22 2.Fundamental Mechanical Properties of Materials sion cycles are applied.The fatigue test measures the number of bending cycles that need to be applied for a specific load un- til failure occurs.Fatigue plays a potentially devastating role in airplane and automobile parts. When subjected to a sudden blow,some materials break at a lower stress than that measured using a tensile machine.The im- pact tester investigates the toughness of materials by striking them at the center while fixing both ends.Toughness is defined as the energy(not the force)required to break a material.A heavy pendulum usually is utilized for the blow.The absorbed energy during the breakage is calculated from the difference in pendu- lum height before and after impact. The creep test measures the continuous and progressive plas- tic deformation of materials at high temperatures while a con- stant stress or a constant load below the room temperature yield strength is applied.The temperature at which creep commences varies widely among materials but is generally above 0.3 times the absolute melting temperature.Lead creeps already at room temperature.We will return to creep in Chapter 6. Leonardo da Vinci (1452-1519)invented already a wire test- ing device in which sand is poured into a bucket(acting as ten- sile load)until the wire breaks. In conclusion,the mechanical properties of materials include ductility,yield strength,elasticity,tensile strength,hardness, toughness under shock,brittleness,fatigue behavior,stiffness, and creep.The question certainly may be raised whether or not it is possible to explain some or all of these diverse properties by one or a few fundamental concepts.We shall attempt to tackle this question in the next chapter. Problems 2.1.What was the original length of a 2.4.What force is needed to plastically wire that has been strained by 30% deform a wire of 2 cm diameter and whose final length is 1 m? whose yield strength is 40 MPa? 2.2.The initial diameter of a wire is 2 cm 2.5.Calculate the ductility of a wire (that and needs to be reduced to 1 cm.Cal- is,its percent area reduction at frac- culate the amount of cold work (re- ture during tensile stressing)whose duction in area in percent)which is initial diameter was 1 cm and whose necessary. diameter at fracture is 0.8 cm. 2.3.Calculate the initial diameter of a 2.6.Calculate the true stress at fracture for wire that has been longitudinally a metal rod whose engineering frac- strained by 30%and whose final di- ture strength is 450 MPa and whose ameter is 0.1 cm.Assume no volume diameter at fracture was reduced by change. plastic deformation from 1 to 0.8 cm.sion cycles are applied. The fatigue test measures the number of bending cycles that need to be applied for a specific load un￾til failure occurs. Fatigue plays a potentially devastating role in airplane and automobile parts. When subjected to a sudden blow, some materials break at a lower stress than that measured using a tensile machine. The im￾pact tester investigates the toughness of materials by striking them at the center while fixing both ends. Toughness is defined as the energy (not the force) required to break a material. A heavy pendulum usually is utilized for the blow. The absorbed energy during the breakage is calculated from the difference in pendu￾lum height before and after impact. The creep test measures the continuous and progressive plas￾tic deformation of materials at high temperatures while a con￾stant stress or a constant load below the room temperature yield strength is applied. The temperature at which creep commences varies widely among materials but is generally above 0.3 times the absolute melting temperature. Lead creeps already at room temperature. We will return to creep in Chapter 6. Leonardo da Vinci (1452–1519) invented already a wire test￾ing device in which sand is poured into a bucket (acting as ten￾sile load) until the wire breaks. In conclusion, the mechanical properties of materials include ductility, yield strength, elasticity, tensile strength, hardness, toughness under shock, brittleness, fatigue behavior, stiffness, and creep. The question certainly may be raised whether or not it is possible to explain some or all of these diverse properties by one or a few fundamental concepts. We shall attempt to tackle this question in the next chapter. 22 2 • Fundamental Mechanical Properties of Materials Problems 2.1. What was the original length of a wire that has been strained by 30% and whose final length is 1 m? 2.2. The initial diameter of a wire is 2 cm and needs to be reduced to 1 cm. Cal￾culate the amount of cold work (re￾duction in area in percent) which is necessary. 2.3. Calculate the initial diameter of a wire that has been longitudinally strained by 30% and whose final di￾ameter is 0.1 cm. Assume no volume change. 2.4. What force is needed to plastically deform a wire of 2 cm diameter whose yield strength is 40 MPa? 2.5. Calculate the ductility of a wire (that is, its percent area reduction at frac￾ture during tensile stressing) whose initial diameter was 1 cm and whose diameter at fracture is 0.8 cm. 2.6. Calculate the true stress at fracture for a metal rod whose engineering frac￾ture strength is 450 MPa and whose diameter at fracture was reduced by plastic deformation from 1 to 0.8 cm