Evaluation of Critical Tearing Energy of Rubber Materials

critical tearing energy rubber

Do you know the critical tearing energy of your rubber material?

Critical tearing energy is an important parameter to study crack growth in rubber under fatigue loading and it’s evaluation becomes imperative for the design and evaluation of rubber products. To prevent crack growth and sudden fatigue failures, one of the technique is to improve the tearing energy of rubber. Evaluation and testing of tearing energy properties is of utmost importance.

In automotive, aerospace and biomedical applications, soft elastomers and rubbers often handle cyclic loads and displacement cycles during their entire service duty cycle. When going through long periods of cyclic loading, catastrophic failure frequently happens becuase of crack formation, growth followed by propagation.

Contact us to evaluate the critical energy of your rubber material. More information at https://www.advanses.com

Join our mailing list and stay up to date on related resources and discussions. We carry out standardized as well as custom testing.

Plastics, Rubber, Composites Product Development and Material Testing

One of the most important component of successful product development and failure analysis programs is the ability to simulate actual material behavior. The definition of material card input data into FEA softwares refers to test data that analysts and engineers must enter into FEA CFD softwares. As the complexity of the simulation increases, the accuracy and fidelity of the material card data becomes top priority to the modeling of actual physics of the system.

We provide Hyperelastic, Viscoelastic Material Characterization Testing data Cards for Input into FEA, CAE softwares

AdvanSES Portfolio

1) Mechanical Testing of Polymers, Metals and Composite Materials
2) Fatigue and Durability Testing
3) Dynamic Mechanical Analysis (DMA) of Materials and Components
4) Hyperelastic, Viscoelastic Material Testing
5) Data Cards for Input into FEA, CAE softwares
6) FEA Services
7) Custom Tests, NI Labview DAQ

At AdvanSES, we provide a full 360 degree static and dynamic characterization of your materials, parts and components. We measure the tension, compression, shear, vibration and dynamic properties of individual components and sub assemblies in accordance to international standards.

Mechanical Testing of Composite Materials

A wide range of standardized and non-standardized mechanical tests on composite materials are carried out to characterize these materials. These tests include tension, compression, flexure, shear, impact and fatigue. It is also imperative that mechanical testing of composites requires use of material testing systems that are capable of performing tests in load control, displacement control, and strain control.

One of the main challenges in testing these type of anisotripic materials is also the requirement that a wide range of fixtures be developed to provide various ways of testing the materials under different conditions.

Our testing engineers are familiar with international standards and range of regulatory requirements. We reglarly characterize composites as per ISO, and ASTM specifications.

Mechanical Testing & Performance Assessment

Uniaxial Tension Test (Directional) (ASTM D638, ISO 527):

The stress (ζ) in a uniaxial tension testis calculated from;

ζ = Load / Area of the material sample ……………………………………..(1)

The strain(ε) is calculated from; ε = δl (change in length) / l (Initial length) ……………..(2)

The slope of the initial linear portion of the curve (E) is the Young’s modulus and given by; E = (ζ2- ζ1) / (ε2- ε1) ……………………………………..(3)

No alt text provided for this image

4 Point Bend Flexure Test (ASTM D6272): 

The four-point flexural test provides values for the modulus of elasticity in bending, flexural stress, flexural. This test is very similar to the three-point bending flexural test. The major difference being that with the addition of a fourth nose for load application the portion of the beam between the two loading points is put under maximum stress. In the 3 point bend test only the portion of beam under the loading nose is under stress.

No alt text provided for this image

This arrangement helps when testing high stiffness materials like ceramics infused polymers, where the number and severity of flaws under maximum stress is directly related to the flexural strength and crack initiation in the material. Compared to the three-point bending flexural test, there are no shear forces in the four-point bending flexural test in the area between the two loading pins.

Poisson’s Ratio Test as per ASTM D3039: 

Poisson’s ratio is one of the most important parameter used for structure design where all dimensional changes resulting from application of force need to be taken into account, specially for 3d printed materials. For this test method, Poisson’s ratio is obtained from strains resulting from uniaxial stress only. ASTM D3039 is primarily used to evaluate the Poison’s ratio. Testing is performed by applying a tensile force to a specimen and measuring various properties of the specimen under stress. Two strain gauges are bonded to the specimen at 0 and 90 degrees to measure the lateral and linear strains. The ratio of the lateral and linear strain provides us with the Poisson’s ratio. 

