Determination of the Effect of Tensile Testing Rate on Hardness: Principles, Methods, and Standards

The rate at which a tensile test is performed has a measurable influence on the hardness and flow behavior of metallic and polymeric materials, making rate-controlled determination essential for comparable, reproducible test results. This technical article explains how tensile testing speed affects hardness values, which international standards govern rate control, and how accredited third-party laboratories design experiments to quantify the strain-rate sensitivity of engineering materials. Understanding these relationships is critical for material selection, quality control, and component qualification across automotive, aerospace, and manufacturing industries.

Technical Overview

The determination of the effect of tensile testing rate on hardness is a systematic evaluation of how the speed of deformation—expressed as stress rate, strain rate, or crosshead separation rate—influences the measured hardness and tensile flow response of a material. Although hardness testing and tensile testing are often treated as separate procedures, both are fundamentally governed by the same underlying plastic deformation mechanisms, including dislocation movement, dislocation multiplication, twinning, and, in polymers, viscoelastic and viscoplastic flow.

Most engineering materials exhibit positive strain-rate sensitivity: as the deformation rate increases, the flow stress increases, and for many material classes the apparent hardness also rises. In body-centered cubic (BCC) steels, for example, elevated strain rates suppress thermally activated dislocation motion, raising both yield strength and macro-hardness values. In face-centered cubic (FCC) aluminum alloys, the effect is weaker but still measurable. In polymers and elastomers, hardness readings are strongly time-dependent because of creep and stress relaxation under the indenter, which is why loading rate and dwell time are strictly specified in the relevant test methods.

From a metrological standpoint, the rate effect introduces a controlled variable that must be either eliminated through standardized rate control or deliberately characterized through rate-sweep experiments. International standards such as ISO 6892-1, ASTM E8/E8M, ASTM E18, ISO 6507-1, ISO 6508-1, and ISO 14577 define precise requirements for test speed, loading rate, and dwell time precisely because uncontrolled rates are a leading source of inter-laboratory scatter. A rigorous determination program therefore combines rate-controlled tensile testing with rate-controlled hardness testing on identical material conditions to establish the correlation and sensitivity coefficients between deformation rate and hardness.

Testing Samples

Metallic Materials

Typical metallic samples include carbon and low-alloy steels, stainless steels, aluminum alloys, copper and copper alloys, titanium alloys, nickel-based superalloys, and cast irons. Samples are prepared as standardized tensile test pieces in accordance with ISO 6892-1 or ASTM E8/E8M, with parallel sections machined to fine tolerances to eliminate geometric strain-rate gradients. Hardness specimens are cut from the gauge sections of tested or untested coupons, surface-prepared by grinding and polishing to a finish appropriate for the selected hardness scale.

Polymeric Materials

Thermoplastics, thermosets, and elastomers show pronounced rate dependence due to viscoelasticity. Tensile specimens are produced per ISO 527-1/527-2 or ASTM D638, while hardness is measured using Shore durometry (ISO 868, ASTM D2240), Ball indentation (ISO 2039-1), or Barcol methods. Because polymer hardness changes with time under load, dwell-time control is as important as loading-rate control.

Coatings, Weldments, and Additively Manufactured Parts

Surface coatings, weld heat-affected zones, and additively manufactured structures frequently exhibit microstructural gradients that respond non-uniformly to deformation rate. Micro- and macro-hardness traverses (Vickers per ISO 6507, Knoop per ASTM E384) are performed at controlled loading rates to map rate sensitivity across these heterogeneous regions.

Testing Items

A comprehensive determination program typically includes the following measured and derived quantities:

• Yield strength (ReH, Rp0.2, or Ry) at each controlled strain rate
• Ultimate tensile strength (Rm) and tensile strength at break
• Uniform elongation, total elongation, and reduction of area as functions of rate
• Strain-rate sensitivity exponent (m-value) derived from flow stress comparisons at multiple rates
• Rockwell hardness (HRB, HRC, etc.) per ISO 6508-1 / ASTM E18 at controlled dwell times
• Vickers hardness (HV) per ISO 6507-1 / ASTM E92 at controlled loading and dwell durations
• Brinell hardness (HBW) per ISO 6506-1 / ASTM E10 with controlled loading rate
• Martens hardness (HM) and indentation hardness (HIT) from instrumented indentation per ISO 14577
• Indentation creep and dwell-time sensitivity indices for time-dependent materials
• Statistical significance analysis (mean values, standard deviations, confidence intervals) comparing hardness across rate conditions

Testing Methods

Rate-Controlled Tensile Testing

Tensile tests are performed on servo-hydraulic or screw-driven universal testing machines equipped with extensometers and, where available, closed-loop strain-rate control. ISO 6892-1 Method A1 specifies control based on estimated strain rates over the parallel length using an open-loop factor or, preferably, closed-loop feedback from the extensometer signal. Typical rate steps include 0.00007 s⁻¹, 0.00025 s⁻¹, and 0.0025 s⁻¹ for the yield region, with higher rates for post-yield hardening evaluation. ASTM E8/E8M provides equivalent stress-rate and strain-rate regimes. For high-rate characterization, servo-hydraulic frames and split-Hopkinson pressure bar setups extend the range to 10²–10³ s⁻¹, enabling correlation with dynamic hardness measurements.

