FMEA and FMECA
4.0 Introduction
Failure
Mode and Effects Analysis (FMEA) and Failure Modes, Effects and
Criticality Analysis (FMECA) are methodologies designed to identify
potential failure modes for a product or process, to assess the risk
associated with those failure modes, to rank the issues in terms of
importance and to identify and carry out corrective actions to address
the most serious concerns.
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Although the purpose, terminology and other details can vary according to type (e.g.
Process FMEA, Design FMEA, etc.), the basic methodology is similar for
all. This document presents a brief general overview of FMEA / FMECA
analysis techniques and requirements.
4.1 FMEA / FMECA Overview
In general, Failure
Modes, Effects and Criticality Analysis (FMEA / FMECA) requires the
identification of the following basic information:
- Item(s)
- Function(s)
- Failure(s)
- Effect(s) of Failure
- Cause(s) of Failure
- Current Control(s)
- Recommended Action(s)
- Plus other relevant details
Most analyses of
this type also include some method to assess the risk associated with
the issues identified during the analysis and to prioritize corrective
actions. Two common methods include:
- Risk Priority Numbers (RPNs)
- Criticality Analysis (FMEA with Criticality Analysis = FMECA)
4.2 Basic Analysis Procedure for FMEA or FMECA
The basic steps for
performing an Failure Mode and Effects Analysis (FMEA) or Failure Modes,
Effects and Criticality Analysis (FMECA) include:
- Assemble the team.
- Establish the ground rules.
- Gather and review relevant information.
- Identify the item(s) or process(es) to be analyzed.
- Identify the function(s), failure(s), effect(s), cause(s) and control(s) for each item or process to be analyzed.
- Evaluate the risk associated with the issues identified by the analysis.
- Prioritize and assign corrective actions.
- Perform corrective actions and re-evaluate risk.
- Distribute, review and update the analysis, as appropriate.
4.3 FMEA Analysis Sub-Sets
Finite Element Analysis (FEA)
Analysis can be tailored to suit particular applications, ranging from
simple conceptual models to detailed production verification. In all
cases the models will provide predicted performance or (if exact loads
are unknown) can be used for trend analysis. For slender structures, BEAM elements
can asses the effects of various section sizes providing a baseline
design which can then proceed to detail design with confidence, and
focusing further analysis effort into the most critical areas. SHELL elements
are suitable for modelling parts, which are constructed of 'thin'
material. Vehicle body shells and composite chassis structures are
efficiently modelled in this way. The most detailed type of model uses SOLID elements
to fully capture the geometry of cast, forged and machined components,
and gives the best stress accuracy. The latest generation P-type analysis software is particularly effective. It uses geometric elements, which mirror the underlying Computer Aided Design (CAD) surface geometry - enabling stress concentration features such as fillet radii and welds to be accurately modelled. The P-type
solver also selectively increases the polynomial order of the elements
to converge to a pre-defined tolerance. The resulting stress accuracy is
of a sufficient quality to allow precise fatigue life predictions.
Accurate analysis predictions rely on knowledge of three sets of information:
GEOMETRY. This can be supplied from 3D CAD
data or constructed within the analysis package. The accurate modelling
of features such as fillet radii for example is critical to the
accurate prediction of stresses.
LOADINGS.
The model must be realistically constrained in a way which corresponds
to the boundary conditions it will see in use. Loading can be applied to
test specifications if known, or recovered from data acquisition.
MATERIAL. Material properties can be assigned to published data or derived from correlation with tested parts.
This is the most elementary form of FEA
and is a powerful and widely used tool in Analysis. Linear static
techniques are ideal for providing information to facilitate stiffness,
strength and fatigue assessment on a wide range of model types. The
insight given by the displaced shape and stress distribution of a
structure can give valuable information as to the critical areas of a
design. The underlying assumption of linear static analysis is that the
displacement and stresses of a structure are directly proportional to
the load applied. Strictly speaking, the results are only valid whilst
the material is stressed below its yield point. Experienced
interpretation allows recommendations, based on the results, even when
the system falls outside the theoretical regime of the linear static
solution.
