Mechanical testing of plastics is a critical component of materials science and engineering that provides essential data for material selection, quality control, and product development. These standardized tests evaluate how plastic materials respond to various types of stress and loading conditions, enabling engineers and manufacturers to make informed decisions about material suitability for specific applications. Understanding the fundamental principles of mechanical testing, the calculations involved, and proper interpretation of results is essential for anyone working with polymeric materials in industries ranging from automotive and aerospace to medical devices and consumer products.
Understanding Mechanical Testing of Plastics
Mechanical testing is used to assess the performance of plastic materials under specified types of stress. Plastic testing is an array of diagnosis processes that assess the physical, chemical, thermal, and mechanical properties of plastic material to confirm a premise of pertinent performance opportunity and consideration of industry standards. The primary objectives of mechanical testing include determining material characteristics, identifying potential weaknesses before products reach the market, and ensuring compliance with industry standards and safety requirements.
The overreaching goal is to determine the sample material's characteristics and to identify possible shortcomings before bringing products to market or selling plastic components to manufacturers down the production chain. More specific reasons to perform a comprehensive set of mechanical tests include: Ensuring compliance with national or international standards and clients' safety and quality requirements. Additional purposes include testing new batches to verify quality consistency, assessing raw materials from new suppliers, and determining whether novel biobased materials perform comparably to traditional plastics.
The choice of properties and testing methods depends on the plastic material's composition and intended use. Different mechanical tests are performed on various plastic forms, such as packaging films versus injection-molded components, because each application demands specific performance characteristics.
Standardization and Testing Protocols
Mechanical testing standards are published by various international and national standardization bodies, including ISO, ASTM, and DIN. ASTM standards, ISO standards and many national standards deriving thereof establish the methodology and the framework parameters for plastics testing, without which good reproducibility of the properties among testing laboratories as well as suppliers and customers is not possible. These standards ensure that test results are consistent, reproducible, and comparable across different laboratories and manufacturers worldwide.
The standards that apply to plastics are mostly industry-specific – and practically countless, as polymer-based materials are so widely used. In some cases, different standards may even apply to similar products within a single industry. For example, the automotive industry presents a unique challenge where each manufacturer may have its own specific testing requirements for plastic components.
The most commonly referenced standards for plastic testing include ASTM D638 for tensile properties, ISO 527 for tensile testing, ASTM D790 and ISO 178 for flexural properties, ASTM D256 for Izod impact testing, and ISO 179 for Charpy impact testing. Understanding which standard applies to your specific application is crucial for obtaining meaningful and compliant test results.
Comprehensive Types of Mechanical Tests
Tensile Testing
Tensile testing is the most common type performed using universal testing equipment to evaluate various mechanical properties that provide vital information about plastic materials. This test method is used to assess the behaviour of plastics when subjected to uniaxial tensile stress. The advantage of the tensile test is that even ductile materials can be tested to complete break point.
The ASTM D638 standard describes the test method for determination of the tensile properties of reinforced and unreinforced plastics. It helps determine essential mechanical properties, including tensile stress, strain, tensile modulus, tensile strength, tensile strength at yield and tensile strength at break. The test involves stretching a standardized specimen at a controlled rate while measuring the applied force and resulting deformation.
Tensile testing involves stretching a specimen and measuring the load carried. The load and deflection data are translated into a stress-strain curve from which a range of tensile properties can be extracted. This stress-strain curve provides a comprehensive picture of how the material behaves under tensile loading, from initial elastic deformation through yield, plastic deformation, and ultimately to failure.
Specimen preparation is critical for obtaining accurate tensile test results. ASTM D638 contains information about specimen shape and specimen sizes, test procedures including environmental conditions as well as accuracy requirements for the corresponding testing machines and extensometers. Different specimen types are used depending on the material form and available sample size, with Type I being the standard specimen and Type IV or V used when material is limited.
Flexural Testing
BS EN ISO 178 defines a 3-point loading method for determining flexural characteristics in rigid and semi-rigid polymers. This method can be used to analyze flexural behavior and calculate flexural strength, modulus, and other flexural stress/strain relationship parameters. Flexural testing, also known as bend testing, evaluates how materials respond to bending forces.
