Materials Science · Units, loads, stress and strain
Applied force and stress over an area
Useful background: Materials, questions and levels of evidence. Each worked case explains the quantities it uses.
Back to the chapterAfter reading, you can
- Calculate average engineering stress and distinguish it from force.
Pulling a strip
Two strips may carry the same pull but have different cross-sectional areas. Force, measured in newtons, describes the applied interaction. Average engineering stress divides the axial force by the original area perpendicular to it: σ = F/A₀. It describes force per area, not the total load.
Use the stated geometry
One pascal is one newton per square metre; one N/mm² is one MPa. This average is useful in a uniform axial region. Near a hole or grip, local stress can differ. The formula does not identify a strength limit unless the selected load corresponds to a defined endpoint.
Worked example
Work through the case
A strip with original area 5 mm² carries 250 N. What is its engineering stress?
- Use original area: 5 mm².
- Divide 250 N by 5 mm².
- Convert 50 N/mm² to 50 MPa.
50 MPa average engineering stress.
Try the reasoning
Double area while keeping force fixed. What happens to stress?
Show the answer and reasoning
It halves.
Area is the denominator.
Does a calculated 50 MPa prove the strip has yielded?
Show the answer and reasoning
No.
Yield requires evidence for the material response at that stress.
What to carry forward
- Force and stress answer different questions.
- State original area and the region where an average is meaningful.
References
Teaching resources and research supporting this lesson’s explanations and examples.