Determine the engineering strain rate for materials testing
Calculation Results
Strain Rate (min⁻¹):–
Strain Rate (sec⁻¹):–
How to Calculate Strain Rate in a Tensile Test
Strain rate is a critical parameter in materials science that describes the speed at which a specimen is deformed during a tensile test. Many materials, particularly polymers and certain metals, are "strain-rate sensitive," meaning their mechanical properties like yield strength and elongation change depending on how fast they are stretched.
The Engineering Strain Rate Formula
In a standard constant-velocity tensile test, the engineering strain rate ($\dot{\epsilon}$) is calculated using the crosshead speed of the testing machine and the initial gauge length of the specimen:
Strain Rate (ε̇) = v / L₀
Where:
v: Crosshead speed or displacement rate (typically in mm/min or mm/s).
L₀: Original gauge length of the specimen (the portion of the specimen where deformation is measured).
Steps for Calculation
Measure Gauge Length: Determine the distance between the gauge marks on your specimen (e.g., 50 mm).
Set Crosshead Speed: Identify the speed at which the machine grips move apart (e.g., 5 mm/min).
Divide Speed by Length: Divide the speed by the length to get the rate in reciprocal minutes.
Convert to Seconds: Divide the result by 60 to obtain the standard SI unit of s⁻¹.
Practical Example
Suppose you are testing a steel bar with an initial gauge length of 50 mm. You set the universal testing machine (UTM) to a crosshead speed of 10 mm/min.
Step 1: 10 mm/min ÷ 50 mm = 0.2 min⁻¹
Step 2: 0.2 min⁻¹ ÷ 60 seconds = 0.00333 s⁻¹
The resulting strain rate for this test is 3.33 x 10⁻³ s⁻¹.
Why it matters: High strain rates (impact tests) generally increase the yield strength of metals but may decrease ductility, making the material behave more brittly. Low strain rates (creep tests) allow for molecular relaxation, especially in thermoplastics.
function calculateStrainRate() {
var v = parseFloat(document.getElementById('crossheadSpeed').value);
var l0 = parseFloat(document.getElementById('gaugeLength').value);
var resultsDiv = document.getElementById('strainResults');
var ratePerMinSpan = document.getElementById('ratePerMin');
var ratePerSecSpan = document.getElementById('ratePerSec');
var interpretation = document.getElementById('interpretation');
if (isNaN(v) || isNaN(l0) || l0 <= 0 || v <= 0) {
alert("Please enter valid positive numbers for both crosshead speed and gauge length.");
resultsDiv.style.display = "none";
return;
}
// Calculation
var strainRateMin = v / l0;
var strainRateSec = strainRateMin / 60;
// Output formatting
ratePerMinSpan.innerHTML = strainRateMin.toFixed(4) + " min⁻¹";
// Use scientific notation for very small or large values in seconds
if (strainRateSec < 0.001) {
ratePerSecSpan.innerHTML = strainRateSec.toExponential(4) + " s⁻¹";
} else {
ratePerSecSpan.innerHTML = strainRateSec.toFixed(6) + " s⁻¹";
}
// Interpretation logic
var msg = "";
if (strainRateSec = 0.0001 && strainRateSec <= 0.1) {
msg = "This is a standard quasi-static strain rate used in most industrial tensile tests (ASTM E8/E8M).";
} else {
msg = "This is a high strain rate. Ensure your data acquisition system is fast enough to capture the peak loads.";
}
interpretation.innerHTML = "Note: " + msg;
resultsDiv.style.display = "block";
}