Mechanical tests

Tensile test:

 

In general, tensile test on metallic materials is used for different purposes.

One of these purposes is the possibility to determine some of fundamental mechanical material parameters, namely the yield strength (Re in MPa); the tensile strength (Rm in MPa);  percentage yield point extension (Ae in %); extension at fracture (A in %); the modulus of elasticity (E in MPa) and the fracture shape (brittle or ductile). All these fundamental mechanical parameters are then used for a proper design, production and final thermal treatment of the metallic component.

Another purpose of the tensile test concerns the possibility of checking the correspondence between the desired/prescribed parameters and the true mechanical parameters obtained after the production of a component. In this case, the testing parameters such as strain rate and strength rate play a remarkable role. Hammer laboratory uses instruments and equipment to carry out the tensile test at very low rate and at ordinary rate according to both UNI EN ISO 6892-1 and ASTM E8-E8M standards. Both standards are Accreditated.

These tensile test are performed under the supervision of the producer, the costumer and qualified inspectors.

 

 

Tensile test at eleveted temperature:

 

Some metallic components are produced for working at elevated temperature (part of
industrial plants where combustion takes place).

In this case, the fundamental mechanical parameters need to be studied and tested as a function of the temperature. Hammer laboratory can perform tensile test from room temperature (about 20°C) up to 900°C according to both UNI EN ISO 6892-2 and ASTM E21 standards. Both standards are Accreditated. The tensile test at elevated temperature can be used to compare different type of metallic material concerning its capacity to deform locally without cracking and thus to accommodate a local stress concentration.

Also the tensile test at elevated temperature are performed under the supervision of the producer, the costumer and qualified inspectors when the final checking of the production is required.

 

Impact Resistance or Charpy test:

 

Metallic alloy are usually very ductile material when subjected to load actions. However, in some circumstances, metallic alloy can behave as brittle material. The impact resistance or Charpy test helps engineers in studying the metallic alloy behaviour as a function of the temperature. In general, metallic alloys become brittle when strongly reducing the exposing temperature due to the arresting of the dislocations motion.

The impact resistance or Charpy test gives rise to an impulsive load on the specimen and the result is expressed in Joule (J) which represents the absorbed energy of the specimen. Low values of absorbed energy corresponds to brittle materials while high values corresponds to ductile materials.

Hammer laboratory can carry out the impact resistance or Charpy test on KV and KCU specimens over a wide range of temperatures (from -196°C to 100°C). The experimental tests are performed according to both UNI EN ISO 148-1 and ASTM E23 standards. Both standards are Accreditated. This test can be performed under the supervision of the producer, the costumer and qualified inspectors.

A single result at a given temperature from the impact resistance or Charpy test is not significant for the design phase. The impact resistance or Charpy test makes sense in determining the transition temperature from ductile to brittle (FATT curve), when studying the effect of changes of the chemical composition on the transition curve, or studying the effect of changes in the direction of the specimen (e.g. longitudinal or transversal) inside a component on the transition curve, or studying the effect of changes in industrial procedure on the transition curve or in studying the effect of changes in the grain size on the transition curve.

 

Hardness test:

 

The hardness test is widely used for metallic materials. This test method is quite simple and can be carried out in a short time. The results of the hardness test are very reproducible and can give several information.

  Three different types of method are suggested in the literature. The first method in named Brinell Hardness test. This method consists of a spheric indenter (usually made of polished tungsten carbide composite ball of diameter 10 mm or lower) which is applied on the material surface under investigation for 10 to 15 seconds. Then, the indentation diameters in the material surface under inspection is measured and converted into Brinell Hardness value. Attention should be paid in the surface preparation of the material under inspection and in the edge effects. The Brinell Hardness test is carried out according to both UNI EN ISO 6506 and ASTM E10-18 (Par. 5.7 not included). Both standards are Accreditated.

  The second method is named Vickers Hardness test and it is suggested for high hardness metallic materials. This method consists of a diamond indenter (in the form of a right pyramid with a square base and with a specified angle between opposite faces at the vertex) which is forced into the material surface under investigation for 10 to 15 seconds. Then, the diagonal length of the indentation left on the material surface under inspection is measured and converted into Vickers Hardness value. A mirror polishing is necessary for the material surface under investigation. The Vickers Hardness test is carried out according to both UNI EN ISO 6507 and ASTM E92-17. Both standards are Accreditated. The Vickers hardness scale and the Brinell hardness scale are very similar each other thanks to the fact the the hardness value is given as the ratio between the applied force and the indentation surface. Moreover, both hardness methods allow to estimate the tensile strength (Rm in MPa) of carbon steel indirectly.

   The third method is named Rockwell Hardness test. This method consists of a conical indenter (usually made of tungsten carbide) which is forced into the surface of a test specimen under two force levels using specific condition. The specified preliminary force is applied and the initial indentation depth is measured, followed by the application and removal of a specified additional force, returning to the preliminary force. The final indentation depth is then measured and the Rockwell values is derived from the difference in the final and initial indentation depths. The Rockwell Hardness scale cannot be correlate to the Brinell and Vickers scales.A