mechanical.qxp 3/4/2005 9:15 AM Page 1 MECHANICAL UNDERSTANDING THE BASICS TESTING Load cell echanical testing deals F The results of tests Crosshead with the response of commonly used to metals to applied forces. M Considered in this article Screw measure the properties of are the common testing techniques heat treated parts provide related to the mechanical failure of AF=σA metals. These include tension (ten- 1 information about how Specimen sile and transverse rupture), torsion, well design requirements hardness, fatigue, creep and stress- are met. Test data also can rupture, and impact tests. Residual stress and statistical analysis of me- help ID production chanical property data are also F Base briefly covered. problems before they Fig. 1 — Typical tensile tester: a screw- driven electromechanical system. (Ref. 1) become critical. Tension Testing Tensile, transverse rupture, and Strain to fracture Daniel H. Herring* other tension tests are widely used Uniform strain s Herring Group Inc. to provide basic design information Necking Fracture Elmhurst, Illinois on the strength of materials and as begins acceptance tests for the specification Offset yield strength Tensile of materials. The basic parameters strength used to determine the stress-strain Fracture stress behavior of a metal are its tensile Engineering stress, strength (ultimate tensile strength), yield strength (tensile yield strength) Engineering strain, e and yield point, elongation (or per- Fig. 2 — Engineering stress-strain curve. cent elongation), and reduction in Intersection of the dashed line with the curve determines the offset yield strength. (Ref. 2) area. The first three are strength properties; the other two, measures this portion of the test, the specimen of ductility. will return to its original length. This Tensile tests are designed to “pull” is called elastic deformation. The pro- a specimen to failure (Fig. 1). The de- portionality between the stress and structive test measures the force and strain is the elastic modulus (mod- stretch of a material during testing. ulus of elasticity or Young’s mod- The test plots “stress” versus “strain” ulus). The modulus of elasticity is de- (Fig. 2). Stress is the (applied) load termined by the binding forces of the divided by the cross-sectional area atoms and since these forces cannot of the test specimen at its center. be changed without changing the Strain is the change in a dimension basic nature of the material, it follows divided by the original dimension, that this is one of the most structure and is measured over the central por- insensitive mechanical properties. tion of the specimen length (where With continued loading and the cross section is constant). stretching, the tensile specimen per- As the specimen is stretched, the manently deforms, exhibiting plastic load required to induce each level of deformation. The yield strength is strain is measured. When the load is the stress at which the specimen first applied the tensile specimen shifts from elastic (recoverable) stretches in proportion to the applied stretching to plastic (permanent) de- * Member, ASM Heat Treating Society load. If the load is removed during formation. By standard convention, HEAT TREATING PROGRESS • MARCH/APRIL 2005 31 mechanical.qxp 3/4/2005 9:16 AM Page 2 the reported yield strength corre- elasticity in shear, torsional yield sponds to a plastic strain of 0.2% strength, and modulus of rupture. (where observable deformation has Any part subject to torsional loading Indent taken place) in service, such as shafts and axles, Diagonal or Tensile strength is the highest should be torsion tested. Test Indenter(s) diameter stress encountered in the tensile test. Torsion testers (Fig. 3) consist of a Brinell Ball indenter, 1–7 mm For many steels this corresponds to twisting head, with a chuck for grip- 10 mm (0.4 in.) (0.04–0.28 in.) the stress at fracture. For very duc- ping the specimen and for applying or 2.5 mm (0.1 in.) tile steels, the stress at fracture is the twisting moment, and a in diameter lower than the tensile strength. For weighing head, which grips the very brittle steels, the yield strength other end of the specimen and meas- is the same as the tensile strength ures the twisting moment, or torque. (and fracture strength). Deformation of the specimen is A common measure of ductility is monitored by a twist measuring de- Rockwell 120° diamond 0.1–1.5 mm the elongation (given as total stretch vice called a troptometer. cone, (0.004–0.06 in.) to failure divided by the initial spec- A torsion test specimen typically 1.6–13 mm (1/16–1/2 in.) imen-center test length, or gage has a circular cross section (the sim- length). Elongation is a dimension- plest geometry for calculation of Rockwell As for 0.1–0.7 mm Rockwell (0.004–0.03 in.) less number, expressed as a per- stress). Since the shear stress in the superficial centage. Another measure of