Reproducible evaluation of material properties. Static Testing Material response to constant loading

Transcription

1 Material Testing

2 Material Testing Reproducible evaluation of material properties Static Testing Material response to constant loading Dynamic Testing Material response to varying loading conditions, including magnitude, cycling, and mode

3 Static Material Testing Evaluation of Material Strength Deformation Fracture Design requirement compliance Standardized Tests Tensile test Compression test Hardness test

4 Tensile Test Uniaxial A straight line axial force is applied to a test sample (typically in the y axis) Hounsfield Tensometer Image courtesy of NSW Department of Education and Training Destructive Force is applied until sample fails.

5 Tensile Test Standard Test Sample (dog bone) Ensures meaningful and reproducible results. Uses a uniform cross section.

6 Tensile Test Procedure Dog bone is created to test specifications Dog bone is secured in tester

7 Tensile Test Procedure A tension force (F) is applied to the dog bone until failure occurs. Simultaneously the applied tension force (F) and dog bone elongation (d) are recorded. F A plot is created from the stored load elongation data. d

8 Tensile Test Data A B F Test sample A and B are 230 red brass. Test sample A has a diameter of in. Test sample B has a diameter of in. If both samples are tested to failure, will the applied tension force and elongation be the same for both tests? NO Why? d

9 Tensile Test Data Load-elongation results are dependent upon sample size. Larger sample indicates larger load-elongation. How can test data be manipulated to represent a material and not an individual test sample?

10 Tensile Test Data To eliminate test results based on sample size, calculate sample stress. Stress is load per unit area. Divide load (F) by the original test sample cross-sectional area (A 0 ) load stress = area F σ = A

11 Tensile Test Data Calculate the stress in the dog bone with a 430 lb applied force. F σ = A 2 area = r 2 area = ( in.) 430 lb in. 2 2 area = in. 35,000 psi

12 Tensile Test Data Manipulating Elongation Results To eliminate test results based on sample size, calculate sample strain. Strain (e) is the amount of stretch per unit length. Elongation (d) under load, divided by the original length (L 0 ) strain = amount of stretch original length

13 Tensile Test Data Calculate the strain in the dog bone with an elongation of in. strain = amount of stretch original length in. ε = 1.000in. =

14 Tensile Test Stress-Strain Curve

15 Tensile Test Stress-Strain Curve Proportional Limit Elastic Range Initial response is linear. Stress and strain are proportional to one another.

16 Tensile Test Stress-Strain Curve Proportional Limit Proportional Limit Stress at which material starts elongating more than the proportion in force.

17 Tensile Test Stress-Strain Curve Modulus of Elasticity (E) The proportional constant (ratio of stress and strain). σ E = = ε stress strain A measure of stiffness The ability of a material to resist stretching when loaded. An inherent property of a given material.

18 Tensile Test Stress-Strain Curve If the load is removed, the test sample will return to its original length. The response is elastic or recoverable. Exaggerated stretch to illustrate principle

19 Tensile Test Stress-Strain Curve Elastic Limit = Yield Point Uppermost stress of elastic behavior. Elastic limit and yield strength mean the same thing. Elastic limit and proportional limit are almost identical, with the elastic limit being slightly higher.

20 Tensile Test Stress-Strain Curve Resilience The amount of energy per unit volume that a material can absorb while in the elastic range. Area under the stress-strain curve. 1 bh 2 Why would this be important to designers? Hint: car bumper

21 Tensile Test Stress-Strain Curve Yield Point When the elastic limit is exceeded. A very small increase in stress produces a much greater strain. Most materials do not have a welldefined yield point

22 Tensile Test Stress-Strain Curve Offset Yield Strength Defines the stress required to produce a tolerable amount of permanent strain. Common value is 0.2%

23 Tensile Test Stress-Strain Curve Yield Point Plastic Deformation Unrecoverable elongation beyond the elastic limit. When the load is removed, only the elastic deformation will be recovered.

24 Tensile Test Strength Properties Stress Strain Curve Plastic deformation represents failure. Part dimensions will now be outside of allowable tolerances

25 Tensile Test Stress-Strain Curve Plastic Deformation without necking Elongation continues, some is permanent Cross-section decreases along entire sample. Load can continue increasing.

26 Tensile Test Stress-Strain Curve Tensile Strength Load bearing ability peaks. Less force is now required to continue elongating. Weakest location begins to decrease in area more than other locations Necking

27 Tensile Test Stress-Strain Curve Plastic Deformation with Necking Sample can now be stretched with less force.

28 Tensile Test Stress-Strain Curve Failure If continued force is applied, necking will continue until fracture occurs. Ductility Amount of plasticity before fracture; The greater the ductility, the more a material can be deformed.

29 Tensile Test Samples Compare the material properties of these three metal samples.

30 Tensile Test Stress-Strain Curve Brittleness Material failure with little or no ductility. Lack of ductility, not lack of strength.

31 Tensile Test Stress-Strain Curve Toughness Work per unit volume required to fracture a material. Total area under the stress-strain curve from test initiation to fracture (both strength and ductility).

32 Tensile Testing Examples steel rebar tensile test steel cylinder tensile test welded steel concrete cylinder metal cable

33 Compression Test Stress and strain relationships are similar to tension tests elastic and plastic behavior Test samples must have large cross-sectional area to resist bending and buckling Material strengthens by stretching laterally and increasing its cross-sectional area Video examples concrete test composite test soda can concrete 2

34 Hardness Testing Resistance to permanent deformation. Resistance to scratching, wear, cutting or drilling, and elastic rebound. Brinell Hardness Test A tungsten carbide ball is held with a 500 lb force for 15 sec into the material. The resulting crater is measured and compared. Brinell testing video

35 Hardness Testing Rockwell Test A small diamond-tipped cone is forced into the test sample by a predetermined load Depth of penetration is measured and compared. Rockwell testing video

36 Resources NSW Department of Education and Training (2011). Retrieved from g/index.htm?signature=%287e02281c-318a-461b-a8ed- 3394db0c4fe6%29 Askeland, Donald R. (1994). The Science and Engineering of Materials, 3rd ed. PWS Publishing: Boston.