Composite Test Methods

Question 1

Explain how strain gauges determine tensile modulus and poisson ratio

Answer 1

• Conducted at 1 or 2 mm/minute
• If the fibres are aligned along specimen axis, E1 is measured
• E2 is measured in samples with fibres aligned at 90 degrees
• The Poisson’s ratios are determined from the 2 strain gauges

Question 2

Describe the test technique used to characterise the mode I inter laminar fracture properties of a unidirectional composite

Answer 2

• Involves opening a cracked body (low energy level)
• Unidirectional laminates that are 3 to 4 mm thick are made with an artificial defect at one end
• Loaded at 1 to 2mm/minute and the crack propagates along the mid plane
• Stable crack propagation, crack length measured by eye

Question 3

Describe the test technique used to characterise the mode II end notch flexure specimen properties of a unidirectional composite

Answer 3

• Shearing a cracked body specimen loaded in 3 point bending

-Loaded at 1 to 2mm/minute

• 20mm wide specimens are then cut from the plate and positioned on simple supports (a/L~0.5)
• Crack propgates in an unstable manner to the centre of specimen directly under the load.
• Impact resistence can be improved by adding liquid rubber to matrix (polymer) during mixing

Question 4

What is NDT

Answer 4

Allows inspection of a material or component without impairing serviceability (used to inspect polymer matrix composites)

Question 5

Explain pulse-echo ultrasonic methods

Answer 5

• Signal is produced by an emitter on one side of the laminate
• ’ embedding’ the signal in water or apply a gel to the emitter( over the surface of the composite) improves the transmission of the signal
• Reflective glass placed behind structure, ultrasonic waves reflected by defects in the composite material.
• The difference between the emitted signal energy and the received signal provides information on the presence of defects
• The signal from the reflective glass passes through the material twice and is most affected by the damage

Question 6

What are A-scans, B-scans and C-scans concerning pulse echo ultrasonic methods?

Answer 6

A-scans: Simplest type, signal displayed on oscilloscope, depth of damage can be estimated, size and nature of the defect is estimated from the strength of the signal

B-scans: Obtain a representation for the cross section of the material by combining Linear A-scan data

C-scans: Data combined over entire surface, light areas show damage and darker areas show undamaged material, can detect damage as small as 2mm across

Question 7

Explain “through transmission” ultrasonic methods

Answer 7

• Separate receiver place behind the structure
• Emitter and receiver scan over the sample at high speed, often , signal is emitted a jet of water and the receiver emits a jet of water to ‘capture’ the signal (signal can only pass through material ONCE)

Question 8

Explain x rays (radiography) use as NDT

Answer 8

Pass ionising radiation through a structure and the level of absorption is recorded by film on the opposite side of the structure (different densities , debonding or inclusions, will contrast)

Usually, the specimen is immersed in an X-ray absorbing liquid prior to inspection. The liquid seeps through the cracks within the composite and then absorbs the X-rays during inspection

Question 9

Explain what happens during acoustic emission

Answer 9

When a crack develops within a composite, during loading, stress waves are released that propagate to the surface of the material producing small surface displacements.

Surface-mounted transducers such as a piezoelectric crystal, convert these displacements into electrical signals

You can count the number of amplifications of the signal to understand how the structure is loaded

Damage locations can be identified by using several transducers and measuring the time of flight of the signal(s) to each sensor.

Depends on damage initiating from pre-existing damage and so is not truly non-destructive.

Question 10

Explain Thermography

Answer 10

Thermal method that involves subjecting the composite to a rapid pulse of heat and measuring the resulting temperature rise

Defects such as delimitation change the heat diffusion rate through the composite, local temperature variations detected using infra-red camera

In the double-sided method, the source and camera are on different sides of the composite and defects appear as cooler regions (large area can be analysed very quickly)