Short Answer:

The breaking stress is the stress developed in a material at the time of its fracture or breaking point. It represents the load per unit original area when the material finally fails during a tensile test. This stress indicates the maximum load a material can sustain before it completely breaks apart.

In simple words, breaking stress shows the final strength of a material before failure. It is usually slightly less than the ultimate stress because, before breaking, the specimen becomes thinner (necks), reducing its actual cross-sectional area.

Detailed Explanation:

Breaking Stress

The breaking stress is the stress corresponding to the point of fracture on the stress-strain curve. It is defined as the ratio of the breaking load (the load at which the material finally fails) to the original cross-sectional area of the specimen. This value indicates the maximum stress a material can handle before it ruptures.

Mathematically, it is expressed as:

Where,

= Breaking stress (in N/m² or MPa)
= Breaking load (maximum load at fracture)
= Original cross-sectional area of the specimen

Breaking stress is a key property that helps in understanding the failure behavior of materials. It is an important consideration in designing structures and machine components that are expected to bear heavy loads safely without failure.

Behavior of Material up to Breaking Stress

When a material is subjected to a gradually increasing tensile load, it passes through several stages before reaching the breaking point:

Elastic Stage:
The material first behaves elastically, meaning it returns to its original shape when the load is removed. The relation between stress and strain is linear, following Hooke’s Law.
Plastic Stage:
Once the elastic limit is crossed, the material enters the plastic stage. Here, permanent deformation begins, and the material will not return to its original shape when unloaded.
Ultimate Stress Point:
As the load continues to increase, the stress also rises until it reaches a maximum value called ultimate stress. Beyond this, necking starts, and the cross-section at one region reduces.
Breaking Point:
Finally, when the load exceeds the capacity of the necked region, the material breaks. The stress calculated at this stage is known as the breaking stress. This point represents complete material failure.

The breaking stress is generally less than the ultimate stress, as the load decreases slightly after necking begins, even though the actual stress (considering the reduced area) may be higher.

Importance of Breaking Stress

Determines Failure Limit:
Breaking stress gives the maximum load-carrying capacity of the material before fracture. It helps in predicting when failure will occur.
Basis for Material Testing:
It is one of the main results obtained from a tensile test, helping engineers understand the mechanical behavior of materials under stress.
Design Safety:
By knowing the breaking stress, engineers can calculate safe working stresses using appropriate factors of safety to ensure components do not reach failure conditions during use.
Material Selection:
Materials with higher breaking stress are preferred for components that must withstand large loads, such as bridges, shafts, and aircraft parts.
Quality and Strength Control:
Breaking stress is used to ensure that materials meet strength standards and perform reliably under load conditions.

Factors Affecting Breaking Stress

Material Type:
Metals, alloys, and non-metals have different internal structures that determine their breaking stress. For example, steel has a higher breaking stress than aluminum or copper.
Temperature:
As temperature increases, most materials tend to soften, leading to a decrease in breaking stress. At low temperatures, materials may become brittle and fail at lower loads.
Heat Treatment:
Processes such as annealing, tempering, and quenching modify the grain structure and directly affect the breaking stress of the material.
Defects and Impurities:
Internal cracks, inclusions, and voids reduce the breaking stress because they act as stress concentration points.
Rate of Loading:
If the load is applied suddenly, the material may exhibit higher apparent breaking stress due to its limited time for deformation.
Surface Condition:
Rough or corroded surfaces reduce the breaking stress since they provide easy points for crack initiation.

Relationship Between Breaking Stress and Ultimate Stress

The ultimate stress is the maximum stress a material can withstand before necking begins, whereas breaking stress is the stress at the instant the material finally fractures.

During the tensile test, after reaching ultimate stress, the specimen starts necking, and the load starts decreasing until the material breaks. Therefore, the breaking stress is slightly less than the ultimate stress because the area reduces during necking.

However, if the stress is calculated based on the actual area at fracture (not the original area), the true stress at breaking may be greater than the ultimate stress, but usually, the nominal value (based on the original area) is used in design calculations.

Practical Applications of Breaking Stress

Machine Design:
Engineers use breaking stress to ensure that machine parts like shafts, rods, and bolts do not reach their failure limits under maximum working loads.
Civil Engineering:
In structural design, materials like concrete and steel are tested for their breaking stress to guarantee safety and durability of bridges, buildings, and dams.
Material Research:
Breaking stress testing helps in studying new materials, composites, and alloys for specific applications like aerospace or automotive industries.
Safety and Reliability Evaluation:
Knowing the breaking stress allows engineers to plan proper maintenance schedules and safety inspections for load-bearing structures.

Conclusion:

The breaking stress represents the final strength of a material — the stress at which it fails completely under loading. It defines the material’s capacity to resist fracture and provides essential information for safe and efficient engineering design. Understanding breaking stress ensures that structures and machines are designed with proper safety margins, avoiding sudden or catastrophic failures during operation.