True Stress Testing via Shadow Imaging
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Solution Overview
Problem
Conventional stress and strain testing systems fail to accurately measure true stress and true strain due to difficulties in consistently and accurately measuring the reduced cross-sectional area of specimens during testing, as they only account for the original gage area and do not adjust for changes in the location of the minimum area, leading to significant errors in the calculation of the true stress-strain curve.
Innovation Solution
A true stress testing system comprising a force input machine, an imaging system with a light source and camera to create a shadow image of the specimen, and a computer that determines the minimum diameter of the specimen at multiple points along its length, allowing for accurate accounting of changes in the position of the minimum diameter over time, thereby providing a more accurate analysis of true stress.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional engineering stress tests use the original gage area for stress calculation, then the testing procedure is simple, but the measurement precision of true stress is poor due to not accounting for necking
Solution Approach 1:
The patent replaces mechanical measurement systems with optical imaging systems. A camera captures images of the specimen during tensile testing, and image processing algorithms automatically determine the minimum cross-sectional area at multiple points along the gauge length. This substitution eliminates the need for complex mechanical measurement devices while achieving high-precision true stress measurement that accounts for necking and area reduction.
Solution Approach 2:
The patent creates a digital copy (image) of the specimen's physical state during testing. By capturing images at multiple points along the gauge length and processing these images computationally, the system determines the minimum cross-sectional area without physically contacting or interfering with the specimen. This copying approach enables accurate measurement of the necked region while maintaining test integrity.
2Measurement precision
If stationary tags are used to measure specimen area, then the measurement system is simple, but the measurement precision is poor because tags cannot move to track the minimum area location
Solution Approach 1:
The patent transforms the measurement system from static (stationary tags) to dynamic (multiple camera positions or movable imaging system). The system captures images at multiple locations along the gauge length, either simultaneously with multiple cameras or sequentially by moving the imaging system. This dynamic approach allows the measurement system to track the moving minimum area location as necking progresses during tensile testing, maintaining measurement accuracy throughout the deformation process.
Solution Approach 2:
The patent divides the measurement task into multiple segments by measuring area at multiple discrete points along the gauge length rather than using a single stationary measurement point. Image processing identifies the minimum cross-sectional area at each segment location, and the system determines the overall minimum area by comparing all segment measurements. This segmentation approach ensures accurate tracking of the necked region regardless of its position.
3Measurement precision
If single-point area measurement is used, then the testing procedure is simple, but the measurement precision of true stress is poor due to inability to account for necking location changes
Solution Approach 1:
The patent replaces time-consuming manual or sequential measurement methods with automated optical imaging and computational analysis. Multiple images are captured during the tensile test, and computer algorithms automatically process these images to determine minimum cross-sectional areas at all measurement points. This substitution significantly reduces processing time compared to manual measurement methods while providing comprehensive multi-point area data for accurate true stress calculation.
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
This approach provides more accurate stress analysis by accounting for changes in the position of the minimum diameter, improving upon theoretical stress calculations and enabling more precise determination of true stress and strain, especially for specimens with non-linear plastic behavior and complex geometries.
Implementation Method 1
The imaging system includes a light source for projecting a light beam in a first direction at the specimen and a camera positioned on an opposite side of the specimen for receiving portions of the light beam not blocked by the specimen. A shadow image of the specimen is thus created via the camera.
Data Source
AI summary
A true stress testing system broadly comprising a force input machine, an imaging system, and a computer. The imaging system includes a light source for projecting a light beam at the specimen in a first direction and a camera positioned on an opposite side of the specimen for receiving portions of the light beam not blocked by the specimen such that a shadow image of the specimen is created via the camera. The computer may determine a minimum dimension of the specimen perpendicular to the first direction at a point in time over a plurality of points along the force axis via the shadow image of the specimen such that the processor accounts for changes in position of the minimum dimension along the specimen. A true stress of the specimen may then be determined according to the minimum dimension of the specimen perpendicular to the first direction at the point in time.