Accelerated life testing is the process of testing a product by subjecting it to conditions (
stress
Stress may refer to:
Science and medicine
* Stress (biology), an organism's response to a stressor such as an environmental condition
* Stress (linguistics), relative emphasis or prominence given to a syllable in a word, or to a word in a phrase ...
,
strain
Strain may refer to:
Science and technology
* Strain (biology), variants of plants, viruses or bacteria; or an inbred animal used for experimental purposes
* Strain (chemistry), a chemical stress of a molecule
* Strain (injury), an injury to a mu ...
, temperatures, voltage, vibration rate, pressure etc.) in excess of its normal service parameters in an effort to uncover faults and potential modes of failure in a short amount of time. By analyzing the product's response to such tests,
engineer
Engineers, as practitioners of engineering, are professionals who invent, design, analyze, build and test machines, complex systems, structures, gadgets and materials to fulfill functional objectives and requirements while considering the l ...
s can make predictions about the service life and maintenance intervals of a product.
In
polymer
A polymer (; Greek '' poly-'', "many" + ''-mer'', "part")
is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
s, testing may be done at elevated temperatures to produce a result in a shorter amount of time than it could be produced at ambient temperatures. Many mechanical properties of polymers have an
Arrhenius type relationship with respect to time and temperature (for example, creep, stress relaxation, and tensile properties). If one conducts short tests at elevated temperatures, that data can be used to extrapolate the behavior of the polymer at room temperature, avoiding the need to do lengthy, and hence expensive tests.
Purpose
ALT is primarily used to speed up tests. This is particularly useful in several cases:
* Low failure - Testing even a very large sample at normal conditions would yield few or no failures in a reasonable time.
* High longevity - The product must be reliable for a much longer time than can be reasonably tested at normal conditions.
* High wear-out - The primary cause of failure occurs over an extended amount of time.
For instance, a reliability test on circuits that must last years at use conditions (high longevity) would need to yield results in a much shorter time. If the test wanted to estimate how frequently the circuits needed to be replaced, then the category of low failure would also be applicable. Furthermore, if the circuits wore out from gradual use rather than extreme use (such as a large sudden shock), the wear out category would be involved. If a sudden shock was the primary cause of failure, a
Highly Accelerated Life Test
A highly accelerated life test (HALT) is a stress testing methodology for enhancing product reliability in which prototypes are stressed to a much higher degree than expected from actual use in order to identify weaknesses in the design or manufac ...
may be more appropriate.
Setting up a test
Designing a test involves considering what factors affect the test object, what you already know about the test object's behavior, and what you want to learn from the test.
Test Conditions
All factors thought to influence the test object should be involved and tests should be conducted at various levels of each factor. Higher stress levels will speed up the test more however the cause of failure or other response measured must not be changed. For instance, melting components in a circuit would alter why the circuit failed. Increasing the number of tests or the number of test objects in each test generally increases how precisely one can infer the test object's behavior at operating conditions.
Picking a Model
A model is an equation that accurately relates a test object's performance to the levels of stress on it. This can be referred to as an acceleration model, with any constants called acceleration factors. The acceleration model is usually related to the types of materials or components tested. A few equations used for acceleration models are the
Arrhenius for high temperature fatigue,
Eyring for temperature and humidity, and the
Blattau model for temperature cycling.
When the model is known in advance the test only needs to identify the parameters for the model, however it is necessary to ensure that the model being used has been well verified. Established models must show agreement between extrapolations from accelerated data and observed data across a range of stress factors.
When the appropriate model is not known in advance, or there exist multiple accepted models, the test must estimate what model fits best based on the context of the test and results from testing. Even if two models fit data at high stresses equally well, they may differ by orders of magnitude at lower stresses. This issue can be approached by more tests at a greater range of stresses however the cause of failure must remain unchanged. A possible pre-experiment approach to minimize this is to estimate what data you expect from testing, fit a model to the data, and determine if one would be able to make reliable conclusions if everything went as expected.
Acceleration Factors
Inference from the results of an accelerated life test requires being able to relate the test object's response (lifespan,
corrosion
Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engine ...
, efficiency, etc...) to the levels of applied stress factors over time.
