Parameter Calibration of Anand Viscoplastic Model for Lead Free Solders
Calibrating Anand viscoplastic parameters for lead free solders demands isothermal tensile and relaxation testing across temperatures and strain rates.
Machine Compliance Compensation
Correction Method
Tensile testing instruments inevitably deform under high loads, adding an unwanted elastic deflection to the recorded extension of the test specimen. To obtain the true strain of the specimen, machine compliance compensation is applied to subtract the machine’s deflection from the raw crosshead displacement. This adjustment is particularly necessary when testing high-stiffness materials where the specimen’s deformation is comparable to the deflection of the load frame.
Displacement Calculation
Deflection of the columns, load cell, and grips is calculated by assuming the testing machine acts as a linear spring. This spring constant is measured by running a test with a high-strength block that does not deform significantly under load. The resulting load-deflection curve represents the compliance of the machine itself.
Calibration Standard
Subtraction of the machine’s elastic deflection from the total measured displacement yields the true specimen extension. This corrected displacement is then divided by the initial gauge length to calculate the true strain. Modern digital controllers perform this calculation automatically during the test.
Measurement Error
Inaccurate strain data can result if the machine’s compliance is not accounted for, leading to an underestimation of the material’s elastic modulus. For this reason, high-precision tests rely on direct strain measurement using contact extensometers or digital image correlation rather than crosshead displacement. However, when extensometers cannot be used, such as in high-temperature furnaces, this digital compensation becomes the primary means of ensuring accurate strain measurements across the entire test range.