A batch of titanium aerospace fasteners failed to be accepted for a single test in 2023. The supplier ran through whole procedure, but everything tight. The grips were tight. The crosshead speed matched the standard. Yield strength readings differed 8% between identical specimens. Bad operator error? No, bad machine compliance. The machine error was crosshead displacement instead of using a contact extensometer; thus the frame deflection would mess up just about any data.
On the face of it, the mere act of applying some tensile loading on a specimen until its break is referred to as tensile testing. However, this may often go astray, and there are different physics associated with the grip and pull that distinguish a reliable result from one that is deceiving. Working on the line of “grip and pull,” the tensile tester is subject to interconvert precision measurement of forces, simultaneous measurement and tracking of deformation, and real-time conversion of mechanical behavior to stress-strain curves that explain material performance.
This guide will give you a better understanding on the issue of how a universal testing machine works in measuring force and elongation of a material, how load cells and extensometers work hand-in-hand, and what hidden errors could refute your results. You will also be given a framework as to when the cross-head travel is accurate and when an extensometer must be attached.
Want to learn more? See our (guide on universal testing machine).
What Is a Tensile Testing Machine?
A tensile testing machine is an accurate test apparatus for controlled uniaxial tension along a standard specimen, thus rendering the latter to fracture due to stretching. This machine also measures the applied force and elongation of the specimen while plotting a stress-strain curve. Ultimate tensile strength, yield strength, elastic modulus, & elongation (as a percentage) are the most important mechanical properties that can be calculated from the curve.
Most modern tensile testers are actually universal testing machines (UTMs) that operate in tensile mode. By changing grips, fixtures, or a combination thereof, the same test frame can double as a compression, bending, tearing, etc., tester. With materials ranging from steel rebar for general building construction to polymer film for medical purposes, one takes the tensile test machine stand as the primary means to determine how a material behaves under tension.
The Physics Behind Tensile Testing
Mechanical principles beg, and the physical laws stand as truths behind the machine’s working. The knowledge of these is what sets apart the mere routine operators from an advanced operator who can identify troubles and justify data.
Newton’s Second Law and Controlled Force Application
The machine is not “tugging” at anything. It controls the speed at which the sample is stretched. The crosshead moves at a regular speed and under controlled conditions, by either the servo motor, ball screw mechanism in electromechanical systems or the hydraulic piston in servo-hydraulic systems. This controlled extension imposes an opposing force on the sample.
The force is not programmed directly. Whenever a material is pulled, the force is generated by the material itself. Softer materials develop lower forces as compared to high-strength alloys, as the latter generate extreme forces. The machine very precisely quantifies this reaction.
Hooke’s Law and the Elastic Region
Hooke’s Law states that the stress developed within the elastic region is directly proportional to the strain. This relationship implies that σ = E Χ ε, with σ denoting the stress, ε denoting the strain and E representing the Modulus of Elasticity or the Young’s Modulus. E is the slope of the initial linear portion of the stress-strain curve.
Beyond the yield strength is a point of irreparable permanent deformation. Even when unloaded, the specimen will not return to its original length. A tensile test records the entire path from elastic loading to fracture.
From Force and Displacement to Material Properties
The machine measures two raw variables:
- Force (F): Measured by the load cell in newtons (N) or pounds-force (lbf)
- Displacement (ΔL): Measured by the extensometer or crosshead position sensor in millimeters (mm) or inches (in)
From these, the software calculates:
- Stress (σ) = Force / Original Cross-Sectional Area (F / A₀)
- Strain (ε) = Change in Length / Original Gauge Length (ΔL / L₀)
These calculations produce the stress-strain curve. Automated algorithms then identify the yield point, the peak stress (UTS), and the fracture point.
Core Components and Their Roles
Every measurement criterion in a tensile test entirely rests on the interaction of four fundamental subsystems; if one of them is misaligned or poorly calibrated, the test shall be worthless.
The Load Frame and Crosshead
The load frame is the backbone of the structure with respect to the drive system, load cell, and grips. Stiffness of the frame is essential for accuracy as any deflection in the frame would be falsely recorded as the elongation of the specimen if being measured by the displacement of the crosshead.
The crosshead moves with the upper grip away from the lower grip during the tensile test. The descent speed is programmable and must be set to a specific standard for your material. Both ASTM E8 for metals and ASTM D638 for plastics cover speed ranges that are acceptable.
