Reliability, Durability and Environmental Robustness

Reliability, durability and robustness answer different questions

The three concepts overlap, but they are not interchangeable. A mechanism may survive a vibration test yet wear out too early; a device may meet average life but fail unpredictably in a humid environment; a protective function may operate reliably while a non-safety feature degrades more often.

Define intended life as an engineering model

“Expected lifetime” is not a marketing phrase to be decided after testing. It is a set of assumptions about calendar time, operating hours, cycles, procedures, patients, actuations, charging, cleaning, transport, maintenance and permitted replacement.

• Define shelf life, transport and storage duration separately from service life.
• Describe daily, weekly and lifetime duty cycles, including peak and unusual demand.
• State whether the device is single-use, reusable, repairable, refurbished or subject to replaceable parts.
• Identify cleaning, disinfection, sterilisation and maintenance cycles.
• Define installation, movement, drops, impacts, cable handling and connector mating.
• Account for battery ageing, consumables, calibration and sensor drift.
• State the conditions that mark end of life and how users recognise them.

Build these assumptions from MTL-102 — Intended Purpose, Users and Use Environments and translate them into measurable design inputs through MTL-103 — User Needs and Design Inputs.

Build one environmental profile from manufacture to disposal

The most severe condition is not always one extreme value. Damage can arise from combinations, transitions and sequences: a cold product moved into humid air, vibration followed by impact, cleaning fluid entering a worn seal, or heat accelerating a chemical process before mechanical loading.

Record normal ranges, foreseeable excursions, frequency, duration, rate of change and combined exposure. Product-specific, collateral and transport standards can supply methods, but the manufacturer must justify the severities against the real environment.

Test failure mechanisms—not just finished products

Reliability work begins by asking how performance could degrade. Use risk analysis, design reviews, supplier knowledge, similar-product history, service records and focused experiments to identify plausible mechanisms.

A generic test list can miss the dominant mechanism. A failure-mechanism review should identify the stress that drives each mechanism, the observable response and the design or process characteristics that control it.

Write requirements that expose the reliability claim

A useful requirement states the function, performance limit, environmental and operating conditions, life or number of demands, allowable degradation and required confidence where a statistical claim is intended.

Connect safety-significant limits to MTL-113 — Essential Performance and Safety Concepts and evaluate failure consequences through MTL-105 — Medical-device Risk Management.

Design for margin, tolerance and graceful degradation

Reliable products result from design choices that reduce stress, tolerate variation and make degradation visible before it becomes unacceptable. Testing should confirm those choices, not substitute for them.

• Reduce loads and stress concentrations before increasing material or component ratings.
• Use derating, tolerance analysis and worst-case analysis where they represent the mechanism.
• Separate wear items from life-limiting permanent assemblies where practical.
• Provide sealing, drainage, ventilation and contamination control appropriate to the environment.
• Design connectors, cables, hinges, latches and controls for foreseeable handling and misuse.
• Detect drift, partial failure and depleted life where loss could otherwise remain hidden.
• Define maintenance, calibration and replacement intervals from evidence.
• Avoid common-cause weaknesses in redundant or protective architectures.

Keep system boundaries and allocations aligned with MTL-112 — Systems Engineering, Architecture and Interfaces.

Reliability is a multidisciplinary design responsibility

Use MTL-114 — Electrical and Electronic Design and MTL-115 — Mechanical Design and Materials for discipline-specific design foundations.

Use a layered reliability and environmental test programme

No single test answers every question. A credible programme combines analysis, component evidence, engineering exploration and controlled verification.

Apply the protocol, sample, configuration and reporting principles in MTL-106 — Verification and Validation.

Accelerate the mechanism—not merely the test clock

Accelerated testing increases a stress or duty rate to produce the same relevant degradation sooner. It is valid only when the acceleration does not introduce an unrealistic failure mechanism, suppress an important recovery effect or change how loads interact.

• State the physical or chemical mechanism being accelerated.
• Justify the relationship between test stress and real-use stress.
• Respect rate limits such as heating, cooling, fluid movement, battery recovery and material relaxation.
• Account for dwell time, sequence and combined stresses.
• Use intermediate inspections to understand the progression of degradation.
• Compare accelerated failures with real or representative-use failures where possible.
• Do not claim a life beyond what the model and evidence support.

