material testing

Composite Material Testing for Drones and UAVs: A Deep Dive

Kartik Srinivas fea abaqus ansys, material testing, Materials Engineering and FEA
Material Characterization Testing of Drones and UAV Materials at AdvanSES

Composite Material Testing for Drones and UAV Applications

Unmanned Aerial Vehicles (UAVs), commonly known as drones, have revolutionized numerous industries, from agriculture and real estate to cinematography and defense. One of the key factors contributing to the versatility and performance of these drones is the use of composite materials in their construction [1,2]. Composite material testing for drones and UAV applications is both difficult and challenging. The use of fiber-reinforced plastic composite materials challenges drone UAV engineers to design and manufacture products with high strength, stiffness and low cost. The demand for more maneuverable, payload effective UAVs is increasing, where composite materials are playing an essential role in the progress of these new high-performance UAV aircrafts with special composite material characteristics like light weight and high strength. These composite materials are distinguished by Young’s modulus as compared to different kinds of metals and aluminum alloys. Multi-rotor type UAVs represent an extremely complex system in terms of design and control. Octacopter, hexacopter and Quadcopter are typical of such multi-rotor designs. Such a type of aircraft is an inherently unstable system, which results from the fact that it cannot independently return to the point of balance (hover) if it loses the functionality of the control loops but will fall or begin to move uncontrollably in space. Furthermore, multirotor UAVs are nonlinear systems since rotor aerodynamic forces and moment characteristics are nonlinear functions with respect to angular velocities and these reasons make the materials used in the manufacturing to be of high quality, load capacity with an infinite fatigue life for the application designed for.

Why Composite Materials?

Composite Material Layered Construction

Composite materials, such as polymers reinforced with carbon fibers (CFRP) and fiberglass (GFRP), are widely used in the manufacturing of drone components, including the fuselage, wings, and landing gear[1].

Polymer composite materials are widely used in various industries, including the manufacturing of drone UAV components, due to their numerous advantages:

High Strength-to-Weight Ratio: Polymer composites, such as those reinforced with carbon or glass fibers, offer a high strength-to-weight ratio[4]. This property is crucial in applications like drone manufacturing, where reducing weight while maintaining strength can enhance performance[4].

Durability: Composites are known for their durability. They do not rust, have high dimensional stability, and can maintain their shape in various conditions. This makes them suitable for outdoor structures and components that are designed to last for a long time.

Design Flexibility: Composites open up new design options that might be hard to achieve with traditional materials. They allow for part consolidation, and their surface texture can be altered to mimic any finish.

Improved Production: With advancements in manufacturing processes, composites are now easier to produce. Digital Composite Manufacturing (DCM), for instance, has made it possible to fabricate composite parts without manual labor.

Material Stability and Insulation: Polymers used in composites offer high material stability against corrosion, good electrical and thermal insulation, and are easy to shape, making them ideal for economic mass production[4].

These advantages make polymer composites an excellent choice for various applications, including the construction of drone components. However, it’s important to note that the use of these materials also necessitates comprehensive testing to ensure safety, reliability, and durability.

Moreover, compared to traditional materials like aluminum, composites can reduce weight by 15-45%, increase corrosion, fatigue, and impact resistance, and reduce noise and vibrations[1].

Testing Composite Materials

Testing composite materials is a critical aspect of ensuring their performance and reliability in various applications, including drone components. Here are some of the common methods used for testing composite materials:

Mechanical Testing: This includes tensile (tension), flexural, impact, shear, and compression testing[1,2]. These tests help determine the material’s strength and deformation under different types of loads.

Physical Testing: This involves tests like water absorption, density, hardness. These tests provide insights into the material’s physical properties and how they might change in different environments.

Thermal Testing: Dynamic Mechanical Analysis (DMA), and Thermomechanical Analysis (TMA) are used to study the material’s thermal properties2.

Moisture Testing: This includes tests like water absorption and moisture conditioning. These tests are crucial for applications where the material might be exposed to moisture.

Analytical Testing: This includes tests like density of core materials, ignition loss, void content, content analysis, and Fourier Transform Infrared Spectroscopy (FTIR). These tests provide a deeper understanding of the material’s composition and structure.