The Load Cell: How Force Is Measured
Load cell is a transducer that changes mechanical force to an electrical signal. Inside it, an elastic element is highly precisely machined that deforms under load. The electrical resistance of the strain gauges fixed to this element changes proportionally with the deformation.
These strain gauges are installed on a Wheatstone bridge circuit wherein the bridge comes to be unbalanced if there are resistance changes. A voltage signal generated is then amplified, digitized and ledgered into a force reading.
Load cells are classified as per the accuracy requirement of ISO 7500-1:
- Class 0.5: ±0.5% accuracy for high-precision R&D and calibration labs
- Class 1: ±1.0% accuracy for standard quality control
Regular calibration to the certified reference weights shall provide traceability.
Grips and Fixtures: Holding the Specimen
Holding a specimen in clamp must be so perfected as to prevent slipping or influence micro stress. The main types of clamps are defined thus:
- Wedge clamps: These auto-tightened by applied stress and are now being used against the specimens
- Pneumatic clamps: Preferably, these apply uniform stress and secure alignment, especially in cases of thin films, textiles, etc.
- Hydraulic clamps: These can clamp down with force suitable for all kinds of structural steel or the other excellent varieties of reinforced steel
The selection of a clamp would directly affect the validity of the test itself. Thus, any slipping of the specimen in the clamp would decrease the force reading, leaving the stress distribution distorted at the early point of the relationship. At the same time, misalignment might distort the axial tension data substantially due to bending moment.
The Extensometer: When Displacement Alone Is Not Enough
An extensometer measures the actual deformation of the specimen’s gauge length, independent of the machine frame. This matters because the load frame itself stretches slightly under high load. This phenomenon is called machine compliance.
When should you use an extensometer?
| Situation | Use Extensometer? | Why |
|---|---|---|
| High-stiffness materials (metals, ceramics, composites) | Yes | Machine compliance error can exceed 2-5% |
| Materials with low elongation (<5%) | Yes | Small errors in displacement produce large strain errors |
| Flexible materials with high elongation (rubber, some plastics) | Optional | Crosshead displacement is often accurate enough |
| ASTM E8 / ISO 6892-1 compliance testing | Usually required | Standards specify direct strain measurement for modulus |
| Quality control with generous tolerances | Optional | Crosshead displacement may be acceptable |
An optical video extensometer is best placed within high-temperature, brittle, or advanced-specimen testing. A contact extensometer, on the other hand, clips directly to the specimen and offers the finest possible precision in ambient-temperature conditions.
Control System and Software
Modern tensile testers have closed-loop PID controllers to ensure that set crosshead speed is maintained no matter how much the material resists. The force/displacement relation is kept synced, stress-strain curves can be viewed in real-time, and the following will all be calculated from the given data:
- Young’s Modulus
- Yield strength (0.2% offset or proof stress)
- Tensile and extension properties
- Percentage elongation at failure
- Reduction in area
While reporting can happen in PDF format or raw CSV files, the supporting exports can be sent to centralized LIMS in mission-critical facilities.
For details on UTM components that apply to compression machines, see our (guide on UTM machine components).
Step-by-Step Tensile Testing Procedure
The procedure for tensile testing consists of a series of consecutive steps. Significant deviations from these steps may introduce errors, which later are very difficult to detect.
Step 1: Specimen Preparation
The sample is machined to a certain standard geometry. Some common shapes include-
- Dog-bone or dumbbell: this configuration ensures that the specimen will break within the gauge length.
- Parallel-sided strip: a common shape for films and textiles.
- Threaded-end or button-end: metals that require alignment fixtures.
The cross-sectional area (A₀) and gauge length (L₀) must be measured accurately to a certain degree of precision using calipers or micrometers. For example, ASTM E8 allows an error in area determination of ± 0.5%.
Step 2: Mounting and Alignment
The specimen is clamped in the upper and lower grips. It is extremely important that the specimen only be aligned. If the specimen is misaligned or crooked, there will be some bending strain, which will hinder the pure axial tension. The risk is minimized by using alignment fixtures and self-aligning grips.
Utilizing a minor pre-load traditionally around 1-2% of estimated maximum force eliminates grip-interface slackness and ensures uniform perfection and consistency.
Step 3: Parameter Input
The operator selects the test standard and enters parameters:
- Crosshead speed: 5 mm/min until fracture for plastics per ASTM D638; 0.5 to 50mm/min for metals per ASTM E8, depending on material and specimen geometry
- Sampling rate: 50 Hz for ductile materials; 1000 Hz or faster for brittle materials;
- Stop condition: drop in force value to 10% of the peak or the specimen breaks
These are stored as reusable test methods in modern thermal analysis systems.
