Machines for safety footwear testing – part 2

Continuing the series on test equipment used to assess safety footwear.

In this article, we continue our consideration of the SATRA test machines that can be used to conduct footwear tests to the European safety footwear requirements as set out in EN ISO 20345:2022 – ‘Personal protective equipment – safety footwear’ and the associated test methods found in EN ISO 20344:2021 – ‘Personal protective equipment – test methods for footwear’. In the article ‘Machines for safety footwear testing – part 1’, we described tests for uppers, linings and tongues, which are listed in the EN ISO 20345 standard under 'basic requirements'. Now we will focus on evaluations conducted on whole footwear.

The basic requirements for whole footwear are set out in section 5.3 of EN ISO 20345:2021. Within section 5.3, sub clauses cover ‘constructional performance’ (5.3.1), ‘toe protection’ (5.3.2), ‘leakproofness’ (5.3.3), ‘specific ergonomic features’ (5.3.4), ‘slip resistance’ (5.3.5), ‘innocuousness’ (5.3.6) and ‘seam strength’ (5.3.7).

EN ISO 20345:2021 classifies footwear as either ‘Class I’ ‘Class II’ or ‘hybrid’. Class I is footwear made from leather and other materials, excluding all-rubber or all-polymeric footwear. Class II footwear is all-rubber (that is, entirely vulcanised) or all-polymeric (entirely moulded) and hybrid is a combination of Class I and Class II constructions. All of the above-mentioned clauses are applicable to Class I footwear except for clause 5.3.3 (leakproofness), which is only applicable to Class II footwear. For Class II footwear, all these clauses are applicable, except clause 5.3.1 (construction performance).

Constructional performance

Construction (clause 5.3.1.1) states that when an insole is used, it shall not be possible to remove it without damaging the footwear. This clause also says that if there is no insole, a permanently attached insock shall be present.

Figure 1: The SATRA STM 566 tensile tester can be used to assess upper/outsole bond or interlayer bond strength

Upper/outsole bond strength (clause 5.3.1.2) indicates the sole bond strength requirements for the attachment of the upper to the outsole, the separation of layers of the outsole or tear failures of the upper or the sole. It references EN ISO 20344:2021 clause 5.2, which highlights details of the tests required. The test can be conducted with SATRA’s STM 566 tensile testing machine fitted with the appropriate jaws (for example, STM 566SB and STM 566F) as shown in figure 1. Preparation of specimens is important, and depends on the particular footwear construction.

EN ISO 20344:2021 illustrates seven types of construction, and indicates where the specimen cuts should be made. A hot knife should be used when separating sole layers at an adhesive surface. This sole bond strength test is not applicable when the bond has been made using a mechanical attachment such as nails, screws or by stitching the components involved.

Toe protection

Toe protection – general (clause 5.3.2.1) describes specific constructional requirements for incorporating toecaps. It states that metallic toecaps shall fulfil the requirements given in EN ISO 22568-1:2019 clause 4.2. and non-metallic toecaps shall fulfil the requirements given in EN ISO 22568-2:2019 clause 4.2.

Internal length of toecaps (clause 5.3.2.2) sets out the minimum length requirements for toecaps against footwear size. The SATRA STM 609TCM toe cap measuring device can be used to obtain the required measurements of toe caps by the method set out in EN ISO 20344:2021 clause 5.3.2.1. Table 5 of EN ISO 20345:2021 gives the minimum internal length for the toecap, depending on shoe size.

Width of toecap flange (clause 5.3.2.3) explains the requirements for the width of the toe cap flange when this element is measured in accordance with EN ISO 20344 clause 5.3.2.2.

Corrosion resistance (clause 5.3.2.4) specifies the corrosion tests applicable to metallic toecaps for Class I, hybrid and Class II footwear. These tests require no special equipment, being visual inspections for the number and size of any corrosion sites after the toecaps are exposed to an aqueous solution of sodium chloride for a prescribed time.

Behaviour of toecaps (clause 5.3.2.5) states that non-metallic toecaps need to conform to the requirements of EN ISO 20345:2021 table 6 when impact tested to ISO 230344:2021 clause 5.6.2. This sets out a number of individual conditioning tests to which new samples of the toe caps should be subjected prior to carrying out impact tests. These cover the effects of i) high temperature, ii) low temperature, and iii) fuel oil. Equipment required for these conditioning tests are an oven with a temperature capability up to 70°C, a freezer with a temperature capability of -20°C and suitable immersion tanks to contain acid, alkali and fuel oil.

Impact resistance of safety footwear (clause 5.3.2.6) specifies the requirements for toe impact resistance of safety footwear, in accordance with the method described in EN ISO 20344:2021 clause 5.4. In turn, this sets out the equipment to be used (which should be in accordance with EN ISO 22568 – the clamping device to hold the specimen, cylinders of modelling clay (used to record the minimum clearance under the toecap during the impact test) and the dial gauge required to measure the height of the clay cylinder. SATRA’s STM 609 safety footwear impact testing machine is suitable for conducting tests to these requirements. In addition, the STM 609 machine can be used to conduct impact tests in line with the requirements of the Canadian Z195-14 test and the American ASTM F2413-11 test.

