Figure "Show stopper during development": The term “show stopper” refers to a situation in which, in the midst of the development process, a property that is absolutely necessary for a technology is discovered to be in violation of a physical law. If merely technical difficulties cause development to fail, then it may be possible to resume development at a later process when the anciliary conditions have improved (Ill. 14-3). Even an altered safety philosophy can justify a renewed attempt as long as only technically solvable problems remain. A typical historical example is perpetual motion devices. It is often difficult to recognize that these violate the first and/or second laws of thermodynamics. This became clear only after expensive and elaborate constructs were assembled and tested. Experience has shown that individuals external to a project have a greater probability of recognizing errors. In some cases, however, their objections were never accpted by the engaged developers. In such a case, costs can increase indefinitely.

Even in engine technology, developments have been undertaken even though they were doomed to fail under the conditions of the time. These include:

• Fan rotor blades made from fiber-reinforced synthetics
• Carbon fiber-reinforced ceramics for hot parts in civilian engines
• Turbine blades made from tungsten-reinforced nickel alloys
• Cr-based alloys (nitrogen embrittlement?)
• Niobium-based alloys (oxidation?)
• Effusion cooled turbine blades (Fig. "Success of cooled turbine blade types")
• Turbine blades made from directionally solidified eutectic materials with ceramic fibers that were formed in situ. In this case, which is depicted in the diagram, operation-realistic tests showed that the thermal strain differences between the matrix and ceramic fibers caused the fibers to break after very few thermal cycles. This eliminated the reinforcing effect. It is interesting that, in the multi-year, “scientific” development phase of these materials, the extent of the problem was either not recognized or not reported. The problem only became known to the users after they had already begun production of prototype parts. One should not ignore the personal difficulties that this caused for the individuals whose existence depended on these projects.

Figure "Resistance to crack growth by strength of materials" (Ref. 14-4): Although this diagram is based on older information from the literature, it clearly shows two tendencies that reveal a rising trend of failure risks in rotor parts of fighter aircraft engines. In one sense, the problem is increasing stress levels. These can be attributed to higher RPM (higher compressor performance) with increased weight reduction. The harder disk materials necessary for realizing the demands show a decreasing resistance to crack growth (structure, Figs. "Factors influencing crack growth" and "Trends of materials applications"). However, an increased resistance to crack growth is desirable in order to safely control any crack growth that occurs under the high LCF loads. A further complication is the fact that crack growth accelerates at higher stress levels (Fig. "Characteristic crack growth"). In addition, high stress levels allow smaller flaws to become capable of growth. Although this is an evolutionary development, the risk is increased. Quality assurance to compensate for the rising risk is correspondingly expensive.

Figure "Technology for additional safety": As discussed in Fig. "Material behavior depending on design and technology", it is of great importance for the development risk, if a technology is merely intended to increase the safety of a part, or to increase its stress resistance, as well. In the first case, the technology will have a safety-increasing effect, in the second, it will increase risk. Typical examples in engine technology include thermal barrier coatings, shot peening, and the inclusion of the crack growth phase. In all of these cases, the technology was used for many years to increase safety, and only used as part of the part design after evolutionary improvements were made.

Figure "Trends of materials applications": The trend towards increasing, highly utilized material strengths tends to lead to a potentially risky bevahior of the components (Fig. "Resistance to crack growth by strength of materials"). This tendency can only be balanced out through increased quality assurance eforts, better analytical control of all relevant loads, damage tolerant concepts (Example "AS HIP"), and extensive experience with tests in near-operating conditions.

Risks for part safety are influence by three main factors:

• design, i.e. loads
• material properties
• failure behavior

Design and loads: The design-conforming operating loads increase with strength. This, in turn, increases the crack growth rate, causes crack growth to occur in smaller initial flaws, and makes residual fractures occur at smaller critical crack lengths (Fig. "Characteristic crack growth"). The consequences can be seen in several examples:
Small material weak points become life-determining flaws. Requirements for serially implementable non-destructive testing methods increase. Finishing processes must be specified and controlled more exactly. Damage such as scratches created during assembly and maintenance work becomes less tolerable.
High mean stresses reduce the tolerable stress amplitude, thus increasing the danger of fatigue damage.

