1. Basic Introduction of Mechanical Testing in Investment Casting
In broad engineering terms, mechanical testing involves applying forces and pressure to a component or system and measuring its response. This is designed to check that the physical and mechanical properties meet the expected requirements and it performs as needed. The mechanical testing has four parameters, which are elongation, yield strength, tensile strength, and hardness.
Elongation refers to an index that describes the plasticity of materials. Its calculation formula is the percentage of the total deformation of the gauge segment after stretching fracture and the original gauge length L: Δ = ΔL / L × 100 %. Material of Δ ≥ 5 % is often called plastic material and material of Δ≤5% is often called brittle material.
Yield strength is the yield limit of metal materials when yielding occurs, the stress of resisting micro-plastic deformation. Yield strength’s unit is Mp. For metal materials without obvious yield, the stress value of 0.2 % residual deformation is stipulated as its yield limit, which is called the conditional yield limit or yield strength. The external force that exceeds this limit will make the parts permanently deactivated and cannot be restored.
Tensile strength is the maximum bearing capacity of a metal under static tensile conditions. It represents the resistance to the maximum uniform deformation of the material. Before stretching sample is subjected to the maximum tensile stress, the deformation is uniform and consistent. But after the excess, the metal begins to show neck shrinkage, generate centralized deformation. For brittle materials without uniform plastic deformation, it reflects the fracture resistance of the material.
Hardness is the local resistance of solids to the invasion of external objects. It’s an index represents the extent to which materials are soft or hard. Due to different test methods, there are different hardness standards. The mechanical meanings of various hardness standards are different. They cannot be converted to each other directly, but they can be compared through tests.
2. Methods of Mechanical Testing: Tensile Test to Determine the Tensile Strength and Elongation at Fracture
The tensile test is the most important testing method in destructive materials testing. A standardized specimen with a known cross section is loaded uniformly with relatively low increasing force in the longitudinal direction. A uni-axial stress state prevails in the specimen until contraction commences.
The ratio of stress to strain can be shown from the plotted load-extension diagram.
3. How JC Casting Controls the Mechanical Testing Quality?
JC Casting has tensile testers in house.
JC Casting tests mechanical properties of products for each batch after casting and heat treatment both in house and outsourced. The performance of heat treatment is qualified and based on the hardness testing results and mechanical testing results.
Before testing the casting parts’ mechanical properties, inspectors calibrate the tensile testing equipment according to rules. This tester is calibrated by qualified third party every year, which is the same as all the in house inspection equipment, on the basis of IATF16949 certificate’s requirements.