What is Torsional Stress?
Torsional stress is one of four common loading mechanisms along with compression, tension, and shear. Torsional stress differs from these because the load is a torque load that is applied transversely rather than longitudinally.
Torsional stress is a form of shear stress experienced by a body when a twisting force is applied. An example of torsional stress can be seen in a car’s axle.
A car’s engine/gearbox will attempt to turn the car’s axle, while the wheels on the other end of the axle will resist the turning. This will cause torsional stress about the center point of the axle.
It is important to understand the torsional physics applied to a load-bearing shaft, or body, to predict the shaft performance.
Torsional stresses may cause elastic or plastic deformation to a shaft, and so shafts must be designed to properly resist torsional stresses.
What is Torsional Strength?
Torsional strength is different from that of torque. Torque is the force that is capable of producing an acceleration whereas torsion describes the twist formed in a body due to torque.
Torsional strength is the Measure of the ability of a material to withstand a twisting load. It is the ultimate strength of a material subjected to torsional loading, and is the maximum torsional stress that a material sustains before rupture. Alternate terms are modulus of rupture and shear strength.
How Torsional Stress works?
Torsional stress works in much the same way a bending moment works by applying a force at a distance along a lever arm.
The only difference is that in a bending moment, the force is applied parallel to the axis. In torsional stress, the force is applied perpendicular to the axis of rotation. This results in a twisting force being exerted.
Torsional stress can be caused by a force couple acting about the center point of a shaft. A couple is two forces that act at the same distance from the point of rotation in an opposite and equal manner.
Examples of a force couple causing torsional stress can be found in anything that rotates including car axles, gear shafts, drills, mandrels, and wind turbine generators. In all of these examples, torsional stress affects the object along the whole length of its axis.
One last thing to note is torsion does cause shear stress. This is because torsional stress is not applied uniformly between the acting point of the force and the point of rotation.
Instead, the force applied starts at zero at the point of rotation and gradually increases along the radius to the acting point of the force.
Torsional Stress Vs Normal stress. What’s the Difference?
Torsional stress may be defined as that shear stress which acts on a transverse cross-section that is caused by the action of a twist. Normal stress and normal strain (which are caused by tension and compression of a member) occur when a force is applied perpendicular to an area.
Normal stress, such as compression or tension, acts normally on the cross-section of an object. Torsional stress, however, works perpendicular to the cross-section about a center point. For compression and tension to occur, both forces must act concentrically.
Torsional Stress Formula
Torque on a shaft is the main cause of shear stress. The torsion induced when torque is applied to a shaft causes a distribution of stress over the shaft’s cross-sectional area. Mathematically, Torsional stress
τ = Tr/J,
Where:
- τ = Torsional stress
- T = Transmitted torque
- r = Distance from the center of rotation
- J = Polar moment of inertia area
Where =Torque applied, radius of cross-section, polar moment of inertia.
The radius is measured in meters and the units for torsional stress are given in newton meters or pascals (Pa).
To calculate the torsional stress, the torsional stress formula must be used. Firstly the value for the variables: the distance between the applied load and the center of rotation, the torque being transmitted, and the polar moment of inertia area must all be identified.
Secondly, the values need to be substituted into the torsional stress equations. Then the formula will lead to a value for the torsional stress in pascals.
There are a few assumptions that are required to be true to allow the torsion equation to effectively analyze the torsional stress of an object.
The assumptions are listed below:
- The material must be the same throughout the body.
- The load should be uniformly distributed along the shaft axis.
- The torque must not exceed the elastic deformation of the material.
- The shaft cross-section has to be circular.
- The length of the shaft must not change during loading.
Torsional Stress Test and How It Performed?
What is Torsional Stress Test?
A torsion test is a mechanical testing method that evaluates the properties of materials or devices under stress caused by angular displacement.
During a torsion test, a specimen is subjected to a twisting or torsional force, which induces a torque.
