Mechanical properties testing of plastic products is a critical quality control and material qualification process that evaluates tensile strength, flexural modulus, impact resistance, hardness, and long-term durability under standardized laboratory conditions. By following internationally recognized standards such as ISO, ASTM, EN, GB, and JIS, accredited third-party laboratories help manufacturers verify material performance, ensure regulatory compliance, and reduce product failure risks. This article provides a comprehensive overview of specimen types, test items, standard test methods, and typical industrial applications of plastics mechanical testing.
Technical Overview
Mechanical properties testing refers to the systematic evaluation of how plastic materials and finished plastic products respond to applied forces, including tension, compression, bending, impact, and shear. Because polymers are viscoelastic materials, their mechanical behavior depends heavily on temperature, humidity, strain rate, loading duration, and processing history. Consequently, all reliable mechanical testing must be performed under strictly controlled environmental conditions using calibrated equipment and standardized test specimens.
The primary objectives of mechanical properties testing for plastic products include:
- Verifying that incoming resins or finished parts meet contractual and regulatory specifications
- Comparing candidate materials during new product development and material substitution
- Supporting quality control in serial production through periodic lot release testing
- Providing data for finite element analysis (FEA) simulation and structural design calculations
- Investigating root causes of field failures and product recalls
- Demonstrating compliance with product safety regulations and customer-specific requirements
Most mechanical tests are conducted with a universal testing machine (UTM) equipped with appropriate fixtures, load cells, and extensometers. Impact behavior is evaluated using pendulum impact testers or instrumented drop towers, while hardness is measured with durometers or Rockwell hardness testers. Results are typically reported as average values with standard deviations, based on a minimum of five specimens per condition as required by most standards.
Testing Samples
Injection-Molded Multipurpose Specimens
The ISO 3167 multipurpose test specimen is the industry-standard sample geometry for rigid thermoplastics. This dog-bone-shaped bar can be directly tested in tension or further machined into specimens for flexural, Charpy impact, and compression testing. Preferred dimensions include ISO 527 Type 1A (gauge length 80 mm, thickness 4 mm) and Type 1B specimens, as well as ASTM D638 Type I specimens (gauge length 50 mm, width 13 mm, thickness 3.2 mm). Proper molding conditions, gate design, and elimination of weld lines are essential, as processing defects can significantly distort test results.
Machined Specimens from Finished Products
When evaluating actual production parts, specimens are machined directly from finished components in designated locations and orientations. This approach captures the true effects of processing, fiber orientation, and part geometry, but requires careful surface preparation to avoid stress concentrations, overheating, or micro-cracks introduced during cutting and milling.
Films, Sheets, and Flexible Materials
Thin polyethylene films, packaging laminates, and flexible PVC are tested using dedicated geometries such as dumbbell-shaped strips (ISO 527-3, ASTM D882) for tensile testing, trouser or Elmendorf tear specimens (ISO 6383, ASTM D1922, ASTM D1004) for tear resistance, and circular discs for falling-dart puncture testing.
Pipes, Tubes, and Profiles
Extruded pipes and tubing are tested using ring segments, longitudinal strips, or full-section specimens according to ISO 6259 and ASTM D638, while profiles are sampled as complete cross-sections or machined coupons depending on geometry.
Finished Products and Components
Assemblies such as closures, brackets, fasteners, and housings are frequently tested as complete units in application-relevant configurations, including top-load compression of containers, closure removal torque, and snap-fit retention force, in addition to standardized coupon testing.
Prior to testing, all specimens must be conditioned in a standard atmosphere of 23 °C ± 2 °C and 50 % ± 10 % relative humidity for a minimum duration (typically 24 to 88 hours) as specified in ISO 291 and ASTM D618, unless the relevant product standard defines other conditions.
