Issue 77

N. A. Alang et al., Fracture and Structural Integrity, 77 (2026) 340-361; DOI: 10.3221/IGF-ESIS.77.20

model validated against experimental results. Despite the increasing use of numerical methods in SPT, only a limited number of studies have considered the influence of initial stress or pre-straining in the Grade 91 steel. Based on available literature, there are very few experimental works and numerical studies on Grade 91 steel that explicitly consider the influence of pre-straining. Therefore, the development of a comprehensive three-dimensional FEM model for SPT of Grade 91 steel, incorporating pre-strain effects, is necessary. For this reason, the present study integrates both experimental and finite element (FE) modelling approaches. Two material conditions, namely as-received and pre-strained materials, are considered and are subjected to small punch loading. Additionally, FE modelling, which incorporates different material hardening data, is employed to simulate the effects of plasticity on material deformation and fracture behaviour. The experimental procedure and modeling techniques used in the present work are systematically reported. ylindrical rods with a diameter of 10 mm and a length of 160 mm were extracted from as-received commercial Grade 91 steel pipes using wire electrical discharge machining (EDM). The rods were extracted in the rolling direction of the pipe. They were carefully machined so that the geometry and the dimensions were accordance to the standard test methods for tension testing of metallic materials ASTM E8 standard [20]. The specimens were prepared for uniaxial tensile and plastic pre-straining tests. Both uniaxial tensile and plastic pre-straining tests were performed using Universal Testing Machine (UTM). A total of nine (9) specimens were prepared for the tests with three (3) specimens used for standard uniaxial tensile testing and the remaining specimens used for pre-straining tests. Prior to the test, a tensile test rig was installed on the machine. After the specimens were properly gripped, an extensometer was attached to the gauge area of the specimen to accurately measure the displacement and strain during the test. A pre-load of 50 N was applied during the test to ensure that there was no initial slip between the specimen and the grips, which could affect the accuracy of the measurements. The testing machine was set to run at a constant displacement speed of 0.375 mm/min during the tests. For standard uniaxial tensile tests, the specimens were strained up to their fracture point to determine the mechanical properties of the material. These properties included yield strength, modulus of elasticity, ultimate tensile strength, ductility and plastic hardening. Subsequently, the properties obtained from the tests were used as input data for the material model in finite element simulations. The remaining six specimens were subjected to predetermined pre-strain levels of 4%, 8%, and 12%, with two specimens tested at each level. Tab. 1 summarizes the test program, while Fig. 1 depicts the uniaxial tensile and pre-straining experimental setups. C M ATERIALS AND METHODS

Specimen ID

Test End Condition

No. of Specimens

Uni-ST

Until Fracture 4% Strained 8% Strained 12% Strained

3 2 2

Uni-PS-4% Uni-PS-8% Uni-PS-12%

2 Table 1: Test program of standard uniaxial tensile and pre-straining tests

The pre-strained specimens were carefully sliced at the mid-section using a high-precision sectioning machine (see Fig. 1). A total of eight (8) specimens were prepared as detailed in Tab. 2. The small punch test was performed at room temperature following the standard test method for small punch testing of metallic materials ASTM 3205 standard [21]. The pre-load of 50 N and displacement rate of 0.375 mm/min were employed as similar to the uniaxial tensile test to avoid any discrepancy due to loading rate effects. The test was stopped when the specimen showed signs of fracture, indicated by an approximate 20% drop in maximum force according to the standard. Punch displacement was recorded using an extensometer and subsequently compared with displacement data obtained from the Universal Testing Machine (UTM). Each specimen condition was tested twice to confirm repeatability of the test results. The small punch test setup is shown in Fig. 2. The yield load, P y , estimated from the load-displacement curves can be correlated with the material yield strength. The yield load was determined by identifying the point where the material deformation transitions from the elastic to the plastic region. In the present study, four different methods for determining the yield loads were used, namely Mao, CEN, t/10 and t/100 [22] [23]. Fig. 3 illustrates the definition of the yield load based on the four methods mentioned above. Furthermore, the maximum load was determined at the point that marks the highest force reached during the test.

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