Highly accelerated limit testing can expose margins and weaknesses, but it is normally an engineering-development method rather than direct proof of a stated lifetime. Accelerated ageing, fatigue and cycle testing each need a mechanism-specific rationale.

Match samples and statistics to the decision

A small number of successful samples may show that a design can work, but it may provide little evidence about the reliability of a production population. The sample rationale should reflect the claim, variability, failure opportunity, risk and decision being made.

• Identify whether the test demonstrates feasibility, verifies a requirement or estimates a population characteristic.
• Represent production materials, processes, suppliers, tooling, variants and software configurations.
• Select worst cases using documented engineering reasoning rather than convenience.
• Define the statistical model, confidence and reliability target before testing when making a quantitative claim.
• Treat every unit, cycle and demand consistently when counting exposure and failures.
• Do not replace failed samples without preserving and investigating the result.
• Separate censored, interrupted, invalid and completed tests in the analysis.

Where evidence is limited, state the uncertainty honestly and combine test results with analysis, supplier evidence and post-market monitoring rather than implying unjustified precision.

Every failure and anomaly is evidence

A device that reaches the final cycle but shows cracking, drift, looseness, corrosion, intermittent behaviour or unusual noise has not necessarily passed. Acceptance criteria should address functional performance, safety, physical condition and latent degradation.

Assess each anomaly against the risk file, essential-performance definition and test validity. Record deviations transparently; retrospective acceptance criteria weaken the credibility of the whole programme.

Transfer reliability-critical characteristics into production

Development tests usually examine few samples. Production controls must preserve the characteristics on which reliability depends across normal process and supplier variation.

• Identify life-limiting materials, components, joints, seals, coatings and processes.
• Control supplier, grade, formulation, geometry and conditions of acceptability.
• Define process parameters and inspections for reliability-critical workmanship.
• Monitor dimensions, forces, leakage, alignment, torque, thermal interfaces and electrical margins where relevant.
• Use incoming and production tests that can detect meaningful drift without damaging the product.
• Retain traceability needed to investigate field failures and lot-specific trends.
• Assess substitutions, tooling, process, firmware and supplier changes against the established mechanisms and margins.

The released evidence should connect through MTL-104 — Design Controls and Technical Documentation, not remain as isolated laboratory reports.

Use post-market data to test development assumptions

Field evidence reveals real duty cycles, environments, user behaviour, maintenance quality and production variation. Complaint counts alone are not enough: exposure and installed population affect the meaning of a trend.

• Capture time in service, use count, environment, configuration, maintenance and failure symptoms where possible.
• Distinguish no-fault-found returns, misuse, wear-out, random failures and systematic weaknesses.
• Examine replaced parts and preserve evidence before repair.
• Trend by age, lot, supplier, variant, geography, environment and usage intensity.
• Compare observed failures with risk estimates and life-test mechanisms.
• Feed credible signals into CAPA, design change, maintenance, labelling and future verification.
• Reconsider claimed life and replacement intervals when assumptions are not supported.

Reliability work across the lifecycle

Common misconceptions

Practical review checklist

• Are shelf life, service life, duty cycles, maintenance and end-of-life conditions defined?
• Does the environmental profile cover manufacture, distribution, storage, operation, cleaning and service?
• Have plausible mechanical, electrical, material, battery, sensing and human-interaction degradation mechanisms been identified?
• Are reliability and durability requirements measurable and linked to risk?
• Does the architecture provide appropriate margin, tolerance and degradation detection?
• Are product variants, suppliers, processes and worst-case configurations represented?
• Does each accelerated test preserve the relevant failure mechanism?
• Are sample size, statistical assumptions and uncertainty appropriate to the claim?
• Are inspections performed before, during and after environmental or life exposure?
• Are all failures, anomalies and deviations investigated and retained?
• Have reliability-critical characteristics been transferred into production controls?
• Are field data compared with development assumptions and used to maintain the evidence?