The term failure means the component/machine fails functionally. (eg: excessive deflection, buckling, creep, corrosion, cracking, fatigue ductile and brittle fracture etc.) whereas fracture means the separation of component into two or more pieces under the action of load (delamination etc).

I think failure and damage means same (not sure about it, someone please correct me if I am wrong)

dear Sajith

I think you right about failure and fracture, but i don't know about difference of damage and failure, too.

Damage leads to failure and failure leads to fracture. When a material is damaged it has not necessarily failed in service and can be repaired somewhat like a shaft that can be built -up through powder metallurgy to its original dimension but when it fails it cannot be used in service, i.e. it has lost its integrity--the property that makes it useful like a shaft that has yielded (plastic deformation) in service. However, when a material fractures it is broken into two or more parts.

I will agree with the comments of others here.

I will just like to add damage and failure is quite ambiguous term.

Damage means that component of the machine is disrupted but it is not necessary that damaged component may not offer services.

In simple words if you have a grinding tool and it is quite old and the turnings are not sharp anymore, then we can say that sample is damaged but the tool may perform the grinding operations.

But if the same tool is unable to perform grinding (intended) operation then we will call that the tool is failed.

But fracture is the term which is related to the breakage of the component in two or more that two parts. There could be different reasons for fracture like over loading, fatigue, creep etc.

I hope it would be clear to you now

The term 'damage' is often used as a parameter quantifying the extent of a material's degradation. For example, the ratio 'degraded modulus/Initial modulus'.

'Fracture' occurs once the 'damage' reaches a critical limit.

'Fracture' can be a reason leading to 'failure'. I agree with Sajith's explanation on 'failure'.

Enough explanation has been given for damage and fracture. The term "failure" is vague and relative. What is "failure" for one system may not be so for the other. In true sense "failure" is connected to the functional requirement of the component. In engineering sense, if you use damage tolerant approach (wherein you can allow the component to accumulate certain level of damage) then attainment of certain damage level is failure (typical example: turbine components). On the other hand if you use defect tolerance approach (wherein you can allow a pre-existing flaw/defect to grow to a certain extent), then defect reaching a critical size is the "failure" (typical example: crack in a thick section component). "Failure" is therefore not connected to whether or not the component/material is separated into two pieces. The definition of "failure" should come from how do you expect the component to perform and when do you decide that you can no longer use the component for the intended purpose. To put it in layman's language, if you find a dent in aero turbine blade, you would have called it failed and replaced with a new one immediately. If you find your screw driver bent, you would definitely not throw it and would continue to manage with it. In the first case, the instrument cannot perform the intended purpose any more whereas in the second case it can still be used !!

I think if in a material propagates crack it is damaged, if a material subject to stress goes to plasticity region it is failed, and if it passes ultimate stress, it is fractured. is it true?

I think, in Fracture mechanics,we have a body with crack(pre-cracked body or cracked body due to the fatigue),and we try to obtain stress and displacement fields around the crack tip and then we discuss about propagation of the crack,but in damage mechanics, we study on any material that it has passed yield point, in other word,in fracture mechanics we discuss about stress intensity factor,energy release rate,...but in damage mechanics we discuss about yield strength,ultimate strength and etc. It seems that damage is more general than fracture.

I think that we have to specify if the discussion concerns common lenguage rather than scientific meanings. Within the framework of structural mechanics the three words have precise differences.

Failure, in my opinion, concerns the loss of seviceability of a structure or a machine. Moreover with reference of strenght of materials often the term failure indicates the ultimate state of a material when one will not precise what brittle or ductile material is described. For ductile materials yield limit is more precise when plastic deformation occurs and failure when, say, a specimen breakes into two parts. Brittle materials breakes with no, or few, plastic deformations then simply fails.

Fracture concerns the growth of a discontinuous surface in the inerior of a specimen where displacement show jumps and the stress approaches zero but at the crac 'tip' where stress diverges, or tends to great value depending on plasticity or elasticity of the material near the crack tip. Fracture mechanics deals with the study of the propagation of cracks within structures.

Damage mechanics concerns the loss of stiffness and strenght during loading process of many materials. It has to be considered that damage depends on the formation af many micro cracks at the micro scale. In damage mechanics the effect of micro-cracks is evaluated at the macroscale using a simple parameter of the stress without considering the actual cracked state.

It is evident that failure and damage concern observation at a great scale and fracture describes the effective mechanism of the strenght of the material at a little scale.

Last consideration concerns the transiction from damage at the micro leve to macrofracture that is that a great amount of micro cracks can transform into a great macro cack responsible of the ultimate fracture on a specimen. Modelling of such transition is the aim of non local models of damage mechanics.