A rectangular cross-section test specimen sitting on two supports is deflected by a loading nose operating on the specimen in the middle of the two supports. The specimen is deflected until it ruptures on its outer edge or reaches a maximum flexural strain of 5%, whichever comes first. This three-point bending configuration creates a combination of tensile, compressive, and shear stresses within the specimen.
This test method enables the strength and dimensional change properties of plastics to be determined when subjected to three-point loading. Three-point loading produces tensile, compressive and shear stresses in the test specimen. The flexural test is particularly useful for materials that are too flexible or too brittle to be tested effectively in pure tension or compression.
These test procedures deal with the determination of flexural properties of reinforced and unreinforced plastics, including electrical insulating materials and high-modulus composites in the form of rectangular bars molded directly or cut from molded shapes, plates or sheets. These test routines are generally applicable to both semi-rigid and rigid materials. These test procedures utilize a three-point loading system that is applied to a simply supported beam.
Compression Testing
Compression testing evaluates how plastic materials behave when subjected to compressive forces. ISO 604 standard applies to compression testing of rigid and semirigid plastics. This test procedure deals with the determination of the mechanical properties of reinforced and unreinforced rigid plastics, including high-modulus composites, when loaded in compression at relatively low uniform rates of loading or straining. Test samples of standard shapes are employed.
The material's displacement is calculated against the applied load to determine compressive properties like compression strength and modulus. During compression testing, a specimen is placed between two parallel plates and subjected to an increasing compressive load until failure occurs or a predetermined deformation is reached. The test provides valuable data about how materials will perform in applications where they must support loads or resist crushing forces.
Compression testing is particularly important for materials used in structural applications, load-bearing components, and packaging materials. The test can reveal different failure modes including buckling, shearing, or crushing, depending on the specimen geometry and material properties.
Impact Testing
Impact testing measures the energy absorbed by a material during fracture and provides information on its toughness. Impact tests evaluate a material's ability to withstand sudden, high-speed loading conditions, which is critical for applications where materials may experience shock loads or sudden impacts during service.
In the case of solid plastics, testing is typically performed by hitting the specimen with a hammer (Charpy or Izod impact tests). For films, a typical impact test is done with the free-falling dart method. These standardized impact tests provide a quantitative measure of material toughness and brittleness.
The Izod impact test is a quick and simple test to facilitate a comparative material assessments. Amongst other things, it is used to investigate the effects of changed formulation, compounding or injection molding conditions on the test specimen. Different effects can be simulated by varying the temperature and the notch shape. The Izod test uses a notched specimen clamped vertically, with the pendulum striking the specimen on the same side as the notch.
The Charpy impact test is a quick and simple test to facilitate a comparative material assessment. Amongst other things, it is used to investigate the effects of changed formulation, compounding or injection molding conditions on the test specimen. In the Charpy test, the test specimens are subjected to stress in a set-up similar to the three-point loading test and broken as soon as they are sufficiently brittle.
These test procedures cover the determination of the resistance of plastics to "standardized" pendulum-type hammers, mounted in "standardized" machines, in breaking standard samples with one pendulum swing. The standard tests for these test routines require samples made with a milled notch. In Test Procedures A, C, and D, the notch produces a stress concentration that increases the probability of a brittle, rather than a ductile, fracture.
Additional Mechanical Tests
Beyond the primary mechanical tests, several specialized tests provide additional information about plastic material properties. Hardness testing measures a material's resistance to indentation and surface deformation. Tear resistance testing evaluates how easily films and flexible materials can be torn. Shear testing determines the material's resistance to forces applied parallel to its surface.
Dynamic mechanical analysis (DMA) is another valuable testing method. Dynamic mechanical analysis (DMA) is another method used to assess the thermal properties of plastics. DMA also provides information about the material's viscoelastic properties, transition temperatures, and much more. This technique applies oscillating stress to a specimen while measuring its response, providing insights into how materials behave under cyclic loading conditions.
Essential Calculations in Mechanical Testing
Stress Calculations
Stress is one of the most fundamental calculations in mechanical testing. The stress, σ, is given by f/A, where A = a x b. In simpler terms, stress equals the applied force divided by the cross-sectional area over which the force is distributed. This calculation provides a normalized measure of the internal forces within a material, allowing for meaningful comparisons between specimens of different sizes.
