Standard Test Method for Plastic Strain Ratio r for Sheet Metal
APP-E517Ensure the specimen edges are polished completely free of blanking burrs; even minor edge roughness skews the transverse width extensometer reading.
Challenge & Testing Gap
Microscopic localized necking and knife-edge indentation from biaxial extensometers introduce uncompensated strain measurement imbalances, severely distorting the computed width-to-length strain slope.
The Solution
Deploy a high-resolution, synchronous axial-transverse extensometer or a non-contact video extensometer array paired with parallel-action pneumatic side grips.
Mechanics & Specimen Behavior
Primary Mechanics
Unaxial longitudinal tensile extension locked within the uniform plastic deformation regime prior to the onset of localized necking.
Specimen Details
Standard sheet-type rectangular tensile specimen machined per ASTM E8/E8M geometry with highly parallel gauge margins.
Mechanical Ratios & Properties
Calculations executed at fixed, uniform plastic strain levels (typically between 15% and 20%) to isolate deep-drawing anisotropic behaviors.
Expert Engineering Commentary
Core Problem Identification
Knife-edge slippage of the transverse strain sensor or out-of-plane specimen bowing, yielding highly volatile or negative r-value data points.
Root Cause Analysis
Inadequate jaw clamping pressure causing microscopic specimen slippage, or burrs along the specimen edges that corrupt transverse tracking.
Hardware Specific Solutions
High-stiffness electromechanical universal testing machine equipped with a multi-channel synchronous strain processing module and dual-axis extensometry.
Analysis & Calculation Standards
Event & Failure Detection
Elastic-to-plastic transition threshold mapping, target plastic strain limit flags, and ultimate localization necking events.
Required Calculations
Plastic Strain Ratio (r-value), Vertical Anisotropy Coefficient, Logarithmic True Axial Strain, Logarithmic True Width Strain.
Statistical Outputs
Batch summary logs reporting mean r-values across orthogonal sheet orientations (0, 45, and 90 degrees), standard deviations, and planar anisotropy (delta-r).
Real-time data matrix engine executing continuous r-value calculations natively at 1000Hz via instantaneous logarithmic true strain ratios.
Additional Commentary
The r-value is a critical index of a sheet metal’s drawability; a high r-value indicates a material that resists thinning under deep-drawing stress states.
For maximum data fidelity, utilize a non-contact video extensometer system to capture width reduction across multiple parallel lines simultaneously.
Common Pitfalls
Extracting width data from post-test manual micrometer measurements instead of continuous real-time extensometry, which ignores elastic springback artifacts.