What is a creep test?

Creep is the deformation of material occurring with time due to an externally applied constant stress. For cushioning materials specifically, it may be defined as the change in thickness of a cushion under static compressive load over a period of time. Creep testing sets out to measure the deformation or change in thickness of a specific material when it’s exposed to a constant level of stress.

Creep can affect palletized or unitized load configurations that place consistent, and oftentimes significant, external stress of products while they’re in storage or in transit. 

With a creep test, packaging suppliers can determine how their product will be affected when placed under constant stress. If the sample material’s physical properties are significantly altered due to the constant stress, this can potentially reduce both the safety and effectiveness of the material in question. Creep material testing provides manufacturers with the insights they need to choose the correct material for their cushioned package systems.

How is creep testing performed?

The creep testing procedures may vary slightly depending on the protocol being followed and the manufacturer’s needs, but the basic process for nearly all creep tests remains the same. In order to perform a creep test, compression force is applied to a sample material. Both the level of external stress and the temperature in the testing area are held constant. The sample material is either tested for a predetermined amount of time. 

During the creep test, engineers record the condition of the sample material at regular time intervals. As time goes on, this data is plotted onto a chart, with one axis representing creep and the other axis representing time. To determine a material’s creep rate, one must calculate the slope of the curve created by this chart. 

What industries should conduct creep testing procedures? 

Creep testing is used across a range of industries to evaluate how certain materials perform when subjected to constant stress. This type of testing can be of use when choosing a cushioning material that will be used with heavier products that may be stored for long periods of time. 

At Westpak, the typical procedures and protocols we follow include: 

• ASTM D2221
• ASTM D3575 Suffix BB

How do you determine the stress-relaxation of a material undergoing creep material testing?

While creep is measured by applying a persistent level of external force to a sample material, stress relaxation can be measured once that force has been removed. As a result of constant pressure being applied, a sample material will typically experience some degree of deformation during creep testing. If the sample material makes it through creep testing without failing, then creep recovery can be measured. 

This means that, once the external pressure is removed from the sample material, engineers will measure the decrease in creep over time. This data can be placed onto a chart that compares the decrease in creep vs. time. By calculating the slope of the resulting curve, one can determine the stress-relaxation rate. 

If you’re in need of creep testing, contact WESTPAK. Our experienced engineers and superb facilities help to ensure that every creep test we conduct produces accurate results that provide you with actionable data and valuable insights.