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Safety Footwear [Sept 2010]

Published: 01st Sep 2010 in OSA Magazine

SATRA is one of the world’s foremost authorities on the testing and certification of protective and safety footwear, and routinely tests products incorporating protective elements. SATRA’s Austin Simmons explains the different types of toe cap and penetration-resistant inserts available, the tests applied and potential problems with footwear.

Protective toe caps are generally fitted to footwear designed to provide impact and compression resistance. Traditionally, these have been made from steel, although aluminium types are also available, and, more recently, plastic or composite non-metallic materials have entered the marketplace.

Both composite and aluminium products are lighter than comparable steel versions, but are generally more expensive. However, they do have advantages in specialist applications, including magnetic sensitive electronics manufacture and petro-chemical industries.

Composite or plastic toe caps are also popular for use at airports, as their non-metallic properties minimise disruption when entering scanned security zones.

Originally, penetration-resistant inserts were made from metal. Developments in high-performance technical textiles have enabled the use of alternative materials to metal, and construction methods that can provide the required level of protection. Such materials are most commonly used in Strobel-stitched constructions in place of a sewn-in sock, where they are adjacent to the foot or directly underneath the footbed.

Globally, there are several standards and certification requirements for safety footwear which specify protective qualities for toe caps and penetration-resistant inserts. These include the Canadian CSA certification scheme based on Z195-02, ASTM standard F2413-05 (replacing ANSI standard Z41-1999) and the Personal Protective Equipment (PPE) Directive 89/686/EEC legislation covering the European Economic Area. All require components to be assessed as an integral part of the footwear.

European requirements

The PPE Directive CE marking requirements cover whole products such as finished footwear and garments and do not apply to sub-assemblies, materials and components. Therefore it is not possible to CE mark toe caps and penetration resistant inserts separately.

However, they can be tested as components using European standard EN 12568 - ‘Requirements and test methods for toe caps and penetration resistant inserts’. This uses similar conditions to those applied to the completed footwear when assessed against EN ISO 20345, but requires higher clearances for toe caps to compensate for potential compromises relating to the softer footwear outsole base.

Toe caps

EN 12568 covers performance of the cap when subject to impact and compression forces. It also includes criteria for the dimensions and corrosion resistance of the metal. For caps produced from material other than metal, the impact test is repeated after various types of pre-conditioning such as at high and low temperatures, plus exposure to a variety of chemicals (see table 2).

When manufacturing footwear for Europe, manufacturers are strongly advised to only source caps that comply with EN 12568 and, where possible, seek evidence of third party testing from an ISO 17025 accredited test organisation such as SATRA.

Regardless of the standard to be achieved, the toe cap design is crucial to good performance. Working on the ‘defended space’ principle, the design must provide adequate strength to limit fracture or distortion to such an extent that toes are not crushed when impacted or compressed in accordance with the standards.

In addition to material strength and thickness and shape, the width of the flange along the bottom edge of the toe cap is an important element as this helps transmit forces to the supporting outsole. Another important property of the toe cap is internal depth. The deeper it is, the more it can distort during the impact or crush before starting to impinge on the wearer’s toes.

While the compression tests in the various standards (such as ASTM, CSA, EN) are very similar, the impact tests do vary slightly in terms of the shape of the striker used to impact the cap, the energy of the impact and the minimum clearance required beneath the cap. Box 1 summarises the requirements.

Clearly, the specification and performance of the actual toe cap plays a vital role in the level of protection afforded in any safety footwear. However, the design and construction of the footwear itself can adversely influence the performance of the toe cap - which is why testing it in-situ is the only way to determine the true level of protection offered to a wearer.

Compromising performance

The performance of toe caps can be compromised in several ways. For the defended space principle to work, not only must it have the necessary strength but the outsole complex immediately below the cap flange must provide the required support, so that the crushing or impact force is efficiently transferred to the ground without the component becoming embedded in the outsole.

For this reason, the support to a toe cap will be more effective if the outsole is produced from a relatively hard compound. Another consideration when designing the outsole will be the alignment of the back edge of the cap with any cleats in the outsole. This is because the area directly above gaps in the cleats will not provide a high level of support to the component and therefore, where possible, should not coincide with the back edge.

Another outsole design feature, which can affect toe protection, is a gradual reduction in overall thickness towards the toe - increasing the degree of toe spring. This can adversely affect the performance of the cap during an impact or crushing incident, as the toe spring allows it to roll forward so that the front wall is lower than the back edge.

As many safety toe caps are designed to transmit forces via the front wall, if this is lowered below the back edge the force transfer mechanism cannot work effectively and the back edge is subject to severe distortion.