These tests help manufacturers understand the properties of the composite materials that go into a finished product. Composite material testing for drones and UAV applications is both difficult and challenging. The data derived from these tests can be used to compare the composite materials against conventional materials. It’s important to note that the specific tests used can vary depending on the type of composite material and its intended application

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)

Uniaxial Tension Testing of Composite Materials

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.

4 Point Bend Flexure Test (ASTM D6272)
4 Point Bend Flexure Test

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. 

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.

Uniaxial Flatwise Compression Testing

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.

Modified Compression Test as per Boeing BSS 7260

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.

Axial Fatigue Test as per ASTM D7791

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.

3 Point Bend Flexure Test

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

Drop Weight low Velocity Impact Test (ASTM D7136, ISO 6603):

The importance of understanding the response of structural composites to impact events cannot be emphasized enough. Low velocity impact occurs at velocities below 10 m/s and is likely to cause some dents and visible damage on the surface due to matrix cracking and fibre breaking, as well as delamination of the material. In some materials, impact tests characterize the face sheet quality and if they are suitable for the application.

Drop Weight low Velocity Impact Test

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.

Conclusion

The use of composite materials in drone manufacturing presents a promising avenue for enhancing UAV performance. However, it also necessitates comprehensive testing to ensure the safety, reliability, and durability of these drones. As the drone industry continues to grow and evolve, so too will the methods for testing and optimizing the use of composite materials in drone construction.

Keywords: UAV, composite materials, drone components, material testing, CFRP, GFRP, finite element analysis, bending test.

References:

  1. M Sönmez, Ce Pelin, M Georgescu, G Pelin, Md Stelescu, M Nituica, G Stoian, Unmanned Aerial Vehicles – Classification, Types Of Composite Materials Used In Their Structure And Applications.
  2. Camil, Lancea et al., Simulation, Fabrication and Testing of UAV Composite Landing Gear. MDPI Journal, https://doi.org/10.3390/app12178598
  3. National Research Council, Airframe Materials and Structures, Enabling Science for Military Systems
  4. Anup Ghosh and Mayank Dwivedi, Advantages and Applications of Polymeric Composites, Processability of Polymeric Composites, Springer, 2019.
  5. Srinivas, Kartik., Mechanical Characterization Testing Of Thermoplastics And Composite Materials, AdvanSES Report, https://www.researchgate.net/publication/339872757_Mechanical_Characterization_Testing_of_Thermoplastics_and_Composite_Materials

Non-linear Hyperelastic Material Characterization Testing for FEA

Kartik Srinivas fea abaqus ansys, material testing, Materials Engineering and FEA
Non-linear Hyperelastic Material Characterization Testing for FEA

The characterization of materials for Finite Element Analysis (FEA) and Computational Fluid Dynamics (CFD) is a specialized process that involves extensive laboratory testing. At AdvanSES, we have become industry leaders in this field, particularly with our focus on the characterization of polymer materials. Through a series of specific tests, we are able to determine the unique properties of each material, thus providing valuable data for FEA and CFD.

Pure Shear

Our testing process begins with a pure shear test. This involves applying uniaxial tension to a test specimen using either a parallel or tangential method. The response of the material to this stress provides a baseline understanding of its characteristics under tension.

Volumetric Compression

We then proceed to a volumetric compression test. This study involves placing a sample of the material under hydrostatic compression deformation. The way the material responds to this form of stress provides valuable data on its behavior under compression.

Uniaxial Compression

Uniaxial compression testing is another key component of our testing process. Here, we evaluate the response of the material when compression stress is applied along a single axis. This test gives us a clear picture of how the material behaves under a single axis of compression stress.

Uniaxial Tension

Uniaxial tension testing involves applying tensile stress to a specimen. The result of this test provides us with further insights into the behavior of the material under tension.

Biaxial Tension

A biaxial tension test involves placing tensile stress on a specimen in two simultaneous directions. This test is particularly useful in understanding the behavior of a material under multiple tensions.

