Appearance
A manufacturer of drill collars (AISI 4145H steel, Ø60 mm bore × 3,000 mm, 285–341 HB) was experiencing fatigue failures at 180,000–320,000 cycles (target 500,000 cycles at 200 MPa bending stress). XRD measurement (Cr-Kα, sin²ψ, 2 mm collimator) revealed a compressive surface residual stress of −350 MPa transitioning to a tensile subsurface peak of +50 MPa at 50–80 µm depth — the crack initiation site. A four-method measurement campaign (XRD, ICHD, DHD, contour method) fully characterised the stress state. A change to two-pass drilling (roughing Vc = 55 m/min, f = 0.18 mm/rev; finishing Vc = 45 m/min, f = 0.06 mm/rev, ap = 0.4 mm) eliminated the tensile peak, producing a monotonic compressive profile (−450 MPa at surface to −50 MPa at 200 µm). Fatigue life exceeded 600,000 cycles — more than double the original.
Residual Stress Measurement Methods for Deep Hole Bores
Method Comparison for Bore Surface Residual Stress Measurement
| Method | Measurement Depth | Depth Resolution | Spatial Resolution (lateral) | Stress Component | Accuracy (±MPa) | Destructive? | Bore Accessibility | Applicable Materials | Standard / Reference |
|---|---|---|---|---|---|---|---|---|---|
| X-ray diffraction (XRD) | 1–20 µm (depends on material and radiation) | N/A (surface average) | 0.5–5 mm (collimator size) | Biaxial (σ₁₁, σ₂₂ in surface plane) | ±20–50 | No (essentially non-destructive) | Requires access for X-ray head (probe diameter 10–50 mm) | Crystalline materials (metals, ceramics) | ASTM E2860, EN 15305 |
| Synchrotron XRD (high energy) | 10–500 µm (depth profiling by energy scan) | 5–50 µm | 10–100 µm | Triaxial (full stress tensor) | ±10–30 | No | Limited (requires beamline access) | Crystalline materials | — |
| Incremental centre hole drilling (ICHD) | 0–2 mm | 5–20 µm (near surface), 20–50 µm (deep) | 1–3 mm (rosette centre hole) | Biaxial (σ₁₁, σ₂₂ at each depth increment) | ±20–50 | Semi-destructive (1–2 mm hole) | Requires strain gauge rosette installation on bore surface | Any material (strain gauge based) | ASTM E837 |
| Deep hole drilling technique (DHD) | Full wall thickness (1–50 mm) | 50–200 µm | 1.5–3 mm (reference hole diameter) | Triaxial (full stress tensor at each depth step) | ±15–40 | Destructive (core removal) | Requires reference hole drilled through bore wall | Any material | — |
| Contour method | Full cross-section (2D map) | 10–100 µm (depends on cutting quality) | 50–500 µm | Uniaxial (stress component normal to cut plane) | ±20–60 | Destructive (sectioning) | Requires wire EDM cutting through bore | Any material (conductive for EDM) | — |
| Slitting method (crack compliance) | Full cross-section (1D profile) | 20–100 µm | N/A (integrated over slit depth increment) | Uniaxial (stress normal to slit plane) | ±15–40 | Destructive (slitting) | Requires sectioning of component | Any material | ASTM E2819 |
| Magnetic Barkhausen noise (MBN) | 10–200 µm (magnetic penetration depth) | N/A (depth-weighted average) | 1–10 mm (probe contact area) | Qualitative (stress + microstructure) | ±50–100 (semi-quantitative) | No | Good (small probe, 5–20 mm diameter) | Ferromagnetic materials only | — |
XRD Measurement Parameters for Bore Surface Residual Stress
| Measurement Parameter | Recommended Value (Steel Bore) | Material-Specific Adjustments | Effect on Measurement Quality | Practical Limitation |
|---|---|---|---|---|
| Radiation (X-ray wavelength) | Cr-Kα (λ = 2.290 Å) for steel; Cu-Kα (λ = 1.542 Å) for aluminium; Ti-Kα (λ = 2.750 Å) for titanium | Higher atomic number materials need shorter wavelengths; heavier radiation gives shallower penetration | Shorter wavelength = deeper penetration (Cu: 5–10 µm vs Cr: 1–5 µm in steel); affects surface specificity | Tube anode must match material; changeover requires tube replacement |
