Unit 5 Fatigue and fracture

AE 2030 FATIGUE AND FRACTURE - 2014 UNIT-5 FUNDAMENTALS FUNDAMENTALS OF FAILURE ANALYSIS Common causes of failure, Principles of failure analysis. Fracture mechanics approach to failure problems. Techniques of failure analysis. Service failure mechanisms - ductile and brittle fracture, fatigue fracture, wear failures, fretting failures, environment induced failures, high temp. failure. Faulty heat treatment and design failures, processing failures (forging, casting, machining etc.),

Common causes of failure

Failure of an aircraft structural component can have catastrophic consequences, with resultant loss of life and of the aircraft. The investigation of defects and failures in aircraft structures is, thus, of vital importance in preventing further incidents. This review discusses the common failure modes observed in aircraft structures, with examples drawn from case histories. What causes failure?

In general, failures occur when a component compo nent or structure is no longer able ab le to withstand the stresses imposed on it during operation. Commonly, failures are associated with stress concentrations, which can occur for several reasons including: • Design errors, e.g. the presence of holes, notches, and tight fillet radii; • The microstructure of the material may contain voids, inclusions etc.; • Corrosive attack of t he mater mater i al, e.g. e.g. pitti ng, can also ge generat nerate e a local stress stress conce concent ntrr ati on. From case histories data, an assessment can be made of the frequency of failure modes (Table 1). Table

Corrosion 29 16 Creep 3 -

T.Manikandan, Asst Professor, Aeronautical Engg Dept, Jeppiaar Engg College, Chennai-119 Page 1

AE 2030 FATIGUE AND FRACTURE - 2014 Wear/abrasion/erosion 3 6 This

reveals that the incidence of fatigue failure dominates the distribution in

aircraft. The detection and rectification of corrosion damage on in-service aircraft, however, consumes more effort than the repair of fatigue cracking. The high occurrence of fatigue failure observed probably reflects the destructive nature of this failure mode, while corrosive attack is generally slower than fatigue, and usually more easily spotted and rectified during routine maintenance.

Common failure modes

Fatigue is a process whereby cracking occurs under the influence of repeated or cyclic stresses, which are normally substantially below the nominal yield strength of the material. Components that fail by fatigue usually undergo three separate stages of crack growth, which are described as follows: • I ni tiati on of a fati gue cr ack. Thi s can be in f lu enced by str ess concentr ation s such as materi al defects or design.

• Propagation of the fatigue crack. This is progressive cyclic growth of the crack. • Final sudden failure. Eventuall y, the propagatin g crack r eaches a cri ti cal size at whi ch th e r emain in g materi al cann ot support t he appli ed loads and sudden ru ptur e occur s.

Fatigue fractures tend to be relatively smooth nea r the origin and show slight roughening of the surface as the crack progresses. There tends to be little or no macroscopic ductility associated with fatigue cracking. Detailed examination of the fracture surface in a scanning electron microscope (SEM) usually shows evidence of fatigue striations (dependant o n the material), which represent one cycle of load and crack propagation. If the magnitude of load cycle remains constant, the striations normally appear closer near the origin, graduall y increasing in spacing as the crack front progresses due to the increasing stress at the crack tip.

Fatigue cracking is the most common cause of structural failure in aircraft, Materials and their

design can be taken into consideration so that the probability of fatigue cracks occurring can be reduced, but it is often the case that the possibility cannot be removed completely. Therefore many aircraft structural components are designed with a safe or inspection-free life, below which T.Manikandan, Asst Professor, Aeronautical Engg Dept, Jeppiaar Engg College, Chennai-119 Page 2

AE 2030 FATIGUE AND FRACTURE - 2014 fatigue cracking should not be a cause for concern. The fact that fatigue failures still occur, however, indicates the complex nature of this problem. There are many variables that influence fatigue, some of which are the mean stress, peak stress, frequency of loading, temperature, environment, material microstructure, surface finish, and residual stresses.

Material surface defects such as forging laps or surface cracking can increase the local

stress, producing a concentration at these points that c ould initiate fatigue much quicker than would be expected. However, many aircraft components are thoroughly inspected by nondestructive techniques after manufacturing and these types of d efects are usually detected and rectified. Stress concentrations caused by surface defects such as scratches and wear tend to be more common as these may not be present at build, but can be introduced during service. Another common cause of stress concentration is corrosion, which can lead to fatigue crack initiation. Ductile or overload failure occurs when a material has been exposed to an applied load at a

relatively slow rate to the breaking point of the material. This results in a ductile fracture of the material, with the fracture surface exhibiting tearing of the metal and plastic deformation. On rapid application of a load, fast fracture or brittle failure can occur. Microscopic examination of brittle fractures reveals intergranular or transgranular facets on the fracture surface.