Testing to the European standard

The EN ISO 20344:2021 clause 5.4 test sets out the following requirements:

an impact energy to which the specimen on test is to be subjected (200 ±4 J)
an impact striker geometry and associated mass (20 ±0.2 kg)
a location for where impacts are to strike the specimen
requirements for anchoring the test specimen during the test
criteria for acceptance after the test.

The toe impact test and compression test specified within EN ISO 20345:2021 are for toe caps in the context of completed footwear. Therefore, the test specimens are prepared from whole footwear, as set out in clause 5.4.2.2 of EN ISO 20344:2021, with the test specimen including a section of sole and the linings and upper in the toe cap region.

Figure 2: The SATRA STM 609 impact testing machine

The SATRA STM 609 impact testing machine (figure 2) produces the specified impact energy by allowing the impact striker and its associated mass to free fall from a specified height, guided by low-friction guide rails. An electric motor drive, incorporated into the machine, is used to hoist the impact striker and its associated mass to the height calculated to deliver the required impact energy. The impact striker and associated mass is then released via the control system. An adjustable catcher mechanism catches the impact striker after the first impact with the specimen, to prevent subsequent re-strikes after any rebound from the initial impact.

A cylinder of modelling clay is positioned within the toe cap to determine the maximum deflection of the toe cap during the test. Modelling clay is used as it has negligible spring-back when compressed, so it will retain the minimum height to which the toe cap is compressed during the impact test, even if the toe cap itself has sprung back. SATRA supplies a calibrated modelling clay (SATRA part number 304976) which conforms to the requirements for this material specified within Annex ‘A’ of EN ISO 20344:2021.

SATRA also supplies a dial gauge (SATRA STM 609HF) with a hemispherical foot conforming to the requirements of clause 5.4.1.4 of EN ISO 20344:2021, which should be used to measure the height of the cylinder after impact. The measured height should then be compared to the minimum clearance, depending on shoe size, as specified within table 6 of EN ISO 20345:2021. In addition to the minimum clearance, the standard requires that toe caps should not develop delamination or any cracks which go through the material – that is, through which light can be seen. The STM 609 safety footwear impact testing machine is supplied with the required clamps for retaining specimens and the formers for producing the modelling clay cylinders. Due to the nature of the test, the test machine needs to be mounted on a mass of at least 600 kg to comply with the EN method, as well as located on a non-elastic floor.

Compression resistance of safety footwear (clause 5.3.2.7) is a crush test applied to the footwear toe region between two parallel surfaces moving together at a controlled rate of 5 ±2 mm/min. The test is conducted in line with the requirements of EN ISO 20344:2021 clause 5.5. The test specimen is subject to a compression load of 15 kN, with a modelling clay cylinder placed to record the lowest height to which the toe cap is compressed during the test. Specimen clamps are as those used for the toe impact test. SATRA’s 20 kN STM 766 tensile testing machine fitted with STM 766CP ‘compression platens’ is suitable for conducting this test, and conforms to the requirements of EN 12568:2010 clause 5.2.3.1.1. The minimum compressed height is determined by measuring the modelling clay cylinder height, which can be done using the SATRA STM 609HF hemispherical foot dial gauge. The minimum compressed height for a particular shoe size is given in table 6 of EN ISO 20345:2021. In addition to the minimum clearance, the standard requires that toe caps should not develop delamination or any cracks which go through the material – that is, through which light can be seen.

Figure 3: Assessing footwear on SATRA’s STM 510 leakproofness testing machine

Figure 4: Air at a controlled pressure is introduced into the boot through the sealing collar

Leakproofness (clause 5.3.3) is applicable to only Class II footwear which has a closed seat region. It references EN ISO 20344:2021 clause 5.7, which sets out the test requirements. SATRA’s STM 510 leakproofness testing machine (figures 3 and 4) can be used to conduct this test. The test equipment consists of a clear-sided, water-filled tank incorporating a frame onto which the boot can be attached, as well as a means of lowering the frame into the water. Attachments are provided for sealing the top of the boot sample across the opening for a wide range of footwear sizes. A supply of air at a controlled pressure is introduced into the boot through the sealing collar. On lowering the boot under the water, the test requires a visual inspection for air leaks. The requirement is that there should be no leakage of air observed by a continued formation of air bubbles from the footwear sample.

The next article in this series will complete the review of the basic requirements for whole footwear within EN ISO 20345:2021, and will cover ‘specific ergonomic features’ (clause 5.3.4), ‘slip resistance’ (clause 5.3.5), innocuousness (5.3.6) and seam strength (5.3.7). As stated in the the previous part of this series, the article is not intended to give a detailed description of the application of EN ISO 20345:2021 or EN ISO 20344:2021. However, it has been set out in order to highlight the main pieces of test equipment that are needed in order to assess the basic requirements for whole footwear as set out in EN ISO 20345:2021 under the headings of ‘sole performance’, ‘toe protection’ and ‘leakproofness’.

How can we help?

Please email test.equipment@satra.com for further information on SATRA test equipment.

Publishing Data

This article was originally published on page 22 of the March 2022 issue of SATRA Bulletin.