The development of SATRA test methods

Reviewing the process involved in the creation of SATRA’s extensive library of test methods.

by Tom Bayes

SATRA’s roots are in the development and creation of test methods and testing machinery in order to measure the performance of footwear materials and constructions. This heritage goes back over 100 years and still forms a major part of the organisation’s work. Since being founded, SATRA has produced more than 460 test methods, many of which have been incorporated into international standards and procurement specifications, as well as being adopted by other organisations under their own numbering systems.

The history of SATRA test methods

In SATRA’s early days, its physical testing department used many methods for measuring the properties of materials for probable wear performance. There were official versions of these tests for the British Standards Institute and the Society of Leather Trades Chemists. Until the late 1950s, however, most of these had not been formally published, other than within leaflets for interested members. These leaflets had reference numbers such as ‘PM/FB3’, which stands for ‘physical method, fibre board method 3’.

From the mid-1960s, these methods were gradually reviewed and made available to all members. They became a numbered series, starting with ‘PM1’. This project took some time to complete, and started a process that still continues today. The ‘PM’ prefix was subsequently replaced by ‘TM’ (standing for ‘test method’). Interestingly, during this formalisation and publication process (and despite being located in the UK), all methods featured metric SI units, except where accepted industry standards dictated that 'imperial' units were to be used – such as square feet for leather area measurement.

Some methods are long lived and have stood the test of time. SATRA has specimens of the leather tested for the 1953 Mount Everest expedition boots, using a test that today is known as SATRA TM47:2019 – ‘Water vapour permeability and absorption’.

Globally accepted

Over time, SATRA test methods became universally accepted within the footwear industry, and many laboratories around the world use SATRA test methods. What sets SATRA apart from general testing laboratories is that SATRA has all the necessary research and development data – as well as the experience – that underlines the methodology behind each test method.

These test methods must stand up to scrutiny and give reliable results. This is true whether they are specified as part of a quality control process, or they are used during development to engineer high-performance products and provide scientific evidence to support commercial claims. The results of tests (especially in the procurement arena) can often determine the future viability of a business, so SATRA’s test methods must measure the desired performance or quality in an unbiased manner and not stifle innovation.

Crucially, SATRA also understands footwear performance requirements. This includes in-depth knowledge of the meaning of results, an understanding of results’ limitations (an important aspect of testing) and very comprehensive guidelines to relate test results to product performance. SATRA test methods are the result of continual research on behalf of the footwear industry (and primarily SATRA members), and often require significant investment. Many test methods are developed after listening to customers’ needs. This is often very challenging as new technology and materials become available.

What is a test method?

A test method is a set of instructions that enables a laboratory to carry out a particular evaluative process, in order to identify the particular performance criteria of a product or material. The test method should make the process clear and without ambiguity, and be written so that it is unambiguous to technicians whose native language may not be English. As a rule, a test method does not contain performance levels or performance guidelines, but it may contain suggested performance levels in an appendix. The test method should describe the apparatus used – especially if it is unique to that test (not standard laboratory equipment). Sufficient details should be provided to enable the production of a suitable device. For instance, SATRA TM230:2017 – ‘Dynamic footwear water penetration test’ assesses whole shoe water resistance, and describes a method for flexing footwear in a depth of water. It has been adopted in the EN ISO 20344 test method standard for safety, protective and occupational footwear, as well as many procurement documents and specifications.

The ongoing development of improved footwear water resistance is very important to many of our customers – especially those competing for tenders of significant size and value. The correct performance levels are as applicable to everyday footwear as to high performance footwear. In this method – SATRA TM230 – the water level plane is defined, as are the flexing frequency and angle, flexing point and various other dimensional requirements, along with their associated tolerances. However, the depth of the water is not defined, and would be classed as a performance level – as is the number of cycles required.

Machine setup is also not defined, as some machines will be more straightforward to set and implement than others, thus permitting competitive machine design. Some machines allow for additional features, such as ‘auto leak detection’. This has been included in the SATRA TM230 test method as an optional way of detecting leaks in footwear where clear and defined performance levels are required on a pass or fail basis, and where very long tests require overnight, unattended testing.

The review process

For existing test methods, there is a rolling process of review implemented by SATRA’s quality team to ensure that each method is current and up-to-date. The test method being reviewed is re-examined by relevant experts, and any questions that arise are presented at regular test method meetings. Suggested changes are often put forward independently of the regular review process, as a result of customer suggestions or members of staff who have become aware of necessary amendments. Test methods evolve for very good reasons. Sometimes, superior methods of testing are discovered, and are often the result of industry innovation. When test methods are written, great consideration is given to ensuring that they are not design restrictive. Even with considerable effort, it is sometimes impossible to foresee how new technology and innovation may develop ways of achieving performance in materials and constructions. Sometimes, test methods written around established technology used to achieve a particular property are unable to fairly measure the performance of new materials and implementations of new technology.