Tensile strength is measured in units of force per cross-sectional area. In the International System of Units (SI), stress is typically expressed in Pascals (Pa), megapascals (MPa), or gigapascals (GPa), where 1 MPa equals 1 million Pascals and 1 GPa equals 1 billion Pascals. In the imperial system, stress may be expressed in pounds per square inch (psi).
The stress calculation must account for the actual cross-sectional area at the point of measurement. For tensile tests, this becomes particularly important as the specimen's cross-section changes during testing. Engineering stress uses the original cross-sectional area, while true stress uses the instantaneous cross-sectional area, which decreases as the specimen elongates.
Strain Calculations
The tensile strain, γ, is equal to the change in length, ∆c/c. Strain represents the deformation of a material relative to its original dimensions and is expressed as a dimensionless ratio or as a percentage. The calculation involves dividing the change in length by the original length of the specimen.
Strain: change in gauge length with reference to the initial gauge-length is a critical measurement in tensile testing. Accurate strain measurement requires precise determination of both the original gauge length and the change in length during testing. This is typically accomplished using extensometers, which are specialized instruments that attach to the specimen and measure displacement with high accuracy.
ASTM refers to "% elongation" ... 0.05% and 0.25% strain. ASTM D638 calculates modulus using the linear portion of the stress-strain curve, dividing strain by the corresponding stress. Different standards may specify different strain ranges for specific calculations, emphasizing the importance of following the appropriate standard for your application.
Like stress, strain can be calculated as engineering strain (based on original dimensions) or true strain (based on instantaneous dimensions). For small deformations typical in the elastic region, these values are nearly identical, but they diverge significantly during plastic deformation.
Young's Modulus (Elastic Modulus)
The tensile (Young's or Secant or 2%) modulus, Esecant, is given by Esecant = σ/γ. Young's modulus, also called the elastic modulus or modulus of elasticity, represents the stiffness of a material in the elastic region. Modulus of Elasticity – A measure of stiffness, reflecting how much a material deforms in response to stress before yielding.
The Young's modulus is calculated from stress divided by strain, and at small strains the true values are equivalent to the nominal values. This calculation provides the slope of the stress-strain curve in the linear elastic region, indicating how much stress is required to produce a given amount of strain.
The elasticity modulus (E modulus) serves as a parameter for comparing different materials and is a measure of stiffness. Materials with high modulus values are stiff and resist deformation, while materials with low modulus values are flexible and deform easily under load. The modulus is expressed in the same units as stress (typically MPa or GPa) since strain is dimensionless.
Modulus is measured by calculating stress and dividing by elongation, and would be measured in units of stress divided by units of ... If the slope is steep, the sample has a high tensile modulus, which means it resists deformation. If the slope is gentle, then the sample has a low tensile modulus, which means it is easily deformed.
Different methods exist for calculating modulus depending on the material behavior. Secant modulus draws a line from zero to a specific point on the stress-strain curve, while tangent modulus uses the slope at a particular point. ISO 527-2 requires modulus to be measured between 0.05% and 0.25% strain. ASTM D638 calculates modulus using the linear portion of the stress-strain curve, dividing strain by the corresponding stress.
Tensile Strength Calculations
Tensile Strength – The maximum stress achieved by the material. Tensile strength is the highest tensile stress that a specimen can reach during a tensile test. This represents the ultimate load-bearing capacity of the material under tensile loading conditions.
Tensile strength is the ability of plastic material to withstand a maximum amount of tensile stress without failure. The stress occurs while the material is being pulled or stretched. It is the point when a material goes from elastic to plastic deformation. The calculation involves determining the maximum force recorded during the test and dividing by the original cross-sectional area.
This tensile strength can occur at a yield point, in which case it is referred to as tensile strength at the yield point. If the tensile strength occurs shortly before failure of the specimen, it is referred to as tensile strength at break. Understanding which type of tensile strength is most relevant depends on the material behavior and application requirements.
Yield Strength Calculations
Yield is defined as the point at which the material transitions from the elastic deformation region to plastic deformation. In other words, the point at which the material begins to deform permanently under stress. Yield strength represents the stress level at which a material begins to exhibit permanent deformation.
Yield strength (A): The stress a material can withstand without permanent deformation. For materials with a well-defined yield point, this value can be read directly from the stress-strain curve. However, many plastics do not exhibit a sharp yield point, requiring the use of the offset method.
