In everyday conversation, “strength” and “toughness” are often used interchangeably to describe a material’s ability to withstand a challenge. However, in materials science, these two properties are fundamentally different concepts describing distinct mechanical behaviors under stress. Strength relates to the force a material can tolerate before it fails, while toughness is a measure of the energy a material can absorb before fracturing. Understanding this separation is necessary for engineers selecting materials for everything from skyscraper supports to consumer product casings.
Defining Material Strength
Material strength describes a material’s capacity to resist an applied load without permanent deformation or failure. When a material is loaded, stress builds within its structure, and strength quantifies the internal resistance offered before a specific limit is breached. This property is represented by two main metrics: yield strength and ultimate tensile strength.
Yield strength is the maximum stress a material can endure before it begins to deform plastically, meaning it will not return to its original shape once the load is removed. For load-bearing structures, this is an important value, as permanent bending or stretching constitutes a failure, even if the material has not completely broken. Ultimate tensile strength is the maximum stress a material can withstand before the onset of failure, which typically occurs after the yield point is surpassed. A simple way to conceptualize strength is the maximum weight a steel cable can bear before it stretches irreversibly or snaps.
Defining Material Toughness
Toughness defines a material’s ability to absorb energy and undergo plastic deformation without fracturing. It is a composite property requiring a balance of both strength and ductility, which is the ability to deform plastically. A tough material sustains damage, such as a localized impact or a growing crack, by distributing the applied energy throughout its volume.
This energy absorption prevents the immediate propagation of a crack, allowing the material to yield and bend over a greater distance before complete failure. Consider glass, which is strong but not tough; it resists the initial load well but shatters instantly upon impact because it cannot absorb energy through deformation. In contrast, a tough material will bend, dent, or tear slowly, providing a visible warning of impending failure rather than fracturing suddenly.
Quantifying These Distinct Properties
The difference between strength and toughness becomes most apparent in the specific laboratory tests used to quantify them. Strength is typically measured using static tests, such as tensile testing, where a standardized specimen is pulled slowly until it breaks. The data from this test is plotted as a stress-strain curve, charting the load applied (stress) against the resulting change in shape (strain).
The ultimate tensile strength is identified as the highest point on this curve, representing the maximum load the material sustained. Toughness is quantified through dynamic methods, such as the Charpy or Izod impact tests, which simulate a sudden, high-speed blow. In these impact tests, a pendulum strikes a notched sample, and the total energy absorbed upon fracture is calculated from the height the pendulum swings to after breaking the sample. Strength is measured as a force-per-unit-area value, indicating resistance to a maximum load, while toughness is an energy value, indicating the total work done to cause failure.
Real-World Material Selection
The choice between a high-strength or a high-toughness material depends entirely on the specific stresses the component will encounter. High-strength materials are selected for applications where the primary concern is resisting a continuous, static load without permanent deformation. Specialized steel alloys or high-performance ceramics are often chosen for load-bearing columns in buildings or internal engine components, where the material must maintain its precise shape under constant stress.
Conversely, high-toughness materials are prioritized in environments likely to face sudden, dynamic impacts or shock loading. Polymers and composite materials, such as those used in vehicle bumpers, crash barriers, or safety helmets, are designed to absorb and dissipate the kinetic energy of a collision. A material that yields or deforms without shattering is preferable in these applications, even if it cannot support as much overall load as a higher-strength material.