5.4.5. Method selection for different items and failure mechanisms#

Once it is clear what needs to be considered in the reliability prediction (see Section 5.4.4), the next step is to decide how each part and failure mechanism should be modelled. The present section gives guidance on this method selection task.

5.4.5.1. Categories and combinations of mechanical failure mechanisms#

The method selection for mechanical parts reliability modelling should always take basis in a good understanding of the failure mechanisms that are of relevance for the considered item, especially the predominant ones. To simplify the presentation in the following sections, mechanical failure mechanisms are grouped into five distinct categories listed in Table 5.4.4.

Table 5.4.4 Categories of mechanical failure mechanisms#

Failure mechanism category

Examples

Distortion

Creep, elastic deformation, plastic deformation, displacement

Fracture/Fatigue

Ductile/brittle fracture, high-cycle fatigue, low-cycle fatigue

Mechanical wear

Adhesive wear, abrasive wear, lubricated wear

Corrosion

Stress-corrosion cracking, galvanic corrosion

Material degradation

Thermal degradation, radiation damage

The probability of failure can for some items be driven by several different failure mechanisms. When using PoF methods, the results for the individual mechanisms have to be combined to estimate the overall part reliability. The correct way of doing this depends on whether the different failure mechanisms can be considered as independent processes or events.

For independent failure mechanisms, it is sufficient to consider one process at once, and to combine the results using normal probability calculus.

However, independence between different failure mechanisms may not always be a valid assumption, e.g. in case of physical interactions between different failure mechanisms processes. Table 5.4.5 gives an overview for which combinations of failure mechanisms such effects may have to be considered. The table distinguishes interactions between different failure mechanisms affecting the same part (left hand side) and between different failure mechanisms affecting different parts (right hand side).

Table 5.4.5 Possible interactions between different mechanical failure mechanisms#

table5_4_5

A simple way to account for such interactions is to select the dominating failure mechanism (e.g. the one with the highest probability of failure) and to consider the effect of other failure mechanism(s) on the dominating failure mechanism. When using structural reliability methods, this can be achieved by adapting the basic variable distributions to account for physical effects related to other failure mechanism processes. The assessment then requires the following steps:

  • Selection of the dominating failure mechanism - e.g. wear of a lubricated bearing.

  • Identification of other failure mechanisms that may interact with the dominating failure mechanism using Table 5.4.5 - e.g. distortion of the membrane compressing the bearing.

  • Identification of physical variables used by the model for the dominating failure mechanism that may be affected by the interaction - e.g. increase of the contact pressure in the bearing.

  • Adjustment of the identified basic variables distributional models to account for the effect of the interaction - e.g. by considering the increased contact pressure.

Finally, it should be noted that besides physical interactions between failure mechanism, there are other possible reason for statistical dependence, such as common cause effects due to random physical variables (e.g. temperature, loads), having an effect on different failure mechanisms and/or parts. An example for explicit common cause modelling in a redundant bolted interface with load redistribution can be found in [BR_MEC_2].

5.4.5.2. Selection of methods for part level mechanical reliability prediction#

The approach for method selection presented in the following is centred around the identified failure mechanisms, see Table 5.4.7 giving an overview of different mechanical part types with associated failure mechanisms and modelling recommendations.

The third column of Table 5.4.7 gives an indication about the relevance of each part type for different classes of mechanical systems introduced in Table 5.4.3 . As has been discussed in Section 5.4.4, it may be sufficient to focus on those parts that dominate system level reliability. In the same way, the reliability of a specific part is often dominated by a single (or very few) failure mechanism(s), allowing to focus the analysis especially with methods that are based on the PoF.

Which failure mechanism(s) dominates reliability depends not only on the part type, but also on the application; e.g. a spring is likely to fail differently when used with static, cyclic or dynamic loading. The “predominant” failure mechanisms listed in Table 5.4.7 should therefore be considered only as a first guidance.

