APPENDIX 1. MATERIAL AND PROCESS CHANGES ON REDESIGNED PARTS
AC 33.19-1 9/27/04 Francis A. Favara Acting Manager, Engine and Propeller Directorate Aircraft Certification Service APPENDIX 1. MATERIAL AND PROCESS CHANGES ON REDESIGNED PARTS A1-1. General .
a. Drive system and structural parts may have small margins of safety that result from operation at high stress levels relative to their material strength. Many redesigns include changes in materials or manufacturing processes relative to the original design part. The margin of safety may be eroded if careful attention isn’t paid to the effects these changes may have on the material strength or stress levels. Consideration should be given to the following effects that material or process changes may have on the redesigned part: (1) A local increase in stress caused by cracks, nicks, gouges, laps, rough surfaces, thin sections, undersize fillet radii, coarse porosity, or gross inclusions.
(2) Reductions in strength, wear resistance, or corrosion resistance caused by coarse microstructure, low hardness, inappropriate heat treatments or surface treatments.
9/27/04 AC 33.19-1 b. The precise effects of these factors may be difficult to quantify in terms of stress or strength. In addition, many of these characteristics are difficult to detect or evaluate by analytical methods. For these and other reasons (related to dimensional and operational variables), substantial reliance should be placed on representative durability testing during certification of drive system and structural reciprocating engine parts.
c. These factors should also be considered when the extent of the redesign is limited to a change in material.
A1-2. Factors That Increase Stress . The following factors may affect mean (steady state) stresses and operational (dynamic) stresses: a. Surface treatment processes (such as carburizing, nitriding, and shot peening) that introduce beneficial surface stresses may also cause increased residual tensile stresses in the core. This can be especially significant in thin sections of parts.
b. Prestress in bolted joints, which is generally beneficial if it is not excessive.
c. Straightening, which can result in residual stresses and risk of cracking.
d. Welding or brazing, which can cause residual stresses.
e. Dimensional changes.
f. Interference fits (assembly stresses).
g. Reduced cross-sectional areas, including wall thicknesses, which can cause increased operational stresses and mean stresses.
h. Manufacturing-related stress raisers, which can cause increased operational stresses.
Manufacturing processes that produce surface stress raisers should normally be discovered during examination of test articles by visual observation and crack inspection procedures.
Subsurface stress raisers resulting in material strength issues are largely controlled by process definition and control, and supported by destructive sampling procedures. Manufacturing related stress raisers include the following: (1) Nicks, deep scratches, and gouges; (2) Gross porosity, large inclusions, cold shuts, and hot tears (castings); (3) Laps and inclusions (forgings); (4) Sharp edges; (5) Corrosion pits; AC 33.19-1 9/27/04 (6) Cracks; (7) Overly rough surface finishes; and (8) Undersize fillet radii.
A1-3. Factors That Reduce Strength . Most of the following factors are especially significant in their effect on fatigue strength, a major design consideration for reciprocating engine parts.
a. Low hardness (specifically true for steels; less significant for cast aluminum alloys).
Low hardness is commonly associated with the of heat treatment processes.
b. Coarse microstructure/grain size, adversely affecting static strength, fatigue strength, and toughness. Coarse microstructure/grain size is commonly associated with the temperatures levels during casting and forging operations, during normalizing (for steels), or during the heat treatment processes.
c. Levels of impurities, inclusions, or porosity (castings).
d. Decarburization of steel surfaces, causing a soft, low-strength surface layer.
Decarburization of steel surfaces is typically associated with the temperature levels during forging operations; some decarburization may be acceptable.
e. Inappropriately specified surface treatments such as carburizing, nitriding, and shot peening.
f. Inappropriately specified plating treatments of high strength steels, resulting in hydrogen embrittlement.
g. Inappropriately specified joining processes such as welding or brazing, which exhibit incomplete fill or, in the case of welding, local section reduction.
APPENDIX 2. NOTES ON MATERIAL STRENGTH EVALUATION
9/27/04 AC 33.19-1 APPENDIX 2. NOTES ON MATERIAL STRENGTH EVALUATION A2-1. Metallographic Analysis. In combination with chemistry and hardness data, metallographic analysis is a fundamental tool for assessing material properties. The metallographic inspection may be expected to provide information of a basic microstructural nature, including grain structure, the presence or absence of unusual intergranular conditions, porosity, and inclusions in the area sectioned. However, there are limits to the amount of information that can be obtained during a general metallographic evaluation. The information gathered depends on the scope of the evaluation. Component idiosyncrasies may go undetected without a thorough understanding of the manufacturing processes. Changes in microstructure between “good” and “bad” material performance are sometimes so subtle as to be indistinguishable.
A2-2. Microstructural Analysis . There are several important characteristics that a general microstructural evaluation may miss or, if detected, may not thoroughly assess in terms of component strength. The following list represents examples of several areas of analytical difficulties: a. Detection of hydrogen embrittlement.
AC 33.19-1 9/27/04 b. Detection of non-homogeneous microstructure.
c. Evaluation of directional properties.
d. Effects of section thickness on the properties of low alloy, high hardenability steels.
e. Detection of local nitrided surface case depth reduction (resulting from grinding/overpolishing).
f. Evaluation of decarburized layer thickness in terms of its effect on the fatigue strength of forged steel components.
g. Evaluation of cast aluminum microstructural and macrostructural variations and their effects on component fatigue strength.
A2-3. Material Strength Reference Data .
a. A large amount of basic material strength data is available in reference books.
Although very valuable, the data, especially that on fatigue strength, should be used cautiously.
Reports published in the handbooks of the American Society for Metals (ASM) indicate that there are significant problems in establishing or confirming fatigue strengths by relying on reference data. “The Selection of Steel for Fatigue Resistance” reports that although the fatigue strength of steel is usually in proportion to the tensile strength, this generalization does not hold in many instances and is not true over wide ranges of tensile strength. “Aluminum Alloy Castings: Mechanical Properties” states that the fatigue strength of cast aluminum is markedly dependent on the casting process. It further states that actual fatigue testing of fully machined cast aluminum parts is the only method of alloy or process selection. “Fatigue Resistance of Steels” states that: “Fatigue tests performed on small specimens are not sufficient for precisely establishing the fatigue life of a part.” “Fatigue Failures” discusses fatigue failures and prediction of fatigue life and includes many of the variables affecting fatigue life. It also identifies that standard fatigue life data (as normally presented in reference books) is usually based on the median life of the specimens tested. In addition, it refers to the significant scatter associated with such fatigue life values.
b. Nevertheless, estimates of basic material fatigue strength should be an essential part of the design or redesign processes; laboratory-derived fatigue data should be used to support this evaluation. When calculating safety factors, avoid overestimating fatigue strengths. The values used should reflect the lower limits of data scatter, rather than median values, when possible.
The fatigue test data presented in curves in the ASM Metals Handbook reports is valuable for assessing alloy steel and cast aluminum fatigue strength scatter. Fatigue strength estimates should also include the effects of such parameters as mean stresses, stress concentrations, surface finishes, and structural temperatures.
c. Complex components, such as cylinder assemblies, that depend significantly on assembly procedures for their strength present an additional difficulty in estimating fatigue strengths (or lives). The durability of such assemblies cannot be fully evaluated based only on 9/27/04 AC 33.19-1 their individual component material properties. Due to uncertainties and operational variables that may affect stress and strength, component analysis should be supported by durability testing.