by James Clinton
Tensile testing determines suitability for use and a material’s performance over time
Tensile testing is a common test method used in force measurement and material testing, primarily to detexine the mechanical behaviour of a component, part or material under static, axial loading.
These tests determine the tensile properties of a material or component. The sample’s deformation is used to characterize its ductility or brittleness as well as important characteristics such as tensile strength, yield point, elastic limit, percent elongation, elastic modulus and toughness.
Material testing quantifies and qualifies the physical characterization of materials—strength, reaction to deformation, and ability withstand an applied force for a period of time.
Material testing involves measurements for stress and strain, which requires knowing the original cross-sectional area of the sample being tested. Common units of measure are N/mm2, MPa, PSI and per cent. Test samples are often prepared to a specified size according to an international testing standard from ASTM, ISO, DIN or other organizations. During a tensile test, the sample’s shape changes as load is applied. The change in the sample’s dimension at various specified forces determines the material’s performance and suitability for a given application or product.
Force Measurement Testing
Force measurement testing generally uses Newtons, pounds-force and kilograms-force. The sample’s cross-sectional area isn’t involved in the measurement result. The most common force measurement is the “peak force” or maximum force value. Force measurement is conducted in the engineering laboratory, in quality control and inspection and on the production floor. Force testing in production has increased substantially in recent years, as more manufacturers recognize that in-situ quality testing helps improve productivity.
Tensile testing may be performed at a very basic level by simply using a handheld force gage to measure the pull force applied on a sample. Extremely sophisticated tensile testing systems with advanced testing software and ancillary instruments—such as extensometers—pull the sample under test at a very precise velocity to a very precise target. Large data sampling produces high resolution data for both force and distance or stress and strain, producing very accurate measurements.
There are a variety of different tensile testing characteristics. Here are some of the more common measurements.
- Tensile strength is the maximum stress measured—often during a destructive test. Stress is the amount of force divided by the original cross-sectional area of the sample under test.
- Elasticity is the material’s ability to be pulled to a stress value, returning to its original length when the stress is removed without showing any permanent deformation. A spring is an example of a product that is designed to have a high elasticity.
- Elastic Limit is the highest degree of stress the material can withstand before it exhibits a permanent set.
- Yield Strength is where the material under test exhibits a permanent set, surpassing its elastic limit. Yield strength is often determined using an arbitrary offset value of 0.2 per cent strain from the elastic slope. A line is drawn parallel with the elastic slope line at an offset of 0.2 per cent. Yield strength is where the offset slope line intersects the stress-strain curve.
- Plasticity is similar to elasticity, with an added element of time.When a material, such as an elastomer, is pulled to a force limit and then held for a period of time, the ability of the elastomer to return to its original shape without permanent deformation is its plasticity characteristic.
- Resilience is the ability to absorb and store energy during a tensile loading application. It is the energy capacity within the elastic range of a material and is characterized as the area beneath the elastic stress-strain curve, expressed in Joules/m3 or Pascals.
- Hysteresis is a material’s loss of energy and its inability to retain its elastic characteristic. Many materials exhibit some degree of hysteresis as they are loaded and unloaded repeatedly over time.
- Stiffness is the material’s ability to resist deformation when a force is applied.
- Modulus of Elasticity measures a material’s stiffness. A stiff material exhibits less deformation under loading.
- Elongation is strain. Elongation is the per cent of change in length from the sample’s original gage length prior to force being applied compared to its length at an applied force.
During tensile testing, a material will deform laterally and axially. Poisson’s Ratio is the comparison of a material’s lateral deformation to the axial or longitudinal deformation.
Tensile Testing Challenges
Non-axial loading is one of the most common causes of incorrect tensile measurements. A slightly off-centre load when using a load sensor or force gauge can result in measurement errors of up to 0.5 per cent. Alignment of the testing string—the load cell, top test fixture, sample and bottom test fixture—is critical.
Using a properly sized force gage or load cell sensor is important to achieving accurate and repeatable results. A general rule is to use a sensor that is between 20 per cent and 80 per cent of the anticipated load measurement. This will avoid or minimize error affects at the low end due to mechanical noise and will help prevent overloading conditions at the upper end of the measurement range. Since most sensors are calibrated and have their accuracy specification based on full scale, the closer you are to zero, the more affect the error has on your measurement.
Having an incorrect test fixture is another common cause of inaccurate tensile measurements. A fixture that is too large or that applies too much gripping force during tensile movement can cause the sample to fracture outside the specified gage length area. The test fixture should be sized to the sample’s expected load characteristics. Wedge-action test fixtures work well on ductile samples but tend to be less reliable on brittle materials, since they apply load onto the sample as axial loading increases. Brittle materials tend to test more consistently with pneumatically operated test fixtures that regulate the gripping force onto the sample.
Improper sample preparation can result in inconsistent and incorrect characterization. When testing to a specific international standard, the sample should be prepared to the prescribed dimensions. Force measurement applications will typically use the component in its finished state. Material testing, however, uses specially prepared specimens in various forms and shapes. Their cross-sections may be round, rectangular or square and they have a known standard gage length.
Testing velocity is another common cause of suboptimal tensile measurement. Where available, testing should be done in accordance to a recognized standard by ASTM, ISO, DIN or others. Test speed is clearly specified in these standards and will ensure a proper measurement is taken.
Tensile results may be significantly affected by temperature. Elasticity can decrease significantly as temperature increases. SMT
James Clinton is a product manager for Force and Material Test Products at The L.S. Starrett Co.