Table of Contents
12.6.1.3 Remedies for LCF Damage
The following provides remedies and preventive measures for mechanically-caused LCF damage. It is important that a specialist selects the suitable measures under consideration of factors such as operation and engine part types, in order to avoid a case of “disimprovement” (Volume 1, Chapter 3). The chapters that deal with individual components (Operating Loads and Part Behavior) contain further information. The topic of thermal fatigue is treated in a separate chapter. The measures can be divided into several larger categories (there is no claim to completeness):
Material selection:
- Select “good-natured” materials, i.e. materials with high breaking strain, high fracture toughness even in the case of corrosion, high dynamic strength, low notch sensitivity.
- Good operating experience.
- Even material properties throughout the entire volume (even in extremely thick and thin cross-sections).
- Good testability (non-destructive testing).
- Unproblematic workability.
- Low residual stresses from blank part manufacture.
- No special tendency to potential weaknesses such as pore formation, undesirable structures, segregation, or inclusions.
Construction:
- Select the stress levels in a way that ensures that the smallest serially detectable flaws are not capable of dangerous growth.
- Keep the life span within the incubation phase, as far as possible.
- Use proper dimensional design to avoid early spontaneous failure of the entire part in case of cracking.
- Avoid use of brittle, strongly bonding coatings in zones under high LCF loads.
- No potentially damaging coatings (e.g. corrosion, diffusion/embrittlement).
- Only prescribe proven auxiliary materials (oil, lubricants, cleaning materials).
- Proper hardening of critically stressed surfaces (shot peening, rolling, piercing, Ref. 12.6.1-20).
- Do not require any potentially damaging assembly procedures (scratches, etc.) One example: joining socket connections.
- Avoid rubbing that could cause potential hot crack danger.
- Avoid fretting zones (contact surfaces with micro-movements) in sensitive materials (e.g. Ti alloys).
Design:
- Do not over-utilize material strength, in order to ensure the largest possible critical crack size.
- Flaws with crack initiation potential must be safely detectable and not be unrealistically small.
- In zones with potential crack initiation danger (e.g. labyrinth racks), the largest possible stress gradient, decreasing in the direction of crack growth, is desirable. This slows crack growth and presents a possibility to discover cracks during inspections.
Production:
- Harden the surface.
- Pre-spin the unfinished parts (induces compressive residual stress in highly-stressed zones).
- Rework damages only after consultation with the responsible specialist personnel (fused deposits, smeared foreign materials, broken tools, etching).
- No unilateral changes to the production guidelines: machining parameters, equipment, machine type, procedure, auxiliary materials (e.g. cooling and cutting media), tools (e.g. grind disk type, turning steel).
- Use only approved marking procedures (engraving, etching, markers, and colors).
- Avoid procedures that could lead to hydrogen absorption.
Assembly/Handling:
- Do not mount any soiled flanges (worm formation).
- Only use approved assembly tools (e.g. made from the proper material).
- Conduct FEMA in the case of new assembly procedures (also with new engine types).
- Be careful of damages (e.g. scratches).
Quality assurance:
- Test parts for structural anomalies (e.g. etching process during production).
- Outturn testing of parts that are the series standard (casting and forging process).
- Optimize non-destructive testing procedures for flaws that increase dwell time effects.
- Coordinate the order of tests with the production procedures (e.g. no penetrative testing after shot peening).
- Sample (cyclical overspeed tests) representative parts that have run for an extended period.
Life span verification:
- Use original parts of the series standard.
- Provide sufficient part
- and material-specific dwell times: note tensile stress
- or compressive stress-sensitive materials.
- Follow-up inspections of specimen fracture surfaces to find signs of dwell time influence (e.g. cleavage cracks, etc.).
Procedure in case of acute damage:
- Inspect comparable parts (e.g. type, production, operating data) for cracking and signs of damage (fretting, etc.).
- Isolate the potentially affected parts: blank part manufacturers, batches, machining, operators, unique operating conditions, etc.
- Risk assessment: safe life span, inspection intervals, priority of parts for tests.
- Develop testing procedures on-site.
