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When failures occur in turbomachinery, they typically result from one or more mechanisms. This may include fatigue, environmentally assisted or stress corrosion cracking, creep rupture, erosion and foreign object damage.
In many cases, the residual stress levels present in the component play an important role in either extending life with favorable residual stresses or diminishing it with unfavorable ones.
Since residual stresses are introduced in practically every step of turbomachinery machining and fabrication processes, their effects must be considered, understood and managed to maximize component life and performance.
By controlling or introducing engineered residual stresses, premature failures can often be mitigated, and the service life of components extended. Accurate quantitative measurement of residual stress in components is necessary to achieve this goal.
Measuring residual stress on a turbine blade via the x-ray diffraction technique[/caption]
Stress corrosion cracking
Stress corrosion cracking (SCC), also known as environmentally assisted cracking, is sometimes characterized as stress-assisted, grain-boundary oxidization (SAGBO) in high-temperature applications. It is a major source of potential failures in the power industry, as well as in process industries, pulp mills, storage vessels, and even aircraft structures.
Figure 1: Synergy is required for stress corrosion cracking to occur[/caption]
Sustained tensile stress (resulting from the superposition of residual and applied stresses) above a material’s SCC threshold is the main component of the stress corrosion cracking triangle (Figure 1).
Other factors are a susceptible material and a slightly corrosive environment. SCC is characterized by selective intergranular corrosion whereby multiple cracks initiate and eventually propagate.
The most obvious solution to SCC is to change the environment. However, that is rarely possible. The addition of a coating can, in principle, break the SCC synergy by sequestering the material from its environment. But this is often impractical or only a short-term solution if the coating wears or degrades over time.
Another approach is to change the material to one less susceptible to SCC. That can be either expensive or unfeasible. Thermal stress relief can be used to help reduce the magnitude of tensile residual stresses to levels below the SCC threshold.
However, if heating reaches the annealing temperature, as would be required to effectively stress-relieve the component, heat treatment itself may negatively affect the desired material’s mechanical properties. Heat treatment is, therefore, only a partial solution in many instances.
Often, the only effective solution to the SCC problem is the introduction of compressive residual stresses. A variety of stress management methodologies can be used to impart compressive residual stresses into a component.
This can include peening, rolling, expanding and their variants. Some are typically applied at the time of fabrication (e.g., shot peening, laser shock peening (LSP), cavitation peening and rolling).
Others can be applied in the field on components already in service (e.g., ultrasonic impact treatment (UIT), flapper peening and split-sleeve cold expansion (Cx). SCC synergy can be broken if the compressive residual stresses imparted are sufficient to overcome the sustained in-service applied stresses and keep the total magnitude of stress below the SCC threshold.
Quantitative residual stress measurement methods, including x-ray diffraction (XRD), can be used to characterize the stress state of components that may be susceptible to SCC either before or after they have been put into service.
This helps verify that a compressive residual stress of sufficient magnitude has been installed (one that overcomes any in-service applied stresses so that the total residual stress level remains below the SCC threshold).
Figure 2: Residual stress vs. depth profiles for ground and shot-peened Ck45 steel[/caption]
Take the case of ground and shot-peened Ck45 steel (Figure 2). Residual stress measurements via XRD were used to compare the effects of grinding and shot peening on the surface and subsurface residual stress state of a component prior to being placed into service. This indicated that the residual stress of the component became more compressive after being shot peened.
When turbomachinery components undergo cyclic loading, they can become susceptible to high-cycle fatigue (HCF) or low-cycle fatigue (LCF). In cases of LCF failures, the in-service stress exceeds the cyclic elastic limit of the material.
LCF is characterized by cumulative fatigue damage associated with failure occurring in the range of 104 to 105 cycles and multiple cracks in highly stressed areas. Residual stresses can evolve with cumulative fatigue damage and can be monitored periodically at predefined inspection intervals to assist in assessing component condition.It has been demonstrated that LCF life is sensitive to changes in residual stress. A 5% to 10% difference in stress can result in a 50% difference in lifespan. Thus, errors in the representation of total stress have a profound effect on life if residual stress is not considered.
The longer fatigue lives, as in the case of HCF, failures generally initiate at precise stress-concentration sites with cracks propagating from a single initiation. Residual stresses remain constant until close to the end of a component’s life; thus, an understanding of the residual stress state at the start of life is critical.
The ways in which surface and near-surface residual stresses have impacted fatigue-life underscore the importance of studying surface treatments and manufacturing processes. If reliable fatigue-life estimates are to be made (for example, via fracture-mechanics predictions), it is necessary to accurately characterize the residual stress fields.
Fatigue life can be extended and failures mitigated by the introduction of engineered residual stresses via a variety of processes, including Cx, LSP, shot peening, cavitation peening, burnishing and UIT (Figure 3).
Figure 3: Bending fatigue S-N curves for Ck45 steel tested in sea water. This illustrates the positive effect of shot peening on fatigue life[/caption]
Such treatments should be optimized for specific application to maximize effectiveness. By obtaining accurate information about the in-service loading spectra, material properties, component geometry, and residual stress fields, more reliable life-assessment and fracture-mechanic predictions can be made.
Erosion and creep should also be considered. The interaction of liquids or particles in the gas path of turbomachinery can result in the erosion of certain components. The loss of material due to erosive impacts can compound into a loss of cross section, the formation of pits, and eventually, the formation of micro-cracks at the surface.
In combination with cyclic loading, the stress concentrations at a crack tip may provide sufficient energy for fatigue cracks to initiate and propagate. Steam turbine blades that operate at relatively high temperatures can be especially susceptible to erosion resulting from the exfoliation of tubing scales in the system or from liquid droplet erosion.
Understanding and managing residual stresses at susceptible locations, such as the leading edge of a blade, can assist in delaying or mitigating the initiation of fatigue cracks where erosion damage is present and presumably unavoidable.
By increasing the effective depth of compressive residual stresses at these locations, tolerance to damage can be improved. In such cases, deeper-reaching engineered residual stress may be required as the allowable material loss increases. However, conventional shot peening alone may be insufficient, and deeper compressive layers may need to be introduced via LSP or UIT.
Creep is the tendency of a solid material to move slowly or deform permanently under the influence of persistent mechanical stresses. This process can be accelerated with increasing temperature. Over time, creep may result in a fracture via creep rupture.
High engineered compressive residual stresses ahead of crack tips delay creep crack initiation and increase creep initiation life. Conversely, high tensile residual stresses due to unmanaged fabrication processes can promote creep crack initiation and decrease the time to crack initiation.
Residual stresses have little or no effect on creep crack growth rates. Therefore, surface and near-surface residual stresses are the most critical when engineered residual stresses are introduced to prevent or delay their initiation.
Since deeper-reaching compressive stress processes, such as LSP or UIT do not impart the highest compressive residual stress at the surface, a double-peening process may be desired for two reasons.
It provides the maximum compressive stresses at and near the surface, as well as providing deeper-reaching compression. Double-peening treatments can be optimized and their effects maximized by measuring residual stress post processes.
Once optimal engineered residual stress fields have been established and formally defined, they can be monitored and audited over time to confirm that components have been correctly processed by a given supplier.
The selected methodology of measurement must be accurate, quantitative and suitable for surface, near-surface and subsurface residual stresses to account for both crack initiation and propagation considerations in life modeling and analysis.
James Pineault is Lab Manager at Proto Manufacturing in Oldcastle, Ontario, a company specializing in XRD equipment and measurement services. For more information call +1 519-737-6330 or visit protoXRD.com.