Introduction to Engineering-Materials 2.1 Stress-Strain Curve Yielding and Yield Strength Yielding occurs in the region between B and C For small changes in stress,the rod experiences a large change in strain The rod stretched substantially even as the force grows only slightly For that reason,the onset of yielding is often taken by engineers as an indication of failure The value of stress in region B- D Yield E C defines a quantity called the stress Yielding yield strength,Sy Strain,a Shanghai Jiao Tong University 11 Introduction to Engineering H.Shao
Introduction to Engineering Introduction to Engineering —— Materials Materials Shanghai Jiao Tong University Shanghai Jiao Tong University Introduction to Engineering Introduction to Engineering H. Shao 11 2.1 Stress-Strain Curve Yielding and Yield Strength Yielding occurs in the region between B and C – For small changes in stress, the rod experiences a large change in strain • The rod stretched substantially even as the force grows only slightly • For that reason, the onset of yielding is often taken by engineers as an indication of failure – The value of stress in region BC defines a quantity called the yield strength, Sy . Strain,ε Stress, σ A B C D E Yielding Yield stress
Introduction to Engineering-Materials 2.1 Stress-Strain Curve Ultimate Strength and Fracture As the load is increased even further beyond point C -O grows to the ultimate tensile strength S at point D The value represents the largest stress that the material is capable of sustaining ■As the test continues The stress in the B Ultimate E figure actually stress Fracture decreases owing to Strain a reduction in the hardening rod's cross- sectional area > The sample fractures at point E Strain.a Shanghai Jiao Tong University 12 Introduction to Engineering H.Shao
Introduction to Engineering Introduction to Engineering —— Materials Materials Shanghai Jiao Tong University Shanghai Jiao Tong University Introduction to Engineering Introduction to Engineering H. Shao 12 2.1 Stress-Strain Curve Ultimate Strength and Fracture As the load is increased even further beyond point C – σgrows to the ultimate tensile strength Su at point D – The value represents the largest stress that the material is capable of sustaining As the test continues The stress in the figure actually decreases owing to a reduction in the rod’s crosssectional area The sample fractures at point E Strain,ε Stress, σ A B C D Ultimate E stress Strain hardening Fracture
Introduction to Engineering-Materials 2.2Elastic Modulus Young's modulus The resistance of a material to elastic (recoverable)deformation under load A stiff material: Has a high Young's modulus Changes its shape only slightly under elastic loads 。A flexible material: Has a low Young's modulus Changes its shape considerably Shanghai Jiao Tong University 13 Introduction to Engineering H.Shao
Introduction to Engineering Introduction to Engineering —— Materials Materials Shanghai Jiao Tong University Shanghai Jiao Tong University Introduction to Engineering Introduction to Engineering H. Shao 13 2.2Elastic Modulus – Young's modulus • The resistance of a material to elastic (recoverable) deformation under load • A stiff material: – Has a high Young's modulus – Changes its shape only slightly under elastic loads • A flexible material: – Has a low Young's modulus – Changes its shape considerably
Introduction to Engineering-Materials 2.2 Elastic Modulus Ceramics 60[ >1000 Porous ceramics 8 100 Glasses 50☐ 90 Metals and alloys 400 Composites 8 200 Woods and wood products 0.08 25 Polymers <0.01 10 Rubbers <0.01 0.1 Polymer foams <0.01 0.5 0.01 0.1 1 10 100 1000 FlexibleYOUNG'S MODULUS(STIFFNESS)(GPa)-stiff Shanghai Jiao Tong University 14 Introduction to Engineering H.Shao
Introduction to Engineering Introduction to Engineering —— Materials Materials Shanghai Jiao Tong University Shanghai Jiao Tong University Introduction to Engineering Introduction to Engineering H. Shao 14 2.2 Elastic Modulus
Introduction to Engineering-Materials 2.3 Strength The strength of a material The resistance to permanent deformation (failure, usually by yielding) A strong material: requires high loads to permanently deform (or break)it >Caution:a stiff material,requires high loads to an elastic deformation Shanghai Jiao Tong University 15 Introduction to Engineering H.Shao
Introduction to Engineering Introduction to Engineering —— Materials Materials Shanghai Jiao Tong University Shanghai Jiao Tong University Introduction to Engineering Introduction to Engineering H. Shao 15 2.3 Strength The strength of a material The resistance to permanent deformation (failure, usually by yielding) A strong material: requires high loads to permanently deform (or break) it Caution: a stiff material, requires high loads to an elastic deformation