ductility elastic range varies linearly from is reduction in area, also dimension- zero at the center of the specimen to Vickers 136° diamond Measure pyramid diagonal, less and expressed as a percentage. a maximum at the surface, it is fre- not diameter Reduction in area is a measure of the quently desirable to test a thin-wall change in cross sectional area at the tubular specimen. This results in a point of failure (change in area di- nearly uniform shear stress over the µ vided by the original area). cross section. Micro- 136° diamond 40 m The transverse rupture test is a Torsion test results can be used to hardness indenter or (0.16 micro-in.) strength test designed for low-duc- validate or expand on the informa- a Knoop indenter tility materials, including carbides tion gleaned from tensile testing. Tor- and powder metallurgy (P/M) ma- sion testing provides a more funda- terials. This destructive test involves mental measure of the plasticity of a µ Ultrasonic 136° diamond 15–50 m bending rather than pulling of the material than does tension testing. It pyramid (0.06–0.2 specimen. Maximum load, specimen directly yields a shear stress-shear micro-in.) dimensions, and test time are used strain curve. In torsion, the critical to calculate the stress needed to shear stress for plastic flow is cause failure. A typical transverse reached before the critical normal rupture strength is 1.5 to 2 times the stress for fracture; while in tension, tensile strength the critical normal stress is reached before the critical shear stress. Torsion Testing ment, heat treaters probably are most Torsion testing is used to deter- Hardness Testing familiar with the hardness tester. Al- mine such properties as modulus of Among mechanical testing equip- most every shop has either a Rock- well-scale or Rockwell superficial- Hydraulic Servo-valve scale tester. The Rockwell scales are power Hydraulic motor Test specimen Rotary Torque cell those most often used for ferrous ma- transducer Radiant terials. Many shops also have Brinell furnace and microhardness or microinden- tation hardness testers. These and other indentation hardness tests are compared in Table 1. Saddle In general, hardness usually im- Lathe bed plies a resistance to deformation. For a metal, the property is a measure of its resistance to permanent or plastic deformation. Hardness measuring Vacuum Furnace pump controller methods can be sorted into three general types, depending upon the manner in which the tests are con- Fig. 3 — Typical torsion tester: a servo-controlled, hot torsion machine mounted on a lathe ducted: scratch hardness, indenta- bed. Shafts and axles are among the parts that should be torsion tested. (Ref. 3) tion hardness, and rebound, or dy- 32 HEAT TREATING PROGRESS • MARCH/APRIL 2005 mechanical.qxp 3/4/2005 9:16 AM Page 3 Table 1 — Comparison of indentation hardness tests (Ref. 4) Method of Surface Tests per Depth1 Load(s) measurement preparation hour Applications Remarks Up to 0.3 mm 3000 kgf Measure Specially 50 with Large forged Damage to specimen (0.01 in.) and for ferrous diameter of ground area diameter and cast parts minimized by use 1 mm (0.04 in.), metals; indent under for measurements of lightly loaded respectively, down to microscope; measurements ball indenter; with 2.5 mm 100 kgf for read of diameter indent will then be (0.1 in.) and soft metals hardness less than a 10 mm (0.4 in.) from tables Rockwell indent in diameter balls 25–375 µm Major, Read hardness No 300 manually, Forgings, Measure depth of (0.1–1.48 60–150 kgf;directly from preparation 900 castings, penetration, not micro-in.) Minor, meter ornecessary on automatically roughly diameter 10 kgf digital display many surfaces machined parts 10–110 µm Major, As for Machined As for Critical A surface test of case (0.04–0.43 15–45 kgf; Rockwell surface, ground Rockwell surfaces of hardening and micro-in.) Minor, 3 kgf finished parts annealing 30–100 µm 1–120 kgf Measure Smooth clean Up to 180 Fine finished Small indent but high (0.12–0.4 indent with surface, surfaces, local stresses micro-in.) low-power symmetrical thin specimens microscope; if not flat read hardness from tables 1–4 µm 1 gf–1 kgf Measure Polished Up to 60 Surface layers, Laboratory test used on (0.004–0.016 indent with surface thin stock, brittle materials or micro-in.) low-power down to microstructural microscope; 200 µm constituents read hardness (0.008 in.) from tables 4–18 µm 800 gf Direct readout Surface better 1200 (limited Thin stock Calibration for Young’s (0.016–0.07 onto meter or than 1.2 µm by speed and finished modulus necessary; micro-in.) digital display (0.004 micro-in.) at which surfaces in 100% testing of finished for accurate operator any position parts; completely work; can read nondestructive otherwise, up display) to 3 µm (0.012 micro-in.) 1.The minimum material thickness for a test is usually taken to be 10 times the indent depth.