Flashcards in 6.2 Machinery hazards Deck (27)
Hazards associated with the use of machinery are normally categorised as 2
mechanical (to do with the moving parts) or non-mechanical (to do with all other aspects).
Mechanical hazards associated with a machine, machine parts or surfaces, tools, work-pieces, loads, or projected solid or fluid materials These can be summarised and remembered with the acronym
EnTICE. En tanglement T raps – crushing, shearing, and drawing-in I mpacts C ontacts – cutting / severing, friction / abrasion, stabbing / puncture E ejection, including high pressure fluid injection.
Non-mechanical hazards arising from the use of machinery include: 10
Electrical (Element C8) Thermal (Element B10) Noise (Element B6) Vibration (Element B6) Radiation (Element B7) Materials and substance (Element B1) Ergonomic (Elements A7, B9 and C5) Slip trip (Element C1) Environmental hazards (Element C4) Hazard combinations (Minor individual hazards can combine to be equivalent to a significant hazard).
The primary causes of material failure are
operating loads, such as stress, impact and frictional loading, and environmental conditions, such as corrosive, high temperature, and high energy environments, with combinations of both often leading to rapid wear-out and failure.
_____ & _____ are the key measurements used to understand the properties of the materials that objects are made of.
Stress and strain
Stress describes the relationship between the applied force and the area over which it acts. It can be expressed as a formula:
S = F/A
Stress is measured in
Newtons per metre squared (Nm-2), which is the same units as for pressure).
Strain is the measurement of the change in the shape of the material / object as a result of the application of the stress force. The precise definition of strain depends on the type of deformation produced. The simplest case is of forces of tension applied to opposite ends of a wire or rod which stretch or extend the length of the wire or rod. In this case:
Strain = Length change / original length
Strain is also caused by the application of stresses other than tension, notably: 4
compression, bending, shearing and torsion,
Materials can fail in more than twenty different recognizable ways. The following common failure modes are specified on the NEBOSH Diploma syllabus: 5
Fatigue Ductile failure Brittle fracture Buckling Corrosive failure.
Fatigue failure occurs when
a material fractures into two or more pieces after being subjected to a cyclic stress (fluctuating load) over a period of time.
The fatigue failure mechanism involves three stages:
Crack initiation - usually at a ‘material inhomogeneity’, such as notch, groove, surface discontinuity, flaw or other material defect. Crack propagation - the applied stress concentrates until it exceeds the local strength of the material and produces a crack. Material rupture - when the crack has weakened the material to a point such that it can no longer support the applied load it will rupture, by shear or by tension.
Ductile materials that are subjected to a tensile or shear stress will elastically or plastically strain to accommodate the load and absorb the energy. Yielding occurs when the material’s yield strength is exceeded and can no longer return to its original shape and size. This is followed by ductile fracture, which occurs when the deformation processes can no longer sustain the applied load.
Brittle fracture occurs when
mechanical loads exceed a material’s ultimate tensile strength, causing it to fracture into two or more parts without undergoing any significant plastic deformation or strain failure.
Buckling occurs when
a material subjected to compressive or torsional stresses can no longer support the load, and it consequently fails by bulging, bending, bowing or forming a kink or other unnatural characteristic.
the deterioration of a metal or alloy and its properties due to a chemical or electrochemical reaction with the surrounding environment.
Galvanic corrosion is
a form of corrosive attack that occurs when two dissimilar metals (such as stainless steel and magnesium) are electrically connected, either through physically touching each other or through an electrically conducting medium, such as an electrolyte. When this occurs, an electrochemical cell can be established, resulting in an increased rate of oxidation of the more anodic material (lower electrical potential). The opposing metal, the cathode, will consequently receive a boost in its resistance to corrosion.
Outline the characteristic features of, and factors that promote, the following types of materials failure: (a) brittle fracture 5 marks (b) ductile fracture 5 marks
(a) A brittle fracture generally occurs without warning or prior evidence of distress. It is a crystalline structure failure with minimal plastic or elastic deformation. There are generally characteristic 'chevron' marks from the point of initiation and the failure is sudden from rapid stress loading. The factors promoting a brittle fracture are high tensile stresses, residual or built in stresses, sudden loading which does not give the material time to deform plastically, case hardening, low temperatures and the degree of brittleness of the material. (b) A ductile fracture generally has a smooth fracture surface with plastic deformation of the material before final fracture. There is evidence of necking and the final fracture is often brittle because there is insufficient material left to sustain a load. This type of failure generally occurs as the result of a single stress overload although other promoting factors include high temperatures, cold work hardening and the plasticity of the material.