Another feature of the outsole complex that can affect toe protection is the profile of the upper surface when viewed in cross section across the width of the outsole. Here, a concave upper surface will increase the central clearance depth under the safety cap, thereby allowing more distortion of the product before injury to the toes is likely to occur.

Penetration resistant inserts

The main requirements of the safety footwear standards for penetration resistant inserts vary slightly (see box 2). EN 12568 includes requirements for both metallic and non-metallic inserts (see table 2).

Methods have been defined for testing penetration resistance of non-metallic inserts after various wet and environmental treatments. These align with the requirements for testing of non-metallic toe caps after chemical and thermal ageing.

Typical force/deformation curves for textile inserts are not the same as for metal inserts. During tests on metal inserts, there is a gradual increase in the penetration load, which drops sharply at the moment that the nail penetrates the insert. This relates directly to the point at which the applied force first stops increasing. However, with textile inserts, the force/deformation curve is usually made up from a number of peaks and troughs until the actual penetration of the nail occurs. Therefore, using the point at which the applied force first stops increasing (for instance, in figure 1 at approximately 550N), does not always relate to penetration of the nail through the insert material.

For various reasons, penetration-resistant inserts do not always extend to the full width of the outsole and the performance standards in the EN ISO 20344 series allow for this by permitting a margin of no more than 6.5mm from the insole edge. However, under compressive situations, the flange of the cap can dig into the outsole complex and pass over the outer edge of the penetration-resistant insert. Metal versions may then spring inside the cap and, because the insert is laterally compressed, it can distort upwards and reduce the clearance under the cap.

To improve performance in the impact and compression test, the penetration-resistant insert has to be fitted so that it extends fully beneath the flange of the cap. Then, during the test, it will act as a base for the cap to sit on and help to resist compression into the sole. In addition, the flange of the cap will sit fully onto the midsole, preventing it moving inside the flange of the cap during tests.

Underfoot cushioning

Most safety footwear incorporates underfoot cushioning in the form of a fitted footbed. However, if the footbed covers the full length of the sole, it will undoubtedly extend into the defended space under the cap. This has the effect of reducing the internal clearance, causing a detrimental effect on the level of protection provided. It is therefore worth considering tapered footbeds with reduced thickness at the toes. Once the toe cap clearance has been assessed as satisfactory, the footbed should not be changed.

Summary

The choice of toe cap and penetration-resistant insert types, and the materials used has never been greater. Manufacturers need to select on the basis of intended market and application and ensure that the footwear has been designed to maximise the effectiveness of protection.

Readers interested in finding out more about how SATRA can work with them on safety footwear development and testing are invited to contact safetyfootwear@satra.co.uk or visit the website www.satra.co.uk/safetyfootwearosa

Author Details

Austin Simmons became CEO in July 2009 after close involvement in developing corporate strategy and initiatives across SATRA’s consumer and industrial product sectors as deputy chief executive for three years. Austin is chairman of the SATRA China Advisory Board. He joined SATRA in 1996 and has a background in consumer products research and testing.

Published: 01st Sep 2010 in OSA Magazine

Author


Austin Simmons


Austin Simmons, Deputy Chief Executive SATRA

SATRA tests and certifies protective footwear, gloves, clothing (including high visibility, motorcycle protective clothing and gloves, protection against mechanical hazards, cutting, chemicals, impact, heat), fall protection, helmets, hearing and eye protection.

SATRA Notified Body scope includes:

  • EC Type examination (Article 10) 
  • Final product Quality control (Article 11A)
  • Quality of production - by monitoring (Article 11B)

These PPE facilities are available to all manufacturers and distributors who are interested in PPE certification and CE marking, regardless of membership status (SATRA is well known as a research and technology centre of excellence), and our team of PPE specialists looks forward to being able to offer you an unparalleled service testing and certifying PPE.

SATRA has established an enviable reputation for PPE testing and certification, and has become a byword for quality and integrity. Indeed, many companies insist on seeing a copy of a SATRA certificate before purchasing PPE. SATRA is also a truly global player, working with major manufacturers and distributors worldwide. We also have offices in Taiwan and China to help Far East manufacturers with their problems locally.

The future holds many challenges but you can be sure that SATRA will remain at the forefront of PPE test and certification and our highly respected experts will continue to provide a high level of quality and customer service backed up by state of the art facilities and equipment.


Austin Simmons

Website:
http://www.satra.co.uk

Email:
info@satra.co.uk

Phone:
+ 44 (0) 1536 410000

info@satra.co.uk
http://www.satra.co.uk
+ 44 (0) 1536 410000

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