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How to Select the Right ALT Approach
byFred Schenkelberg, CRE, CQE
Senior Reliability Engineer, Ops A La Carte [email protected] // www.opsalacarte.com // (408) 710-8248
ASQ Silicon ValleyQuality Conference
Objectives
• Introduce thought process to select ALT approach
• To discuss trade-off decisions involved
• To enable you to select the right ALT approach for your situation
Presenter BiographyFred Schenkelberg is a Senior Reliability Engineering Consultant at Ops A La
Carte. He is currently working with clients using reliability assessments as a starting point to develop and execute detailed reliability plans and programs. Also, he exercises his reliability engineering and statistical knowledge to design and conduct accelerated life tests.
Fred joined HP in February 1996 in Vancouver, WA. He joined ESTC, Palo Alto, CA., in January 1998 and co-founded the HP Product Reliability Team. He was responsible for the community building, consulting and training aspects of the Product Reliability Program. He was also responsible for research and development on selected product reliability management topics.
Prior to joining ESTC, he worked as a design for manufacturing engineer on DeskJet printers. Before HP he worked with Raychem Corporation in various positions, including research and development of accelerated life testing of polymer based heating cables.
He has a Bachelors of Science in Physics from the United States Military Academy and a Masters of Science in Statistics from Stanford University. Fred is an active member of the RAMS Management Committee and currently the IEEE Reliability Society Santa Clara Valley Chapter President.
• Respect constraints– Time, expenses, sample size, accuracy
• Use use conditions– Environment, frequency, loading
• Minimize assumptions– Statistical, modeling, variability
• Focus on failure mechanisms– Stress, damage, measurement
Guiding Principles
With no constraints
• Build products and deploy, measure time to failure for every unit.
– Real time– Real stresses– Real variability– “Customer” defines failure
Paperclip Experiment 1
• What is use profile?
• Environment
• Probability of success
• Duration
• Failure definition
• Benefits– Perfect results– No assumptions
• Problems– Takes time– No Sales, or– Customer risk
No Constraints
• Use product more often– Increase use cycles per day– Maintain other stresses at use
levels– Measurement system detects
specific failure
• Assumes specific action leads to failure– Wear, fatigue, accumulation– Must understand or assume failure
mechanism
Time Compression
Acceleration Factor (AF)
Time shiftTime shift
Paperclip Experiment 2
• Assume clipping action leads to eventual failure
• How many cycles?
• What forces, angles?
• How many?
Time Compression
• Benefits– Less time– May use a sample– Simple extrapolation
• Problems– Not suitable for time
dependent failure mechanisms
– Nor, 24/7 loaded products
Stress to Life Relationship
Paperclip Experiment 3
• Assume bend angle is related to damage accumulation
• Angles?
• Other variables?
• Benefits– Creates AF model – Product of use specific
• Problems– 1 or 2 stresses or
failure mechanisms– Logistically challenging
Stress to Life Relationship
Acceleration Model• A model exists describing the stress
to life relationship– Arrhenius, Peck, Coffin-Manson, prior
work – The failure mechanism,
use profile, and environment within applicable limits of model
• If failure mechanism doesn’tchange, may use comparative testing in absence of model, giving up on accuracy of result.
Paperclip Experiment 4
• Assume failure mechanism is the same for new product
• Best to run till failure and conduct detailed failure analysis
Acceleration Model
• Benefits– Single stress to apply– Fewer samples
• Problems– Assumptions
multiplying– May miss new failure
mechanism
ALT w/ AF model
• Peck’s Relationship
• Norris-Landzberg
Step Stress
• Variation of acceleration model ALT, with samples experiencing step increases in stress– Quicker than acceleration model method– May require more samples– Requires careful control of stress and failure
detection– Best with accumulated damage failure
mechanisms
Degradation
• Useful when performance decays over time in a measurable way– Often used with an acceleration model– Failure is defined by threshold value– Non destructive measurements permit
repeated measures on samples
ALT Limits
• Extrapolation
• Stresses
• Failure Mechanisms
Wearout
Sample Size
Success Testing
Weibull Success Testing