The Stress-Strain Microprobe (SSM) uses an automated ball indentation technique to obtain flow data from a localized region of a test specimen or component, providing an essentially non-destructive measurement of yield strength—especially suited to complex or highly variable microstructures such as weldments and weld heat affected zones.
Abstract
The Stress-Strain Microprobe (SSM) uses an automated ball indentation technique to obtain flow data from a localized region of a test specimen or component. This technique is used to rapidly determine the yield strength and microstructural condition of a variety of materials including pressure vessel steels, stainless steels, and nickel-base alloys. The SSM provides an essentially non-destructive technique for the measurement of yield strength data. This technique is especially suitable for the study of complex or highly variable microstructures such as weldments and weld heat affected zones. In this study 119 distinct SSM determinations of the yield strength of eight engineering alloys are discussed and compared to data obtained by conventional tensile tests. The sensitivity of the SSM to the presence of residual stresses is also discussed.
Introduction
A Stress-Strain Microprobe (SSM) system was procured to obtain reliable tensile data from specimens and components where standard tensile tests were not feasible (e.g., the heat affected zone (HAZ) of a weldment) or desirable (e.g., an in-service component).
Test Technique
SSM data were obtained using a portable (Model Number PortaFlow-P1) SSM test system. This system has been described in detail in References 1 - 4. The test method is based on strain controlled, multiple indentations (at a single penetration location) of a polished surface by a spherical indenter. Typical indenter diameters are 62 and 20 mils although other sizes have been used for specific purposes. This test method, also referred to as Automated Ball Indentation, consists of cyclically loading and unloading a spherical indenter into a polished surface under computer control. Loads are increased monotonically in each successive cycle. Applied loads and resulting displacements are measured during both loading and unloading using standard load cell and linear variable differential transformer (LVDT) transducers. A typical load displacement plot is shown in Figure 1 for AISI 304L stainless steel.
A first-degree polynomial fit is used to analyze this load-displacement data. These data are used to derive a true stress-true plastic strain curve, yield strength, and other tensile-type data. A typical true stress-true strain plot including the 0.2 percent offset yield strength for AISI 304L stainless is shown in Figure 2. Determination of yield strength using the SSM does not involve the extrapolation of SSM data, obtained at relatively high strains (Figure 1), back to the onset of yielding. Such extrapolations, Reference 5, result in significant deviations from true yield strengths.
Instead, the yield strength is calculated using a SSM test parameter and empirical correlations for distinct material classes (steels, nickel-base, etc.). This technique allows yield strengths to be reliably obtained for alloys of the same material class which have been subjected to property altering processes such as cold work, hot work, heat treatment, welding, and exposure to neutron radiation.
Tables 1 and 2 provide a summary of the materials, SSM derived yield strengths and, where available, actual tensile yield strength data. These tensile tests utilized several specimen designs and were conducted in accordance with the requirements of ASTM Standard Test Method E8.
Test Results
Tables 1 and 2 summarize yield strength data obtained from both SSM and standard tensile testing of materials with strengths ranging from about 30 to 185 ksi. A total of 119 distinct SSM tests were performed on sections of weldments, cut and mounted for (destructive) metallographic examination, the grip ends of actual tensile specimens, actual lifting and handling components which were returned to service after SSM (non-destructive) testing and tensile specimens subjected to simultaneous axial loading (Table 3). Comparing average values, all SSM yield strength data are within 6 percent and most SSM yield strengths are within 1 to 3 percent of the tensile yield strength data.
SSM profiles across weldments, from base metal through the HAZ and into the weld metal, have demonstrated that the HAZ in the ASTM A508 Class 4 (MIL-S-23194 Composition F) weldments are significantly (20 percent) stronger than the weld and base metal. While some variations in yield strength were noted a similar trend was not observed for the HAZ of the AISI 304L and the nickel-base Alloy 600 and 690 weldments.
Finally, the magnitude of the standard deviation (as indicated by 1σ values in Tables 1 and 2) in SSM yield strength data ranges from 0.6 ksi for AISI 4340 base metal to 6.8 ksi for data obtained in the weld metal of an Alloy 690 plasma arc weldment. In the absence of other sources of variability (e.g., significant microstructural differences) this range in 1σ may be due to residual stress.
To assess the sensitivity of SSM tests to residual stress, tests were performed on Composition F, base metal specimens with and without the application of known tensile stresses. A ring fixture, Figure 3, was used to maintain the applied load during SSM testing. The specimen gage section was rigidly supported during the test to minimize the effect of bending.