Corrosion is the chemical degradation of metals as a result of a reaction with the environment. It

usually results in failure of components when the metal wastes to such an extent that the remaining material cannot support the applied loads or the corrosion renders the component susceptible to failure by some other mode (e.g. fatigue). Extensive work has been carried out on the rates and types of corrosion observed in different materials so that selecting a suitable material in terms of corrosion resistance for a known environment is relatively straightforward. In aircraft structures, however, the strength to weight ratio can be a more desirable property than corrosion resistance and in these circumstances the most suitable material cannot always be used. In cases like this, measures must be taken to limit corrosion, which most commonly involve the use of a coating, such as a paint system, to act as a barrier to the environment. There are various forms of corrosion that exist, each of which


AE 2030 FATIGUE AND FRACTURE - 2014 poses different problems to aircraft structures. The most common types of corrosion observed are discussed below: • Uniform corrosion, as its name suggests, is corrosion that occurs without a ppreciable localized attack, resulting in uniform thinning. • Pitting corrosion is a localized form of attack, in which pits develop in a material causing localized perforation of the material. Pitting corrosion occurs when one area of a metal surface becomes anodic with respect to the rest of the surface of the material. The pits formed by this type of attack are generally very small and, therefore, difficult to detect during routine inspection. Pitting attack can cause failure b y perforation with very little weight loss to the material. • Crevice corrosion occurs when localized changes in the corrosive environment exist and lead to accelerated localized attack. These changes in the localized corrosive environment are generated by the existence of narrow crevices that contain a stagnant environment, which results in a difference in concentration of the cathode reactant between the crevice region and the external surface of the material. Crevices can be formed at joints between two materials, e.g. riveted, threaded, or welded structures, contact of a metal with a nonmetallic material, or a deposit of debris on the metal surface. • Galvanic corrosion occurs when dissimilar metals are in direct electrical contact in a corrosive environment. This results in enhanced and aggressive corrosion of the less noble metal and protection of the more noble metal of the bimetallic couple. This type of corrosion can be recognized by severe corrosion near to the junction of the two dissimilar metals, while the remaining surfaces are relatively corrosion-product free. Galvanic corrosion is generally a result of poor design and materials selection. • Stress corrosion cracking is a mechanical-environmental failure process in which tensile stress and environmental attack combine to initiate and propagate a fracture. Failure by stress corrosion cracking is frequently caused by simultaneous exposure to an apparently mild chemical environment and to a tensile stress well below the yield strength of the material. The stress required for failure can originate from in-service conditions or from residual stress during component manufacturing. Hydrogen embrittlement is a failure process that results from the retention or absorption of

hydrogen in metals, usually in combination with applied tensile or residual stresses. It most T.Manikandan, Asst Professor, Aeronautical Engg Dept, Jeppiaar Engg College, Chennai-119 Page 4

AE 2030 FATIGUE AND FRACTURE - 2014 frequently occurs in high-strength steels (>1100 MPa). For aircraft components, the common source of hydrogen embrittlement is hydrogen absorption du ring manufacturing processes such as pickling and electroplating.

Service failure mechanisms Brittle Fracture

Basically, brittle fracture is a rapid run of cracks through a stressed material. The cracks usually travel so fast that you can't tell when the material is about to break. In other words, there is very little plastic deformation before failure occurs. In most cases, this is the worst type of fracture because you can't repair visible damage in a part or structure before it breaks.

In brittle fracture, the cracks run close to perpendi cular to the applied stress. This perpendicular fracture leaves a relatively flat surface at the break. Besides having a nearly flat fracture surface, brittle materials usually contain a pattern on their fracture surfaces. Some brittle materials have lines and ridges beginning at the origin of the crack and spreading out across the crack surface.

Types of Brittle Fracture Transgranular Fracture

The first type of fracture is transgranular. In transgranular fracture, the fracture travels through the grain of the material. The fracture changes direction from grain to grain due to the different lattice orientation of atoms in each grain. In other words, when the crack reaches a new grain, it may have to find a new path or plane of atoms to travel on because it is easier to change direction for the crack than it is to rip through. Cracks choose the path of least resistance.


AE 2030 FATIGUE AND FRACTURE - 2014 when a crack has changed in direction through the material, because you get a slightly bumpy crack surface.

Intergranular fracture

The second type of fracture is intergranular fracture. Intergranular fracture is the crack traveling along the grain boundaries, and not through the actual grains. Intergranular fracture usually occurs when the phase in the grain boundary is weak and brittle ( i.e. Cementite in Iron's grain boundaries). Transgranular fracture cuts through the puzzle pieces, and intergranular fracture travels along the puzzle pieces pre-cut edges.

