# Tensile Testing - Material Failure Prediction

• Corsan
In summary: And what is meant by failure? Yielding? Breaking? Excess deformation? Exactly what is meant by "we have been asked to look at material properites"? Not sure. Thanks for your help. Not sure. Thanks for your help.
Corsan
Hi all,
This is not really a homework problem, more of a 'what-if' scenario but I'm not sure which section to put it in.
For a science project we have been asked to look at material properites and come up with a project (which will have a presentation but that's another story).
I'm looking at tensile testing as we have one at our school and I'd like to use that as I think it will be an interesting project.

## Homework Statement

What I'd like to know if possible, is there a generic equation that I can use to predict the failure load of a material?
All the equations I've used require a set force, however tensile testing continually pulls the material (increasing load?) until failure.

## Homework Equations

Not sure. Shear flow?

## The Attempt at a Solution

As I mentioned it is a theoretical project for the time being so I haven't got anything to show.

Thanks for any help

Last edited:
Corsan said:
Hi all,
This is not really a homework problem, more of a 'what-if' scenario but I'm not sure which section to put it in.
For a science project we have been asked to look at material properites and come up with a project (which will have a presentation but that's another story).
I'm looking at tensile testing as we have one at our school and I'd like to use that as I think it will be an interesting project.

## Homework Statement

What I'd like to know if possible, is there a generic equation that I can use to predict the failure load of a material?
not really. Steel for example has different failure loads depending on its chemical composition and molecular structure
All the equations I've used require a set force, however tensile testing continually pulls the material (increasing load?) until failure.

## Homework Equations

Not sure. Shear flow?

## The Attempt at a Solution

As I mentioned it is a theoretical project for the time being so I haven't got anything to show.

Thanks for any help
You can look up the breaking strength of the material, say for a high strength steel, medium steel, hi strength aluminum rod, etc., and compare with the testing results. You should plot the stress versus strain or load versus extension during the tests for showing the behavior of the material as the load increases.

PhanthomJay said:
not really. Steel for example has different failure loads depending on its chemical composition and molecular structurecorrect, increasing load You can look up the breaking strength of the material, say for a high strength steel, medium steel, hi strength aluminum rod, etc., and compare with the testing results. You should plot the stress versus strain or load versus extension during the tests for showing the behavior of the material as the load increases.
Thanks for the response.

Taking it a level further, what about joints? e.g. a Lap Joint (riveted?). Is it possible to predict the yield force?

Last edited:
Corsan said:
Thanks for the response.

Taking it a level further, what about joints? e.g. a Lap Joint (riveted?). Is it possible to predict the yield force?
No. The rivets or bolts would fail in shear at the ultimate shear stress of the rivet or bolt. This again must be given as the property of the material from which the bolt or rivet is made. The strength of the bolt has nothing to do with the yield strength of the bar/rod.

PhanthomJay said:
No. The rivets or bolts would fail in shear at the ultimate shear stress of the rivet or bolt. This again must be given as the property of the material from which the bolt or rivet is made. The strength of the bolt has nothing to do with the yield strength of the bar/rod.

Hi,
So if I wanted to predict the failure load of the joint I would simply compare the shear stress of the rivet and the yield strength of the material?

Then it's just a case of making the joint and subjecting to load until the joint fails, the failure load should tie in with the above.
I think that's correct?

I thought you were looking at tensile stressing a rod, but now you've focused on joints which can get a bit difficult to analyze because shear stress in a bolt is just one method of joint failure; the joint could also fail by bearing pressure of the bolt on the steel, or tearout failure at the bolt hole, or failure of the rod itself. And what is meant by failure? Yielding? Breaking? Excess deformation? Exactly what is meant by "we have been asked to look at material properites"? .

