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

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AdvanSES Mechanical Testing, Analysis & FEA Engineering Services

1) An Independent, design analysis and mechanical testing laboratory.
2) More than 2 decades of product testing and application expertise in mechanical, and materials engineering.
3) State of the art materials and mechanical testing laboratory with qualified engineers.
4) Innovative design, analysis and testing solutions for a wide range of industries

Our Services

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 Characterization Testing
5) Data Cards for Input into FEA, CAE softwares
6) FEA Services
7) Custom Test Setups with NI Labview DAQ

Material Testing, Product Engineering and Failure Analysis Services

Quality Control and Data Integrity in Mechanical Testing Of Engineering Materials and Products

Quality Control

Quality control refers to the process of systematically detecting errors in the laboratory testing results to ensure both that the accuracy and reliability of test results are maintained and best possible testing results are supplied to customers. Unreliable and inaccurate testing results can result in faulty failures, degraded field performance of engineering materials and products. it is therefore of great importance to ensure all results provided are accurate, reliable and consistent.

Alfort and Beaty define quality control as;

“Quality control is the mechanism by which products are made to measure up to the specifications determined from the customer’s demands and transform into sales, engineering and manufacturing requirements. It is concerned with making things right rather than discovering and rejecting those made wrong. Quality control is a technique by means of which products of uniform acceptable quality are manufactured.”

A mechanical and materials testing laboratory tests all kinds of materials at all stages of product engineering, from the raw material stage to performance characterization and durability testing of finished ready to market products.

The range and types of instruments to test these materials and product range from simplest to complex. Instruments such as density meter and hardness meters are the simple instruments, while SEMs, fatigue test benches, high strain rate equipments etc., are complex instruments that also have a significant learning curve. Only qualified engineers and analysts would be conducting the tests with the help of calibrated instruments to make sure that the data obtained is reliable and accurate.

Achieving quality in a mechanical and materials testing laboratory requires the use of many tools, instruments and machinery. These include UTMs, hardness meters, fatigue testing rigs, and also various custom made test benches. An established maintenance schedule, calibration, quality assurance program, training and quality control are pre-requisites. Calculations and maintenances of QC Statistics for systematic analysis of historical standard deviations, covariances, uncertainty calculations etc., is also required.

Data Integrity

Data integrity refers to completeness, consistency and accurateness of the raw data generated in the testing laboratory during the course of its work. It means that the raw data has to be reliable, consistent and accurate and that no modifications, changes or deletions cannot be caried out by any person or machine.

Raw data in the quality control laboratory can be generated by testing machnes, DAQ systems, and computer systems as well as by laboratory staff as paper records and reports. Ensuring integrity of data starts from the proper design of the procedural documents, level of access provided to authorized persons, physical reliablility of the infrastructure and training of laboratory personnel. An appropriately designed procedure is uniquely named and numbered has sufficient leeway for records to be stored comfortably digitally and physically and distribution are strictly controlled at all levels.

Having established all the QC standard protocols at AdvanSES, we take pride in our work and our protocols are available for audit at any time.

Finite Element Analysis (FEA) Consulting Services

AdvanSES has a history of completing FEA projects for customers from automotive, aerospace, biomedical and consumer durables background. All our projects are delivered using state of the art commercial FEA softwares.

We offer a complete range of Finite Element Analysis FEA consulting services for solving structural, thermal, fatigue, and fluid flow pressure problems. We work with our customers to analyze product behavior, predict service life, and understand failures. Our FEA engineers help our customers make early design choices. Our proactive approach helps our customers expedite products into the market.

Some our strengths in Finite Element Analysis (FEA) are detailed below;

  1. Non-linear Materials: Our regular work includes characterization and implmentation of complex material models for hyperelasticity, elasto-plasticity and viscoelasticity. We can work on any kind of materials to implement them successfully in FEA models.

2) Verifications and Validations: Any kind of simulation without a strong verification and validation basis will mostly fail on expert scrutiny. We have extensive experience in verifications and validations procedures in our laboratory. We can replicate field service conditions, setup custom test rigs and characterize products under static and dynamic loads.

3) Contact-Impact: We offer implementation of full physics in contact, drops and high speed impact analysis.

Contact us for a quick quote.

UNIAXIAL TENSION TEST: THE MOTHER OF ALL MECHANICAL TESTS

UNIAXIAL TENSION TEST: THE MOTHER OF ALL MECHANICAL TESTS

INTRODUCTION:

In engineering design and analysis, stress-strain relationships are needed to establish and verify the load-deflection properties of an engineering component under service loads and boundary conditions. From the tensile testing carried out to evaluate materials, various mechanical properties such as the yield strength, Young’s modulus, Poisson’s ratio etc. are obtained. Strain hardening and true stress-strain etc. values can be calculated by means of conversion using equations from the stress-strain curve. The uniaxial tensile test is the primary method to evaluate the material and obtain the parameters. Uniaxial tension test is also the primary test method used for quality control and certification of virtually all ferrous and non-ferrous type of materials.

Standards for tensile testing were amongst the first published and the development of such standards continues today through the ASTM and ISO organizations. Reliable tensile data, which is now generated largely by computer controlled testing machines, is also crucial in the design of safety critical components automotive, aerospace and biomedical applications.