DUCTILE FRACTURE

This signifies large plastic deformation, and occurs after extensive plastic deformation prior to and during the propagation of the crack. This requires considerable energy which is absorbed in forming dislocations and other imperfections (defects) in metals. In a ductile fracture, there are three successive events involved: (i) Test sample or specimen begins necking and mi nute cavities form in the necked region. The plastic deformation is concentrated in this region and indicates that the formation of cavities is closely linked to plastic deformation, hence to the dislocation movement, thereby taking the longest time in the fracture process, (ii) The cavities coalesce and form minute crack at the centre of the test specimen,


AE 2030 FATIGUE AND FRACTURE - 2014 (iii) The crack propagates outwardly to the surface o f the specimen by a shear separation in a direction 45° to the tensile axis, resulting in a familiar “cup and cone” type fracture. The various stages involved in ductile fracture are shown in Fig. Necking begins at the points of plastic instability where the increase in strength due to strain-hardening, fails to compensate for the decrease in cross-sectional area. Th is occurs at maximum load, the formation of a neck introduces a tri-axial state of stress in the region. A hydrostatic component of tension acts along the axis of the specimen at the centre of the necked region. Many fine cavities form in this region (Fig.c), and under continued straining these grow and coalesce into a central crack. This crack grows in a direction perpendicular to the axis of the specimen until it approaches the surface of the specimen. It then propagates to the surface of the specimen in a direction roughly 45° to the tensile axis to form the cone part of the fracture (Fig.e).

FATIGUE FRACTURE

A far more common mode of failure is the fracture of a structural member that has been subjected to many cycles of a fluctuating load. Failure occurs even though the load amplitude may be much less than the static yield strength of the material. This form of fracture is known as fatigue fracture , or simply fatigue.


AE 2030 FATIGUE AND FRACTURE - 2014 Fatigue here refers to a progressive failure of a material after many cycles of load. A form of failure known as low-cycle fatigue also occurs in which the strains are much larger than those in high-cycle fatigue. The behavior of a material under cyclic load is influenced by many factors, but of prime importance are the amplitude of the load and the presence in the material of regions of stress concentration. The fatigue characteristics of a material are also affected by the type of loading (bending, torsion, tension, or a combination of the three); the specimen size, shape, and surface roughness.

Fatigue Failures

Metal fatigue is caused by repeated c ycling of of the load. It is a progressive localized damage due to fluctuating stresses and strains on the material. Metal fatigue cracks initiate and propagate in regions where the strain is most severe.

The process of fatigue consists of three stages:

sudden fracture of the remaining cross section.

Wear failure

Wear may be defined as damage to a solid surface caused by the removal or displacement of material by the mechanical action of a contacting solid, liquid, or gas. It may cause significant surface damage and the damage is usually thought of as gradual deterioration. While the terminology of wear is unresolved, the following categories are commonly used.



Adhesive wear


AE 2030 FATIGUE AND FRACTURE - 2014 Adhesive wear has been commonly identified by the terms galling, or seizing. Abrasive wear, or abrasion, is caused by the displacement of material from a solid surface due to hard particles or protuberances sliding along the surface. Erosion, or erosive wear, is the loss of material from a solid surface due to relative motion in contact with a fluid that contains solid particles. More than one mechanism can be responsible for the wear observed on a particular part.

Fretting Fatigue Fretting fatigue is the combination of tribiological and fracture behaviour of materials which are in contact together and having small relative movement at the same time. Due to fretting, fatigue lifetime is significantly reduced as compared to that when no fretting takes places. This is because of the high stress gradients that are generated at the counter surfaces of the two connected bodies. Many failures in mechanical components due to fretting fatigue have been reported and studied in literature, e.g. threaded pipe connections, bolted and riveted joints, bladedisk attachment in gas and steam turbines, shrink-fitted shaft and aero-engine splined couplings, wires and so on. T.Manikandan, Asst Professor, Aeronautical Engg Dept, Jeppiaar Engg College, Chennai-119 Page 9

AE 2030 FATIGUE AND FRACTURE - 2014 Fretting fatigue failure process can be divided into two different categories of mechanical failure process namely, fretting and fracture failures . Fretting is related to tribological behavior of interface surfaces of bodies which are in contact and fracture behavior is more related to fracture mechanics properties of bulk material. The fraction of fretting fatigue lifetime spent in crack initiation and in crack propagation depends on many factors, e.g. contact stresses, amount of slip, frequency, environmental conditions, etc.., and varies from one application to another.

