Torsion-Based Stress-Strain Estimation Beyond Uniform Elongation
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Solution Overview
Problem
Existing methods for estimating the stress-strain curve in metal materials beyond uniform elongation suffer from inaccuracies due to necking, anisotropic yield functions, and the need for large-scale equipment, making them impractical and error-prone.
Innovation Solution
A method involving torsion tests on round-bar specimens with varying radii to estimate stress-strain curves using torsional torque and angle relationships, converting sheared stress and shear strain into uniaxial tension stress and strain, utilizing a compact torsion test machine.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If bulging test or compression test is used to acquire stress-strain curve beyond uniform elongation, then large deformation is applicable and necking is less likely to occur, but conversion to uniaxial tension relationship introduces error depending on anisotropic yield function
Solution Approach 1:
The invention extracts only the necessary information (stress-strain relationship) directly from torsion test data without requiring conversion through anisotropic yield functions. By performing torsion tests on specimens with different initial radii, the method directly obtains the stress-strain curve in the plastic region, eliminating the need for complex conversion processes and reducing errors associated with yield function assumptions.
Solution Approach 2:
Instead of converting from biaxial or compression test results to uniaxial tension relationships, the invention inverts the approach by directly measuring torsion behavior and extracting the uniaxial equivalent stress-strain relationship from the torsion data. This inversion eliminates the need for anisotropic yield function conversions.
2Measurement precision
If bulging test is used with DIC for strain measurement, then direction of strain is considered, but stress is estimated to be constant regardless of direction leading to error
Solution Approach 1:
The invention extracts stress information directly from the applied torsional moment and specimen geometry, rather than estimating stress from hydraulic pressure assumptions. This direct extraction method provides accurate stress values that account for the actual loading conditions, eliminating errors from constant stress assumptions.
3Reliability
If bulging test is used, then special test machine is required, but this reduces versatility and practicality
Solution Approach 1:
The invention uses a universal torsion test machine that can perform tests on various specimen types and materials. The torsion test apparatus is a standard equipment found in most material testing laboratories, making the method highly versatile and practical for different applications without requiring specialized bulging test machines.
4Reliability
If compression test is used to avoid necking, then stress-strain curve can be obtained, but friction with jig and influence of jig require correction using finite element analysis
Solution Approach 1:
The invention extracts the stress-strain relationship directly from torsion test data without requiring finite element analysis corrections. By using torsion testing with specimens of different radii, the method directly obtains the plastic region stress-strain curve without the friction and jig influence problems that plague compression tests.
5Measurement precision
If uniaxial tensile test with DIC is used to estimate stress-strain beyond uniform elongation, then theoretical calculation of tensile load is used, but assumptions about anisotropic yield function and associated flow rule may deviate from actual conditions
Solution Approach 1:
The invention extracts the stress-strain relationship directly from torsion test measurements without relying on theoretical calculations based on anisotropic yield functions or associated flow rules. This direct measurement approach eliminates errors from assumptions about material behavior models.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Accurately estimates stress-strain relationships beyond uniform elongation without necking, using a compact setup, reducing errors associated with large-scale equipment and anisotropy.
Implementation Method 1
a first torsion test step of applying a torsional torque T1 around an axis of a first round-bar specimen having a reduced section of a round bar-shape having a first radius R1 to perform a first torsion test for shear deformation of the reduced section
Data Source
Figure 1~2
Figure 3~4
Figure 5(a)~5(b)
AI summary
A stress-strain curve estimation method according to the present invention includes a step (S1) of performing a torsion test of a first round-bar specimen 10 to acquire a relationship between a torsional torque and a torsional angle, a step (S3) of performing a torsion test of a second round-bar specimen 10 having a reduced section 11 having a radius smaller than that of the first round-bar specimen 10 to acquire a relationship between a torsional torque and a torsional angle, a step (S5) of obtaining a relationship between sheared stress and shear strain of surface based on the relationships between the torsional torques and the torsional angles acquired by the torsion tests of the first round-bar specimen 10 and the second round-bar specimen 10, and a step (S7) of converting the sheared stress and the shear strain of surface into stress and strain in simple tension to estimate a relationship between stress and strain.