Vibration
analysis can give you a valuable insight into the response of a
structure as it is subjected to dynamic forces through one-off impact,
cyclic, transient or random loading. A crucial application is the
assessment of natural oscillations and harmonics of structures, the
identification of mode shapes and their frequencies. Weak or overly
stiff areas of a component or structure can be identified, tuned or
damped. This can remove the likelihood of operational resonance and
associated amplified response. A "specialism" is the employment of
forcing functions, (a simple half sine wave to a complex seismic event)
to structural models, revealing the magnitude and location of the
stresses at different operating frequencies.
Fatigue
is responsible for the majority of premature operational failures. It
occurs when repeated loading generate stresses, which although below the
material's static allowable stresses, are above the level where
microscopic damage is initiated, and if continued will result in
cracking. Fatigue analysis identifies the potential crack initiation
sites and indicates how long a component can be expected to last within
its allowable loading envelope. The opportunities for savings are great -
For high cycle fatigue a small reduction in stress can lead to a large
increase in the life of a component. In many cases including rotating
and reciprocating parts in high-speed machines it may be straightforward
to define the loads. Where the situation is more complex and random
events occur either a loading simulation or data acquisition can be used
to define the load sets. Rationalising the data can identify the
individual events that cause most of the damage to a component. The
smallest radii on a complex casting or the production weld detail can
now be modelled and analysed against comprehensive loading spectra.
A
valuable technique for analysis of components and systems whose
stiffness changes as the material is strained. Either the load direction
changes, it has non-linear material properties or the load path has
altered. For many structures the effect of buckling is critical, the
initiation of which is often difficult to assess for complex geometry,
loading or boundary conditions. This analysis allows us to predict the
onset of instability and extends understanding still further by allowing
the resultant internal load redistribution to be clearly recognised. A
classic application is a plastic snap-fit connection. In addition to the
non-linear material properties, the solution's boundary condition would
include clearance, gaps, sliding joints and the effect of friction.
This
analysis steps through the real time of a dynamic load event allowing
materials to strain and yield, loads to redistribute, contacts to be
made and inertia to be transferred. Impact analysis is one of the main
applications of this technique. The non-linear geometry and material
capabilities allow the large deflection and plastic collapse to be
modelled. The event could be controlled as with a golf club hitting a
ball or unpredictable as with a drop test or an aircraft bird strike. We
can assess the effect on the structure and optimise it for strength,
mass or energy absorption. Non Linear Dynamic capability includes the
analysis of coupled fluid structure interaction. The properties of the
dynamic contact region between the fluid and the structure can be
predicted and the stress and deflection results recovered. Manufacturing
processes such as pressing and forging can also be analysed using this
technique. Modelling the material flow within the forging process allows
the final deformed shape and the residual stresses in the component to
be predicted. Potential sources of production defects such as voids or
cracks can be identified and removed before commitment is made to
production tooling.
Multi-body systems (MBS)
analysis is a technique for evaluating the relationships between
several associated and moving bodies. MBS technique apply to prescribed
kinematic linkages, where the rate characteristics and space envelopes
need deriving, through to high speed dynamic systems including inertial
effects and the interaction of flexible elements. Resulting load data
can be extracted from the MBS model and directly applied to FEA models
increasing confidence and accuracy of analysis predictions. Information
to identify the fundamental parameters controlling system performance -
at a stage in the design programme where changes may be economically
effected.
- Computational Fluid Dynamics
Like FEA the Computational Fluid Dynamics (CFD)
methods break down the fluid continuum into many cells whose
interactions can be described by relatively simple equations. The
applications and capabilities of the CFD code usually include;
Multi-phase, combustion, unsteady flows, moving boundaries, supersonic
aerodynamics, and particulate erosion. Almost any physical parameter can
be recovered including pressures, velocities, forces, temperatures and
mass fractions of individual species. CFD can study conditions,
which would be impossible, or impractical to investigate physically,
providing realistic values where only estimates were available. It can
cost effectively support physical testing, reducing time-scales and
providing information more enlightening than any wind tunnel method.
