Understanding Mechanical Properties in Metal: Types of Strength

Strength is a foundational consideration in material selection and mechanical design. Components must possess sufficient strength to support expected loads without permanent deformation or catastrophic failure, while also balancing factors like weight, cost, manufacturability, and fatigue life.

Strength is the ability of a metal to withstand an applied load without failure or plastic deformation.

It reflects the material’s capacity to resist forces that attempt to deform or fracture it and is typically measured in terms of stress, which is force per unit area. Stress is typically represented in megapascals (MPa) or pounds per square inch (psi).

Use this table of contents to click ahead:

• Types of Strength
• Types of Failures
• Metal Strength Comparison Chart

Types of Strength

Strength is not a single mechanical property, but rather a general term that encompasses several specific types of resistance to different loading conditions, and is determined through mechanical testing such as tensile, compression, shear, or impact tests.

Tensile strength is the maximum stress a metal can withstand when being stretched or pulled before it breaks. It represents the peak of the stress-strain curve in a tensile test.

Yield strength, also known as yield point, is the stress at which a metal begins to deform plastically. Below this stress, the metal will return to its original shape when the load is removed. Beyond it, permanent deformation occurs.

Compressive strength is a metal’s ability to resist deformation and failure under compressive (pushing) forces. For ductile metals, compressive strength is often similar to tensile strength, though brittle materials may fail more readily under compression.

Shear strength is the resistance to sliding or shearing forces that are acting parallel to a part’s surface. This property is critical in applications involving fasteners or torsional loads.

Fatigue strength, also known as endurance limit, refers to the highest stress a metal can withstand for an infinite number of cycles without breaking under repeated or fluctuating loading.

Impact strength is a metal’s ability to resist sudden and forceful impact loads without fracturing. Often measured using standardized tests like the Charpy or Izod impact test.

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In addition to understanding the specific types of strengths found in metals, it's important to note the other factors that influence strength and why one material might be better than the other for your project.

A metal’s strength is also dependent on material composition, microstructure, temperature, strain rate, and loading conditions.

Higher strength doesn’t always translate to better performance across the board; it often comes with trade-offs like reduced ductility, increased brittleness, or poor machinability.

If your component calls for a metal with high strength, consider all the types of strength and determine what is most appropriate and relevant based on the part’s end use, the product’s overall application, the loading environment, and failure risks.

Types of Failures

Failure arises when a material is unable to fulfill its intended purpose due to deformation, cracking, or other types of damage related to the part's strength.

Tensile Failure

Tensile failure occurs when a material is subjected to a stretching force that results in breaking or fracturing. This mode of failure involves the material elongating and ultimately separating under stress.

Tensile failure occurs when a material is subjected to a tensile force that exceeds its ultimate tensile strength, causing it to break. This is exemplified in tensile testing, where materials are pulled apart to observe their behavior until they fail.

In a typical tensile test, ductile materials exhibit a "neck" formation as they elongate and locally thin before fracturing. At this neck, the material's cross-section diminishes until it ultimately breaks. Conversely, brittle materials do not exhibit necking; they fracture abruptly with little deformation.

Tensile failure in structural components results in elongation and eventual breakage. In ductile materials, this type of failure is preceded by considerable stretching, while in brittle materials, it happens suddenly. This mode of failure significantly affects load-bearing structures like bridges, cables, and pressure vessels.

Compressive Failure

Compressive failure occurs when materials are subjected to compressive forces beyond their capacity, leading to deformation or collapse. The mode of failure is influenced by the material's ductility or brittleness.

Compressive forces can lead to different types of failure. Ductile materials may experience buckling or barreling, whereas brittle materials are prone to cracking, crushing, or shattering. The nature of compressive failure depends on the material's characteristics and its response to axial stress.

During compression, ductile materials typically expand sideways, exhibiting "barreling" as a result of the Poisson effect. Conversely, brittle materials tend to crack or shatter upon reaching their compressive threshold. Additionally, columns or walls may buckle if the design is inadequate to bear the applied load.

Compressive failure greatly affects elements such as columns, pillars, and structural supports. This type of failure can result in the collapse of whole structures, posing significant safety hazards in buildings, bridges, and pressure vessels.

Shear Failure

Shear failure occurs when forces induce one section of a material to slide against an adjacent section, generating a shear force that results in material separation. This type of failure is prevalent in beams, joints, or bolted connections under substantial loads.

Shear failure occurs when forces act parallel to the material's surface, leading it to slip or slide over adjacent surfaces. Failure happens when the material's ultimate shear strength is exceeded.

Shear failure is evident in structural joints or beam cross-sections, especially where rivets or bolts connect materials. When subjected to heavy loads, the material may fail along the shear plane, leading to the separation of the connected parts.

Structural joints or connections are susceptible to shear failure, potentially causing the separation of vital components. In large structures, shear failure can lead to disastrous collapses if key load-bearing elements are unable to withstand the shear forces.

Buckling

Buckling is a type of failure that happens when slender structural components, like columns or beams, are exposed to compressive forces and bend sideways. This failure mode is particularly hazardous as it can happen abruptly and without prior indication.

Buckling takes place when a structural element is exposed to compressive forces surpassing its critical buckling threshold. Elements such as boundary conditions, geometric flaws, and slenderness ratios significantly influence a member's vulnerability to buckling.

Buckling frequently occurs in slender columns or plates subjected to compressive forces, causing the member to deform laterally and lose its ability to bear loads. The member may bend, twist, or buckle in a manner that undermines its structural integrity.

Buckling results in instability within structural components, potentially leading to abrupt collapses. It is a significant issue in tall structures, thin-walled pressure vessels, or bridge supports, where a loss of stability may result in catastrophic failure.

Fatigue Failure

Fatigue failure happens when a material experiences repeated or varying loads over time, even if these loads are beneath the material's yield strength. Small cracks develop and expand over numerous cycles, eventually causing the material to fracture.

Fatigue failure occurs due to cyclic or fluctuating loads exerted on a material over time. These repeated stresses, even when below the material's yield strength, can initiate and propagate micro-cracks, progressively weakening the material with each cycle until it ultimately fractures.

Fatigue failure is identified by the presence of "beach marks" on the fracture surface, indicating the advancement of crack growth through numerous load cycles. The fracture surface may reveal distinct areas where cracks began, spread, and eventually led to the final breakage.

Fatigue failure poses a significant risk as it can happen even in materials that appear to be functioning within safe stress thresholds. Components exposed to cyclic loading, including aircraft parts, machinery, and bridges, are vulnerable to fatigue failure if cracks are not identified and addressed promptly.

Impact Failure

Impact failure occurs when a material is subjected to a sudden and forceful impact or blow, leading to its inability to withstand the applied stress. This type of failure can manifest in two primary forms: ductile or brittle.

The nature of the failure—whether it is ductile, characterized by significant deformation before fracture, or brittle, where the material fractures with little to no prior deformation—depends significantly on the rate at which the load is applied and the inherent properties of the material.

Factors such as the material's toughness, temperature, and microstructure play crucial roles in determining how it will respond to impact forces. In engineering applications, understanding the conditions that lead to impact failure is essential for selecting materials that can endure sudden loads without catastrophic failure, ensuring the reliability and safety of the components in their operational environments.

Metal Strength Comparison Chart

The comparison chart below ranks commonly used CNC machining metals from strongest to weakest by tensile strength and shows typical property values. Units are MPa.

Please note, these values are general guidelines used for the sake of comparison and education. Consult your machining partner or request to review the data sheet for the materials in their inventory.