Torsion testing can be split into two distinct categories:
- Testing raw materials like metal wires or plastic tubing to determine properties such as shear strength and modulus, or
- Functional testing of finished products subjected to torsion, such as screws, pharmaceutical bottles, and sheathed cables.
The most common mechanical properties measured by torsion testing are modulus of elasticity in shear, yield shear strength, ultimate shear strength, modulus of rupture in shear, and ductility.
Torsion testing provides valuable information about a material’s ability to resist deformation when subjected to torsional loads.
How to perform a torsion test?
Typically, a longitudinal sample is placed in a torsion tester and one end of the sample is twisted around the long axis until failure, during which the force, or in the case of rotation the torque, and the displacement, or in the case of rotation the angular displacement, are recorded.
Performing a torsion test is a crucial process in materials testing, helping engineers and scientists understand a material’s response to twisting forces. To conduct a torsion test, follow these steps:
#1. Sample Preparation: Begin by preparing a cylindrical sample of the material you want to test. Ensure it has a uniform diameter and length to obtain accurate results.
#2. Fixturing: Secure the sample in a testing machine equipped with specialized grips designed for torsion. One end of the sample is fixed while the other is free to rotate.
#3. Measurement Setup: Connect sensors to measure torque (twisting force) and angular displacement. These sensors record data throughout the test.
#4. Loading: Gradually apply torque to the sample. The twisting force induces angular deformation in the material.
#5. Data Collection: Continuously monitor and record torque and angular displacement data. This information helps in understanding the material’s behavior under torsion.
#6. Analysis: After completing the test, analyze the data to determine the material’s shear modulus, torsional strength, and other relevant properties.
#7. Reporting: Summarize the results in a clear report, including the testing conditions, equipment used, and the material’s performance.
What is the purpose of Torsional Stress?
Many products and components are subjected to torsional forces during their operation. Torsion testing is necessary when engineers wish to change or update the materials used in these products.
For example, the metal used in vehicle drive trains experiences a complex combined loading when in use, with torsion being the main component.
An engineer trying to design a more fuel-efficient vehicle may need to change the material of the driveshaft in order to reduce vehicle weight.
Torsional testing can help the engineer identify an appropriate material that will possess the required torsional strength while also contributing to the goal of light weighting.
The scope of torsion testing extends beyond raw materials; it encompasses finished products as well.
Biomedical tubing, switches, fasteners, and an array of other devices confront torsional stresses in their daily service.
By subjecting these products to torsion tests, manufacturers simulate real-life service conditions, scrutinize product quality, verify designs, and ensure the adequacy of manufacturing techniques.
Types of Torsional Stress Test
Torsion tests can be performed by applying only a rotational motion or by applying both axial (tension or compression) and torsional forces.
Types of torsion testing vary from product to product but can usually be classified as failure, proof, or product operation testing.
- Torsion Only: Applying only torsional loads to the test specimen.
- Axial-Torsion: Applying both axial (tension or compression) and torsional forces to the test specimen.
- Failure Testing: Twisting the product, component, or specimen until failure. Failure can be classified as either a physical break or a kink/defect in the specimen.
- Proof Testing: Applying a torsional load and holding this torque load for a fixed amount of time.
- Functional Testing: Testing complete assemblies or products such as bottle caps, switches, dial pens, or steering columns to verify that the product performs as expected under torsion loads.
Popular torsion testing applications
Torsion testing finds widespread applications in multiple industries. Some key applications include:
- Wire, Metals, Tools
- Plastics, composite materials
- Screws, Fasteners, Rods, shafts
- Torsion Springs
- Biomedical Products
- Automotive Components
- Aerospace Components
- Calibration of torque sensors
Most common standard torsion tests
- ASTM F543 – Axial and Torsion Testing of Bone Screws
- ASTM A938 – Torsion Testing of Metallic Wire
- ISO 7800 – Torsion Testing of Metallic Wire
- ISO 594/ISO 80369 – Testing of Conical Luer Fittings
- ISO 7206 – Endurance and Fatigue Testing Artificial Hip Implants