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ISO 13997 cut resistance testing
How to ensure that cut-resistant personal protective equipment is fit for purpose.
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Cut-resistant fabrics are manufactured in many forms and for a variety of purposes, including clothing, footwear, gloves and even head protection. In most cases, any risk from severe cutting hazards should be removed from the work activity – for example, by the use of guards – leaving the operative to require protection against only relatively minor hazards from small blades and cutters.
These minor cutting hazards often cannot be removed due to the nature of the job (such as cutting operations requiring precision and dexterity afforded by hand cutting). Nevertheless, although the potential damage from cutting by a single blade is somewhat less than that from a circular saw or laser cutter, it can still be fatal. The use of cut-resistant personal protective equipment (PPE) should, therefore, always be considered where there is a risk of being cut.
Most minor cut incidents occur to the hands, simply because these are normally the parts of the body closest to the operation involving a sharp blade. Other parts of the body can usually be protected by modification of cutting practice or with the use of additional guards, but the hands are (in most cases) required to carry out the cutting operation, and so cannot be removed from the process.
Gloves with mechanical protection are classified in EN 388:2016+A1:2018 – 'Protective gloves against mechnical risks'. This is based on five characteristic elements of protection: abrasion, blade cut, tear, puncture and impact resistance, with each element divided into protection levels. Similar tests are carried out on fabrics used in other types of protective clothing, with minimum levels based on the expected hazards.
For gloves, blade cut resistance is divided into five levels, with level 5 being the most resistant and level 0 offering the least protection. Traditionally, the level is based on the cut index gained by a test using a rotating blade being drawn along fabric samples until cut-through occurs. This is commonly referred to as the ‘coup test’. However, where fabrics have a high resistance to cutting, this test can often create anomalous results due to blunting of the test blade. In such cases, an alternative method is used as specified in ISO 13997:1999 – 'Protective clothing – Mechanical properties – Determination of resistance to cutting by sharp objects' (see figure 1).
The ISO 13997 method uses a straight blade drawn across a small piece of the fabric until cut-through takes place. The principle of this test is to vary the load required to be applied to the blade in order to facilitate cut-through in a known distance. In comparison to the coup test, the blade travels only a short distance, which means that blunting of the blade plays a much less significant part.
An ISO 13997 blade cut tester consists of a straight blade (of known sharpness) fitted to a carriage, which is capable of horizontal movement to draw the blade across the sample. The fabric sample is mounted on a curved surface. In turn, this is placed on top of a series of levers in order to apply a force from below the sample holder onto the blade, which simulates a mass being placed on top of the blade itself. The blade is drawn across the sample at a set speed, with the distance travelled until cut-through (referred to as ‘stroke length’) being recorded. Typically, cut-through is indicated by the point where an electrical contact is achieved between the blade and the holder. Therefore, where fabrics include steel threads, it is necessary to place a sheet of thin paper or plastic film between the sample holder and the fabric to prevent electrical contact through the fabric itself.
The test procedure begins by carrying out a number of cuts using a variety of masses applied to the blade to gain a suitable range of cut lengths. This is, typically, five cuts in the range of 5 to 15 mm, five cuts in the range of 15 to 30 mm and five cuts in the range of 30 to 50 mm (cut lengths below 5 mm or above 50 mm are ignored). Using this data, a scatter graph can be drawn by plotting stroke length against applied load.
From this graph, an estimate can be gained for the applied load necessary to gain a 20 mm stroke length before cut-through by plotting a trend line through the data points (an exponential plot often gives a fairly good correlation). Using this estimate, a further five cut tests are carried out, with the graph re-plotted. If the average of these five cuts is within a suitable tolerance from 20 mm (±2 mm), a further estimate is taken from the new graph and recorded as the final result. If the average of the five cuts is outside the tolerance, the new estimate is used for a further five cuts, with the results used for a final re-plot. The final estimate from this third graph then becomes the final test result. The test result is based on the estimated force required to generate a 20 mm stroke length, in Newtons.
Blades for this test are made to a specification set by the test method. Each batch is checked for average sharpness, defining a sharpness correction factor. Batches of blades with too low a value for sharpness, or with too great a variation in sharpness, are rejected. When blades are accepted, each batch is assigned with a correction factor, which is used to normalise the results of each cut test. For instance, a blade sharpness correction factor of 0.5 would result in each stroke length being halved. A new blade is used for every single cut test – and discarded after use – to ensure that blunting does not play a significant factor in the results.
Although the main purpose of the ISO 13997 cut test is to provide data for comparison between fabrics (the larger the force, the higher the cut resistance), requirements are included in EN 388 for cut-resistant gloves.
It is also worth noting that due to the differences in the two methods used for cut resistance, there is no correlation between the two methods – a material could achieve different levels using various test methods. It is also worth noting that the methods have different benefits, which make them more suitable for a variety of material types. However, it is widely considered that the ISO 13997 method is generally more accurate for high levels of cut resistance. The trade-off is that due to the need for large numbers of individual cuts (each using a brand new blade), the ISO 13997 method is significantly more time-consuming and costly to carry out.
Further information on SATRA's PPE certification and testing services is available at www.satra.com/ppe
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SATRA is able to offer testing and, where applicable, EU type-examination of fabrics for cut resistance, as well as a large variety of other properties. Please email firstname.lastname@example.org for more information. SATRA also manufactures the ISO 13997 cut tester and rotating blade cut tester. Contact email@example.com for details regarding purchasing these machines.