How to choose an Accelerated Aging Test

What Is Accelerated Aging?

Accelerated aging exposes a material to higher stress than it experiences in operation, then records changes in properties. Temperature, humidity, ultraviolet light, ozone, and salt fog are the usual stresses. The point is to compress years of service into weeks of chamber time.

The model works only as long as the same reaction is prevalent at both temperatures. Go beyond the glass transition temperature, the softening temperature, or the plasticizer movement temperature, and the extrapolation no longer reflects reality. Photodegradation, ozone degradation, and salt degradation do not follow the Arrhenius equation at all. These processes depend on the dose of exposure to radiant energy of a known wavelength, fog time, or cycling time. No published standard converts exposure time to field-life years.

Types of Accelerated Aging Testing

The stress has to match the mechanism you expect. Six categories cover most programs.
Dry heat aging. Specimens are placed in a circulating-air oven at a constant temperature with no load applied, and their properties are checked for retention. ASTM D3045 covers plastics, and ASTM D573 covers rubber.

• Temperature and humidity aging. Heat and moisture can cause hydrolysis, adhesion loss, and delamination. Cyclic tests per ASTM E1171 and pressure HAST for electronic components fall into this category.
• Photo and weathering exposure. ASTM G154 for fluorescent UV and ASTM G155 for xenon arc both simulate different sections of the solar energy spectrum. With the right filters, xenon can simulate complete sunlight, including the visible and near-infrared wavelengths that cause colorant fading. Fluorescent UV focuses on short wavelengths, causing faster chain scission.
• Ozone and chemical exposure. ASTM D1149 strains an elastomer specimen statically at a constant ozone concentration and checks for surface cracking. At very low concentrations, actual damage occurs; that is why the test is conducted at parts per hundred million.
• Salt and cyclic corrosion. ASTM B117 consists of continuous neutral salt fog. ASTM G85 and ASTM D5894 involve additional testing phases such as drying, cycling, and UV to better mimic real-world conditions for coated metals.
• Thermal cycling and combined stress screening. MIL-STD-883 Method 1010 uses thermal cycling to expose microelectronic packages to extreme temperatures to load CTE mismatches in the solder joints and seal. The step stresses employed by HALT/HASS provide operating and destruct limits, hence making it a design tool.

Accelerated Aging Test Methods and Standards

Accelerated Aging Test Methods and Standards

ISO counterparts cover much of this. ISO 4892 parts 2 and 3 correspond to xenon arc and fluorescent UV, ISO 9227 to salt spray, ISO 188 to air oven aging of rubber. ASTM G151 sits above the individual light source methods and sets the general practice for laboratory exposure devices.
Look at the correlation as described by G151. Findings from one type of exposure equipment cannot be used for another, and you should never apply an acceleration factor between laboratory and outdoor exposure unless it was generated for that particular material and location.

Also read: ASTM G155: Xenon Arc Weathering Test for UV Resistance and Material Durability.

How to Choose the Right Accelerated Aging Test

Work through four questions in order.

1. Which failure mode are you trying to reproduce? Oxidation embrittlement suggests oven aging. Chalking and color change suggest either xenon or fluorescent ultraviolet radiation. Adhesive joint cracking during humid operation is due to thermal and humid cycling. Cracking on stretched rubber gasket is caused by ozone. Answer this question first, because otherwise, wrong stresses will give us clean data about something useless.
2. How much stress can the material take before the mechanism changes? Consider the glass transition, softening temperature, and migrating or volatilizing additive. F1980 suggests that the temperature be in the range of 40°C to 60°C for medical packaging polymers for the reasons above. PVC, heat-activated adhesive, and foam with remaining blowing agent all misbehave beyond this temperature.
3. What property will you measure, and where is the unaged control? The aging without the baseline will provide a value without any point of reference. Establish from the outset that you will be testing tensile strength and elongation, seal peel strength, gloss at 60°, and color difference ΔE. Make sure the control runs on the same equipment in the same week.
4. Do you need a life prediction or a pass/fail screen? For service-life extrapolation, exposure at more than three temperatures is required to calculate the activation energy. If exposure is done in only one temperature, it only provides the ranking of the formulations. Indicate which one is required since it increases the number of specimens.
   Another schedule-spoiling factor: interim pull tests. An F1980 test lasting 91 days at 55 degrees Celsius involves testing at 23, 46, 68, and 91 days, for five specimens per property per interval. This means 20 specimens per property, not counting the unaged specimens, at the planning phase, not when the oven is scheduled.

Accelerated Aging FAQs