Fracture can be modeled using damage mechanics. Very nice examples are shown in "Ductile fracture initiation and propagation modeling using damage plasticity theory" by Xue and Wierzbicki (2007). On the other hand, damage mechanics can be used to reach almost failure values while the body is computationally treated as a continuous medium/continuum.

Damage is a physical discontinuity in the object or material. It can be introduced either during manufacturing or service stage. The damage can impair usefulness or normal functioning of the object or material. The damage can be of micro, meso or / and macro scale. Damage characterizes the state of the object or material. Quantitative evaluation of damage location, shape, size, evolution and effect can be performed based on experimental, analytical and numerical techniques.

Failure is the state or condition of not meeting a desirable or intended objective of the object or material. Product or material failure ranges from failure to sell the product or material to its fracture. Failure characterizes functioning or lack of functioning of the product or material to its desirable or intended requirement.

A fracture is the separation of an object or material into two or more pieces under the action of applied / induced stress. Fracture strength or breaking strength is the stress when a specimen fractures. 

Damage components can not be repair due to became old.

Failure components can be repair.

Fracture components can not be use and maybe can be repair to use but the properties is not same to initial properties of part. however, the fracture means to separate and break to two or more than two small parts

What does mean by failure in mechanical engineering?:- The failure of engineering materials is almost always an undesirable event for several reasons; these include human lives that are put in jeopardy, economic losses, and the interference with the availability of products and services.

Even though the causes of failure and the behavior of materials may be known, prevention of failures is difficult to guarantee. The usual causes are improper materials selection and processing and inadequate design of the component or its misuse.

Fundamentals of fracture:

Simple fracture is the separation of a body into two or more pieces in response to an imposed stress that is static (i.e., constant or slowly changing with time) and at temperatures that are low relative to the melting temperature of the material.

The applied stress may be tensile, compressive, shear, or torsional; the present discussion will be confined to fractures that result from uniaxial tensile loads. For engineering materials, two fracture modes are possible: ductile and brittle. Classification is based on the ability of a material to experience plastic deformation.

Read more:

https://www.mechanicalsite.com/2019/12/failure.html

Damage: It refers to the state of material where the properties of the material (e.g. stiffness) has degraded due to the formation of defects, voids, micro-pores etc.

Failure: The term failure has no single meaning, it is mostly context or application-specific. For instance in the design of machine elements generally yielding is considered as a failure while in some other cases fracture is considered as a failure; Also the term failure refers to structural failure such as buckling. It is basically a state where a component or structure fails to perform as desired or designed.

Fracture: It's the phenomenon which breaks an object into two or more pieces. At this stage, the material can no longer take the load. In general, Crack propagates to make fracture happen.

Dear Mohammd,

Very good topic.

Damage-

Damage is the actually loss of material due to a mechanical action. Just one example, damage of pipeline can be defined as localized damage resulting from contact between the pipe and an object. Damage leads to material failure, and failure leads to fracture of materials.

Fracture-

In a simple way it is defined, this is the separation of an object or material into two or more pieces under the action of applied / induced stress. Fracture is a common failure mode for equipment/system/devices with movable structures. The root cause of mechanical fracture can be overload, mechanical shock, fatigue, or stress corrosion. Mechanical fracture of material is caused by disconnection of atomic bonds or molecular bonds. It is classified into brittle fracture and ductile fracture according to size of strain at the time of fracturing. Fracture mechanics is a methodology that is used to predict and diagnose failure of a part with an existing crack or flaw. The presence of a crack in a part magnifies the stress in the vicinity of the crack and may result in failure prior to that predicted using traditional strength-of-materials methods. Fracture mechanics, like the other branches of mechanics, is concerned with predicting the response of a system to external disturbances.

Failure-

The term is applied to mechanical systems failure. Failure characterizes functioning or lack of functioning of the product or material to its intended requirement. In the case of non-typical vibrations, the main causes are: defective bearings and gears, electrical and mechanical failures in motors, problems caused by misalignments, unbalances or unstable bases, bent shafts, pulley or belt failures, mechanical gaps, aerodynamic or hydraulic problems, etc. To understand the mechanism of the failure one need to understand that responsible caused which can be of different kinds, often multiple factors play a role at the same time. It might be include like corrosion, welding of contacts due to an abnormal electric current, return spring fatigue failure, unintended command failure, dust accumulation and blockage of mechanism, etc. Seldom only one cause or hazard can be identified that creates system failures. The real root causes can be traced back to some kind of human error, e.g. design failure, operational errors, management failures, maintenance induced failures, specification failures, etc.. By carefully observation failure conditions can be described by identifying whether failures are reproducible or transient, and hypothesizing what combination of conditions and sequence of events led to failure is part of the process of fixing design flaws.

Ashish

References

1. Failure Analysis of Engineering Materials and Structures, Editors: Boukharouba, Taoufik, Elboujdaini, Mimoun, Pluvinage, Guy (Eds.)