When bridge designers select steel, they must consider not only its ability to withstand tension but also its performance under bending forces. This is where bending tests prove invaluable—simulating real-world stress conditions to provide engineers and researchers with critical data.
Bending tests, also known as flexural tests, are destructive material evaluations used to assess bending strength and other key properties. Widely applied to plastics, fiber-reinforced polymers (FRP), metals, and ceramics, these tests reveal how materials behave under uniaxial bending stress, guiding optimal material selection and application.
At their core, bending tests apply flexural forces to materials while observing their responses. Three primary methodologies exist based on loading points and support configurations:
- Single-point bending: Fixes one end of the specimen while loading the other, primarily calculating flexural modulus.
- Three-point bending: Supports both specimen ends while applying central loading—the most common testing method.
- Four-point bending: Uses dual indenters to load two points between supports, creating more uniform stress distribution.
Standardized cylindrical specimens are typically centered on fixtures with parallel support rollers spaced proportionally to specimen diameter. A test punch descends at constant speed, incrementally loading the specimen until fracture or predetermined deformation occurs. The peak applied force during testing is termed fracture force.
Advanced optical systems with high-resolution cameras now enable precise specimen imaging. Single cameras suffice for flat specimens, while dual-camera setups handle complex geometries. Technicians apply random dot patterns or utilize existing surface structures, with correlation algorithms tracking deformation through pixel coordinate analysis.
Maximum bending stress occurs at the specimen's center (point of greatest deflection), where bending moment peaks. From this central pressure point, moment decreases linearly toward supports. Materials experience compression on inner surfaces and tension on outer surfaces, with stress diminishing inward toward neutral fibers—creating non-uniform stress distribution.
When partially plastically deformed specimens are unloaded, residual stresses and resulting torque remain effective, partially reshaping the sample.
Below plastic deformation thresholds, ductile materials exhibit purely elastic bending stress. As stress increases, yield strength is first exceeded in peripheral regions, causing plastic deformation (material flow). The limit yield point represents maximum bending stress before permanent edge deformation occurs.
For materials like steel, the limit yield point exceeds yield strength by 10-20% due to linear stress progression. When edge fibers surpass yield strength, elastic interior fibers resist flow movement. Unlike brittle materials, ductile specimens undergo extreme plastic deformation without fracturing—testing concludes when exceeding yield points.
Brittle specimens fracture without visible material flow, making yield point determination complex. Fracture deflection—the maximum pre-fracture deformation—varies with support width (greater distances permit larger deflections). For many brittle materials like thermosets and fiber-reinforced plastics, bending tests often replace tensile tests which would cause premature fracture.
The most prevalent configuration uses two supports and one central loading point. While practical, it introduces transverse forces alongside compression/tension forces—a limitation addressed by four-point bending.
This method replaces the single indenter with dual loading points, creating constant bending moment between them without transverse forces. Though more accurate for fiber-reinforced materials, the specialized fixtures are costlier and more complex to operate.
Standardized bending tests—conducted via three or four-point configurations—either fracture specimens or induce plastic deformation (in ductile materials). Modern optical metrology now delivers significantly more accurate results than traditional measurement techniques, advancing material science capabilities across industries.