No alt text provided for this image

Flatwise Compression Test as per ASTM D695: 

The compressive properties of 3d printed materials are important when the product performs under compressive loading conditions. The testing is carried out in the direction normal to the plane of facings as the core would be placed in a structural sandwich construction. The test procedures pertain to compression call for test conditions where the deformation is applied under quasi-static conditions negating the mass and inertia effects.

No alt text provided for this image

The test procedures pertaining to compression call for test conditions where the deformation is applied under quasi-static conditions negating the mass and inertia effects.

Modified Compression Test as per Boeing BSS 7260:

Modified ASTM D695 and Boeing BSS 7260 is the testing specification that determines compressive strength and stiffness of polymer matrix composite materials using a loading compression test fixture. This test procedure introduces the compressive force into the specimen through end loading.

No alt text provided for this image

Axial Fatigue Test as per ASTM D7791 & D3479:

ASTM D7791 describes the determination of dynamic fatigueproperties of plastics in uniaxial loading conditions. Rigid or semi-rigid plastic samples are loaded intension (Procedure A) and rigid plastic samples are loaded incompression (Procedure B) to determine the effect of processing, surface condition, stress, and such,on the fatigue resistance of plastic and reinforced composite materials subjected to uniaxial stress for a large number of cycles.The results are suitable for study of high load carrying capability of candidate materials. ASTM recommends a test frequency of 5hz or lower.The tests can be carried out under load/stress or displacement/strain control. The test method allows generation of stress or strain as a function of cycles, with the fatigue limit characterized by failure of the specimen or reaching 10E+07 cycles.The maximum and minimum stress or strain levels are defined throughan R ratio.

No alt text provided for this image

3 Point Bend Flexure Test (ASTM D790):

Three point bending testing is carried out to understand the bending stress, flexural stress and strain of composite and thermoplastic 3d printed materials. The specimen is loaded in a horizontal position, and in such a way that the compressive stress occurs in the upper portion and the tensile stress occurs in the lower portion of the cross section.This is done by having round bars or curved surfaces supporting the specimen from underneath. Round bars or supports with suitable radii are provided so as to have a single point or line of contact with the specimen. The load is applied by the rounded nose on the top surface of the specimen. If the specimen is symmetrical about its cross section the maximum tensile and compressive stresses will be equal. This test fixture and geometry provides loading conditions so that specimen fails in tension or compression.

No alt text provided for this image

For most composite materials,the compressive strength islower than the tensile and thespecimen will fail at thecompression surface. Thiscompressive failure isassociated with the localbuckling (micro buckling) ofindividual fibres.

Summary:

A variety of standardized mechanical tests on unreinforced and reinforced 3d printed materials including tension, compression, flexural,and fatigue have been discussed.

Mechanical properties of 3d printed polymers, fiber-reinforced polymeric composites immensely depend on thenature of the polymer filament, fiber, and the layer by layer interfacial bonding. Advanced engineering design and analysis applications like Finite Element Analysis use this mechanical test data to characterize the materials. These material properties can be used to develop material models for use in FEA softwares like Ansys, Abaqus, LS-Dyna, MSC-Marc etc.

References:

1) Coutney, T.H., Mechanical Behaviour ofmaterials, Waveland, 1996.

2) Dowling, N.E., Mechanical Behaviour of materials, engineering methods for deformation, fracture and fatigue, Pearson,2016.

3) Ian McEnteggart, Composites Testing:Challenges & Solutions.

4) V. Shanmugam et al., The mechanicaltesting and performance analysis ofpolymer-fibre composites preparedthrough the additive manufacturing. PT, 21.

Heat Deflection Temperature (HDT) and Vicat Softening Temperature Testing

HDT stands for Heat Deflection Temperature. The heat deflection temperature of a reinforced or unreinforced polymer material is a measure of polymer’s resistance to distortion under an applied load at elevated temperatures.

Applications of HDT tests include:

1) Identify suitable material for injection molding application.
2) Identify suitable material for elevated temperature application.
3) Identification and grading of materials as per their properties.

The test specification for the HDT is ASTM D 648 and ISO 75.

Factors Influencing Thermal Performance of Polymer Materials

HDT tests typically test for the short term performance of the materials under loads at elevated temperatures. The following factors play a significant part in the performance prediction of the materials under the test conditions.

1) The total time material is exposed to elevated temperatures.
2) The rate of temperature increase.
3) The specimen dimensions and part geometry.

