Crack formation stage

Crack formation in materials can be triggered by a wide range of factors:

• Mechanical stress: excessive tensile, compressive or bending loads can lead to cracking.

• Material fatigue: repeated loads, even below the load limit, lead to the formation of microcracks.

• Environmental influences: UV radiation, ozone, chemical substances or moisture can weaken the material.

• Ageing: over time, many materials lose elasticity and strength, which increases their susceptibility to cracking.

• Processing defects: production defects such as air inclusions, stress cracks or uneven material distribution can promote cracks.

The crack formation stage is typically determined by standardised test methods. These methods simulate loads or environmental influences to evaluate the material’s resistance:

Tensile test

• Description: the material is stretched until it breaks in order to measure the load at which cracks form.

• Parameters: tensile strength, elongation at crack formation.

Fatigue tests

• Description: repeated loads are applied to the material to observe the formation of microcracks.

• Parameters: number of cycles to crack formation.

Ozone and UV tests

• Description: the material is exposed to UV radiation or ozone in a controlled environment to test its crack resistance.

• Parameters: time to crack formation, visibility of the cracks.

Impact tests

• Description: the material is subjected to a sudden impact to measure its susceptibility to cracking under sudden loads.

• Parameters: energy absorbed until crack formation.

The crack formation stage is a key indicator of the quality and longevity of a material:

• Product life: materials with higher crack resistance have a longer service life.

• Safety: crack resistance is essential, particularly in safety-critical applications such as seals, O-rings or vehicle components.

• Cost savings: materials with an optimised crack formation stage reduce failures and maintenance costs.

• Quality control: testing the crack formation stage helps detect processing errors and material defects at an early stage.

A material’s crack resistance is determined by several factors:

Factor	Influence	Examples
Material composition	Determines basic strength and elasticity	EPDM is more resistant to ozone than NBR
Temperature	Higher temperatures reduce strength	Cracks in frost due to embrittlement
Processing	Inhomogeneous compounds or inclusions promote cracking	Irregular vulcanisation
Type of load	Tensile, bending or impact loads have different effects	Tensile tests vs. fatigue tests

Knowing the crack formation stage is crucial for many industries:

Automotive industry

• Rubber seals, O-rings: testing resistance to UV, ozone and temperature changes.

• Tyres: analysis of crack formation under dynamic load and wear.

Construction industry

• Facade seals: testing resistance to weathering.

• Insulating materials: analysis of cracks caused by mechanical or thermal stress.

Medical technology

• Elastomer components: ensuring the longevity and functionality of medical devices.

• Resistance to sterilisation: crack testing after repeated sterilisation.

Evaluating the crack formation stage involves some challenges:

• Long-term tests: time-consuming tests that simulate long-term loads.

• Complex factors: crack formation is often an interplay of mechanical, chemical and thermal influences.

• Material variability: fluctuations in material composition can affect the results.

• How is the crack formation stage measured in practice? By standardised tests such as tensile tests, fatigue tests and UV resistance tests.

• Which materials are particularly crack-resistant? EPDM and FKM have high crack resistance, particularly to environmental influences.

• Can cracks be repaired afterwards? In some cases, e.g. surface cracks, repairs can be carried out, but often with limitations.

• How can crack formation be minimised? Through optimised material formulations, careful processing and regular quality control.

The crack formation stage is an essential indicator of the quality and durability of materials. It helps identify weak points early and optimise products in a targeted way. In the rubber and plastics industry, analysing the crack formation stage plays a central role in making materials safe and durable for demanding applications.