4. Result discussion and conclusions

The chapter presented here gives basic overview on issues related to small-size specimen testing in the field of the fracture mechanics tests. Some theoretical background and the relation between values obtained on small- and full-sized specimens for all regimes of the fracture behavior ranging from the lower shelf behavior up to the upper shelf region are shown. Some possibilities on how to resolve the size issue influence of the fracture toughness parameters and the reasons for differences obtained during the evaluation in the first chapter part were presented. An overview of size requirements for a valid value determination of the fracture toughness is also given. The subsequent experimental part is demonstrating results of the fracture toughness determination for three materials covering transition and upper shelf region behavior. As an important part of the fracture toughness tests are tensile properties determination. The chapter is dealing with miniature specimen testing; thus mini tensile tests are presented here for the basic property determination that is necessary for fracture toughness test preparation, execution and assessment.

Testing in the transition region and evaluation with the use of the Master Curve approach yielded very good result comparability between miniaturized and full-size specimens for the material investigated. Testing program spanning over five specimens' geometries agrees very well with published results and confirms reliable result determination in this region even with the use of the miniaturized specimens including 4-mm-thick mini-CT specimens and threepoint-bend specimens of cross section 2 � 4 mm2 . The upper shelf behavior with the stable crack extension was investigated for stainless steel and Ti-alloy produced by the additive manufacturing process. In the case of the stainless steel, four specimens' geometries were investigated, and multiple specimens' as well as single specimens' approaches were applied. Standard-sized specimen results yielded very good agreement with the results achieved for subsized three-point bend specimens, while the mini-CT specimen yielded values of about 60% of those ones obtained for standard-sized specimens. In the case of Ti-alloy, due to very limited amount of the experimental material, typically, e.g., for AM parts, mini specimens were investigated only. Both considered specimens' geometries yielded repeatable results. However, the CT specimens yielded significantly lower fracture toughness values of about 40% of those obtained for three-point-bend specimens. Large difference between these specimens' geometries results is in agreement with other published studies and results presented here for the stainless steel.

The results obtained here point out the fact that there is currently no available general solution for size effect description in the fracture toughness determination approaches so far. Varying agreement is found for various materials. Therefore, for a reliable "size-independent" value determination, the material of the interest has to be investigated and size effect quantified. It seems that the J-integral-based assessment has rather limited reporting value and better description has to be established for size-independent fracture toughness evaluation in the upper shelf. Lower transition region is well described by the Master Curve approach including size effect in the evaluation. Generally considered, there is no need in all cases to obtain sizeindependent values. These can be the case such as property assessment of the components of small wall thickness, where plain strain condition is in reality not predominant. Cases when local property anisotropy is being evaluated, just ratio among different locations and/or orientations, are considered. Typical examples of the materials produced with small wall thickness exhibiting high ration of property anisotropy are materials and components produced by the additive manufacturing processes. In these cases, there is generally hardly any chance to obtain "size-independent material properties" due to the reason that if produced in different wall thicknesses, different properties are achieved, and thus considered wall thickness has to be directly assessed.

As it can be seen in many cases, no real size-independent values are possible to achieve for the material, and thus small-size techniques are the only way to characterize the properties. These values are related just to the component and the process considered; however, valuable information are provided allowing component design and process optimization. Miniaturized specimen-based techniques for the fracture toughness determination were demonstrated here as a tool providing deeper insight into the material fracture behavior for better understanding of the material behavior in cases when limited amount of the experimental is available.
