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Compressive Deformation

What Does Compressive Deformation Mean?

Compressive deformation in the context of buried pipelines can be defined as the deformation of the pipe material when it is subjected to compressive forces. The material will undergo deformation without failing if the compressive forces are below its breaking strength. Once the forces exceed that threshold, the pipe will fail. Compressive forces can be either stress or strain and can be measured using compressive strength tests.

A compressive strain is produced when a body decreases in length when equal and opposite forces try to compress it. Compressive stress is a dimensionless property that expresses the relationship between an object's change in length (ΔL) to its original length (L). If a compressive force is applied to the pipe:

The compressive strain is expressed as ε = -ΔL/L.

Based on Hooke's law, the relation between stress and strain is expressed as

σ = Eε,

where σ is stress, E is Young's modulus, and ε is strain.

The compressive deformation of a material is considered negative while tensile deformation is considered positive. When a tensile force is applied to a material, it develops a stress corresponding to the applied force (P), contracting the cross-section and elongating the length (ΔL).

The tensile strain ε is expressed as ε = ΔL/L.

Based on Hooke's law, the relation between stress and strain is expressed as

σ = Eε,

where σ – Stress, E – Young's modulus, and ε – Strain

On receiving tensile force, the material expands in the axial direction (longitudinal strain) while contracting in the transverse direction (transverse strain). The ratio between the longitudinal and transverse strains is known as Poisson's ratio.

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Trenchlesspedia Explains Compressive Deformation

Pipelines are extensively used to transfer liquid and gas products over long distances. Some pipelines are constructed above ground while some are buried out of sight. For the safe design of a buried pipe, engineers must know the types, sources, and calculation methods of all forces exerted on the pipe.

Pipe Loading

Buried pipelines are subject to different kinds of loads depending on their location. Buried pipes under highways will be subject to the dead load of the soil above it and varying live loads from moving traffic. Pipelines buried in remote locations may only have to deal with the dead load of the soil above it and occasional live loads.

Pipelines also experience initial stresses such as hoop tension and longitudinal contraction caused by internal pressure, axial stress due to temperature change, and ring bending due to overburden pressure and pipe laying process.

Pipe Materials

Pipes are either rigid such as concrete, vitrified clay, and cast iron pipes, or flexible such as high-density polyethylene (HDPE), polyvinyl chloride (PVC), and glass-reinforced plastic (GRP) pipes. Flexible pipes are designed to deform to a certain extent when compressive forces act on them. When the load above the pipe reaches a critical load or deflection, the pipe can become structurally damaged by buckling. Rigid pipes are capable of self-supporting loads with little help from backfill. However, beyond permissible loading conditions, a rigid pipe will fail by fracturing or cracking before considerable deflection has taken place.

Compression Tests

Compression tests are carried out to determine the behavior of a material when crushing loads are applied. The test is conducted by applying compressive pressure to a test specimen on a testing machine. Various parameters of the material are calculated and plotted as a stress-strain diagram. It is then used to determine qualities such as elastic limit, yield strength, and compressive strength. Compression testing allows manufacturers to assess the integrity and safety of materials.

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