The offset method, typically using a 0.2% offset, involves drawing a line parallel to the elastic portion of the stress-strain curve but offset by a specified strain value (usually 0.2% or 0.002). The intersection of this offset line with the stress-strain curve defines the yield strength. This standardized approach ensures consistent yield strength determination across different materials and testing laboratories.
Elongation and Strain at Break
Elongation – The change in gauge length in respects to the original gauge length until the specimen yields; a higher value indicates greater ductility. Elongation at break represents the total strain a material can withstand before fracture and is typically expressed as a percentage.
The calculation involves measuring the final gauge length at the point of fracture, subtracting the original gauge length, dividing by the original gauge length, and multiplying by 100 to express as a percentage. This value provides important information about material ductility and formability.
For brittle plastics and plastics, which do not exhibit a yield point, strain at break is measured directly with a mechanical or non-contact measuring extensometer. In the case of thermoplastics that do present a yield point, the nominal strain at break is measured from the travel of the testing machine's pulling grip. This distinction is important because materials that neck or yield may concentrate strain in a localized region, making extensometer measurements unreliable for total elongation.
Impact Energy Calculations
Impact energy represents the total energy absorbed by a specimen during impact testing. However, the only value determined is the impact energy consumed. This energy is calculated from the difference in potential energy of the pendulum before and after striking the specimen.
The results of all test procedures are reported in terms of energy absorbed per unit of specimen width or per unit of cross-sectional area under the notch. This normalization allows for meaningful comparison between specimens of different sizes. Impact energy is typically expressed in joules per meter (J/m) or foot-pounds per inch (ft-lb/in).
The impact strength calculation accounts for the specimen dimensions, particularly the width and thickness at the notch location for notched specimens. Higher impact energy values indicate greater toughness and resistance to brittle fracture under high-speed loading conditions.
Flexural Strength and Modulus
Flexural strength represents the maximum stress in the outer fibers of a specimen during bending. The calculation uses beam theory equations that account for the applied load, specimen dimensions, and support span. For a three-point bend test, the flexural stress is calculated using the formula that incorporates the load, span length, specimen width, and specimen thickness.
Flexural modulus is calculated from the slope of the initial linear portion of the load-deflection curve, similar to how tensile modulus is determined from the stress-strain curve. The calculation converts the load-deflection data into stress-strain data using the specimen geometry and test configuration, then determines the slope in the elastic region.
The Stress-Strain Curve: A Comprehensive Analysis Tool
The stress-strain curve is the primary graphical representation of mechanical test data and provides a wealth of information about material behavior. The load and deflection data are translated into a stress-strain curve from which a range of tensile properties can be extracted. Understanding how to read and interpret this curve is essential for proper analysis of mechanical test results.
Elastic Region
The initial portion of the stress-strain curve represents the elastic region, where the material deforms reversibly. Elastic deformation: The material returns to its original dimensions. Deformation is reversible and non-permanent. Valid for small strains. In this region, the relationship between stress and strain is linear, and the slope of this linear portion equals Young's modulus.
When stress is removed in the elastic region, the material returns completely to its original dimensions with no permanent deformation. This behavior is governed by the stretching and compression of atomic bonds within the material, which act like microscopic springs.
Yield Point
Materials first deform elastically when you release the stress, and they return to their original shape. Then, with more force, they deform plastically. This is yield, i.e., when you release the stress, they have permanently been stretched into a new shape. The yield point marks the transition from elastic to plastic deformation.
It measures the stress a plastic can withstand at the yield point, i.e., when an increase in strain is not provoked by an increase in stress. Beyond the yield point, the material begins to flow plastically, and permanent deformation occurs even if the stress is removed.
Plastic Region
Plastic deformation: The material does not return to its original dimensions. This process is permanent and irreversible. In the plastic region, the material undergoes permanent deformation as atomic planes slip past one another within the material's structure.
The plastic region may exhibit strain hardening, where the material becomes stronger as it deforms, resulting in an upward-sloping stress-strain curve. Alternatively, some materials may exhibit strain softening or necking, where localized deformation occurs and the stress decreases with increasing strain.