Tensile testing is also important for polymeric materials as they depend strongly on the strain rate because of their viscoelastic nature. Polymers exhibit time dependent deformation like relaxation and creep under service applications. Polymer properties also show a higher temperature dependency than metals. Multiple temperatures and strain rates are generally used to fully characterize polymer materials.


Figure 1: Uniaxial Tension Test on a Material Sample

Figure 2 shows sample uniaxial stress-strain results from testing a metal specimen. The X axis depicts the strain and Y axis the stress. The stress (σ) is calculated from;

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

The strain(ε) is calculated from;

ε = δl (change in length) / l1 (Initial length) ……………………………………..(2)

The slope of the initial linear portion of the curve (E) is the Young’s modulus and given by;

E = (σ2- σ1) / (ε2- ε1) ……………………………………..(3)

Point A in the graph shows the proportional limit of the material beyond which the material starts to yield. When this point is not clearly visible or decipherable in a test, the off yield strength at B is taken by offsetting the strain (F-G) by 0.2 % of the gauge length. Similarly, extension by yield under load (EUL) is calculated by offsetting the strain 0.5% of the gauge length. The region between points A and B on the graph is also purely elastic, with full recovery on the unloading of the metal, but it is not essentially linear.

Figure 2: Sample Uniaxial Stress-Strain Results from a Metal Specimen

TRUE STRESS-STRAIN CURVE:
Figure 2 shows the engineering stress-strain curve where the values of stress beyond the proportional limit do not give the true picture of stress in the sample as the cross-sectional area of the sample is assumed to be constant. The engineering stress-strain values can be converted to true stress-strain values by the following relation;

σt = σe (1 + εe) = σeλ , ……………………………………..(4)

εt = ln (1 +εe) = ln λ, where λ = initial length / final length …………………………………..(5)


Figure 3: Sample Uniaxial Stress-Strain Results for a Polymeric Rubber Material

Figure 3 shows typical uniaxial stress-strain results from a test on a 40 durometer rubber material. Unlike the results for the metal specimen the elastomer test results do not have or exhibit a yield limit. The material extends in the classical ‘S’ shape and results in a fracture at the end of the tests. Polymeric rubber materials exhibit the following characteristics;
• The load-deflection behavior of an elastomer is markedly non-linear.
• The recoverable strains can be as high 700 %.
• The stress-strain characteristics are highly dependent on temperature and rate effects are highly pronounced.
• Nearly incompressible behavior.
• Viscoelastic effects are significant.
Typical test results for rubber materials show the values of modulus at 100%, 200% and 300%. Modulus represents stress in such results.

SPECIFIC MECHANICAL ISSUES IN TESTING:

1. Strain Rate
2. Extensometry
3. Alignment and Gripping
4. Testing Machine Frame Compliance
5. Young’s Modulus Measurement
6. Specimen Geometry

Strain rate range of different material characterization test methods

1) Quasi-static tension tests 10-5 to 10-1 S-1
2) Dynamic tension tests 10-1 to 102 S-1
3) Very High Strain Rate or Impact tests 102 to 104 S-1

A fundamental difference between a high strain rate tension test and a quasi-static tension test is that inertia and wave propagation effects are present at high rates. An analysis of results from a high strain rate test thus requires consideration of the effect of stress wave propagation along the length of the test specimen in order to determine how fast a uniaxial test can be run to obtain valid stress-strain data.

IMPORTANCE OF THE UNIAXIAL TENSION TEST:
At the basic level apart from giving us an understanding about the ultimate strain and stress capabilities of the material, tensile tests provide us with information about the factor of safety that needs to be built-in the products using these materials.
1) Fatigue life of engineering materials can be calculated from tensile tests carried out on notched and unnotched specimens.
2) Aging and other environmental effects can be incorporated in the test procedure to characterize the material, as well as predict service life using techniques like Arrhenius equation.
3) Endurance limits in design calculations are calculated from the results obtained from uniaxial tension tests.
4) In manufacturing of rubber materials and products, it is used to determine batch quality and maintain consistency in material and product manufacturing.
5) Electromechanical servo based miniature tensile testing machines can be developed to study material samples of smaller size.

REFERENCES:
1. Dowling, N. E., Mechanical Behavior of Materials, Engineering Methods for Deformation, Fracture and Fatigue Prentice-Hall, NJ, 99
2. Roylance, D., Mechanical Properties of Materials, MIT, 2008.
3. Gedney, R., Tensile Testing Basics, Tips and Trends, Quality Magazine, 2005.
4. Loveday, M. S., Gray, T., Aegerter, J., Testing of Metallic Materials: A Review, NPL, 2004.
5. Srinivas, K., and Pannikottu, A., Material Characterization and FEA of a Novel Compression Stress Relaxation Method to Evaluate Materials for Sealing Applications at the 28th Annual Dayton-Cincinnati Aerospace Science Symposium, March 2003.
6. Ong, J.H., An Improved Technique for the Prediction of Axial Fatigue Life from Tensile Data, International Journal of Fatigue, 15, No. 3, 1993.
7. Manson, S.S. Fatigue: A Complex Subject–Some Simple Approximations, Experimental Mechanics, SESA Annual Meeting, 1965.
8. Yang, S.M., et al. Failure Life Prediction by Simple Tension Test under Dynamic Load, International Conference on Fracture, 1995.