Tensile testing at elevated temperature example and dependence on coupon preparation method - A433

 

Tensile testing at elevated temperature example and dependence on coupon preparation method
Foundational knowledge article
Document Type	Article
Document Identifier	433
Prerequisites	Materials science Material properties Polymer properties

Introduction

Tensile testing serves as a fundamental mechanical evaluation for plastic materials, providing essential data on elasticity, strength, and ductility under uniaxial loading. By stretching a standardized specimen to failure, this test generates a stress-strain curve that reveals key properties: Young’s modulus (stiffness), tensile strength (maximum stress), elongation at break (fracture strain), and yield strength (onset of plastic deformation in ductile materials). Globally, two principal standards govern these tests—ISO 527 and ASTM D638 — which despite methodological variations, both aim to ensure reproducible data for material selection and design. Reliable tensile testing hinges on three critical factors: specimen preparation, controlled conditions, and precise instrumentation. Additional challenges are encountered when testing at elevated temperatures due to both test frame effects and material effects as thermal energy overrides microstructure-driven properties.

Significance

Tensile testing of polymers is essential for understanding their mechanical behavior under real-world operating conditions; however, thermal effects significantly influence performance. As temperature increases, polymers typically exhibit reduced stiffness, lower yield strength, and enhanced ductility due to increased molecular chain mobility. Standard tensile tests conducted at room temperature may not accurately predict material behavior in higher-temperature environments, such as automotive under-the-hood components, aerospace structures, or industrial applications. By performing tensile tests across the range of temperatures expected during operation, the change in under thermal stress can be measured aiding in material selection and design optimization.

Scope

This paper provides an overview of tensile testing methodologies for plastics, highlighting the key differences between ASTM D638 and ISO 527 standards. It examines the challenges associated with elevated temperature tensile testing. The discussion then narrows to a demonstration of these effects on PERT (polyethylene of raised temperature resistance), presenting comparative tensile data across three temperatures (23°C, 60°C, and 95°C) and three preparation methods (injection molding, compression molding, and extrusion).

Tensile Testing

Tensile testing is a fundamental mechanical test used to characterize the behavior of plastic materials under axial stretching forces. It provides critical data on elasticity, strength, ductility, and failure modes, which are used for material selection, quality control, and product design. It involves applying a uniaxial load to a standardized specimen until fracture occurs. A stress-strain curve is then generated from the force displacement curve and is used to determine properties of the material.

The properties that are commonly extracted from the test are:

• Young’s modulus: stiffness of the material in its elastic deformation region.
• Tensile strength: maximum stress the material can withstand before breaking or yielding.
• Elongation at break: percent strain at which the material fractures.
• Yield strength: stress at which plastic deformation begins in ductile samples.
• Curve shape: reveals whether a material is brittle (e.g., polystyrene) or ductile (e.g., polyethylene).

Two primary standards govern tensile testing of plastics, namely ISO 527 and ASTM D638, providing similar data for engineering applications despite having different methodologies (see Figure 1).

Figure 1. The Comparison of ASTM D638 and ISO 527 Tensile Test Standards

The accuracy and reliability of tensile testing results for plastic materials fundamentally depend on three critical factors:

• Proper specimen preparation
• Controlled testing conditions
• Precise equipment calibration

Specimen preparation covers the standardized geometry (dog bone-shaped samples are universally employed to ensure failure occurs within the gauge length not the grips), method of preparation (injection, compression, extrusion, etc.) and orientation (if anisotropy is expected). These specimens must then undergo proper environmental conditioning, with tests typically conducted under controlled laboratory conditions of 23°C and 50% relative humidity to ensure consistency and comparability of results across different testing sessions and laboratories. The testing equipment itself plays an equally vital role, requiring the use of a properly calibrated universal testing machine (UTM) equipped with precise load cells and crosshead control systems to apply and measure forces and displacement accurately throughout the duration of the test.

Beyond these fundamental requirements, several material-specific factors significantly influence test outcomes and must be carefully considered. Plastics inherently exhibit viscoelastic behavior, meaning their mechanical properties demonstrate pronounced strain rate sensitivity, where the speed of deformation directly affects the measured properties. Temperature represents another critical variable, as elevated temperatures generally reduce plastic's stiffness and tensile strength while simultaneously increasing its ductility and elongation at break, highlighting the importance of maintaining consistent thermal conditions during testing. Furthermore, many engineered plastics display anisotropic behavior, particularly fiber-reinforced or oriented polymers, where mechanical properties vary significantly depending on the direction of testing relative to the material's internal structure or fiber alignment. This directional dependence requires careful specimen orientation during preparation and testing to obtain meaningful, representative data.

Conducting tensile tests at elevated temperatures introduces several complications, from the device point of view:

• Difficulty in handling: leads to low throughput and/or extended testing times.
• Thermal expansion of fixtures and specimens: can artificially inflate strain measurements if not corrected.
• Softening at high temperatures: may lead to specimen slippage or premature failure at the clamping points.
• Temperature gradients across the specimen: can cause non-uniform deformation.

Additionally, from a material perspective, several factors must be considered:

• Oxidative degradation in air: can alter material properties during prolonged testing
• Viscoelastic creep: becomes significant at elevated temperatures and may introduce time-dependent artifacts
• Sensitivity of amorphous polymers near their glass transition temperature: even minor temperature fluctuations can produce disproportionate responses and therefore require very tight thermal control
• Presence of overlapping thermal transitions: can further complicate data interpretation.

An example of a tensile testing set-up is shown in Figure 2 where notably an optical extensometer is employed to bypass the influence of temperature drift of contact extensometers and minimize expansion effect on measurement. In Figure 3 an example of the typical curves for an injection molded PERT material is provided demonstrating the decrease in elastic slope, the reduction of tensile strength and the transition from ductile failure at 23°C, through cold-drawing at 60°C into a more rubber-like behaviour at 95°C.

Figure 2. Experimental Set-up for Tesile Testing at Temperature: (a) Sample Mounted Inside Temperature Chamber, (b) Optical Extensometer

Figure 3. Load-Displacement Curves of a Single Polymer Material (PERT) Tested at Different Temperatures

Processing methods also influence the mechanical behavior of polymers. Semi-crystalline polymers exhibit crystallinity-dependent mechanical properties. In a recent study a polymer was prepared in three different manners; the compression-molded samples showed 63.5% crystallinity, followed by extruded samples at 58.0% and the injection-molded variant at 55.0%. These crystallinity differences are directly manifested in mechanical performance. At 23°C, compression-molded specimens exhibited 19% higher tensile yield strength and 36% greater modulus than the injection-molded counterpart, the elevated temperature testing (60°C and 95°C) demonstrated progressive property convergence, where compression-molded samples showed just 5% higher yield strength at 95°C versus 19% at 23°C. These mechanical results across processing methods and service temperatures agree with established structure-property relationships and summarized in Figure 4.

Figure 4. Elastic Modulus and Yield Strength as a Function of Temperature for a Single Polymeric Material Using Three Different Preparation Methods Demonstrating How Their Values Converge as the Measurement Temperature Approaches the Melting/Softening Point

References

Literally "without structure", randomly coiled molecular polymer chains.

The glass transition temperature (T_g) is the temperature region where the polymer transitions from a hard, glassy material to a soft, rubbery material. It is one of the most important properties of any amorphous polymer.

Periodic 3-D, repeating array of molecules.