 ```python
import pydpf.core as pdpf
import numpy as np

def calculate_average_stress(result_file):
    # Create model and get node IDs
    model = pdpf.Model(result_file)
    node_ids = [node.id for node in model.nodes]

    # Find minimum amount of surrounding elements to get minimum volume
    min_volume_elements = pdpf.find_minimum_volume_elements(model, node_ids)

    # Read volume by element and find minimum list of elements by node to get volume check
    volume_by_element = pdpf.VolumeByElement(model)
    volume_check = pdpf.VolumeCheck(model, min_volume_elements, volume_by_element)

    # Create workflow to compute equivalent stress averaged on elements
    seqv = pdpf.EquivalentStressSequence(model)
    workflow = pdpf.Workflow(model, [seqv])

    # Apply dot product seqv.volume, sum up those on the list of elements, and divide this sum by the total volume on these elements
    elements_volume = np.array([volume_by_element[e.id] for e in min_volume_elements])
    avg_stress = np.dot(workflow.run()[seqv][min_volume_elements], elements_volume) / np.sum(elements_volume)

    # Plot equivalent elemental stress and volume averaged elemental equivalent stress
    pdpf.plot_equivalent_stress_elemental(model, workflow.run()[seqv])
    pdpf.plot_volume_averaged_equivalent_stress_elemental(model, workflow.run()[seqv], volume_check)

    # Use the operator with the same algorithm that has been implemented
    return avg_stress
```