Aeroengine Safety

Table 17.4-1: In engine construction, the marking requirements and the extent of the information that must be included depend on the safety relevance, i.e. the safety class, of the part or part group. The information can be taken from specifications and guidelines.

Figure "Markings, requirements and application": The demands on markings and the processes by which they are made (Tables 17.4-2 and 17.4-3) are determined by widely varying requirements from the entire life cycle of the part.

Construction design: The location of the marking in a part zone subjected to as little stress as possible, and which will not have any negative effects on part behavior during assembly, operation, and repair.

Finishing: Simple and safe application of with as little work as possible.

Logistics: Sufficient information content, including that required for sufficient process tracking in case of damage. Good and safe readability. This includes markings intended to aid part tracking during the finishing process.

Assembly: Marking of the part position (e.g. weight position, angle) is essential for assembly. The position of the markings on the part is generally provided along with the contained information. It must be ensured that markings are not applied to contact surfaces or fittings with tight tolerances. If marking processes alter the dimensions (raised areas), there is a danger of stress peaks. The contact areas of flanges may settle during operation and cause threaded connections to loosen.If gaps are created, leaks will occur.

Operation: No interference with the operating behavior. This primarily refers to the part life span and safety, as well as its function (e.g. aerodynamic requirements). At the same time, it must be ensured that the marking will not be unallowably altered during operation, e.g. through oxidation
or wear. If problems occur that necessitate the identification of parts in their assembled state, externally visible (e.g. boroscopy) markings are advantageous.

Repair: Processes such as high-temperature soldering, abrasive blasting, etching (before penetrant testing), or diffusion coating can influence the area of the marking. This can affect the readability of the marking and at the very least make misinterpretations more likely.

Figure "To be considered when marking parts": The marking process must follow requirements. These must be known to the person, generally the design engineer, who specifies the type of marking used.

Assembly and centering surfaces: Impressing marking processes throw up material that is displaced. These raised areas can prevent flange surfaces from contacting, resulting in a gap. In housings or lines under internal pressure, this gap can allow a leakage flow with various types of damaging influence:

• Hot gas escaping with the danger of overheating and fires.
• Oil or fuel leakages with fires.
• Decreased efficiency due to leakage losses.
• Impressions with notch effects damage flange surfaces. Micro-movements can exacerbate this effect.
• Localized powerful flange pressure with stress concentrations and notch effects.
• Settling of the flange connection can have various effects: loosening, friction wear (fretting), rotor unbalances, dynamic overstressing of the insufficiently prestressed threaded connection.Similar considerations apply to positive-fitting contact surfaces such as fir tree roots (“3”) and dovetail roots. These surfaces are subjected to especially high dynamic fatigue stresses, so dangerous stress levels are reached even with only minor notch effects.
  

Centering surfaces (“2”), such as ring surfaces on rotor disks, have tight tolerances. Joining during assembly can be made more difficult. If axial grooves are created due to fretting and/or wear tracks, there is a danger of LCF cracking. Especially threatened parts are thin rotating ring-shaped cross-sections with large diameter, in which high tangential stresses can be expected.
Therefore, it is always recommended to avoid dimensional changes that may cause interference, or to remove them carefully in accordance with specifications if they do occur.

Good readability demands sufficient marking size, i.e. digit/letter size. This means good accessibility for the marking process and the reading process. In addition, the permanent marking must be positioned so it is sufficiently protected from operating influences.
Part stresses: Markings should generally not be located in highly-stressed, especially life-determining part zones. Even in relatively small highly-stressed parts with a lot of functional surfaces (contact, aerodynamic), it should be possible to find a suitable surface for marking (examples in frame). Ultimately, only the design engineer or strength department can determine suitable surfaces and, if necessary, explicitly specify which surfaces should not be marked. Unsuitable areas for marking include the relatively large surfaces of rotor disk hubs (“4”). These can fundamentally be classified as being under high dynamic fatigue loads (LCF). Marking must not be done either in the bores nor on the face sides without explicit approval by the responsible technical department. Markings on the disk annulus, just below the blade grooves, are also unallowable as this area must always be assumed to be subject to powerful tangential loads.
Fundamentally, edges are not suitable for markings. They are always likely to be subject to high loads and have a tendency to corner cracks. For this reason, a minimum distance between the marking area and the edge must be observed (frame, right diagram, “5”).
Typical part zones suitable for marking are depicted in the frame using examples from compressors and turbine rotor blades.

Surfaces with coatings such as diffusion coatings or applied coatings (e.g. galvanic coatings) are generally not suitable for subsequent marking. Brittle coatings, such as Al diffusion coatings for oxidation protection on hot parts, must not be marked. It may be possible to mark the area before coating. However, it is important that the marking does not interfere with the coating process. Examples of this include the disturbance of galvanic deposition or the diffusion of a diffusion coating due to a painted marking.

Part stiffness and strength must be sufficient to prevent overstressing, especially plastic deformations, during the marking process. Depending on the clamping and application, plastic deformations with damages can occur in part zones that are not in the marking area (Fig. "Damages by unsuitable markings"). It is entirely possible, in parts with complex geometries, to be unable to recognize any connection between the two part areas. Examples of especially filigreed parts are integral compressor stators, sheet metal parts of the type common in combustion chambers, cooled turbine blades, and thin-walled housings. The primary result of plastic deformation is warping, i.e. insufficient dimensional accuracy. Plastic deformation also means the buildup of residual stresses that can dangerously lower the dynamic fatigue strength under tension (spring-back, Fig. "Straightening difficulties"). An additional problem can occur if cracks form in brittle coatings and their notch effect significantly lowers the dynamic fatigue strength. Even if the cracks are in an area under low operating stresses (Fig. "Cracking of brittle coatings by deformation"), they can unallowably worsen the protective effect of the coating. In hot parts, increased local oxidation can be expected. Brittle applied coatings such as thermal barriers can separate unnoticed even after sufficient elastic deformation of the base material, causing them to fail surprisingly early during operation.

