Using tensile testers – part 3
Concluding this series of articles on how tensile testing equipment can be utilised by companies producing footwear.
In this third part of the series, examples are given of how a tensile tester can be used to assess the strength of footwear assemblies. The test methods described in this article can be utilised as part of the validation of a new product design or for the quality auditing of production footwear. All the SATRA tests mentioned in this article can be conducted using the SATRA STM 566 tensile tester, together with the appropriate jaws.
In parts one and two, the main focus was on tests for the assessment of material properties or the performance of components. In this third article, examples are given of methods which can be used to verify the assembly of materials and components typically found in completed footwear. The first four tensile test methods described below relate to heel attachments and heel component attachments. If footwear is placed onto the market with poor quality heel attachment – due to either inadequate design or poor manufacturing processes – the result may be an injury to the wearer, as well as a loss of customer confidence in the brand. This can also lead to high replacement and compensation costs.
SATRA TM113:1996 – ‘Measurement of the strength of attachment of heels to footwear and the backpart rigidity of such footwear’ allows an assessment to be made of the strength of heel attachment in completed footwear. The method is applicable to all footwear with separately attached heels. When conducting this test, the shoe is clamped at the forepart and the heel is pulled backwards at a constant rate. The deformation due to a specified force is determined, as well as the force required to detach the heel or to cause some other failure. The corresponding type of failure is recorded. Specialist jaws are available from SATRA for carrying out this test. For more substantial heels, the attachment of the heel jaw is accomplished by drilling a hole through the heel at a specified position. The jaw pivot bar is located in this hole, allowing the heel to pivot during the test. For slender heels, the attachment is made via a pivoting clamp (figure 1).
Before commencing the test, a measurement is taken from prescribed points on the specimen. Load is applied to the specimen at a separation rate of 100mm/min until 200N has been applied. As this load is reached, the distance between the two prescribed points is re-measured. The test then continues up to a load of 400N before the load is reversed back to zero. At this point, another dimensional check is taken.
The load is then reapplied at a steadily increasing rate until the heel becomes detached or some other failure occurs, and the load and failure mode are recorded. The force at the point of failure is recorded, along with the mode of failure. Other aspects (permanent set and, if required, backpart deformation) are also reported. Although this is relatively simple and inexpensive test to carry out, it can make a very valuable contribution to determining the overall performance of the heel attachment, and hence protect against costly failures in wear.
In addition to testing the overall heel attachment, evaluations can also be carried out on the capabilities of the components used within the attachment. For example, SATRA TM96:1995 (2017) – ‘Security of heel pins’ allows for an assessment of the heel pin (nail) holding strength of a material, or the heel nail pull-out force on a completed shoe. The method, which is carried out with specialist jaws, is mainly applicable to plastic heels for women’s footwear, but can also be used for wooden heels.
SATRA TM96 contains two methods: ‘Method 1 – Heel pin holding strength’ and ‘Method 2 – Heel pin pull-out force’. Each of these methods sets out the specimen preparation, the number of test specimens and the procedure for conducting the tests, along with the required results including the arithmetic mean of heel nail holding strength. Also needed are the arithmetic mean of depth of nail penetration for Method 1, and the arithmetic mean heel nail pull-out force for Method 2. SATRA can provide the jaws required to conduct SATRA TM96.
SATRA TM11:1993 – ‘Pin holding strength of insole materials’ (figure 2) is intended to simulate the force required to pull the head of a nail through a material. The test is carried out with both wet and dry specimens. The method is mainly applicable to footwear insole backpart materials, but can be also used with all types of semi-rigid material. The strength of a heel assembly depends on each component and the interfaces between them. Another aspect to be considered, in addition to those mentioned earlier in this article, is the ability of the backpart materials to resist the pull loads applied via the heads of the heel nails when lateral loads are applied to the heel. SATRA TM11 allows a consideration of the retention of the nail in the materials which are attached to the heel by the nail.
