What are the standard test methods for determining the polymer tensile properties? How do the testing standards vary for evaluating plastics, films, elastomers and other polymer materials? What type of test specimens are required for tensile testing polymers?

Tensile Properties and the Stress-Strain Curve

Figure 1. Stress-strain behavior of various polymer materials. Source: Chemical Retrieval on the Web (CROW)

Tensile properties are evaluated by placing a test specimen between two clamps in a tensile or universal tester, which is a type of mechanical test equipment. The clamps are pulled apart using hydraulics or mechanical (servo screw drive) to apply tension until the sample breaks. The force applied is measured as well as the elongation (deformation or strain) induced in the sample. Since the cross-section of the polymer sample is known, a plot of stress versus strain can be generated. Tensile properties such as break tensile strength, tensile modulus or secant modulus and percent elongation at break can be determined.

Figure 2. Table-top, single-column testing machine for high accuracy testing of polymers, composites and other materials. Source: United Testing Systems Inc.Polymers, especially elastomers, are viscoelastic materials, so they do not deform like metals or elastic-plastic materials. Metals deform elastically below their yield point and then plastically or permanently above the yield point. Viscoelastic polymers behave like a combination of viscous liquid and elastic solid depending on the stresses and stress rates applied and released. The temperature and strain rate applied during testing can significantly alter the tensile strength and mechanical response of polymers.

While ductile plastics or tough polymers exhibit a yield point, many brittle plastics or composites and elastomers do not exhibit a true yield point or proportional limit where a material deformation transitions from linear and elastic to non-linear and permanent. As a result, we cannot determine the yield strength (YS) of certain polymers. Materials engineers often use a specific permanent offset strain of 0.1%, 0.2% or 0.5% to determine a yield point or proof stress in metals or plastics without a proportional limit. Some polymers may not exhibit any linear elastic deformation, so a true Young’s elastic modulus cannot be calculated. However, a secant modulus at a specific stress or strain can be calculated.

Rubbers and elastomers exhibit elastic deformations or enormous elongations of 300 to 1000 percent. In elastomer testing (ASTM D412), tensile stress is reported at various elongations such as tensile Figure 3. Brookfield CT-3 Texture Analyzer is a powerful universal testing machine that offers both compression and tensile testing for food and low strength polymers. Source: AMETEK Brookfieldstress at 200 percent elongation or 200 percent modulus. Certain brittle polymer materials have very limited deformation (less than 5 percent) and their stress-strain curve is linear until failure. Brittle polymer-based materials include rigid plastics, plastics below their glass transition point, fiber reinforced polymer matrix composites and high strength synthetic fibers, e.g. aramid (Kevlar®), melt-spun liquid crystal polymer (LCP) Vectran® or UHMWPE Spectra®. While composites and synthetic fibers can have high tensile strength, they will fail catastrophically when their break strength is exceeded.

While testing the mechanical or tensile properties of polymer engineering materials for design engineering purposes, testing natural polymers in foods, paper, wood and other materials is important, too. For instance, our foods consist of many natural polymers, such as starches, proteins, cellulose, gluten, pectin, gelatin and collagen, which also require tensile and texture testing. Alligator tastes like chicken, but the texture is very rubbery, which might explain why the alligator meat market is a fraction of the poultry market. Improper processing or alteration of natural polymers in foods can produce unpleasant or rubbery textures. The Brookfield Ametek’s CT3 Texture Analyzer in figure 3 is an example of a universal tester designed to test the tensile and burst strength of foods as well as more abstract “texture” properties such as spreadability, tackiness, consistency and firmness. Tensile testing is important in developing the proper feel and texture in the natural polymer in foods through processing and chemical modification.

Material Testing Standards

Tensile testing standards are available from ISO, SAE, JIS, DIN and ASTM. ASTM D638 – Standard Test Method for Tensile Properties of Plastics and ISO 527-1: Plastics – Determination of Tensile Properties – Part 1 are the most widely specified plastic tensile testing standards. The ASTM International organization is the globally recognized leader with over 12,000 ASTM standards related to materials test methods, specifications, classifications, guides and practices. ASTM International was founded as the “American Society of Testing and Materials” in 1898; now it is the primary materials and testing standards development body internationally.

“Polymer materials” include a broad group of materials available in many different forms such as plastics, thin plastic films, elastomers, foams or cellular materials, adhesives, coatings, resin matrix composites, synthetic fibers, yarns, tapes, textiles and coated fabrics.

Specific ASTM test methods have been developed for different types and forms to accommodate varying mechanical property behaviors. The table in figure 4 shows the properties determined based on the test methods in the ASTM standard. Head-to-head comparison of different polymer material types (sheeting, tapes, yarn, plastics, rubber, etc.) is difficult because the different ASTM methods evaluate different properties.