Vicat softening temperature tests are used to identify the temperature at which a needle of specified dimensions penetrates into a plastic material specimen for a specified distance under applied loading conditions. The test specification for the Vicat softening temperature testing is ASTM D1525 and ISO 306.

Compared with the Heat Deflection Temperature (HDT), Vicat softening temperature test measures the temperature at which the specimen loses its stiffness and softens. HDT test measures the temperature at which the specimen loses its load bearing capability. The Vicat softening point is closer to the actual melting or softening point of the polymer. Vicat softening temperature is typically always higher than the HDT for a polymeric material as the penetration load is always higher than a bending load on a material specimen.

Biaxial Tension Extension Testing of Rubber, Elastomers and Polymeric Thin Films

AdvanSES offers biaxial tension testing of rubber, elastomers and polymeric thin films. Biaxial testing is important for hyperelastic and viscoelastic characterization of elastomers and polymers for Finite Element Analysis FEA Applications. 

We carry out tests under the following biaxial tension deformation modes;

1) Single Stretch.

2) Multiple Cyclic Loading.

3) Single Stretch followed by Stress Relaxation Step.

Hyperelastic and Viscoelastic material characterization testing is carried out under the following deformation modes;

  1. Uniaxial Tension Testing
  2. Planar Shear Testing
  3. Volumetric Compression Testing
  4. Uniaxial Compression Testing
  5. Biaxial Tension Testing

AdvanSES offers a choice of different capacities of load cells for biaxial testing high and low hardness materials, non-contact measurements and capacity to mold material test samples. 

Biaxial Tension Extension Testing of Rubber, Elastomers and Polymeric Thin Films

Biaxial Tension Testing of Rubber is a key property for input into FEA softwares like Abaqus, Ansys, MSC Marc, LS-Dyna. Biaxial tensile testing is a non-traditional but highly accurate testing technique for mechanical characterization of soft and hard materials. Typical materials tested in biaxial tension are silicone and natural rubber elastomers, composites, polymeric thin films, and biological soft tissues.

Polymer Testing & Finite Element Analysis FEA

Simulation of plastics and polymers

Application of engineering polymers and plastics are widespread because of the wide range of material properties, and lower manufacturing cost offered by them. Plastics and polymers are also mixed with glass and carbon fibers to increase the stiffness and provide higher load handling capabilities. Polymer Testing & Finite Element Analysis FEA go hand in hand for such material and product development.

Thermosetting plastics are used in the manufacture of hard, high temperature resistanting parts and a matrix for composites is used for strength and stiffness. Elastomers such as natural rubber are used in automotive applications for vibration isolation and load bearing properties.

Material Properties

The first step in non-linear analysis of polymers and plastics is material characterization of these highly time and frequency dependent strain rate sensitive materials.

Finite Element Analysis (FEA)

Finite Element Analysis FEA of polymer testing is critical to understanding the perfomance of polymer materials and their judicious use in industrial products. Design optimization iterations are then carried out to finalize the design for optimum stress-strain distribution and high fatigue life.

Polymers exhibit a combination of elastic and viscous response to deformation forces. At low temperatures the stiff elastic behaviour is dominant. At high temperatures a viscous or fluid like behaviour is exhibited. At room temperatures a combination of viscous and elastic behaviour (Viscoelastic) is exhibited. The challenge is to test, model and simulate this unique behaviour of this class of material so that the complete material property can be mapped on the product.

Rubber Material Testing and Pre-Conditioning Techniques for Characterization

A proper treatment of the rubber material testing service conditions and material degradation phenomena like strain softening is of prime importance in the testing of rubbers specimens for FEA material characterization. The accuracy and reliability of obtained test data depends on how the mechanical conditioning and representational service conditions of the material have been accounted for in the test data. To simulate a component in unused and unaged conditions, the mechanical conditioning requirements are different than the ones for simulating a component that has gone through extensive field service and aging under different environmental conditions.

To simulate performance of a material or component by Finite Element Analysis (FEA) it should be tested under the same deformation modes to which original assembly will be subjected. The uniaxial tension tests are easy to perform and are fairly well understood but if the component assembly experiences complex multiaxial stress states then it becomes imperative to test in other deformation modes. Planar (pure shear), biaxial and volumetric (hydrostatic) tests need to be performed along with uniaxial tension test to incorporate the effects of multiaxial stress states in the FEA model.