Creep and Stress Relaxation

The final testing stage is the creep and stress relaxation test. This involves a uniaxial tensile test followed by the maintenance of the elongation on the specimen for a specified duration. By observing the material’s response over this period, we can gain valuable insights into the long-term behavior of the material under stress.

Our laboratory is located at Plot No. 49, Mother Industrial Park, Zak-Kadadara Road, Kadadara, Taluka: Dehgam, District: Gandhinagar, Gujarat 382305, India.

For more information about our services and how we can assist with your material characterization needs, give us a call at +91-9624447567 or send us an email at [email protected].

Mechanical Testing of Automotive, Railways, Aerospace components

Kartik Srinivas fea abaqus ansys, material testing, Materials Engineering and FEA , , ,

Installation of mechanical testing structural load frame for carrying out testing high capacity load bearing components. Frames are now available for comprehensive engineering validation of automotive, railways, aerospace components and structures made from polymers, metallic and composite materials.

We can now test and break products and materials from 5N to 500KN.

Discover more at http://www.advanses.com

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.

More information at https://www.advanses.com

Installation of mechanical testing structural load frame for carrying out testing high capacity load bearing components

Heat Deflection Testing of Plastics: Ensuring Material Quality, Reliability and Performance

Kartik Srinivas failure analysis, life prediction, material testing, Materials Engineering and FEA, service life prediction

Introduction:

Plastics are ubiquitous in our daily lives, used in a wide range of applications due to their versatility, durability, and cost-effectiveness. However, when exposed to elevated temperatures, some plastics may deform or lose their structural integrity, compromising their functionality and safety. This is where heat deflection testing plays a crucial role in assessing a plastic material’s ability to withstand heat and maintain its shape under load. In this blog post, we will delve into the importance of heat deflection testing for plastics and explore its significance in ensuring material reliability and performance.

Are you curious about the performance of plastics under high temperatures? Discover the significance of heat deflection testing for plastics, ensuring their reliability and performance in various applications.

Understanding Heat Deflection Testing:

Heat deflection testing, also known as heat distortion testing or HDT, is a standardized method used to evaluate a plastic material’s ability to resist deformation under load at elevated temperatures. It determines the heat deflection temperature (HDT) or the temperature at which a specific deformation or deflection occurs in the plastic specimen. This testing method helps manufacturers and engineers select the most suitable plastic materials for applications involving high temperatures.

AdvanSES VST HDT Apparatus

Importance of Heat Deflection Testing:

2.1. Ensuring Material Reliability:

Heat deflection testing provides vital insights into a plastic material’s ability to maintain its structural integrity when exposed to elevated temperatures. By subjecting plastics to controlled heating and measuring their deformation under load, manufacturers can identify materials that can withstand the intended operating conditions without significant deformation or failure. This ensures that the final products will perform reliably and maintain their shape, avoiding costly failures or safety hazards.

2.2. Performance Optimization:

Different plastics exhibit varying thermal properties, including their response to temperature changes. Heat deflection testing helps engineers optimize material selection for specific applications where exposure to heat is a concern. By comparing HDT values of different plastics, manufacturers can choose materials with higher HDT values that can withstand higher temperatures, resulting in improved product performance and longevity.

 Conducting Heat Deflection Testing:

3.1. Standards and Test Methods:

Heat deflection testing follows established standards such as ASTM D648 (Standard Test Method for Deflection Temperature of Plastics Under Flexural Load in the Edgewise Position) and ISO 75 (Plastics—Determination of Temperature of Deflection Under Load). These standards provide specific guidelines for test specimen preparation, loading conditions, and temperature ramp rates, ensuring consistent and comparable results across different laboratories.

3.2. Test Equipment:

Heat deflection testing requires specialized equipment, typically including a testing machine capable of applying a load on the plastic specimen and a heating chamber or furnace to control the temperature. The test machine measures the deflection of the specimen while it is subjected to a specified load at increasing temperatures until a predefined deflection value is reached.

Applications of Heat Deflection Testing:

Heat deflection testing is essential in numerous industries where plastics are used in high-temperature environments. Some key applications include automotive components, electrical enclosures, consumer electronics, aerospace parts, and industrial equipment. By subjecting plastic materials to rigorous heat deflection testing, manufacturers can ensure the long-term performance and reliability of their products.