Therefore, both crack initiation and propagation phases are important in analyzing fretting fatigue problem. In fretting fatigue experiments, it is very difficult to detect the crack initiation phase; the onset of crack growth and the crack growth rate because damage and cracks are always hidden between the counterpart surfaces. Therefore, numerical modelling techniques for analyzing fretting fatigue crack initiation and propagation phases are very desirable. Environment Induced Failure

Exposing a material to high temperatures is another cause of fatigue in materials. Thermal expansion, and contraction will weaken bonds in a material as well as bonds between two different materials. For example, in space shuttle heat shield tiles, the outer covering of silicon tetraboride (SiB4) has a different coefficient of thermal expansion than the Carbon-Carbon Composite. Upon reentry into the earth's atmosphere, this thermal mismatch will cause the protective covering to weaken, and eventually fail with repeated cycles. Another environmental affect on a material is chemical attack, or corrosion . Small pits may form on the surface of the material, similar to the effect etchin g has when trying to find dislocations. This chemical attack on a material ca n be seen in unprotected surface of an automobile, whether it be by road salt in the winter time or exhaust fumes. This problem can be solved by adding protective coatings to the material to resist chemical attack. The problem of corrosion fatigue is complex, and it must be dealt with on the basis of tests of each combination of material and environment. The action of the c yclic stress in destroying protective surface films and of the corrosive agent in producing surface pits and reaction products leads to reciprocal aggravation of both corrosion and fatigue of the metal. Fatigue can also be induced by the relative motion of two surfaces in contact; this form of failure is known as . fr etting f atigue T.Manikandan, Asst Professor, Aeronautical Engg Dept, Jeppiaar Engg College, Chennai-119 Page 10

AE 2030 FATIGUE AND FRACTURE - 2014

High temperature failure

The effect of service environment on material performance at elevated temperature can be divided into three main categories: 1. Mechanical effects, such as creep and stress rupture. 2. Chemical effects, such as oxidation. 3. Microstructural effects, such as grain growth and o veraging

The fatigue strengths of metals generally increase as temperatures decrease; however, the notch sensitivity and tendency to brittle fracture of some metals greatly increase at low temperatures. At higher temperatures fatigue lives generally decrease, but the analysis is complicated by the presence of the creep mechanism of failure as well as fatigue.

Principles of failure analysis

Failure of engineering components occurs by several mechanisms, which can be arranged in order of importance as follows:

•Corrosion, which can be defined as the unintended destructive chemical or electrochemical reaction of a material with its environment, represents about 30 % of the causes of component failures in engineering applications.

•Fatigue, which occurs in mater ials when they are subjected to fluctuating load s, represents about 25 % of the causes of component failures in engineering applications.

Discuss the following: (i) Probit method

(ii) PROT method

Probit method : This method one in which several specimens are tested for a given no.of

cycles at each of4 or 5 stress values near the stress of interest first the desired life is determined, for example 107cycles. T.Manikandan, Asst Professor, Aeronautical Engg Dept, Jeppiaar Engg College, Chennai-119 Page 11

AE 2030 FATIGUE AND FRACTURE - 2014

PROT method : Test a no. of specimens starting from a low stress of about 60% to 70%

of its estimated fatigue strength stress in increased continuously in each c ycle at a constant rate will fracture, then test a no. of speci mens at a different rate of increase in loading, starting from the same low stress.

Discuss a few fatigue problems encountered in subsonic and supersonic aircraft. To design and construct efficient fatigue resistant supersonic transport (SST) and subsonic transport structures reliable for 50,000 hrs in the severe environment (particularly in Mach 3 condition) is a considerable challenge. The successful operation of the aircraft in this severe environment generally depends on the following factors. 56˚C to 2000˚C. (i) Temp. vari ation of above - (ii ) A combin ation of m echani cal and th ermal stresses (ii i) U p to 50000 hr s of operati ng l if e un der the above conditi ons. As a fi r st step in providin g optimum fati gue resistance as well as many other f un ctional r equi rements data on (i ) Service loads (ii ) Temp (ii i) A tmosphere conditi ons I n addi ti on to th ese general problems, sever al exampl es of more specif ic f atigue data are invol ved in the design of SST.

56˚C, 30˚C and 1000˚C. (a) Conventi onal; f atigue data at above - (b) Ext rapolati on of shor t and lon g term f atigue sin ce 3000 hr s and 5000 hr s. (c) Simult aneous load and temperatur e effects (d) F ail safe design s, crack arr estors and redun dant str uctur es for hi gh r eli abil ity. (e) At crui sing al ti tudes (about 17,000 ft ) th e ozone atmosphere may impact the f atigue resistance of metals. (f ) B ecause the win g ski n acts more li ke a 2D plan e than a 1D beam sur face and th e T.Manikandan, Asst Professor, Aeronautical Engg Dept, Jeppiaar Engg College, Chennai-119 Page 12

AE 2030 FATIGUE AND FRACTURE - 2014 f uel tan ks and the fu selage are in tern ally pr essur ized there are bi-ax ial str esses f atigue data un der bi -axi al stresses are requi r ed. (g) Prevention of explosive f ail ur e of pr essur ized fu selage. (h) Contr ol of cracki ng due to soni c fatigue. (i) Preventi on of cumul ative damage.


Unit 5 Fatigue and fracture

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