Ultimate Tensile Strength and Fracture
Ultimate strength (B): The maximum stress a material can withstand. This point represents the peak of the stress-strain curve and indicates the maximum load-bearing capacity of the material. Beyond this point, the material may neck or begin to fail, and the stress decreases until final fracture occurs.
Finally, they break; this is ultimately tensile stress or breaking point. The breaking point or fracture point represents the end of the stress-strain curve, where the specimen separates into two pieces. The stress and strain at this point provide important information about the material's ultimate ductility and failure characteristics.
Interpreting Mechanical Test Results
Material Classification Based on Behavior
Mechanical test results allow materials to be classified based on their stress-strain behavior. In general, fibers have the highest tensile moduli, and elastomers have the lowest, and plastics have tensile moduli somewhere in between fibers and elastomers. This classification helps engineers select appropriate materials for specific applications.
Rigid plastics exhibit high modulus and strength but limited elongation. They can withstand significant stress but fracture at relatively low strains. Examples include polystyrene, poly(methyl methacrylate), and polycarbonate. These materials are suitable for applications requiring dimensional stability and load-bearing capacity.
Flexible plastics show moderate modulus and strength with higher elongation capabilities. They can undergo significant deformation before failure, making them suitable for applications requiring toughness and impact resistance. Examples include polyethylene and polypropylene.
Elastomers like polyisoprene, polybutadiene and polyisobutylene have completely different mechanical behavior from the other types of materials. Elastomers have very low moduli. You can see this from the very gentle slope of the pink plot, but you probably knew this already. Elastomers can undergo very large deformations and return to their original shape, making them ideal for seals, gaskets, and flexible components.
Strength versus Toughness
Understanding the distinction between strength and toughness is crucial for proper interpretation of mechanical test results. Strength refers to the maximum stress a material can withstand, while toughness represents the total energy absorbed before failure, which is related to the area under the stress-strain curve.
While it's good for materials in a lot of applications to have high moduli and resist deformation, in the real world it's a lot better for a material to bend than to break, and if bending, stretching or deforming in some other way prevents the material from breaking, all the better. So when we design new polymers, or new composites, we often sacrifice a little bit of strength in order to make the material tougher.
A material can be strong but not tough (brittle materials that fracture at low strain), or tough but not particularly strong (ductile materials that deform extensively). The ideal material for many applications balances both properties, providing adequate strength while maintaining sufficient toughness to prevent catastrophic failure.
Comparing Results to Specifications
One of the primary purposes of mechanical testing is to verify that materials meet specified requirements. This involves comparing test results to material specifications, industry standards, or customer requirements. Key parameters to evaluate include minimum tensile strength, minimum elongation at break, modulus range, and impact resistance thresholds.
Plastic manufacturers may on occasion test batches to ensure mechanical properties are consistent. Varying extrusion temperature, injection pressure, or cooling rates will be transferred to tensile properties. ASTM D638 is an accepted method to ensure materials meet specifications before shipping. This quality control testing ensures batch-to-batch consistency and compliance with specifications.
Environmental and Processing Effects
Plastic's mechanical properties are extremely sensitive to temperature. Test results must be interpreted in the context of the testing conditions, particularly temperature, humidity, and strain rate. Materials that perform well at room temperature may exhibit dramatically different properties at elevated or reduced temperatures.
This is short term data obtained over a limited time period and usually under ideal laboratory conditions. Due to the majority of plastic materials being sensitive to temperature, the likely working conditions for the product or component need to be borne in mind when studying performance data. Real-world performance may differ from laboratory test results due to environmental factors, long-term loading, and other service conditions.
Processing conditions also significantly affect mechanical properties. Factors such as molding temperature, cooling rate, orientation, and crystallinity all influence the final properties of plastic parts. Understanding these relationships helps in optimizing processing parameters to achieve desired mechanical performance.
Statistical Analysis and Variability
Mechanical test results inherently contain some degree of variability due to material heterogeneity, specimen preparation variations, and testing uncertainties. Proper interpretation requires statistical analysis of multiple test specimens to determine average values, standard deviations, and confidence intervals.
Typically, a minimum of five specimens should be tested for each condition to obtain statistically meaningful results. Outliers should be identified and investigated to determine whether they represent true material variability or testing errors. The coefficient of variation (standard deviation divided by mean) provides a measure of data scatter and testing precision.
