Acoustical and Optical Determination of Mechanical Properties of Inorganically-Bound Foundry Core Materials

Abstract

Inorganically-bound sand cores are used in many light-metal foundries to form cavities in the cast part, which cannot be realised by the mould itself. To enable FEM simulations with core materials, their mechanical properties have to be measured. In this article, we adapt methods to determine the Young's and shear modulus, the Poisson ratio and the fracture strain of sand cores. This allows us to fully parametrise an ideal brittle FEM model. We found that the Young's and shear modulus can be obtained acoustically via the impulse excitation technique. The fracture strain was measured with a high-speed camera and a digital image correlation algorithm.

Keywords: 3-point-bending; Poisson’s number; Young’s modulus; elastic properties; fracture strain; fracture strength; inorganic sand core materials; shear modulus; water-glass.

Figure 1

Test-bench for determining elastic properties via impulse excitation technique. The main components are a microphone, a specimen, an automatic hammer system and the frame of the test-bench.

Figure 2

Algorithm for determining elastic parameters via impulse excitation technique and FEM simulation. The measured and calculated eigenfrequencies are compared and the Young’s modulus and Poisson ratio are optimised such, that the error between the measured and the calculated eigenfrequencies is minimal.

Figure 3

Meshed specimen for FEM calculation of natural frequencies and eigenmodes.

Figure 4

Eigenmodes 1–6 of the specimen obtained by FEM simulation.

Figure 5

Test setup for a 3-point-bending experiment with high-speed imaging. The camera is oriented orthogonal to the specimen’s surface.

Figure 6

Spectral analysis of the impulse excitation data. The doted lines mark natural frequencies based on FEM calculations, with Young’s modulus and the shear modulus calculated analytically using f1 and t1, respectively.

Figure 7

Spectral analysis of the impulse excitation data. The doted lines mark natural frequencies based on FEM calculations. The Young’s and shear modulus of the FEM material model result from a error minimisation between the peaks in the spectrum and the calculated eigenfrequencies ($<12$ kHz).

Figure 8

Strain calculation via digital image correlation for a 3-point-bending (3PB) experiment and 10 points in time ($t_1$–$t_{10}$). Only the area under the punch is analysed. Starting at $t_2$, the strain starts to localise until the fracture is complete at $t_9$, which is still undetectable by the human eye. We added a scale to the plots