Effect of Testing Temperature on Mechanical Behavior Outline: • Ductility • Resilience • Toughness • Hardness • Example • Slip Systems • The yield and tensile strengths ………… with increasing temperature. • Ductility ……………. with temperature. Dr. M. Medraj Mech. Eng. Dept. - Concordia University Mech 221 lecture 12/1 Dr. M. Medraj DUCTILITY Mech. Eng. Dept. - Concordia University Mech 221 lecture 12/2 DUCTILITY Ductility measures the amount of plastic deformation that a material goes through by the time it breaks. • Ductility is a measure of how much strain a given stress produces. • Highly ductile metals can exhibit significant strain before fracturing, whereas brittle materials frequently display very little strain. • An overly simplistic way of viewing ductility is the degree to which a material is “forgiving” of local deformation without the occurrence of fracture. Two measures of ductility: 1) Percent Elongation (%El ) % El = Final length - Initial length x 100 Initial length Brittle materials: %EL 5% at fracture 2) Percent Reduction In Area Ductile materials: %EL and %RA both 25% Initial Area - Final Area %RA = x 100 Initial Area Dr. M. Medraj Mech. Eng. Dept. - Concordia University Mech 221 lecture 12/3 Dr. M. Medraj Mech. Eng. Dept. - Concordia University Mech 221 lecture 12/4 RESILIENCE Typical Mechanical Properties of Metals Ability of material to absorb energy during elastic deformation and then to give it back when unloaded. • Measured with Modulus of Resilience, Ur • Ur, is area under - curve up to yielding: U r 0 y d • Assuming a linear elastic region: Ur 1 2 y y 1 2 y y 2y E 2E • Units are J/m3 (equivalent to ……) Dr. M. Medraj Mech. Eng. Dept. - Concordia University Dr. M. Medraj Mech 221 lecture 12/5 Mech. Eng. Dept. - Concordia University Mech 221 lecture 12/6 Toughness Measurement: Impact Testing TOUGHNESS Ability to absorb energy before fracture • • Charpy and Izod tests measure impact energy or notch toughness Charpy V- notch (CVN) most common Charpy Test • • • • Toughness is the area under - curve up to fracture. • - Similar to Resilience (same units J/m3). - Larger area tougher material. • • So tough materials have a combination of ……….. and ……….. • Can be measured by an impact test (Chapter 8). Dr. M. Medraj Mech. Eng. Dept. - Concordia University Mech 221 lecture 12/7 • use standard sized bar specimens with a central notch weighted pendulum released from a height, h impacts the specimen behind the notch (stress concentration) fracture of specimen occurs and energy is absorbed the pendulum travels to point, h´, where h´< h obtain the amount of absorbed energy from scale Dr. M. Medraj Mech. Eng. Dept. - Concordia University Mech 221 lecture 12/8 HARDNESS Vickers Hardness is a measure of the material’s resistance to localized plastic deformation (e.g. dent or scratch) Brinell HARDNESS Quantitative Hardness: Different types of quantitative hardness test has been designed • Rockwell • Brinell • Vickers • Knoop Dr. M. Medraj Mech. Eng. Dept. - Concordia University Mech 221 lecture 12/9 • The depth or size of indentation is measured. Rockwell Moh’s scale, determined by the ability of a material to scratch another material: from 1 (softest = talc) to 10 (hardest = diamond) Knoop Qualitative Hardness: • Usually a small indenter (sphere, cone, or pyramid) is forced into the surface of a material under conditions of controlled magnitude and rate of loading. • The tests somewhat approximate, but popular because they are easy and nondestructive (except for the small dent). Where, P (the applied load) is in kg, D is the indenter's diameter d is the diameter of the resulted indentation Dr. M. Medraj Example Mech. Eng. Dept. - Concordia University Mech 221 lecture 12/10 Plastic Deformation A cylindrical metal specimen having an original diameter of 12.8 mm (0.505 in.) and gauge length of 50.80 mm (2.000 in.) is pulled in tension until fracture occurs. The diameter at the point of fracture is 6.60 mm (0.260 in.), and the fractured gauge length is 72.14 mm (2.840 in.). Calculate the ductility in terms of percent reduction in area and percent elongation. Why metals could be plastically deformed? Why the plastic deformation properties could be changed to a very large degree by forging without changing the chemical composition? Why plastic deformation occurs at stresses that are much smaller than the theoretical strength of perfect crystals? Plastic deformation – the force to break all bonds in the slip plane is much higher than the force needed to cause the deformation. Why? These questions can be answered based on the idea proposed in 1934 by Taylor, Orowan and Polyani: Plastic deformation is due to the motion of a large number of …………….. Dr. M. Medraj Mech. Eng. Dept. - Concordia University Mech 221 lecture 12/11 Dr. M. Medraj Mech. Eng. Dept. - Concordia University Mech 221 lecture 12/12 Dislocations allow deformation at much lower stress than in a perfect crystal, How?! Dislocations and Plastic Deformation Under applied shear stress, dislocations can move by breaking bonds CONSECUTIVELY (rather than simultaneously) Requires less energy. This is the reason why experimental shear strength is lower. Deformation by dislocations movement is called SLIP. • The combination of C-P plane (the slip plane) and C-P direction (the slip direction) is called a …………... Recall: The movement of the dislocation (to the right in this sequence) requires the breaking (and formation) of only ONE set of bonds per step. SLIP SYSTEMS DEPEND ON THE CRYSTAL STRUCTURE OF THE MATERIAL! Dislocations move in ………………. directions within ……………………. planes. Dr. M. Medraj Mech. Eng. Dept. - Concordia University Mech 221 lecture 12/13 Dr. M. Medraj Mech. Eng. Dept. - Concordia University Mech 221 lecture 12/14 Dislocations and Plastic Deformation Next Topic: Phase Diagrams The more slip systems available, the easier it is for dislocations to move, which is why (on the average) FCC and BCC metals are more ductile than HCP metals. Dr. M. Medraj Mech. Eng. Dept. - Concordia University Mech 221 lecture 12/15 Dr. M. Medraj Mech. Eng. Dept. - Concordia University Mech 221 lecture 12/16