Advanced Considerations in Mechanical Testing
Temperature-Dependent Testing
The test is possible within a temperature range from –40°C to +230°C. Many applications require materials to perform across a wide temperature range, necessitating mechanical testing at various temperatures. Temperature-controlled chambers allow testing at elevated or reduced temperatures to simulate service conditions.
The heat deflection temperature (HDT) is the temperature at which a standard test specimen deflects 0,025 mm under a standardized load of either 0.455 MPa or 1.82 MPa. The higher this temperature is, the better the plastic is suited for use in elevated temperatures. Heat deflection temperature testing provides specific information about thermal performance under load.
Strain Rate Effects
The rate at which a specimen is deformed significantly affects the measured mechanical properties. For ASTM D638, the test speed is determined by the material specification. For ISO 527, the test speed is typically 5 or 50 mm/min for measuring strength and elongation, and 1 mm/min for measuring modulus. Different test speeds are specified for different property measurements.
Polymeric materials are viscoelastic, meaning their response depends on both the magnitude and rate of applied stress. Higher strain rates generally result in higher strength and modulus but lower elongation. This rate-dependence must be considered when interpreting results and selecting appropriate test speeds.
Specimen Preparation and Quality
The quality of test specimens directly affects the reliability of test results. Specimens must be prepared according to standard specifications, with proper dimensions, smooth surfaces, and no defects or damage. Machining marks, scratches, or other surface imperfections can act as stress concentrators and lead to premature failure.
For injection-molded specimens, processing conditions must be controlled and documented. Molding parameters such as temperature, pressure, and cooling rate affect the final properties. Specimens machined from molded parts may exhibit different properties than directly molded test specimens due to differences in molecular orientation and residual stresses.
Gripping and Alignment
Secure, consistent gripping pressure helps prevent slippage — a common issue observed with plastics that thin undergoing stress. Pneumatic side action grips with serrated jaw faces are recommended for rigid plastics. Their pneumatic design ensures a consistent clamping force, even as the material becomes thinner during testing. Proper gripping is essential for obtaining accurate test results.
Specimen alignment is equally critical. Eccentric loading, where the applied force is not perfectly aligned with the specimen axis, introduces bending stresses that can significantly affect results. Modern testing machines include alignment fixtures and procedures to minimize eccentric loading effects.
Extensometry and Strain Measurement
Modulus of elasticity is one of the most important calculated properties of ASTM D638. To capture modulus accurately, you need an appropriate strain-measuring device. Extensometer recommendations for ASTM D638 depend on material elongation, throughput goals, calculation requirements, and whether you need to test at high or low temperature.
Extensometers provide accurate strain measurement by directly measuring the deformation of the specimen gauge section. Contact extensometers attach to the specimen using clips or knife edges, while non-contact optical extensometers use video or laser technology to track specimen deformation without physical contact. The choice of extensometer depends on the material, expected strain levels, and testing requirements.
Applications of Mechanical Testing Data
Material Selection and Design
Determining material mechanical characteristics is critical in the research and development efforts of plastics and rubber industries. When developing production processes, it is essential to identify the mechanical qualities needed to meet the material specifications of new goods. A thorough study of new materials and products is required to optimize these manufacturing processes.
Mechanical test data enables engineers to select materials that meet the specific requirements of their applications. By comparing the mechanical properties of different materials, designers can identify candidates that provide the necessary strength, stiffness, toughness, and other characteristics required for successful performance.
Quality Control and Assurance
Testing new batches to verify that their quality is comparable to previous batches. Assessing the quality of plastic raw materials provided by a new supplier. Regular mechanical testing as part of quality control programs ensures that materials consistently meet specifications and that any variations in properties are detected before defective products reach customers.
The results are essential for end-product quality control during the manufacturing of plastics and to ensure that products meet specification requirements for end use. This ongoing testing provides confidence in product quality and helps identify process variations that may affect material properties.
Research and Development
In R&D, tensile testing determines the effect of additives, fillers, or reinforcement in raw materials. For example, the addition of glass fibers to polypropylene doubles tensile strength from 30 MPa to over 80 MPa and reduces elongation at break by half. Mechanical testing is essential for developing new materials and formulations.