Material properties and failure behavior: The yield strength ratio is the relationship of the breaking strength to the yield strength. As loads increased, material variants with higher yield strengths and similar breaking strengths were introduced. The yield strength ratio fell.The greater the difference between the yield strength and the breaking strength, the higher the probability that plastic deformation will occur before the ultimate failure (fracture). Warnings, such as serious imbalances, enable earlier safe detection.
If the fracture strain decreases as strength increases, the material becomes more brittle. This is to be expected when maximizing strength. This makes it more difficult to break down notch stresses without damage through plastic flowing. In hot part materials, as thermal reistance increases, the probability of high residual stresses increases, as well. High creep resistance places tight limits on possible stress relief. The higher residual stress acts as a mean stress and lowers the cyclical load capacity (Fig. "Residual stresses and thermal strength"). Overstress can lead to brittle fractures even in thin cross-sections and low critical fracture toughness (Fig. "Forced fracture behavior by section thickness").
With higher strengths, the sensitivity of materials to environmental influences generally increases. These are corrosion types that initiate and accelerate cracking (Volume 1, Chapter 5.4).
If the increase in material strength and operating loads is paired with smaller grain sizes (e.g. powder-metallugrical materials), the unfavorable fracture-mechanical characteristic values can additionally increase risks. For example, the crack growth rate can increase (Ill. 12.2-8; Example "AS HIP", Excerpt 5) and the fracture toughness can decrease. This acts like a stress increase, causing smaller flaws to be capable of crack growth, and making critical crack lengths shorter.
Small grain sizes can have the advantage of better ultrasonic inspections (smaller disturbances, “grass”). Parts made from the same material, yet conventionally produced using casting and forging, can have larger grains than powder-metallurgical versions. This can result in considerably worse ultrasonic inspections, which increases the probability of larger flaws, thus worsening the strength utilization.

Figure "Residual stresses and thermal strength": High thermal resistance can considerably reduce the usable fatigue strength of parts in unexpected ways. The high thermal resistance and the structural influence sometimes prevent residual stresses from the forging process (Fig. "LCF fracture of a fan disk") and/or heat treatment to be sufficiently reduced through subsequent annealing treatments to make the mean stress increase is negligible (top diagram). The thicker the cross-sections, the greater the expected residual stresses. The residual stresses can have a life-determining effect on part zones in which this would not be expected (Fig. "LCF fracture of a fan disk"). If larger cross-sections, i.e. volumes, in a disk are under high tensile stress, it increases the probability of crack growth-capable flaws and thus also reduces the usable strength (Fig. "Causes of cracks below th surface").

Figure "End of a revolutionary technology" (Example "AS HIP"): This case is an excellent example of the risks of introducing new technologies in highly stressed serial components. The depicted fighter was undergoing testing. It crashed after the low-pressure turbine disk burst. The probable damage cause was an LCF fracture, which originated in a critical, highly stressed zone (arrows in right detail). The output power of this engine type had been increased considerably. The low-pressure turbine, which was especially highly stressed due to the single-stage design, had a disk made from ultra-high tensile strength material (Ref. 14-7). The material was an AS HIP version of a powder-metallurgical material, i.e. no subsequent forging. This material is distinguished by fine, even grains. Disadvantages include notch sensitivity, low crack toughness, and rapid crack growth. The dangers of this kind of combination of damage-promoting influences should serve as an important experience for anyone who works on the development of these technologies.
It can be assumed that the high loads made the crack growth phase extremely difficult to control. The major advantage of the powder-metallurgical material, aside from its high strength, was the exceptionally cost-effective AS HIP process, which resulted in semi-finished parts with dimensions that were very close to those of the finished part. This production process uses a hollow metal form (can), which is filled with metal powder in a vacuum and welded shut (Ref. 14-10). The filled capsule is then subjected to hot isostatic pressing (HIP) under high argon pressure (>1000 bar) and high temperatures, creating a dense material. The form is then removed.

This damage provides the following lessons:

• The informational value of an accumulated total run time of several engines and/or superstructural parts, especially if this is greater than several thousand hours, cannot be used as a sufficient argument for the serial application of a technology.

• New design concepts (in this case, a single-stage turbine) that considerably increase part loads demand more than mere increases in material hardness.

• The use of higher-strength materials reduces damage tolerance and demands additional fracture mechanical considerations (Fig. "Technology for additional safety").

