It is a stress-strain diagram. Let’s divide the diagram into 3 different zones: 1, 2, 3, each characterized by its own typical trend.

ZONE 1: Linear law between stress and strain (Hook’s Law). The slope of such a line is a measure of the elastic module of the material. If during this phase you stop applying the force, the preserved elasticity of the material brings it back to the original dimension from which you started (i.e. the origin of the axes). Elastic modulus, also known as Young's modulus, is a fundamental property of materials that measures their stiffness or resistance to elastic deformation under stress. This property is essential in engineering and materials science, as it determines a material's ability to support loads and maintain its shape. The area of each zone (kg x m) represent the necessary energy to deform the sample of (unknown) material under test.

ZONE 2: The law between stress and strain is no more linear. Even stopping to apply the force, the original elasticity of the material is damaged and lost and it is impossible to bring back to the material to its original dimension. Noticeable is the negative slope in this zone. The deformation continues with decreasing force up to a minimum and the slope changes sign again, presumably due to work hardening of the material (as reported in some literature).

ZONE 3: The minimum point reached can be considered the beginning of actual failure, characterized by large deformations with sm aller forces, which again follow a law that is substantially linear here, but is not necessarily the case in all materials.

(Refer to the enclosed document)

There can be several reasons why the stress-strain curve of your concrete model in Abaqus may exhibit a certain behavior.

You can utilize the following solutions.

Material Properties

Concrete Model

Failure Criteria

Loading Conditions

Utilizing websites such as https://caeassistant.com/blog/fiding-reaction-force-on-a-body/

Abaqus distinguishes very well in the graphical results between action and reaction by presenting two separate graphs with different trends, especially in the case of dynamic loads. Refer to:

https://caeassistant.com/wp-content/uploads/2019/11/Fiding-Reaction-Force-on-a-Body.webp

In this case, however, the question was asked on a single graph without specifying whether it concerns active load and reactive load combined together.But it is not the case. So I am inclined to assume that the positive slope of Zone 3 is the result of work hardening, just as the specific literature mentions about it (search for: work hardening effect in unreinforced concrete). Anyhow, in case anyone has a different, specific explanation, I'd appreciate hearing about it.

For further details see also:

https://roccomorelli.blogspot.com/2024/01/why-does-stress-strain-curve-in-abaqus.html