| Diffraction plane (hkl) | (211) for ferritic steel; (311) for aluminium; (213) for titanium | Select high-angle diffraction peak (2θ > 120°) for optimal strain sensitivity | Higher 2θ angle = higher strain sensitivity (differential accuracy improves as cot(θ)) | Peak must have adequate intensity; low-intensity peaks increase measurement time |
| Measurement angles (ψ) | 11–13 ψ angles in range 0° to ±45° (sin²ψ method) | Minimum 9 angles for biaxial stress; add ψ-negative for shear stress detection | More ψ angles = better statistics but longer measurement time; 13 angles typical | Equipment geometric constraints may limit maximum ψ angle to ±40° |
| Collimator size | 2 mm (standard); 1 mm (high spatial resolution); 5 mm (high speed) | Smaller for steep stress gradients; larger for homogeneous regions | Smaller collimator = better spatial resolution but lower X-ray intensity (longer measurement) | Minimum practical collimator: 0.5 mm (insufficient intensity below this) |
| Measurement time per ψ angle | 30–120 seconds (depends on material, collimator, and tube power) | Longer for coarse grain materials (e.g., aluminium) and small collimators | Longer = better statistics (lower uncertainty) | Practical limit: ~15 minutes total per measurement point for production |
| Number of measurement points along bore | 5–10 points (axial distribution) | More points for long bores (> 1,000 mm) or where stress varies with depth | More points = better depth-dependent characterisation | Access time: each point requires repositioning of XRD head in bore |
| Statistical uncertainty | ±20–50 MPa (95% confidence) | Depends on peak intensity, number of ψ angles, and material grain size | Lower uncertainty from: higher peak counts, more ψ angles, finer grain size | Fundamental limit: ±10–15 MPa for ideal laboratory conditions |
Typical Residual Stress Profiles from Deep Hole Drilling Operations
| Drilling Method | Material | Process Parameters | Surface Stress (MPa) | Peak Compressive Stress (MPa) | Depth of Peak Compression (µm) | Crossover Depth (to tensile) (µm) | Subsurface Stress Characteristic | Fatigue Life Implication |
|---|---|---|---|---|---|---|---|---|
| BTA drilling (single pass) | AISI 4140 (28 HRC) | Vc = 80 m/min, f = 0.12 mm/rev, p = 100 bar | −280 ± 30 | −350 ± 30 | 30–50 | 50–80 (tensile peak +30 MPa at 80 µm) | Compressive at surface → tensile subsurface peak → gradual decay to zero | Reduced: tensile subsurface peak serves as fatigue initiation site |
| BTA drilling (finish pass) | AISI 4140 (28 HRC) | Vc = 45 m/min, f = 0.06 mm/rev, ap = 0.4 mm | −420 ± 35 | −450 ± 35 | 10–20 | 150–250 (no tensile — monotonic compressive decay) | Monotonic compressive decay from surface to bulk | Excellent: no tensile region, high compressive magnitude |
| Gun drilling | 316L stainless | Vc = 60 m/min, f = 0.03 mm/rev, p = 80 bar | −320 ± 30 | −380 ± 30 | 20–40 | 80–120 (tensile −10 to 0 MPa at 120 µm) | Compressive at surface → near-neutral in subsurface → zero | Good: minimal tensile region, adequate for most applications |
| Gun drilling | Ti-6Al-4V | Vc = 35 m/min, f = 0.025 mm/rev, p = 100 bar | −250 ± 40 | −300 ± 40 | 15–30 | 40–60 (tensile +50 to +100 MPa at 60 µm) | Compressive at surface → tensile subsurface peak → decay | Reduced: tensile peak significant relative to material yield strength |
| BTA drilling (aggressive) | AISI 4145H (32 HRC) | Vc = 100 m/min, f = 0.20 mm/rev, p = 80 bar | −150 ± 40 | −200 ± 40 | 50–80 (tensile +80 MPa at 80–120 µm) | 50–100 (dual tensile peaks +50 to +80 MPa) | Compressive surface → strong tensile subsurface → compressive → decay | Poor: dual tensile peaks create multiple potential crack initiation sites |
| Skiving + roller burnishing | SAE 1026 (180 HB) | Burnishing: 0.15 mm interference, 2 passes | −550 ± 50 | −600 ± 50 | 50–100 | 200–400 (monotonic decay — no tensile) | Deep monotonic compressive profile | Excellent: deep compression, work-hardened surface layer |
Measurement Procedures and Process Optimisation
