For materials without a distinct yield point, the stress that leaves a permanent strain of 0.2% after unloading. It is found where a line parallel to the elastic line, offset by 0.2% strain, meets the stress–strain curve.
Japanese name: 0.2%耐力
0.2% offset yield strength appears in 1 practice question across 1 topic, including Mechanical properties and fracture.
A tensile test of mild steel shows a yield point where the stress drops suddenly, but aluminum alloys, copper alloys, austenitic stainless steels, high-strength steels and others pass smoothly from the elastic to the plastic range. The 0.2% offset yield strength is the stress that leaves a permanent strain of 0.2% (a strain of 0.002) on unloading, adopted by convention as the yield strength. The JIS and ISO tensile test standards denote it R_p0.2, and in English it is called the 0.2% offset yield strength or proof stress.
Shift the straight elastic line of the stress–strain curve by 0.2% along the strain axis and read the stress where it intersects the curve. For example, for an aluminum alloy with a Young's modulus of 70 GPa and a proof stress of 280 MPa, the elastic strain at that point is 0.4%, so the total strain is about 0.6%. If strain is calculated from crosshead displacement, deformation of the grips and the machine itself is added and the elastic slope appears too low, so to read the proof stress correctly, strain is measured with an extensometer attached to the specimen.
The 0.2% offset yield strength is not the limit below which no plastic deformation occurs (the elastic limit) but a conventional value that already includes a small plastic deformation. In fact, minute plastic strains occur even at lower stresses. Some applications use a 0.1% or 0.01% offset, and the smaller the offset, the lower the value. Tensile strength is the maximum load divided by the original cross-sectional area, a different quantity. The ratio of proof stress to tensile strength (the yield ratio) expresses the margin between yielding and fracture.
In the static strength design of machine parts, the allowable stress is often set by dividing the 0.2% offset yield strength by a safety factor, as a criterion for avoiding permanent deformation in service. It is also convenient for comparing steels that have a yield point with alloys that do not on the same basis. However, higher-strength materials tend to have higher yield ratios and can approach fracture soon after exceeding the proof stress, so elongation and toughness are also checked. The proof stress in compression or at high temperature can differ from that in room-temperature tension, so data matching the service conditions are needed.
Answer: B. Take the stress where a line parallel to the elastic line, offset by a strain of 0.2%, intersects the stress–strain curve
The 0.2% offset yield strength is defined as the stress that leaves a permanent (plastic) strain of 0.2% when the load is removed. On the stress–strain curve, shift the elastic line by 0.002 along the strain axis and read the stress where it meets the curve. Reading the stress at a strain of 0.2% directly differs from the definition because that strain includes elastic strain, and it gives a lower value. The maximum load corresponds to the tensile strength, a different quantity from the offset yield strength. For materials such as mild steel that show distinct upper and lower yield points, the yield point value is used.
The yield point is read from an actual sudden drop or plateau in stress on the stress–strain curve, and there are upper and lower yield points. The 0.2% offset yield strength is a value defined by the convention of 0.2% permanent strain for materials that show no yield point.
It was chosen by convention as a permanent strain small enough to hardly affect a part's function yet reliably measurable with an extensometer. Depending on the application, such as precision springs, offsets of 0.1% or 0.01% are also used.
In the tensile tests of JIS Z 2241 and ISO 6892-1 it is written R_p0.2. Textbooks and design data may also write it as σ0.2 or σ_y, and the unit is MPa (N/mm²).
The 0.2% offset yield strength is defined as the stress that leaves a permanent (plastic) strain of 0.2% when the load is removed.
For materials such as aluminum alloys that show no distinct…