Material stiffness degradation phenomena like Mullin’s effect at high strains and Payne’s effect at low strains significantly affect the stiffness properties of rubbers. After the first cycle of applied strain and recovery the material softens, upon subsequent stretching the stiffness is lower for the same applied strain.

Despite all the history in testing hyperelastic and viscoelastic materials, there is a lack of a methodical and standard testing protocol for pre-conditioning. Comprehensive studies on the influence of pre-conditioning are not available. Readers are referred to Austrell[] and Remache, et al,[]. There are no guidelines for sample pre-conditioning in ASTM D412. However, British Standard BS 903 suggests to perform 5 cycles of pre-conditioning to improve test reproducibility.

There are five (5) different techniques to carry out pre-conditioning and material testing of general elastomer samples.

1. The first technique pertains to a mechanical test where the the testing of the sample is carried out under one single stretch at a speed recommended by the ASTM D412 specification.

2. In the second testing technique the sample is stretched at a constant speed up to the maximum relevant strain brought back to the initial position. Cyclic stretching is then carried out for anywhere for 1 to 7 cycles. The speed of the initial stretch and the number of subsequent cyclic stretches needs to be same. The data curve is then shifted to the origin by zeroing out the stress and strain and used for curve-fitting procedure.

3. In the third testing protocol the sample is stretched to a fraction of the full material stretch capability and brought back to the initial position and cycled back again to this fractional position anywhere from 2 to 7 cycles. The stretch in the sample is then increased to the second level and cycled back to initial position multiple times. The sample is stretched back to an even higher position and cycled back. This progressively continues until the maximum strain capability of the material is reached. This protocol is known as progressive pre-conditioning.

4. In the fourth testing technique the sample is stretched to a fraction of the total stretch capability and relaxed for anywhere from 30 seconds to 120 seconds as per the material. The sample is then again stretched to a higher limit and relaxed again. This is continued until the maximum stretchable capability of the material. This particular technique stretches the material and allows the material to fully creep and relax at each interval so that all the stress softening is accounted for in the test data.

5. In the fifth testing technique the sample is tested under a single stretch but the speed or the rate of stretch is very slow. This ultra-slow speed test is carried out so that the material can creep, relax, and the cross-links in the elastomer are given enough time to expand-contract and come to a balanced position during the stretching. This technique is a combination of the two above testing techniques.

These five (5) testing protocols involve the stretching of the material to different limits under different conditions and suitably cycling the material. The suitability of one testing technique over the other is debatable and one should adopt the technique that most closely resembles the operating conditions of the material and what one expects to back out from the Finite Element Analysis. Figures (1.11) through (1.14) shows the results from using the different testing protocols on a 55 durometer Natural Rubber Compound that finds general application in an automotive engine mount.

Figure 1.11: Uniaxial Tension Test Results

Figure 1.12: Single Stretch Followed by 3 Cycles of Stretching to the Maximum

Figure (1.12) shows the results from comparisons carried out on a uniaxial compression test for an engine mount material characterization. Two protocols were employed to carry out the material characterization. Progressive straining and cycling was carried out first. The material was strained 3 times before reaching the ultimate strain of approximately 75 %. The material was subsequently tested using ultra slow straining protocol. As can be seen the test data output for FEA input is the same using both the techniques. This result confirms the observation by Austrell in his work on conditioning of material samples for characterization i.e., Mullins effect can be negated when enough time is allowed for the material to relax, creep, and flow during the rubber material testing.

Figure 1.13: Progressive Pre-conditioning with Stretching to 3 levels and Cyclic Stretches

Figure 1.14: Progressive Relaxation, Stretching and Relaxation to Maximum Levels

1.2 Guidelines for Rubber Material Testing

Rubber compounds are formulated from recipes of ingredient materials. Depending on the time, location and environment while mixing the compounds, properties are known to vary from batch to batch.

• All testing to characterize a material compound should be performed on the same batch.

• Laboratory validations will help to correlate test specimen slabs against the real components to make sure they have identical cure history.

• Small compression buttons can be extracted from components and compared with slab data.

• The testing should be carried out at the temperature at which the component is expected to perform under field service conditions.

• For seals and o-rings, aging in oils and solutions can be carried out prior to testing.

• Input of more than one test data type in FEA software will increase simulation accuracy by an order of magnitude

Izod Charpy Impact ASTM D256, ISO 180

The Izod Charpy impact test is an ASTM and ISO standard method of determining the impact resistance of thermoplatic and composite materials and is an important test for characterization of materials. The test is very much similar to the Charpy impact test but uses different type of specimens for testing. The Izod impact test differs from the Charpy impact test in that the test sample is held in a cantilevered beam configuration as opposed to a three-point bend configuration. Both notched and un-notched  Izod Charpy impact test  provide important material properties.