Figure 4. Table of several ASTM standard tensile test methods for polymer materials. Source: ASTM & ADMET

Another major plastic testing standard from ISO, ISO 527, consists of the following parts, under the general title Plastics — Determination of tensile properties:

— Part 1: General principles
— Part 2: Test conditions for molding and extrusion plastics
— Part 3: Test conditions for films and sheets
— Part 4: Test conditions for isotropic and orthotropic fiber-reinforced plastic composites
— Part 5: Test conditions for unidirectional fiber-reinforced plastic composites

Test Specimens

Figure 5. Certified plastic and elastomer polymer specimen cutting dies as per ASTM, JIS, DIN, ISO standards. Source: Qualitest International Inc.The dimension and shape of the tensile test specimens must be fabricated according to the standards for valid test results. A dog bone or dumbbell specimen is used to tensile test plastic materials per ASTM D638. The plastic dog bones are typically molded in a die to produce test specimens depending on the specific requirements of the applicable tensile testing standards. Flat dog bone specimens can also be die cut from a plastic sheet or machined from a block. ASTM standards that require cutting dies include ASTM D412, D624, D638, and D882. ASTM D882 and ISO 527-3 are standard test methods for evaluating the tensile properties of thin plastic sheeting.

Different types of test specimens or methods for specimen preparation are sometimes referenced within a single standard. For instance, ASTM D412, the test method for the determination of the tensile stress-strain properties of elastomers, use dog bone specimens in Part A and ring-shaped samples in Part B. The ISO 37:2017 standard references additional test standards for test specimen fabricated by injection molding, machining or compression molding:

ISO 293, Plastics — Compression molding of test specimens of thermoplastic materials
ISO 294-1, Plastics — Injection molding of test specimens of thermoplastic materials
ISO 295, Plastics — Compression molding of test specimens of thermosetting materials
ISO 2818, Plastics — Preparation of test specimens by machining
ISO 10724-1, Plastics — Injection molding of test specimens of thermosetting compounds (PMCs)
ISO 20753, Plastics — Test specimens

Grips and Fixtures

Figure 5. Specialized test grips with rubber faces for elastomer testing per ASTM D412. Source: United Testing Systems Inc.The grips used to hold specimens in polymer tensile testing are another important consideration. The grip should not introduce a flaw or tear because these flaws could be an initiation point for failure, which would invalidate the test as would any failure outside of the gage length zone. Specialized grips are made with rubber faces for elastomer testing per ASTM D412, such as the rubber dog bone grips from United Testing Systems shown in figure 6. Films and tapes also require specialized grips for tensile testing because films and pressure-sensitive tapes can break at relatively low forces. Low-strength tapes and films can be tested with rubber or flat metal grip faces. Stronger tapes, reinforced tapes, webbing, braids, and cloth may require a grip with a serrated metal face. Very strong sheet materials require a wrap or capstan grip in order to properly hold the sample without slippage.

Advanced Tensile Testing Methods

Figure 6. Miniature, in-situ miniature material tensile stage designed for use in scanning electron microscopes (SEMs), and light microscopes (LMs). Source: MTI InstrumentsIn advanced tensile testing, the impact of environmental conditions and temperature on tensile properties is evaluated. A polymer's tensile "static" properties at room temperature under dry conditions are typically higher compared to tensile test results performed under elevated temperatures and "dynamic" testing. For instance, certain polymers will have reduced tensile strengths when tested in a solvent or chemical bath or under repeated lower load cycles. Tensile creep and fatigue strength are considered dynamic properties, which are evaluated in specialized test equipment. Tensile fatigue strength tests measure a polymer's performance under repeated load cycles. Creep tests are performed by exposing a material to a lower load at an elevated temperature where a material will exhibit "cold flow."

In certain cases, materials engineers may want to visually observe a polymer material deform during tensile testing in order to understand deformation, failure mechanisms or how the environmental media is interacting with or degrading a polymer test specimen. MTI Instruments Inc. has developed an in-situ miniature material tensile microscope stage designed for use in scanning electron microscopes (SEMs), atomic force microscopes (AFMs) and light microscopes (LMs), which was initially designed and successfully deployed for metal testing. The dual leadscrew design symmetrically loads samples while keeping them centered within the scopes field of view. The miniature tensile also has optional heaters for observation of a material tensile response under elevated temperature conditions. The observation of fiber reinforcement debonding or pull-out, delamination and other composite failure mechanisms might be observable with an MTI tensile testing microstage.