domande d'esame maurizio vedani

Ultrasonic Testing

By placing the transducer in contact with the surface to be analyzed by means of a conduction medium (oil, fats, gels), introducing the waves into the material. As the diameter of the smallest detectable defect is equal to half the wavelength (d = λ/2), it is important to choose the best sensitivity for the material analyzed. In fact, for coarse structures (like in foundry products) low frequencies can be adopted, while for materials containing finer defects also high frequencies can be used. By moving the transducer and the detector along the material analyzed, any difference in the structure of the material causes a modification in the signal, therefore cracks perpendicular to the signal are easier to detect as they involve a wider area. We can work with reflection or transmission method. The type of defect can be understood by comparing the signal obtained to a reference one with known defects. This method can be applied in case of welded joints or in general to detect bulk defects.also with great thicknesses in any material with a good penetration depth. Pros:

• Detect bulk defects
• Need just one side of the material
• Quick
• Detect position as well as size of the defects
• High sensitivity
• Great thicknesses
• On-site analysis

Cons:

• Only materials with good penetration depth, that is low acoustic attenuation
• No complex geometries
• Skilled operators

Extrinsic mechanism try to affect the path of the crack during its propagation. Mesoscopic mechanism try to optimize the fibrous texture of the material to deviate crack propagation from a straight path:

• Crack arrester, there is a delamination ahead of the crack tip that reduces triaxiality (to plane stress) and crack-tip acuity, so less stress intensification, that contribute positively to arrest the propagation of the crack
• Crack divider, pancake structure, instead of a bulk piece, the crack propagates across different planes, so the triaxiality is lost and we reach the better toughness condition of plane

The distinction between type I, II and III residual stresses depends on their extension:

• Type I: they have the size of the component itself (macroscopic)
• Type II: they develop among crystalline grains
• Type III: they belong to the atomic scale

Type one residual stresses can develop, for example, during hot rolling: for small rolls and high deformations, the surface tends to deform but it is prevented by the matrix. On the contrary, for big rolls, the matrix tends to expand but it is prevented by the surface, which is stuck by the roll. In the first case, we have a compressed surface and a tensed matrix, while in the second case, the opposite situation occurs.

Type two residual stresses are at the scale of dimension grains. The main cause for their formation is that different grains have different deformability to the same macroscopic loading due to their different crystallographic orientation. They may be produced when a material containing two different constituents is deformed. If one

constituent is tougher or has a different coefficient of thermal expansion than the other one, they deform in a different way and therefore induce residual stresses. Type three residual stresses can be caused by inclusions of interstitial atoms leading to the elongation/reduction of the atomic bonds nearby, like in steel where there is C inside Fe lattice. Bulk defects that can be found in castings are: - Gas porosity: bubbles of gasses may be trapped into the molten metal when it solidifies due to decreasing solubility. It is generally found in interdendritic position. The most common gasses are hydrogen (small and bright spots) and oxidant gas (big and dark). - Shrinkage porosity: it is caused by volume reduction due to solidification. It can be distinguished into macro-porosity, when liquid can't fill all the gaps in the solid (final spongy surface), and micro porosity, when liquid metal can't counterbalance the shrinkage during solidification. - Inclusion defects, as happens for

alumina in aluminum or non metallic

phase or refractory particles

Cracks and hot tears, growing in the most stressed zones like during

cooling shrinkage in presence of geometrical constrain

According to Ashby, there are in general up to six different mechanisms by which a solid polycrystalline metal can be plastically deformed:

1. plasticity ruled by tangential stress exceeding the ideal shear stress of the τ lattice, id
2. plasticity by glide of dislocations
3. plasticity by twinning glide and climb of dislocations
4. plasticity at high temperature due to (dislocation creep) in the crystal lattice
5. plasticity induced by vacancy flow (diffusion) (Nabarro Herring creep) grain boundary
6. plasticity induced by vacancy flow (diffusion) at (Coble creep)

Depending on material and testing (applied stress and temperature) conditions, different mechanisms can be activated. Which of these mechanisms is really activated at a certain stress-temperature condition is represented in Ashby maps:

There are

two methods, both destructive.

