Tensile & compression testing
A specimen is gripped at both ends and stretched at constant speed until it breaks while force and elongation are recorded. Dog-bone shapes concentrate deformation in a narrow gauge section so failure happens away from the grips.
Our ruleSimulations prepare you for the bench but don't replace hands-on practice.
Uniaxial tensile
A specimen is gripped at both ends and stretched at constant speed until it breaks while force and elongation are recorded. Dog-bone shapes concentrate deformation in a narrow gauge section so failure happens away from the grips.
How it differs from the standard method
- Use an extensometer or video gauge for modulus; crosshead strain includes machine compliance.
- Precondition soft tissues with a few cycles before the measured ramp.
- Report rate, temperature, gauge length and failure location.
1Stiffness and strength from force and deformation
Mechanical testing loads a specimen in a controlled way and records force against deformation. Dividing force by the original cross-section gives engineering stress, and dividing the change in length by the original length gives strain.
Uniaxial tensile
Dog-bone or strip specimens are pulled to failure: modulus, yield, ultimate strength and elongation of films, fibres, tendons and skin.
Unconfined compression
Discs or cylinders are compressed between flat platens, free to bulge sideways: the usual test for hydrogels, foams, cartilage.
Nanoindentation
A hard tip of known shape is pressed in by nanometres to microns; modulus and hardness come from the unloading curve (Oliver–Pharr).
AFM
A cantilever with a sharp or spherical tip indents cells or soft gels with pico- to nanonewton forces; Hertz fits give local moduli and maps.
2Why the same material gives different numbers
Most biomaterials are viscoelastic: the faster you load them, the stiffer and stronger they appear, and hydrated gels and tissues are also poroelastic, because water has to flow out as they are squeezed. A modulus therefore means little without the rate, temperature and hydration it was measured at.
3Good practice
- Follow a standard geometry and rate (for example ISO 527 or ASTM D638 for plastics) or report yours fully.
- Measure each specimen's cross-section; measure strain on the specimen, not the crosshead, when the modulus matters.
- Test hydrated materials immersed, slowly enough (or by stress relaxation) to reach equilibrium, with low-friction platens and a defined preload.
- In AFM, calibrate deflection sensitivity and spring constant, keep indentation under about 10% of film thickness, and fit the model that matches the tip.
Key terms
Practise these as flashcards →Slope of stress against strain in the initial linear region; a measure of stiffness.
Force over original area, and change in length over original length.
Stress at which deformation stops being recoverable; for many polymers the peak before necking.
Combined elastic and viscous behaviour, so the response depends on loading rate and time.
Rate dependence of a water-filled network caused by fluid flowing through it as it deforms.
Softening of an elastomer or tissue after the first loading cycles; the reason tissues are preconditioned.
The force–indentation relation for a sphere on an elastic half-space, F ∝ E·√R·δ^1.5.
The deformation of frame, load cell and grips, which adds to crosshead travel.
When to use it
- Checking that a scaffold, hydrogel or film matches the stiffness of the tissue it is meant to replace or model.
- Measuring the strength and failure behaviour of sutures, grafts, fibres and engineered tissues.
- Mapping local stiffness of cells, matrices or thin coatings at micron scale (AFM, nanoindentation).
- Comparing formulations or processing conditions under a fixed, standard protocol.
Compared with alternatives
Shear moduli of soft gels and their frequency and time dependence, without cutting specimens.
Where the gels being compressed come from and how their crosslinking sets stiffness.
Porous architectures whose stiffness these tests check.
Chemistry and morphology that explain a mechanical result.
Sub-techniques
Choosing one here switches the whole page, including the protocol and the lab, to it.
A specimen is gripped at both ends and stretched at constant speed until it breaks while force and elongation are recorded. Dog-bone shapes concentrate deformation in a narrow gauge section so failure happens away from the grips.
- Use an extensometer or video gauge for modulus; crosshead strain includes machine compliance.
- Precondition soft tissues with a few cycles before the measured ramp.
- Report rate, temperature, gauge length and failure location.