Standard Test Method for Open-Hole Tensile Strength of Polymer Matrix Composite Laminates
APP-D5766Inspect the internal perimeter of the drilled hole under a microscope before testing; any evidence of fiber fraying or ply delamination invalidates the specimen.
Challenge & Testing Gap
Localized delamination and micro-cracking at the machined hole perimeter can propagate prematurely if jaw alignment is out-of-plane.
The Solution
Utilize rigid hydraulic wedge grips coupled with precise laser alignment systems to enforce absolute collinearity.
Mechanics & Specimen Behavior
Primary Mechanics
Direct axial tensile pulling of a flat composite coupon containing a centrally located, unreinforced drilled hole until macro-breakdown occurs.
Specimen Details
Constant cross-section flat rectangular strip laminate containing a precision-machined 6mm central circular hole.
Mechanical Ratios & Properties
Standard geometry dictates a 250mm length, 36mm width, and a highly controlled 6:1 width-to-hole-diameter ratio baseline.
Expert Engineering Commentary
Core Problem Identification
Premature longitudinal splitting at the hole margins or grip-line failure instead of clean transverse net-section fracture across the hole diameter.
Root Cause Analysis
Poor machining quality of the central hole (inducing hidden micro-cracks or delamination deltas) or angular misalignment within the wedge jaws.
Hardware Specific Solutions
High-capacity hydraulic wedge grips with fine-serrated diamond-cut jaw faces and a dedicated non-contacting optical strain mapping core.
Analysis & Calculation Standards
Event & Failure Detection
First-derivative sharp load-drop threshold capture tracking the sudden, catastrophic structural breakdown of the net cross-section area.
Required Calculations
Open-Hole Tensile Strength (computed using the gross cross-sectional area ignoring the hole), Ultimate Load, and Failure Code logging.
Statistical Outputs
Mean open-hole tensile limits, standard deviation profiling, coefficient of variation (CV%), and structural failure mode distribution mapping.
1000Hz real-time continuous data capture tracks micro-second acoustic/load drop events to log structural damage onset at the hole perimeter.
Additional Commentary
Open-hole tensile testing evaluates a laminate’s sensitivity to structural stress concentrations, a critical performance metric for bolted composite assemblies.
Use solid backup plates and a high-RPM diamond-tipped core drill to machine the central hole to eliminate backside exit delamination damage.
Common Pitfalls
Calculating the final tensile strength using the net area (subtracting the hole width) instead of the gross area as strictly mandated by the standard.