Development of Non-Destructive Indentation Fracture Toughness Methodologies for High Strength Alloys Case study: High strength rail steels and aluminum
Creators: Okocha, Stephen I
Publisher: name: University of Alberta Library
Year: 2024

<p>Rail derailment comprising largely from main track derailments frequently occur in the Canadian railway network. Reports have shown that some of these derailments experienced a train-initiated emergency brake application caused primarily from broken rails especially at extreme cold temperatures. These rail derailments thus require continuous monitoring of the rail structures for their integrity. However, using traditional means of estimating the structural integrity for an in-situ examination would not be feasible since samples need to be removed from existing structures. Hence, there is a desired need to adopt alternative methods for determining the structural integrity of in-service rail steels. The objective of this research project is to develop efficient, non-destructive indentation testing methodologies to establish the fracture toughness and mechanical properties of high-strength rail steels for implementation in current or potentially future technologies needed for continuous monitoring of rail infrastructures or verification of rail steel’s properties. In this research project, two indentation testing methods are developed to quantify the fracture toughness of high-strength rail steels. The project can be broadly divided into two major parts.<br>
Part I, reported in chapter 2 and 3, involves establishing the fracture toughness (KIC,pred ) of high strength rail steels using a modified critical fracture strain model and establishing mechanical properties of the rail steels via instrumented ball indentation test methods. 9 different rail steels were investigated to obtain the mechanical properties, which are vital parameters needed for the implementation of the modified critical fracture strain model. The modification to this model focused on determining the equivalent plastic fracture strain and equivalent plastic strain from tensile and indentation tests, respectively, as well as the characteristic distance needed for estimating fracture initiation. The study showed that stress triaxiality played a vital role by decreasing the ductility required to initiate fracture, which in conjunction with the prior austenite grain sizes are crucial for establishing the characteristic distance. The KIC,pred defined based on the modified critical fracture strain model from tensile tests showed a strong correlation with KIC determined from ASTM standard method. KIC,pred from indentation also offers the opportunity for ascertaining the fracture toughness non-destructively by focusing on the equivalent plastic strain and pressure at the tip of the indenter<br>
In Part II, which is reported in chapter 4, 5 and 6, involved the use of both flat-end cylindrical indenter rods and spherical indenters for estimating the fracture toughness using three different indenter sizes. The 2nd method for fracture toughness estimation was based on a modified limit load approach using the concept of virtual load and indentation depths analysis. Critical apparent stress intensity factor via J-integral was determined by combining virtually determined J-integral in the form of an apparent stress intensity factor (KJ) and extrapolating it to zero with the contact radius to imitate a sharp crack. The first phase of Part II showed that a chamfered cylindrical indenter is preferable to a flat-ended cylindrical indenter in estimating the fracture toughness due to the lower stress singularity at the edge of the indenter. For the second phase of Part II, the study focused on investigating between the chamfered indenter and the spherical indenter since the stress constraint is continuous with increasing depth. The study showed that the modified limit load analysis can also be employed for spherical indenters only when the average contact pressure is replaced with hardness estimated via expansion cavity model (ECM) approach. Comparison between the chamfered indenter and spherical indenter for fracture toughness showed that spherical indenters required lower indentation depths as well as smaller plastic zone size development at the material’s substrate than the chamfered indenter. For the third phase of Part II, the modified limit load analysis using spherical indentation is applied to rail steels.<br>
The fracture toughness for rail steels using the modified limit load model via spherical indentation showed a better measurement of KIC having an average value range from -2.66 – 2.51% difference while the modified critical strain model approach projected an average measurement of KIC range from -8.13 – 6.32% difference from the ASTM KIC measurement. In the end, indentation testing offers the opportunity for ascertaining the fracture toughness of high strength rail steels thus idealizing the condition of a non-destructive testing for structural integrity assessment in the future.</p>