Rate- and Dwell-Controlled Hardness Testing

Hardness measurements are conducted with strict control of the variables that emulate rate effects: the loading rate to reach the total test force, the dwell time at full force, and the removal rate. For Rockwell testing per ISO 6508-1 and ASTM E18, the force application duration and dwell time directly shift measured values on rate-sensitive materials such as low-carbon steels and soft aluminum alloys. Vickers and Brinell testing per ISO 6507-1, ISO 6506-1, ASTM E92, and ASTM E10 similarly require controlled force application times and dwell durations, typically 10–15 seconds, with longer dwells for polymers. Instrumented indentation per ISO 14577 records the complete force–depth curve, allowing continuous hardness determination across a programmable loading-rate window and enabling quantitative extraction of rate sensitivity parameters.

Correlation and Determination Procedure

The determination follows a structured experimental matrix:

• Prepare homogeneous sample lots from a single material condition with verified microstructure and heat-treatment status
• Perform tensile tests at a minimum of three to five logarithmically spaced strain rates with at least three replicates per rate
• Perform hardness tests on the gauge sections and on undeformed reference blanks at matched effective loading rates and dwell times
• Measure post-tensile hardness gradients along the gauge length to relate accumulated strain, strain rate, and local hardening
• Compute flow-stress ratios and the strain-rate sensitivity exponent m from the tensile data, and correlate these with hardness shifts between rate conditions
• Convert and cross-check hardness scales using ASTM E140 or ISO 18265 only within validated material classes, noting that conversion tables do not correct for rate effects
• Report all rates, dwell times, machine control mode, temperature, and calibration status to ensure traceability and reproducibility

Equipment and Calibration

Testing machines are calibrated and verified per ISO 7500-1 or ASTM E4 for force accuracy, with extensometers classified per ISO 9513 or ASTM E83. Hardness testers are indirectly verified using certified reference blocks per ISO 6508-2, ISO 6507-2, ISO 6506-2, and ASTM E18, E92, and E10 annexes. Temperature is maintained at 10–35 °C for standard tests, or 23 ± 2 °C where polymer viscoelastic response makes thermal control critical.

Applications

Rate-sensitive hardness and tensile determinations support a wide range of engineering and quality functions:

• Automotive crash and structural analysis, where strain rates from 10¹ to 10³ s⁻¹ govern energy absorption and formed-component performance
• Metal-forming process design, including rolling, forging, stamping, and high-speed forming, where flow stress at process strain rates controls load prediction and die design
• Aerospace fastener, landing-gear, and engine-component qualification, where dynamic loading conditions must be reflected in material data sheets
• Pipeline and pressure-vessel assessment, where rate effects on toughness and hardness interplay with sour-service hardness limits (e.g., NACE MR0175 / ISO 15156 hardness criteria)
• Weld procedure qualification and heat-affected-zone hardness surveys at controlled rates to ensure comparability across laboratories
• Additive manufacturing parameter qualification, where layered microstructures can show unusual rate sensitivity
• Polymer component design, where Shore hardness and tensile modulus vary with loading rate and service temperature
• Inter-laboratory comparison and measurement-uncertainty evaluation, where uncontrolled rate differences are a known source of reproducibility scatter
• Incoming material inspection and supplier qualification programs executed by accredited third-party laboratories using standardized rate-control schemes

FAQ

Why does tensile testing rate affect hardness measurements?

Both hardness and tensile deformation are controlled by the same plastic-flow mechanisms. At higher deformation rates, thermally activated dislocation glide has less time to proceed, so higher stresses are required to deform the material. This raises flow stress and, because hardness is essentially a measure of resistance to localized plastic flow, the measured hardness value increases accordingly. In polymers, reduced time for chain relaxation produces the same net effect.

Is the rate effect the same for all materials?

No. Body-centered cubic steels and polymers show strong rate sensitivity; face-centered cubic aluminum and copper alloys show moderate sensitivity; and some austenitic and precipitation-hardened alloys can behave nearly rate-insensitive at room temperature under standard test speeds. The magnitude must therefore be determined experimentally for each material class and condition.

Which standards specify test-rate control?

Key standards include ISO 6892-1 and ASTM E8/E8M for tensile testing of metals, ISO 527-1/527-2 and ASTM D638 for plastics, ISO 6506-1, ISO 6507-1, and ISO 6508-1 with their ASTM counterparts E10, E92, and E18 for hardness testing, ISO 14577 for instrumented indentation, and ASTM E140 / ISO 18265 for hardness conversions. Machine calibration is covered by ISO 7500-1, ASTM E4, and extensometer standards ISO 9513 and ASTM E83.

How is the strain-rate sensitivity exponent determined?

The m-value is obtained by testing identical specimens at multiple strain rates and comparing flow stresses at fixed true strain values, typically at 5–10% plastic strain. The exponent is calculated from the slope of log flow stress versus log strain rate. A parallel comparison of hardness values across the same rate conditions establishes the empirical hardness-rate correlation used in quality control.

Can hardness conversions correct for rate effects?

Standard conversion tables such as ASTM E140 and ISO 18265 assume nominally identical test conditions and do not compensate for strain-rate differences. Conversions should only be applied within validated material classes, and rate-sensitive evaluations should rely on directly measured hardness at controlled loading rates and dwell times rather than converted values.

What should a test report include for this determination?

A compliant report identifies the material and its condition, specimen geometry and orientation, the testing machine control mode (stress rate, strain rate, or crosshead speed), all applied rate values and dwell times, extensometer class, calibration traceability, individual and mean hardness and tensile results, statistical analysis of rate effects, and full identification of the test standards applied.