Measurements were taken at applied stresses of 0, 20, 34.9 and 57.5 ksi or, 0, 20, 35 and 58 percent of the reported uniaxial yield strength for this material. As listed in Table 1 SSM testing determined that Composition F yield strength was 99.6 ksi ± 1.9 ksi, in excellent agreement with the material certification data.
SSM test results as a function of applied stress are listed in Table 3. Baseline data (zero applied load) were obtained on two distinct specimens. These results, 91.6 and 92.7 ksi, are about 7 ksi less than the value reported in Table 3. In order to ensure that the SSM equipment was operating properly, calibration tests were performed on a certified test block. This calibration testing produced acceptable values. Accordingly, the basis for this difference in the baseline (zero applied load) yield strength of Composition F is not apparent at this time. Two SSM tests were conducted at each of the applied load levels.
Discussion
Comparison with Tensile Data
Excellent agreement between SSM derived and tensile-type 0.2 percent offset yield strengths for a wide range of materials demonstrates that reliable flow-type data can be obtained from test specimens and actual components. The localized nature of this test method allows the determination of strength profiles in complex structures such as weldments.
In some weldments, significant increases in yield strength have been noted in the HAZ. For a given finite strain, this gradient in yield strength indicates that the HAZ would sustain a higher stress than either the base or weld metal. Therefore, based just on stress, the HAZ of a Composition F weldment is potentially more susceptible to, say, environmental effects than either the base or weld metal. Clearly, the microstructure of the HAZ may also play a role in the susceptibility of the weldment to environmental effects.
Sensitivity to Residual Stresses
Residual stresses may affect the SSM derived yield strength. Flow properties measured by the SSM should include contributions due to all types of stresses. This concept is illustrated, Figure 4, using a typical load-deflection curve. From the origin - point A, (zero applied load and zero displacement) the uniaxial loading of a member results in an initial linear deflection - point B, until sufficient load has been applied to cause a permanent or plastic deviation from linear behavior - point X. At an offset displacement of 0.2 percent, the load at point X corresponds to the material’s 0.2 percent offset yield strength.
Now, consider a specimen with an applied load, point B - Figure 4, to simulate a residual stress, and subject that specimen to SSM testing. From point B a reduced load and a reduced displacement is required to reach point X - the offset yield strength - as compared to the unstressed condition. Accordingly, a specimen with an applied load should appear to have a reduced yield strength. Conversely, the presence of an initial compressive load, point C, should result in an apparent increase in the yield strength as measured by the SSM.
In this discussion it is assumed that the stress states caused by the SSM test and the applied stress are identical (and therefore additive) and that the material is homogeneous and isotropic. In reality, while the bulk stresses due to an applied load in a tensile specimen are largely uniaxial, the SSM spherical indenter produces a complex, tri-axial stress state. Furthermore, typical engineering materials are neither completely homogeneous nor isotropic. Accordingly, some deviation from this idealized view would be expected.
Yield strength data are plotted as a function of applied stress in Figure 5. Interestingly, the trend from zero to the maximum applied stress for SSM measured yield strength is a monotonically decreasing function if the 99.6 ksi (Table 1) value for the baseline (zero applied stress) is used. If the 92 ksi (Table 3) baseline yield strength is used then the apparent yield strength increases before decreasing as the applied stress is increased. However, in either case and based on this data it appears that the SSM is sensitive to the presence of applied stresses.
Conclusions
The SSM has been used to obtain reliable flow-type data from a wide range of engineering materials. The localized nature of the method allows strength gradients, which may exist in complex structures such as weldments, to be resolved. For example, a significant difference in the yield strength of the HAZ (as compared to either the base or weld metal) was noted for a Composition F, low alloy steel weldment. Finally, the technique appears to be sensitive to the presence of residual stresses. Accordingly, the SSM potentially can be used to probe a test sample or component of known yield strength to estimate the residual stress level. However, residual stress sensitivity must also be considered as a possible source of error when flow data are obtained using the SSM.
