Cruciform Specimen Machining Using EDM and a New Design Verification for Biaxial Testing Abhishek Raj, Pundan Kumar Singh, Rahul Kumar Verma, and K. Narasimhan (Submitted December 25, 2019; in revised form March 27, 2020; published online July 6, 2020) Cruciform flat specimen is mostly used to investigate experimentally the in-plane biaxial mechanical behavior of sheet metals. In this study, a modified cruciform specimen design is proposed and validated against the performance of current standard design. The existing standard cruciform specimen prepared through laser cutting for sheet metal can accurately provide biaxial stress–strain curve through biaxial tensile test by ensuring superior homogeneous stress and strain field in gauge section. However, preparation of such specimen using laser cut is cumbersome, expensive and prone to errors. The current standard cruciform specimen design features slotted arm of fixed length and width with specific gauge. Few other designs also feature reduced gauge section to localize failure. Both need high precision of laser cutting and machining to prepare specimens which can achieve biaxial stress–strain curve for more than 4% equivalent plastic strain level. The proposed specimen design features running slits in all four arms of specimen with uniform gauge section machined using electrical discharge machining (EDM) owing to higher accuracy and minimum heat-affected zone (HAZ). This technique also avoids machining effects with an understanding that proposed specimen design is to determine the yield behavior of material only. Also, specimen fabri- cated from as received sheets and without any thickness reduction in gauge area allows the user to keep full thickness in biaxial condition. The modified design qualifies the commonly accepted performance criteria for cruciform specimen, i.e., stress and strain distribution in the gauge section is found to be homogeneous and the biaxial stress–strain curve can be generated until 4% of equivalent plastic strain. The proposed specimen design is evaluated experimentally by conducting biaxial test at various stress ratios, and it is compared with alike experiments using standard specimen design. One automotive grade material, inter- stitial-Free high-strength (IFHS) steel sheet of thickness 0.70 mm is tested in this study to validate the specimen design. Keywords biaxial testing, cruciform specimen, Digital image correlation (DIC), Electrical discharge machining (EDM), laser 1. Introduction Most of the engineering structures undergo multiaxial state of stress during their life cycle. Plane stress case is a special case of multiaxial stress state. Often, for sheet metal forming, the plane stress case corresponds to biaxial state of stress in sheet metals. Biaxial state of stress in sheet metals is limited to tension–tension loading since other modes like tension–com- pression, compression–tension and compression–compression could lead to buckling of thin sheet cruciform specimen. Certain phenomenological constitutive material modeling (Ref 1) of sheet metal involves the use of biaxial yield stress to accurately predict the material behavior. To this end, biaxial tensile test of sheet metal with cruciform specimen or bulge test with tubular specimen using a special testing machine (Ref 2-4) is often preferred in which the required stress ratios in the arms are achieved by changing the proportion of load or displace- ment. One of the major advantages of cruciform specimen (Ref 4, 5) is the existence of a homogeneous stress–strain distribu- tion in the gauge area of the specimen, which is not influenced by contact, friction or other effects introduced by the test equipment. The external load is transmitted directly onto the biaxial tensile deformation area, which is realized by a load transfer mechanism through the arms of the cruciform speci- men. As the deformation is homogeneous in the gauge section, stress calculation in rolling and transverse direction of cruci- form specimen can be done through normalizing the respective loading arm with its cross-sectional area. The average stress calculated through this method is equal to instantaneous stress in the gauge section as the deformation and load transmission are homogeneous. The new international standard ISO 16842:2014 (Ref 5, 6) is only valid for determining the yield locus and hardening behavior close to equivalent plastic strain of 4 to 5% in biaxial tensile tests. It cannot be used for an analysis of larger deformations. Few researchers have also attempted to conduct biaxial tensile test using uniaxial test machine (Ref 7-9), wherein a special easy-to-use cost-effective fixture is developed for universal testing machine. An extensive review of biaxial testing with cruciform specimen is done by (Ref 10-12), and the testing method is also standardized (ISO 16842) (Ref 5). Inhomogeneous load transfer during biaxial testing has led to the development of various cruciform geometries and test equipment as reviewed by Hannon and Teirnan (Ref 12). Abhishek Raj, Research and Development, Tata Steel Limited, Jamshedpur 831007, India; and Indian Institute of Technology- Bombay, Mumbai 400076, India; Pundan Kumar Singh and Rahul Kumar Verma, Research and Development, Tata Steel Limited, Jamshedpur 831007, India; and K. Narasimhan, Indian Institute of Technology-Bombay, Mumbai 400076, India. Contact e-mail: abhishek.raj@tatasteel.com. JMEPEG (2020) 29:4716–4724 ÓASM International https://doi.org/10.1007/s11665-020-04921-8 1059-9495/$19.00 4716—Volume 29(7) July 2020 Journal of Materials Engineering and Performance