• Fracture-mechanical charateristic values, such as fracture toughness, critical crack length, initial flaw size, crack growth, and notch-sensitivity, must be known, understood, and, if possible, optimized for the specific application.

• Safe life design alone is not sufficient. It must be combined with a damage tolerant concept. Therefore, the crack initiation and growth phases must be taken into account (Fig. "Configuration oriented safe life").

• A new technology must be understood in all its influences on part safety. This includes knowledge of flaws in the semi-finished parts: size, distribution, and probability.

• Proof-of-concept tests must directly verify the required properties. Indirect verification is not sufficient. The results of a burst overspeed test should not be used to draw conclusions regarding the LCF life of a highly-utilized part.

• An isolated case is usually only the first case (Volume 1, Ill. 2-3.2).

• A spectacular damage incident can easily mean the end of a technology, at least temporarily.

Figure "New damage types of new technologies": With new materials and/or production technologies, specific damage forms must be expected. The introduction of directional solidified materials (left diagram, see Fig. "Grain boundaries influencing thermal fatigue") promoted radial crack initiation along the grain boundaries at the blade tip, followed by material break-outs and performance losses.
In single-crystal turbine blades (right diagram), during the finishing process (casting, annealing) or at high operating temperatures, recrystallization occurs near plastic deformations.. In this process, grains are created which tend to crack initiation and complete material break-outs under operating loads (creep, thermal fatigue).

Figure "Unexpected effects of new technologies": New technologies always come with surprises. Glasses and high-strength ceramics with glass phases (sintering aid) in atmospheric conditions exhibit extraordinary behavior. If these materials are under sufficiently high tensile stress, normal atmospheric moisture can incite stress corrosion cracking. In this case, cracking occurs, typically at small nicks (e.g. damage such as scratches or impact marks) with sub-critical crack growth to fracture (Ref. 14-26). A typical example is a car windshield crack (high tension residual stresses in safety glass) originating in a small impact mark from a rock. The crack growth to sudden shattering of the windshield can be observed over several days. A similar effect can be seen in household objects made of glass (bottom diagrams) and can also be found in medical technology (ceramic implants).
The failure of pyrometers (top diagram), which are used to monitor part temperatures in the turbine area, can be attributed to the described effect in several cases. If the fibers of the optical fibers to the sensor are subjected to high tensile stress in the fastening area, and moisture is present, individual glass fibers will break. This leads to gradual deterioration of the sensitivity of the pyrometer, and increases the danger of excessively high temperature levels.

Figure "New technologies unpredictable damage behavior": Unforeseen behavior under unusual loads, such as in the case of damage, heavy rubbing, or FOD, can prevent or restrict the introduction of a new technology. The following are several examples:
Typical situations with unusual behavior occur during damage incidents.
High deformation rates, such as impact loads, lead to brittle behavior in metallic and synthetic materials (Volume 2, Ill. 8.1-13). The degree of embrittlement is material-specific. Impact loads occur in situations such as containment incidents, bird strikes, or under the influence of other ingested foreign objects or fragments from within the engine itself. The embrittlement can be influenced considerably by the operating temperatures. Contrary to expectations, pronounced embrittlement can be observed at very high temperatures, as well as at very low temperatures (Fig. "Indications of turbine blade overheating"). The special tendency of some synthetic resins to embrittlement under impact stress can, for example, prevent their use in spinners made from fiber-reinforced synthetics. The reduced impact resistance of single-crystal materials at operating temperatures leads to considerably greater consequential damages than in less sensitive, but also less heat-resistant, multi-crystal materials.

The increasing risk with the use of high-strength materials can also be related to their unfavorable fracture-mechanical specific values. If this behavior is not correctly assessed, realization in serial applications is threatened (Fig. "Trends of materials applications", Example "AS HIP"). Increased notch sensitivity (e.g. in titanium alloys) can cause unexpectedly large consequential damages after damage due to foreign objects or during a damage process (e.g. haircut of a blade stage).
Notches and cracks in hard, brittle, and firmly-bonding coatings, which can be caused by foreign objects, can cause dynamic strength to drop considerably more sharply than it would in uncoated material.
Extreme rubbing at blade tips and in labyrinths causes a great deal of material removal from the contact surfaces. If this material is ignited, it can cause a dangerous dust explosion (Volume 2, Ill. 9.4-6).