The
drive for ever-more compact products has placed increased demands on
thermal efficiency. Whether preventing or facilitating heat transfer,
our thermal analysis techniques makes optimisation of conductive,
connective and radiant mechanisms attainable, allowing novel solutions
to be quantified early in the design process. From heat sources and
boundary conditions the temperature distribution within a system
computed. A critical application of the results is the affect on a
structural component, due to stresses induced by expansion. CFD
techniques allow flow over a body from natural or forced convection
modes to be considered whilst taking into account the changes in the
material's heat transfer coefficients and the heat transferred to the
fluid.
Closing the loop
between "virtual" and "reality". Data acquisition is the process of
collecting real time or statistical data from existing systems and
components in operation to derive the static and dynamic loading for
application to finite element models and to verify actual results with
predicted performance.
Equipment can be
used to acquire strain, displacement, acceleration and temperature. The
equipment is portable, modular and capable of operating in harsh
environments and the information generated would be used to validate and
verify the modelled analysis.
4.4 Risk Evaluation Methods
A typical failure
modes and effects analysis incorporates some method to evaluate the risk
associated with the potential problems identified through the analysis.
The two most common methods, Risk Priority Numbers and Criticality
Analysis, are described next.
4.5 Risk Priority Numbers
To use the Risk Priority Number (RPN) method to assess risk, the analysis team must:
- Rate the severity of each effect of failure.
- Rate the likelihood of occurrence for each cause of failure.
- Rate the likelihood of prior detection for each cause of failure (i.e. the likelihood of detecting the problem before it reaches the end user or customer).
- Calculate the RPN by obtaining the product of the three ratings:
RPN = Severity x Occurrence x Detection
The RPN can then be
used to compare issues within the analysis and to prioritize problems
for corrective action. This risk assessment method is commonly
associated with Failure Mode and Effects Analysis (FMEA).
4.6 Criticality Analysis
The MIL-STD-1629A
document describes two types of criticality analysis: quantitative and
qualitative. To use the quantitative criticality analysis method, the
analysis team must:
- Define the reliability/unreliability for each item, at a given operating time.
- Identify the portion of the item’s unreliability that can be attributed to each potential failure mode.
- Rate the probability of loss (or severity) that will result from each failure mode that may occur.
- Calculate the criticality for each potential failure mode by obtaining the product of the three factors:
Mode Criticality = Item Unreliability x Mode Ratio of Unreliability x Probability of Loss
- Calculate the
criticality for each item by obtaining the sum of the criticalities for
each failure mode that has been identified for the item.
Item Criticality = SUM of Mode Criticalities
To use the qualitative criticality analysis method to evaluate risk and prioritize corrective actions, the analysis team must:
- Rate the severity of the potential effects of failure.
- Rate the likelihood of occurrence for each potential failure mode.
- Compare failure
modes via a Criticality Matrix, which identifies severity on the
horizontal axis and occurrence on the vertical axis.
These risk assessment methods are commonly associated with Failure Modes, Effects and Criticality Analysis (FMECA).
4.7 Applications and Benefits
The Failure Modes,
Effects and Criticality Analysis (FMEA / FMECA) procedure is a tool that
has been adapted in many different ways for many different purposes. It
can contribute to improved designs for products and processes,
resulting in higher reliability, better quality, increased safety,
enhanced customer satisfaction and reduced costs. The tool can also be
used to establish and optimize maintenance plans for repairable systems
and/or contribute to control plans and other quality assurance
procedures. It provides a knowledge base of failure mode and corrective
action information that can be used as a resource in future
troubleshooting efforts and as a training tool for new engineers. In
addition, an FMEA or FMECA is often required to comply with safety and
quality requirements, such as ISO 9001, QS 9000, ISO/TS 16949, Six
Sigma, FDA Good Manufacturing Practices (GMPs), Process Safety
Management Act (PSM), etc.
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