Uncertainty Sources in Bore Residual Stress Measurement
| Uncertainty Source | XRD (±MPa) | ICHD (±MPa) | DHD (±MPa) | Contour (±MPa) | Magnitude Control | Mitigation Strategy |
|---|---|---|---|---|---|---|
| Peak position determination | 10–25 | N/A | N/A | N/A | Higher peak counts, more ψ angles | Use centroid method instead of peak maximum; increase counting time |
| Material elastic constants | 5–15 | 5–10 | 5–10 | 5–10 | Accurate literature values or independent measurement | Measure elastic constants by tensile test on same material batch |
| Stress-free lattice parameter (d₀) | 10–30 | N/A | 10–30 | 10–30 | Critical for triaxial analysis | Measure from powder sample of same material; use compliance correction |
| Strain gauge misalignment | N/A | 10–25 | N/A | N/A | Rosette alignment to principal axes | Use precision alignment fixture; verify with optical microscope |
| Hole eccentricity (ICHD) | N/A | 10–20 | N/A | N/A | Drilled hole position relative to rosette centre | Use orbital drilling (ECM or air-abrasive) for precise hole |
| Wire EDM cutting damage (contour) | N/A | N/A | N/A | 15–30 | EDM recast layer and thermal damage | Optimise EDM parameters (rough cut + finish cut + polishing) |
| Reference hole drilling (DHD) | N/A | N/A | 10–25 | N/A | Hole straightness and diameter variation | Use gun drilling for reference hole; measure hole straightness |
| Material grain size | 10–30 | N/A | N/A | N/A | Large grains cause spotty diffraction rings | Use oscillation (ψ or φ) during measurement; larger collimator |
| Surface roughness | 5–20 | 5–15 | 5–10 | 5–15 | Rough surface affects X-ray penetration and strain gauge bonding | Polishing (for XRD: electropolish to remove machining-affected layer) |
| Temperature variation | 2–5 | 2–5 | 2–5 | 2–5 | Thermal expansion stresses | Control temperature to ±1°C; measure reference at same temperature |
Process Optimisation Using Residual Stress Data
| Process Change | Effect on Surface Residual Stress | Effect on Subsurface Profile | Effect on Fatigue Life | Implementation Complexity | Cost Impact |
|---|---|---|---|---|---|
| Reduce feed rate (f −50%) | More compressive: −150 to −250 MPa → −300 to −450 MPa | Reduces tensile peak magnitude; shifts crossover depth deeper | +100–300% improvement | Low (CNC parameter change) | Neutral (reduced MRR but longer tool life) |
| Reduce cutting speed (Vc −30%) | More compressive: −200 to −300 MPa → −350 to −450 MPa | Reduces tensile peak; may increase surface compression depth | +50–200% improvement | Low (CNC parameter change) | Neutral (reduced MRR) |
| Increase coolant pressure (+50%) | More compressive: −50 to −100 MPa shift | Reduces thermal contribution to tensile stress | +25–75% improvement | Medium (pump upgrade if pressure limited) | Moderate (pump energy cost) |
| Add finish boring pass (ap = 0.3–0.5 mm) | Eliminates tensile subsurface peak entirely | Changes from compressive-tensile-compressive to monotonic compressive decay | +200–500% improvement | Medium (two-pass programming + tool change) | Moderate (additional pass cycle time) |
| Add roller burnishing after drilling | Surface: −450 to −600 MPa; depth: 200–400 µm compression | Deep compressive layer without tensile subsurface region | +300–800% improvement | High (additional operation + burnishing tool) | High (additional capital equipment) |
| Add stress relief heat treatment (150–200°C) | Minimal change (< 50 MPa reduction) | Reduces all stress magnitudes by 10–30% | −50 to +25% (may reduce beneficial compression) | Medium (furnace capacity required) | Moderate (energy + cycle time) |
| Optimise guide pad geometry (increased land width) | More compressive: −50 to −100 MPa shift | Increases burnishing-induced compression depth | +25–75% improvement | High (tool redesign) | Low (one-time tool modification cost) |
FAQ
What is the best method for measuring residual stress in a deep hole bore?
