Fracture toughness testing methods; what is the difference between fracture strength and tensile strength?

The application of sintered NdFeB permanent magnets primarily relies on their magnetic properties; however, depending on the operating environment 

and conditions, requirements extend beyond magnetism to include mechanical and chemical properties. For instance, magnets used in high-speed rotating 

machinery must withstand significant centrifugal forces, vibrations, or extreme accelerations (3g–5g). Additionally, stresses encountered during installation 

can lead to issues such as flaking, edge chipping, corner breakage, or cracking. Consequently, specific mechanical performance standards are essential.

What are the mechanical properties of a material?

The mechanical properties of a material generally include strength, hardness, plasticity, and toughness; each of these parameters carries a distinct physical significance.

Strength refers to a material's maximum capacity to resist failure caused by external forces. Based on the nature of the applied force, strength is categorized as follows:

Tensile strength: The ultimate strength limit when the external force is a pulling (tensile) force.

Compressive strength: The ultimate strength limit when the external force is a pushing (compressive) force.

Flexural strength (or bending strength): The ultimate strength limit when the external force acts perpendicular to the material's axis, causing it to bend.

Fracture toughness testing methods; what is the difference between fracture strength and tensile strength?

Hardness refers to a material's ability to locally resist the indentation of a hard object into its surface; it serves as an indicator for comparing the relative hardness 

of different materials. Higher hardness indicates a greater capacity for the metal to resist plastic deformation.

Plasticity refers to the ability of a solid substance to resist deformation under a given external force; specifically, it is the material's capacity to undergo permanent 

deformation without fracturing when subjected to external forces.

Toughness represents a material's ability to absorb energy during the processes of plastic deformation and fracture. Greater toughness implies a lower likelihood 

of brittle fracture. In materials science and metallurgy, toughness refers to a material's resistance to fracture when subjected to deforming forces; it is defined as 

the ratio of the energy absorbed prior to fracture to the material's volume.

Mechanical Properties of Sintered NdFeB

Sintered NdFeB is a brittle material characterized by high hardness and brittleness—meaning it possesses high strength but low toughness. It exhibits virtually 

no plastic deformation before fracturing, failing while still within the elastic deformation stage.

The figure below compares the maximum energy product (BH)m and fracture toughness of various permanent magnet materials. It can be seen that sintered 

NdFeB possesses the highest (BH)m, while its fracture toughness remains comparable to that of Sm2Tm17, SmCo5, and ferrite permanent magnets; this is because 

they are all brittle permanent magnet materials based on intermetallic compounds. Bonded rare-earth permanent magnets, Fe-Cr-Co alloys, and magnet steels 

exhibit superior fracture toughness, yet their (BH)m values are significantly lower than that of sintered NdFeB.

Fracture toughness testing methods; what is the difference between fracture strength and tensile strength?

Three common indicators are used to describe the mechanical properties of brittle materials:

Fracture toughness: This generally reflects the material's strength in the presence of propagating cracks; the unit is MPa·m¹/². Testing requires equipment such 

as a tensile testing machine, stress sensors, an extensometer, and a dynamic strain amplifier with signal conditioning, and the specimens must be prepared as thin sheets.

Impact strength (impact fracture toughness): This reflects the energy absorbed by the material during fracture under impact stress; the unit is J/m². Measurements 

of impact strength are highly sensitive to factors such as specimen dimensions, shape, machining precision, and testing conditions, resulting in significant data scatter.

Flexural strength: This measures the material's resistance to fracture under bending, typically using the three-point bending method. Due to the ease of 

specimen preparation and testing, it is the most common metric used to describe the mechanical properties of sintered NdFeB magnets.

We have compiled approximate ranges for the mechanical property indicators of sintered NdFeB magnets based on data from various researchers using 

different testing methods; as these are brittle materials, the test data exhibit considerable scatter.

The high strength and low toughness characteristic of sintered NdFeB permanent magnets are determined by their inherent crystal structure; additionally, 

two other factors influence their flexural strength and offer pathways for enhancing it.

The neodymium (Nd) content affects the strength of sintered NdFeB; experimental results indicate that, under certain conditions, higher Nd content leads to 

higher material strength.

The addition of other metallic elements also influences the strength of sintered NdFeB. Adding specific amounts of titanium (Ti), niobium (Nb), or copper (Cu) 

improves the impact fracture toughness of the permanent magnets, while the addition of a small amount of cobalt (Co) enhances their flexural strength.

The relatively limited comprehensive mechanical properties of sintered NdFeB are a key factor restricting its application in a wider range of fields. If the material's 

toughness could be improved without compromising—or while actually enhancing—its magnetic properties, sintered NdFeB would play a more significant role in 

sectors such as the military and aerospace, ushering in a new era of development.