Note: experience has shown that, unlike cracks under high-frequency vibrations, which usually cause fatigue in the HCF range, LCF cracks under low-frequency loads (e.g. startup/shutdown cycles) can be caught in periodic inspections before they cause part failure and catastrophic damage.
Figure "Damage accumulation and dwell times" (Ref.12.6.1-2): The easiest possibility for estimating the total damage from creep stress and dynamic loads is a linear superposition of the damages. The estimation is analogous to the Miner Rule for a linear damage accumulation of multi-stage stresses (Fig. "Dynamic fatigue life span estimations (Miner rule)"). It can technically be applied to all types of low-cycle problems.
- Any order and size of load peaks.
- Any shape for the load/time progress.
- Free of contradiction from pure dynamic damage to pure creep loads.
It is especially important to ensure that the specimens used are representative of the material conditions in the part (Fig. "Lifespan verification by cyclic spin test ").
The various uses do not mean that the results always closely reflect reality. Dynamic damages and creep damages must occur simultaneously (left diagram). Centrifugal force-induced loads in rotor parts during the startup/shutdown cycle fulfill this demand fairly well. Dwell times during relief must be considered if they contribute to recovery processes. If thermal stress occurs due to the temperature changes, then the conditions for such a simple estimation are generally not met. The thermal stresses are usually phase-shifted relative to the stresses induced by centrifugal force. Therefore, the temperatures change under loads. In the case of thermal fatigue, relaxation processes cause the maximum stress to change (Fig. "Thermal fatigue promoting fatigue fractures"). If the maximum stresses are reached at very different temperatures, it can incite different creep mechanisms (Ill. 12.5.1-8). This would make the linear superposition completely unusable.
Another method of damage calculation is strain range partitioning (not shown), which takes into account plastic flow and creep (and relaxation).
Figure "Lifespan verification by cyclic spin test " (Refs.12.6.1-8, 12.6.1-12, and 12.6.1-22): LCF tests of specimens made from the titanium alloy Ti-6Al-4V yielded important realizations that make spectacular damages in large rotor parts (Examples 12.6.1-2 and 12.6.1-3) more understandable. Ref. 12.6.1-19 describes the material-specific sensitivity to compressive stress (e.g. Ti-6Al-4V, Waspaloy, etc.) or tensile stress (e.g. CrNi18/8 steels, IN100) during the dwell time. A dwell time sensitivity, in which fatigue and creep processes occur, was discovered in many materials.
The crack growth rate in Ti-6Al-4V can be several times greater with dwell times (test data: 5 minutes and 45 minutes) at the stress maximum than under dynamic loads without dwell times (test frequencies: 0.3-25 Hz). Interestingly, Ti-6Al-4V is sensitive to compressive stress (Ref. 12.6.1-19). In this case, the compressive stresses induced during LCF loading have an especially damaging effect during the “rest phase”.
- The effect of crack acceleration depends on the micro-structure and therefore also the heat treatment.
- The dynamic loads must act in an unfavorable direction (perpendicular) relative to the orientation of the sensitive structure.
- At lower operating temperatures (such as 20°C), the effect is considerably stronger than at high ones (the effect is negligible at 75°C).
- Marine atmospheres, i.e. watery NaCl solutions, accelerate crack growth.
- At temperatures below 75°C, strain-induced hydrogen embrittlement (also see Volume 1, Chapter 5.4.4) ahead of the crack tip seems to be the source of the dwell effect. It must be noted that the effects are subject to various influences. There are evidently titanium alloys with varying degrees of sensitivity (e.g. IMI 685, Example "Importance of proper testing"). The temperature up to which the dwell effect occurs also seems to be higher in especially sensitive structures. Supporting evidence for this can be found in high-pressure compressor damage (Example "Dwell time fatigue"). Temperatures here can be assumed to be around 200°C. This sensitivity is most likely related to the blank part production process (Example "Dwell time fatigue"), whereby part sizes play an important role. The larger the blank part, the more likely that unfavorable structures will result.
The dwell time sensitivity must be determined for design-relevant data of the materials and, if available, must be considered when determining LCF life spans. Compressive stress-sensitivity and/or tensile stress-sensitivity must also be taken into account in this process.
References
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