Strain Aging Test

Dynamic strain aging (DSA) is a phenomenon in which interstitial atoms (carbon, nitrogen) diffuse rapidly enough to pin moving dislocations during plastic deformation, causing discontinuous yielding and serrated stress–strain curves (Portevin–Le Châtelier effect). It results in a local reduction in work-hardening rate and uniform elongation, and a corresponding increase in yield strength. In pipeline and structural steels, DSA can significantly reduce toughness and ductility — particularly in the HAZ after welding — and its occurrence must be characterised and accounted for in design code compliance and fitness-for-service assessment.

Static strain aging (SSA) occurs when a material is first deformed plastically and then aged at elevated temperature (or ambient temperature over a longer time), allowing dislocations to be pinned before the next deformation event. The classic example is pipe pre-strain followed by FBE coating at 200°C–250°C. Dynamic strain aging (DSA) occurs when aging and deformation proceed simultaneously — the interstitials re-pin dislocations during ongoing plastic flow. Both are relevant to pipeline reeling: SSA affects the material after each pass over the reel drum; DSA affects the strain response during each pass. The hysteresis cycle test captures both effects simultaneously within the cyclic loop data.

The ±2% amplitude corresponds approximately to the maximum plastic bending strain experienced by a standard-wall pipeline during reel-lay over a typical reel drum geometry. DNV-ST-F101 and ISO 3183 Annex H adopted this level as the baseline qualification amplitude because it covers the majority of practical reel-lay installation scenarios. The ±4% amplitude represents a more severe condition applicable to high-grade, thick-wall pipes on tighter-radius reel drums or deep-water applications, and is also used in seismic design qualification where larger inelastic demand must be demonstrated. Both are well-established levels with an extensive body of comparative database in the industry.

The number of cycles depends on the application and specification requirement. For pipeline reeling qualification per DNV-ST-F101, the standard protocol applies a single complete cycle (one compressive half-cycle followed by one tensile half-cycle, or vice versa) to represent one pass over the reel and one reverse bend over the aligner. More complex programmes apply 3, 5, or 10 cycles to represent multiple installation passes or to characterise the transient hardening/softening behaviour to its stabilised state. For FEA model calibration, sufficient cycles to reach saturation (typically 5–20) are required to identify the saturated kinematic backstress. SMT can configure the test to the required number of cycles as specified by the client or applicable standard.

The Bauschinger effect is the reduction in yield strength in the reverse loading direction following prior plastic deformation in the forward direction. In a cylindrical hysteresis cycle test, it appears as a lower compressive yield point (or early onset of reversed plastic flow) on the second half of the first cycle compared to the initial tensile yield. The effect is quantified by the Bauschinger factor or stress ratio comparing the reverse yield stress to the forward peak stress. For pipeline steels, the Bauschinger effect is a critical parameter because it determines the residual stress state and springback after forming operations, and directly affects the reeling simulation in FEA. The hysteresis loop dataset from SMT's test captures the Bauschinger response from the first reverse loading half-cycle.

Yes — and this is a common programme structure for pipeline and pressure vessel material qualification. The sequence is: (1) extract sub-sized Charpy or CTOD specimens from the parent material; (2) apply the strain aging cycle(s) to a cylindrical specimen extracted from the same heat; (3) verify mechanical property retention (typically by re-testing tensile specimens subjected to the same strain-aging sequence). Additionally, full-section cycled specimens can be sectioned post-test for metallographic examination to characterise microstructural changes. SMT offers all required test types from a single laboratory, coordinated by a dedicated Job Manager, with consistent specimen traceability across the programme.

The most widely used cyclic plasticity models that can be calibrated from hysteresis loop data include: the Armstrong–Frederick kinematic hardening model (calibrated from a single half-cycle); the Chaboche multi-surface kinematic hardening model (calibrated from multiple superimposed backstress terms, typically requiring 5–10 cycles per amplitude level); and combined isotropic–kinematic hardening models (requiring loops at stabilisation for isotropic term identification). Calibration is performed directly from the experimental stress–strain data using optimisation routines in MATLAB, Python, or commercial FEA preprocessors. SMT provides the complete numerical loop dataset formatted for direct input into these calibration workflows.

The cylindrical hysteresis cycle test (TA120NNE, TA130NNE) is performed on SMT's servo-hydraulic machines under the ISO/IEC 17025 quality system, but is not within the current ACCREDIA accreditation scope (No. 00588). The accredited fatigue tests are force-controlled axial fatigue per ASTM E466 and ISO 1099. The hysteresis cycle test benefits from the same equipment calibration, extensometer traceability chain, and technical competence applied to accredited tests. For clients requiring accredited mechanical data, SMT's broad accreditation scope covering tensile, Charpy, fracture mechanics, and fatigue is available for the complementary tests in a combined qualification programme.