Step 4: Executing the Test
The crosshead will move at the programmed constant speed. The force-measuring transducer within the load cell reads the force applied by the grip system. The extensometer would be utilized to provide feedback on gauge-length elongation sensibly; however, the software is now capable of displaying the curve in real time.
For brittle materials like ceramics or cast iron, failure occurs suddenly with little plastic deformation. The specimen neck becomes thinner gradually in ductile materials, like mild steel or in some of the polymers and few other materials before ultimate fracture.
Step 5: Data Analysis and Reporting
After the fracture, the software calculates the various mechanical properties:
- Young’s modulus : Slope of the linear elastic range
- Yield Strength : The stress at which deformation becomes permanent (or 0.2% offset)
- UTS: The maximum stress reached during the test
- Elongation: (Final gauge length − original gauge length)/original gauge length × 100%
The report contains the raw data, the stress-strain curve, and the decision of the test as pass or fail, provided the limits were set while programming.
Understanding the Stress-Strain Curve
A stress-strain diagram offers the most important piece of information following tensile testing in general. Almost all the information regarding the mechanical behavior of the material in tension can be gathered with such a diagram.
The Elastic Section
When the stress acting on it is low, the behavior of a specimen is governed by linearity, as shown by stress-strain behavior. The slope of this line is called the Young’s modulus (E) and serves as a measure of stiffness: when the slope is very steep, the material is stiff (steel), and vice versa is true for a low slope.
The specimen here shows complete recovery of the length back to its original length if the load is removed. The proportionality limit marks the end of the linear part of the curve. The elastic limit defines the end of an irreversible deformation. For most engineering purposes, these two points are looked upon as the same.
The Yield Point
Yield strength is the stress beyond which a material yields permanently. In materials with distinct yield points (such as mild steel), the curve abruptly drops or elbows. In materials without identifiable yield points (like aluminum alloys or many plastics), the 0.2% offset is used: the offset line, parallel to the initial stage, is drawn from 0.2% strain to intersect with the defining yield point on the stress-strain curve.
The Plastic Region and Ultimate Tensile Strength
During the formation of the yield point, the material becomes permanently deformed. It rises until the ultimate tensile strength is reached, the maximum resistance to stress which the material can withstand. Necking lags a bit behind the maximum strength. Necking, the local reduction in the cross-sectional area which gives the appearance of a drop in the engineering stress, begins at the ultimate tensile strength. It increases the actual stress even onward as engineering stress is the stress based on the original area.
Fracture Point
A fracture is complete when the specimen breaks into two pieces. The total strain at fracture is usually stated as a percentage, which reflects the preexisting elongation. Typically, ductile materials elongate 20% or higher, while brittle materials break at less than 2% elongation.
The extent of reduction in area measures the extent of the diminish in the cross-section caused by the fracture. This is another good measure of ductility.
Common Errors and How to Avoid Them
Data from tensile tests could be null even if carried out exactly due to five prevalent mistakes. Detecting these mistakes and trying to prevent them comprises the fulcrum of the validity of tests.
1. Grip Slippage
The wedging or wane grip is a location where the specimen is slipping back and forth through the grip, whereas it should be stretching. This allows the force reading to obfuscate any proper reading, albeit typically reading low, and will lay an artificial plateau in the stress/strain curve.
Suggestions: Yamaha-style grips and mating serrated inserts are suitable for virtually all soft specimens like polymers; jaws are for hard specimens like steel alloy. Any imposed condition must prevent slippage.
2. Specimen Misalignment (Bending Strain)
If a specimen is not mounted exactly perpendicularly to the load direction, it turns and starts getting some bending strain. This sets off premature yielding on its one side and creates scatter in data for the yield strength.
Solutions: Use alignment fixtures or self-aligning grips. Visually check the straightness of the fixture against a straightedge before each test. The procedure for alignment verification as given in ASTM E1012.
3. Machine Compliance
Machine compliance refers to the elastic deformation of the test frame, load cell, and grips during testing. For strain measurement involving crosshead displacements, the adds from the frame deflection and the specimen stretching. This error can be significant, about 1-5% for stiff materials.
Solutions: The use of direct contact extensometer to test the modulus and strength for extension. When lacking a proper extensometer, a compliance correction curve is advisable.
