African Fusion March 2021

eNTSA small sample analysis

microscope image with the load/displace- ment curve assists in accurately identifying critical crack initiation during post-test evaluation. With this information the criti- cal strain energy density (wc) can be calcu- lated, which can be used as a ranking tool for describing the qualitative toughness of plant material. In his work on the effect of graphitisa- tion on the fracture toughness, Grewar – Modelling the Effect of Graphitization on the Fracture Toughness (J_IC) of Service Exposed ASTM A-515 Gr. 65 material by the Small Punch Test Method – also showed good correlation between Yield Strength mea- sured by traditional methods versus Yield Strength estimated using small punch test analysis. The data as presented in Figure 7 shows the correlation between punch load and deflection data on the same graph as the true stress and strain plot, demonstrat- ing that the simulation method enables SPT alone to be used to determinematerial properties for stress versus strain. Creep property analysis and SPCT A typical Small Punch Creep Testing (SPCT) sample is the same as that used for SPT (see Figure 8). eNtsa has developed and is run- ning a fleet of sixteen SPCT platforms that use a ceramic ball and punch to generate creep data for both the petrochemical and power generation industries. Key design aspects of the include: • A lever armmechanism for dead weight loading. • A ceramic ball-punch configuration. • An argon purged sample furnace. • Inline load monitoring. • Deflectionmonitoring via the punch rod. • Dual, indirect sample temperature con- trol & monitoring. Test sample preparation, calibration and meticulous loading and control of all parameters, variables and test environment are critical for obtaining repeatability in this type of test setup and environment. As part of a validation process for the SPCT methodology, eNtsa first embarked on a zero creep life study by doing tests on a section of material known to have failed in service due to creep exhaustion. The zero creep life aimed to determinewhether a de- crease in life to rupture could be observed as samples are approaching the failure region. This rupture curve of zero-creep samples at the point of failure would then represent the zero creep life ‘end point’ for ongoing SPCT tests. For the zero-creep test, a sample was removed from a ruptured carbon steel steampipe bend as shown in Figure 9. Core

ric finite element model, before the actual physical punch test characteristics aremod- elled by calculating the nonlinear material properties via the Ramberg-Osgood func- tions. The nonlinear parameters are ad- justed until the response suitably matches the actual load versus displacement results. This correlation in actual versus simulated results is shown in Figure 6. When the simu- lation results suitably match the test curve up to the region of crack initiation, then the calculated parameters are considered usable for further analyses. From a very early stage in eNtsa’s in- volvementwith the SPTmethodology itwas decided to focus on strain energy density as a ranking tool rather than attempting to quantify fracture toughness. The abil- ity of the eNtsa SPT platform to provide a synchronised video of the real-time digital

with Acoustic EmissionMonitoring. The first step is to performa physical test on the disc (Figure 5), which is used to plot an experi- mental small punch curve, of punch load (N) versus displacement (mm) (Figure 6). The next phase of analysis is aimed at determining thematerial’s nonlinear stress and strain properties using the axisymmet-

Figure 5: Schematic illustration of applying the punch load via a ball onto the SP Disc.

Figure 6: SPT load versus displacement data (blackline) with modification of the nonlinear material properties to achieve a suitable load versus displacement correlation superimposed (red circles).

Figure 7: Typical Engineering data extracted from SPT after converting Punch load/deflection to Stress/Strain data.


March 2021


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