Properties of Materials
Application Properties 使用性能
Properties performed when the materials are applied,including mechanical properties, physical properties and chemical properties.
Processing Properties 工藝性能
Properties performed when the materials are machined,including casting, forming, joining and machining, etc.
Many materials when in service, are subjected to forces or loads. Mechanical properties are most essential in engineering!
Important mechanical properties of materials include
strength 強度 plasticity 塑性 hardness 硬度 impact toughness 沖擊韌性
fracture toughness 斷裂韌性 fatigue 疲勞 viscoelasticity 滯彈性
1.1 Strength and plasticity 強度與塑性
Testing method: tensile testing .Specimen geometries: recommended by ASTM or GB
The tensile testing machine is designed to elongate the specimen at a constant rate, and to continuously and simultaneously measure the instantaneous applied load (with a load cell) and the resulting elongations (using an extensometer). The specimen is mounted by its ends into the holding grips of the testing apparatus.
A stress-strain test typically takes several minutes to perform and is destructive; that is, the test specimen is permanently deformed and usually fractured. The output of such a tensile test is recorded on a strip chart (or by a computer) as load or force versus elongation.
Engineering Stress
工程應力
Yield Strength 屈服強度
However, most engineering materials have no obvious yield point on stress-stain curve, thus it is usually given as the stress required causing 0.2% plastic strain, know as the 0.2% offset yield stress ?0.2. 條件屈服強度
Ultimate Tensile Strength 極限抗拉強度
The ultimate tensile strength is the maximum strength reached in the engineering stress-strain curve.
If the specimen develops a localized decreased in cross-sectional area (commonly called necking 頸縮), the engineering stress will decrease with further strain until fracture occurs since the engineering stress is determined by using the original cross-sectional area of the specimen.
Tensile strengths may vary anywhere from 50MPa for an aluminum to as high as 3000 MPa for the high-strength steels.
Ductility 塑性
Ductility of metals is a measure of the degree of plastic deformation that has been sustained at fracture.
A material that experiences very little or no plastic deformation at fracture is termed as brittle 脆性的. (approximately having a fracture strain of less than about 5%)
A knowledge of the ductility of materials is important for at least two reasons.
(i) It indicates to a designer that the degree to which a structure will deform plastically before fracture.
(ii) It specifies the degree of allowable deformation during fabrication operations.
Ductility 塑性
Ductility may be expressed quantitatively as either
percentage elongation (δ，斷后伸長率) or percentage reduction in area (ψ，斷面收縮率).
δ = (final length – initial length) / initial length ? 100% = (lf – l0) / l0 ? 100%
ψ = (initial area – final area) / initial area ? 100% = (Ao – Af) / Ao ? 100%
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