KAPL-P-000189, p.9 · SSM = Stress-Strain Microprobe · HAZ = Heat Affected Zone
| Zone | Method | Individual values (ksi) | Mean ± 1σ (ksi) |
|---|---|---|---|
| Composition F — Gas Tungsten Arc | |||
| Base Metal | SSM | 102.4, 102, 95.4, 100.3, 97.6, 103.2, 99.2, 99.1, 97.6, 99.5 | 99.6 ± 2.4 |
| Base Metal | Tensile Test | 100.5 | 100.5 |
| HAZ | SSM | 123.9, 116.7, 116.6, 126.0 | 120.8 ± 4.9 |
| Weld Metal | SSM | 105.8, 97.8, 100.6, 100.3 | 101.1 ± 3.4 |
| Weld Metal | Tensile Test | 103 | 103 |
| AISI 304L Stainless Steel — Tungsten Inert Gas | |||
| Base Metal | SSM | 27.4, 29.9, 29.3, 31.1, 29.6, 32, 29 | 29.8 ± 1.5 |
| Base Metal | Tensile Test | 31.4 | 31.4 |
| HAZ | SSM | 33.1, 32.8, 32.2, 33, 36.9, 37, 37.8, 36.2 | 34.9 ± 2.3 |
| Weld Metal | SSM | 31.9, 30, 34.2, 36.1 | 33.1 ± 2.7 |
| Alloy 600 — Tungsten Inert Gas (Set 1) | |||
| Base Metal | SSM | 40.3, 48.7, 46.2, 54.1 | 47.3 ± 5.7 |
| HAZ | SSM | 42.4, 42.4, 40.6, 45.9, 41.2, 44.8, 40.2, 45.2 | 42.8 ± 2.2 |
| Weld Metal | SSM | 41, 35.3, 38, 42.8 | 39.3 ± 3.3 |
| Alloy 600 — Tungsten Inert Gas (Set 2) | |||
| Base Metal | SSM | 49.2, 51.3, 51, 51, 58 | 52.1 ± 3.4 |
| HAZ | SSM | 42.1, 47, 55.1, 56.1, 47.7, 45, 54.2, 54.4, 44.7, 48.5 | 49.5 ± 5.1 |
| Weld Metal | SSM | 47.9, 44.2, 53.3, 53.6 | 49.8 ± 4.5 |
| Alloy 600 — Submerged Arc | |||
| Base Metal | SSM | N/A | N/A |
| Base Metal | Tensile Test | N/A | N/A |
| HAZ | SSM | N/A | N/A |
| Weld Metal | SSM | 57.8, 57.2, 56, 57.5, 57.9, 56.3 | 57.1 ± 0.9 |
| Weld Metal | Tensile Test | 56.0 | 56.0 |
| Alloy 690 — Plasma Arc Weld | |||
| Base Metal | SSM | 51.4, 54.1, 51.1 | 52.2 ± 1.7 |
| HAZ | SSM | 51.8, 53.3, 47.9, 51.9 | 51.2 ± 2.3 |
| Weld Metal | SSM | 37.6, 38, 51.7, 46.3 | 43.4 ± 6.8 |
Table notes: (1) Heat Affected Zone. (2) Stress-Strain Microprobe. (3) Average ±1σ for data listed above. Alloy 600 — Tungsten Inert Gas is reported in the source as two stacked data sets, preserved here as Set 1 and Set 2.
KAPL-P-000189, p.10 · SSM = Stress-Strain Microprobe
| Material | Location / Method | Individual values (ksi) | Mean ± 1σ (ksi) |
|---|---|---|---|
| PM 690 (Powder Metallurgy) | Base Metal — SSM | 59.4, 61.4, 63.8, 63.1, 64.5 | 62.5 ± 2.1 |
| Base Metal — Tensile Test | 60.5 | 60.5 | |
| 17-4 PH (Precipitation Hardened) | Base Metal — SSM | 186.2, 184.9, 182.2 | 184.4 ± 2.0 |
| Base Metal — Tensile Test | 185 | 185 | |
| AISI 4340 — Heat 1 | Base Metal — SSM | 135.8, 136.4, 133.2 | 135.1 ± 1.7 |
| Base Metal — Tensile Test | 138.4 | 138.4 | |
| AISI 4340 — Heat 2 | Base Metal — SSM | 98.9†, 100.0, 99.3 | 99.4 ± 0.6 |
| AISI 4340 — Lifting & Handling Components (SSM) | Loc. 1 | 124.2, 123.8, 123.2 | 123.7 ± 0.5 |
| Loc. 2 | 125.8, 124.2, 126.3 | 125.4 ± 1.1 | |
| Loc. 3 | 128.1, 124.1, 123.3 | 125.2 ± 2.6 |
Table notes: (1) Powder Metallurgy. (2) Precipitation Hardened. (3) Stress-Strain Microprobe. (4) Average ±1σ for data listed above. † See transcription note 1.