The testing conditions are governed by many parameters, as below:

  • the dimensions of the usually rectangular cross section of the sample below the notch;
  • the height of the hammer at the start position, determining its velocity at impact;
  • the mass of the hammer which together with the velocity determines its kinetic energy at impact;
  • the sharpness, or tip curvature, of the notch;
  • the temperature of the sample.

Results of impact tests are expressed in terms of:

  • Amount of energy absorbed (N-m) or 
  • Amount of energy absorbed per unit cross sectional area (N-m/cm2)

Applications include:

  • Measure of the energy required to crack the material.
  • Test materials and grade them as per their impact property and use the grading for different applications.
  • Develop new materials suitable for use in automotive and aerospace impact applications.

Fatigue Testing of Materials and Products

AdvanSES offers Fatigue Testing of Materials and Products using a variety of stress and strain controlled testing machines for characterizing field service behaviors of engineering products, medical devices, and automotive aerospace components.

Fatigue testing involves the application of cyclic loading to a test specimen or a product. Unlike monotonic tests in which loading increases until failure, the applied load is cycled between prescribed maximum and minimum levels until a fatigue failure occurs, or until the predetermined number of loading cycles have been applied.

The loading is applied to assess long term fatigue behavior, vibration isolation properties, and observe failure causing mechanisms.

Figure 1: Fatigue and Static Testing Systems at AdvanSES

AdvanSES offers Fatigue Testing of Materials and Products for;

1) Material Samples
2) Full Scale Component Level Testing
3) Measure Material and Product Degradation
4) Elevated Temperature Testing
5) Incorporation of Aging Mechanisms in Testing

Benefits of Fatigue Testing at AdvanSES

With material and product testing at AdvanSES, you can be sure of:

  1. ISO 17025:2017 Accredited Facility
  2. Knowledgable Testing and Engineering Personnel
  3. Commitment to Understanding your Materials and Products
  4. Identify root cause if Required
  5. Short Timelines
  6. Detailed Testing Reports that Help You Make Accurate Decisions

AdvanSES Fatigue Testing Procedure

The AdvanSES approach to fatigue testing of materials and products is as follows;

  • The machine, environment chamber, extensometer, loadcell etc., are all calibrated and verified.
  • The test samples or products to be tested are prepared.
  • The test parameters are set, environment chamber is placed and the test environment is brought upto the required parameters.
  • The machine is started and runs the set cyclic load, stress, strain ranges according to the parameters predetermined for the test samples.
  • The test is run for the set number of cycles and periodically checked, photographed for signs of degradation, fatigue or until failure.
  • Results and further engineering steps are discussed.
  • All relevant test data with information is reported.

Materials Testing Laboratory

Materials Testing Laboratory Services

Materials Testing Laboratory at AdvanSES uses highly precise, repeatable and reliable techniques that quantify and measure quality control and performance characteristics of materials, such as mechanical properties, design properties and durability properties

Our material testing laboratory is ISO 17025:2017 accredited and equipped to age, test and analyze materials for a wide-range of industries and applications. We can perform testing as per ASTM, ISO standards and also as per your specified criteria. Our test data and results help design engineers make better designs. Our test data help scientists and engineers determine whether materials, products and aging criteria meet the requirements of design engineers, quality control department and whether they products and materials are suitable for their intended applications.

We can mold, machine and creat test specimens and samples for efficient turnaround. We can also mold and manufacture these test specimens for your own in-house material testing.

ALL ABOUT QUALITY

AdvanSES material testing laboratory is ISO 17025:2017 accredited. Any kind of samples can be tested at our laboratory using the latest techniques. We can mould specimens, extract samples from failed or in field service products, age and make any kind of samples for your testing convenience.

AdvanSES provides answers to your material questions that meet your quality requirements:

  • Accredited by NABL ISO/IEC 17025.
  • Testing performed to ASTM, ASME, ISO standards and customer specifications.
  • Quality system complies with ISO 9001.

Material testing services at AdvanSES provides you with valuable insights and answers about:

  • Characterisitics of materials and products
  • Mechanical properties
  • Finite element analysis
  • Durability of your mateirals and products.
  • Vulnerability to fatigue failures