Figure "Development time of fiber fanblades" (Examples 14-2 and 14-3): This diagram shows fiber-reinforced synthetic fan blades from two engine generations. The left diagram is based on a large, first-generation fan engine from the early 1970s. Despite similar design characteristics (long chord lengths, no clappers, dovetail blade roots), early development was a disastrous failure for the developer due to poor bird strike behavior.
The right diagram shows a fan blade from a third generation engine from the 1990s. The size difference is striking. The pronounced metallic front edge (titanium alloy) also attracts notice.
Serial realization was made possible primarily by the large mass, which means that equal bird strike energy will place considerably less stress on the large blade than it would on the small blade from the earlier development. In addition, 3-D reinforcements (against delamination) and improved fibers and matrix were used (impact resistance, strength). The integrated metallic leading edge guaranteed sufficient FOD behavior and erosion resistance. These properties have now proven themselves through very good operating behavior over long operating periods in a large number of engines. Their advantages include outstanding bird strike behavior and extremely high fatigue strength.
In order to use the now successful blade (Ref. 14-23), it was necessary to keep the circumferential speed of the blade below a threshold value (Fig. "Examples of successful fiber reinforced components"). Despite this, major problems occurred during verification of the part`s suitability for serial implementation. The blades, which were deflected considerably by bird strikes and containment, were overstressed by the sharp edges of the spacer segments that form the hub contour, causing the blades to fracture in the leaf section. The remedy for this was the use of less rigid spacer segments. The containment also had to be matched to the material behavior of the blades. This was aided by the demonstrated unique failure mechanism of the blades. Unlike fractures in metallic blades, the fractures in this case occurred well above the root.

Figure "Reintroduction of early technologies ": Contrary to expectations, technologies that have proven successful in older engine types cannot simply be used in modern engines. The high static and dynamic loads, as well as increased aero- and thermodynamic demands, require important adaptations that require development efforts approaching those necessary for the development of completely new technologies. Even the use of of traditional materials that are frequently used in older engines can lead to major problems in modern engine types with considerably higher part loads. A typical example are Al alloys in the fans or high-pressure compressors of fighter aircraft engines. The lack of a pronounced fatigue strength (bottom right diagram), combined with low strength levels and a sensitivity to erosion and corrosion, increase the risk of dynamic fatigue considerably. The thin, sharp-edged profiles of modern compressors further increase the problem.
Experience has shown that consequential damages following a blade failure and/or FOD lead to haircuts in large sections of the compressor (top diagram). This is primarily due to the large deflection caused by the low E modulus and the low blade weight (Volume 1, Ill. 5.2.2.3-5).
Another example is sheet metal constructions. The housings of early engine types differ from the cast parts of modern engines through especially complex welded constructions made from sheet metal and forged parts. Every so often, the suggestion is made to return to the older designs due to cost advantages they seem to offer. However, it is usually forgotten that these welded parts require a great deal of know-how. Conceptualizing sufficiently deformation-free weld patterns and training the necessary skilled specialist personnel is a long-term task. Even though the use of modern welding robots can minimize expensive labor costs, extensive finishing developments are indispensible. However, it may be worthwhile to consider these possibilities once again, since the possibilities offered by CAD, such as the simulation of joining processes, welding deformation, and strength calculation of complex structures, have opened up new perspectives.

Figure "Success of cooled turbine blade types": The cooling of turbine blades is a key technology that positively influences the fuel consumption, performance, and weight of an engine. Some concepts, such as effusion cooling and the use of monolithic ceramics, have not yet found their way into serial applications. In both cases, the reason was unsafe operating behavior due to an insufficiently safely controllable damage mode under influence of foreign objects (carbon impact). In addition, effusion cooling has problems with erosion, plugging, oxidation, and thermal fatigue. The blade with the “ceramic shirt” has problems with thermal fatigue and force transmission from the supporting metal core into the ceramic.
Therefore, intensively cooled cast blades have become the standard technology. These also have problems, but they are safely controllable within the required life span.
Modern engine types use turbine blades with ceramic thermal barrier coatings. This technology has thus far shown itself to be life span-guaranteeing. If the thermal barrier function becomes a component of the design, its reliability attains a higher degree of importance (Figs. "Material behavior depending on design and technology" and "Technology for additional safety").
Both serial technologies attained their current levels of maturity over years of evolutionary development, which makes the introduction of other, newer technologies very difficult.