How one factors in the effect of time depends largely on what one is measuring. For instance, a test that is measuring lifespan may look only at the mean time to failure of the test objects, or it may try to
fit a
statistical distribution
In statistics, an empirical distribution function (commonly also called an empirical Cumulative Distribution Function, eCDF) is the distribution function associated with the empirical measure of a sample. This cumulative distribution functio ...
to the data. This is usually referred to as a life distribution, the
probability density function
In probability theory, a probability density function (PDF), or density of a continuous random variable, is a function whose value at any given sample (or point) in the sample space (the set of possible values taken by the random variable) can ...
of which represents the proportion of products failing at a given time. Several distributions for this purpose are the
exponential
Exponential may refer to any of several mathematical topics related to exponentiation, including:
*Exponential function, also:
**Matrix exponential, the matrix analogue to the above
* Exponential decay, decrease at a rate proportional to value
*Exp ...
,
Weibull
Weibull is a Swedish locational surname. The Weibull family share the same roots as the Danish / Norwegian noble family of Falsenbr>They originated from and were named after the village of Weiböl in Widstedts parish, Jutland, but settled in Skà ...
, log-normal, and
gamma
Gamma (uppercase , lowercase ; ''gámma'') is the third letter of the Greek alphabet. In the system of Greek numerals it has a value of 3. In Ancient Greek, the letter gamma represented a voiced velar stop . In Modern Greek, this letter re ...
distributions. In any case, the parameters would be related to the test subjects and the levels of the stress factors being tested.
As a simplified example, consider a test object with a life distribution that roughly matches a normal distribution. Tests at various stress levels would yield different values for the mean and standard deviation of the distribution. (its parameters) One would then use a known model or attempt to fit a model to relate how each stress factor influenced the distributions parameters. This relation would then be used to estimate the life distribution at operating conditions.
Step-Stress Accelerated Life Test
A step stress ALT is a variant of ALT that tests a component at multiple stress levels, one after the other. Components that survive one test are immediately subjected to the next. These are widely modeled under the assumption that survival life of a product depends only on the current level of stress and how many test subjects have failed so far. Step stress ALT can increment low to high, high to low, or through a mix of levels. A step stress ALT test that is interested in extrapolating a life distribution to constant operating conditions must be able to relate the life distribution observed under changing stresses to one of constant stresses.
See also
*
Research and development
Research and development (R&D or R+D), known in Europe as research and technological development (RTD), is the set of innovative activities undertaken by corporations or governments in developing new services or products, and improving existi ...
*
Product management
Product management is the business process of planning, developing, launching, and managing a product or service. It includes the entire lifecycle of a product, from ideation to development to go to market. Product managers are responsible for ...
*
Service life
A product's service life is its period of use in service. Several related terms describe more precisely a product's life, from the point of manufacture, storage, and distribution, and eventual use.
Service life has been defined as "a product's ...
*
Reliability (engineering)
Reliability engineering is a sub-discipline of systems engineering that emphasizes the ability of equipment to function without failure. Reliability describes the ability of a system or component to function under stated conditions for a specifi ...
*
Highly Accelerated Life Test
A highly accelerated life test (HALT) is a stress testing methodology for enhancing product reliability in which prototypes are stressed to a much higher degree than expected from actual use in order to identify weaknesses in the design or manufac ...
*
Accelerated aging
Accelerated aging is testing that uses aggravated conditions of heat, humidity, oxygen, sunlight, vibration, etc. to speed up the normal aging processes of items. It is used to help determine the long-term effects of expected levels of stress wit ...
*
AFT model
"Aft", in nautical terminology, is an adjective or adverb meaning towards the stern (rear) of the ship, aircraft or spacecraft, when the frame of reference is within the ship, headed at the fore. For example, "Able Seaman Smith; lie aft!" or " ...
*
Cox model
Proportional hazards models are a class of survival models in statistics. Survival models relate the time that passes, before some event occurs, to one or more covariates that may be associated with that quantity of time. In a proportional haza ...
*
Fatigue testing
Fatigue testing is a specialised form of mechanical testing that is performed by applying cyclic loading to a ''coupon'' or structure. These tests are used either to generate fatigue life and crack growth data, identify critical locations or de ...
*
Fault injection
In computer science, fault injection is a testing technique for understanding how computing systems behave when stressed in unusual ways. This can be achieved using physical- or software-based means, or using a hybrid approach. Widely studied phys ...
References
{{Reflist
Environmental testing