Research On The Determination Of Tensile Properties Of Metal Materials By Continuous Ball Indentation Technique

Continuous ball indentation method is a simple technique with the advantage of easy operation, which can be used to evaluate the mechanical properties of metal materials at the scene. Compared with conventional tensile test, this technique can be performed on in-service components and apply to assess the mechanical properties of small-sized parts. Therefore, it has a wide application future. In this dissertation, the theoretical analyses and experimental studies were carried out to investigate how to characterize the tensile properties including constitutive relation, strength properties, plasticity index and anisotropic-orthotropic plastic parameter of metal materials by continuous ball indentation method. According to the differences in material category and property, appropriate models are established, which extend its scope of application. The main contents and conclusions are as follows:(1) In order to optimize the experimental conditions, the influence of different experimental factors on the indentation test curve was studied systematically, which made the indentation test curve react to the real response of metal materials. The unloading curves of ball indentation test fed back not only the restoring deformation but also the plastic deformation. The influence of the plastic deformation on the measurement results of the contact stiffness was discussed. The new pile-up/sink-in characterization parameter was defined by the plastic deformation. And the relationship between the pile-up/sink-in parameter and the plastic parameters was established by finite element analysis. The optimized model was used to determine the constitutive relations and strength properties of three metallic materials. In addition, conventional tensile tests were also performed for comparison. Results reveal that the constitutive relations and strength properties of metallic materials can be effectively evaluated by continuous ball indentation.(2) Owing to the difference of stacking fault energy, there is not only power law hardening material but also linear hardening material. According to the characteristics of linear hardening material, a modified indentation model was proposed. The formula for calculating strength properties were established by means of new constitutive relation. The optimized model was applied to evaluate the constitutive relations and strength properties of austenitic stainless steels. Conventional tensile tests were also carried out for comparison. The results reveal that the constitutive relation and strength properties of austenitic stainless steel can be represented effectively with continuous ball indentation.(3) With the purpose of assessing the plasticity index of metal material by continuous ball indentation, a damage variable was introduced based on the concepts of continuum damage mechanics and the relationship between damage and elastic modulus was established. This relationship was designed for proposing the concept of the critical indentation depth h+ which can be applied to judge the failure time of metal material. The critical strain corresponding to the critical indentation depth was used to calculate the percentage reduction of area Z. To verity this model, the percentage reduction of area of three metallic materials was evaluated by both continuous ball indentation and conventional tensile tests. The results show good agreement in Z between indentation and tensile tests and the deviation is less than 20%, which demonstrate the feasibility of model.(4) In order to calculate the anisotropic-orthotropic plastic parameter of metal materials by continuous ball indentation, the yield stress ratio m was used to simplify the anisotropic-orthotropic plasticity model. The responses of anisotropic-orthotropic plastic metal materials in continuous ball indentation tests were discussed experimentally and numerically. Indentation curve was insensitive to anisotropic-orthotropic plastic metal materials. According to the sensitivity of indentation morphology, the relationship between the indentation depth ratio hz/hx and the yield stress ratio m was set up, which can be applied to the characterization of the anisotropic-orthotropic plastic parameter.