How to Measure Fatigue Crack Growth Propagation

Written by Travis Parkman, Product Specialist Mechanical EIT

Rubber can sustain large reversible deformations and is typically used in belts, hoses, seals and, of course, tires. These applications involve a continuous and repeating force being exerted onto the material, known as a cyclic loading condition. Small cracks can initiate and grow during normal operating conditions even when this cyclic loading is well below the material’s maximum strength. The incremental growth of these cracks, which is a function of the cyclic loading’s amplitude and frequency, will lead to premature failure. This will reduce your product’s life cycle; more importantly, product failure can lead to severe injury and loss of life [1].

Due to the importance of reducing premature failure, research focusing on the durability of rubber has significantly increased in the past few decades. For example, tire companies are comparing the performance of different fillers, rubbers, manufacturing processes and environmental ageing conditions on crack growth behaviour to maximize safety and service life [2].

HOW CAN YOU OBJECTIVELY AND EFFICIENTLY QUANTIFY THE CRACK PROPAGATION OF A MATERIAL?

Even though the perfect solution for measuring crack growth behaviour does not yet exist, there are several different approaches; we will be focussing on two of the more prominent methods shown in Figure 1, flexural and tension testing.

Figure 1.Classification of deformation modes for crack propagation tests.

Rubber flexural crack growth testing follows ISO 132 standards, which cover different flexing approaches [3]. The premise of flexural testing is to measure the change in crack length as a repeating bending deformation is delivered to a cracked specimen. Straining the sample through flexing easily replicates the deformation that tire sidewalls, belts, and footwear experience during service. It has been found that the closer the laboratory experiment reproduces the operating conditions, the more promising the correlation. The exact method of delivering this bending deformation varies with different testing equipment.

Despite this reasonable approach, there are some limitations [4].

– The strain applied through a bending deformation is difficult for the testing equipment to control, which lowers your accuracy.

– Current technology limitations prevent accurate tracking of the crack growth.

Since most products are rarely subjected to one form of deformation, some researchers have found the limitations of flexural testing to outweigh the benefits. Tension crack growth testing with pure shear samples is another popular approach, which follows the ISO 6943 standard [2]. This testing approach involves clamping a cracked specimen on both sides and cyclically deforming it in tension. Like flexural testing, the change in crack length is measured through a camera. Although this method does not recreate the real-life deformation; the applied strains can be accurately controlled and cracks can be easily measured in a non-objective manner [4].

In summary, both approaches have their limitations and benefits, making the process of quantifying rubber fatigue complicated. However, with being aware of these limitations and understanding your product, you can still effectively compare the crack growth behaviour of different materials, fillers, and manufacturing processes.

VALIDATING A NEW TIRE FILLER IN TERMS OF CRACK GROWTH BEHAVIOUR

For example, a research group in China designed an experiment to test the feasibility of using graphene (GE) as an alternative filler to carbon black and silica.  They used a Metravib DMA+1000 to quantify crack growth behaviour for each compound [5]. The group filled a common rubber blend, natural rubber-solution polymerized butadiene styrene rubber (NR-SSBR), with GE. The study compared two different GE filler concentrations, 1 wt.% (NR-SSBR-GE1) and 3 wt.% (NR-SSBR-GE1).

Figure 2. Tearing energy vs. crack growth rate for the rubber composite measured with the DMA+1000 [5].

Figure 2 shows the crack growth rate (dc/dn) plotted against tearing energy, which is the required energy for the crack to propagate across a unit area. Referencing Figure 2, the raw SSBR compound experiences a large amount of crack growth at low tearing energies. This is an unfavourable trait since the crack aggressively grows even when a small amount of energy is delivered to the sample. Meanwhile, the GE-filled rubber compounds have a relatively lower crack growth rate. Upon further inspection of the images provided from the high-resolution camera, the group was able to relate this lower crack rate to surface peeling on the crack, shown in Figure 3.

Figure 3. Side view of a crack captured with the Metravib crack growth system showing the formation of secondary cracks [5].

Surface peeling increased the area of the crack and caused the formation of secondary cracks, dissipating the tearing energy away from the crack tip; meaning that more force and energy were required to propagate the crack [5].

In conclusion, the research group was able to confidently show that the GE is a viable filler material in reducing the crack growth rate of the rubber blend, specifically with a 3 wt.% [5]. If you are interested in other ways rubber tire performance is characterized, check out Using Dynamic Mechanical Analysis (DMA) to Develop Sustainable Tires.

Works Referenced