ANALYSIS OF IMPACT ENERGY TO FRACTURE UNNOTCHED CHARPY SPECIMENS MADE FROM RAILROAD TANK CAR STEEL

This paper describes a nonlinear finite element analysis (FEA) framework that examines the impact energy to fracture unnotched Charpy specimens by an oversized, nonstandard pendulum impactor called the Bulk Fracture Charpy Machine (BFCM). The specimens are made from railroad tank car steel, have different thicknesses and interact with impact tups with different sharpness. The FEA employs a Ramberg-Osgood equation for plastic deformations. Progressive damage and failure modeling is applied to predict initiation and evolution of fracture and ultimate material failure. Two types of fracture initiation criterion, i.e., the constant equivalent strain criterion and the stress triaxiality dependent equivalent strain criterion, are compared in material modeling. The impact energy needed to fracture a BFCM specimen is calculated from the FEA. Comparisons with the test data show that the FEA results obtained using the stress triaxiality dependent fracture criterion are in excellent agreement with the BFCM test data. INTRODUCTION The loss of lading from railroad tank cars involved in accidents is commonly caused by failures in three general locations: (1) tank end cap or head, (2) side of the tank or shell, and (3) damage to fittings and valves. Failures in the tank car head and shell occur from collisions with objects, such as couplers and wheels from adjacent cars, broken rails, etc. Research is ongoing within the government and the industry to improve the safety of railroad tank cars carrying hazardous materials (hazmat). Scaled and full-scale impact testing have been performed in the past to examine the puncture resistance of railroad tank cars [1-2], but the mechanics of material failure under such loading conditions are not well understood. Moreover, no industry-accepted standard currently exists to quantify the puncture behavior of materials. In this paper, a nonlinear finite element analysis (FEA) approach is described that examines the fracture of unnotched specimens subjected to pendulum impact loading. Different material failure criteria are used in conjunction with the nonlinear FEA to simulate the tests and to calculate fracture energy. The criteria are based on the state of stress and strain in the impacted specimen. Moreover, a failure criterion based on stress triaxiality, which is a parameter related to the state of stress, provides excellent agreement with the test data for fracture energy. PENDULUM IMPACT TESTING Previous research to examine the resistance of tank car steels to impact loading has focused on fracture toughness or Charpy V-notch (CVN) energy. Industry-accepted procedures exist to conduct these types of tests, which use specimens containing a pre-existing crack (usually fatigue-sharpened) or a stress concentration or notch. However, the physical significance of fracture toughness or impact energy in a structure without a pre-existing crack is unclear. Pendulum impact testing, such as the standard CVN test, has been used to examine the impact resistance of materials for over a century because it is relatively simple, inexpensive, and rapid to perform [3]. In addition, the physical interpretation of the test is clear. The energy available to fracture the specimen is proportional to the initial height of the swing hammer above a reference level (y1 in Figure 1). The energy remaining in the hammer is characterized by the height to which it recovers (y2), and the weight of the striker times the difference (y1-y2) represents the energy absorbed by the specimen. The physics of the pendulum test are the same whether the specimen This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States. Approved for public release; distribution is unlimited. 2 contains a notch or not. However, more energy is needed to fracture an unnotched specimen compared to one containing a notch. Figure 1. Schematic of Pendulum Impact Test An oversized, nonstandard pendulum impactor was built by Southwest Research Institute (SwRI) to examine the fracture energy of different tank car steels (see Figure 2). The size of the fixture was necessary in order to achieve the levels of energy needed to fracture the unnotched specimens. This pendulum test fixture, called the Bulk Fracture Charpy Machine (BFCM), was constructed specifically to conduct studies to assess puncture behavior. That is, the measurement of fracture energy is used to assess the puncture resistance of tank car steel. Figure 2. Oversized, Nonstandard Pendulum Impactor Figure 3 shows a drawing of a BFCM test specimen. The trapezoidal ends of the specimen self-engage into the test fixture, so they are held fixed as the impact load is applied through the pendulum. In this drawing, the test section is 6 inches long, and the specimen width is 1 inch. Figure 3. BFCM Test Specimen Two batches of normalized TC128-B steel were used in the tests, and are identified as Materials 1 and 2 in Table 1 with their respective tensile properties. Properties for Material 1 are averages of tensile test results in longitudinal and transverse orientations of tank car plates, whereas properties for Material 2 were obtained from tensile tests in the longitudinal orientation. Material 1 appears to have slightly higher yield and ultimate tensile strengths than Material 2. Table 1. Mechanical Properties for Normalized TC-128B Material 1 Material 2 Ultimate tensile strength 90.6 ksi 87.3 ksi Yield strength 64.2 ksi 59.2 ksi Elongation 27.5% 27% Reduction in area 58% 59% Two impact tups were used in the tests: a blunt tup with a 0.5 inch wide contact surface (Figure 4), and a sharp tup with a 0.125 inch wide contact surface (Figure 5). Table 2 indicates combinations of BFCM test conditions in terms of impact tup, specimen material and specimen thickness.