What Is Flexing?
The flex resistance test simulates in the laboratory the repeated flexing motion footwear materials undergo during walking. The specimen is flexed at a defined angle, defined speed and set number of cycles, then examined for cracking, creasing, coating separation and surface degradation.
Purpose of Flexing
The purpose is to predict whether the footwear will crack at the flex point over its service life. The most common failure mode in footwear is cracking and sole separation at the flex point.
Which Products Is Flexing Applied To?
- Footwear upper leather and synthetic leather materials
- Sole materials (rubber, PU, TPU)
- Coated and patent leathers
- Lining materials
- Safety footwear
In Which Sectors Is Flexing Used?
Standards Related to Flexing
- TS EN ISO 17694 (footwear upper and lining materials — flex resistance)
- TS EN ISO 17707 (soles — flex resistance)
- TS EN ISO 5423 (moulded plastics footwear)
- TS EN ISO 20344 (protective footwear test methods)
- SATRA TM55 / TM161
Flexing Conditions
- Conditioning
- At the temperature and relative humidity defined in the standard
- Number of cycles
- Set by the specification; staged inspection may be performed
- Temperature
- At room temperature, or at low temperature if required (cold flex)
Equipment for Flexing
- Bally flexometer
- Sole flexing (Ross flex) machine
- Low-temperature chamber
- Magnifier and imaging system
Sample Requirements for Flexing
- Specimens cut to the dimensions defined in the standard
- Specimen orientation (back/belly direction on the hide) must be marked
- In the sole test the initial cut is made per the standard
- A separate specimen for each material and colour
Duration of Flexing
Duration is determined directly by the number of cycles. High-cycle programmes run for days on the machine; low-temperature tests add conditioning time.
Flexing Procedure
- 1Specimens are conditioned and mounted in the machine as defined.
- 2In the sole test, the initial cut defined in the standard is made.
- 3Flexing is applied at the defined angle and speed.
- 4The specimen is removed and examined at the defined stages.
- 5Crack length is measured and surface degradation is recorded.
- 6The result is reported together with the number of cycles.
Flexing Acceptance Criteria
The acceptance criterion is the maximum permitted crack length or degree of damage at the cycle count specified. For protective footwear, product standards such as TS EN ISO 20345 define their own requirements.
Evaluation of Results
The type of damage is recorded: surface cracking, coating separation and sole tearing each point to different manufacturing issues. The low-temperature test is assessed separately, since a material that passes at room temperature can become brittle in the cold.
Flexing Report
- Specimen identification and material type
- Standard applied and number of cycles
- Test temperature
- Stage-by-stage observations and crack measurements
- Photographs
Accreditation
EUROLAB holds internationally recognised accreditation obtained from accreditation bodies party to the IAF and ILAC multilateral recognition arrangements, under ISO/IEC 17025 for laboratory testing, ISO/IEC 17020 for inspection and surveillance, and ISO/IEC 17021-1 and ISO/IEC 17065 for system and product certification. This chain ensures the reports and certificates issued are accepted worldwide.
Whether this test can be offered within accredited scope depends on your product and the standard applied; since accreditation is method-based, the scope is confirmed in writing at the quotation stage.
Scope and authorisationsRelated Regulations
- (EU) 2016/425 Personal Protective Equipment Regulation (for protective footwear)
Footwear and Leather Testing — What Does This Testing Field Cover?
The section below describes the field that the Flexing test belongs to; the conditions and acceptance criteria of this particular test are given in the sections above. The scope specific to your product is confirmed in writing at the quotation stage.
Footwear and leather testing assesses the physical durability and chemical safety of a product together. Footwear is a MULTI-COMPONENT product: the sole, upper, lining, insole, heel and bonding line are tested both individually and together. One component being good does not mean the product is good; failure usually appears where components join.
On the physical side, the most common field failure is SOLE-UPPER SEPARATION. Bond strength is therefore the most critical measurement in the field and is a direct indicator of the bonding process (surface preparation, adhesive selection, pressing conditions). Next come sole abrasion and flex resistance: a shoe is flexed hundreds of thousands of times over its life.
Slip resistance is the most important parameter for safety and its measurement depends on the surface: wet ceramic, oily steel and dry floor give different results. Slip resistance is therefore reported not as a single number but with the surface and the direction of slip stated.
A separate body of work applies to protective footwear, and this is the most commercially critical distinction. Toe protection (impact and compression), penetration resistance, antistatic behaviour, heat and cold insulation, electrical insulation and fuel resistance are assessed under personal protective equipment requirements. CE is mandatory for these products and the category must be chosen correctly; an ordinary footwear test report does not satisfy that file.
On the chemical side, CHROMIUM (VI) has a special place in leather. The chromium (III) used in tanning can oxidise into chromium (VI) during storage and ageing; that is, a leather that was compliant at production may exceed the limit months later. The time of measurement and the history of the sample therefore affect the result, and measurement after ageing may be required.
Other chemical items are aromatic amines from azo dyes, pentachlorophenol, short-chain chlorinated paraffins, dimethyl fumarate and phthalates. These items come from REACH restrictions and brand restricted substance lists.
There is NO CE marking on consumer footwear; the obligation runs through material composition labelling, compliance with chemical restrictions and general product safety. Labelling has a rule of its own: the type of material used for the upper, the lining and insole and the outer sole is shown by pictograms.
Comfort and functional claims (waterproofness, moisture management, breathability) are supported by separate tests, and these claims require evidence if used in marketing copy.
Method groups in this field
Bonding and strength
Sole-upper bond strength, seam strength, heel attachment strength, strap and buckle strength.
Abrasion and flexing
Sole abrasion, flex resistance (dry and at low temperature), insole abrasion.
Slip resistance
Forward and backward slip measurements on wet ceramic, oily steel and dry floor.
Protective properties
Toe impact and compression resistance, penetration resistance, antistatic behaviour, heat and cold insulation, fuel and chemical resistance.
Leather chemistry
Chromium (VI), azo dye amines, pentachlorophenol, short-chain chlorinated paraffins, pH and fat content.
Comfort and function
Waterproofness, water vapour permeability, moisture management, colour fastness and the effect of perspiration.
Commonly confused in this field
- Footwear claiming a protective function falls under PPE legislation and requires CE; a consumer footwear test report does not satisfy that file.
- A chromium (VI) result depends on the age of the sample and its storage conditions; leather compliant at production may exceed the limit later.
- Slip resistance is not a single number; it cannot be interpreted without stating the surface and the direction.
- A result obtained for one component does not cover the whole product; the joints are tested separately.
- Labelling rules run on material pictograms; an incorrect pictogram causes problems in customs and market surveillance.
- A waterproofness claim is not verifiable unless the test used and the number of flex cycles after which it was measured are stated.
Which decisions these results support
- Compliance with the brand restricted substance list
- The PPE category and CE file for protective footwear
- Supplier and lot-based acceptance control
- Root cause analysis of field complaints (bond opening, sole separation)
- Substantiation of functional claims
Related Tests
Frequently Asked Questions About Flexing
How many cycles should I request?
The number of cycles is set by the product category and the specification. Everyday footwear and work footwear do not carry the same expectation. If your specification gives no figure, the target market's product standard is used.
When is the cold flex test required?
For products intended for use in cold climates. Some PU and TPU soles become glassy at low temperature, and a material that never cracks at room temperature can fail quickly in the cold. If your claim covers cold-weather use, the test must be run at low temperature.