ST TE D T) BEN TES INT RAL PO XU3- L E (F By:- Krishna Kr. Hansdah
FLEXURAL TESTIn engineering mechanics, flexure or bending characterizesthe behavior of a slender structural element subjected to anexternal load applied perpendicularly to a longitudinal axisof the element.Typical Materials used for Flexural Test1.Polymers2.Wood And Composites3.Brittle Materials
WHY PERFORM A FLEXURE TEST?A flexure test produces tensile stress in the convex side of the specimenand compression stress in the concave side. This creates an area of shearstress along the midline. To ensure the primary failure comes from tensileor compression stress the shear stress must be minimized. This is doneby controlling the span to depth ratio; the length of the outer span dividedby the height (depth) of the specimen.For most materials S/d=16 is acceptable.Some materials require S/d=32 to 64 tokeep the shear stress low enough.
The Flexural test measures the force required to bend abeam under 3-point loading conditions.The data is often used to select materials for parts that willsupport loads without flexing.Flexural modulus is used as an indication of a material’sstiffness when flexed.
3-POINT BEND TESTIn this test a specimen with rectangular or flat cross-section is placed on twoparallel supporting pins. The loading force is applied in the middle by meansloading pin.The supporting and loading pins are mounted in a way, allowing their freerotation about:-axis parallel to the pin axis;-axis parallel to the specimen axis.
It provides values for the modulus of elasticity in bending , flexural stress ,flexural strain and the flexural stress-strain response of the material.Advantage -ease of the specimen preparation and testing. Disadvantage -the results of the testing method are sensitive to specimen and loading geometry and strain rate.Flexural strength or modulus of rupture -The stress required to fracture aspecimen in a bend test.Flexural modulus - The modulus of elasticity calculated from the results of abend test, giving the slope of the stress-deflection curve.
TESTING METHODFor a rectangular cross section,Calculation of the flexural stress ,Calculation of the flexural strain ,Calculation of flexural modulus ,
where,= Stress in outer fibers at midpoint, (MPa)= Strain in the outer surface, (mm/mm) = flexural Modulus of elasticity,(MPa)P= load at a given point on the load deflection curve, (N)L= Support span, (mm)b = Width of test beam, (mm)d= Depth of tested beam, (mm)D= maximum deflection of the center of the beam, (mm)m = The gradient (i.e., slope) of the initial straight-line portion of the loaddeflection curve,(P/D), (N/mm)