Indentation hardness
Hardness measurements quantify the resistance of a material to plastic deformation. Indentation hardness tests compose the majority of processes used to determine material hardness, and can be divided into two classes: "microindentation" and "macroindentation" tests. Microindentation tests typically have forces less than . Hardness, however, cannot be considered to be a fundamental material property. Classical hardness testing usually creates a number which can be used to provide a relative idea of material properties. As such, hardness can only offer a comparative idea of the material's resistance to plastic deformation since different hardness techniques have different scales.
Mental toughness
Mental toughness has been equated with better understood constructs from psychology such as resilience and hardiness. The term resilience is often incorrectly used interchangeably with mental toughness, and researchers have found the two constructs are positively associated with one another. However, psychologists define resilience as a positive adaptive "process" of coping with stress and adversity, as opposed to a collection of psychological attributes or personality traits. Hardiness has been suggested as a similar construct to mental toughness. Hardiness has typically been constructed as a personality trait, which is generally seen as stable throughout the lifetime. This differs from the conceptions of mental toughness offered by both Jones et al. and Gucciardi et al. These authors both conceive of mental toughness as unstable, arising in development, fluctuating over time, and varying for an individual performer between different sport and life scenarios. This definitional dilemma plagues the use of the term mental toughness and if mental toughness exists as a valid construct it may on occasion be maladaptive. Evidence to support this contention is derived from a study of overtraining behaviors and mental toughness by Tibbert (2013). She reported that "the MT attributes of mental self-concept and task familiarity displayed moderate curvilinear correlations with sport-specific recovery scales of the RESTQ-Sport. The curvilinear correlations reflect decreasing recovery at the highest levels of MT. The results suggest that some attributes of MT may relate to increased ability to recover whereas other attributes are associated with lower recovery (p.2-3). Arguably mental toughness is more closely linked with goal fixedness rather than adaptability and a flexible mindset, attributes which are central to resilience.