PhanthomJay said:
I thought you were looking at tensile stressing a rod, but now you've focused on joints which can get a bit difficult to analyze because shear stress in a bolt is just one method of joint failure; the joint could also fail by bearing pressure of the bolt on the steel, or tearout failure at the bolt hole, or failure of the rod itself. And what is meant by failure? Yielding? Breaking? Excess deformation? Exactly what is meant by "we have been asked to look at material properites"? .
Hi,
I'll try and put some more meat on the bone.

I'd like to analyze a single lap joint, riveted together which is then put into a tensile testing machine and subjected (pulled apart) until failure.
By failure I mean breaking, as in the joint can hold no more force.

So would I then need to complete several equations?
From my research I found these failure modes of joints on a British website (http://www.roymech.co.uk/Useful_Tables/Rivets.html).

• Shearing through one section of the rivet (single shear).
• Shearing through two sections of the rivet (double shear).
• Compressive bearing failure of the rivet.
• Shearing of the plate(s) being joined.
• Bearing failure of the plate(s) being joined.
• Tearing of the plates between the rivets.

Also shown on that website is the equations for the above.
I believe it's a case of doing all of the calculations and the lowest force is the failure mode?

However, again I fall back to the issue that the equations require a force. Do I simply rearrange the equation so I can solve for force?
Thanks

Rivets went out of fashion over 50 years ago, having been replaced, for the most part, by high strength bolts.

Ideally, for economy of design, you'd like the joint to fail in bearing, tear out, and shear at the same time the steel member fails. But since you're looking at joint failure, be sure to oversize the member for tensile load, so that it is is the joint and not the member that fails. Bearing pressure failure by crushing may be difficult to identify, and shear tearout can be eliminated by proper edge distances and bolt spacings. You should calculate what is required for bolt/joint design to eliminate these failur modes, to ensure your failure will be by ultimate shear stress in the bolts. The greater the number of bolts, the greater will be the force required to fail them. Since bolts come in various diameters and various strengths, be sure to know the properties of the particular bolts you will use. Remember that the tensile force applied to the steel member is not the same as the force applied to the bolt. This must be calculated.

PhanthomJay said:
I thought you were looking at tensile stressing a rod.
Corsan: Would you be interested in tensile testing a rod? I think I would recommend it. It would be simpler, but still complicated enough, I would think.

## 1. What is tensile testing and why is it important?

Tensile testing is a method used to determine the mechanical properties of a material, such as its strength, ductility, and toughness. It involves pulling a material specimen until it breaks, while measuring and recording the force and displacement. This information is important for predicting the behavior of a material under different conditions and for selecting the appropriate material for a specific application.

## 2. How is a tensile test performed?

A tensile test involves preparing a specimen with a specific shape and dimensions, usually in the form of a dogbone or cylindrical shape. The specimen is then placed in a tensile testing machine and pulled at a constant rate until it breaks. The force and displacement data are recorded and used to calculate the material's mechanical properties.

## 3. What factors influence the results of a tensile test?

The results of a tensile test can be influenced by several factors, including the type and quality of the material being tested, the testing equipment and method used, the temperature and humidity during testing, and the specimen preparation and handling. It is important to control these factors to ensure accurate and reliable test results.

## 4. What can be learned from a tensile test?

A tensile test can provide valuable information about a material's strength, stiffness, ductility, and toughness. It can also indicate the presence of defects or imperfections in the material and help identify potential failure modes. This information is useful for designing and selecting materials for various applications and for understanding the behavior of materials under different loading conditions.

## 5. How is material failure predicted from a tensile test?

Material failure prediction involves analyzing the results of a tensile test and comparing them to the known properties and behavior of the material. By understanding the material's response to different levels of stress and strain, it is possible to predict when the material will fail under a given set of conditions. This information is essential for ensuring the safety and reliability of structures and products made from the tested material.

Replies
4
Views
10K
Replies
6
Views
2K
Replies
3
Views
1K
Replies
12
Views
2K
Replies
1
Views
4K
Replies
14
Views
1K
Replies
8
Views
11K
Replies
4
Views
1K
Replies
1
Views
6K
Replies
1
Views
2K