Material Properties and Fracture
Laboratory Report
This comprehensive final report of ENGN 1440: Mechanical Properties of materials combined all of the laboratory exercises in the course to create a unified analysis of the mechanical properties of copper and martensitic steel. A parallel aim of the report is to explain the temperature dependence of fracture toughness in BCC metals.
Overview
This laboratory investigated how the mechanical behavior of copper changes with temperature, strain rate, annealing, and alloying (with zinc to create brass). This was accomplished with uniaxial tensile tests conducted for copper and brass as well as uniaxial compression tests conducted in annealed copper at different temperatures and strain rates. In the tensile tests, it was seen that alloying decreased elasticity and ductility but increased strength. Annealing increased ductility but decreased strength. In the compression tests yield strength, flow stress at 6% strain, and work hardening capacity decreased with increasing temperature. With increasing strain rate the yield stress and flow stress increased, but the work hardening capacity decreased. This laboratory also examines the toughness of 4140 martensitic steel as measured through Charpy impact tests. A ductile to brittle transition was observed in the steel. This transition was fully understood with the aid of scanning electron microscopy, by which images of the Charpy fracture surfaces were taken. Ductile fracture in the annealed copper tensile specimen and intergranular brittle fracture in an alumina specimen are also looked at providing a representative picture of fracture mechanisms.
Key Results
This laboratory examined the mechanical properties of copper when alloyed to form brass, when annealed, and when compressed at different strain rates and temperatures. Additionally, the effect of temperature on 4140 steel was determined and fracture mechanics were studied both for steel as well as for annealed copper and alumina. The principal findings were as follows:
• Annealed copper is more ductile than as-received copper and has a lower yield point, which can be attributed to recovery and recrystallization processes that form strain-free grains.
• Annealing both copper and brass promotes greater work hardening compared to as-received samples with preexisting cold work.
• Alloying copper (to form brass) strengthens it through solid solution hardening.
• Annealed copper fails fibrously as indicated by the cup-and-cone fracture surface and rationalized through the presence of deep microvoids seen through microscopy.
• With increasing temperature, the yield stress, flow stress at 6% plastic strain, and work hardening of annealed copper under compression decrease.
• With increasing strain rate the yield stress and flow stress at 6% plastic strain increase under compression, but the work hardening decreases. These findings only hold for elevated temperature.
• Copper, an FCC metal, does not experience a ductile to brittle transition with temperature while martensitic 4140 steel, with a BCC matrix, does.
• Martensitic 4140 steel exhibits brittle transgranular fracture though cleavage at low temperatures and ductile transgranular fracture facilitated by decohesion of cementite particles at elevated temperatures.
• Alumina exhibits brittle intragranular fracture.