When conducting this test, the force required to push a stepped metal rod through a hole drilled in a test specimen is measured. The test is carried out with one dry test specimen and one that has been soaked in water. Specialist jaws can be supplied by SATRA to conduct SATRA TM11.
Another assembly assessment for heels which can be tested using a tensile tester is the attachment of top-pieces (top-lifts). SATRA TM108:2020 – ‘Strength of top-piece attachment’ (figure 3) has been designed to determine the force required to detach the top piece from a shoe heel. For a top-piece which is moulded around a spigot, the method also can be used to assess the top-piece to spigot attachment strength. The method is applicable to all types of top-pieces, and specialist jaws (which are available from SATRA) are required to carry out this test.
In the development of this test method, the means of applying the load and the direction of loading has been carefully considered for different heel types. This is reflected in the details given in the method for specimen preparation and specimen mounting. The maximum force(s) recorded in the test, the type(s) of failure and the median of the maximum forces should be reported.
The following SATRA test methods are examples where other aspects of footwear construction can be assessed using a tensile tester.
SATRA TM118:1992 – ‘Strength of sandal toe posts’ allows for a determination to be made of the strength of a toe post and its attachment. The method is applicable to all types of sandals with a toe post. In order to conduct this test, a thin and rigid square template – having a hole drilled close to each corner and a keyhole-shaped slot cut to accommodate the toe post – is placed around the toe post on the insole of the test specimen. Four holes which match the holes in the template are drilled through the sole. Two loops of thick wire are threaded through both the template and the sole. The toe post is held in one jaw of the tensile tester and the ends of the wire loops are held in the opposing jaw. The force required to pull the post from the sole or some other failure such as the toe post breaking or detaching from the straps, is measured. This force and the mode of failure are recorded.
SATRA TM120:2001 – ‘Strength of attachment of straps and nailed or stapled uppers’ allows an assessment to be carried out of the strength of attachment of straps, nailed (pinned) or stapled uppers, to the sole of completed footwear. The method is applicable to all types of footwear with uppers having straps less than a certain width, or uppers attached to the sole by means of nails or staples.
When conducting this test, an increasing force is applied to either strap or to a strip cut from the upper containing one or more nails or staples until failure occurs. The force is applied at right angles to the plane of the sole unit. This force at failure is determined and the type of failure recorded. The test method sets out the details of how the specimen is prepared and attached to the tensile tester, so the load can be applied correctly across the tensile tester jaws. The test method also sets out the different types of failure which can occur so these failure modes can be identified and recorded in the test report. This test allows a number of different aspects of attachments to soles to be assessed, and can help to determine if a product is fit for purpose or, alternatively, needs to be improved. The careful consideration of failure modes should act as a guide as to what needs to be changed when looking for an improvement in the performance of the product.
SATRA TM181:2017 – Strength of buckle and strap attachments’ (figure 4) allows the strength of a buckle and strap attachment in completed footwear to be assessed. When conducting a test, a test specimen containing a buckle and/or strap is gradually stretched by a tensile tester until failure occurs. The breaking force and the type and position of failure are determined. Depending on how the specimens are cut and the clamping positions selected, the method can be used to determine: i) the strength of buckle and attachment, ii) the strength of the strap in the closed buckle, iii) the strength of the strap attachment, or iv) the strength of the strap. SATRA TM181 sets out the details for how to prepare the specimens applicable for the characteristic to be assessed. This test and the individual elements within the test can be carried out with standard flat jaws fitted to the STM 566 tensile tester.
These SATRA test methods are examples of the many SATRA test methods developed for use with a tensile tester. The three parts in this series of articles have given an overview of the application of a number of these methods. The SATRA STM 566 tensile tester can be used to conduct these tests, along with a wide range of specialist jaws required for a number of the tests.
How can we help?
Please email firstname.lastname@example.org for more information on test methods or for guideline advice for SATRA members. Contact email@example.com for more advice on SATRA’s STM 566 or STM 766 tensile test machines and the associated range of jaws.
This article was originally published on page 26 of the February 2021 issue of SATRA Bulletin.