Standard Test Method for Compressive Properties of Rigid Plastics
APP-D695Always verify that the upper spherically seated platen is unlocked and free to float during alignment, then locked or allowed to self-level upon initial contact.
Challenge & Testing Gap
Extraneous machine compliance and localized platen bedding deform measurements, severely inflating apparent crosshead displacement and skewing modulus accuracy.
The Solution
Utilize a self-aligning spherically seated compression platen coupled with a direct-on-specimen displacement transducer or non-contacting extensometer.
Mechanics & Specimen Behavior
Primary Mechanics
Uniaxial axial compression loading directed parallel to the major axis of the specimen.
Specimen Details
Right cylinder or rectangular prism. Standard blocks measure 12.7mm x 12.7mm x 25.4mm; sheets require an anti-buckling support jig.
Mechanical Ratios & Properties
Slenderness ratio design requires length to be exactly twice the principal width or diameter to delay structural buckling.
Expert Engineering Commentary
Core Problem Identification
Thin sheet polymer specimens buckle elastically out-of-plane, masking true compressive yield and ultimate strength numbers.
Root Cause Analysis
Insufficient slenderness ratio combined with rigid, non-adjusting flat compression face interfaces.
Hardware Specific Solutions
Hardened steel upper compression platen with an integrated spherical seat, paired with a low-friction anti-buckling support guide frame.
Analysis & Calculation Standards
Event & Failure Detection
Automated 0.2% offset compressive yield detection, or peak force tracking for specimens that fracture abruptly.
Required Calculations
Compressive Strength, Compressive Yield Stress, Compressive Modulus (Young’s Modulus), and Offset Yield Strength.
Statistical Outputs
Mean, standard deviation, min/max range limits across a standard 5-sample batch.
Real-time software system automatically executes a virtual compliance correction profile when physical extensometers are unfeasible.
Additional Commentary
True modulus calculations under D695 strictly demand an ASTM E83 Class B-2 extensometer or direct LVDT contact between the platen faces.
For unreinforced specimens thicker than 3.2mm, bypass the anti-buckling jig completely and utilize standard solid prisms to isolate pure material behavior.
Common Pitfalls
Reporting calculations derived purely from raw crosshead travel without accounting for load frame deflection under stress.