Testing Items
Core mechanical properties commonly evaluated for plastic materials and products include:
- Tensile strength, tensile stress at yield, tensile stress at break, tensile modulus, and elongation at yield and at break
- Flexural strength and flexural modulus
- Charpy and Izod notched and unnotched impact strength
- Instrumented puncture and falling-dart impact resistance
- Shore hardness (Shore A, Shore D), Rockwell hardness, Ball indentation hardness, and Barcol hardness
- Compressive strength and compressive modulus
- Shear strength and bearing strength
- Tear resistance (Elmendorf, trouser, Graves methods) and puncture resistance for films
- Creep behavior under sustained loading and stress relaxation
- Fatigue strength under cyclic loading
- Environmental stress cracking resistance (ESCR)
Tensile Properties
Tensile testing quantifies the fundamental load-bearing capability of a plastic. Key outputs include Young's modulus in the initial linear region, yield stress and strain for ductile materials, and ultimate tensile strength with elongation at break. The test also reveals the ductile-to-brittle transition behavior of the material.
Flexural Properties
Three-point bending provides flexural strength and flexural modulus, which are among the most widely specified properties for rigid plastics used in structural and semi-structural applications such as appliance housings, automotive interior trim, and furniture components.
Impact Resistance
Impact tests measure the energy absorbed during rapid fracture. Notched impact strength assesses notch sensitivity and toughness, while unnotched values reflect overall material ductility. Instrumented impact testing additionally records force-displacement curves, peak force, and total absorbed energy, enabling detailed failure-mode analysis.
Hardness
Hardness indicates surface resistance to indentation and correlates loosely with modulus. Shore A is applied to soft, elastomeric materials; Shore D to hard plastics; Rockwell scales (for example, M and R) to engineering thermoplastics; and Barcol hardness to reinforced thermoset composites.
Long-Term Mechanical Behavior
Creep tests under constant load (ISO 899-1, ASTM D2990) and fatigue tests under cyclic loading (ASTM D7774, ISO 13003) characterize performance over extended service lifetimes, which is essential for pressure pipes, load-bearing brackets, and continuously stressed assemblies.
Testing Methods
Tensile Testing
Tensile testing is performed per ISO 527-1 and ISO 527-2, ASTM D638, GB/T 1040, and JIS K 7113. Specimens are gripped in a universal testing machine and extended at standardized speeds, typically 1 mm/min for modulus determination and 5, 50, or 500 mm/min for strength measurement, until failure. An extensometer or video strain measurement system is used for accurate strain recording. Films are tested per ISO 527-3 or ASTM D882 with strip specimens and appropriate grip pressure to prevent slippage or premature tearing.
Flexural Testing
Three-point bending per ISO 178, ASTM D790, and GB/T 9341 is the dominant flexural method. A support span of 16 times the specimen thickness is standard, with a test speed calculated from the span and specimen depth. Four-point bending per ASTM D6272 is used when a larger region of uniform maximum stress is required, reducing the influence of surface defects at the loading nose.
Pendulum Impact Testing
Charpy impact testing is conducted per ISO 179-1, ASTM D6110, and GB/T 1043, with the specimen supported horizontally and struck by a pendulum at the middle, typically edgewise with defined notch types (A, B, C). Izod impact testing per ASTM D256, ISO 180, and GB/T 1843 uses a vertically clamped cantilever specimen struck near the notch. Both methods report absorbed impact energy normalized by specimen width or cross-sectional area, and both can be performed notched or unnotched, instrumented or conventional.
Falling-Dart and Instrumented Impact Testing
For films, dart-drop testing per ASTM D1709 and ISO 7765-1 determines the mass causing 50 % failure (F50). Instrumented falling-weight impact per ISO 7765-2, ASTM D5628, and ASTM D5420 records full force-deflection data on rigid plaques, enabling determination of peak force, energy to peak force, and total failure energy.
Hardness Testing
Shore durometer testing per ISO 868 and ASTM D2240 measures indentation depth under a spring-loaded indenter (Type A for soft materials, Type D for hard materials). Rockwell hardness per ISO 2039-2 and ASTM D785 applies major and minor loads with steel balls of defined diameter. Ball indentation hardness per ISO 2039-1 and Barcol hardness per ASTM D2583 are used for engineering plastics and thermoset laminates, respectively.
