L.S. Starrett's L3 systems are an example of the tech you need to create and perform a test, analyze results and manage the data.Click image to enlargeby 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
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.

Here's a look at tensile testing data: The elastic limit (left) is the greatest amount of stress a material can take. The elastic modulus (right) represents stiffness under stress.Click image to enlarge

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.

Testing Instrumentation
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 strength (left) is the point of maximum stress on the stress-strain curve, shown here by the blue rectangle. Yield strength (right) is the point where stress causes a permanent set.  Click image to enlarge

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.

Look where you're going

by Kip Hanson

Automated vision machines measure parts quickly and accurately

If you have a handful of parts to check, you'll probably pick up a micrometer. If you're going to measure a truckload of them, only an automated solution will do. But measurement systems come in a variety of shapes and sizes, and offer more options than a new car. Deciding 

HR-530 Series Rockwell hardness testers

Mitutoyo Canada has introduced the latest Rockwell Hardness Testers in its HR-530 Series.

Crunching the Numbers

by Andrew Brooks

NDT comes out of the cold–and cuts data volumes down to size

Manufacturing spurs Canada's biggest job gains in 7 months

A surge in employment in the manufacturing sector helped Canada's economy gain 58,000 jobs in May, the biggest gain in employment in seven months, according to the latest figures from Statistics Canada.

Factory operations and machining control software

Okuma America Corp. has released Connect Plan, a software-based system for collecting and measuring data to adjust processes for increased productivity.

How will electric cars transform manufacturing?

If the global automotive industry and its suppliers are to satisfy changing market demands for a complex blend of powertrain options over the coming years, they will need to adopt a more flexible manufacturing model.

New laser tracker available in bundle offer

Hexagon Manufacturing Intelligence recently announced the launch of the Leica Absolute Tracker AT960-SR, a lower-budget addition to the AT960 range of laser trackers.

Next-level Efficiency

by Noelle Stapinksy

Enterprise resource planning (ERP) software helps manufacturers improve productivity and provides a clear view of an entire operation

Manufacturing: What’s Next?

by Noelle Stapinsky

Around this time last year, corporate CEOs and economists alike were throwing their arms up in the air as they watched all forecasts, economic analyses and five year business plans become obsolete.

Computer gauge connects to multiple devices

Marposs Corp. has updated its Merlin Plus embedded gauge computer, which now features six USB and two ethernet ports enabling connectivity to multiple data collecton devices and the ability to measure up to 250 characteristics.

A Material Difference

by andrew Brooks

As additive manufacturing matures, new processes, powers and powders enter the market

Robotic Metrology: Closer and Closer

by Andrew Brooks

Robotic metrology continues to make strides, but challenges remain

Reshoring for the Rebound

by Michael Ouellette

No one is sure how supply chains will change after coronavirus, but change will come

Drive Time?

by Noelle Stapinsky

In the face of a global pandemic, the questions facing Canada’s automotive supply sector will shape the industry for years to come

The Future Now

by Andrew Brooks

As computing technology becomes more powerful and pervasive, Industry 4.0 moves from sci-fi to reality

Stay In Touch

twitter facebook linkedIn