There is no single "best" method for residual stress measurement in deep hole bores — the optimal approach depends on the depth of interest, spatial resolution requirements, whether destructive analysis is acceptable, and the material being measured. For practical purposes, the methods are selected based on the measurement depth regime: (1) Surface and near-surface (0–20 µm) — X-ray diffraction (XRD) is the standard method. It is non-destructive (or minimally destructive, requiring only access for the X-ray head), provides biaxial stress measurement with ±20–50 MPa accuracy, and is applicable to all crystalline materials. For deep hole bores, the main limitation is access: the XRD probe must fit inside the bore (minimum bore diameter approximately 15–20 mm for standard probes, 5–10 mm for specialised small-probe systems). The measurement depth is limited to 1–20 µm depending on the X-ray wavelength and material. (2) Surface to 2 mm depth — incremental centre hole drilling (ICHD) is the standard method. A strain gauge rosette is bonded to the bore surface, and a small hole (0.5–2 mm diameter) is drilled incrementally through the centre of the rosette. The strain relief measured at each depth increment is used to calculate the residual stress profile. ICHD is semi-destructive (leaves a 1–2 mm hole) and provides depth-resolved biaxial stress with ±20–50 MPa accuracy. For deep bores, rosette installation on the bore surface is challenging but achievable with specialised tooling (bore-mounted strain gauge installation tools). (3) Full wall thickness (2–50 mm) — the deep hole drilling technique (DHD) is the most established method for thick-section components. A small reference hole (1.5–3 mm diameter) is gun-drilled through the bore wall, and the diameter of this hole is measured precisely before and after trepanning a core (typically 5–10 mm diameter) containing the reference hole. The diameter change gives the triaxial residual stress profile through the wall thickness. DHD is destructive (removes a core sample) but provides the most complete through-thickness stress profile. (4) Full 2D cross-sectional map — the contour method provides a complete 2D residual stress map across a cross-section. The component is cut in half by wire EDM, and the surface contour of each half is measured (by CMM or laser scanning). The difference between the measured contour and a flat plane is used to calculate the residual stress normal to the cut plane. The contour method is destructive but provides unique 2D mapping capability. For most deep hole drilling applications where the primary concern is the surface and near-surface stress (0–200 µm), the recommended measurement strategy is: XRD for surface measurement (fast, non-destructive, direct measurement) followed by ICHD for the near-surface profile. If through-thickness stress information is required (for thick-walled pressure vessels or drill collars), add DHD or the contour method. The measurement campaign should always include multiple measurement locations along the bore length (typically 5–10 points) to capture the axial variation in residual stress caused by the changing process conditions with depth.
How does the residual stress profile affect fatigue life of deep hole drilled components?
The residual stress profile in deep hole drilled components has a direct and dominant effect on fatigue life because the total stress experienced by the component is the superposition of the applied cyclic stress and the residual stress from manufacturing. For a deep hole bore subjected to cyclic bending or pressure loading, the critical locations are at the bore surface and at any subsurface peak in the residual stress profile. The relationship between residual stress and fatigue life follows these principles: (1) Compressive residual stress at the surface is beneficial because it reduces the effective tensile stress experienced during the loading cycle. A surface compressive stress of −400 MPa superimposed on a cyclic tensile stress of +200 MPa means the net surface stress during the tensile portion of the cycle is only −200 MPa (compressive), which is insufficient to initiate a fatigue crack. The same component without residual stress would experience a tensile stress of +200 MPa at the surface, which could initiate a crack in 10⁵–10⁶ cycles depending on the material. (2) A tensile subsurface peak is detrimental because it creates a region of elevated tensile stress below the surface where a fatigue crack can initiate and grow. This is the failure mechanism observed in the drill collar case study: the subsurface tensile peak of +50 MPa at 50–80 µm depth became the crack initiation site because the combined applied + residual stress exceeded the material's fatigue limit at that depth. (3) The depth of the compressive layer matters: a deep compressive layer (200–500 µm from roller burnishing or low-feed finish boring) provides protection against surface damage and wear that could otherwise remove the beneficial compressive layer. A shallow compressive layer (< 50 µm) can be easily removed by minor surface damage or corrosion. (4) The residual stress profile interacts with the applied stress gradient: for bending loads, the maximum applied stress is at the surface, so surface compression is most important. For pressure vessel loading (hoop stress), the maximum applied stress is at the bore surface, again favouring surface compression. For components with both bending and pressure loading, the full depth profile must be considered. The quantitative relationship between residual stress and fatigue life can be estimated using the modified Goodman diagram or the Smith-Watson-Topper (SWT) parameter, which accounts for the mean stress effect. A typical result for steel drill collars (AISI 4145H) is: surface compression of −400 MPa with monotonic compressive decay to 200 µm depth provides a fatigue limit (infinite life) at ±250 MPa bending stress, compared to ±150 MPa for the as-received (no residual stress) condition — a 67% improvement. For finite life design (10⁵ cycles), the improvement is typically 2–5× in cycles to failure, depending on the applied stress level.