4. Incorrect Crosshead Speed
Crosshead speed can drastically affect the resultant mechanical properties of materials sensitive to strain rate. Polymers are commonly known to have this characteristic. A 10x increase in speed can increase measured yield strength for some plastics by more than 10 to 20%.
Solutions: Always run the test at the recommended crosshead speed per the standard criteria, and ensure that speed is recorded in every test report.
5. Inadequate Data Sampling Rate
All of a sudden and at once brittle materials will fracture. Therefore, any data sampling system incapable of recording at the required speed (which may be as fast as 1 kHz or, in some cases, as slow as 0.01 Hz) will miss out on detecting the peak load.
Solution: Use a sampling rate of at least 1,000 Hz for brittle materials. A range of 100-500 Hz is suitable for most metal-type materials and ductile plastics.
Upon assuming the responsibilities of the role of Quality Assurance Manager with an automotive plastics supplier in Brazil in 2022, Maria Santos discovered that tensile strength values ranged by 15% for the same batch of polypropylene when tested at the laboratory. She suspected the material cause. She found three faults among the test setups: the pneumatic grips were set to the same pressure for all specimen thicknesses; crosshead speed varied between 5 and 50 mm/min according to the operator performing the test; no extensometers were used to determine modulus.
The standardized crosshead speed and grip pressure through material thickness, locked the same speed through the elimination of ASTM D638, and added an extensometer for modulus testing in the form of a clip-on that could be easily mounted on the test specimen. Within one month variability was reduced to 3%. It was never a material problem, it was always a measurement system problem.
ASTM and ISO Standards for Tensile Testing
Compliance with international standards ensures that your test results are accepted by customers, regulators, and accreditation bodies worldwide.
Metals
- ASTM E8 / E8M: Tension testing of metallic materials at ambient temperature
- ISO 6892-1: Metallic materials, tensile testing at ambient temperature
- ASTM E21: Elevated temperature tension tests for metals
Plastics and Polymers
- ASTM D638: Tensile properties of plastics
- ISO 527: Plastics, determination of tensile properties
- ASTM D1708: Tensile properties of plastics by use of microtensile specimens
Composites
- ASTM D3039: Tensile properties of polymer matrix composite materials
- ISO 14125: Fibre-reinforced plastic composites, determination of flexural properties
Textiles, Films, and Packaging
- ASTM D5034: Breaking strength and elongation of textile fabrics (grab test)
- ASTM D882: Tensile properties of thin plastic sheeting
- TAPPI T494: Tensile breaking properties of paper and paperboard
Machine Accuracy
- ISO 7500-1: Verification of static uniaxial testing machines, tensile/compression testing machines
- ASTM E4: Practices for force verification of testing machines
Always verify that your machine’s load cell and extensometer calibration certificates are current and traceable to recognized national standards.
Need to understand safety standards for the universal testing machine? Review our complete (astm iso standards universal testing machine guide.)
Conclusion
The working principle of the tensile testing machine is simple yet elegant, all while demanding highly precise actions. A machine employs a force on a sample as it pulls it, and force and elongation are measured when this experiment takes place. This quality of data being recorded relies completely on understanding the cell physics, history, gripper selection, alignment, and crosshead speed control.
These are the five main takeaways:
- Force measurements operate electrically and not mechanically: Strain gages in the load cell amplify the deformation into a voltage signal within a Wheatstone bridge.
- Crosshead displacement is not always the same as specimen elongation: The presence of machine compliance means you need to use an extensometer for a material of high stiffness or low elongation.
- The need to pick the right pair of grips to prevent slipping: Choosing the wrong grips creates a false plateau on the stress-strain curve.
- it’s all about the speed for the polymers: Optimal crosshead speed would be the competitor with the standard yielding like results.
- The pitfalls of sampling rate when analyzing brittle materials: Too low a sampling rate will miss the peak load at break.
David Chen, who works as a materials engineer for an electronics company in Shenzhen, and his team applied these same principles in 2004 while testing the flexible circuit board. Their action included the substitution of pneumatic grips for the wedge grips, installing an extensometer with video feed for the weak repairs, and sticking to ASTM D858 minutes. Within the lab, using these specific techniques diminished the test fluctuations from a shocking figure of 12% to a new figure of 2.5%. The data was so magnificent to carte blanche an agreement for supplying for the major brand of a smartphone.
Understanding the way the machine works is the first step toward trustworthy data.
