placement should concentrate on them.
- 📋 Keep records: placement diagrams, gage types, bridge wiring, calibration, and test data should be fully archived as part of the test report to support future service and re-testing.
5. Frequently Asked Questions ❓
❓ Q: Is strain gaging mandatory for the external hydrostatic test under PVHO-1?
A: The guideline recommends gaging to monitor structural behavior and instability precursors, but as its foreword and abstract state, actual placement is not made a mandatory minimum by this guideline or by PVHO-1; whether and how to gage depends on the test objectives and the specific hull.
A: The guideline recommends gaging to monitor structural behavior and instability precursors, but as its foreword and abstract state, actual placement is not made a mandatory minimum by this guideline or by PVHO-1; whether and how to gage depends on the test objectives and the specific hull.
❓ Q: Is a uniaxial gage sufficient?
A: Only where the stress direction is known and the geometry is regular (e.g., the circumferential direction of a cylinder mid-shell). For complex stress fields near openings, attachments, and head transitions, biaxial or triaxial rosettes are needed to resolve the principal stresses.
A: Only where the stress direction is known and the geometry is regular (e.g., the circumferential direction of a cylinder mid-shell). For complex stress fields near openings, attachments, and head transitions, biaxial or triaxial rosettes are needed to resolve the principal stresses.
❓ Q: How does external-pressure testing differ from internal-pressure testing in monitoring purpose?
A: Internal-pressure testing focuses on pressure retention and leakage, with strain monitoring mainly validating strength; the limit state for external-pressure testing is buckling/collapse, so strain monitoring focuses on capturing stiffness softening and instability precursors.
A: Internal-pressure testing focuses on pressure retention and leakage, with strain monitoring mainly validating strength; the limit state for external-pressure testing is buckling/collapse, so strain monitoring focuses on capturing stiffness softening and instability precursors.
❓ Q: Can the guideline’s examples be copied directly?
A: No. The guideline explicitly states the two examples are illustrative only and establish no code requirements. The gage count for each hull is a function of chamber configuration, openings, attachments, actual as-built geometry, and weld details, and must be designed for the specific hull.
A: No. The guideline explicitly states the two examples are illustrative only and establish no code requirements. The gage count for each hull is a function of chamber configuration, openings, attachments, actual as-built geometry, and weld details, and must be designed for the specific hull.