Compression Testing
Compressive properties are measured per ISO 604, ASTM D695, and GB/T 1041 using block or cylinder specimens with anti-buckling guidance where required, reporting compressive strength, compressive modulus, and strain at yield or failure.
Tear and Shear Testing
Elmendorf tear testing per ASTM D1922 and ISO 6383-2 measures propagation tear energy in films; trouser tear per ISO 6383-1 and Graves tear per ASTM D1004 measure initiation resistance; ASTM D624 covers trouser, angle, and crescent specimens for flexible elastomers and thermoplastic elastomers. Shear strength per ASTM D732 uses a punch-type fixture.
Conditioning and Test Report Requirements
All tests must follow documented conditioning per ISO 291 or ASTM D618, and the test report should identify the material, specimen preparation method, specimen dimensions, conditioning, test standard and speed, individual and mean results, standard deviation, and the test laboratory's accreditation status.
Applications
Mechanical properties testing supports product development and compliance across virtually every industry that uses plastics:
- Automotive: validation of interior trim, bumpers, battery housings, and under-hood components against OEM specifications, including impact performance at low temperatures
- Packaging: film tensile and seal strength, tear resistance, puncture resistance, and container top-load performance for PET bottles and injection-molded closures
- Electrical and electronics: housings, connectors, and insulating parts requiring verified flexural strength, hardness, and impact resistance per IEC-referenced material standards
- Medical devices: material qualification of catheters, syringes, IV components, and instrument handles under ISO 10993-1 biocompatibility-relevant and mechanical performance frameworks
- Construction: pipes, fittings, profiles, and geosynthetics verified against EN, GB, and ISO product standards for long-term hydrostatic and structural performance
- Consumer goods and appliances: durability verification of housings, gears, latches, and structural parts under repeated loading and drop conditions
- Recycling and sustainability: characterization of recycled and bio-based resins to demonstrate mechanical equivalence to virgin materials
In regulatory contexts, mechanical test data from accredited third-party laboratories is routinely used to support certifications under UL, EN, and GB product standards, export documentation, and customer part approvals.
FAQ
Why must specimens be conditioned before mechanical testing?
Plastics are moisture- and temperature-sensitive. Conditioning at 23 °C and 50 % relative humidity per ISO 291 or ASTM D618 stabilizes the specimen state, ensuring results are reproducible and comparable between laboratories. Hygroscopic materials such as PA6 and PA66 may additionally require drying or specific moisture content control.
Should I choose ISO or ASTM standards?
Both systems are internationally accepted, but specimen geometry and test parameters differ, so results are generally not directly interchangeable. ISO methods (including GB/T equivalents, which largely align with ISO) dominate in Europe and Asia, while ASTM methods prevail in North America. The choice should follow the governing product standard or customer specification.
What is the difference between Charpy and Izod impact testing?
Charpy (ISO 179-1, ASTM D6110) tests a horizontally supported specimen struck at mid-span, while Izod (ASTM D256, ISO 180) tests a vertically clamped cantilever specimen struck above the notch. The notch faces the striker in both cases, but clamping and stress states differ, so the two results are not directly comparable.
How many specimens are required per test?
Most standards require a minimum of five specimens per condition, with additional specimens when scatter is high. Impact testing may require 10 or more specimens for statistical determination of failure thresholds, such as in dart-drop F50 methods.
When is Shore A versus Shore D hardness used?
Shore A applies to soft, flexible materials such as soft PVC, TPE, and elastomers, while Shore D applies to hard plastics such as PP, POM, PA, and PS. Values above approximately 90 Shore A overlap with the lower Shore D range, in which case Shore D is preferred.
How long does mechanical testing take?
Short-term static tests such as tensile, flexural, hardness, and impact are typically completed within a few working days including conditioning. Creep, fatigue, and long-term hydrostatic strength programs run from weeks to more than a year, depending on the intended service-life demonstration.
Can finished products be tested instead of molded coupons?
Yes. Machining coupons from production parts or testing complete assemblies reflects actual processing conditions and is common for part qualification, but standardized molded coupons remain the reference method for material data sheets and material designation.