Researchers use mechanical testing to understand structure-property relationships, optimize formulations, and develop materials with improved performance. By systematically varying composition, processing conditions, or other parameters and measuring the resulting mechanical properties, researchers can identify optimal material designs.
Regulatory Compliance
Medical devices, food packs, and electronic components often require mechanical testing according to ASTM D638 to meet ISO, FDA, or ASTM regulation standards. Polyethylene tubing that is FDA-approved is one such instance where tensile properties uniformity must be assured in order to ensure safe use in medical devices.
Testing also proves the compliance of plastics with the regulatory standard bodies, such as ASTM and ISO, that set down standards for the performance of materials and their safety. Many industries have specific regulatory requirements for mechanical properties, and testing provides the documentation needed to demonstrate compliance.
Failure Analysis
When plastic components fail in service, mechanical testing of the failed parts and comparison materials helps identify the root cause. Testing can reveal whether the failure resulted from material defects, improper material selection, processing issues, or service conditions exceeding design limits. This information guides corrective actions to prevent future failures.
Common Challenges and Best Practices
Addressing Necking and Non-Uniform Deformation
Plastics often do not deform homogeneously. Strain can concentrate in a small region due to necking. For materials that neck or have a yield point, percentage elongation at break cannot be reported via the extensometer because necking may occur outside the extensometer's gauge length.
Necking presents challenges for accurate strain measurement and requires careful consideration of measurement methods. Therefore, nominal strain must be used to report percent elongation after yield. Using an extensometer for strain at break is only acceptable when strain is homogeneous and the material does not exhibit necking or yield.
Managing Test Variability
The tensile impact test frequently has a large spread of measured values. Some tests, particularly impact tests, inherently exhibit greater variability than others. Understanding the expected variability for each test type helps in determining appropriate sample sizes and acceptance criteria.
Minimizing variability requires careful attention to specimen preparation, testing procedures, environmental control, and equipment calibration. Following standardized procedures and maintaining consistent testing conditions reduces variability and improves data reliability.
Selecting Appropriate Standards
While ASTM D638 and its ISO counterpart ISO 527-1 / ISO 527-2 are technically equivalent, they do not delivery fully comparable results, since the specimen shapes, the test procedure and determination of the results differ in a few aspects. While ISO 527-2 is similar to ASTM D638 — both measure the tensile properties of plastics — they are not technically equivalent due to differences in specimen dimensions, test speeds, and specific procedural requirements. Manufacturers often choose the appropriate standard based on regional preferences and customer requirements. ASTM D638 is common in North America, ISO 527-2 is widely used in Europe and Asia, and manufacturers in China commonly test to both.
Understanding the differences between standards and selecting the appropriate one for your application and market is essential. When comparing data from different sources, verify that the same standard was used or understand how differences in standards may affect the results.
Documentation and Reporting
Comprehensive documentation of test conditions, specimen details, and results is essential for meaningful interpretation and future reference. Test reports should include the standard used, specimen dimensions, testing speed, temperature and humidity conditions, equipment used, and any deviations from standard procedures.
Complete stress-strain curves should be retained along with calculated properties. This raw data allows for reanalysis if questions arise and provides a complete record of material behavior. Digital data storage and management systems facilitate organization and retrieval of test data.
Future Trends in Mechanical Testing of Plastics
Advanced Testing Techniques
Emerging testing technologies continue to expand the capabilities of mechanical testing. Digital image correlation (DIC) provides full-field strain measurement across the entire specimen surface, revealing strain distributions and localized deformation that traditional extensometers cannot detect. This technology is particularly valuable for studying complex deformation patterns and validating finite element models.
High-speed testing equipment enables characterization of material behavior at strain rates approaching those experienced in crash events and other high-speed loading scenarios. This data is essential for applications where impact and crash performance are critical, such as automotive and protective equipment applications.
Automation and Data Analytics
Automated testing systems increase throughput and consistency while reducing operator influence on results. Robotic specimen handling, automated data acquisition, and integrated analysis software streamline the testing process and improve efficiency.
Advanced data analytics and machine learning algorithms are being applied to mechanical testing data to identify patterns, predict material behavior, and optimize formulations. These tools can process large datasets from multiple tests to extract insights that would be difficult to discern through traditional analysis methods.
