The ASTM D638 standard measures the ability of reinforced and non-reinforced plastics to withstand tensile forces. It is an essential test in a world where plastic packaging leads the market, since this test can predict the protective capabilities of the material to tensile forces, which are quite common during shipping and handling, as well as in other situations.
At Safe Load Testing Technologies we have prepared this guide to learn everything there is to know about the ASTM D638 protocol: its characteristics, how it is performed and the equipment that is required to perform it, as well as the advantages of implementing it and of packaging optimization.
What does ASTM D638 testing involve?
The ASTM D638 standard is defined as a testing method to determine the tensile strength of both reinforced and non-reinforced plastics. This protocol is performed on “dog bone” shaped samples and under different pre-treatment, temperature and humidity conditions as per the different speeds of the testing machine.
In other words, the test is performed by applying tensile forces to the sample and measuring how its properties change under extreme tension or stress.
In this regard, the ASTM D638 protocol is related to the ISO 527-2 test, since the purpose of both is to demonstrate the resistance of a plastic to tensile forces. However, in its website, ASTM clarifies that the technical content of both tests is different, and therefore they are not interchangeable. The companies that are required to measure this parameter usually perform just one of the tests, with the ASTM testing method being the favorite in the North American market and area of influence.
What are the properties measured by the ASTM D638 protocol?
The most common properties measured by this testing method are the following:
- The tensile force or the amount of force that can be applied to a sample before it fails (deforms) beyond repair or breaks. In a tensile test, the tensile force is the maximum tension that can be applied to the sample.
“Failing” in a material testing environment means a change in material from the point in which it still behaves as an elastic (when the tensile force is removed, it can return to one same point) to a point in which it behaves like a plastic (where the deformation is permanent).
All tensile tests performed in a universal testing machine can be used to create a graph containing the recorded force-deformation data, which is useful to determine the elastic and plastic behavior of a material, as well as to quantify its maximum resistance or its breaking point.
This concept can be applied in different ways and is also included in the material compression test. Measuring tensile forces is one of the key aspects of the ASTM D638 standard, but it is also important in metal testing.
- The elastic modulus, or how much a material can deform when subjected to tensile forces. In other words, it calculates the resistance of a material to a non-permanent deformation.
- Elongation, or the increase in length after rupture when compared to the original length.
Here, the ductility of a material is being measured: its ability to plastically deform in a sustained manner without rupturing. The greater the elongation, the greater the ductility, and therefore, the less fragile the material.
In the case of the ASTM D638 test, it can be used to measure the elongation at the point of deformation and the elongation at the point of rupture.
- Poisson’s ratio, an elasticity constant that measures the ratio between how much a material can be stretched and how thin it becomes during this process.
This ratio is calculated again both under tensile forces and under compression forces and takes into account the compression changes experienced along an axis when a material is expanded from the opposite axis.
Measuring Poisson’s Ratio requires the use of a transverse strain gauge.
Types of samples that can be gauged
The ASTM D638 method is applied to rigid plastic samples with sizes ranging from 1.00 mm to 14 mm thick.
Preparing the samples for testing first requires applying the ASTM D5947-18 protocol, which outlines methods for measuring the physical dimensions of solid plastic samples. This step is a requirement, since the size of the samples will affect the results later during the performance of the ASTM D638 method.
Once this test has been performed and the exact dimensions of the samples to be tested have been determined, the ASTM D638 method outlines 5 sample types.
However, the most common (type I) has a thickness of 3.2 mm, a length of 165 mm, a width of 13 mm and a gauge length of 50 mm, and is die-cast.
If the sample is flat, it is shaped like a “dumbbell” or “dog bone,” so that any rupture resulting from the tensile force takes place at the centre of the sample. Furthermore, this method also allows for the testing of tube or rod-shaped materials, which must also be moulded into a dumbbell or dog bone shape.
Samples whose thickness is less than 1.00 mm must be tested using the ASTM D882 standard, which provides for a testing method to measure the tensile properties of thin plastic materials.
It is particularly important to keep in mind the sample preparation process to obtain accurate results with this testing method. Even more so when performing comparison tests between two or more materials. In this case, it is fundamental to ensure that the samples undergo the same processes. There is also, however, the option of including different sample preparation methods as part of the comparison test method.
Required machinery to perform the ASTM D638 test
The ASTM D638 test must be performed in a universal testing machine or a tensile tester. The most common equipment is electromechanical, though hydraulic systems are also used.
These machines can combine the application of tensile and compression forces, in addition to flexion and adhesion.