Conclusion:

Heat deflection testing is a vital aspect of evaluating plastics’ performance and reliability when exposed to high temperatures. By conducting this testing, manufacturers and engineers can select appropriate materials for specific applications, optimize product performance, and minimize the risk of deformation or failure. Ultimately, heat deflection testing contributes to the overall quality, safety, and longevity of plastic-based products in various industries.

Looking to optimize plastic materials’ performance in high-temperature environments? Contact our experts today to learn how heat deflection testing can enhance the reliability and longevity of your products.

Plastic Material Testing: Ensuring Quality and Safety

Kartik Srinivas failure analysis, fea abaqus ansys, life prediction, material testing, Materials Engineering and FEA
AdvanSES Testing Laboratory

Plastic Material Testing: Ensuring Quality and Safety

Plastic materials have become an integral part of our lives, from the packaging of our daily essentials to the construction of our homes and buildings. However, the use of plastics has also raised concerns about their impact on the environment and human health. Therefore, it is essential to test plastic materials to ensure their quality and safety. At AdvanSES plastic material testing is carried out under the strict and rigorous quality control as per ISO 17025:2017 testing conditions.

Plastic material testing involves analyzing the physical, chemical, and mechanical properties of plastic materials. These tests provide valuable information about the durability, strength, and chemical resistance of plastics, which are critical factors in determining their suitability for specific applications.

Types of Plastic Material Testing

Fatigue Testing at AdvanSES

There are various types of plastic material testing, each serving a specific purpose. The most common types of tests include:

  1. Tensile Testing: This test measures the strength of plastic materials under tension, providing valuable information about their mechanical properties.
  • Impact Testing: This test evaluates the ability of plastic materials to withstand sudden impact, which is critical in applications such as packaging and transportation.
  • Fatigue Testing: This test evaluates the ability of plastic materials to withstand long term service loads, the mechanical service life of the materials and parts can be predicted from fatigue testing.
  • Thermal Analysis: This test measures the thermal properties of plastic materials, such as their melting and crystallization behavior.
  • Chemical Resistance Testing: This test evaluates the resistance of plastic materials to various chemicals, providing important information about their suitability for use in specific environments.
  • Flammability Testing: This test evaluates the ability of plastic materials to resist ignition and combustion, providing critical information for applications such as building construction.

At AdvanSES, we provide plastic and composite material testing under all the above mentioned parameters, you can be worry free about our test data and results as we are ISO 17025:2017 accredited.

Benefits of Plastic Material Testing

Plastic material testing offers numerous benefits, including:

  • Quality Control: Plastic material testing helps to ensure that plastic materials meet quality standards, reducing the risk of product failure and liability.
  • Cost Savings: By identifying potential defects or weaknesses in plastic materials early on, testing can help to reduce production costs and minimize waste.
  • Safety: Plastic material testing ensures that plastic materials are safe for use in specific applications, protecting both consumers and the environment.

    We can provide a quick quote for your plastic and composite material testing needs within a business day, try giving us a call or email and we would be happy to assist with any of your testing needs.

Conclusion

Plastic material testing plays a critical role in ensuring the quality and safety of plastic materials. By analyzing their physical, chemical, and mechanical properties, testing provides valuable information about their suitability for specific applications. By optimizing this blog post for search engines, we can ensure that this important information reaches a wider audience, promoting greater awareness of the importance of plastic material testing.

Viscoelasticity and its Testing

Kartik Srinivas material testing, Materials Engineering and FEA

Viscoelasticity refers to the ability of a material to exhibit both viscous and elastic behavior under deformation. Viscous behavior involves the material exhibiting resistance to deformation that gradually decreases over time, while elastic behavior involves the material returning to its original shape after deformation. Viscoelastic materials exhibit both of these behaviors simultaneously, meaning they exhibit both viscosity and elasticity.