Sustainability and Bio-Based Materials
Determining whether the performance of a novel biobased material is similar to that of traditional plastics. As the plastics industry moves toward more sustainable materials, mechanical testing plays a crucial role in evaluating bio-based and recycled plastics to ensure they can serve as viable alternatives to conventional petroleum-based materials.
Testing of recycled plastics presents unique challenges due to potential degradation and contamination. Mechanical testing helps assess the quality of recycled materials and determine appropriate applications where they can perform reliably.
Multiscale Testing and Modeling
Integration of mechanical testing at multiple length scales, from nano-indentation to full-scale component testing, provides a comprehensive understanding of material behavior. Combining experimental testing with computational modeling enables prediction of performance under conditions that are difficult or expensive to test experimentally.
Finite element analysis (FEA) uses mechanical property data from standardized tests to predict the behavior of complex components under realistic loading conditions. This approach reduces the need for extensive prototype testing and accelerates product development.
Practical Guidelines for Conducting Mechanical Tests
Pre-Test Preparation
Successful mechanical testing begins with proper preparation. Specimens should be conditioned at the test temperature and humidity for a sufficient time to reach equilibrium, typically 24-48 hours. This conditioning ensures that test results reflect the material's properties under the specified environmental conditions rather than transient effects.
Verify that specimens meet dimensional requirements and are free from defects. Measure and record specimen dimensions accurately, as these measurements directly affect calculated stress and strain values. Even small errors in dimension measurement can lead to significant errors in reported properties.
During Testing
Monitor the test in real-time to identify any anomalies such as specimen slippage, premature failure at grips, or equipment malfunctions. Modern testing software typically displays the stress-strain curve as it develops, allowing operators to verify that the test is proceeding normally.
Record all relevant test parameters including actual test speed, temperature, humidity, and any observations about specimen behavior. Note the failure mode (brittle fracture, ductile failure, grip failure, etc.) as this information aids in interpreting results.
Post-Test Analysis
After testing, examine failed specimens to verify that failure occurred in the gauge section rather than at the grips or other stress concentrations. Grip failures or failures at specimen defects invalidate the test and require retesting with new specimens.
Calculate all required properties according to the applicable standard, using the correct formulas and specimen dimensions. Verify that calculated values are reasonable and consistent with expected material behavior. Outliers should be investigated to determine whether they represent true material variability or testing errors.
Equipment Calibration and Maintenance
Regular calibration of testing equipment ensures accuracy and reliability of results. Load cells, extensometers, and displacement transducers should be calibrated according to manufacturer recommendations and applicable standards. Calibration records should be maintained to demonstrate ongoing compliance with accuracy requirements.
Routine maintenance of testing machines, grips, and accessories prevents equipment-related problems that could affect test results. Worn grips, damaged fixtures, or improperly functioning equipment can introduce errors and should be repaired or replaced promptly.
Conclusion
Mechanical testing of plastics provides essential data for material selection, quality control, product development, and regulatory compliance. Understanding the various test methods, proper calculation procedures, and correct interpretation of results is fundamental for anyone working with polymeric materials. The stress-strain curve serves as a comprehensive representation of material behavior, revealing elastic properties, yield characteristics, ultimate strength, and failure modes.
Accurate calculations of stress, strain, modulus, strength, and other properties require careful attention to specimen dimensions, testing procedures, and applicable standards. Proper interpretation of results considers not only the numerical values but also the material's behavior under different loading conditions, environmental factors, and processing history.
As materials technology advances and new applications emerge, mechanical testing continues to evolve with improved techniques, automation, and analytical methods. Whether evaluating traditional plastics, developing novel bio-based materials, or assessing recycled polymers, mechanical testing remains an indispensable tool for ensuring that plastic materials meet the demanding requirements of modern applications.
By following standardized procedures, maintaining proper equipment, and applying sound engineering judgment in interpreting results, engineers and technicians can generate reliable mechanical property data that supports informed decision-making throughout the product lifecycle. The investment in proper mechanical testing pays dividends in improved product performance, reduced failures, and enhanced customer satisfaction.
For further information on plastic testing standards and best practices, consult resources from organizations such as ASTM International, the International Organization for Standardization (ISO), and industry-specific technical associations. These organizations provide comprehensive standards, training materials, and technical support to help ensure accurate and meaningful mechanical testing of plastic materials.