Fatigue calculations that demonstrate the safety of HASS

Abstract

Highly Accelerated Stress Screening (HASS) is a production test process that has become a widely accepted method for detecting process or component changes in a product that could reduce product reliability. The stress profiles that are used in HASS are based on the results of Highly Accelerated Life Testing (HALT) on the product. In both HALT and HASS, Repetitive Shock (RS) vibration systems are used to create the vibration stresses. RS vibration will, by design, fatigue a product much more quickly than vibration from an ElectroDynamic (ED) system. The broad spectrum and high acceleration shocks that characterize an RS system are very unlike the ED vibration that is typically used in Design Verification Testing (DVT) or is specified for use in Environmental Stress Screening (ESS). Because the product stress induced by a RS system is so different, there can be a lingering concern that HASS may remove too much life from the product. This paper uses classical methods of fatigue calculation to calculate the fraction of life removed from a product by HASS stresses, and to show that these conservative calculations clearly demonstrate that HASS will not remove an unacceptable amount of life from a product.