1. Stoney equation: it consist in removing a thin layer of material and measure the consequent deflection caused by the releasing of residual stresses. Almentest also exploits this equation.
2. Hole drilling method: The principle involves the introduction of a small hole into a component containing residual stresses and the subsequent measurement of the locally relieved surface strains around the hole with a strain gauge rosette. The local residual stress (at hole center) can then be calculated from these strains using formulae and calculations derived from experimental and Finite Element analyses.

Procedure: a hole is drilled in the component at the center of a special strain gauge rosette which has been previously glued on the surface. The hole is performed stepwise, with small increments. At each increment, data about strain relaxation are recorded. Main critical point:

• It is important that a suitable drilling method be used to avoid alterations, e.g. abrasive.

jetmachining or high-speed air-turbine drilling.
There is no advantage in making measurements beyond a depth roughly equivalent to the drill diameter, since no additional strain can be measured on the surface.
The basic hole drilling analysis assumes that the material is isotropic, linear elastic and that stresses do not vary significantly with depth, especially within the length of the hole.
The analysis only applies where residual stress values do not exceed half the yielding of the material.
In the case of thin plate (i.e. h < 0,4D), one only set of gauge measurements is necessary.
Otherwise, measurements should proceed stepwise with small increments (for instance, increments of 0,05D could be used), to achieve a correct evaluation of average residual stress filed along the thickness. Moreover, the measured values should increase with increasing depth of measurement and be uniform along thickness.
Steel alloys' workability can be plotted as a function of temperature.
In castalloys ductility is lower due to solidification defects, big grains, low-melting second phase, insoluble compound and micro-segregation at GB which decrease the melting point, and brittle phase. Always avoid to cold work a cast alloy! The hot ductility gap is present in wrought alloys. The plot obtained can be divided into three different regions: the first one (cold working) and the third one (hot working) highlight a ductile behavior, while in the second region (700-1100 °C) there is a ductility gap. It can become very clear in presence of alloys like Al, Nb, N or V and it becomes critical during straightening of continuous casting billets deformed after solidification (a liquid core may still be present) or during hot plastic deforming at the critical temperature range. This low deformability region can occur under two different conditions, both take to intergranular fracture: Presence of a thin proeutectoid ferrite film (α-Fe is softer than austenite) on grain boundaries, that

could promote the nucleation of microvoids or crack formation due to strain localization

In the absence of proeutectoid ferrite, this process can occur due to

• precipitates at grain boundaries leading to PFZ (precipitate free zones) weaker than the grain core promoting the formation of microvoids

Nitrogen increase the ductility gap due to the formation of titanium and aluminum nitrides.

Increasing in workability of wrought parts:

Region I: At low temperature, the amount of ferrite(α-Fe) is higher. This decreases the strain concentration in the film.

Region III: Above the critical recrystallization temperature, microvoids are isolated from the other ones as soon as they nucleate, so that microvoid coalescence is prevented. The increasing temperature also motivates the increase in workability.

3 kinds of short cracks:

• Microstructurally: cracks have a size comparable to microstructure feature, the crack’s plastic zone is fully contained in one or few grains.
• Mechanically:

cracks have smaller size than the geometrical feature that promotes the intensification of the stresses that induce the crack's nucleation. These types of cracks are fully surrounded by the plastic zone of the macroscopic notch, so their growth aren't much affected by microstructural variables.

- Chemically: they grow in a chemically aggressive environment that promotes their growth also if value of ∆K is lower than the threshold value in an inert environment.

Evolution during time: The growth rates of short cracks are significantly different from those of long cracks in the same material for the same ∆K value. Short cracks have its own da/dN trend and not all the short cracks end their life or propagate by the same mechanism, in fact the hypothesis of homogeneous material is totally lost at the microstructural scale.

The liquid penetrant method is a non-destructive technique able to detect surface defects by following the steps listed below:

Accurate cleaning of the piece

Application of the penetrant on the entire surface for a sufficient time, from 5 to 30 minutes, so that it can penetrate into the defects Removal of the exceeding liquid Application of a developer, generally a powder, uniformly and homogeneously on the surface, in order to take out the liquid that remain inside the defects, and make them visible. Detection of defects Generally, the liquid penetrant is fluorescent to make the detection easier. It penetrates the cracks because of capillarity, not of gravity, so it works even on non-horizontal surfaces. The advantages of this analysis method are that it is cheap and it doesn't require qualified operators and it can be applied on any material. The disadvantages are that it requires a clean