Members developing new technology who have concerns relating to possible issues with established test methods are welcome to contact SATRA in complete confidence for advice. SATRA actively seeks this kind of customer involvement, which is a key part of test method development and evolution.

New test methods

SATRA develops new test methods at a rate of two or three each year. There are rules regarding the production of test methods, and the developer must provide evidence that a new method is actually required and does not duplicate others. Very rarely, superior methods are produced that in effect make previous techniques redundant, in some cases, this means that old methods may be withdrawn. More often, ‘preferred methods’ may be established, with older tests allowed to remain in the system as they may still be referred to in many specifications in use.

The term ‘preferred method’ will typically refer to a test that has been proven to give the most reliable and realistic results, but it also may refer to a method that is particularly good at relating to real wear in footwear. For example, while there are many ways of determining the breathability of materials, very few are suitable for the performance conditions required in footwear. The same is true for material flexing tests, in which the evaluation must replicate the nature of the folding and creasing that occurs in footwear.

SATRA can advise on the most appropriate preferred method that should be used to get the most meaningful results. However, there are often other test methods that give a result in a shorter time, are more cost efficient or, in a particular situation (to a production schedule of in-factory testing), give a correlating result.

Again, using SATRA TM230 as an example, while it is the preferred test, it is time-consuming to complete. SATRA TM444:2012 – ‘Water resistance of footwear – centrifuge method’ is also offered, which provides a precise assessment of footwear integrity while allowing for 100 per cent non-destructive testing for water resistance. SATRA TM230 is a reliable method for long-term water resistance and durability, and SATRA TM444 offers a comparable quick test to assist with factory production quality assurance.

Occasionally, a new test will be so different from anything that has gone before that SATRA will decide to reserve the published method for members’ use for a period of time. This often applies when the method is particularly advanced. A particular test method and item of machinery may be only available to members for a period of time, giving them priority access to the method and a potential competitive edge over other footwear manufacturers.

Methods are often produced in conjunction with new test machinery development, especially if the test is completely new. This is not always the case and occasionally existing machines can be used or adapted with accessories.

The most likely need for the development of a new test method will be due to the introduction of new technology, new materials or a new type or application of footwear. SATRA regularly assists members with product development and advanced testing to support commercial claims. There may be properties and uses of new designs and materials that cannot be distinguished by traditional and established means.

As an example, SATRA TM446:2018 – ’Resistance to waterborne abrasive particulates’, is a test method that arose out of a number of questions and information requests that were directed to SATRA’s innovation and development  department. It became clear that there was a need to test whole footwear for water resistance in very real conditions. We saw many examples of wear returns where the failures had not been caused directly by the presence of water, but by the small particles that may be suspended in it. Established water resistance tests use very sterile water, whereas a river, mud or the surf zone on a beach contains waterborne silt, which can be very abrasive. This abrasive silt is very damaging to threads and textiles, and can work its way into the construction of the footwear, becoming trapped for a considerable period of time. This can drastically reduce the lifetime and performance of the product.

Using SATRA TM230 as a starting point, various methods were investigated to maintain a specific density of abrasive particles suspended in the water. This was more challenging than we anticipated – many experiments resulted in the abrasive settling and forming a sand bar in the water container. Therefore, SATRA’s research team developed an idea involving a water container with a base plate fed with air. This proved to be very effective and was able to maintain the suspension for several days. It was so effective that the design was patented.

The new technique was promoted to our members and came into regular use, it was at this point that the decision was taken to formalise the process, and SATRA TM446 was established. Many advanced tests are developed in this way – directly as a result of SATRA members’ needs and requests.

The process behind producing a method, whether a modification of a current test or a completely new method involves a committee of experts. Once changes are proposed, the method enters a system in which other experts and quality managers review each change until a consensus allows for the method to be published. With complex methods, this can mean some time can pass before the method is published, although in most cases the quality process can be reasonably fast. SATRA methods are ‘controlled documents’, so when they are issued to customers, details such as issue number are retained. As a result, the customer can be notified of any subsequent re-issues. The same system is used internally at SATRA – methods are issued and registered to individual departments, to ensure that the latest controlled issue is always available.

Publishing Data

This article was originally published on page 16 of the October 2021 issue of SATRA Bulletin.

Other articles from this issue »

---