Failure at Mode Level – Failure Modes

In Step 4: Failure Analysis of the AIAG-VDA 7-Step FMEA approach, the first activity is to identify failure modes at the function level.

👉 A failure mode describes:
“How does the function fail to meet its requirement?”

Getting this right is critical, because failure effects and causes are built on these definitions. Poorly written failure modes lead to incomplete or misleading risk analysis.

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What is a Failure Mode in FMEA? #

• A Failure Mode is the deviation from the intended function or requirement.
• It must be specific, clear, and observable.
• It always relates to the function defined in Step 3: Function Analysis.

📌 A failure mode is not the effect (what happens after failure) or the cause (why it fails).

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Characteristics of a Good Failure Mode #

1. Linked directly to the function & requirement.
2. Written in clear, technical terms.
3. Observable or measurable (e.g., “weld strength below 3 kN” not “weak weld”).
4. Independent of effect or cause.

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Examples of Failure Modes in FMEA #

1. DFMEA – Electric Motor

• Function: Provide rotation at 2000 ± 50 RPM.
• Requirement: Maintain torque of 20 Nm.
• Failure Modes:
  • Motor rotates below 1500 RPM.
  • Motor fails to rotate.
  • Torque output < 20 Nm.

2. PFMEA – Bolting Process

• Function: Secure bolt to 100 ± 5 Nm torque.
• Requirement: Apply torque within tolerance.
• Failure Modes:
  • Under-torque (<95 Nm).
  • Over-torque (>105 Nm).
  • Bolt not installed.

3. PFMEA – Welding Process

• Function: Join two metal sheets with spot weld.
• Requirement: Weld strength ≥ 5 kN.
• Failure Modes:
  • Weld nugget not formed.
  • Weld strength < 3 kN.
  • Weld cracks during inspection.

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How to Identify Failure Modes – Step-by-Step #

1. Start with the Function (from Step 3).
   • Example: “Seatbelt restrains passenger.”
2. Define the Requirement (measurable criteria).
   • Example: “Withstand 10 kN tensile force.”
3. Ask: How can this requirement fail?
   • Seatbelt tears below 10 kN.
   • Seatbelt buckle fails to lock.
   • Webbing does not retract properly.

📌 This structured method ensures failure modes are always tied back to function.

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Common Mistakes in Writing Failure Modes #

• Writing vague phrases like “not working” or “failure of system.”
• Mixing effects with modes (e.g., “vehicle won’t start” → effect, not mode).
• Writing causes instead of modes (e.g., “loose wiring” → cause, not mode).
• Listing subjective descriptions (“bad weld”) instead of measurable deviations.

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Case Study – DFMEA for Airbag System #

• Function: Deploy airbag in crash event.
• Requirement: Deploy within 30 ms.
• Failure Modes:
  • Airbag does not deploy.
  • Airbag deploys late (>30 ms).
  • Airbag deploys without crash.

👉 Each failure mode is specific, measurable, and directly tied to the requirement.

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Why Defining Failure Modes is Critical #

1. Foundation for Risk Analysis: Severity, occurrence, and detection ratings are all applied to failure modes.
2. Customer Safety: High-severity failure modes (e.g., airbag not deploying) drive urgent preventive actions.
3. Completeness: Ensures no risk is overlooked at system, subsystem, or process level.

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Key Takeaways #

• Failure Mode = How the function fails to meet the requirement.
• Must be specific, measurable, and independent of effect/cause.
• Clear failure modes ensure effective Failure Effect and Cause Analysis.
• This step is the starting point of Step 4: Failure Analysis in FMEA.

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Next Lesson #

👉 Continue with Lesson 3.5.2: Effects of Failure (End User / Downstream Process)