How does the two-pass BTA drilling strategy affect residual stress compared to single-pass drilling?
The two-pass BTA drilling strategy (roughing pass followed by finish boring) produces a fundamentally different and more favourable residual stress profile compared to single-pass drilling. The mechanisms and outcomes are: (1) Single-pass BTA drilling — the cutting edge removes material while the guide pads burnish the bore surface. The guide pads apply high normal pressure (200–500 MPa contact pressure) to the bore surface, creating severe plastic deformation and compressive residual stress to a depth of 30–80 µm. However, the cutting action generates high temperatures (600–900°C at the cutting edge), and the thermal gradient creates a tensile stress region just below the burnished layer. When the cutting heat conducts into the bore wall and then cools, the contraction of the subsurface material is constrained by the cooler bulk material, generating tensile residual stress. The resulting profile is typically: compressive at the surface (−200 to −400 MPa) → tensile subsurface (+20 to +80 MPa at 50–150 µm depth) → compressive again (from the elastic-plastic boundary at 150–300 µm) → gradual decay to zero. The subsurface tensile peak is the most detrimental feature for fatigue life. (2) Two-pass drilling with finish boring — the roughing pass removes most of the stock (typically 80–90% of the material). The finishing pass removes a thin layer (0.3–0.5 mm depth of cut) at conservative parameters (low feed, moderate speed). The finishing pass removes the subsurface tensile peak from the roughing pass and establishes a new, shallow compressive layer. Because the finish pass uses a low feed rate (f = 0.05–0.08 mm/rev), the heat generation is substantially lower than the roughing pass, and the resulting profile is monotonic compressive decay: high surface compression (−400 to −500 MPa) → gradually decreasing compression → zero at 150–250 µm depth. No tensile peak is present because the thermal gradient from the finish pass is insufficient to overcome the mechanical compressive effect. (3) Process window — the finish boring depth of cut must be sufficient to remove the tensile peak from roughing: typically 0.3–0.5 mm, depending on the roughing parameters. If the finish pass depth of cut is too small (< 0.2 mm), the tensile peak may remain beneath the new surface. If it is too large (> 0.8 mm), the finish pass itself may generate a new tensile peak. The recommended finish pass depth of cut is 0.3–0.5 mm, which removes the roughing-induced tensile zone (typically at 50–150 µm depth) and establishes a new, stable compressive profile. (4) Economic trade-off — the two-pass strategy increases cycle time by 30–50% (adding the finishing pass) but can increase tool life by 2–4× (roughing tools are retired earlier, finishing tools have very low wear due to conservative parameters) and improve component fatigue life by 2–5×. For fatigue-critical components (drill collars, landing gear, pressure vessels), the two-pass strategy is strongly recommended despite the additional cycle time.
What are the practical challenges of measuring residual stress inside deep holes?
