The concept of objectivity in science means that qualitative and quantitative descriptions of physical phenomena remain unchanged when the phenomena are observed under a variety of conditions. For example, physical processes (e.g. material properties) are invariant under changes of observers; that is, it is possible to reconcile observations of the process into a single coherent description of it.
Physical processes can be described by an observer denoted by . In Euclidean three-dimensional space and time, an observer can measure relative positions of points in space and intervals of time.
Consider an event in Euclidean space characterized by the pairs and where is a position vector and is a scalar representing time. This pair is mapped to another one denoted by the superscript. This mapping is done with the orthogonal time-dependent second order tensor in a way such that the distance between the pairs is kept the same. Therefore, one can write:
By introducing a vector and a real number denoting the time shift, the relationship between and can be expressed
The one-to-one mapping connection of the pair with its corresponding pair is referred to as a Euclidean transformation.
A physical quantity like displacement should be invariant relative to a change of observer. Consider one event recorded by two observers; for , point moves to position whereas for , the same point moves to . For , the displacement is . On the other hand, for , one can write:
The velocity can be obtained by differentiating the above expression:
By reorganizing the terms in the above equation, one can obtain:
is a skew tensor representing the spin of the reference frame of observer relative to the reference frame of observer (Holzapfel 2000). To simplify the mathematical notation, the arguments of functions will no longer be written.
From the above expression, one can conclude that velocity is not objective because of the presence of the extra terms and . Nevertheless, the velocity field can be made objective by constraining the change of observer to:
The material time derivative of the spatial velocity returns the spatial acceleration. By differentiating the transformation law for the spatial velocity, one can obtain:
which can be rewritten as the following:
Just like the spatial velocity, the acceleration is not an objective quantity for a general change of observer (Holzapfel 2000). As for the spatial velocity, the acceleration can also be made objective by constraining the change of observer. One possibility would be to use the time-independent rigid transformation introduced above.
The general condition of objectivity for a tensor of order can be applied to a scalar field for which . The transformation would give:
Physically, this means that a scalar field is independent of the observer. Temperature is an example of scalar field and it is easy to understand that the temperature at a given point in a room and at a given time would have the same value for any observer.
Euclidean transformation of others kinematic quantities
where represents the material coordinates. Using the chain rule, one can write:
From the above equation, one can conclude that the deformation gradient is objective even though it transforms like a vector and not like a second order tensor. This is because one index of the tensor describes the material coordinates which are independent of the observer (Holzapfel 2000).
The Cauchy traction vector is related to the Cauchy stress tensor at a given point by the outward normal to the surface such that: . The Cauchy traction vector for another observer can be simply written as , where and are both objective vectors. Knowing that, one can write:
The three stress tensors, , and , studied here were all found to be objective. Therefore, they are all suitable to describe the material response and develop constitutive laws, since they are independent of the observer.
It was shown above that even if a displacement field is objective, the velocity field is not. An objective vector and an objective tensor usually do not conserve their objectivity through time differentiation as demonstrated below:
Objectivity rates are modified material derivatives that allows to have an objective time differentiation. Before presenting some examples of objectivity rates, certain other quantities need to be introduced. First, the spatial velocity gradient is defined as:
where is a symmetric tensor and is a skew tensor called the spin tensor. For a given , and are uniquely defined. The Euclidean transformation for the spatial velocity gradient can be written as:
Substituting in the above equation, one can obtain two following relations:
Substituting the above result in the previously obtained equation for the rate of an objective vector, one can write:
where the co-rotational rate of the objective vector field is defined as:
and represents an objective quantity. Similarly, using the above equations, one can obtain the co-rotational rate of the objective second-order tensor field :
This co-rotational rate second order tensor is defined as:
The principal of material invariance basically means that the material properties are independent of the observer. In this section it will be shown how this principle adds constraints to constitutive laws.
In order to find the restriction on which will ensure the principle of material frame-indifference, one can write:
A constitutive equation that respects the above condition is said to be isotropic (Holzapfel 2000). Physically, this characteristic means that the material has no preferential direction. Wood and most fibre-reinforced composites are generally stronger in the direction of their fibres therefore they are not isotropic materials (they are qualified as anisotropic).