KAPL-P-000189, p.11 · Two SSM tests at each applied load level
| Applied Stress (ksi) | Apparent Yield Strength (ksi) |
|---|---|
| 0 | 91.6, 92.7 (1) |
| 20 | 98.8, 96.7 |
| 34.9 | 90.2, 92.4 |
| 57.5 | 86.8, 83.5 |
(1) Based on SSM tests of tensile specimens subsequently loaded in tension. Yield is about 7 ksi less than previous SSM average for the same material.
Figures
The original figures are plots and schematics. Their captions and the numeric axis labels are transcribed below for reference.
- Depth, mm (top axis)
- 0.03, 0.05, 0.08, 0.10, 0.13, 0.15, 0.18, 0.20, 0.23, 0.25
- Load, lbs. (left axis)
- 50, 100, 150, 200, 250, 300, 350, 400, 450, 500
- Load, N (right axis)
- 222, 445, 667, 890, 1112, 1334, 1557, 1779, 2002, 2224
- Depth, mils (bottom axis)
- 1.00, 2.00, 3.00, 4.00, 5.00, 6.00, 7.00, 8.00, 9.00, 10.00
- True Stress, ksi (left axis)
- 15, 30, 45, 60, 75, 90, 105, 120, 135, 150
- True Stress, MPa (right axis)
- 103, 207, 310, 414, 517, 621, 724, 827, 931, 1034
- True Strain, % (bottom axis)
- 0.02, 0.04, 0.06, 0.08, 0.10, 0.12, 0.14, 0.16, 0.18, 0.20
- Labels
- Specimen (Composition F); Ring (X-750)
- Axes
- Load (vertical); Displacement (horizontal)
- Labelled points
- A (origin: zero load, zero displacement); B (applied tensile load); C (initial compressive load); X (0.2% offset yield point)
- SSM Yield Strength, ksi (left axis)
- 70, 75, 80, 85, 90, 95, 100
- Applied Stress, ksi (bottom axis)
- 0, 20, 40, 60, 80, 100
- Legend
- ◇ SSM Data — Non-tensile Specimens · ○ SSM Data — Tensile Specimens
References
- FM Haggag, ‘In-Situ Measurements of Mechanical Properties Using Novel Automated Ball Indentation System’, ASTM STP 1204, p. 27, 1993.
- United States Patent, Number 4,852,397, ‘Field Indentation Microprobe for Structural Integrity Evaluation’, 8/1/89.
- FM Haggag et al., ‘The Use of Field Indentation Microprobe in Measuring Mechanical Properties of Welds’, in Proceedings of the 2nd International Conference on Trends in Welding Research, (edited by SA David and JM Vitek), p. 843. ASM International, Metals Park, OH (1989).
- FM Haggag et al., ‘Use of Automated Ball Indentation Testing to Measure Flow Properties and Estimate Fracture Toughness in Metallic Materials’, Applications of Automation Technology to Fatigue and Fracture Testing, ASTM STP 1092, (edited by AA Braun, NE Ashbaugh, FM Smith), p. 188. ASTM, Philadelphia, PA (1990).
- P Au, et al., ‘Flow Property Measurements From Instrumented Hardness Tests’, in Non-Destructive Evaluation in the Nuclear Industry, p. 597. ASM, Metals Park, OH (1980).
Transcription Notes & Accuracy Checks
- AISI 4340 — Heat 2, first SSM reading (Table 2). Printed in the source as “8.9”. The original is a degraded scan that explicitly warns “Portions of this document may be illegible.” The leading digit is clipped; the value is reconstructed here as 98.9. With 98.9, 100.0, 99.3 the mean is 99.4 and 1σ is 0.6 — exactly the values the paper reports (99.4 ± 0.6). ↩
- All individual SSM and tensile readings were transcribed and then cross-checked by re-averaging them and recomputing the sample standard deviation. The results reproduce the paper’s printed means and 1σ values to within the paper’s own one-decimal rounding.
- Two values differ from the recomputed figure by 0.1 ksi due to rounding in the original, not transcription: PM 690 base-metal SSM mean (readings average 62.44; paper prints 62.5) and Alloy 600 — Submerged Arc weld-metal SSM 1σ (readings give 0.79; paper prints 0.9). Individual readings and reported means agree in both cases.
- The text reports the Composition F SSM yield strength as 99.6 ksi ± 1.9 ksi (p.6), while Table 1 lists the base-metal SSM average as 99.6 ± 2.4 ksi. Both figures are reproduced exactly as they appear in their respective locations.
- Values are reproduced as printed, including mixed precision (e.g., “102”, “103”, “126.0”, “100.0”). Apostrophe glyphs shown as a placeholder box in the scan were normalized to a standard apostrophe. ↩