ASTM D7137 PDF  - Standard Test Method for Compressive Residual Strength Properties of Damaged Polymer Matrix Composite Plates

1. ASTM D7137 

is the standard test method for determining the residual compressive strength of polymer matrix composite laminates after impact damage.

Unlike pristine compression tests, this method evaluates how well a composite structure performs after it has been damaged, typically by a low-velocity impact per ASTM D7136.

The test is especially relevant for:

• Aerospace primary and secondary structures

• Automotive composite panels

• UAVs and defense components

• Sporting goods and pressure shells

The output is commonly referred to as Compression After Impact (CAI) strength.

2. Real-World Engineering Example

Aircraft fuselage and wing skins are classic ASTM D7137 use cases.

A carbon fiber fuselage panel may experience:

• Tool drops during maintenance

• Bird strikes

• Runway debris impact

Even if the surface damage appears minor, internal delamination can severely reduce compressive load-bearing capacity. ASTM D7137 quantifies how much compressive strength remains, ensuring the structure still meets safety margins without immediate replacement.

This data directly informs:

• Damage tolerance design

• Repair vs. replace decisions

• Certification allowables

3. Test Specimen Overview

The CAI specimen is a rectangular composite laminate that has already been impacted.

Typical specimen characteristics:

• Material: CFRP or GFRP laminate

• Layup: Quasi-isotropic or application-specific

• Thickness: Defined by laminate schedule

• Damage: Controlled impact per ASTM D7136

The impact site is centered on the specimen and becomes the critical failure region during compression.

4. Test Fixture and Boundary Conditions

ASTM D7137 requires a guided compression fixture designed to:

• Prevent global Euler buckling

• Allow local failure at the impact site

• Maintain proper specimen alignment

Key fixture features:

• Anti-buckling side rails

• End loading platens

• Clearance for local out-of-plane deformation

The fixture is mounted directly into a universal testing machine (UTM).

5. Universal Testing Machine Requirements

A standard servo-electric or servo-hydraulic universal testing machine is used.

Typical setup includes:

• Compression platens or CAI fixture interfaces

• Load cell sized for expected failure load

• Crosshead displacement control

• Optional extensometry (global strain not always required)

Loading mode: Compression
Control mode: Displacement-controlled
Loading rate: As specified in the standard (quasi-static)

Proper alignment is critical — even small eccentricities can invalidate CAI results.

6. Test Procedure (High Level)

1. Impact specimen per ASTM D7136

2. Visually inspect and document damage

3. Install specimen into CAI fixture

4. Mount fixture into the UTM

5. Apply compressive load until failure

6. Record maximum load and failure mode

Failure typically initiates at the impact damage zone as:

• Local buckling

• Delamination growth

• Fiber microbuckling

7. Calculations and Formulas

Compression After Impact Strength

The primary result reported is CAI compressive strength:

σCAI=PmaxA\sigma_{CAI} = \frac{P_{max}}{A}σCAI​=APmax​​

Where:

• σCAI\sigma_{CAI}σCAI​ = Compression After Impact strength (MPa or psi)

• PmaxP_{max}Pmax​ = Maximum compressive load before failure

• AAA = Original cross-sectional area of the specimen

Cross-Sectional Area

A=b×tA = b \times tA=b×t

Where:

• bbb = specimen width

• ttt = laminate thickness

Note: The damaged area is not subtracted — strength is normalized to the full cross section.

8. Data Interpretation

ASTM D7137 results are typically compared against:

• Undamaged compression strength

• Design allowables

• Certification minimums

A significant reduction in CAI strength highlights:

• Poor damage tolerance

• Susceptibility to delamination

• Need for tougher resin systems or modified layups

9. Why ASTM D7137 Matters

ASTM D7137 bridges the gap between lab testing and real-world structural performance. For composite structures that rarely fail in pristine condition, CAI testing provides a realistic and conservative measure of safety.

It is a cornerstone test for:

• Aerospace qualification programs

• Material screening and comparison

• Structural health and survivability analysis