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ENVIRONMENTAL STRESS SCREENING


More Degrees of Freedom for HALT


Six Degree of Freedom vibration testing gives more options for Highly Accelerated Life Testing in the automotive industry, as CentraTEQ explains.


by mainstream manufacturing for both reliability testing and for environmental stress screening (ESS). Stressing the product within the


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operational boundaries of its endurance (ie without over-stressing it), accelerates the life cycle and allows the manufacturer to simulate the extended life of the product in a relatively short space of time. Stress screening has been traditionally carried out in single axes, vertically or horizontally with an upright electrodynamic shaker table or with the shaker swivelled into the horizontal position to actuate a slip table, on which the test subject is mounted.


ACCELERATING LIFE TESTING Highly complex new products, particularly in the rapidly changing environment of automotive engineering, is placing more emphasis on the need for assessing reliability at the component, sub-assembly and completed product levels.


Such testing consumes valuable


development time and there’s an imperative within the industry to perform life testing more quickly without over- stressing through artificially induced acceleration or unrealistic test profiles. To meet this challenge, a group of researchers from King Design Industrial proposed a technique for Highly Accelerated Life Testing (HALT) with 6 degrees of freedom. In a paper presented at a sound and vibration conference in Hsinchu, Taiwan, the scientists discussed the comparative results of traditional shaker tables, air driven HALT and the


ith its origins in military product and reliability assurance, vibration testing was quickly picked up


❱ ❱ Multiple actuators provide vibration in three axes simultaneously for highly accelerated life testing


proposed technique of Electrodynamic HALT (ED-HALT). By performing the test cycle in 6 degrees of freedom, all three axes (X, Y and Z) are simultaneously actuated creating vectored vibration forces that provide greater realism in comparison to the expected application environment of the product. “The ED-HALT system is something entirely unique and presents a significant advantage over traditional pneumatic HALT systems, particularly in terms of control at low frequency,” explains Jim Flanagan of CentraTEQ.


DEVELOPMENT AND ESS CYCLES For product development, the researchers proposed a step-wise increase in the gRMS value on the primary axis to find the operating limit of the device under test (the point at which its function can be recovered) and the destruct limit (from which recovery isn’t possible). The margin between the two provides an indication of the failure margin for the product and therefore its robustness so that the design team can make adjustments if necessary to make the product more robust. With the margins established,


production Environmental Stress Screening (ESS) profiles can be set up to test products within their operating margins before shipment.


PROFILE REALISM Some industries, such as aerospace and automotive, have the advantage of wide access to realistic vibration profiles that their products are expected to encounter in their working lives. There is a wealth of data for automotive suppliers to access based on tests performed on proving grounds. Such data is used for creating realistic environments in simulators and also for creating vibration test profiles.


❱ ❱ 6 DoF HALT tables can be installed in climatic chambers for multi modal environmental testing


With 6 degrees of freedom, the profiles obtained from track data can be used as input directly into the ED vibration device, something that can’t be achieved with single axis or tri-axial set-ups. With realistic profiles and greater life


acceleration through the use of 6 DOF tables, test cycles don’t have to be as long and the automotive industry can take another step towards achieving its goal of shorter product development lead times. 6 DOF shaker tables are available in the UK from Centrateq and can be used singularly or within climatic chambers.


Climatic & Vibration Testing Vol 2 No. 1 /// 9


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