Testing viscoelastic materials is important in order to understand their behavior under various conditions and to determine their mechanical properties. Some common tests for viscoelastic materials include:

  1. Creep testing: This involves subjecting the material to a constant load and measuring the amount of deformation over time. This test helps to determine the material’s long-term deformation behavior.
Figure 1: Creep Testing
  1. Stress Relaxation testing: This involves subjecting the material to a constant deformation and measuring the amount of stress over time. This test helps to determine the material’s long-term stress behavior.
Figure 2: Stress Relaxation Testing
  1. Dynamic mechanical analysis (DMA): This involves subjecting the material to cyclic loading and measuring its response. DMA can provide information on the material’s storage modulus, loss modulus, and damping coefficient.
Figure 3: Dynamic mechanical testing
  1. Tensile testing: This involves stretching the material until it breaks, and measuring the amount of force required to break it. Tensile testing can provide information on the material’s strength and stiffness. Tensile testing can be carried out by including cyclic effects, suitable pre-conditioning and multiple test steps.
Figure 4: Mechanical tensile testing
  1. Rheological testing: This involves subjecting the material to shear stress and measuring its response. Rheological testing can provide information on the material’s viscosity and shear modulus.
Figure 5: Melt flow index testing

Overall, these tests help to determine the viscoelastic behavior of materials, which is important in a wide range of applications, from materials science and engineering to medicine and biology.

AdvanSES is an ISO 17025:2017 accredited independent material testing laboratory. Give us a quick call to help us more on your requirements and how we can satisfy them to the best of our abilities.

Low Velocity Impact Testing

Kartik Srinivas failure analysis, life prediction, material testing ,

Low Velocity Impact Damage Testing:

Glass and Carbon fiber reinforced thermoplastic composite materials are widely used in automotive, and aerospace applications to make structures light-weight without losing on the stiffness and load carrying capability. Properties like high toughness, high Young’s modulus, low strains, and manufacturability for specific applications make these materials indispensable to modern industry. Most of these applications include high strain, low and high velocity impacts and it becomes imperative to study the behaviour of these materials under such conditions.

AdvanSES offers testing capabilities for testing as per ASTM D7136 (Drop Weight Impact), ASTM D256 (Izod Impact Test), ASTM D6110 (Charpy Impact Test) wherein the damage resistance of these materials and products can be studied. Low Velocity Impact Testing forms one of our core range of services for material and product testing.

Evaluation of Critical Tearing Energy of Rubber Materials

Kartik Srinivas failure analysis, fea abaqus ansys, life prediction, material testing , , ,
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.

Rubber Material Testing and Pre-Conditioning Techniques for Characterization

Kartik Srinivas fea abaqus ansys, material testing, service life prediction

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

AdvanSES Materials Testing Services

Kartik Srinivas failure analysis, fea abaqus ansys, life prediction, material testing

We at AdvanSES are capable of developing a custom testing protocol for compliance with international standards or for quality assurance. Materials testing services offered by AdvanSES include:

Composition: Whe you need to know with certainty what materials are used in the manufacture of thermoplastics, rubber materials etc.

Shear Test: Materials testing designed to measure shear strength of rubber and composites. These tests show how much stress a specimen can take before failure and is often times used to test and compare the strength of adhesives.

Flexural Test: When a product like an I-beam or girder used in construction must support a predetermined amount of weight without sagging, a flexure materials test is often performed to verify that the specimen can withstand a certain level of stress without flexing.

Environment and High Temperature Exposure Test: When it comes to determining the lifespan of materials, especially elastomer materials intended for outdoor use, exposure to high temperature and oils is carried out to check the degradation of materials.

Tensile/Compression Tests: From plastics and metals to adhesives and rubbers, tensile/compression testing is a form of materials testing that places specimens under precise compressive loads to measure their ability to withstand compression before deformation occurs.

Fatigue Tests: Fatigue tests are important to determine the endurance or breaking load a material can withstand before failing as well as the number of repeated loading cycles it can endure. Fatigue testing looks at a materials limit to withstand stresses and environment degradation. We can conduct stress controlled and strain controlled high cycle fatigue tests from room temperature to 250C on material samples, parts and components.

Applications of Materials Testing:

1) Quality Control
2) Regulatory Compliance
3) Design Development
4) Failure Analysis
5) Performance Prediction
6) Finite Element Analysis Material Constants Data