Other companies choose universal testers that are customized based on the needs of the material to be tested: higher speeds or larger-than-average frames, for materials that have large elongation capabilities. This is particularly important in current times when the resistance of plastics is improving.
Likewise, the machines offer customization options which include adding more security measures, as well as an option that allows for the process to be automated. Automated systems take care of measuring the sample, loading it, testing it and removing it. This makes it possible to have a mostly operator-free operation, which in turn reduces the possibility of human error and costs while improving the efficiency of the process.
As for the clamps, pneumatic serrated systems are generally included (which can be used with materials other than plastics, such as fibres, cables and textiles). Pneumatic systems operate using pressurized air, keeping the force constant. This is essential when obtaining accurate results while performing the ASTM D638 method since the thickness of the sample will change.
There are kits available that are specifically designed to prepare and cut the sample as per the ASTM D5947-18 method mentioned above.
Lastly, testing can require the use of a strain gauge – a sensor that measures the deformation, pressure or load of a sample. This machine determines the capability of a material to change the nominal value of its resistance when subjected to (in this case) tensile forces. The strain gauge will show the resulting change in length compared to the initial length.
The most common option consists of a system that can be attached to the testing machine. There are also strain gauges that do not require constant operation by an operator, which are suitable for ASTM D638 testing in environmental chambers.
To obtain results as per the ASTM D638 test, the system must be compliant with the ASTM E83 Class B-2 standard.
How to perform the test
Once the sample is ready, the ASTM D638 testing method can be performed. The process is divided into the following phases:
1. Sample loading in a way that it is secured between to clamps that will exert the force. The strain gauge is also attached at this time.
This is an important step in which the operator must ensure that the sample is not tilted to either side in order to obtain accurate results.
This step, as well as any verification, can be performed visually or by using specific sample alignment mechanisms.
2. Establish the speed at which the clamps will separate. This speed can range from 1 to 500 mm/min depending on the shape of the sample.
3. The test ends when reaching the point of rupture or permanent deformation, which is when results and analyzed and evaluated.
Why you should measure a plastic’s tensile force
The ASTM D638 testing method provides companies with highly useful information. More specifically, it allows companies to have a deeper knowledge of the plastic’s characteristics, which they may use to choose an optimized packaging material that offers a high level of protection.
In this regard, it is part of a widespread trend in package material testing (in this case, plastic packaging) that is performed in order to guarantee the resistance of the components to various contingencies. In the case of ASTM D638, it is applied to measure the tensile properties of plastics.
These testing methods allow packaging materials to be thoroughly known, thereby becoming an essential part of the design process, with sights on obtaining an effective performance and a more sustainable package. Having a deep knowledge of the physical and chemical properties of your packaging material allows engineers and designers to provide increasingly effective solutions.
As a result, packaging material tests guarantee that the package is able to protect a good against unforeseen circumstances during transportation and storage.
Some of the practical reasons why you would want to test your packaging material include:
- Testing in a controlled environment whether the package will withstand the conditions of the distribution cycle. This prevents possible losses or problems, and any necessary changes are performed in due time.
- Having a better understanding of the distribution process.
- Making better decisions using the obtained information in regard to the design of the package and the distribution process.
- Reduce unnecessary losses from shrinkage or other accidents by using optimal packaging.
- Ensure that the company meets packaging standards and protocols, industry requirements and the customer’s expectations.
Testing both the package’s materials and the package via transport simulation systems is useful in order to guarantee the safety of the product being transported and its properties, as well as to ensure the safety of the transport operator. Companies that address these issues, therefore, benefit from a competitive advantage that sets them apart.
ASTM standards: what they are and why you should comply with them
ASTM International is one of the largest standard organizations in the world, composed of expert volunteers. It is a nonprofit organization whose purpose is to develop quality standards that reflect an international consensus for various industries.
The protocols of the ASTM standards bring together the expertise of producers, consumers, government regulations and researchers. This information then translates into technical documents such as the ASTM D638 method, which become guidelines for the processes of dozens of industries and trades.
There are multiple ASTM standards related to packaging, many of them with the purpose of establishing protocols to guarantee the safety of packages during shipping & handling.
At Safe Load Testing Technologies we help companies in the plastics and other sectors to optimize their packages based on these standards, as well as those of other organizations such as ISTA and ISO. Our transportation simulation systems are designed to comply with these protocols and to optimize and certify the packaging, products or loads of companies of various sectors. If you are looking for solutions such as these, get in touch with us to discuss how we can be of assistance.