Figure "Damages by unsuitable markings": Marking methods differ in characteristics such as readability, processing time, and costs, as well as in their potential for causing damage (also see Tables 17.4-2 and 17.4-3).

Stamped markings: The notch effect of the relatively deep impact notches promote dynamic fatigue cracks. This is especially true for thermal fatigue. Evidently, protective hardening in the area around the stamped marking are broken down, allowing the notch to exert its full potential influence. Beneficial compressive stresses can be expected directly around the stamped markings. However, cracks were discovered in stamped markings during the electropolishing of turbine blades made from a forged Ni alloy. These cracks were probably stress corrosion cracking.
The impact force from stamping can overstress/plastically deform insufficiently supported filigreed parts even in areas away from the marking, as well as causing cracking in brittle coatings (bottom left detail, Fig. "To be considered when marking parts").
A noticeable upsurge of material can be expected around stamped markings, which is not acceptable in connecting, centering, sealing, or contact surfaces.

Vibration marking: Similar to stamping, but not as intensive, this type of marking with a vibrating hard metal needle can also result in a significant notch effect. Micro cracking and a noticeable reduction in dynamic fatigue strength can be expected around vibration markings on brittle coatings such as Al diffusion coatings. In this case, as with all electrically powered processes, accidental arc burning due to unsuitable equipment (cables) must be avoided with certainty.

Electro-chemical etching (ECE): In this case, it is important to ensure that the material is not damaged (AlMg alloys). Suitable processing parameters must prevent grain intergranular corrosion. The significant, if brief, electrical current can damage the part through arc burning at poor contact points (Fig. "Causes of local overheating"). Experience has shown that damaged cables (to the tools, to the part) or careless management of the electrical current to the part can result in damaging arc burning (Fig. "Dynamic fatigue lowered by electric arcs").

Spark pen/electro-etching pen: This process uses electrical sparks. Its operating principle is similar to electrical-discharge machining with regard to its effect on the surface. This means that damages can be expected in the recast layer (Ills. 16.2.1.2-1 and 16.2.1.2-5 ), including embrittlement, high tensile residual stress levels, and micro cracking. They result in a significant loss in dynamic fatigue strength, which means that this marking method is generally not recommended and should certainly not be used on highly stressed parts. In addition, there is a danger that dangerous electrical arcs may form in the contact area of the electrical supply to the part (usually through an attached sheet metal section). Flawed electrical cables can also result in electrical arcs forming in part zones far away from the area being marked (Fig. "Dynamic fatigue lowered by electric arcs").

Laser marking: If the surface melts and/or is vaporized, one should expect similar damages to those resulting from processes that use electrical arcs. Cracks, embrittlement, and excessively high tensile residual stresses can be expected even with relatively low beam energy (Ref. 17.4-1). Even laser coloring, in which tarnishing is induced as a form of marking, can lead to structural changes. Some materials may be sensitized by this process, making intergranular corrosion more likely (Fig. "Schweißteile"). Tensile residual stresses and a reduction in dynamic fatigue strength can also be expected.

Marking pens and paints: In contrast to the “permanent” processes discussed above, pens and paints are used for temporary marking. There is a danger that unallowable paints may be used. Components such as coal, zinc, copper, or lead react/diffuse with the part material during finishing (e.g. heat treatment, Fig. "Unsuitable paints and markings") or operation. This can negatively impact characteristics such as strength, toughness, or oxidation resistance.

Table 17.4-2 and Table 17.4-3: Every marking process has a specific application that is dependent on its advantages and disadvantages, as well as potential risks (Fig. "Damages by unsuitable markings"). In these two tables, an overview of these characteristics has been compiled for commonly used marking methods.

Figure "2 D data matrix part marking" (Refs. 17.4-2 and 17.4-3): Since about the year 2000, a direct part marking (DPM) process using a 2D data matrix has been gradually introduced. This also includes an automated reading method: direct part mark identification (DPMI). This process es especially well suited to the demand of high-tech fields:

• does not take up much space;
• holds a large amount of data;
• high reading accuracy in the range of 6 x standard deviations (6s, i.e. 3 errors in one million readings);
• simple, fast, and inexpensive to install
• not sensitive to environmental factors.

These conditions are achieved through the use of a checkerboard-like cell matrix (top left frame). The number of cells, i.e. the size of the cell matrix, can be adjusted to the application and specified. The information is contained in the so-called data region. The single cells are marked in accordance with the code, which can occur in various ways (top left diagram). The matrix code contains instructions for corrective methods and error monitoring (clock track) in the upper and right cell rows. This means that decoding and error control is possible even after 60% damage. In order to orient the matrix for reading, a positional recognition is integrated. This consists of a continuous angle that is one cell wide. It is formed by the left and bottom rows (finder “L” pattern).

In order to ensure sufficiently accurate readability, the cell size should be adjusted to the mean part roughness Ra (bottom left diagram).
The positional accuracy and shape of the markings in the cells, as well as possible distortion of the matrix, must meet a certain minimum standard (top right frame).
Certain part-specific standard procedures are accepted for creating markings in engine construction (bottom right frame). Advantages and problems with the marking processes are treated in Fig. "Damages by unsuitable markings" and Tables 17.4-2 and 17.4-3.