Percent Shear™, Lateral Expansion,
The current ASTM E 23 methods for measuring percent shear fracture area are tedious and inaccurate. While the use of a lateral expansion gage is adequate for standard specimens, miniature and subsize specimens often require an alternative procedure. MPM has developed a new technology for accurate shear and lateral expansion measurement. In addition, the system can be used for Charpy notch and other important dimensional verification. MPM has designed three optics and imaging systems to meet the needs of industry for accurate measurements on Charpy test specimens before and after impact testing. Both the 7200 and the 7300 systems can also be used as a digital optical comparator and also for Brinell indentation test measurements. Detailed information is available in our journal article.
Figures 1 and 2 (below) show the hardware used for acquiring fracture surface and notch images. The system consists of a camera, optics, lighting, and data acquisition software. The hardware in Figure 1 is the 7200 system, which includes a telecentric lens for accurate and precise quantitative measurements such as the notch radius, notch depth, and included angle. The hardware in Figure 2 is the 7100 system, and it is used in cases where only percent shear measurements are needed. The 7200 system can be upgraded for remote use to produce the 7300 system. The 7300 is very useful in radioactive or other harsh environments or in cases where continuous zoom magnification is needed.
Percent Shear Measurement
An accurate digitized image of the fracture surface is used for the percent shear measurement. Figures 3 and 4 show the versatility of the system as the percent shear of a conventional and an atypical specimen are calculated. Certain features of the specimen shown in Figure 4 do not follow the typical fracture trends found in classical Charpy bar fractures. The specimen in Figure 4 exhibited a fracture normal to the main crack plane, and this area should not be included in the brittle fracture area definition. With the digital imaging system, the percent shear of any fracture surface is measurable. In addition to outlining the surface as shown, the software package includes five equations (Figure 5) which use the instrumented striker characteristic load data to calculate percent shear. This latter method is very effective for materials where the brittle region is difficult to resolve. The instrumented striker results can also be used to check optical measurements, and this approach is very effective for quality assurance verification.
Lateral Expansion Measurement
Lateral Expansion (LE) measurements require calibration to real world coordinates. A high-accuracy reticle as shown in Figure 6 can be used for this purpose. Additionally, MPM has developed two-dimensional calibration software that re-maps the pixels to account for second-order effects (e.g. minor lens distortion and optical axis non-perpendicularity). The LE measurement is made in accordance with ASTM E23 directly on the fracture surface as shown in Figure 7. The final result is calculated based on the measurements made on the two specimen halves (Figure 8).
Charpy Notch And Specimen Dimension Verification
The system can also be used to measure the Charpy notch radius (Figure 9), the notch included angle (Figure 10), and the notch depth (Figure 11). Measurement of Charpy specimen dimensions, such as the notch radius and included angle, requires a telecentric lens and backlighting to achieve high accuracy (Figure 12). In most cases, the notch radius is measured using magnifications ~200X. Lower magnifications are used for the measurement of the notch depth, notch angle, and cross section. The system can be provided with fully automatic measurement and acquisition of key Charpy specimen parameters including notch radius, depth, and angle. The system can be provided with a Charpy-specimen alignment fixture and the software can be used in manual or automatic measurement modes. In automatic mode the notch characteristics can be measured with a single click (Figure 13).