Ductile fracture, also known as plastic fracture or tensile overload fracture, refers to a type of fracture that occurs when the load applied to a metallic material exceeds its yield strength, resulting in significant macroscopic plastic deformation before fracture. Therefore, it is also referred to as ductile overload fracture.
A typical example is the fracture of a smooth tensile specimen under uniaxial static loading, as shown in the figure below. The fracture surface exhibits a clear necking phenomenon near the break. Macroscopically, the fracture surface has a cup-and-cone shape: the central region appears fibrous and dark gray in color, generally oriented perpendicular to the tensile stress direction, while the outer region features shear lips that form at approximately a 45° angle to the tensile axis.
In practical applications, the shape of components and the forces they experience can be quite complex, so the macroscopic fracture morphology features may not be obvious. However, the primary basis for distinguishing between ductile fracture and brittle fracture is the presence or absence of significant macroscopic plastic deformation near the fracture.
From the perspective of the fracture mechanism in metallic materials, there are two main types: slip separation and ductile dimple fracture.
Slip separation refers to the phenomenon where, under external force, the atoms within the crystal structure of the material experience relative sliding along specific crystal planes and directions due to shear stress, which is known as slip. When this slip occurs discontinuously or unevenly, or when it is hindered at grain boundaries, phase boundaries, or other interfaces, small gaps or cracks form at these locations, leading to slip separation. Pure slip separation fracture is relatively rare in metallic materials.
Ductile dimple fracture is the more common type. The mechanism for crack formation and propagation in this mode is as follows: metallic materials contain various discontinuities such as voids, inclusions, second-phase particles, grain boundaries, and phase boundaries. When subjected to overload stress (exceeding the yield strength), stress concentrations develop in these local regions, causing plastic deformation. As the deformation progresses, microvoids form at these discontinuities or interfaces. As strain continues to increase, the microvoids grow and coalesce, eventually connecting to form microcracks. Under continued stress, these microcracks slowly expand until they reach a critical size, leading to fracture. These microvoids are referred to as ductile dimples (or plastic pits). The typical morphology of ductile dimples observed under SEM is shown in the figure below, where second-phase particles or inclusions can also be seen at the bottom of the dimples.
Under different stress states, the morphology of ductile dimples on a ductile fracture also varies. Generally, they can be classified into three types: equiaxed dimples (under tensile stress), tear dimples (under tearing stress, Mode I), and shear dimples (under shear stress, Mode II and Mode III).
In practical fracture morphology analysis, it is often observed that all three types of ductile dimple morphologies can appear. This is generally due to the complex stress state experienced by the material, or, under simple stress conditions, as the crack propagates, local stress changes occur, resulting in differences in dimple morphology.
Moreover, we cannot simply determine that a fracture is ductile based on the presence of numerous dimples in the local fracture area. Ductile dimples are not a necessary and sufficient condition for ductile fracture, because in real situations, many mixed fractures can occur, such as quasi-cleavage fractures. Therefore, it is still necessary to combine both macroscopic and microscopic analysis to determine the fracture type, understand the failure mechanism, identify the root cause of the fracture failure, and propose suggestions for improving the material, component design, manufacturing processes, and usage environments.
Under what conditions is ductile fracture more likely to occur? Any factor that increases ductility (reduces brittleness) will promote the occurrence of ductile fracture. The following points summarize the key factors:
Superior microstructure: Different microstructures may lead to different fractures under the same conditions. For example, tempered martensite has better ductility, while pearlite + ferrite has relatively lower ductility. The former is more likely to experience ductile fracture.
Fine-grained structure: Generally, the finer the grains, the better the ductility. Additionally, the defects in fine-grained structures are often smaller, so higher stress is required for fracture.
Tough inclusions or second-phase particles: Tough inclusions or second-phase particles generally do not reduce the material's ductility. Sometimes, a plastic second phase can even improve the material's ductility. As shown in the diagram below, the ductile dimples contain a significant amount of Type A sulfide inclusions, but the material still exhibits ductile overload fracture.
Pure raw materials: Improving the material's purity, such as paying attention to the introduction of external impurity elements during steelmaking, reducing harmful impurities in the material, and minimizing the possibility of intergranular brittleness and second-phase brittle inclusions.
Good structural design: Avoiding stress concentrations, such as reducing sharp corners, notches, etc., and designing a structure that ensures uniform load distribution.
Good operating environment (temperature, medium conditions, etc.): Minimizing exposure to corrosive media and low-temperature environments. If such conditions are required, the environmental sensitivity should be considered when designing the material.
For ductile fracture, in practical applications, it allows for significant deformation without sudden fracture. Therefore, from this perspective, compared to brittle fracture, ductile fracture is a more acceptable mode of fracture. However, if the material design, production, and usage processes are properly controlled at each stage, unnecessary fracture incidents can be avoided, reducing property losses for both businesses and nations.
