This is just an Excerpt from a larger document, click here to view the entire document.Introduction
To meet increasing competition, get products to market in the shortest possible time, and satisfy demanding customer expectations, industry is turning to sophisticated methods and techniques of testing. Many of today's products are capable of operating under extremes of environmental stress and for thousands of hours without failure. Traditional test methods are no longer sufficient to identify design weaknesses or validate life predictions.
Accelerated testing is an approach for obtaining more information from a given test time than would normally be possible. It does this by using a test environment that is more severe than that experienced during normal equipment use. Since higher stresses are used, accelerated testing must be approached with caution to avoid introducing failure modes that will not be encountered in normal use. Accelerating factors used, either singly or in combination, include:
More frequent power cycling
Higher vibration levels
More severe temperature cycling
Accelerated testing falls into two main categories, each with a specific purpose:
Accelerated Life Testing - life estimation
Accelerated Stress Testing - problem/weakness identification (or confirmation) and correction
The differences between these two categories, although subtle, are significant and include the underlying assumptions upon which the test is based, the models utilized in constructing the test, the test equipment and chambers used, the way in which the test itself is conducted, and the manner in which the resulting data is analyzed and interpreted. Table 1 compares the two main categories of accelerated testing.
Table 1: The Two Main Categories of Accelerated Testing
Purpose and Approach
Accelerated Life Testing (ALT)
Uses a model relating the reliability (or life) measured under high stress conditions to that which is expected under normal operation to determine length of life
an understanding of the anticipated failure mechanism(s)
a knowledge of the magnitude of the acceleration of this failure mechanism, as a function of the accelerating stress
Accelerated Stress Testing (AST)
Uses accelerated environmental stresses to precipitate latent defects or design weaknesses into actual failures to identify design, part or manufacturing process problems which could cause subsequent failures in the field.
Requires a thorough understanding, or at least a workable knowledge, of the basic failure mechanisms. Estimation of item life may, or may not, be a concern.
The level at which accelerated test is performed is very important. Some accelerating techniques are appropriate only for part level testing, while others can be used only for higher levels of assembly, and a very few techniques may be applicable for both part level and assembly level. The underlying assumptions and modeling approaches which may be perfectly legitimate at the part level may be totally invalid for tests performed on higher level equipment and viseversa. Table 2 provides information on testing at the two major levels; equipment and component.
Table 2: Levels at which Accelerated Testing is Performed
Usually quite limited; seldom performed. Creating a valid model relating rate of equipment failures at a high stress to that at normal operating conditions is extremely difficult. Also, it is very difficult to formulate stress conditions that do not change the failure mechanisms occurring within the equipment.
One example of an accelerated test that can be used effectively on equipment is that of increasing the duty cycle, e.g., for a system normally running only during one shift, or avionics equipment operating only a few hours before and during a flight. In such cases a higher duty cycle could easily be used during the test. the system undergoing test could be operated continuously for three shifts a day or the avionics equipment might be cycled continuously, with only enough time between simulated flights to permit the temperature within the equipment to stabilize to non-operating conditions. Although the failure rate per operating hour does not change, the number of failures accrued per day is increased. This type of accelerated testing is commonly done in reliability qualification test, and although it is not usually recognized as such, this is actually a form of accelerated testing.
Components (parts) tend to have fewer failure modes than equipment. Thus, it is far easier to identify a stress which can effectively accelerate the rate of failure without seriously changing the failure mechanism.
There are usually one or more dominant failure mechanisms accelerated by a given stress, e.g., dielectric breakdown of capacitors as a function of voltage, or corrosion as a function of humidity. In this case it is usually relatively easy to find an acceleration model relating failure rate as a function of operating stress. For this reason accelerated life testing is used extensively for components and the technique is highly recommended for most types of parts and for most part applications.