The remaining columns of Table 5.4.7 provide recommendations on the modelling of the selected parts and failure mechanisms. The focus is on the methods making reference to the physics of failure, in particular structural reliability methods as outlined in Section 5.4.6 and Section 5.4.7.

The application of the Handbook approaches is restricted to selected parts, allowing the possible use of the NSWC Handbook [NR_MEC_1] as a scaling method (which is based the physics of failure) to adapt statistical estimates based on data samples that are more relevant for space applications than the NSWC data, see Section 5.4.3.1.5 for discussion. The NPRD Handbook [NR_MEC_5] is not considered any further.

In Table 5.4.7, statistical methods are mentioned only where a prediction based on a pure physics of failure approach is unlikely to be successful. However, as has been discussed already in Section 5.4.3.2, statistical methods are generally preferred over physics of failure approaches provided that sufficient and relevant data is available to perform the prediction. The failure mechanisms listed in Table 5.4.7 can provide guidance for judging the relevance of a specific data set, always keeping in mind the strong variability in mechanical parts reliability when seemingly identical parts are applied under different conditions.

Also a combined approach with Bayesian inference updating of a prior derived from the physics of failure, using relevant data even with limited sample size, is generally preferred over a pure physics of failure approach, see Section 5.4.3.2.4 and Section 5.4.6.5 for guidance.

Table 5.4.6 Mechanical part types, associated failure mechanisms and modelling recommendations#
Structural reliability methods NSWG 2011 handbook
Part type Description Mechanical item category Predominant failure mechanism Modelling recommendation Potential failure mechanisms Limit state function Simplified method Model available Space relevance of NSWC method
Bearings A device that constrains relative motion to only the I Active desired motion, and reduces friction between moving parts I Active Lubrication wear Structural reliability methods

wear

fatigue &fracture

distortion

generic

method not applicable due to predominance of lubrication wear over fatigue in space
Springs An elastic object that stores mechanical energy, II Passive used to exert forces or to provide flexibility. II Passive Load loss, creep (distortion) Statistical methods, NSWC adjusted, or Structural (but difficult to model creep)

fatigue &fracture

distortion

depends on the type and application; fatigue/fracture less relevant in space
Gears and splines A toothed machine part such as a wheel or cylinder I Active that meshes with another toothed part to transmit motion or to change speed or direction I Active Lubrication wear Structural reliability methods

wear

fatigue &fracture

distortion

generic

application questionable due to space specific lubrication and rotation speeds
Shafts A rotating device, usually of circular cross section that transmits torque from one part to another I Active Wear (when contact is present),otherwise fatigue Structural reliability methods

wear

fatigue &fracture

distortion

generic

method is maintenance related and thus not applicable to space
Hinges Ajoint that holds two parts together such that one II Passive can swing relative to the other II Passive Increased friction or kinematic issues Structural reliability methods

distortion

wear

generic

only some general considerations in NSWC
Mirrors and lenses A piece of glass or similar material designed to reflect (mirror), or to concentrate or disperse light rays. IV Structural Fracture Structural reliability methods

fatigue &fracture

distortion

degradation

generic

NA
Flexural pivots A flexure which is engineered to be compliant in one or more angular degrees of freedom.

I Active

II Passive

Fatigue Structural reliability methods

fatigue &fracture

NA
Fusible Wires A device consisting of metal strips soldered with a fusible alloy that melts at a specific temperature, allowing the pieces to separate II Passive Quality issue or random failure Statistical methods

fatigue &fracture

distortion

generic

NA
Seals and Gaskets A device placed between two surfaces to prevent the flow of gas or fluid from one region to another

I Active

II Passive

III Fluidic

Wear, degradation, installation damage NSWC adjusted, Statistical methods or Structural reliability methods

wear

distortion

degradation

generic

method may be applicable but requires base failure rate adjustment
Structural component (ind.threaded fasteners,inserts, brackets) A mechanical component designed to sustain loads or pressures, provide stiffness or stability, or provide support or containment. IV Structural Fracture Structural reliability methods

fatigue &fracture

distortion

generic

NA
Table 5.4.7 Mechanical part types, associated failure mechanisms and modelling recommendations#

