A modified diametrical compression test technique (MDCTT) for mode II fracture toughness of iron powder compact A.A. Alabi a , S.M. Tahir a, ⁎, N.I. Zahari a , M.A. Azmah Hanim a , M.S. Anuar b a Department of Mechanical and Manufacturing Engineering, Faculty of Engineering, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia b Department of Process and Food Engineering, Faculty of Engineering, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia abstract article info Article history: Received 29 March 2017 Received in revised form 15 June 2017 Accepted 25 June 2017 Available online 08 July 2017 A sound knowledge of the fracture properties of a powder metallurgy (PM) component is essential for a design engineer to predict or prevent sudden fracture failure. Though iron powder is the most used base material in the PM industries, only its tensile mode fracture toughness (K IC ) has been documented. This lack of adequate data can be traced to the non-availability of suitable test technique. In this paper, we present a new test approach for determining the mode II (shear mode) fracture toughness (K IIC) of iron powder compact. This method is known as the modified diametrical compression test technique (MDCTT). The MDCTT combined the qualities of diametrical compression test specimen with the pure shear mode failure of a riveted lap joint. The fracture toughness of copper powder compacts was also studied to corroborate the results of this technique. A compari- son of the ratios KIIC  KIC for both powder compacts with theoretical predictions made using the maximum tangen- tial stress criterion show strong agreement. Hence, the MDCTT is a suitable technique for determination of the K IIC values for iron and other metallic powder compacts. © 2017 Elsevier B.V. All rights reserved. Keywords: Mode II fracture toughness MDCTT CDCTT Relative density Riveted joint 1. Introduction Over the years, different methods, involving different specimen ge- ometry have been proposed for the determination of mode II fracture toughness of different materials. Prominent among them are: Four- point asymmetric bending test [1–4]; diagonally loaded square plate specimen [5]; compact-tension-shear specimen, [6–9]; Semi-circular bend specimen [10–13]; and Cracked straight through Brazilian disc [14–18]. The Brazilian disc test, also known as the diametrical compres- sion test technique (DCTT) is shown in Fig. 1. It is a versatile method that has been used to determine the mode I, mode II and mixed mode fracture toughness of a wide range of brittle materials simply by manip- ulating the compression load, P, and the crack angle, α. In 1995, by International Society for Rock Mechanics (ISRM) adopted a standard sample for the determination of mode I fracture toughness, K IC, for rocks and related materials [19]. Over the years, sample size, notch size, and notch configuration have been modified to accommodate pe- culiarities of other materials. A modified version of the ISRM sample having a less complicated notch profile, compressed as shown in Fig. 2, has been successfully used to determine the K IC of iron powder [17]. In this paper, this sample is referred to as the conventional diamet- rical compression test technique (CDCTT) sample. Unlike the K IC that has standardized experimental methods for determining its value for almost all known powdered materials, there is none for K IIC . The DCTT uses a trial and error method to obtain notch angles for which pure shear failures occur. The failures evaluated at these critical angles are termed mode II fracture toughness. The guide for the trial and error method is to choose crack angles that are within the range of some sug- gested theoretical values. See Table 1. The trial and error method has been successfully used to obtain K IIC for rocks, marbles, and polymers because test samples of large diameter with relatively high green strength and uniform mechanical properties could easily be produced from these materials. This makes handling and angle measurement easy. Unfortunately, small test samples are necessary for iron powder compacts if one must reduce the risk of cre- ating high inhomogeneous density variation. The lack of sufficiently large diameter makes it almost impossible to measure crack angles of iron powder compacts via trial and error method. Perhaps this explains why no research works have reported experimental values for the K IIC of iron powder compacts. Theoretical predictions using the maximum tangential stress (MTS) criterion and the minimum strain energy density (MSED) criterion show that the ratio of K II to K I for a given material using the DCTT lies within the range 0:87 ≤ KII  KI ≤ 1:0 [10,22–24]. Shear occurs when two equal but opposite forces act on a body. It causes a shift in contacting planes or surfaces. Fig. 3(a) illustrates a riveted joint subjected to two equal but opposite forces, P, not acting in the same plane. Solid mechanics have proven that the rivet pin shown in Fig. 3(b) failed due to pure shear [25]. This concept of shear Powder Technology 319 (2017) 356–364 ⁎ Corresponding author. E-mail address: su_mtahir@upm.edu.my (S.M. Tahir). http://dx.doi.org/10.1016/j.powtec.2017.06.058 0032-5910/© 2017 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Powder Technology journal homepage: www.elsevier.com/locate/powtec