NDT encompasses a range of test processes that produce no harmful effects on the material or structure under test. NDT techniques include: 6
Simple visual examination of surfaces Dye penetrant techniques Radiography (Gamma and X-ray) Ultrasonic testing Eddy currents Magnetic particle inspection.
Dye penetrant outline
The liquid penetrant is drawn into the surface-breaking crack by capillary action, and excess surface penetrant is then removed. A developer (typically a dry powder) is then applied to the surface, to draw out the penetrant in the crack and produce a surface indication. Fluorescent penetrants are usually used when the maximum flaw sensitivity is required, and can detect cracks as narrow as 150 nm.
The radioactive source is placed on one side of a specimen and a photographic film on the other side, an image is obtained on the film of the thickness variations in the specimen, whether these are surface or internal.
Ultrasonic methods use beams short wavelength (1 to 10 mm) and high-frequency (0.1 to 20 MHz) mechanical waves (vibrations) transmitted from a small probe and detected by the same, or other, probes.
Eddy current testing outline
Eddy current testing places a coil carrying an AC current close to the specimen surface, or around the specimen. The current in the coil generates circulating eddy currents in the specimen close to the surface, which in turn affect the current in the coil by mutual induction. Flaws and material variations in the specimen affect the strength of the eddy currents.
Magnetic particle inspection (MPI) outline
The specimen is magnetised either locally or overall, and if the material is sound the magnetic flux is predominantly inside the material. If, however, there is a surface-breaking flaw, the magnetic field is distorted, causing local magnetic flux leakage around the flaw. This leakage flux is displayed by covering the surface with very fine iron particles, applied either dry or suspended in a liquid. The particles accumulate at the regions of flux leakage, producing a build-up which can be seen visually, even when the crack opening is very narrow. Thus, a crack is indicated as a line of iron powder particles on the surface.
X-ray radiography is a non-destructive testing (NDT) technique that is sometimes used to examine the internal structure of fuel tanks in the wings of large passenger aircraft. (a) Outline the principles of operation when using X-ray radiography for this application. 4 marks (b) Other than visual inspection, explain why different forms of NDT might not be appropriate for this application. 4 marks (c) Identify TWO disadvantages in the application of X-ray radiography. 2 marks
(a) Radiation would be generated by a portable electrically powered machine such as a generator focussed into a beam by lead shielding. The beam would be directed to penetrate the wing skin to reach a film or electronic sensor placed inside the tank on the opposite side of the wing. An image of the intervening structure within the wing is created on film after photographic development or on the electronic sensor after electronic processing, with defects being revealed by the contrast between light and dark areas. The image has light areas where less radiation reaches the film or sensor after passing through unbroken or sound structure, whereas the image has darker areas where more radiation reaches the film or sensor through voids or cracks in a structural element which is faulty. This part of the question was not well answered with many candidates showing confusion between radioactive sources and x-rays generated by a machine. (b) 4 other forms of NDT which might not be appropriate for the scenario include: gamma radiography, which would be impractical and too expensive eddy current can experience spurious defect indications and does not provide a permanent record dye penetrant shows only surface cracks with accessibility to tanks being impossible magnetic particle testing does not work on non-ferrous metal and there would again be the problem of tank inaccessibility there would be no indication of fault location from tap testing. (c) Disadvantages in the application of x-ray radiography include: the manual handling of heavy equipment the possibility of exposure to x-rays training in interpretation skills is necessary for the operators its use is relatively expensive.
The NEBOSH Diploma specifies the following four major accidents to illustrate the causes and effects of materials failure:
Brent Cross 1964 (Overloading, bending moments, buckling of crane jib) Markham Colliery 1973 (fatigue failure of winding engine brake rod) Littlebrook D 1978 (corrosion of hoist cage wire ropes) Ramsgate Walkway Collapse 1994 (fatigue failure of bearing mount welds due to torsional stresses ‘roll’).