Part type

Description

Mechanical item category

Predominant failure mechanism

Modelling recommendation

Structural reliability methods - Potential failure mechanisms

Structural reliability methods - Limit state function

Structural reliability methods - Simplified method

NSWG 2011 handbook - Model available

NSWG 2011 handbook - Space relevance of NSWC method

Bearings

A device that constrains relative motion to only the I Active desired motion, and reduces friction between moving parts

I Active

Lubrication wear

Structural reliability methods

wear

fatigue &fracture

distortion

generic

method not applicable due to predominance of lubrication wear over fatigue in space

Springs

An elastic object that stores mechanical energy, II Passive used to exert forces or to provide flexibility.

II Passive

Load loss, creep (distortion)

Statistical methods, NSWC adjusted, or Structural (but difficult to model creep)

fatigue &fracture

distortion

depends on the type and application; fatigue/fracture less relevant in space

Gears and splines

A toothed machine part such as a wheel or cylinder I Active that meshes with another toothed part to transmit motion or to change speed or direction

I Active

Lubrication wear

Structural reliability methods

wear

fatigue &fracture

distortion

generic

application questionable due to space specific lubrication and rotation speeds

Shafts

A rotating device, usually of circular cross section that transmits torque from one part to another

I Active

Wear (when contact is present),otherwise fatigue

Structural reliability methods

wear

fatigue &fracture

distortion

generic

method is maintenance related and thus not applicable to space

Hinges

Ajoint that holds two parts together such that one II Passive can swing relative to the other

II Passive

Increased friction or kinematic issues

Structural reliability methods

distortion

wear

generic

only some general considerations in NSWC

Mirrors and lenses

A piece of glass or similar material designed to reflect (mirror), or to concentrate or disperse light rays.

IV Structural

Fracture

Structural reliability methods

fatigue &fracture

distortion

degradation

generic

NA

Flexural pivots

A flexure which is engineered to be compliant in one or more angular degrees of freedom.

I Active

II Passive

Fatigue

Structural reliability methods

fatigue &fracture

NA

Fusible Wires

A device consisting of metal strips soldered with a fusible alloy that melts at a specific temperature, allowing the pieces to separate

II Passive

Quality issue or random failure

Statistical methods

fatigue &fracture

distortion

generic

NA

Seals and Gaskets

A device placed between two surfaces to prevent the flow of gas or fluid from one region to another

I Active

II Passive

III Fluidic

Wear, degradation, installation damage

NSWC adjusted, Statistical methods or Structural reliability methods

wear

distortion

degradation

generic

method may be applicable but requires base failure rate adjustment

Structural component (ind.threaded fasteners,inserts, brackets)

A mechanical component designed to sustain loads or pressures, provide stiffness or stability, orprovide support or containment.

IV Structural

Fracture

Structural reliability methods

fatigue &fracture

distortion

generic

NA

5.4.5.3. Selection of methods for assembly level mechanical reliability prediction#

Reliability prediction for assemblies of mechanical parts can either be made bottom-up, combining part level inputs with system level reliability methods, or top-down, using inputs directly collected at the level of the considered assembly. In general, similar considerations apply as for part level mechanical reliability prediction, leading to the following modelling recommendations:

  • Statistical (or combined) methods are suitable provided that relevant failure data is available directly at the level of the considered assembly.

  • Physics of failure methods based on Structural reliability theory are suitable if the assembly level probability of failure is dominated by one or very few failure mechanisms (see also the discussion in Section 5.4.4).

  • Bottom-up modelling starting from part level analysis is recommended in case none of the abovementioned conditions is fulfilled.

It should be noted that the NSWC Handbook [NR_MEC_1] contains models also for some items that are better considered as assemblies rather than parts, including e.g. electric motors, solenoids and transducers. For reasons already discussed in Section 5.4.3.1.5, the direct use of these models is not recommended for space applications.