Materials Lab Report

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  • Topic: Fracture mechanics, Fracture toughness, Beam
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MECH3362: Materials II
Laboratory Report

Brittle Rupture of Glass Plates

Guirguis Rezk SID 199991603 Objectives:
  To understand mechanisms of the fracture of brittle materials (in this case, glass) To recognize the variations in deformation and failure mechanisms which occur with a difference in flaw size, (i.e. between scratched and unscratched samples) Defining flaw sizes in glass plates by employing linear elastic fracture mechanics (LEFM) Utilizing the Weibull statistics approach in predicting probable failure in brittle materials.

 

Testing Procedure:
1. The thickness (t) and width (w) of the rectangular glass sample is measured. The first glass slide is placed in the Instron 3-point bending apparatus with the support span set at 60mm (See Figure 1 –picture for illustrative purposes only) Support Span 60mm Figure 1: Instron 3-point bending apparatus Downward Motion – Application of load

2.

Glass Sample

Page 1 of 7 G Rezk SID 199991603

3.

The loading beam (Silicon Carbide) is lowered at 10mm per second onto the plate. The load is applied until the glass breaks against the two supports (also of Silicon Carbide). The values for maximum load and deflection are recorded from the apparatus. Steps 1-4 are repeated for ten different samples. With a piece of emery paper scratch the surface of the glass slides along the width in the centre on one side only of a glass sample (see Figure 2)

4. 5. 6.

t

w Scratches Figure 2: Glass plate dimensions and orientation of scratches

7.

Steps 1-6 are repeated for 10 samples ensuring that the scratched surface is placed face down in the testing apparatus. Data is recorded as per step 4.

8.

Representative Sketches:

P
��

�� �� 2 V (Shear Force) �� 2

�� �� 2

�� �� 2

M (Moment)
Page 2 of 7 G Rezk SID 199991603

Definitions of Load, Deflection and Specimen Span:
Original position of glass sample

Load

Deflection

Specimen Span
Load: The maximum force applied to the centre of the specimen span before fracture (measured in N). Deflection: The maximum deflection of the centre of the plate before fracture (measured in mm). Specimen Span: The distance between the two supports (measured in mm).

Specimen Geometry:
Specimen No:

Scratched w (mm) t (mm) 24.42 24.22 24.16 24.28 24.29 24.35 24.4 24.29 24.25 24.23 1.02 1.03 1.04 1.05 1.06 1.05 1.04 1.05 1.03 1.05

Unscratched w(mm ) t (mm) 24.38 24.34 24.36 24.25 24.20 24.20 24.23 24.19 24.23 24.14 1.00 1.00 0.99 0.99 1.01 1.01 1.01 1.00 1.01 1.01

1 2 3 4 5 6 7 8 9 10

Page 3 of 7 G Rezk SID 199991603

Three-Point Bending Formulae:
Flexural Stress: ������ = ������ �������� Young’s Modulus: ���� ���� �� = ������ ���� If assuming Linear Elastic Fracture Mechanics (LEFM) then: ���� �� = ���� �� �� = ������ ������ = �������� �������� ��

P: Load L: Specimen Span w: Width t: Thickness Stiffness: �� = �� ��

Mass Moment of Inertia: �� = ������ ����

��: Deflection

Load and Deflection Readings:
Scratched
Specimen No: Load (N) Deflection (mm)

Un-Scratched
Load (N) Deflection (mm)

1 2 3 4 5 6 7 8 9 10

17.60 11.79 17.02 17.45 16.16 14.28 15.87 14.07 11.28 13.10

0.48 0.41 0.47 0.44 0.43 0.46 0.42 0.38 0.37 0.37

28.08 26.68 25.48 32.78 20.16 22.85 21.58 24.67 25.80 36.00

0.77 0.70 0.64 0.87 0.54 0.6 0.58 0.63 0.64 0.98

Observations in deformation & failure mechanisms:
Observations from the results are: 1. 2. 3. Scratched specimens have a significantly lower deflection before failure Scratched specimens also failed under considerably lower loads. Scratched specimens exhibit consistent failure mechanisms with fracture nearly always parallel to the scratches on the surface and linear, while unscratched specimens had inconsistent and jagged fracture edges.

Page 4 of 7 G Rezk SID 199991603

(a)

Fracture Edges

(b)

Figure 3: Failure mechanisms for (a) unscratched and (b) scratched glass plates

Tabulated Results of Young’s...
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