Measuring residual stress inside deep holes presents significant practical challenges that limit the application of standard measurement methods designed for flat or externally accessible surfaces. The specific challenges for each method are: (1) XRD on bore surfaces — the X-ray probe must be positioned inside the bore at the measurement location. Standard XRD systems have probes 30–80 mm in diameter and 100–200 mm long, limiting measurements to bores > 30 mm diameter. For smaller bores (10–30 mm), specialised miniaturised probes are available but with reduced X-ray power (requiring longer measurement times) and limited ψ angle range (reducing measurement accuracy). The probe must be aligned with the bore surface at a fixed distance (typically 10–20 mm standoff), and the alignment must be maintained within ±0.1 mm as the probe moves between measurement points along the bore length. For bores < 10 mm diameter, XRD measurement is generally not practical with current technology. (2) ICHD on bore surfaces — strain gauge rosettes are designed for flat surfaces. Installing a rosette on a curved bore surface requires: a flexible backing rosette that conforms to the bore curvature (available for bore diameters > 15 mm); a specialised bore-mounted installation tool that presses the rosette against the bore surface with controlled pressure during adhesive curing; and a precision drilling guide that centres the drilling tool over the rosette centre. The drilling operation (typically air-abrasive or orbital drilling) must be performed inside the bore, requiring a specialised drilling endoscope or right-angle drilling attachment. (3) DHD for through-thickness measurement — the reference hole (1.5–3 mm diameter) must be gun-drilled through the bore wall from the bore surface outward, requiring gun drilling capability from within the bore. The trepanning operation removes a 5–10 mm diameter core surrounding the reference hole, which must be extracted from the component. For long bores, the measurement is typically performed at 3–5 axial locations, requiring sectioning of the component. (4) The contour method — requires cutting the component in half through the bore centreline using wire EDM, completely destroying the component. For a long bore, the measurement provides a 2D stress map on the cut plane, but only one component can be cut per measurement campaign. The practical recommendations for bore residual stress measurement are: for routine production monitoring, use MBN (magnetic Barkhausen noise) as a fast, non-destructive screening tool, calibrated against XRD measurements at representative locations; for process development and qualification, use XRD at 5–10 locations along the bore (for bores > 20 mm diameter) complemented by ICHD at 2–3 critical locations; and for failure analysis or high-value components, add DHD or contour method for complete through-thickness characterisation.
How are residual stress profiles interpreted to guide process improvement?
Residual stress profiles from deep hole drilled bores are interpreted by identifying three key features: the surface stress magnitude, the subsurface profile shape (monotonic decay vs. tensile peak), and the depth of the compressive layer. Each feature provides distinct guidance for process improvement: (1) Surface stress magnitude — a surface compressive stress of −200 to −600 MPa is typical for BTA and gun drilling, depending on parameters. If the surface stress is less compressive than −200 MPa (e.g., −50 to −150 MPa), this indicates insufficient mechanical deformation from the guide pads, possibly due to: low feed rate causing excessive rubbing rather than cutting (increased thermal stress); low coolant pressure reducing the hydrodynamic lubrication and increasing frictional heating; or worn guide pads reducing the burnishing pressure. The corrective action is to increase feed rate (if below 0.05 mm/rev), increase coolant pressure (to 100–180 bar), or replace guide pads. (2) Subsurface profile shape — the most critical feature for fatigue life. A monotonic compressive decay (stress → zero without crossing into tension) is the ideal profile. A compressive → tensile → compressive profile indicates that thermal effects from the cutting process have created a tensile layer beneath the surface. The corrective action depends on the tensile peak magnitude and depth: for tensile peaks < +50 MPa at depths < 100 µm, reducing cutting speed by 10–20% or increasing coolant pressure by 30–50% often eliminates the peak; for tensile peaks > +50 MPa or depths > 100 µm, a finish boring pass (0.3–0.5 mm depth of cut at low feed) is the most reliable corrective action. (3) Depth of compressive layer — a shallow compressive layer (< 50 µm) provides limited protection against surface damage, wear, or corrosion pitting. A deep compressive layer (> 150 µm) provides robust protection. To increase the compressive layer depth: increase coolant pressure (deeper cooling reduces the thermal gradient); reduce cutting speed (less thermal input reduces the depth of thermally affected material); or add a roller burnishing operation after drilling (can extend compression to 200–500 µm depth). (4) Axial variation — comparing residual stress profiles at multiple axial positions (e.g., bore entry, mid-point, and bottom) reveals the effect of drill tube extension on the stress state. If the stress profile varies significantly with depth (e.g., compressive at entry but tensile at the bore bottom), this indicates that drill tube dynamics (increased vibration at longer extensions) are affecting the stress state. The corrective action is to apply stability lobe diagram methodology to select chatter-free parameters at the critical depth, or to add a steady-rest or vibration damper to the drill tube.
This article provides an overview of residual stress measurement methods for deep hole drilling applications. Method selection, measurement procedures, and data interpretation depend on the specific component geometry, material, and performance requirements. Consulting with residual stress measurement specialists and validating measurement results through fatigue testing is recommended for safety-critical applications. The technical data presented here reflects published research and documented case studies as of 2026.